39_02.Intermediate Weld Discontinuities _compressed.pdf

WSS Study Guide WD2.1 Intermediate Weld Discontinuities @ Lesson 2 Objectives After completing this lesson you should be able to: & Identify the types and causes of distortion $+sJgsseses 9 Describe surface irregularities Explain the formation and causes of slag inclusions Describe the forma...

WSS Study Guide WD2.1 Intermediate Weld Discontinuities @ Lesson 2 Objectives After completing this lesson you should be able to: & Identify the types and causes of distortion $+sJgsseses 9 Describe surface irregularities Explain the formation and causes of slag inclusions Describe the formation, types, location and causes of porosity Categorize and describe the formation, detection and correction of cracks 2.14 Distortion 2.14.1 Causes The welding operation involves the application of heat and fusion of metal in localized sections of the weldment. Stresses of sufficient magnitude may occur, due to thermal expansions and contractions, which will cause distortion of the structure. The most frequently seen types of welding distortions are shown in Figure 32. It should be recognized that when distortion occurs it is not always in the simple form of distortion as shown. Distortion quite often OCcurs in compounded forms, such as bending or angular, as well as any combination of the simple forms. @® Page 40 Copyright © 2015 CWB Group Industry Services WSS Study Guide WD2.1 Intermediate Weld Discontinuities Shrinkage Longitudinal shinkage and transverse shrinkage Transverse shrinkage Angular Distortion Caused by transverse shrinkage Bending Distortion Caused by longitudinal shrinkage Buckling Caused by longitudinal shrinkage (also to a minor degree, by transverse shrinkage) Most often when welding large, thin plates or sheets G22] Page 41 Copyright © 2015 CWB Group Industry Services Types oƒ distortion , Longitudinal shrinkage WSS Study Guide WD2.1 Intermediate Weld Discontinuities Distortion may have a number of contributing causes such as: 1. Improper welding parameters. Lack of control of the current, voltage and travel speed can result in an increased heat input. 2. Improper weld pass sequencing. For weldments that may be prone to distortion, a procedure for weld pass sequencing should be developed and followed. Simply spreading out the welding to even out the heat into the weldment will help in reducing distortion ïf a procedure for weld pass sequencing is not available. 3. Improper joint preparation. An example of improper joint preparation causing increased distortion is a bevel-groove weld with an included angle larger than specified on the welding procedure. A larger included angle requires more filler metal to fill the joint, adding more heat to the joint and causing distortion. Inspection of the joint preparation should be performed before fit-up of the joint. 4. Improper joint fit up. An example of improper joint fit up causing increased distortion is a root opening that is too large. Increasing the root opening of a groove weld significantly increases the amount of filler metal required to fill the joint. Care should be taken when fitting up joints for welding and inspection of the fit-ups should be performed before welding to avoid distortion. 5. Joint design. Joint preparation should be carefully considered at the design stage. For example, thick joints can benefit from the use of double-V-, J- or U grooves. m Page 42 Copyright © 2015 CWB Group Industry Services WSS Study Gụide WD2.1 cwbgroup Intermediate Weld Discontinuities 2.15 Surface Irregularities Surface irregularities include badly shaped surface ripples, excessive spatter, inadequately filled craters, overly filled craters and arc strikes or stray arcs. They are typically the result of the welder or welding operator using incorrect welding parameters or technique. Bead irregularities are discontinuities that may be considered defects Ïf they are excessive. Bead profiles with irregular ripple patterns, when excessive, show portions of crater between ripple spacing, as shown in Figure 33. This may be considered a defect due to the increased likelihood Of cracking in these regions from the exposed craters. riG.23, 8ead irregularities An ininarive of the. WSS Studv Guide WD2.1 . cwbgroup' Intermediate Weld Discontinuities Spatter, as shown ïn Figure 34, is a discontinuity that is indicative of improper welding techniques and increase the likelihood of other associated discontinuities. Final weld passes or single pass fillet welds with excessive spatter are problems when painting operations are to be performed. Spatter removal is expensive, as it is very labour intensive. ^ =4 Em Excessive spadtter Tack welds left for inclusion in the completed weld may cause cracks. lf small tacks are made on a large, cold surface, the resulting rapid cooling (quench effect) of the weld metal and heat-affected zone may result in cracks, or micro-fissures that wÏïll develop into cracks given time and stress. Stray arc strikes, as shown in Figure 35, may cause cracks or micro-fissures due to similar rapid cooling. ^ _ _ FiG.25i Arc strikes up Indusi WSS là Guide WD2.1 Intermediate Weld Discontinuities Stray arc strikes, either with the electrode or holder, are more serious than expected. They may create a quenched and embrittled condition in the weld zone of alloy steels and are inadvisable even on mild steel, where high static or normal fatigue stresses may be encountered. Repair of such damage may be difficult and costly, involving grinding and probably preheating in the case of alloys. 2.16 Slag Inclusions Slag Inclusions are oxides and other nonmetallic solids that are sometimes found as elongated or multifaceted inclusions in welds. Slag is always produced when welding with covered electrodes, tubular flux cored electrodes and submerged arc electrodes/flux, serving as scavengers of impurities in the molten metal pool. In addition, it forms a blanket over the weld to control cooling rate and exclude atmospheric oxygen and nitrogen from the hot metal surface. During the welding process, fluxes from slag are forced below the surface of the molten metal by the stirring action of the arc. While the weld metal remains molten, slag generally has time to float to the surface of the weld due to its lower density. A number of factors may affect the ability of the slag to rise to the surface of the weld resulting ïn slag that is trapped subsurface in the weld metal. Some of these factors are: $ $ High viscosity weld metal Rapid solidification of weld metal (heat input too low for the application) $ $ $ Improper manipulation of the electrode Improper interpass cleaning Weld discontinuities from a previous weld pass Rapid solidification of the weld metal sometimes does not allow enough time for slag to float to the surface of the weld metal. This is usually due to incorrect setting of welding parameters, resulting in a heat input that is too low for the application. High-speed welding applications result in low heat inputs and fast cooling, and procedures need to be developed to ensure that slag has adequate time to escape. Page 45 Copyright © 2015 CWB Group Industry Services An Iniative øf thế WSS Study Guide WD2.1 . cwbgroup Intermediate Weld Discontinuities z —- Covered electrode \ Slag rising to HN surface of slag solidifles weld pool Plus Core wire Droplets of filler metal coated with flux or slag entering molten pool through arc Base metal Previously deposited weld bead MPRenGiog an. riG.36| Formation oƒ slag m In multipass welding, insufficient cleaning between weld passes can leave portions of the slag coating in place, which is then covered by subsequent Dasses. Such slag inclusions are often characterized by their location at the edge of the underlying metal deposits, where they tend to extend ^ longitudinally along the weld. Slag lines can be either intermittent or continuous. lí the prior pass produces a bead that is too convex, or ïf the arc has undercut the joint face, ït will be difficult to remove the slag between the groove and the ^ = deposited metal. When the slag is left in place, it is covered by subsequent passes (see Figures 37). Trapped Slag Convex bead Undercui FiG.37) Trapped slag | WSS là Guide WD2.1 Intermediate Weld Discontinuities The proper choice of welding electrode is also important as some electrodes are specially formulated. For example, flux cored electrodes designed for out-of-position welding have a fast freezing slag system. The use of these electrodes for high-speed welding applications may result in slag inclusions. The proper choice of electrodes for high-speed welding are those designed for welding in the flat and horizontal positions with more fluid slag systems. Selecting the wrong size of electrode may also result in slag inclusions. In making a root pass, the electrode may be so large that the arc strikes the side of the groove instead of the root. The slag will roll down into the root opening and become trapped under the root layer. This is because the arc failed to heat the root area to a sufficiently high temperature to allow the slag to float to the surface. Slag inclusions are usually trapped below the surface and require NDT methods capable of detecting below-surface discontinuities. The majority of slag inclusions may be prevented by proper preparation of the groove before each bead is deposited (including sufficient cleaning) and using care to correct any contour that would be difficult to fuse fully with the arc. Page 4 Copyright © 2 9 WSS Study Guide WD2.1 % ® cwbgroup Intermediate Weld Discontinuities 2.16.1 Tungsten Inclusions Tungsten inclusions are characteristic of the tungsten inert gas welding process. lf the tungsten electrode comes into contact with the weld metal, tungsten particles can be trapped in the deposited metal. Applications that employ pure tungsten are prone to this because the electrode end is molten while welding and will spït droplets of tungsten across the electrode electrode and being arc if overheated. Improper sharpening of the tungsten can also result in tungsten inclusions. Sharpening the to a fine point may result in the tip of the electrode melting off deposited in the weld metal. 2.16.2 Copper Inclusions Copper inclusions occur when pieces of the copper sheath of a carbon arc-air electrode fall into the groove and are subsequently welded over. Continuous electrode processes use copper contact tips and copper-alloy nozzles. lf these parts contact the weld pool, copper inclusions can be created. Copper inclusions may also occur during magnetic particle testing of welds. The current supplied to create the magnetic field may be passed through copper conductors (prods). lf there is poor contact between the prods and the steel when the current is applied, sparking will occur and copper particles may be melted into the structure. This tựpe of problem should be carefully avoided due to the propensity for crack propagation from the embedded inclusions. 2.16.3 Oxidation In pipe and tube welding of components for critical service (e.g., nuclear plants), some specifications forbid the presence of oxides on the internal surface of the welds. In these cases, the internal surface of the pipe/tube weld joint is filled with a constant supply of inert gas during welding. This practice is commonly called purging. lf the gas flow is inadequate, oxides wÏïll form and cause the weld to be rejected. Control of the gas supply is, therefore, an essential operation to produce sound welding. Page 48 Copyright © 2015 CWB Group Industry Services WSS Studý Guide WD2.1 cwbgroup Intermediate Weld Discontinuities 3. Metallurgical Discontinuities 3.1 Porosity The term porosity is used to describe gas pockets trapped in the solidifving weld metal. At high temperatures, gases can dissolve into the liquid metal. These gases originate from impurities in the base metal, contamination on the joint surfaces, incomplete shielding of the molten pool or contaminated weld filler metal. The amount of gas that can be absorbed by the liquid is much more than can be contained in the solid metal. When the welding process is applied correctly, the amount of absorbed gas is reduced and porosity is prevented. Gas bubbles escape here Consumable electrode Gases absorbed here Base metal Weld solidifying here tìm Formation oƒ porosity An ininanve of the WSS Study Guide WD2.1 . cwbgroup Intermediate Weld Discontinuities Porosity can occur in a variety of patterns, sizes, shapes and quantities in any position in the deposited weld metal. Some porosity may appear on the surface of the weld, and therefore can be visually detected. However, when porosity is sub-surface, non-destructive testing methods, such as radiography or ultrasonic inspection, is necessary to detect it. | WSS sua Guide WD2.1 1 ặ¡ cwbgroup Intermediate Weld Discontinuities Causes of Porosity: In multipass welding, the location of porosity in relation to the depth on the cross-section of the weld may assist in determining the probable cause. ln many cases, porosity is accumulative as subsequent passes are deposited. To avoid building up the density of the porosity to a point where a completed weld would be unacceptable, it should be removed entirely prior to the addition of further passes. .v.$--2$}. The probable causes of porosity may be categorized as: Moisture Chemistry and structure of the parent material Surface impurities and contaminants Shielding gas Welder technique Air contaminants Insufficient flux coverage Slag residue (improper interpass cleaning) 3.1.1 Moisture Moisture pick-up on flux coated electrodes, or on the surface of flux-cored wire that has been exposed to humid conditions, will cause porosity. The same situation pertains to externally applied flux in welding processes such as submerged arc and electroslag. To avoid moisture, the consumables, including fluxes, should be stored under controlled conditions. Various codes and standards may dictate procedures for the proper storage of weld consumables. Storage conditions for SMAW will be governed by the type of flux, with basic fluxing systems requiring storage temperatures above 120 °C (250 °F). This ensures moisture levels are kept at an acceptable level to produce a weld deposit with a low hydrogen designation. For SMAW and SAW processes, consumable manufacturers typically give baking instructions for electrodes and fluxes that may have been exposed to the atmosphere. Page 51 Copyright © | 2015 CWB Group Industry Services WSS Study Guide WD2.1 —¬ An initiative ØI cwbgroup thế Intermediate Weld Discontinuities 3.1.2 Chemistry and Structure of the Parent Material Under normal conditions, it is important to select the proper filler metal to match the chemistrv and properties of the material to be welded. In cases of relatively high-sulphur content metals, parent material porosity is commonly encountered. Other elements, such as zinc coating on galvanized steels, may also create excessive porosity after welding. In some applications, for example in welding T-stiffeners in shipbuilding, welding over primers is common practice. Specially formulated electrodes and welding procedures developed for welding over primer have been used in this case. Porosity is commonly encountered when welding the second side of a fillet weld because gases have little room to escape. Welding procedures for these situations can be verified by performing fracture tests on the second side welded and taking porosity counts, which are evaluated against the applicable standard. 3.1.3 Surface Contaminants When fabricating metals, the surfaces may be in contact with certain contaminants that can cause porosity. Some of these contaminants are: $ oil $ grease $ paint $ oxides Page 52 Copyright © 2015 CWB Group Industry Services WSS Study Guide WD2.1 cwbgroup Intermediate Weld Discontinuities Rust and mill scale can also absorb contaminants and become a source of DOTOSity. ,v.Ÿ.S._.scs-.2e..e Methods of preparing material for welding often introduce contaminants. Some of these methods include: shears band saws abrasive grinding wheels mechanical nibblers oxy-fuel apparatus Tools, such as air grinders, aïr chipping or aïr scaling guns, may deposit films of oil, grease or moisture on the surfaces to be welded. Inline filters should be used to remove moisture from the air system before it reaches the aïr powered tool. Carbon steel wire brushes used on stainless steels may also be a cause of porosity. Contaminants that cause porosity may also be picked up when recovering unused flux during or after submerged arc welding operations. 3.1.4 Shielding Gas Porosity associated with shielding gases is often caused by poor distribpution within the arc and surrounding areas, insufficient or excessive shielding gas flow rates, or impurities collected in the gas through hoses, connections and the torch or gun assemblies. It is possible for the shielding gas to be contaminated but this occurs mainly where bulk systems have not been properly maintained. However, shielding gas mixtures dissociate over time resulting in an uneven mixture of the gases. Loose fittings and connections may allow atmospheric gases to enter the gas hoses and assemblies and cause porosity. roup Industry Services WSS Study Guide WD2.1 - ® so Intermediate Weld Discontinuities 3.1.5 Welder Technique 2soeses$ In manual welding applications, the following may cause porosity: Taulty manipulation of the electrode excessive arc voltages incorrect electrode angle incorrect weave techniques 3.1.6 Air Contaminants Welding operations located in close proximity to painting operations may result in porosity problems. 3.1.7 Insufficient Flux Coverage Insufficient flux coverage in submerged arc welding may cause scattered surface porosity. 3.1.8 Slag Residue Slag left on the surface of tack welds may cause porosity. Interpass cleaning operations should be such that all slag is removed before depositing the next pass. Surface porosity may be detected by visual examination if the indications are large enough or by a nondestructive method that is suitable for detecting surface discontinuities. Porosity below the surface may be detected by NDT methods capable of detecting subsurface discontinulities. Relevant codes and standards should be referenced to determine the acceptance limits of porosity. Usually this is determined by the size, number and spacing of the porosity, and also the service requirements of the weldment. For example, small porosity indications are common when welding aluminum, and and are acceptable as long as the porosity indications remain small and scattered throughout the weldment. Repairing a weld with porosity that is determined to be a defect requires grinding or gouging to remove the porosity and re-welding. Care must be taken when removing weld metal that a desirable groove profile is achieved for the repair weld passes. @ Page 54 (ø 2015 CWB Group Industry Services nh WSS Study Gúide WD2.1 Intermediate Weld Discontinuities 3.2 Cracking 3.2.1 Solidification Cracking solidification cracking is a phenomenon that results in longitudinal cracks ¡in the weld metal. During the solidification of weld metal, grains begin to grow from the fusion boundary towards the central region of the weld pool. Some alloying elements and impurities are rejected ahead of the growing crystals. Their presence lowers the freezing temperature substantially below that of the first liquid to solidify. As solidification takes place, the weld and surrounding material are progressively cooling, and this gives rise to contraction strains across the weld. When solidification is almost complete, and grains begin to meet, the low melting point liquid may not have much ductility and the contraction strains produce cracking. Solidification cracks rm Solidifcation cracks WSS Study Guide WD2.1 An Iniiaiveof the . cwbgroup Intermediate Weld Discontinuities The risk of solidification cracking is increased when the depth of the weld deposit (dimension Y) is greater than the width (X) as shown in the illustration for fillet and groove welds. màn -- -†o-width ratio 3.2.2 Hot Cracks Hot cracks are cracks located in the weld metal and may be either longitudinal or transverse. The development of hot cracks in welds results from the combined effects of metallurgical and mechanical factors at the grain boundaries. Some metals are prone to hot cracking, for example high temperature alloys and high sulphur steels. rân up Industry Services Hot cracks | WSS Study Guide WD2.1 Intermediate Weld Discontinuities 3.2.3 Hydrogen-lnduced Cold Cracking (HICC) Hydrogen-induced cold cracks are defects that form as the result of the contamination of the weld microstructure by hydrogen. Unlike solidification cracking and hot cracking, which occur during or soon after welding, hydrogen-induced cold cracking occurs hours, days, weeks or even months after welding. Cracking usually occurs at low temperatures, generally below 150 °C (302 °P). The following four factors must be present for hydrogen-induced cracking 1o 0ccur: $ Thesusceptibility of the weld metal or heat-affected zone to hydrogen embrittlement, which is related to the chemical composition, cooling rate and the resulting microstructure $ $ Hydrogen content Thestress at the point of crack initiation $ Lowtemperature By eliminating one of these four factors, hydrogen-induced cracking cannot occur. The most economical method of ensuring that that hydrogen-induced cold cracking does not occur is to use a low-hydrogen Drocess. +$+s$«›. . ce cec Sources of hydrogen include: Group Industry Services Moisture in the electrode Moisture in the electrode coating Hydrogen in the electrode coating, for example, cellulose Humid air Grease, oil or moisture on the plate Paint on the plate WSS Study Guide WD2.1 An Ini5alive of the ! cwbgroup Intermediate Weld Discontinuities Moisture on electrodes ⁄“ Moisture in electrode coating Humid air Hydrogen in electrode coating (e.g., Cellulose) Grease or Moisture on plate - Hydrogen dissolves in weld pool ¬ rên Ì⁄ Sources oƒ hydrogen Hydrogen is soluble ïn iron at high temperatures; however, the solubility Ïs very low at room temperature. As the molten weld hydrogen is rejected from the solution and becomes solidifying weld pool. It diffuses and collects at grain discontinuities of any type. This creates pressure and levels, which can develop into cracks. ¬ ~^ Hydrogen Heat affected zone (HAZ) ki ® rm Page 58 Copyright © 2015 CWB Group Industry Services Diffusion of hydrogen metal solidifies, entrapped in the boundaries or at results in high stress