CSWIP day4

We use Non Destructive Testing (NDT) when we wish to assess the integrity of a structure without destroying it The 4 of the common types of NDT used when assessing weldments are:

Procedure First the work must be cleaned thoroughly thor oughly,, then a  penetrant is applied for a specified time *

Once the contact time has elapsed, the penetrant is removed and a developer is then applied * Any penetrant that has been drawn into a crack by capillary action will be drawn out into the developer * Two types of penetrants are: 1) Colour contrast 2) Fluorescent Penetrant* Penetrant*

Method

Apply Penetrant

Clean then apply Developer

Result* Result*

Advantages

Disadvantages

1) Low operator skill level

1) Highly clean metal

2) All materials (Non Porous)

2) Surface flaws only

3) Low cost method

3) Extremely messy

4) Simple equipment

4) No permanent record* record*

Procedure

First the work must be cleaned and a whitener applied for contrast. A magnetic magnetic flux is then applied by permanent magnet, electro magnet, or straight current* A magnetic ink is applied which will concentrate in areas of flux leakage, as those caused by flaws *

The weld length must be crossed at 90 ° by the magnetic field* The types of magnetic media used are: 1) Wet ink  2) Dry powder 3) Fluorescent ink *

Method

Contrast paint

Magnet & Ink

Result* Result*

Advantages

Disadvantages

1) Low operator skill level

1) Fe Magnetic metal only

2) Sub surface flaws

2) De-magnetize after use

3) Relatively cheap

3) Can cause arc strikes #

4) Simple equipment

4) No permanent record* record*

# When using the straight current prod technique

Procedure

First the work must be cleaned thoroughly thorou ghly,, then a couplant coup lant is applied to increase sound transmission * A probe is then applied with the correct angle for the weld  preparation and sound waves are transmitted *

Any imperfections will rebound the sound waves causing a signal to occur on the cathode ray tube *

Method Apply Couplant

Sound wave

Result* Result*

CRT display

Signal rebounded rebounded from Lack of fusion

Advantages

Disadvantages

1) Can find lack of fusion

1) High operator skill

2) Most materials

2) Difficult to interpret

3) No safety requirements requirements

3) Requires calibration

4) Portable/instant results

4) No permanent record* record*

Procedure

A film is placed inside ins ide a cassette between lead screens. It is then placed to the rear of the object to be radiographed A radiographic source, is exposed to the work and film for a  pre-calculated time* Any imperfections in line with the beam of radiation will be shown on the film after exposure and development* development * The 2 types of radiation used in industrial radiography: 1) 2)

X rays (from Cathode Ray Tube) Gamma rays (from a Radioactive Isotope)* Isotope)*

Method Load film

Exposure to Radiation

Radioactive source IQI

Film cassette

Interpret Graph

Developed Graph

Latent image on the film

Advantages

Disadvantages

1) A permanent record ?

1) High operator skill

2) Most materials

2) Difficult interpretation

3) Assess root pen’ in pipe

3) Lack of sidewall fusion

4) Gamma ray is portable

4) Safety requirements requirements* *

 New TWI Video “Non Destructive Testing” 30 Minutes

Weld Repairs:

Weld repairs can be divided into two specific areas: 1)

Production repairs

2)

In service repairs* repairs*

Production repairs are usually identified by the Welding Inspector, or NDT operator during the process of inspection, or evaluation of reports to the code, or applied standard* standard *

A typical defect is shown below: *

Prior to repair the defect may need to undergo the following:

1) 2) 3) 4) 5) 6) 7)

A defect analysis and report An assessment of defect extremity An excavation procedure NDT procedures A welding repair procedure Welder approval to the the approved repair procedure procedure Any subsequent treatments procedures i.e. PWHT *

Plan View View of defect with drilled ends

Side View View of defect excavation* excavation*

Completed repair *

*

NDT confirmation of successful repair:

After the excavation has been filled the weldment should then be undergo a complete retest using NDT to ensure no further defects have been introduced by the repair.  NDT may also need to be further applied after any additional post weld heat treatment has been carried out *

Residual welding stresses are defined as those stresses that remain inside a material after welding has been carried out Stresses are caused by the heat of welding, which produces local expansion and contraction to take place If a metal was heated & cooled uniformly no stresses would remain, as expansion & contraction would be uniform Welding causes only o nly local heating and cooling conditions to exist, hence some residual stresses to remain in the metal* metal*

Residual stresses can have very complex patterns in welded constructions. In simple butt welded plates they may be indicated as shown below:

Tensile 

Tensile 

Compressive  *

We can say s ay that expansion/contraction has three directions.

Transverse Weld

Plan View of plate

Longitudinal End View of plate

Short transverse

One effect of welding related stresses is distortion Distortion is the movement of material in one area caused by expansion and contraction, cont raction, and local elastic/plastic movement that misshapes the component* The various types of distortion produced are caused by the directions and amount of expansion and contractional stresses involved, and the ability of the material to resist the stress without the formation of elastic/plastic strain, or deformation It is this deformation that produces distortion in a product *

Transverse distortion

Longitudinal distortion

The volume of weld metal in a joint join t will also effect the amount of local expansion and contraction Hence the more volume of weld metal then the overall ov erall amount of distortion will be higher *

Many methods are used to control the effects of distortion. Perhaps the best of these is to to pre set the materials to allow distortion to bring it to its final shape. This method is called offsetting, or pre-setting* a) Practical

b) Practical c) Impractical* Impractical*

Other forms of distortion control stop the movement of material from occurring by using such methods as clamping, jigging, strong backs, and tacking etc * These methods will reduce the distortion, but will also amass the maximum amount of residual stresses to exist *

Back-step & balance welding are sequences which may also  be used to control the effects effects of distortion *

All heat treatments applied to metals are cycles of 3 elements.

Temp

1)

Heating

2)

Soaking

3)

Cooling 2

1

3

Time

Annealing:

Used to make metals soft and ductile For steels, the component is heated above its UCT, UCT, or upper critical temperature, soaked for 1 hour/25mm of thickness and left in the furnace to cool Produces a coarse grain structure & low toughness *

Normalising: Used to make steels tough As for annealing, but the steel is removed from the furnace after soaking to cool in still air Produces a fine grain structure with good toughness *

Hardening:

Used to make some steels harder Used to increase the hardness of some plain carbon & alloy steels. Plain carbon > 0.3% The cycle is the same as previously but the cooling is rapid i.e. Quenched in water, oil, but sometimes air *

Tempering: empering :

Used after hardening to balance the properties of Toughness & Hardness The temperature range is from 220 –  220 –  723  723 °C The cooling part of the cycle should not be too rapid, but over heating will over temper the steel*

PWHT:

Used after welding to release residual stresses, caused by welding operations *

Stress

Force/Stress required to induce plastic strain *

Y

The effect of heat on the position of the yield point*

Strain By heating the steel, the yield point is suppressed/reduced relieving residual stresses as plastic strain at a much lower level of stress *

Pre-Heating:

Used mainly on steels to retard the cooling rate of a hardenable steel and reduce the hardening effect (Martensite formation) Is also used to help diffusion of Hydrogen from the HAZ of hardenable steels to avoid hydrogen cracking. cracking . Typically Typically < 350 °C Is also used to produce a more uniform rate of cooling, and control distortion, distort ion, or effects of high contractional strains *

*

Name Neutral

Oxidising

Carburising

Pictorial view

Uses

Fusion welding most metals. Flame temp > 3,200 °C* Bronze Bronz e Welding Welding * Hard surfacing & fusion & Brazing Aluminium & alloys *

A jet of pure oxygen reacts with high temperature iron (>1100  C) to produce Fe 3 O4 by exothermic reaction. This dross is then removed by the pressure of the oxygen jet * Different types of fuel gases may be used for the pre-heating flame in oxy fuel gas cutting: i.e. Acetylene. Hydrogen. Propane. Etc* Etc*

By adding Iron powder to the flame we are able to cut most metals “Iron Powder Injection”* Injection”* The high intensity of heat and rapid cooling will cause hardening in low alloy, alloy, and medium, or high C steels s teels They are thus pre-heated to avoid the hardening effect effect*

Slightly rounded top edge caused  by too close a nozzle gap

The “Kerf” 50mm

Flutes

MMA electrodes are specially produced for cutting and gouging. Oxy/Arc and Arc/Air are arc cutting process that produce lots of fume and arc air produce a high noise level. All these processes require good extraction, and ear protection is vital vi tal for arc/air  Both oxy arc and arc air use special types of electrodes and gas supplies, which will be described by the course lecturer *

Safety is the responsibility everyone. As respected officers, it is the duty of all welding inspectors to ensure that safe working practices are strictly followed. Safety in welding can be divided into several s everal areas, some of which are as follows: (Areas to be expanded briefly by the lecturer/presenter) 1) Welding/cutting elding/c utting process safety.* safety.* 2) Electrical safety. safety.* 3) Welding fumes & gases. Use & storage of gases.* gases.* 4) Safe use of lifting equipment.* equipment.* 5) Safe use of hand tools and grinding machines.* machines.*

Careful consideration should be given to safety hazards when using a particular welding process. This may include: * 1) The hazards of infra red and/or ultra violet light 2) The hazards of heat, burns and fire 3) The hazards of toxic and non toxic gases from process, coatings, or purging, and gases stored at high pressure 4) The hazards of working with high voltages & currents 5) The hazards of working in confined spaces *

Careful consideration should be given to safety when using gas, or arc cutting systems by:* 1) Removing any combustible materials from the area 2) Checking all containers to be cut are fume free and have a Permit to Work  3) Providing ventilation and extraction where required 4) Ensuring good gas safety is being practised 5) Keeping oil and grease away from oxygen 6) Using ear defenders when arc air cutting

Safe working with electrical power is mostly common sense. sens e. Ensure that insulation is used where required and that cables and connections are in good condition. (Check the duty cycle) * Gases should be stored s tored separately, separately, and cylinders should be secured when used in tthe he vertical position, especially oxygen * Exposure to dangerous welding fumes and gases from electrodes,  plating, i.e.ozone, nitrous oxide, phosgene, cadmium, beryllium are to name just a few. Always use extraction/breathing systems. If in any doubt, stop the work *

From the points that have been covered in the safety lecture, and/or the Video, complete the safety check list exercise in your course text* text *

“Weldability” is a term used in welding engineering to describe “Weldability” the ease of which a material can be welded by the common welding processes and still retain the properties for which it was designed*

If we say that a material has limited weldability, it means that we need to take special measures to ensure that the properties as required are maintained * Most materials are weldable with certain processes. The weldability of steel is mainly dependant on carbon carb on content weldability* & alloying, though most steels have a degree of weldability*

1) Hydrogen induced HAZ cracking in Low Alloy Alloy Steels 2) Hydrogen weld metal cracking in Micro Alloy Steels 3) Solidification cracking in Ferritic steels 4) Lamellar tearing in Ferritic steels 5) InterInter-granular corrosion in Stainless Steels* Steels*

Steels are classified into groups as follows:* follows:* Plain Carbon Steels:* Steels:*

1) Low Carbon Steel 0.01 –  0.01 –  0.3%  0.3% Carbon* 2) Medium Carbon Steel 0.3 –  0.3 –  0.6%  0.6% Carbon* 3) High Carbon Steel 0.6 –  0.6 –  1.4%  1.4% Carbon* Plain carbon steels contain only iron & carbon as main alloying elements, traces of Mn Si Al S & P may also be present *

An Alloy steel is one o ne that contains more than Iron & Carbon as a main alloying elements* elements*

Alloy steels are divided into 2 groups:* groups:* Low Alloy Steels< 7% extra alloying elements* *

High Alloy Steels> 7% extra alloying elements

The following basic foundation information on metallurgy will not form any part of your CSWIP examination* examination*

A most important function in the metallurgy of steels, is the ability of iron to dissolve carbon in solution * The carbon atom is very much smaller than the iron atom and does not replace it in the atomic structure, but fits between it *

Iron atoms

Carbon atoms*

Iron is an element that can exist in 2 types of cubic structures, depending on the temperature. This is an important feature *

At temperatures below Ac/r Ac/r 1, (LCT) iron exists like this* this *

α

Alpha iron

This structure occurs below 723 °C and is  body centred, centred, or BCC in structure It can only dissolve up to 0.02% Carbon

*

Also known as Ferrite

or BCC iron* iron*

Compressed representation could appear like this

At temperatures above the Ac/r 3, (UCT) iron exists like this* this*

γ Gamma iron This structure occurs above the UCT in Plain Carbon Steels and is FCC in structure. It can dissolve up 2.06% Carbon Also called Austenite

or FCC iron*

*

Compressed representation could appear like this

If steel is heated and then cooled slowly in equilibrium, then exact reverse atomic changes take place* place * If a steel that contains more than 0.3% Carbon is cooled quickly, quickly, then the carbon does not have time to diffuse out of solution, hence trapping the carbon in the BCC form of iron. This now distorts the cube to an irregular cube, or tetragon* tetragon * This supersaturated solution is called Martensite and is the hardest structure that can be produced in steels* steels *

If some steels are cooled quickly their structure looks like this* this*

Martensite can be defined as: A supersaturated supersaturated solution of carbon in BCT iron (Body Centred Tetragonal) Tetragonal)

It is the hardest structure we can produce in steels* *

Compressed representation could appear like this

Solubility of Carbon in BCC & FCC phases of steels* steels* Ferrite:

Low carbon solubility. Maximum 0.02%* 0.02%*

Austenite:

High carbon solubility. Maximum 2.06%* 2.06%*

Martensite:

The hardest phase in steels, which is produced  by rapid cooling from the Austenite phase It mainly occurs below 300 C* °

The atomic structures of plain carbon steels have been briefly identified and explained in this lecture * To summarize the effect of increasing the hardness of steels by thermal treatment, it can be said that the formation of Martensite is caused by the entrapment of carbon in solution, produced  by rapid cooling from temperatures above the Upper Critical *

In plain carbon steels there must be sufficient carbon to trap. In low alloy steels however, the alloying elements play a significant part in the thermal hardening of steels *

Crack type: Location: Steel types:

Microstructure:

H² HAZ & weld metal cracking a. HAZ (Longitudinal) b. Weld metal (Transverse) a. All hardenable steels including: b. HSLA steels c. Quench & Tempered steels Martensite* Martensite*

Occurs when: Hydrogen Hardness Stress Temperature emperat ure

is above 15 ml/100 gm weld metal is above 350 VPN is greater than 0.5 of the yield stress is below 300 ºC* ºC*

Cellulosic electrodes produce hydrogen as a shielding gas Hydrogen produced from oil, or paint on plate

H2 Martensite forms from γ

Hydrogen absorbed in a long, or unstable arc

Hydrogen crack

H2

H2 diffuses to γ in HAZ* HAZ*

HSLA or Micro-Alloyed Steels are high strength steels s teels that derive their high strength from finite alloying * Typically the level of alloying is in the region reg ion of 0.05% and elements such as vanadium molybdenum and titanium. are used. It would be impossible to match this micro alloying in the electrode due to the effect of losses across an electric arc * It is however important to match the strength s trength of the weld to the strength of the plate, and so a simple way of matching weld strength must be found and utilised * To find a simple method we would need to look at the effect of increasing carbon content on the properties of iron *

Increasing the carbon content will increase the strength, but will also increase greatly the formation of martensite in the weld. This may now produce H2 Cracks across weld* 0.83 % Carbon (Eutectoid) *

Tensile Strength

Hardness

Ductility 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.2 1.4 1.6 % Carbon

H2 HAZ Cracks in Alloy steels* steels*

Transverse Weld Cracks in HSLA Steels* Steels* Low ductility weld metal

Longitudinal contractional strain

1) Main Mainta tain in calc calcul ulat ated ed preh prehea eats ts,, and never allow the inter pass temperature to go below the pre-heat value * 2) Use Low Hydrogen processes with short arcs & ensure consumables are correctly baked & stored as required *

3) If using a cellulosic E 6010 for the root run, insert the “Hot pass” as soon as possible. pos sible. (Before HAZ < 300 °C)* 4) Remove any any paint, oil or moisture from the plate or pipe *

5) Carry out any specified PWHT as soon as possible* 6) Avoid any restraint, and use high ductility weld metal *

Crack type: Location: Steel types: Microstructure:

Solidification cracking Weld centre (longitudinal) High sulphur & phosphorus steels. Columnar grains In direction of solidification* solidification*

Occurs when: Liquid iron sulphides are formed around solidifying grains. High contractional strains are present High dilution processes are being used. There is a high carbon content in the weld metal* metal *

1) The first steps in eliminating this problem would be to choose a low dilution process, and change the joint design* 2) Grind and seal in any lamination and avoid further dilution * 3) Add Add Manganese to the electrode to form spherical Mn/S which form between the grain and maintain grain cohesion* 4) As carbon increases the Mn/S ratio required requir ed increases exponentially and is a major factor. Carbon content % should  be a minimised by careful control in electrode and and dilution * 5) Limit the heat input, hence low contraction, & minimise restraint*

Liquid Iron Sulphide films

Solidification crack *

Contractional strain

Add Manganese to weld metal *

Spherical Mn Sulphide balls form between solidified grains Cohesion and strength  between grains remains

Contractional strain

Crack type: Location: Steel types: Microstructure:

Lamellar tearing Below weld HAZ High sulphur & phosphorous steels Lamination & Segregation* Segregation*

Occurs when: High contractional strains are through the short transverse direction. There There is a high sulfur content in the base metal. There is low through thickness ductility in the base metal. There is high restraint on the work *

Restraint

Lamellar tear High contractional strains

Plate to be tested * Through thickness tensile test* test*

Contractional strain*

Full fusion compound welded cruciform joint*

Machined test piece*

The test piece is machined from the cruciform joint and  placed under tension. If Lamellar Lamellar tearing was present it it would fail at a low value *

Assessment of susceptibility to Lamellar Tearing: Tearing:

•

Carry out through thickness tensile test

•

Carry out cruciform welded tensile test

•

Carry out Ultra-sonic testing

•

Carry out penetrant testing of plate edges

•

Carry out full chemical analysis (S < 0.05%)* 0.05%)*

Methods of avoiding Lamellar Tearing:* Tearing:* 1)

Avoid restraint* restraint*

2)

Use controlled low sulfur plate *

3)

Grind out surface and butter *

4)

Change joint design *

5)

Use a forged T piece (Critical Applications)* Applications)*

Re-design weld* Grind and infill with ductile weld metal *

Control restraint*

For critical work a forged T piece may be used * Forged T Piece

Crack type:

Inter-granular corrosion

Location:

Weld HAZ. (longitudinal)

Steel types:

Stainless steels

Microstructure: Occurs when:

Sensitised grain boundaries* boundaries*

An area in the HAZ has been sensitised by the formation of chromium carbides. This area is in the form of a line running parallel to and on both sides of the weld. This depletion of chromium will leave the effected grains low in chromium oxide which is what produces the corrosion resisting effect of stainless steels. If left untreated corrosion and failure will be rapid* rapid *

During the welding of stainless steels, a small grain area in the HAZ, parallel to the weld will form chromium carbide at the grain boundaries. This depletes this grain of the corrosion resisting chrome oxide We say that the steel has become “Sensitised” or has become sensitive to corrosion* corrosion*

1) Use Use Sta Stabi bili lise sed d Stai Stainl nles esss Ste Steel elss* 2) Use Low Carbon Stainless Steels ( Below .04%)* .04%)* 3) A sensitised Stainless Steel may be de-sensitised by heating it to above 1100 °C where the Chrome carbide will be dissolved. The steel is normally quenched from this temperature to stop re-association* re-association*

WIS 5 E Only Attempt the end of course Multi Choice question quest ion paper answering 30 questions Time allowed 30 minutes

Attempt the specific question paper answering 4 from 6 questions Time allowed 1 hour hou r

WIS 5 Only Practice observing & reporting using the pipes or  plates and forms provided Use your nominated pipe code for all your visual inspections.

Practice this up and till the end of the day, or as directed by your course lecturer *