Penetrants TESTING

DYE PENETRANT TESTING CONSULTANTS

1

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CONTENTS

1.

INTRODUCTION

01

2.

PRINCIPLES OF DYE PENETRANT TESTING

04

3.

CLASSIFICATION OF DYE PENETRANT TESTING METHODS

10

4.

FLOW CHART OF VARIOUS PROCESS COMBINATIONS

13

5.

FLOW CHART FOR PROCESS SEQUENCE OF VISIBLE

14

DYE AND FLOURESCENT TYPE PENETRANTS, 6.

DUAL SENSITIVITY PENETRANTS PROCESS SEQUENCE

15

7.

TYPES OF PENETRANTS

17

8.

THE PROCESS OF EMULSIFICATION

23

9.

DEVELOPERS

32

10.

ULTRAVIOLET LIGHT

38

11.

CLEANING OF TEST SURFACE

40

12.

STANDARD SPECIMEN

43

13.

SYSTEM PERFORMANCE CHECK / KDM PANELS

44

14.

EVALUATION

55

15.

INSPECTION

51

16.

TEST PROCESS

56

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INTRODUCTION LIQUID PENETRANT TESTING Liquid penetrant test method enhances the visibility of surface breaking flaws

such

as

cracks, fissures, crevices

and

pores. It

can

be

used

very

successfully regardless of component size and can tolerate complicated part geometry. Liquid penetrants can seep into various types of minute surface pores or openings by capillary action, because of this, the process is well suited for detection of all types of surface cracks, laps, porosity, shrinkage areas, laminations and similar type of discontinuities. It is extensively used for the inspection of cast and wrought and welded products of both ferrous and non ferrous metals, powder metallurgy parts , ceramics, plastics, and glass objects. Penetrant testing is used on metals such as aluminium, magnesium, brass, copper, cast iron, steel, stainless steel, titanium and other common alloys. It can also test other materials, including glazed ceramics, plastics, molded rubber, powdered metal products and glass. Some limitations are, The discontinuity to be detected must be open to the surface and the interior free from foreign materials. The test surface should not be porous. The material under test must not be susceptible to damage from the

liquids used for the examination The test process has temperature

limitations [ 10 to 52 0 C ]. [ using special materials, the range is -15 to 200 0 C part temperatures ]. As

Special requirements, Penetrant materials must be

designed with a low sulfur and halogen content to avoid harmful effects on the test parts. Stainless steels are especially susceptible to corrosion when exposed

to

chlorine

and

Carbon

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steels

to

sulfur. Titanium

is

extremely

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susceptible to embrittlement when in contact with halogens. High Nickel alloys are also affected by sulpher and halogens. These harmful chemicals can be found in penetrant materials but are limited to 1% by weight of content. Common penetrant materials attack PVC, making it brittle, which leads to cracking. Liquid oxygen compatible penetrant materials must be used when testing parts that will be in contact with either liquid or gaseous oxygen. Method : A penetrating liquid is applied to the clean and dry test surface and allowed to enter the discontinuity opening over a period of time. The liquid soaks into material flaws that are open at the test surface. After a suitable penetration time, the excess surface penetrant is removed and the test surface is dried. A developer is then applied in a thin uniform coating which acts as blotter and draws some of the entrapped penetrant out of the discontinuities. The penetrant stains the developer and the indication becomes visible. The surface flaw becomes increasingly visible to the eye, because the dye spreads in the developer and effectively broadens the trace. The dye indicator appears either red or blue on a white background for color contrast penetrants or appears Yellow – green or orange - red on a dark violet background, when the surface is illuminated by an ultraviolet lamp for a fluorescent test process. The sensitivity of the test depends on the size of the discontinuity opening and not on the length. Crack width is the more important determinant of a penetrant's ability to detect cracks. Tight narrow cracks, regardless of length, are more difficult for penetrants to find. Fatigue cracks and forging cracks are tighter and require more judicious processing and higher sensitivity penetrants to locate than casting cracks of the same length. The crack limit for this inspection method is approximately 0.5 µm crack width. Detection of surface flaws depends on the general condition and finish of the test surface. The depth of the anticipated defect should be three times deeper than the general surface roughness "valley" depth. Defect detect- ability can be further diminished by normally acceptable surface conditions such as

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tool marks, scratches, scale, edges, grooves, notches, rough weld surfaces etc, where penetrant is trapped. Although the dye penetrant test is a very easy process, it is also very easy to make mistakes and make the test ineffective. Care must be exercised by the operator in order to perform all the steps of the examination correctly.

Sensitivity – equal or better than MT – better than RT for surface discontinuities 1 μm x 10 μm x 50 μm can be detected. Applications – on all materials – metals(ferrous and nonferrous) nonmetals (rubber ,plastic etc) all type of defects (open to surface) Normally not applicable to porous materials (unfired ceramics and powder metallurgical parts) Presently Filtered particle penetrant for porous materials has been developed. Rough surfaces - > 125 μm-background poses problem in using penetrant testing. Highly coloured (visible or fluorescent) organic dye liquid which is also surface active in nature (called penetrants) is applied on to the clean surface of the component and allowed sufficient time for penetration into discontinuities. The excess surface penetrant on the component is removed. This leaves a clean surface of the component with pentrant residing in the discontinuities. At this point Of time, developr, which is highly absorptive in nature, is applied. The developer brings back or bleeds out the penetrant thereby providing an indication in a

contrasting

background of white colour of developer. 1.Preparation of part 2. Application of penetrant PREPARED BY S.MUTHUKUMAR, B.Tech lll

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3. Removal of excess surface penetrant 4. Development 5. Inspection and Evaluation 6. Post Cleaning

Physical principles

Clean surface of Component Penetrant application

Removal of excess surface penetrant

Application of developer

After removal of excess surface penetrant and before application developer

Inspection and Interpretation

Fig.1. Principle of Penetrant Testing

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liquid penetrant inspection depends mainly on penetrant effectively wetting the surface of a solid work piece or specimen, flowing over that surface to form a continuous and reasonably uniform coating and the migrating into cavities that are open to the surface. So this ability of given penetrant to flow over a surface and enter surface cavities depends principally on the following:  Cleanliness of the surfaces  Configuration of the cavity  Cleanliness of the cavity size of surface opening of the cavity  Surface tension of the fluid  Ability of the liquid to wet the surface  Contact angle of the liquid The cohesive forces between molecules of liquid causes the surface tension, a best example for this phenomenon is tendency of liquid over the surface such as droplet Of water, these contracts into sphere, in such droplet, surface tension is counter balanced by internal hydrostatic pressure of the fluid. When the liquid comes into contact with a solid surface, a cohesive forces responsible for surface tension competes with the adhesive forces between the molecules of the liquid and solid surfaces,, these two forces jointly determines the contact angle between the liquid and wet surface,b There are are three types of liquid wets the solid surface in relation to contact angle,  If contact angle is < 90 degrees

liquid having good wetting ability

 If contact angle is > 90 degrees

liquid having poor wetting ability

 If contact angle is = 90 degrees

liquid having poor wetting ability

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The property of a liquid called capillarity is explained as the process of increase in the meniscus or of the fluid in a thin tube over the water level when this tube is immersed in water in container, this capillary rise is also closely related to wetting angle between the liquid that wets the tube and the inner surface of the tube, Wetting angle between capillary tube And liquid Less than 90 degrees then meniscus of the liquid will be concave and liquid level in the tube raises in the tube over the liquid level in a container – having good wetting ability

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Physical principles Capillary pressure:  h = 2T cos θ  wρg 

 

Wetting ability

θ

good, 90°

Wetting angle between capillary tube And liquid

greater

than 90 degrees

then

meniscus of the liquid will be convex and liquid level in the tube reduces in the tube over the liquid level in a container – having poor wetting ability This is due to hydrostatic pressure immediately under the meniscus is reduced by the distribution of surface tension in the concave surface, and the liquid is pushed up by the hydraulically transmitted pressure of the atmosphere at the free surface of the liquid outside the capillary tube. The height to which the liquid rises is directly proportional to the surface tension of the liquid and to the cosine angle of contact, and it is inversely proportional to the density of the liquid and the radius of the capillary tube, This above explained similar phenomena is applicable for the penetrants entering the discontinuity and seeping out when developer is applied.The term viscosity is also related to rate at which the liquid will flow and it is having negligible effect in penetrating ability. highly viscous liquids are not suitable for not suitable for penetrants, since rate of flow is slower one consequently requires longer time to migrate into fine flaws. PREPARED BY S.MUTHUKUMAR, B.Tech lll

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The principle of liquid penetrant testing is based on the ability of some liquids to enter a discontinuity opening and then re-emerge from it when the excess penetrant is removed from the surface. Capillary action is the means by

A liquid drop on a ideally smooth, clean surface at equilibrium γL



γSL



  







γSG

poor wetting Good wetting θ < 90 γ SL

θ>90 γL

γL COSθ

γ SG

L = LIQUID- GAS, SG= SOLID-GAS, SL= LIQUID-SOLID FOR GOOD SPRE ADING γ SG > γSL The above description is for a liquid drop on a solid surface and above the liquid is gaseous m edium

. which a liquid enters a discontinuity opening. This action is what causes a piece of sponge to absorb liquid Capillary action is a phenomenon in which water or other liquids will rise above the normal liquid level in a small bore or capillary tube due to the attraction of the molecules in the liquid for each other and for the wall of the tube [ cohesion and adhesion ]. Cohesion is interaction between two surfaces of the same material in contact, makes them cling together [ with two different materials the similar phenomenon is called adhesion ]. According to kinetic theory, cohesion is caused by attraction between particles at the atomic or molecular level. Surface PREPARED BY S.MUTHUKUMAR, B.Tech lll

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which

causes

SAIMIRA

10 liquids

to

form

spherical

droplets, is

caused

by

cohesion. The distance the liquid will rise up the tube of a given diameter and material is a function of three factors; Surface tension, Wetting ability, Tube open or close at the top end. Liquid will rise less in a closed end tube. The practical circumstances, a penetrant encounters during testing is more complex. Cracks, for example are not capillary tubes, but simulate the basic interaction between a liquid and a solid surface, which is responsible for the migration of penetrant into its open space. This same interaction acts again and penetrant emerges from the discontinuity when the excess penetrant is removed

from

the

surface

Factors influencing Penetrant infiltration into discontinuities       

1. surface tension of the liquid penetrant 2. discontinuity configuration constant 3. surface coatings and contaminants 4. additives and contaminants in the liquid penetrant 5. mechanical obstructions 6. temperature of the test object 7. roughness of the interior walls of the discontinuity

Normally, the penetrants need to have a very low contact angle and the commercial penetrants have contact angles between 0 –5.. Contact angle depends on the solid surface to be wetted. Water-glass has a contact angle of 0 deg. compared to water-silver which is 90 deg

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Once the liquid wets the surface, the ability of the liquid t Surface tension Is one of the most important properties which determines the penetrating ability of a liquid. Low surface tension liquids provide better penetration and spreads well on part surface. Wetting ability is

also determines the penetrating ability of a liquid. Ability

to wet or spread on the surface is related directly to the contact angle between the liquid and the surface at the point of contact. To have a good wetting ability, the contact angle must be small. Penetrants used for testing have contact angle of 50 or less. Brightness The dye in the penetrant should be highly stable and bright enough to be visible in very thin film. Volatility is the speed with which a liquid evaporates. Penetrant should be non volatile to allow long penetration and inspection time. The penetrant must not dry during the examination period. Flash point is the temperature at which flammable vapor is given off. For safety purpose, a penetrant should have higher or no flash point. Chemical Innertness is the ability of a material not to interact when mixed with or brought into contact with other materials. Penetrant should be as inert and non corrosive as possible towards the materials to be tested. Solubility is the ability of a material to be dissolved into another material. Penetrant must be soluble in order to be easily removed from the surface of the part being examined. Creep is the ability of

small amounts of liquid in

discontinuities to com back out to form an indication. Tolerance for Contamination is the ability to tolerate small amounts of foreign substances and not affect unfavorably the action of a penetrant. A penetrant is a compound of several ingredients and a little water, acids, detergents and degreaser solvents may upset the balance and cause the penetrant to lose some or all of its important properties. Toxicity, Skin irritation and odor No penetrant should contain poisonous, corrosive or skin irritating material or have an offensive odor. PREPARED BY S.MUTHUKUMAR, B.Tech lll

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Penetrant properties points to remember 1. Low surface tension and contact angle, capable of smoothly and evenly spread on the part surface. 2. Low viscosity, capable of entering small discontinuity openings faster. 3. Non volatile. 4. Easy removal from part surface but not from the discontinuities. The penetrant should get removed with no dye precipitation. 5. Capable

of

emerging

from

discontinuities after

excess penetrant

is

removed. 6. High visibility and contrast in small quantities and thin films on part surface. 7. Does not corrode the test surface. 8. Must be Non Toxic The rise in the capillary or enter into the openings is determined by surface tension (T) Hence the main properties of penetrant are T and θ . The ability of the liquid to spread or wet the solid surface is related to the contact angle θ, which quantifies the resultant adhesive and cohesive forces, The contact angle is defined as the angle between the solid surface and the tangent drawn to the liquid at the point of contact. It can be seen that spreading ability and contact angle are inversely related. Colour contrast ratio of visible dye is ~1:10

(The light reflected by the white

background to bright red of the dye). Colour contrast ratio of the fluorescent dye is ~1:100 (light emitted by the indication to the light emitted by the dark background) Because of this colour contrast ratio, the indication is better seen in the case of fluorescent indication . The human eye brings an effect called halation effect, the ability to magnify the indication property of penetrant is dye solouble capability to produce easily visible indications,finally this penetrants must be easily removed by suitable solvents or remover or emulsifier without precipitating the dye, PREPARED BY S.MUTHUKUMAR, B.Tech lll

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The performance of a penetrant is achieved by a combination of controlled physical and chemical properties ; Viscosity Is related to the rate at which a liquid will flow under some applied force. It affects the speed of penetration through the discontinuity opening. A low viscosity liquid is used for

faster penetration and also less

drag out in immersion. There are mainly three types of penetrants namely  Liquid Penetrant è TYPE ∗

Fluorescent >>> Type (I) ;

Visible

>>> Type (II)

∗  Excess Penetrant Removal è Method ∗

Water Washable Method (A)

∗

Post-Emulsifiable, Liphophilic Method (B)

∗

Solvent removable Method (C) *

Post-Emulsifiable, Hydrophilic

Method (D)

 Developer è Form ∗

Dry Developer >> Form (a)

∗

Water-soluble >> Form (b)

∗

Water-suspended

∗

Non-aqueous

>> Form (c)

>> Form (d)

 Solvents for pre-cleaning è CLASS ∗

halognated è Non-Flammable Class (1)

∗

non-halognated è Flammable Class (2)

Application of penetrant Penetrant can be applied by immersion, dipping ,spraying, swabbing and pouring PREPARED BY S.MUTHUKUMAR, B.Tech lll

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Dwell time : The total time the penetrant is contact with the test surface including the time required for application and for drain . Dwell time = application time + drain time Normally 5-30 mts. Depends on the size and nature of discontinuity and the material and surface condition of the material etc. The only requirement is that a thin layer of penetrant should be present for the specified time (dwell time) on the surface to be inspected Removal of excess penetrant The excess surface penetrant on the surface is carefully removed without affecting the penetrant that is residing in the discontinuity. In any NDT method the Signal to Noise ratio is important and this should be as high as possible S- Volume of penetrant that has entered into the discontinuity N- Unremoved excess surface penetrant in the discontinuity This is an important step as the unremoved excess surface penetrant will affect subsequently by affecting the contrast of the indication (excessive background) and if any penetrant in the discontinuity is disturbed the volume of the penetrant indication will get reduced (The volume of penetrant is already very small) It is obvious that in PT to achieve a high S/N ratio, the excess penetrant on the surface should be removed as completely as possible and the penetrant is the discontinuity should not be lost by overwashing Penetrant removal methods Dissolve and remove 1. Disperse (emulsify) and remove by water washable/post emulsification 2. Simply water wash if penetrant contains built in Emulsifier (WW) In each case, care need be exercised so as to completely remove the excess penetrant on the surface completely and at the same time the penetrant in the discontinuity is not affected. Wash – don’t over wash . In the case, welds, when need be inspected without flushing the crown, the surface oughness poses problems giving background colouration thereby affecting the visibility of the

ndication. Hence Solvent removable and water washable

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variations are normally employed. Here overwashing tendency of these methods, istaken to advantage, meaning, the surface is

much free from the background

colouration. Of course, the sensitivity suffers. Sensitivity depends on the amount of retained penetrant in the discontinuity DEVELOPING After

the removal of the excess surface penetrant , in developing step, the

penetrant from the discontinuity is brought to the surface so as to form an visible indication. Developer functions. The developer functions are 1.To assist the natural seepage of the penetrant in the discontinuity and extract or blot out the penetrant so as to form

an visual indication (mechanism of visual

indication) 2. To provide a contrasting base which enhances the detection an indication. Blueblack to yellowish green: white to red 3. To spread the penetrant so as to increase the apparent size of the indication 4. To mask some confusing indications Mechanism of developing action : Natural seepage is assisted and the absoptive developers blot out more penetrant from the discontinuity. The thickness of the penetrant layer is increased to the levels above the threshold visibility .( Some fluorescent penetrant indications can be seen without developing with high intensity black light -3000 microwatt/sq.cm) Developer has two variations: Dry and Wet. In the case of wet, aqueous and non aqueous.As the sensitivity of wet developers are better than, dry developers and since in weld inspection, less sensitive methods namely water washable and solvent removable are employed , it is better to use wet developers (mostly nonaqueous) in weld inspection in spite surface roughness. Normally for high surface roughness, dry powder is better suited . FLOW CHART ILLUSTRATES VARIOUS PROCESS COMBINATIONS PREPARED BY S.MUTHUKUMAR, B.Tech lll

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VISIBLE

WATER

EMULSIFIE R

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FLOURESCENT

DUAL SENSITIVITY

SOLVENT

WATER

SOLVENT

WATER

WET

DRY

WATER SUSPENDED PARTICLE

WATER SOLOUBLE

NONAQUEOUS WET

FLOW CHART FOR PROCESS SEQUENCE OF VISIBLE DYE AND FLOURESCENT TYPE PENETRANTS, PREPARED BY S.MUTHUKUMAR, B.Tech lll

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WATER WASHABLE

POST EMULSIFIED

SOLVENT REMOVAL

APPLY PENETRANT

APPLY PENETRANT

APPLY PENETRANT

APPLY EMULSIFIER

SOLVENT WIPE

WATER WASH

DRY

APPLY WATER BASED WET DEVELOPER

APPLY DRY POWDER DEVELOPER

DRY

APPLY DRY POWDER DEVELOPER

APPLY NON AQUEOUS WE DEVELOPE R

INSPECT POST CLEAN

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DUAL SENSITIVITY PENETRANTS PROCESS SEQUENCE

DUAL SENSITIVITY PENETRANTS PROCESS SEQUENCE

PRECLEAN APPLY PENETRANT

SOLVENT WIPE

WATER WASH APPLY DRY POWDER DEVELOPER

DRY APPLY NON AQUEOUS WET DEVELOPER

INSPECT POST CLEAN

Developing action at the end of developer time reveals an indication. In this step the indications are interpreted as to the origin of the indication namely whether True or false indications and if true indication whether relevant or nonrelevant and if PREPARED BY S.MUTHUKUMAR, B.Tech ASNT NDT LEVEL lll

DYE PENETRANT TESTING CONSULTANTS relevant

as

to

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19

the

nature

of

discontinuities

linear

or

volumetric.

Inspection environement depends on the type of dye - Visible or fluorescent dye white light for visible and black light for fluoresecent. Types of penetrants Penetrants are generally a chemical solution of highly stable dyes, either visible or fluorescent, in a mixture of surface active agents with a blend of highly refined hydrocarbon distillate. The carrier liquid is practically colorless and transparent. The dye provides a high contrast indication against the background. Petroleum distillates, which are relatively nonvolatile and used to dilute both

water

washable

and

non - water

washable

penetrants, contribute

to

penetrant performance in several ways. As a diluent, a light petroleum distillate dissolves light organic soils on surfaces and in flaws, and it assists crack penetration with its low surface tension and affinity for metal surfaces. Also, petroleum distillates naturally fluoresce a light blue color and, in a minor way, contribute to penetrant brightness. Petroleum distillates are source of halogens and sulphur. Water

based

fluorescent

penetrants [ using

water

as

diluent

which

represents at least 50% of a penetrant ] have been developed for easy disposal, previously used as non-certified applications, are now approved. Water based fluorescent penetrants are restricted to available

in

sensitivity

level

1

and

2. The

water washable method and penetrants

are

liquid

oxygen

compatible, suitable for use as a leak detector and for inspection of plastics. The penetrants are compatible with water based precleaners, e.g., hot alkaline, which for environmental reasons have replaced petroleum solvents and vapor degreasers. Being compatible with the aqueous cleaner, these penetrants have tolerance for rinse water carry over. Also, flaw entrapped water is less likely to impair the inspection process, such as would happen when flaw entrapped water repelled a petroleum based penetrant. PREPARED BY S.MUTHUKUMAR, B.Tech lll

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Water in water based penetrants will evaporate, and regular, testing with a refractometer is necessary to measure the loss. Periodic water additions are part of the maintenance criteria for these penetrants. Longer penetration time [ 30 mins ] improves sensitivity and dry developers are less suitable for these penetrants. Selection of penetrant type depends on, •

Sensitivity required. [ tightness of the opening ]

•

Number of articles to be tested.

•

Surface condition of the part to be tested.

•

Configuration of the test specimen.

•

Availability of water, electricity, compressed air and other equipments.

•

Suitability in environment where the test will be performed.

•

Cost of inspection. In weld inspection, lower sensitivity methods,namely water Washable and

solvent removable are employed due to surface roughness of the weld. Hence, use of high sensitivity penetrant namely fluorescent dye penetrant may be preferred. Visible Dye : Visible dye or color contrast penetrant inspection makes use of a dye that is visible in ordinary light. These penetrants contain a highly stable bright red or purple dye, so that the indications produce a definite contrast with the white background of the developer powder. The dyes are visible in very thin film.

Color contrast penetrant can not achieve the level of sensitivity which is

possible with the fluorescent penetrants because of higher viscosity and inability to enter very tight cracks. As a general rule of thumb, visible penetrant examination is roughly equal to fluorescent penetrant examination of level 1 sensitivity. Discontinuities in the range of 50 µ in NiCr panels are routinely detected and under the right conditions and processing 30 µ discontinuities PREPARED BY S.MUTHUKUMAR, B.Tech lll

can

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advantage of using a visible penetrant is that it can be used with ordinary shop lighting and from a small portable solvent removable kit at any location. They are specially suitable for field applications and where darkening the inspection area is impractical.

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Sensitivity of penetrants All fluorescent methods are more sensitive than visible dye penetrant Penetrant Type Properties

Visisble dye

Fluorescent dye

Indication

Bright red colour

Yellowish green(on excitation) Dark (max. 2 ft.candles)

Background

White colour of the developer Colour contrast ratio ~1:10 Visibility(seeability) Medium Halation Normal effect(ability of the eye to magnify the indication) Sensitivity Medium

~1:100 Very high High

High

In weld inspection, lower sensitivity methods, namely water Washable and solvent removable are employed due to surface roughness of the weld. Hence, use of high sensitivity penetrant namely fluorescent dye penetrant may be preferred.

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Visible penetrants are less vulnerable to contamination from cleaning fluid that can significantly reduce the brightness of a fluorescent indication. Visible penetrants are used in maintenance, repair and in manufacturing processes where a visible indication helps in locating the area for subsequent repair work, such as large rough castings. These penetrants are also used for through leak testing of heat exchangers and tanks, where penetrant is applied to one side and developer to the other. Effect of Temperature: Penetrants are normally developed for use from about 10 to 520 C. However they can be used as low as 4 0 C but penetration time must be increased by 2 to 3 times. At low temperature, penetrant becomes highly viscous and sluggish. Ideal operating temperatures for normal use of penetrants lie between 10 0 C

and

380 C.

Penetrant

testing

at

higher

part

temperature

has positive

advantage. The molecular movement of a liquid is directly related to the liquid's absolute temperature. The higher the temperature, the greater the movement and viscosity decreases. Thus, a penetrant, in contact with a heated part, will be less viscous and fast penetrate into flaws by displacing any gas or liquid from the interior of the discontinuity. Heat vaporizes discontinuity entrapped solvents and moisture, which might otherwise interfere with penetration. Heat liquefies heavy oils and waxes present from

previous

processing, facilitating penetrant

displacement. Thus, a

heated

penetrant will more effectively fill a crack. In addition, when penetrants are heated, dwell time may be reduced because of faster penetration. During the dwell time, the part temperature will fall and the viscosity of the penetrant will increase. This will resist over removal. Similarly, the developing step is also faster, penetrant molecules are more readily absorbed by the developer layer, and the developer dries faster. When inspecting heated surfaces with water washable penetrants, it would be prudent to wipe the surface with a solvent or cleaner resistant to high temperatures, since boiling water may not rinse excess penetrant satisfactorily. High temp certified penetrant materials shall be used. PREPARED BY S.MUTHUKUMAR, B.Tech lll

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Fluorescent Dye [ Type I ] : Fluorescent penetrant uses minerals or chemical compounds

which

emit

Fluorescent

penetrant’s

visible

light

when

exposed

response

is

maximum

when

to

ultraviolet

exposed

to

light.

365 nm

wavelength ultraviolet light.

The dye absorbs this energy and emits between 520 nm to 620 nm wavelengths

which

is

visible

as

brilliant

yellow-green. Orange - red

type

penetrants are also available. The efficiency of fluorescent dyes in converting UV light to visible light may be reduced by prolonged exposure to UV light. Elevated temperature can reduce the fluorescence of penetrant in open tanks. Fluorescent penetrants are designed for different sensitivity levels ;  Level 1/2

Ultra low,

 Level 1

Low sensitivity 50μm NiCr crack panels.

 Level 2

Normal sensitivity 40μm, general purpose use.

 Level 3

High sensitivity 20 - 30μm.

 Level 4

for castings and rough surfaces.

Ultra high sensitivity 10μm extremely critical use.

The actual certifications for sensitivity level ½ to 4, involves the use of a series of titanium and nickel alloy panels containing very small, laboratory generated fatigue cracks. Fluorescent penetrants are more sensitive than visible dye because they have lower viscosity, can penetrate smaller openings, and PREPARED BY S.MUTHUKUMAR, B.Tech lll

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have excellent visibility. It is easier to detect something glowing in the dark than to observe a small colored area on a white background. Fluorescent indications

are

many

times

brighter

than

their

dark

surroundings. The

brightness of indications depends on the incident UV - light intensity and minimum 1000 micro watts / cm2 is required at the test surface. Background white light illumination is to be limited to 20 Lux. Fluorescent

penetrants

require

a

developer

with

lower

particle

concentration, because the white background provided by the developer is not necessary. Developer

is

required

only

for

the

blotting

action. Fluorescent

indications are visible in very thin film. For these penetrants, removal of excess penetrant by solvent wiping requires more care and is a problem when a large surface area is to be cleaned. Water washable type will reduce this problem. For fluorescent penetrant testing, it is very important to ensure that the part surface is completely free from nitric, sulphuric, chromic acids, alkaline permanganate, acid

chromate

solutions, acid

ferric

solutions, peroxides,

persulphates etc. These common cleaners all degrade or kill the fluorescence completely.

Dual sensitivity

penetrants [ Type III ] :

These

penetrant

contains

a

combination of red visible dye and orange fluorescent dye, such as ‘By Lux’ from Sherwin. The test object is first viewed under white light for red dye indications

followed

by

ultraviolet

light

in

a

darkened

area

for

orange

indications. These penetrants provide two levels of inspection sensitivity. One advantage is that flaws located during UV examination are marked by visible

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DYE PENETRANT TESTING CONSULTANTS indications

for

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26

repeat

examination

or

repair

work.

Another advantage is that flaws found in one mode can be verified in the second

mode. However, the brightness

of

the

visible

red

color

and

the

fluorescent color are less than the individual visible dye and fluorescent penetrants. Important : Fluorescent penetrant should never be used on surfaces which have been processed previously with color contrast penetrant. In many cases residues of dyes persist in a defect, the fluorescent process suffers not only from the residue reducing the amount able to get into the defect, residues of color dyes will compete for light and effectively kill the fluorescence of any entrapped fluorescent penetrant in subsequent tests. 1% visible penetrant can stop the fluorescence. If fluorescent inspection is required, then the part shall be cleaned thoroughly to remove any color penetrant residue from the interior of the discontinuities. Applying a reversible penetrant developer, which contains a fluorescent dye, reacts with,

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and thus is quenched by, the red dye. When the surface is viewed under UV illumination, the residual spots of red dye can be detected, which

stands out

as dark spots against a fluorescent background. Once the removal of red penetrant

dye

has

been

completed

and

verified

by

this

procedure, the

developer is removed by rinsing with water. Fluorescent Brightness : is the amount of visible light given off when a fluorescent dye is exposed to UV light. It depends on ; * the thickness of the penetrant film. * the intensity of the UV light. * amount of fluorescent dye and its capability to absorb UV light. * the efficiency of the dye in converting the

released electrons to visible

light. Penetrants were examined by placing a drop of used and new penetrant next to each other on a paper towel for a visual comparison. Most penetrant specifications require replacement when the brightness drops by more than 10%. Under test situations, the human eye may detect a change of about 15% or more in brightness. Fluorescent brightness measurement can be performed using a approved Photo fluorometer Note : The general rule is penetrant that

to use the lowest sensitivity

fluorescent

reveals the discontinuities of interest.

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Post Emulsification penetrants : These penetrants are not directly water washable and the danger of overwashing the penetrant from the discontinuities is reduced. An emulsifying agent is used that makes the surface penetrant soluble in water so that the excess penetrant can be removed by water rinse and then a developer can be applied. These penetrants have better penetrating ability than water washable penetrants and can detect minute flaws. These processes are widely used with fluorescent penetrants but much less with color contrast penetrant. Emulsifiers are

liquids

used

to

render

the

excess

penetrant

water

washable. The

manufacturers carefully formulate the emulsifiers depending upon the penetrant to be used. Penetrants and emulsifiers are used as a system, and emulsifier from

one

manufacturer

may

not

perform

adequately

on

a

different

manufacturer’s penetrant. There are two types of emulsifiers used in the removal processes ; They are Lipophilic emulsifiers and Hydrophilic removers [ detergents ] .Lipophilic emulsifiers [ Method B ] are oil based and contain blends of surfactants, esters and high boiling point hydrocarbon distillates. They are colored other than green to allow easy identification. These emulsifiers are used as supplied, and function by mechanical and chemical action. The commonest method for application of lipophilic emulsifiers is direct immersion followed by drainage. After the emulsifier has coated the surface of the part, mechanical action starts to removes some of the excess penetrant as the mixture drains from the part. During the emulsification time, the emulsifier diffuses into the remaining penetrant film on the surface of the part and PREPARED BY S.MUTHUKUMAR, B.Tech lll

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render it water washable. The emulsifier is fast acting, thus making the emulsification [ or contact time ] very critical. The emulsifier continues to act as long as it is in contact with the penetrant, therefore, the rinse operation should take place

quickly

to avoid over - emulsification. The

component

may

be

immersed rapidly in a large volume of water to stop the action of the emulsifying agent. On removal from the water, the wash is completed by spray rinsing. Spray application followed by drainage has also been used successfully. Brush application of lipophilic emulsifiers is prohibited, because this would mechanically mix the emulsifier into the penetrant resulting in non uniform emulsification.

Flowing

the

recommended

because it

emulsifier may

not

on

be

the

component

possible

to

surface

cover

the

is

not

component

surfaces rapidly enough to ensure uniform emulsification. It is essential that complex shaped components are rotated during the drainage stage so that the various surfaces receive similar processing. The three properties of lipophilic emulsifiers that control the washing characteristics are activity, viscosity, and water tolerance.

Specific contact

times for lipophilic emulsifiers should be established for each application. The contact time can vary between 60 and 180 seconds depending on the type of the emulsifying agent, the penetrant in use and the surface condition of the components. Lipophilic emulsifiers are used at temperatures between 15 and 250 C. Lipophylic emulsifier are miscible with penetrants in all concentrations. However, if

the

concentration

of

penetrant contamination

in

the

emulsifier

becomes too great, the mixture will not function affectively as a remover. A specification requirement is that, lipophilic emulsifiers be capable of tolerating 20% penetrant contamination without a reduction in performance. The emulsifier is to be replaced when its cleaning action is less than that of new material. Since lipophilic emulsifiers are oil - based, they have a limited tolerance for water. When the tolerance level is reached, the emulsifier starts to thicken and will eventually form a gel as more water is added. Specification requires that PREPARED BY S.MUTHUKUMAR, B.Tech lll

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lipophilic emulsifiers be formulated to function adequately with at least 5% water contamination and that lipophilic emulsifiers be replaced when the water concentration reaches the limit.

Flow diagram for PE lipophilic  Method Pre clean

D removal process ASTM E 165 dry

Apply penetrant

dwell

Dip in Emulsifier

rinse

Time is critical! !

reject

inspect Post clean

Non Aqueous developer

dry

OK

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Hydrophilic removers are applied by immersion in an aqueous solution. The concentration for immersion varies between the range of 2.5% to 10%, 20% and

up to

30 %

depending

on

brands

and

conditions

of

agitation. The

manufacturer of the emulsifier can provide the proper information concerning this concentration. Concentrations higher than recommended by the emulsifier manufacturer or the qualified percentage is prohibited. The immersion time varies from 20 seconds to 5 minutes depending on the penetrant, emulsifier concentration, surface roughness and agitation. The surface active agent in the remover combines with a small quantity of penetrant from the surface and prevents the penetrant from recombining with the remaining surface penetrant. A slight agitation is necessary to remove the colloidal suspension of penetrant – emulsifier from the surface and to expose fresh penetrant. The preferred method of agitation is mechanical.

Agitation using compressed air can cause heavy foaming and may introduce PREPARED BY S.MUTHUKUMAR, B.Tech lll

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application

is

by

immersion, hydrophilic

emulsifiers

malfunction as a result of penetrant accumulation. Contamination increases when part geometry includes cavities, or areas which entrap penetrant to be carried over to the emulsifier tank. Emulsifier with lower concentration gets contaminated faster. The 30% concentration, if used, tolerates some three times more penetrant contamination as the 10% concentration does. Everything being equal, a 30% concentration lasts three times longer than a 10% concentration, and is economical because of less frequent disposals. Hydrophilic emulsifiers are infested by fungus and algae and the tanks must remain covered when not in active use. 30% concentration has more resistance to this infestation. Hydrophilic remover solutions are also applied by spray or as foam. The concentration of the solution tend to be much lower up to 5%. However, higher concentration may be used depending on the manufacturers recommendation. Immediately following the remover spraying, a freshwater rinse of the entire part is required to stop the action of any remover remaining on the surface of the

part. Hydrophilic

removers

minimize

background

fluorescence

on

part

surfaces as well as bleed out of excess penetrant from hollow parts. The concentration of the detergent solution is critical to the success of the process and must be controlled. Water loss in hydrophilic emulsifiers results in more concentrated and more active solutions, a cause for overemulsification with resultant failure of the penetrant process. To monitor the concentration, the

refractive

index

of

the

working

solution

is

generally

measured and compared against graphs generated by measuring the refractive index of solutions of known concentrations, using a Refractometer [ an optical device with a reading scale ]. A drop of the working solution is placed on the proper spot on the refractometer and a reading is made on the scale and then converted into percentage by using a table or a graph applicable for the particular remover and supplied by the emulsifier manufacturer. The graph may be generated in the workstation using prepared solutions of the standard

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remover from 50% of the working strength to 50 % above. A minimum of 5 solutions of different strengths should be prepared for this purpose.

Hydrophilic emulsifier Method D Pre clean

Post clean

dry

Inspect With BL

Apply penetrant

Apply developer

Pre rinse

dwell

Inspect With BL

Collect The bulk Excess penetrant

Solvent removable penetrant process : Penetrants which cannot be removed directly with water can be removed by use of organic solvents. The solvents used are quite versatile and dries quickly without a residue and a developer can be applied on the dry surface. Normally, the same type of solvent is used for pre cleaning and for removing excess penetrant from the test surface. Typical applications involve in situ inspection or testing of a specific location on the component when it is not practical to remove the penetrant using water, which would involve subsequent drying of the component. Three types of solvent removers are used :  Volatile hydrocarbon distillates.  Halogeneted hydrocarbon solvents [ non flammable ]. PREPARED BY S.MUTHUKUMAR, B.Tech lll

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 Volatile aliphatic alcohols. The flammable cleaners are potential fire hazards but are free from halogens, while the non flammable cleaners contain halogens, which render them unsuitable for some applications.

Solvent removable penetrant may be

applied to the test surface by spraying, dipping, flooding or brushing. For a small test area, brushing

controls applied penetrant and removal of excess

penetrant is easier. Excess penetrant removal is performed by wiping the part surface in one direction only, with a clean and lint free cloth or absorbent paper. The proper procedure is to make a single pass, then fold the cloth to expose a clean surface for each succeeding wipe. This will remove most of the penetrant. The remaining traces are then removed by wiping with a new cloth or paper lightly wetted with the solvent remover. After each pass, the surface of the cloth is examined. If there is more than a trace of penetrant on the cloth, the cloth is folded to expose a clean surface, remoistened with the solvent, and the surface is wiped again. The process is repeated until there is little or no trace of penetrant on the cloth. Cleaning should be done quickly, using a minimum of cleaner. If the operation is prolonged or if excessive amount of the cleaner is used, some penetrant may be removed from discontinuities. The surface is then viewed under suitable light to ensure that the excess penetrant is removed. For application of developer, the surface is dried by evaporation or wiping.

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Non flammable, volatile solvents can be used in the vapor phase in automatic processing stations [ like vapor degreasing ]. The solvent should be redistilled frequently to avoid heavy contamination with penetrant, since the penetrant will start to co-distil with the solvent and cause heavy background. Note : The volatile organic solvents used for penetrant removal are the most aggressive and great care must be taken in their use to prevent overremoval. For manual processing, the solvent remover must be used by wiping with a cloth. Use of a solvent remover by spraying or flooding will results in over wash or dilution of the penetrant and the indications will become diffused and weak. The optimum sensitivity is achieved when the brightest indications appear against a thin developer contrasting background. When normal cleaning cannot sufficiently remove penetrant from aswelded surfaces, machined parts with sharp inside corners, or parts with depressions or pits, such areas can be further cleaned with a commercially available cotton swab or with a cotton swab on a toothpick wetted with the solvent.

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some hints 1. Due to surface roughness of

unflushed welds, it is better to use, water

washable and solvent removable methods to avoid undesirable background. 2. In the case of Solvent removable pentrants, usual precaution of wiping with solvent moistened lint free cloth may not yield the desired results because of excessive background. Sometimes, the solvent is directly applied to the weld to overcome this difficulty. Of course fine cracks may be missed 3. In the case of TIG and MIG where better surface is obtained, it is better to go solvent wiping method 4. Inspection with multiple pass welds with penetrants requires that each weld bead is inspected prior to the next pass to provide the next the best reliability. Cleaning after each testing 5. 5.Water washable fluorescent penetrants vessels or other

can be used on large pressure

large structures by washing with a hose and air

drying.

Developent with solvent suspended developer

can provide good sensitivity

Water washable penetrants can be washed more

readily from most weld

6. 6. It is important to remove all the of the slag from testing.

welds before penetrant

Grinding off the ripples on rough welds is

desirable. Grinding

should not smear the metal 7. 7. Brazed joints can discontinuities similar to weld beads that can be detected by PT.

A braze that does not wet the surface is

indication of a poor joint.

Developers : Normally

a

developer

is

used

to

complete

the

penetrant

process.

Developers are fine white powders applied as dry or with a liquid carrier to PREPARED BY S.MUTHUKUMAR, B.Tech lll

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form a thin uniform coating on the test surface. Developer acts like a sponge with

very

fine

random

capillary

paths. When

the

penetrant

contacts

the

developer, it spreads through these paths by capillary action [ known as blotting or reverse capillary action ]. The penetrant gradually diffuses into the developer and indications become larger than the discontinuity opening. The white color of the developer provides contrast with color contrast penetrants and also provides a darker background for fluorescent indications. It should be noted that the developer itself does not produce indications but simply absorbs the penetrant already present below it and makes it more visible. Developer should be applied as soon as possible or within 30 minutes after the removal of the excess penetrant. Dry developers [ Form A ] :

[ blends of chemically inert white powders ]

Dry developers are a non toxic fluffy absorbent white powder that is used mostly with florescent penetrant process. These dry powders do not normally provide sufficient white background for contrast with color contrast penetrants. This may be achieved by use of electrostatic spray application but is not always controlled easily. Dry powders must be used on dry surface only. Dry developers are the least sensitive. The dry powders should be used in a closed container or, if sprayed, inside a booth with good extraction. Where the throughput of components is low, components can be placed on a grid in a tank and powder applied using a scoop or from a pear shaped rubber puffer. Excess powder is then removed by gentle air blast, shaking or tapping. Dry powder may also be applied by brushing on rough test surfaces. Dry powder is best applied by placing the component in a dust storm cabinet which may be of the tunnel type or top loading. The cabinet is sealed to be dust proof. Most cabinets work on a preset cycle so that once started the cabinet cannot be opened until the cycle is complete. The dry powder developer is then agitated with dry and oil free compressed air, which blows the powder into a cloud around the component to coat the surface. It is PREPARED BY S.MUTHUKUMAR, B.Tech lll

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" extract and return " system with the cabinets,

since the fine particles take a very long time to settle from the air. Dust chambers should be fitted with heaters to maintain a dry inside atmosphere. Installing a fan at the bottom of the container to blow the powder eliminates most of the problems associated with the use of compressed air. Dry powders can be applied after charging them in a electrostatic spray system. Some dry powders are of a chemical composition which does not become

sufficiently

charged

and

this

point

should

be

checked

before

attempting to use this method of application. Dry developers should cling to dry metallic surfaces in a fine film of dust. The adherence of the powders should not be excessive, as the amount of black light available to energize indications will be reduced. In automatic processing systems, dry powders are applied by passing the part through a conveyor in a fluidized bed tunnel system. This application relies on producing a controlled air / powder mixture which passes over the components and coat the surfaces.Extremely fine dry developer powders are available for application by dipping the dried part into it. This process has the disadvantage that the developer gradually becomes contaminated by fluorescent flecks and produces false indications. For purpose of storage and applications, the dry powders should not be hygroscopic and they should remain dry. If they pickup moisture when stored in areas of high humidity, they agglomerate or lump up and lose their ability to flow and dust the surface adequately. The developer should be checked daily for caking and contamination. Dry powder developers can dry the skin and irritate the eyes and the respiratory system. Use of rubber gloves and respirators is desirable.

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Developer properties  Good blotting action. Must be easily wetted by penetrant and draw the maximum amount of penetrant from the defect.  Fine grained  Show indications from small amount of penetrant.  Mask out interfering penetrant and part surface background.  To form a thin and uniform coating.  Non fluorescent.  Easily cleaned from the surface.  Non flammable.  Non toxic and non corrosive.  Developer selection :  Select wet developer in preference to dry on very smooth test surfaces.  Select dry developer in preference to wet on very rough test surfaces. Cleaning and re inspecting a rough surface is difficult, if a wet developer was used for a prior inspection.  Wet

developers

accumulates

can

developer

not at

be

used

certain

reliably

locations

where such

part as

configuration

threads. Excess

developer can mask out indications. Dry developer can be used.  Solvent suspended developers are very effective for flaws with narrow opening, but are not satisfactory for finding wide shallow defects. PREPARED BY S.MUTHUKUMAR, B.Tech lll

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Aqueous [ Wet ] developers : These developers are used in penetrant inspection stations. Aqueous developer uses water as the carrier of the developer particles. The developer is used as a solution [ Form B ] or as suspension [ Form C ]. Water based developer includes a wetting agent, corrosion inhibition system and dispersing medium. These developers are applied directly on a water washed surface and after drying a solvent washed surface. Parts are dried immediately after excess developer has drained from the component. These

developers

are

less

sensitive

when

compared

to

non - aqueous

developers. Water Soluble developers [ Form B ] : [ Blends of white crystalline powders to be dissolved in water ] Supplied as soluble white powders to be dissolved in water at a concentration recommended by the manufacturer [ 40 - 60 g / lit ] to prepare a transparent working solution. The prepared bath is completely soluble and does not require agitation. If the solutions are stored in open tanks, precipitate may form due to loss of water by evaporation. The concentration of the solution is monitored by measuring the specific gravity with a hydrometer. Water soluble developers are not recommended for use with water washable penetrants because of the potential to wash the penetrant from within the flaw unless the developer application is very carefully controlled. Water solution developers can be applied by immersion followed by drainage in a stainless steel tank. The developer may be applied by flow-on, brush or a wet spray. When spray application is used, it is important, that the spray is wet and the droplets coalesce to form a continuous layer on the test surface. The part surface must be dried actively after the developer application to form an uniform layer. The dried developer film is water soluble and can be removed after inspection by simple water rinsing. PREPARED BY S.MUTHUKUMAR, B.Tech lll

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Soluble developers contain inorganic compounds which ionizes in the solution. Dipping different metals in contact with each other and use of uncoated basket

produce

galvanic

corrosion. The

contaminated

developer

solution

becomes gray to dark gray in color. Water Suspended developers [ Form C ] : [ Blends of white inert powders and surfactants for suspension in water ] supplied as insoluble white powders to be suspended in water at a concentration recommended by the manufacturer [ around 70 g / lit ] to give a working preparation. Water suspension developers may be applied by immersion in an agitated bath or by spraying. The immersion tank should be of stainless steel with mechanical stirrer to prevent settling. Air agitation should be avoided since the developers contain wetting agents and corrosion inhibitors which may

become degraded by chemical reaction with the oxygen or carbon

dioxide in the air. If the developer is to be applied by spraying, the design and operation of the spray apparatus must allow continuous agitation of the reservoir of developer and the application must allow the suspension to arrive wet at the test surface. Foaming must be avoided because when foam bubbles break they leave holes in the developer coating. The amount of powder in suspension must be carefully maintained. Too much or too little developer on the test surface can seriously affect sensitivity. It is essential that the

developer bath is monitored by checking the specific

gravity and the wetting characteristics. Water suspension developer is applied before drying, the developing time decreases because the heat from the drying operation helps penetrant back to the surface opening and the developer film being already in place, the developing action begins at once. Non

Aqueous

Suspension

developers

[ Form D ] : [ Chemically

inert

white

pigments suspended in volatile liquid carriers ] PREPARED BY S.MUTHUKUMAR, B.Tech lll

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These consist of developer particles suspended in a volatile organic solvent. The use of a quick drying solvent as carrier makes non aqueous wet developer the most sensitive for detection of minute cracks. The solvent combines with the penetrant in the cracks, and the result is a less viscous liquid which bleeds into the developer fairly quickly as the solvent evaporates. Non aqueous suspension developers must be applied by spraying on a dried part surface. Spraying may be by aerosol can or conventional spray gun. Since volatile solvents are used in these developers, efficient extraction facility must be provided in the inspection area. The suspended particles separate on standing [ settles to the bottom of the container ] and so must be agitated before use to an homogeneous mixture. When aerosol cans are used, this is done by shaking the can. For a spray gun application, the reservoir must be equipped with an efficient agitator. Alternative methods of application, such as immersion, flow-on or brushing on will cause loss of sensitivity.

A developer coat which is just thick enough to mask the metal surface is about right. If the coat is too thick, the penetrant might not blot through to the surface. If it is too thin, it might not blot well enough. Spraying should be performed from a distance of 8 to 10 inches from the test surface. Holding the can too close to the surface will result in excessive developer being applied every time.

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Starting the spray path from one side of the part, moving the spray onto and across the part, and finishing the spray pass beyond the part, is the preferred technique. Additional spray passes are required to ensure full, but minimum coverage. This method results in even coat without puddles that can result when the spray path is begun by aiming directly at the part. The solvent shall not be allowed to accumulate on the test surface or the penetrant will be diluted and the indications will become diffused and weak.

It is wise to test

the aerosol can for its uniform spraying ability before using it on the test part. Ultraviolet light : Inspection environment Visible dye penetrant indications are inspected under white light and fluorescent dye penetrant indications are viewed under blach light (ultraviolet light?) Electromagnetic radiation spectrum Visible and ultraviolet spectrum Visible light spectrum - 380 nm to 770 nm or ( V to R ) or

3800 Ao to 7000 Ao

Ultraviolet spectrum - 10 nm to 380 nm or 100 -3800 Ao Black light spectrum - 320 nm to 380 nm or (with peak at 365 nm) Hg arc vapour lamp produces ultraviolet light in addition to visible light. The filters are used to remove the harmful portion of UV (below 320 nm) and also visible light which affects the fluorescence of the fluorescent dye. The filters that are commonly used give black light with maximum peak intensity at 365 nm.. Colour – part of the white light that is reflected by the surface that is flooded with the white light Fluorescent colour – Emission of visible light (any colour) on the impingement of high energy radiation on the surface. In PT & MT, the impingement of black light (UV) leads to emission of yellowish green light Ultraviolet radiation is a band of wavelengths within the electromagnetic spectrum and below visible [ white ] light. Ultraviolet wavelengths are too short to be seen by the human eye. The boundary between visible light [ white ] and UV [ black ] light is a wavelength of 400 nm [ nano meters ] or [ 4000 Angstroms ].  UV - spectrum is divided into three ranges : PREPARED BY S.MUTHUKUMAR, B.Tech lll

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 UV - A [ 400 - 300 nm ] - Long Wave.  UV - B [ 300 - 280 nm ] - Medium Wave.  UV - C [ 280 - 180 nm ] - Short Wave. Ultraviolet A is not considered harmful to the human eye. Ultraviolet B can cause sunburn as well as be harmful to the eyes. Ultraviolet C is used for germicidal purposes and is harmful to the eyes and the body. Daylight viewing is considered to be a range of 540 through 570 nm, with the average being 555 nm. The fluorescent materials used in NDT for both the magnetic and penetrant inspections are selected to provide maximum fluorescent properties with UV - A excitation in the 365 nm wavelength. In fact these materials normally fluoresce under excitation of any wavelength, from 320 - 400 nm. The fluorescence, which is reflected back is in the range of 540 - 620 nm and is visible as yellow-green to the human eye. Any excessive white light

in the

inspection area [ especially those in the green / yellow wavelengths ] will decrease the contrast of indication to background, for this reason, specifications limit the ambient white light level to 20 lux maximum. Black light sources are normally high pressure mercury arc bulbs and low pressure mercury vapor discharge tubes. Low pressure mercury vapor light sources have a deep purple coating on the inside surface of the bulb to filter out undesired UV wavelengths

and blocks the visible light generated by the

bulb. Mercury vapor arc [ HID ] lamps used for NDT have only two lines of UV A energy in significant intensities. One of these bands is at 365 - nanometer wavelength. New micro power gas discharge technology [ MPXL ], is recognized to create a much higher contrast of fluorescent indication to background, thus improving the operator's visual acuity. MPXL spot lamps produce in excess of 90,000 micro watts / cm 2 from 12 inch distance, and allow inspection to be performed in areas where previously it would have been impossible. PREPARED BY S.MUTHUKUMAR, B.Tech lll

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The light intensity of ultraviolet lamps is measured with a suitable UV light meter. Most of the UV intensity measurement meters for NDT usage are designed for use on mercury vapor light sources at 365 nanometers. Fluorescence : is the emission of electromagnetic radiation by a substance as the result of the absorption of electromagnetic or corpuscular radiation of greater

energy

than

that

characterized by the fact

of

the

fluorescent

radiation. Fluorescence

is

that it occurs only so long as the stimulating

radiation is maintained. UV light filter : Ultraviolet lamps produce both the harmful ultraviolet radiations and visible light. The deep purple filter in the lamp allows only the wavelengths between 320 and 400 nanometers to pass through and eliminates the harmful ultraviolet radiations. The dark color of the filter also cuts off sufficient visible [ white ] light. UV radiations below 320 nm can burn the retina of the eyes and also the skin of the exposed person. Examiners must protect themselves from the harmful UV light by ensuring that the filter used in the UV lamp is in perfect condition. Any cracked or damaged filter shall be immediately replaced or the Lamp shall be rejected. A lamp with a damaged filter must not be used. Looking directly at the lamp with a proper filter will not cause any harm but may cause temporary blurring of vision. Cleaning of test surface Paint Removal : The paint film must be completely removed to expose the surface of the examination area. Paint removers are chemical bond release agents or solvent strippers for the removal of paint and corrosion inhibitor coatings. They are applied by dipping or brushing and work well with heat to accelerate the removal process. The paint strippers is removed and the surface is cleaned by detergents or solvents.

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Ultrasonic Cleaning :

is

SAIMIRA

46

performed

on

large

number

of

small

parts

by

immersion or dipping the part in a ultrasonic cleaning tank with almost any kind of cleaning solution. Solvents and alcohol work well with ultrasonic agitation. It is the most effective method for opening up defects by removing contaminants imbedded in the discontinuity cavity. Ultrasonic cleaning has been found to be effective in removing penetrant entrapped in discontinuities. Tight flaws require 2 hours of cleaning. For geometrically complex components, huge benefits result from combining the chemical action of a solvent with ultrasonic agitation. Ultrasonic cleaning

is based on the principle of producing cavitations in a

liquid solvent in which the component to be cleaned is immersed. Large quantities of tiny bubbles are created in the solvent [ cavitations ] by high intensity compression waves. The collapsing pressure of the bubbles forces them into crevices where the fluid penetrates between contamination and the work piece surface. This action forces the contaminant off the work piece leaving it totally clean. The frequency of ultrasonic vibration is 30 to 40kHz although higher frequencies may sometimes be used. Cleaners are available as static

solvents

or

ultrasonic

types.

Etching : This process uses acids or alkaline solutions to dissolve smeared metal from the surface. Etching is performed after any operation that could mechanically

remove

material

from

the

surface,

such

as

sand

blasting,

machining, grinding or power brushing, to assure that the cracks are opened to the

surface. These

mechanical

operations

tend

to

smear

and

close

the

discontinuity openings.

PREPARED BY S.MUTHUKUMAR, B.Tech lll

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Etching on steel is accomplished using 5 to 20% of acid solutions or 20% solution of ammonium per sulphate. The solution is applied for a specific period of time, neutralized and rinsed. The etching time is depends on the amount of material that is to be removed. Etching usually requires a separate certification because of the inherent danger to the test operator and the component being used. Acid entrapment from etching can have disastrous effects on the penetrant inspection. Sodium hydroxide

caustic

often

used

to

etch

aluminum

parts

does

not

affect

penetrants but can reduce the brightness of indications. Careful cleaning of both

acid

and

caustic

etches

before

penetrant

inspection

is

highly

recommended. Drying the Part’s surface : Components that require the use of cleaning agents for the pre cleaning process must be dried completely before penetrant application. When water based cleaners are used during pre cleaning, the final step is a wash with clean water and the parts are thoroughly dried. Water or cleaning liquid residue will hinder entrance of penetrant into discontinuities. The normal way of removing water from cleaned parts before penetrant application is to heat them in a temperature controlled re - circulating air oven for one hour at around 125 0 C. An alternative is to immerse the components in a dewatering fluid which is then itself removed by vapor degreasing. The Part surfaces are to be dried again after removal of excess penetrant and the stage at which surfaces are dried depends on the type of developer to be used. When dry powders or non aqueous solvent suspension developer is used, the surfaces must be completely dry before application. When water soluble or water suspended

developer is used, the parts are dried after the

application of the developer. Drying temperatures of 60 - 80°C [ 140 - 176°F ] is normal for oven drying. Ovens should be thermostatically controlled with supply of clean dry re circulating air and should be capable of drying the parts in 10 minutes. Care should be taken so as not to raise the surface temperature of the part above PREPARED BY S.MUTHUKUMAR, B.Tech lll

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600 C. In an oven, drying of small and large parts together is not recommended because of dissimilar drying time. Excessive drying can damage the part as well as evaporation of the penetrant, which can impair the sensitivity of the inspection. Drying components before penetrant application may require heating them to 1200 C for at least 1 hour in an oven operating at ambient pressure. The component must cool down to 40 0 C before the application of the penetrant. A vacuum drying oven can dry the component within 10 minutes. Vacuum drying ovens can maintain a working pressure as low as 10 milli bars absolute and this can be achieved within 2 minutes. At this pressure, water boils at a temperature below 50 C. However, atmospheric water vapor will condense on work pieces removed from a drying oven operating at this temperature. Vacuum drying ovens equipped with external heating elements in their base and wall to maintain a working temperature at nominal 400 C can reduce drying times to less than 10 minutes. The residual temperature of approximately 40 0 C is achieved after drying, which is ideal for penetrant application. The ovens are smaller in size and can accommodate a single, relatively large work piece, or many smaller components. With recommended care, the oven provides a long and efficient service enhancing the drying process after precleaning. Standard / Test samples : Quench cracked aluminium block : These blocks are made of ASTM B 209, SB 211 or 2024 - T3 aluminum approximately 2" wide X 3" long X 3/8’’ thick pieces. The blocks are heated and quenched to produce an overall circular crack pattern. A groove is machined in the middle of the block to separate the cracks into two zones. Since the cracks are uncontrolled, the blocks are used for comparing the performance of penetrants only and not for absolute evaluations. PREPARED BY S.MUTHUKUMAR, B.Tech lll

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At the center of the face, an area approximately 1 inch in diameter shall be marked with 950 0 F temperature indicating crayon or paint. The marked area shall be heated with a blow torch to a temperature between 950 The

area

should be

heated

within

4 minutes

to

produce

a

0

and 975 0 F. temperature

difference within the block. The block is then immediately quenched in cold water which produces a network of relatively tight and uniform crack pattern on the face. The block shall then be dried by heating to approximately 300 0 F. It is because of tight crack pattern, that color contrast penetrants are not normally checked with this block. These penetrants are not as sensitive as fluorescent penetrants. The cracking will be symmetrical on either side of the separation groove but the depth and the width of the cracks produced is uncontrolled and unknown. For the color contrast penetrants comparison only, the 950

0

F cracked block is heated a second time to 800

0

F and quenched

again to open the cracks wider. New and used penetrants or different brands of penetrants can be processed and then compared side by side. A penetrant in use can be checked for its crack detection ability and indicating brightness against fresh penetrant samples. One half of the block is processed with the used penetrant and the other half with the fresh penetrant. The whole plate is then processed to completion in the relevant way. The indications from each half can be tested under identical illumination on the same alloy with the same surface condition, resulting in a true comparison of relative sensitivity. The slight potential for error with this device arises with the requirement that the discontinuities in both halves be as identical as possible. If cracks detected on the used penetrant side is not as complete as the fresh penetrant side, the used penetrant is considered contaminated and should be discarded. If the brightness of the used penetrant indications appear to be below 90 % of the brightness of the fresh penetrant indications, the used penetrant is discarded.

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Use of penetrant outside their normal operating temperature range [ 16 52

0

C ]

can

processed. One

be

qualified

block

is

using

heated

these or

blocks. Two

cooled

to

the

separate proposed

blocks

0

to

are

examination

temperature and maintained at that temperature throughout the processing cycle. The other block is processed within the normal operational temperature of the penetrant. If the indications obtained under the proposed conditions are essentially the same as obtained with the normal conditions, the proposed technique shall be considered to be qualified for use. These blocks can be reused by careful cleaning. A 30 minutes soak in isopropyl alcohol clears the induced cracks of residual penetrant. System performance check / KDS Panels : The two outer edges have 25 mm wide strips of brittle chrome plating of 0.1 mm thickness. Five controlled induced cracks of varying dimensions are produced in pairs. The sheet is sheared into two equal sections and they are virtual twins. Both the sections are processed for side by side comparison. System performance checks involve processing a test specimen with known defects to determine if the process will reveal discontinuities of the size required. The purpose of the known defect Standard is not to verify the sensitivity level of the penterant, but to monitor the performance of the entire penetrant examination system. Further, The specimen must be processed following the same procedure used to process production parts. A system performance check is typically required daily, at the reactivation of a system after maintenance or repairs, or any time the system is suspected of being out of control. As with penetrant inspection, results are directly dependent on the skill of the operator, therefore each operator should process a panel. Known Discontinuity Standards [ KDS panels ] : The panels come in pairs having known discontinuities in known locations. Two KDS panels are produced simultaneously from a single sheet of stainless steel, so they are precisely matched as to plating thickness, crack size, and surface roughness. The center PREPARED BY S.MUTHUKUMAR, B.Tech lll

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section is grit blasted to specification roughness. This rough section is used to compare washability of different types of penetrant.

Standard / Test samples 2: KDS panels permit the monitoring of in use penetrants as well as side by side comparison to new, unused penetrant. They are rugged and can withstand daily system processing. A simple 30 min soak in isopropyl alcohol clears the induced cracks of residual penetrant. Cracked nickel chromium plated panels : The nickel chromium test panels are ideal penetrant sensitivity comparators. The panels are available with crack of 10, 20, 30, 50 microns and come in matched pairs. The size refers to the depth of the nickel plating. Matched pairs enables studies of the effects of PREPARED BY S.MUTHUKUMAR, B.Tech lll

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process changes as compared to a standard process or comparing used material vs fresh material. A thick brass panel measuring 1-1/2" x 4", plated with brittle nickel, on one side. The nickel plating is then plated with a very thin layer of chromium for protection. The panel is stress loaded on one side, which produces fine lateral cracks through the plating. These cracks are then used for the penetrant system performance check. Two panels containing identical or nearly identical crack patterns are used to perform the checks. The depth of the plating can be controlled, and panels are available with plating thickness of 10µm [ 0.0004" ]. Nickel chromium panels have been sectioned and micro-photographed. The photographs reveal crack width to be 1/20th of the depth. So, the width of the cracks in the 10µm plated panel would be 0.5µm [ 0.00002" ]. Under laboratory conditions, high sensitivity penetrants both water washable and non waterwashable find the flaw lines in these 10µm panels. So, theoretically, the penetrant that can show cracks in a 10µm nickel chromium sensitivity panel, is expected to

find

a

flaw

measuring

0.5µm

x

0.5µm

x

10µm.

Apart from length, another measure of concern should be flaw width, since a penetrant's ability to enter a crack depends more on a crack’s width than its length. For example, cracks in forgings are inherently tighter and more difficult to reveal than cracks of equal length in castings or extrusions. Cracks under compression, squeezed tight and having virtually no width, are often impossible to show with penetrant inspection. Limitations : A problem with test pieces is that once a penetrant process has shown all the discontinuities in a test piece no other process can appear more sensitive. Similarly if the discontinuities are difficult to show and can be found only by use of the most sensitive process all other processes appear equally ineffective. The size of the reference discontinuities itself increase with use. The size of the indications must be recorded [ photograph / transfer lacquer replica ] when the block is first used and compared with the indications obtained with subsequent uses. PREPARED BY S.MUTHUKUMAR, B.Tech lll

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Important : Several precautions are necessary when using the penetrant test panels. The panels must be thoroughly cleaned as quickly as possible after each use. Penetrant residues when left in discontinuities become very viscous and extremely difficult to remove. In cases of severe contamination by previous residues, the PSM5 and NiCr test panels can be boiled in 5 to 20% detergent solution for 30 minutes. This is followed, after cooling, by a water rinse then a rinse with acetone. In between uses they should be kept in dry acetone [ flammable ] or 1,1,1, trichloroethane. If water is allowed to contaminate either of the solvents, the discontinuities can become corroded. Solvents are best kept dry by use of molecular sieves or silica gel. Some specifications limit the number of times that a test panel can be reused. The left picture shows crack comparisions on Ni-Cr

→ panels and the

right shows washability test for two different penetrants Evaluation is the process of deciding the severity of the condition after the discontinuity indications have been interpreted. Evaluation leads to the decision as to whether the part must be rejected, to be repaired or directly accepted for use. In liquid penetrant testing, discontinuities are judged on the size of the indication and not on the actual size of the discontinuity opening visible at the surface. The indication enlarges with time and it is better to evaluate the indications at the minimum development time allowed by the procedure. Recommended development time as per ASME Sec V is minimum 7 to maximum 30 minutes. Evaluation should begin with an over all examination of the test surface to determine that the work piece has been properly processed and is in satisfactory condition for inspection. Usually, the indications are classified as linear or rounded. Linear type is an indication with a length greater than three times the width. It will

appear

as

a

continuous, straight

or

jagged

line. These

indications

represents more harmful discontinuities. Cracks, seams, deep scratch, undercut, PREPARED BY S.MUTHUKUMAR, B.Tech lll

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lack of fusion at surface, rolling and forging laps, cold shut etc produce linear indications. The same flaws may show up as an intermittent line indicating that the flaw may be partially closed at the surface.

A rounded indication is defined as a circular or elliptical and with a length of three times or less than its width. Porosity or a cavity at surface produces such indications. Color contrast penetrant indications : Indications will appear in red against white background at the locations of the discontinuities. Depth or size of the internal cavity may be co-related with the brightness of color, speed of bleeding and dye spread. Usually, a crack, deep scratch or similar opening will show up as a solid line. Tight crack or a lap show as a broken line. Gross porosity will produce large

rounded

indications. Fine

porosity

is

indicated

by red

dots. Porous

material will produce randomly distributed tiny red dots. Crater crack may produce round indication because they tend to trap large amount of penetrant. Indications with a light pink color may indicate excessive cleaning. Diffused or weak indications are difficult to interpret and the part should be

PREPARED BY S.MUTHUKUMAR, B.Tech lll

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reprocessed. In many cases they turn out to be false indications caused by improper processing. Fluorescent indications : These indications are viewed under ultraviolet light in a darkened area. The

indication

glows

brilliant

yellow

green

or orange

red

against

violet

background of the developer. Porosity will show as glowing spots. Cracks will show as fluorescent lines. Where a large discontinuity has trapped a quantity of penetrant, the indication will spread wide on the surface. Experience in the use of this method allows conclusion to be drawn from the extent of this spread with respect to the relative size of the discontinuities. Poorly processed part will have broad fluorescent patches. Acceptance standard for welds : ASME Sec VIII Appendix 7 a. relevant indications : indications with major dimensions greater than 1 / 16 in. [ 1.6 mm ]. b. linear indications :

an indication having a length greater than three

times the width. c. rounded indications : an indication of circular or elliptical shape with the length equal to or less than three times the width. An indication in excess of the limits specified below is not acceptable : a. Relevant linear indications; b. Relevant rounded indications greater than 3 / 16 in. [ 4.8 mm ] c. Four or more relevant rounded indications in a line separated by 1 / 16 in. (1.6 mm) or less [ edge – to – edge ]

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Inspection : Development time : After application of the developer, it must be allowed to work. If the size of the discontinuity opening is small, the penetrant bleeds out slowly into the developer.  Dry powder developer rely upon capillary action, which is slower. The developer should be left on the surface for a minimum of ten minutes before inspection of indications.

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 A water base [ aqueous ] developer is to be dried by circulating hot air or heating in a temperature controlled oven. The minimum development time is 5 minutes, after the developer layer becomes dry.  A minimum of 5 minutes developing is also needed for non aqueous developer. This time is taken from the point when the developer layer appears to be dry. Watching the indication form immediately after developer application and awareness of the type of defects that may be encountered beforehand is extremely helpful. The indication is not the same size as the discontinuity opening, the developer will magnify the discontinuity size to make it more visible to the eye for easier detection. Penetrants continue to bleed out into the developer and indications become larger over a period of time. Wise usage of minimum and maximum development time is therefore highly recommended. Gross and large discontinuities will begin to form indication immediately. Tight and extremely small discontinuities will take long time to form. For evaluation of indications for accept / reject condition, a development time of 7 to 30 minutes is generally used after the developer layer becomes dry. Lighting :

All

penetrant

examinations

must

be

performed

under

recommended light levels, as written in the test procedure. To see faint indications with color contrast penetrants, adequate white light is required. Inspection should be carried out in good white light of at least 500 lux at the examination surface. Inspection of fine detail may require up to 2000 lux. In natural daylight, penetrant indications appear deep red against the white developer background. Under strip light [ tube lamps ], indications appear somewhat darker than in the natural daylight. For fluorescent penetrant inspection the test surface must be examined under ultraviolet [ black light ] of peak wavelength around 365 nm and intensity of at least 1000 micro watts / cm 2 at the examination surface with a minimum of 6 inches distance between the test surface and the light source. Inspection PREPARED BY S.MUTHUKUMAR, B.Tech lll

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must be carried out in a darkened area where the ambient white light level should not exceed 20 lux. The interior of the inspection booths should be painted mat black to avoid distracting reflections. Brightness of indication depends on the intensity of incident ultraviolet light and the darkness of the surrounding. If fluorescent inspection is to be performed under normal lighting condition, then the minimum intensity should be 3000 micro watts / cm 2 at the examination surface. The output of UV lamp falls with time, the effective output also falls as bulbs, reflectors, or filters become dirty. Once a week the optical parts of the lamp should be cleaned with a dry cloth when the lamp is cold. The UV light must be checked for presence of

cracks in the filter. A lamp with cracked

filter must not be used. UV light intensity is to be checked when first used each day for required intensity with a calibrated UV light meter, in the darkened room condition and after allowing a 5 minutes warm up period. Fluorescent

indications

appear

brilliant

yellow-green

or

orange-red

depending on the type of the dye in the penetrant. The darker the area of inspection, the more brilliantly the indications will glow. Dark adaptation for fluorescent inspection : When evaluating fluorescent indications, the examiners must allow their eyes to adjust when first entering the darkened inspection area. Normally, 5 minutes are enough time for the eyes to adapt to the darkened condition. Inspection should not start until the examiner's eyes have adapted to the darkened

conditions. Dark

adaptation

time

can

be

shortened

by

wearing

suitable red eye glasses in white light areas. Note : Photosensitive eyeglasses should not be used during inspection because these glasses darken in the presence of UV radiation. Examiners performing inspection under ultraviolet light should consider wearing yellowish or special ultraviolet filter goggles to reduce eyestrain. Evaluation of indications : PREPARED BY S.MUTHUKUMAR, B.Tech lll

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Penetrant inspection provides indirect indication of flaws by showing zones of penetrant accumulation. Penetrant may be accumulated on the part, due to presence of flaws, part configuration or improper processing. It cannot always be determined at first glance whether an indication is real, false or non relevant. A real indication is caused by a flaw such as a crack, porosity or surface breaking defects. Non relevant

and

false

indications, are

the

indications, that

may

be

result of

the

interpreted erroneously as a flaw. Non

relevant

are

true

indications, which

occurs

as

a

component geometry or configuration such as keyways, splines, nut and bolt threads, press fit parts, riveted, assembled part or very rough surfaces such as a weld bead. Tool marks, loose scales and scratches may also produce non relevant indications. A False indication is an accumulation of penetrant, not caused by a discontinuity in the work piece. It may appear as a result of improper handling or poor processing during the inspection process. This may appear in the form of excessive penetrant background or streaking from insufficient or improper penetrant

removal. Common

causes

are, penetrant

contaminated

developer,

penetrant residue on inspection table, dirt, lint, or finger prints from handling during the test process. For fluorescent penetrants, inadequate cleaning of oil and grease, can also be a source of false indications.

Verification of Flaw Indication : Swabbing the indication with a very fast drying solvent dampened, cotton tipped applicator, re-application of developer and witnessing the re-appearance PREPARED BY S.MUTHUKUMAR, B.Tech lll

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DYE PENETRANT TESTING CONSULTANTS of

an

indication

can

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60 also

greatly

assist

acceptance or rejection decision should

the interpretation

process. An

be made before this test. The ability

of an indication to re-appear is called persistence. This will alert the inspector to the fact that the discontinuity has depth or volume. Swabbing an indication is an useful technique to verify false or retest a non relevant indication. However, swabbing should not be done until the indication has been evaluated for size. If allowed by the specific procedure, indications may be evaluated by wiping the indication with a fast drying solvent-dampened swab, allowing the area to dry, and redeveloping. Redevelopment time shall be as long as the original development time, except non aqueous redevelopment time shall be 3 min minimum. If no indication reappears, the original indication is considered false. This procedure may be performed twice for any given original indication. Spraying non aqueous developer onto a cotton swab and wiping, removes the indication, cleans the surface and reapplies developer in one step. Whether or not the flaw indication re appears after the solvent wipe depends on ; 1. how volatile the solvent is, 2. how much solvent is applied, 3. which applicator is used, 4. finger pressure exerted, 5. developer used …… powder or non aqueous, 6. developing time before verification, 7. developing time after verification, 8. number of wipes.

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Test Process : Surface cleaning : Careful cleaning of the test surface and inside of the discontinuities is an extremely important step for success of the penetrant inspection because the penetrant method works by allowing a liquid to enter a PREPARED BY S.MUTHUKUMAR, B.Tech lll

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defect which is open to the surface. If there is dirt in the crack, there is no room for the penetrant to enter it, and the process will not work. Surfaces to be examined are cleaned so that they are free from dirt, scales, oil, grease, paint, rust and corrosion and discontinuities are free inside of water, oil or other contaminants. This is known as pre cleaning. Following cleaning, the parts shall be thoroughly dried. Penetrant application : After drying, the entire surface to be tested must be wetted with a layer of the penetrant and kept wet during the entire dwell time. This can be achieved by immersion, flow - on, spraying or brushing the liquid. The temperature of the test surface should be between 10° C and 52° C. Rubbing the surface with a penetrant soaked cloth is not permitted. Penetrant Dwell : The penetrant is left on the test surface for sufficient time to allow penetration into the discontinuity openings. The time involved depends on the viscosity of the penetrant, temperature of the part surface and the tightness of the discontinuity opening. Penetration time used is generally between 10 and 30 minutes. At low temperatures, longer penetration time shall be used because the viscosity of penetrant increases. This time is known as dwell time. Emulsification : For post emulsification type penetrant, the emulsifier is applied after the completion of the dwell time.

Emulsification makes the

penetrant water washable. Emulsification time varies between 1 to 3 minutes, depending on the type of emulsifier, the penetrant , the surface condition and actual time shall be determined by practical tests. Removal of excess penetrant : The purpose of removing excess penetrant is to free the surface of penetrant so that the penetrant, which has entered a discontinuity, will be readily visible when it re-emerges onto the surface. After the dwell time, excess surface penetrant is to be removed with a solvent wipe, water spray rinse, or the penetrant has to be emulsified so that it can be washed-off with water rinse. Excess penetrant removal depends on the removal characteristic of the penetrant being used. The removal of excess PREPARED BY S.MUTHUKUMAR, B.Tech lll

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penetrant is a delicate procedure because it is essential not to remove penetrant from the discontinuities. Penetrant is so easily removed from smooth polished surfaces that special procedures may be required to prevent over removal. Rough surfaces reduce removability by retaining penetrant in the indentations or recesses by preventing the emulsifier from evenly combining with the surface penetrant. A completely cleaned surface may remove some or all of the penetrant trapped in discontinuities. To make sure that the surface has not been over - washed, the cleaning may allow a very low level penetrant background to remain. A slight shading of penetrant should be visible in the developer layer. During this cleaning operation, the surface has to be checked for

residual

penetrant

using

suitable

illumination.

Developer application : To make the penetrant indication clearly detectable a developing medium is used. Developer may be applied by dusting or dipping for dry powder or spraying or immersion for water base developers. Non aqueous

wet

developers

developer should

should

be

best

applied

by

spraying

only. The

then be allowed to dwell on the part surface for sufficient

time [ usually 7 to 30 minutes ] to permit it to draw penetrant out of any surface flaws to form magnified visible indications of such flaws. This is known as developing time. Longer times may be necessary for tight cracks. Inspection of indications : Inspection is then performed under ordinary light for color

contrast

penetrants

or

under

ultraviolet

illumination

for

fluorescent

penetrants to detect any flaws which may be present.

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Post Cleaning : After the completion of examination, the test surface is cleaned

to

remove

developer

and

penetrant. In

some

industries, such

as

nuclear, the failure to post clean a part can have very detrimental results. For parts that will be in contact with liquid oxygen systems, the failure to post clean a part could result in a fire and possible serious injury. If penetrants and developers that contain halogen and sulphur products are used, can have detrimental effects on some metals and must be cleaned according to approved technical data. Penetrant and developer residue tend to absorb moisture and cause corrosion. The removal of dry developer and non aqueous developer is easy. Wet aqueous developer are difficult to remove because they are baked onto the part during the drying process. Soluble developers can be removed by water rinse. The longer the developer remains on a part the harder it is to remove. The use of steam with detergents is probably the most effective of all methods. PREPARED BY S.MUTHUKUMAR, B.Tech lll

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in liquid oxygen systems, require total penetrant removal after the inspection. Entraped penetrant in discontinuities can be removed most effectively by hot tank solvent cleaning or ultrasonic cleaning. Penetrant inspection normally leaves the part’s surface clean and exposed and thus making it susceptible to corrosion. All efforts should be made to protect

the

parts

surfaces

from

corrosion

effectively

after

the

penetrant

inspection.

Penetrant Testing using fluorescent and visible dyes

Glossary of Terms of Liquid Penetrant Inspection Background: (fluorescent or colour contrast) residue of penetrant over general surface of part during inspection. Background Colouration: colouration remaining after incomplete removal of dye penetrant from the test surface. Background Fluorescence: fluorescence remaining after incomplete removal of dye PREPARED BY S.MUTHUKUMAR, B.Tech lll

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penetrant from the test surface. Bath: refers to penetrant materials into which parts are dipped or totally immersed. Black Light: light radiating with a wavelength ranging between 360 to 400 nm. i.e. just below visible light and used to excite fluorescence. Black Light Filter: a filter passing black light but opaque to visible and far ultraviolet wavelengths. Bleedout: the action of penetrant exuding from discontinuities on to the surface of a part. Blotting: the soaking action developer has on penetrant as it bleeds from a discontinuity, increasing sensitivity and contrast. Capillary Action: the action by which the level of a liquid in contact with a solid surface is elevated or depressed to a level different from the liquid not in contact with the solid's surface. Clean:free of solid or liquid contaminants. Colour Contrast Dye: dye used in penetrant to give sufficient colour intensity to allow good contrast to the background being viewed under white light. Colour Contrast Penetrant: a penetrant utilizing colour contrast dye. Dark Adaptation: adjustment of the eyes when one goes from a bright area to a darkened area allowing better visibility in dim light. Degreasing Fluid: agents used to clean oil and grease from a part prior to applying penetrant. Developer: a white powder applied to a part to draw excess penetrant from discontinuities after initial removal of penetrant from part. Also, provides a contrasting background to view against. Developer, Dry: light, fluffy, dry absorbent powder. Developer, Nonaqueous: absorbent powdered materials suspended in a nonaqueous liquid.

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Developer, Wet: absorbent powdered materials suspended in water. Development Time: time a developer is allowed to remain on the surface of a part being inspected. Dragout: The loss of fluid as a result of carry-over. Drain Time: time allowed for excess penetrant to flow off part after it has been immersed in a bath. Drying Time: time in which a washed or wet-developed part is in the hot air drying oven. Dwell Time (Penetration time): time that a penetrant is in contact with a part's surface. Dye Penetrant: a penetrating fluid containing a dye which is visible under normal (white) light. Emulsifier: an agent that when added to an insoluble penetrant renders it soluble allowing the mixture to be washed off the part. Emulsion: a stable mixture of water, oil and emulsifier. Emulsification Time (soak time): period which emulsifier is allowed to mix with the liquid penetrant prior to water rinsing. Evaluation: process of deciding on the severity of a discontinuity after it has been interpreted, and involves acceptance or rejection of the part. False Indications: irrelevant indications. Family: the complete series of materials of a specific manufacturer to perform a specific type or process of LPI. Flash Point: lowest temperature at which a substance will decompose to form a flammable gaseous mixture. Fluorescence: property of emitting light as a result of and only during absorption of radiation of a shorter wavelength. Fluorescent Penetrants: penetrants used to reveal surface discontinuities which become visible when irradiated with black light. PREPARED BY S.MUTHUKUMAR, B.Tech lll

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Hydrophilic: refers to type of emulsifier that is water based. Indication:marks a discontinuity. In LPI it is the presence of bleed out. Interpretation:same as evaluation. Irrelevant Indication: indication resulting from poor technique and not associated with a material discontinuity. Leak Testing: LPI technique where penetrant is applied to one side of a part and observations are made from the other side to locate any through-wall discontinuities. Lipophilic: refers to type of emulsifier that is oil based. LOX-Safe Penetrant: penetrant system designed to be compatible or nonreactive in the presence of liquid oxygen. Monochromatic Light: light of one wavelength. Nonrelevant Indications: LPI indications resulting from conditions not associated with a material discontinuity. Penetrability: the property of a penetrant that allows it to enter very fine openings. Penetrant: a liquid possessing properties enabling it to enter very fine openings such as cracks. Polar Attraction: electrostatic attraction between positive and negatively charged ions. Reproducibility: the reproducing of one or more LPI indications (primarily for statistical data). Reference Pieces: test pieces containing controlled artificial discontinuities or natural discontinuities, used for checking the efficiency of the penetrant testing method and materials. Seeability: the ability of an indication to be seen by an observer under adverse conditions e.g. poor background contrast or outside light (fluorescent method). Soak Time: same as emulsification time. Solvent Cleaning: process of removing excess penetrant from a surface by wiping or PREPARED BY S.MUTHUKUMAR, B.Tech lll

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washing with a solvent. Solvent Removable Penetrant: a penetrant that must be removed by a suitable solvent. Standard Cracked Test Panel: an intentionally cracked test panel having separate areas for the application of different penetrant materials so a direct comparison can be obtained. Water Washable Penetrant: a penetrant with suitable emulsifying agents incorporated to render it directly water washable. Surface examination (such as with liquid penetrant examination - PT) is not performed for damage mechanisms known to initiate from the inside surface as it would not be effective in these situations. A disadvantage of dye penetrant testing is that cracks not open to the surface can not be detected.

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The ABC's of Nondestructive Weld Examination Introduction The philosophy that often guides the fabrication of welded assemblies and structures is "to assure weld quality." However, the term "weld quality" is relative. The application determines what is good or bad. Generally, any weld is of good quality if it meets appearance requirements and will continue indefinitely to do the job for which it is intended. The first step in assuring weld quality is to determine the degree required by the application. A standard should be established based on the service requirements. "Whatever the standard of quality, all welds should be inspected." Standards designed to impart weld quality may differ from job to job, but the use of appropriate examination techniques can provide assurance that the applicable standards are being met. Whatever the standard of quality, all welds should be inspected, even if the inspection involves nothing more than the welder looking over his own work after each weld pass. A good-looking weld surface appearance is many times considered indicative of high weld quality. However, surface appearance alone does not assure good workmanship or internal quality. Nondestructive examination (NDE) methods of inspection make it possible to verify compliance to the standards on an ongoing basis by examining the surface and subsurface of the weld and surrounding base material. Five basic methods are commonly used to examine finished welds: visual, liquid penetrant, magnetic particle, ultrasonic and radiographic (X-ray). The growing use of computerization with some methods provides added image enhancement, and allows real-time or near realtime viewing, compar ative inspections and archival capabilities. A review of each method will help in deciding which process or combination of processes to use for a specific job and in performing the examination most effectively.

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Visual Inspection (VT) Visual inspection is often the most cost-effective method, but it must take place prior to, during and after welding. Many standards require its use before other methods, because there is no point in submitting an obviously bad weld to sophisticated inspection techniques. The ANSI/AWS D1.1, Structural Welding Code-Steel, states, "Welds subject to nondestructive examination shall have been found acceptable by visual inspection." Visual inspection requires little equipment. Aside from good eyesight and sufficient light, all it takes is a pocket rule, a weld size gauge, a magnifying glass, and possibly a straight edge and square for checking straightness, alignment and perpendicularity. "Visual inspection is the best buy in NDE, but it must take place prior to, during and after welding." Before the first welding arc is struck, materials should be examined to see if they meet specifications for quality, type, size, cleanliness and freedom from defects. Grease, paint, oil, oxide film or heavy scale should be removed. The pieces to be joined should be checked for flatness, straightness and dimensional accuracy. Likewise, alignment, fit-up and joint preparation should be examined. Finally, process and procedure variables should be verified, including electrode size and type, equipment settings and provisions for preheat or postheat. All of these precautions apply regardless of the inspection method being used. During fabrication, visual examination of a weld bead and the end crater may reveal problems such as cracks, inadequate penetration, and gas or slag inclusions. Among the weld detects that can be recognized visually are cracking, surface slag in inclusions, surface porosity and undercut. On simple welds, inspecting at the beginning of each operation and periodically as work progresses may be adequate. Where more than one layer of filler metal is being deposited, however, it may be desirable to inspect each layer before depositing the next. The root pass of a multipass weld is the most critical to weld soundness. It is especially susceptible to cracking, and because it solidifies quickly, it may trap gas PREPARED BY S.MUTHUKUMAR, B.Tech lll

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and slag. On subsequent passes, conditions caused by the shape of the weld bead or changes in the joint configuration can cause further cracking, as well as undercut and slag trapping. Repair costs can be minimized if visual inspection detects these flaws before welding progresses. Visual inspection at an early stage of production can also prevent underwelding and overwelding. Welds that are smaller than called for in the specifications cannot be tolerated. Beads that are too large increase costs unnecessarily and can cause distortion through added shrinkage stress. After welding, visual inspection can detect a variety of surface flaws, including cracks, porosity and unfilled craters, regardless of subsequent inspection procedures. Dimensional variances, warpage and appearance flaws, as well as weld size characteristics, can be evaluated. Before checking for surface flaws, welds must be cleaned of slag. Shotblasting should not be done before examination, because the peening action may seal fine cracks and make them invisible. The AWS D1.1 Structural Welding Code, for example, does not allow peening "on the root or surface layer of the weld or the base metal at the edges of the weld." Visual inspection can only locate defects in the weld surface. Specifications or applicable codes may require that the internal portion of the weld and adjoining metal zones also be examined. Nondestructive examinations may be used to determine the presence of a flaw, but they cannot measure its influence on the serviceability of the product unless they are based on a correlation between the flaw and some characteristic that affects service. Otherwise, destructive tests are the only sure way to determine weld serviceability. Radiographic Inspection (RT) Radiography (X-ray) is one of the most important, versatile and widely accepted of all the nondestructive examination methods - Fig. 1. Fig. 1 - Radiography is one of the mostFig. 2 - Thicker areas of a specimen PREPARED BY S.MUTHUKUMAR, B.Tech ASNT NDT LEVEL lll

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being x-rayed or higher density important, versatile and widely

material absorbs more radiation and

accepted examination methods.

the corresponding areas on the radiograph will be lighter

X-ray is used to determine the internal soundness of welds. The term 'X-ray quality," widely used to indicate high quality in welds, arises from this inspection method. Radiography is based on the ability of X-rays and gamma rays to pass through metal and other materials opaque to ordinary light, and produce photographic records of the transmitted radiant energy. All materials will absorb known amounts of this radiant energy and, therefore, X-rays and gamma rays can be used to show discontinuities and inclusions within the opaque material. The permanent film record of the internal conditions will show the basic information by which weld soundness can be determined. "Radiography is one of the most widely accepted NDE methods." X-rays are produced by high-voltage generators. As the high voltage applied to an Xray tube is increased, the wavelength of the emitted X-ray becomes shorter, providing more penetrating power. Gamma rays are produced by the atomic disintegration of radioisotopes. The radioactive isotopes most widely used in industrial radiography are Cobalt 60 and Iridium 192. Gamma rays emitted from these isotopes are similar to PREPARED BY S.MUTHUKUMAR, B.Tech lll

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X-rays, except their wavelengths are usually shorter. This allows them to penetrate to greater depths than X-rays of the same power, however, exposure times are considerably longer due to the lower intensity. When X-rays or gamma rays are directed at a section of weldment, not all of the radiation passes through the metal. Different materials, depending on their density, thickness and atomic number, will absorb different wavelengths of radiant energy. The degree to which the different materials absorb these rays determines the intensity of the rays penetrating through the material. When variations of these rays are recorded, a means of seeing inside the material is available. The image on a developed photo-sensitized film is known as a radiograph. The opaque material absorbs a certain amount of radiation, but where there is a thin section or a void (slag inclusion or porosity), less absorption takes place. These areas will appear darker on the radiograph. Thicket areas of the specimen or higher density material (tungsten inclusion), will absorb more radiation and their corresponding areas on the radiograph will be lighter - Fig. 2. Whether in the shop or in the field, the reliability and interpretive value of radiographic images are a function of their sharpness and contrast. The ability of an observer to detect a flaw depends on the sharpness of its image and its contrast with the background. To be sure that the radiographic exposure produces acceptable results, a gauge known as an Image Quality Indicator (IQI) is placed on the part so that its image will be produced on the radiograph. IQls used to determine radiographic quality are also called penetrameters. A standard hole-type penetrameter is a rectangular piece of metal with three drilled holes of set diameters. The thickness of the piece of metal is a percentage of the thickness of the specimen being radiographed. The diameter of each hole is different and is a given multiple of the penetrameter thickness. Wire-type penetrameters are also widely used, especially outside the United States. They consist of several pieces of wire, each of a different diameter. Sensitivity is determined by the smallest diameter of PREPARED BY S.MUTHUKUMAR, B.Tech lll

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wire that can be clearly seen on the radiograph. A penetrameter is not an indicator or gauge to measure the size of a discontinuity or the minimum detectable flaw size. It is an indicator of the quality of the radiographic technique. Radiographic images are not always easy to interpret. Filmhandling marks and streaks, fog and spots caused by developing errors may make it difficult to identify defects. Such film artifacts may mask weld discontinuities. Surface defects will show up on the film and must be recognized. Because the angle of exposure will also influence the radiograph, it is difficult or impossible to evaluate fillet welds by this method. Because a radiograph compresses all the defects that occur throughout the thickness of the weld into one plane, it tends to give an exaggerated impression of scattered-type defects such as porosity or inclusions. An X-ray image of the interior of a weld may be viewed on a fluorescent screen, as well as on developed film. This makes it possible to inspect parts faster and at lower cost, but image definition is but image definition is possible to overcome many of the shortcomings of radiographic imaging by linking the fluorescent screen with a video camera. Instead of waiting for film to be developed, the images can be viewed in real time. This can improve quality and reduce costs on production applications such as pipe welding, where a problem can be identified and corrected quickly. By digitizing the image and loading it into a computer, the image can be enhanced and analyzed to a degree never before possible. Multiple images can be superimposed. Pixel values can be adjusted to change shading and contrast, bringing out small flaws and discontinuities that would not show up on film. Colors can be assigned to the various shades of gray to further enhance the image and make flaws stand out better. The process of digitizing an image taken from the fluorescent screen - having that image computer enhanced and transferred to a viewing monitor - takes only a few seconds. However, because there is a time delay, we can no longer consider this "real time." It is called "radioscopy imagery." PREPARED BY S.MUTHUKUMAR, B.Tech lll

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Existing films can be digitized to achieve the same results and improve the analysis process. Another advantage is the ability to archive images on laser optical disks, which take up far less space than vaults of old films and are much easier to recall when needed. Industrial radiography, then, is an inspection method using X-rays and gamma rays as a penetrating medium, and densitized film as a recording medium, to obtain a photographic record of internal quality. Generally, defects in welds consist either of a void in the weld metal itself or an inclusion that differs in density from the surrounding weld metal. Radiographic equipment produces radiation that can be harmful to body tissue in excessive amounts, so all safety precautions should be followed closely. All instructions should be followed carefully to achieve satisfactory results. Only personnel who are trained in radiation safety and qualified as industrial Fig. 3 - Applications for magnetic particle testing include inspecting plate edges prior to welding, in process inspection of each weldpass or layer, postweld evaluation and repairs.

radiographers should be permitted to do radiographic testing. Magnetic Particle Inspection (MT) Magnetic particle inspection is a method of locating and defining discontinuities in magnetic materials It is excellent for detecting surface defects in welds, including PREPARED BY S.MUTHUKUMAR, B.Tech lll

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discontinuities that are too small to be seen with the naked eye, and those that are slightly subsurface. This method may be used to inspect plate edges prior to welding, in process inspection of each weld pass or layer, postweld evaluation and to inspect repairs Fig. 3. It is a good method for detecting surface cracks of all sizes in both the weld and adjacent base metal, subsurface cracks, incomplete fusion, undercut and inadequate penetration in the weld, as well as defects on the repaired edges of the base metal. Although magnetic particle testing should not be a substitute for radiography or ultrasonics for subsurface evaluations, it may present an advantage over their methods in detecting tight cracks and surface discontinuities. With this method, probes are usually placed on each side of the area to be inspected, and a high amperage is passed through the workplace between them. A magnetic flux is produced at night angles to the flow of current - Fig. 3. When these lines of force encounter a discontinuity, such as a longitudinal crack. they are diverted and leak through the surface, creating magnetic poles or points of attraction. A magnetic powder dusted onto the surface will cling to the leakage area more tenaciously than elsewhere, forming an indication of the discontinuity. For this indication to develop, the discontinuity must be angled against the magnetic lines of force. Thus, when current is passed longitudinally through a workpiece, only longitudinal flaws will show. Putting the workpiece inside a solenoid coil will create longitudinal lines of force (Fig. 3) that cause transverse and angular cracks to become visible when the magnetic powder is applied. Although much simpler to use than radiographic inspection, the magnetic particle method is limited to use with ferromagnetic materials and cannot be used with austenitic steels. A joint between a base metal and a weld metal of different magnetic characteristics will create magnetic discontinuities that may be falsely interpreted as PREPARED BY S.MUTHUKUMAR, B.Tech lll

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unsound. On the other hand a true defect can be obscured by the powder clinging over the harmless magnetic discontinuity. Sensitivity decreases with the size of the defect and is also less with round forms such as gas pockets. It is best with elongated forms, such as cracks, and is limited to surface flaws and some subsurface flaws, mostly on thinner materials. Because the field must be distorted sufficiently to create the external leakage required to identify flaws, the fine, elongated discontinuities, such as hairline cracks, seams or inclusions that are parallel to the magnetic field, will not show up. They can be developed by changing the direction of the field, and it is advisable to apply the field from two directions, preferably at right angles to each other. Magnetic powders may be applied dry or wet. The dry powder method is popular for inspecting heavy weldments, while the wet method is often used in inspecting aircraft components. Dry powder is dusted uniformly over the work with a spray gun, dusting bag or atomizer. The finely divided magnetic particles are coated to increase their mobility and are available in gray, black and red colors to improve visibility. In the wet method, very fine red or black particles are suspended in water or light petroleum distillate. This can be flowed or sprayed on, or the part may be dipped into the liquid. The wet method is more sensitive than the dry method, because it allows the use of finer particles that can detect exceedingly fine defects. Fluorescent powders may be used for further sensitivity and are especially useful for locating discontinuities in corners, keyways, splines and deep holes. "MT may have an advantage over RT and UT in detecting tight cracks and surface disconfinuifies."

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similar to liquidSAIMIRA penetrant inspection except vividly coloreddyes visible under ordinary light are used.

Liquid Penetrant Inspection (PT) Surface cracks and pinholes that are not visible to the naked eye can be located by liquid penetrant inspection. It is widely used to locate leaks in welds and can be applied with austenitic steels and nonferrous materials where magnetic particle inspection would be useless. Liquid penetrant inspection is often referred to as an extension of the visual inspection method. Many standards, such as the AWS D1.1 Code, say that "welds subject to liquid penetrant testing ... shall be evaluated on the basis of the requirements for visual inspection." Two types of penetrating liquids are used - fluorescent and visible dye. With fluorescent penetrant inspection, a highly fluorescent liquid with good penetrating qualities is applied to the surface of the part to be examined. Capillary action draws the liquid into the surface openings, and the excess is then removed. A "developer" is used to draw the penetrant to the surface, and the resulting indication is viewed by ultraviolet (black) light. The high contrast between the fluorescent material and the object makes it possible to detect minute traces of penetrant that indicate surface defects.

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Dye penetrant inspection is similar, except that vividly colored dyes visible under ordinary light are used - Fig 4. Normally, a white developer is used with the dye penetrants that creates a sharply contrasting background to the vivid dye color. this allows greater portability by eliminating the need for ultraviolet light. The part to be inspected must be clean and dry, because any foreign matter could close the cracks or pinholes and exclude the penetrant. Penetrants can be applied by dipping, spraying or brushing, but sufficient time must be allowed for the liquid to be fully absorbed into the discontinuities. This may take an hour or more in very exacting work. Liquid penetrant inspection is widely used for leak detection. A common procedure is to apply fluorescent material to one side of a joint, wait an adequate time for capillary action to take place, and then view the other side with ultraviolet light. In thin-walled vessels, this technique will identify leaks that ordinarily would not be located by the usual air test with pressures of 5-20 Ib/in 2. When wall thickness exceeds 1/4 in., however, sensitivity of the leak test decreases.

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Fig. 5 - Ultrasonic inspection detects discontinvities both on and below the weld surface. Compact, portable equipment makes it easy to use in the field.

Ultrasonic Inspection is a method of detecting discontinuities by directing a highfrequency sound beam through the base plate and weld on a predictable path. When the sound beam's path strikes an interruption in the material continuity, some of the sound is reflected back. The sound is collected by the instrument, amplified and displayed as a vertical trace on a video screen - Fig. 5. Both surface and subsurface defects in metals can be detected, located and measured by ultrasonic inspection, including flaws too small to be detected by other methods. The ultrasonic unit contains a crystal of quartz or other piezoelectric material encapsulated in a transducer or probe. When a voltage is applied, the crystal vibrates rapidly. As an ultrasonic transducer is held against the metal to be inspected, it imparts mechanical vibrations of the same frequency as the crystal through a couplet material into the base metal and weld. These vibrational waves are propagated through the material until they reach a discontinuity or change in density. At these points, some of the vibrational energy is reflected back. As the current that causes the vibration is shut off and on at 60-1000 times per second, the quartz crystal intermittently acts as a receiver to pick up the reflected vibrations.These cause pressure on the crystal and generate an electrical current. Fed to a video screen, this PREPARED BY S.MUTHUKUMAR, B.Tech lll

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current produces vertical deflections on the horizontal base line. The resulting pattern on the face of the tube represents the reflected signal and the discontinuity. Compact portable ultrasonic equipment is available for field inspection and is commonly used on bridge and structural work. Ultrasonic testing is less suitable than other NDE methods for determining porosity in welds, because round gas pores respond to ultrasonic tests as a series of single-point reflectors. This results in lowamplitude responses that are easiIy confused with "base line noise" inherent with testing parameters. However, it is the preferred test method for detecting plainer-type discontinuities and lamination. Portable ultrasonic equipment is available with digital operation and microprocessor controls. These instruments may have built-in memory and can provide hard-copy printouts or video monitoring and recording. They can be interfaced with computers, which allows further analysis, documentation and archiving, much as with radiographic data. Ultrasonic examination requires expert interpretation from highly skilled and extensively trained personnel. Table 1 - Reference Guide to Major Methods for the Nondestructive Examination of Welds Inspection

Equipment

Enables

Advantages Limitations

Method

Required

Detectiort of

Visual

Magnifying

Surface flaws - Low cost.

Applicable to Should always

glass

cracks, porosity,Can be

surface

Weld sizing

unfilled craters, applied while defects only. method of

gauge

slag inclusions work is in

Provides no

inspection, no

Pocket rule

Warpage,

permanent

matter what

record.

other

process,

Straight edge underwelding, permitting

Remarks

be the primary

Workmanship overwelding,

correction of

techniques are

standards

poorly formed

faults.

required.

beads,

Gives

Is the only

misalignments, indication of

"productive"

improper fitup incorrect

type of

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procedures.

inspection. Is the necessary function of everyone who in any way contributes to the making of the weld.

RadiographicCommercial X-Interior ray or gamma macroscopic units made

When the

Requires skill X-ray

indications

in choosing

flaws - cracks, are recorded angles of

inspection is required by

especially for porosity, blow on film, givesexposure,

many codes

inspecting

holes,

a permanent operating

and

welds,

nonmetallic

record.

specifications.

castings and inclusions,

equipment,

When viewed and

Useful in

forgings.

incomplete root on a

Film and

penetration,

fluoroscopic indications.

welders and

processing

undercutting,

screen, a low-Requires

welding

facilities.

icicles, and

cost method safety

processes.

Fluoroscopic burnthrough.

of internal

precautions.

Because of

viewing

inspection

Not generally cost, its use

equipment.

interpreting

qualification of

suitable for

should be

fillet weld

limited to those

inspection.

areas where other methods will not provide the assurance required.

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Magnetic

Special

Excellent for

Simpler to

Applicable to Elongated

Particle

commercial

detecting

use than

ferromagnetic defects parallel

equipment.

surface

radiographic materials only.to the magnetic

Magnetic

discontinuities - inspection.

powders - dry especially or wet form;

Permits

surface cracks. controlled

Requires skill field may not in

give pattern; for

interpretation this reason the

may be

sensitivity.

of indications field should be

fluorescent for

Relatively

and

viewing under

low-cost

recognition of two directions

ultraviolet

method.

irrelevant

at or near right

patterns.

angles to each

light.

applied from

Difficult to useother. on rough surfaces. Liquid

Commercial

Surface cracks Applicable to Only surface In thin-walled

Penetrant

kits containingnot readily

magnetic anddefects are

vessels will

fluorescent or visible to the

nonmagnetic detectable.

reveal leaks not

dye

unaided eye.

materials.

ordinarily

penetrants

Excellent for

Easy to use. used

and

locating leaks inLow cost.

effectively on usual air tests.

developers.

weldments.

hot

irrelevant

assemblies.

surface

Application

Cannot be

located by

equipment for

conditions

the developer.

(smoke, slag)

A source of

may give

ultraviolet

misleading

light - if

indications.

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used. Ultrasonic

Special

Surface and

Very

Requires high Pulse-echo

commercial

subsurface

sensitive.

degree of skill equipment is

equipment,

flaws including Permits

in interpreting highly

either of the

those too small probing of

pulse-echo

developed for

patterns.

weld inspection

pulse-echo or to be detected joints transmission by other

inaccessible Permanent

type.

methods.

to

Standard

Especially for

radiography. readily

reference

detecting

patterns for

subsurface

purposes.

record is not The transmission-

obtained.

type equipment simplifies

interpretation lamination-like

pattern

of RF or video defects.

interpretation

patterns.

where it is applicable.

Choices Control Quality A good NDE inspection program must recognize the inherent limitations of each process. For example, both radiography and ultrasound have distinct orientation factors that may guide the choice of which process to use for a particular job. Their strengths and weaknesses tend to complement each other. While radiography is unable to reliably detect lamination-like defects, ultrasound is much better at it. On the other hand, ultrasound is poorly suited to detecting scattered porosity, while radiography is very good.

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The Five P's are 1.

Process Selection. The process must be right for the job.

2.

Preparation. The joint configuration must be right and compatible

3.

with the welding process.

4.

Procedures. The procedures must be spelled out in detail an followed

5.

during welding.

6.

Pretesting. Full-scale mockups or simulated specimens should be used

7.

to prove that the process and procedures give the desired standard of

8.

quality.

9.

Personnel. Qualified people must be assigned to the job. Whatever inspection techniques are used, paying attention to the "Five P's" of weld quality will help reduce subsequent inspection to a routine checking activity. Then, the proper use of NDE methods will serve as a check to keep variables in line and weld quality within standards.

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