CSWIP 3.1 exam prep notes.doc

CSWIP 3.1 Preparation Material

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SECTION 1- TYPICAL DUTIES OF WELDING INSEPCTORS: → BS EN 970 Non destructive examination of fusion welds – visual examination → Welding inspector should have good vision – in accordance with EN 473, should be checked every 12 months → Condition for visual examination – BS EN 970 states minimum illumination shall be 350 lux but recommends min 500 lux → Direct inspection should enable the eye to be within 600mm from the surface being inspected, in a pst to give viewing angle not less than 30° → Aids to visual inspection – the use of mirrored boroscope or a fibe optic system viewing system. → Magnifying lens is used to avoid visual examination. It should be X2 to X5 → Duties of welding inspector – before welding, during welding, after welding → WPS is Welding Procedure Specifications → Welding inspector should also ensure inspection aids that will be needed are in good condition, calibrated. → Safety consciousness is the duty of all employees.

SECTION 2 – TERMS AND DEFINITIONS: → Brazing: Melting point of filler metal is above 450°C but always below the melting temperature of the parent metal. → Welding: An operation in which two or more parts are united by means of heat or pressure or both, in such a way that there is continuity in the nature of the metal between these parts. → Cruciform joint needs more pre-heating. → For corner joint Fillet weld is done. → For Lap joint, fillet or resistance butt weld is done. → Types of welds

Butt weld Fillet weld Autogenous weld (fusion weld without filler metal) Slot weld (make hole and weld) Plug weld (keep 2 parent metals in contact, apply current)

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→ Autogenous weld: A fusion weld made without filler metal which can be achieved by TIG, plasma, electron beam, laser or oxy-fuel gas welding. → Types of joints

Homogeneous Heterogeneous Dissimilar / Transition joint

→ Heat affected Zone (HAZ): The part of the parent metal that is metallurgically affected by the heat of welding or thermal cutting, but not melted. (The weakest & hardest part of joint) → Full penetration weld: A welded joint where the weld metal fully penetrates the joint with complete root fusion. In US it is called Complete Joint Penetration weld (CJP) → Partial penetration weld: A welded joint without full penetration. In US Partial Joint Penetration weld (PJP) → Fusion line / Fusion boundary / Weld junction. → Toe: Toes are points of high stress concentration and often they are initiation points for different types of cracks (eg., fatigue cracks, cold cracks) → Excess weld filament – reinforcement, overfill. →

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→ Angle of bevel: For MMA weld on carbon steel plate, the angle is 25-30° for V preparation. → Included angle for single V preparation is around 70°. → Root face effects root penetration. 1-2mm for common welding processes. → Root Gap is gap b/w two plates. Generally 14mm. → Root Radius: In case of MMA, MIG/MAG and oxy-fuel gas welding on carbon steel plates, the root radius has a value of 6mm for single and double U preparations and 8mm for single and double J preparations. → Land: usually present in weld preparations for MIG welding of aluminum alloys. → If thickness is more, then single U prep is made instead of V to avoid distortion/bend. → Single V prep – flame or plasma cutting, cheap and fast. → Single U prep – by machining, slow and expensive. → Single V prep with backing strip: for full penetration welds with increased current. Permanent types are made of same material as being joined and are tack welded in place. Disadvantages- poor fatigue resistance, probability of crevice corrosion between parent metal and backing strip, difficult to examine by Temporary types include copper strips, ceramic tiles and fluxes.

NDT.

→ BS EN ISO 9692 – weld preparations → Run (pass): Metal melted or deposited during one passage of an electrode, torch or blowpipe. → Layer: A stratus of weld metal consisting of one or more runs.

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→ Design Throat Thickness: Distance from root to centre of weld. Denoted by ‘a’ → Leg length: Distance from the actual or projected intersection of the fusion faces and the toe of a fillet weld, measured across the fusion face. Denoted by ‘z’.

→ Shape of fillet welds: → MITRE fillet weld. a=0.707 x z → Convex fillet weld. → Concave fillet weld: the formula doesn’t apply. → Asymmetric fillet weld: formula not valid here. Cross section is not an isosceles triangle. → Deep penetration fillet weld: It is produced using high input welding processes (ie SAW or MAG with spray transfer). Greater arc penetration. → Compound of butt and fillet welds.

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Welding positions:

3. WELDING IMPERFECTIONS AND MATERIAL INSPECTION → Classification of imperfections according to BS EN ISO 6520-1 1. Cracks 2. Cavities 3. Solid Inclusions 4. Lack of fusion and penetration 5. Imperfect shape and dimensions 6. Miscellaneous imperfections → Cracks: Type of cracks. a. Longitudinal cracks b. Transverse cracks c. Radiating (cracks radiating from common point) d. Crater (end of weld / weld metal only / star crack) e. Branching Cracks can be situated in weld metal, HAZ, parent metal. Depending of their nature these cracks can be a. Hot (Solidification or Liquation cracks) b. Precipitation induced c. Cold (Hydrogen induced cracks) d. Lamellar tearing – problem of parent metal only. a. Solidification cracks: also called Centre Line Cracking / Hot shortness. It occurs during solidification of metal. Reason – high carbon, sulphur/zinc in metal. Carbon adds hardness to material. Sulphur comes from parent metal / oil, grease etc. Melting point of sulphur is less - 115°C and carbon steel m.p. 1500°C. During welding if sulphur is available in parent metal, it will be added in weld. Sulphur comes in the centre and top during welding as it will be in the liquid state whereas the carbon steel of parent metal solidifies. Possibility of cracks due to stress. It also occurs if, depth-to-width ratio of the solidifying weld bead is large (deep & narrow), Disruption of heat flow condition occurs, eg stop/start condition.

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Liquation crack: see fig. also called hot crack. Arc strike creates martensite grain structure. It is done by welder due to immediate release of electrode from the spot it cools down and cracks are formed due to immediate cooling. b. Hydrogen induced cracks: also known as – Cold, delayed or underbead/toe cracking. It occurs primarily in the grain-coarsened region of the HAZ. Causes:

Hydrogen level > 15ml/100g of weld metal deposited. Stress > 0.5 of the yield stress Temperature < 300°C Susceptible microstructure > 400 HV hardness.

To avoid these cracks, Apply preheat, maintain specific interpass temp, post-heat, apply PWHT, reduce weld metal hydrogen by proper selection of welding process/consumable, use multi instead of single run technique, use dry shielding gas, use aurstenitic or nickel filler, clean rust from joint, reduce residual stress, blend the weld profile.

→ d. Lamellar tearing: parent metal only. It occurs only in rolled steel products and its main distinguishing feature is that the cracking has a terraced appearance. A thermal contraction strain occurs in the through-thickness direction of steel plate. It breaks in Z direction only. To avoid lamellar tearing use Z-grade steel. It has high through thickness ductility. Method to determine whether metal is Z grade steel or not – STRA (Short Transverse Reduction Area Test)

To avoid Lamellar tearing avoid welding or take solid T or solid L joint, put clamps or restraint control, apply high ductility material on surface of parent metal.

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→ Cavities: Gas cavity essentially spherical shape trapped within the weld metal. present in various forms: - isolated - uniformly distributed - clustered (bunch of pores together) - linear porosity (in straight line, usually occur along the side wall result in lack of fusion) - elongated cavity (ovule shape) - surface pore (visible at surface) Causes: Damp fluxes or corroded electrode (MMA), grease/hydrocarbon/water contamination of prepared surface, air entrapment in gas shield (MIG/MAG, TIG), incorrect/insufficient deoxidant in electrode, filler or parent metal, too high an arc voltage or length, gas evolution from priming paints/surface treatment, too high a shielding gas flow rate which results in turbulence (MIG/MAG, TIG). → Worm holes: causes due to bad surface, laminated work surface, crevices in work surface due to joint geometry. → Surface porosity: Visible on the surface. Causes: damp or contaminated surface or electrode, low fluxing activity (MIG/MAG), excess sulphur, loss of shielding gas due to long arc or high breezes (MIG/MAG), too high a shielding gas flow rate which results in turbulence (MIG/MAG, TIG). → Crater pipe: Shrinkage cavity at the end of weld run. Main cause shrinkage during solidification. Lack of welder skill. Slope out (TIG) – decreases the current flow and welding can be done slowly. Use it when the welding is about to finish. → Solid inclusions

→ Slag inclusions: Due to flux coating of an electrode. Slag should be removed by the welder. Slag causes lack of internal fusion. Particularly in MMA process.

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→ Flux inclusions: Flux trapped during welding. MMA, SAW, FCAW. To avoid- use good electrode and proper current. → Oxide inclusions: Oxide trapped during welding. Irregular shape, thus differs in appearance from the gas pore. Occurs specially in case of Aluminum alloys. → Tungsten inclusion: During TIG welding. Typical working voltage for TIG is 1012 volts. HF start voltage is 20,000 volts. It may damage electronic equipment. Insulation should be provided. It becomes expensive. Causes: Contact of electrode with weld pool, inadequate shielding gas, inadequate tightening of collet, extension of electrode beyond normal distance. → Lack of fusion: lack of side wall fusion, lack of fusion, lack of inter run fusion, lack of root fusion

→ Lack of penetration: Incomplete penetration, incomplete root penetration.

→ Undercut: causes- melting of top edge due to high welding current or high travel speed, attempting a fillet weld in horizontal vertical PB position with leg length >9mm, excessive or incorrect weaving, incorrect electrode angle, incorrect shielding gas selection 100% co2. → Excess weld metal: causes- excess arc energy, shallow edge preparation, incorrect electrode size, too slow travel speed, wrong polarity used (DC –ve MMA, SAW) It will become a prb as the angle of weld toe can be sharp leads to fatigue cracking. → Excess penetration: causes- weld heat input too high, incorrect weld preparation ie., excessive root gap, thin edge preparation, lack of backing, use of electrode unsuited to weld pst, lack of welder skill. → Overlap: An imperfection a the toe of the weld caused by metal flowing on to the surface of the parent metal without fusing to it. Causes- poor electrode manipulation (MMA), High heat input/low travel speed causing surface flow of fillet weld, incorrect pst of weld. In fillet weld undercut at top and overlap at base is called sagging.

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→ Linear misalignment: welder and inspector both responsible. It is structural preparation problem. It increases linear shear stresses at joint and induce bending stress. → Angular distortion: same as linear → Incompletely filled grove: welder is responsible. Causes- insufficient weld metal, irregular weld bead surface. → Irregular width: Causes- welder eye sight prb, severe arc blow. → Root concavity: causes- excessive backing gas pressure, incorrect prep/fit up, lack of welder skill. → Burn through: insufficient travel speed, excessive welding current, lack of welder skill, excessive grinding of root face, excessive root gap. → Stray arc: arc strike can produce hard HAZ, which may contain cracks – martensite. → Spatter: high arc current, long arc length, magnetic arc blow, damp electrodes, wrong selection of shielding gas. → Torn surface / grinding mark / chipping mark / underflushing / misalignment of opposite runs / temper colour (visible oxide film)

4. DESTRUCTIVE TESTING → Destructive Testing  Quantitative Tests & Qualitative Tests. → Quantitative Tests – Measure a mechanical property such as Tensile strength, hardness or impact toughness. Carried out for welding procedure qualification. a. Transverse Tensile Test b. All weld tensile test c. Impact toughness test (charpy v-notch test, unit Joules) d. Hardness testing e. Crack Tip Opening Displacement Test (COTD) → Qualitative Tests – to verify the joint is free from defects, they are of sound quality. a. Bend Test b. Fracture Test c. Macroscopic examination

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→ Transverse Tensile Tests: Welding procedure qualification always requires this test to show that the strength of the joint satisfies the design criteria. Tensile strength of entire joint is measured. Component is to be cooled for both sides. Tensile Strength or Ultimate tensile strength = If load is in KN then, load = KN x 1000 = N The test is intended to measure the tensile strength of the joint and thereby shows the basis of the design, the base metal properties, remains the valid criterion. Acceptance Criteria: If the test piece breaks in the weld metal, it is acceptable provided the calculated strength is not less than the minimum tensile strength specified, which is usually the minimum specified for the base metal material grade. Eg: If parent metal tensile strength is 1000 N/mm2 and metal breaks from the weld at 960 N/mm2 – it is reject. Should be more than or equal to 1000. As per ASME IX code if parent metal breaks at 95% ie 950 N/mm2 – it is acceptable. → All weld tensile tests: Specimens are subjected to a continually increasing force in the same way that transverse tensile specimens are tested. Yield or proof stress are measure by means of extensometer that is attached to the parallel length of the specimen and is able to accurately measure the extension of the gauge length as the load is increased. Ductility: Ability of the material to stretch before getting fractured or break. Tensile ductility in two ways, % elongation = change in length x 100 / original length % reduction in area = change in area x 100 / original area

→ Impact toughness Test: Charpy V notch test pieces have become the internationally accepted method for assessing resistance to brittle fracture by measuring the energy to initiate and propagate, a crack from a sharp notch in a standard sized specimen subjected to an impact load. There are standard dimenstions for smaller sized specimens, for example 10 x 7.5mm and 10 x 5mm, angle 45° Specimens are machined from welded test plates. Notch pst located in different pst according to testing requirements but typically in centre of the weld metal and at pst across HAZ.

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Unit of impact toughness test is ‘Joules.’ Ft-lbs in US specification. Toughness decreases at high temp & low temp. Impact specimens are tested at a temperature that is related to the design temperature for the fabricated component. C-Mn and low alloy steels undergo sharp change in their resistance to brittle fracture. SS grade 316L is used to provide good toughness at low temp. CS has high carbon. Direct breaks, high strength. Transition: Change in state of material. Brittle material fracture – Flat and rough, crystalline surface, featureless or smooth, chevron marks. Ductile material fracture – Rough and torn, shear lips, fibrous surface, plastic or permanent deformation takes place, reduction in area, lateral expansion. Acceptance criteria: Three samples are tested and the average value is taken. Values are compared with those specified by application standard or client. After this test additional info about toughness is provided which can be added in test report.

→ Hardness Testing: Generally known as resistance to scratch. Ability of material to resist indentation. Hardness of metal is resistance to plastic deformation. This is determined by measuring the resistance to indentation by a particular type of indenter. Types of methods, i. Vickers hardness test – square based diamond pyramid indenter ii. Rockwell hardness test – Diamond cone indenter or steel ball iii. Brinell hardness test – Ball indenter (mainly used to measure hardness of base metal, tungsten ball usually used.)

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readings in HV, HB, HRV specimen used for macroscopic examination can be used for hardness testing. HEZ will give maximum hardness.