Presentation on theme: "WELD DEFECTS ( WELD DISCONTINUITIES ), DISTORTION AND ITS CONTROLS"— Presentation transcript:
1 WELD DEFECTS ( WELD DISCONTINUITIES ), DISTORTION AND ITS CONTROLS Ramanatha C.Scientist ‘F’ (Retd.)Gas Turbine Research EstablishmentBangalore
2 Types of Welding Fusion Welding Manual Metal Arc Welding Shielded Metal Arc weldingSubmerged Arc WeldingGas Tungsten Arc Welding (GTAW) or (TIG)Gas Metal Arc Welding (GMAW) or (MIG)Flux Cored Arc WeldingResistance Welding- Spot Welding and Seam WeldingElectrogas Welding (EGW)Electron Beam Welding (EBW)Laser Beam WeldingHigh Frequency WeldingNon – Fusion or Solid State WeldingDiffusion WeldingUltrasonic WeldingFriction Welding
3 DefinitionThe word “Defect “ should be used carefully, as it implies that a weld is defective and requires corrective measures or rejection, In some cases repairs may be made unnecessarily and solely by implication without a critical engineering assessment. Consequently the engineering community has recently begun to use the word “Discontinuity” or “Flaw” instead of “Defect”, Discontinuity may be defined as interruptions in the desirable physical structure of a weld. The significance of a weld discontinuity should be viewed in the context of the fitness-for-purpose of the welded construction. Fitness-for-purpose is a concept of weld evaluation that seeks a balance between quality, reliability and economy of welding procedure. It is not a constant varies depending in the service requirements of a particular welded structure as well as on the properties of the material involved.
4 Classification of the Causes of Discontinuities DesignrelatedStructural DetailsChoice of the wrong type of weld joint For a given applicationUndesirable changes in cross sectionWelding process RelatedUndercutSlag inclusionsPorosityTungsten inclusionBacking piece left onShrinkage voidsOxide inclusionsLack of fusion (LOF)Lack of penetration (LOP)CratersMelt – throughSpatterArc Strikes (Arc burns)UnderfillMetallurgicalCracksFissuresFisheyeSegregationLamellar Tearing
5 Design Related Discontinuity Misalignment (hi-lo)Amount of a joint is out of alignment at the root.Causes: 1. Carelessness.2. Due to joining of different thicknesses(Transition thickness)Prevention: 1. Workmanship2. Transition angles not to exceed 2.5 to 1
6 Welding Process Related Discontinuities Discontinuities resulting from welding process include:Undercut: A groove melted into the base metal adjacent to the toe or root of a weld and left unfilled by weld metal.Slag inclusion: Non metallic solid material entrapped in weld or between weld metal and base metal.Porosity: Cavity-type discontinuities formed by gas entrapment during solidification.Tungsten inclusions: Particles from Tungsten electrodes. Which results from improper gas Tungsten arc welding procedures.Backing piece left on: Failure to remove material placed at the root of a weld joint to support molten weld metal.Shrinkage voids: Cavity type discontinuities normally formed by shrinkage during solidification.Oxide inclusions: Particles of surface oxides which have not melted and are mixed into the weld metal.
7 Welding Process Related Discontinuities Contd…. Lack of Fusion: A condition in which fusion is less than complete.Lack of Penetration: A condition in which joint penetration is less than that specified.Craters: Depression at the termination of a weld head or in the molten weld poolMelt-through: A condition resulting when the arc melts through the bottom of a joint welded from one side.Spatter: Metal particles expelled during welding which do not form a part of the weld.Arc strikes (arc burns): Discontinuities consisting of any localized remelted metal, or change in the surface profile of any part of a weld or base metal resulting from an arc.Underfill: A depression on the face of the weld or root surface of the adjacent base metal.
8 INCOMPLETE PENETRATION This type of defect is found in any of three ways:1) When the weld bead does not penetrate the entire thickness of the base plate.2) When two opposing weld beads do not interpenetrate.3) When the weld bead does not penetrate the toe of a fillet weld but only bridgesacross it.Welding current has the greatest effect on penetration. Incomplete penetration is usually caused by the use of too low a welding current and can be eliminated by simply increasing the amperage. Other causes can be the use of too slow a travel speed and an incorrect torch angle. Both will allow the molten weld metal to roll in front of the arc, acting as a cushion to prevent penetration. The arc must be kept on the leading edge of the weld puddle.
9 LACK OF FUSIONLack of fusion, also called cold lapping or cold shuts, occurs when there is no fusion between the weld metal and the surfaces of the base plate.Causes:1. Poor welding technique.2. Use of a very wide weld joint.3. Very low travel speed and attempting to make too large a weld ina single pass.4. Low welding voltage.5. Presence of Oxide layer.
12 UNDERCUTTINGUndercutting is a defect that appears as a groove in the parent metal directly along the edges of the weld. It is most common in lap fillet welds, but can also be encountered in fillet and butt joints.Causes:1. Improper welding parameters; particularly the travel speedand arc voltage.2. Excessive welding currents.
14 PorosityPorosity is gas pores found in the solidified weld bead. These pores may vary in size and are generally distributed in a random manner. However, it is possible that porosity can only be found at the weld centre. Pores can occur either under or on the weld surface.The most common causes of porosity are atmosphere contamination, excessively oxidized work piece surfaces, inadequate deoxidizing alloys in the wire and the presence of foreign matter.Atmospheric contamination can be caused by:1) Inadequate shielding gas flow.2) Excessive shielding gas flow. This can cause aspiration of air into the gas stream.3) Severely clogged gas nozzle or damaged gas supply system (leaking hoses, fittings, etc.)4) An excessive wind in the welding area. This can blow away the gas shield.
16 Weld Defects, Their Possible causes and corrective action POROSITY A. Inadequate shielding gas coverageCorrective ActionsIncrease the shielding gas flow to displace all air from the weld areaDecrease the shielding gas flow to avoid turbulence and entrapment of air in the gasRemove Spatter from the interior of the gas nozzle.Eliminate drafts (from fans, open doors, etc.) blowing into the welding arc.Use a slower travel speed.Reduce the nozzle-to-work distance.Hold the gun at the end of the weld until the molten crater solidifiers
17 Weld Defects, Their Possible causes and corrective action POROSITY ( Contd..) Possible Causes Corrective actionsB. Electrode contamination C. Workpiece contamination D. Arc Voltage too high E. Excess nozzle -to- work distanceUse only clean and dry electrode wireEliminate pickup of lubricant on electrode in the wire feeder or conduitRemove all grease, oil, rust, paint and dirt from work surfaces before welding.Use a more highly deoxidizing electrode.Reduce operating VoltageReduce electrode extension.
18 Weld Defects, Their Possible causes and corrective action Lack of penetration Possible Causes Corrective actionsImproper joint preparationb. Improper welding Techniquec. Inadequate heat inputJoint design must be adequate to provide access to the bottom of the grove while maintaining proper nozzle-to-work distance and arc characteristicsReduce root face heightProvide or increase the root opening in butt jointsPosition the electrode at the proper travel angle to achieve maximum penetrationKeep the arc on the leading edge of the weld poolIncrease electrode feed to obtain higher welding currentsMaintain proper nozzle-to-work distance
19 Weld Defects, Their Possible causes and corrective action Excessive melt through Possible Causes Corrective actionsReduce the electrode feed rate and arc voltageIncrease the travel speedReduce excessive root openingIncrease root face heightExcessive heat inputImproper joint preparations
27 Metallurgical Discontinuities Cracks: Fracture type discontinuities characterized by a sharp tip and high ratio of length and width to opening displacement.Fissures: Small crack-like discontinuities with only a slight separation (opening displacement) of the fracture surfaces.Fisheye: A discontinuity found on the fracture surface of a weld in steel that consists of a small pore or inclusion surrounded by a bright, round area.Segregation: A non-uniform distribution or concentration of impurities or alloying elements which arises during the solidification of the weld.Lamellar tearing: A type of cracking that occurs in the base metal or heat affected zone (HAZ) of restrained weld joints that is the result of inadequate ductility in through-thickness direction of steel plate.
28 CrackingThis can occur due just to thermal shrinkage or due to a combination of strain accompanying phase change and thermal shrinkage. In the case of welded stiff frames, a combination of poor design and inappropriate procedure may result in high residual stresses and cracking. Where alloy steels or steels with a carbon content greater than about 0.2% are being welded, self cooling may be rapid enough to cause some (brittle) martensite to form. This will easily develop cracks. To prevent these problems a process of pre-heating in stages may be needed and after welding a slow controlled post cooling in stages will be required. This can greatly increase the cost of welded joins, but for high strength steels, such as those used in petrochemical plant and piping, there may well be no alternative.
29 Factors Promoting Hot Cracking Welding Current Density (High Levels promote cracking)Heat Distribution (Joint design)RestraintCrack sensitivity of electrode materialDilution of weld metalImpurities (Eg. Sulphur & Phosphorous)Preheating (Increase liability to cracking)Welding Procedures (High Speeds, long arcs increase sensitivity)
30 Types of Cracking Solidification Cracking This is also called centreline or hot cracking. They are called hot cracks because they occur immediately after welds are completed and sometimes while the welds are being made. These defects, which are often caused by sulphur and phosphorus, are more likely to occur in higher carbon steels.A schematic diagram of a centreline crack is shown below:
31 Types of cracking (Contd..) Hydrogen induced cracking (HIC)It is also referred to as hydrogen cracking or hydrogen assisted cracking, can occur in steels during manufacture, during fabrication or during service. When HIC occurs as a result of welding, the cracks are in the heat affected zone (HAZ) or in the weld metal itself.Four requirements for HIC to occur are:a) Hydrogen be present, this may come from moisture in any flux or from other sources. It is absorbed by the weld pool and diffuses into the HAZ.b) A HAZ microstructure susceptible to hydrogen cracking.c) Tensile stresses act on the weldd) The assembly has cooled to close to ambient - less than 150oCHIC in the HAZ is often at the weld toe, but can be under the weld bead or at the weld root. In fillet welds cracks are normally parallel to the weld run but in butt welds cracks can be transverse to the welding direction.
32 Reheat Cracking Mo-V and Mo-B steels susceptible Due to high temperature embrittlement of the heat-affected zone and the presence of residual stressCoarse-grained region near fusion line most susceptiblePrevention byLow heat input weldingIntermediate stress relief of partially completed weldsDesign to avoid high restraintRestrict vanadium additions to 0.1% in steelsDress the weld toe region to remove possible areas of stress concentration
33 Lamellar TearingOccurs in thick plate subjected to high transverse welding stressRelated to elongated non-metallic inclusions, sulfides and silicates, lying parallel to plate surface and producing regions of reduced ductilityPrevention byLow sulfur steelSpecify minimum ductility levels in transverse directionAvoid designs with heavy through-thickness direction stress
34 Residual Stress & Distortion Residual Stresses also referred to as internal stresses , initial stresses , inherent stresses reaction stresses and locked in stresses, arc stresses that continue to exist in a body even after the removal of all the external loads. Thermal stresses occur in parts during welding due to the localized application of heat. Residual stresses and distortion remain after the welding process is completed. Thermal stresses, residual stresses and distortion sometimes cause cracking and mismatching of joints. Correcting unacceptable distortion is costly and in some cases impossible.Distortion in a weld results from the expansion and contraction of the weld metal and adjacent base metal during the heating and cooling cycle of the welding process.
35 Causes of residual Stresses Residual stresses in metal structures occur for many reasons during various manufacturing stages. They may occur during forming and shaping of metal parts by processes such as shearing, bending, machining and grinding. They also occur during fabrication processes such as welding. Residual stresses are classified into two types: 1. Residual stresses produced by mismatch. When bars of different lengths are forcibly connected , tensile stresses are produced in the shorter bar and compressive stresses are produced in the longer bars. 2. Residual stresses produced by uneven distribution of non elastic strains. When materials are heated uniformly, thermal stress is not produced, as they also expand uniformly. However, residual stress is expected when materials are not heated uniformly. Also, residual stresses are produced when unevenly distributed non elastic strains, or plastic strains exist.Before welding; Free stateAfter welding; Stressed state
36 Residual Stresses developed during welding Fig : Residual stresses developed during welding of a butt joint.
37 Distortion after Welding Fig : Distortion of parts after welding : (a) butt joints; (b) fillet welds. Distortion is caused by differential thermal expansion and contraction of different parts of the welded assembly.
38 Prevention or Minimization of distortion Several ways can be used to minimize distortion caused by shrinkage:Do not overweldControl fitup.Use intermittent WeldingUse as few weld passes as possiblePlace welds near the neutral axisBalance welds around the neutral axisUse backstep weldingAnticipate the shrinkage forcesPlan the welding sequenceRemove shrinkage forces after weldingMinimize welding timeUse the smallest leg size permissible when fillet welding.
39 Prevention or Minimization of distortion (Contd..) For groove welds, use joints that will minimize the volume of weld metal. Considerdouble-sided joints instead of single-sided joints.Weld alternately on either side of the joint when possible with multiple-pass welds.Use low heat input procedures. This generally means high deposition rates andhigher travel speeds.Use welding positioners to achieve the maximum amount of flat-position welding.The flat position permits the use of large- diameter electrodes and high-deposition-rate welding procedures.Weld toward the unrestrained part of the member.Use clamps, fixtures, and strongbacks to maintain fitup and alignment.Prebend the members or preset the joints to let shrinkage pull them back intoalignment.Sequence subassemblies and final assemblies so that the welds being madecontinually balance each other around the neutral axis of the section.
40 Prevention or Minimization of distortion (Contd..)
41 Prevention or Minimization of distortion (Contd..)
42 Other Techniques for Distortion Control Water-Cooled Jig Various techniques have been developed to control distortion on specific weldments. In sheet-metal welding, for example, a water-cooled jig is useful to carry heat away from the welded components. Copper tubes are brazed or soldered to copper holding clamps, and the water is circulated through the tubes during welding. The restraint of the clamps also helps minimize distortion.
43 Other Techniques for Distortion Control (Contd..) StrongbackThe "strongback" is another useful technique for distortion control during butt welding of plates. Clips are welded to the edge of one plate and wedges are driven under the clips to force the edges into alignment and to hold them during welding.
44 Other Techniques for Distortion Control (Contd..) Thermal Stress Relieving (Heat Treatment)Except in special situations, stress relief by heating is not used for correcting distortion. There are occasions, however, when stress relief is necessary to prevent further distortion from occurring before the weldment is finished.PreheatPreheat reduces the temperature differential between the weld region and the base metalReduces the cooling rate, which reduces the chance of forming martensite in steelsReduces distortion and shrinkage stressReduces the danger of weld crackingAllows hydrogen to escape
45 Interaction of Preheat and Composition CE = %C + %Mn/6 + %(Cr+Mo+V)/5 + %(Si+Ni+Cu)/15Carbon equivalent (CE) measures ability to form martensite, which is necessary for hydrogen crackingCE < no preheat or postweld heat treatment0.35 < CE < preheat0.55 < CE preheat and postweld heat treatmentPreheat temp. as CE and plate thickness
46 Other Techniques for Distortion Control (Contd..) Post-Weld Heat TreatmentThe fast cooling rates associated with welding often produce martensiteDuring postweld heat treatment, martensite is tempered (transforms to ferrite and carbides)Reduces hardnessReduces strengthIncreases ductilityIncreases toughnessResidual stress is also reduced by the postweld heat treatmentPostweld Heat Treatment and Hydrogen CrackingPostweld heat treatment (~ 1200°F) tempers any martensite that may have formedIncrease in ductility and toughnessReduction in strength and hardnessResidual stress is decreased by postweld heat treatmentRule of thumb: hold at temperature for 1 hour per inch of plate thickness; minimum hold of 30 minutes