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Mailam India Ltd.Mr. R.D.PennathurWELDING METALLURGYMailam India Ltd.Mr. R.D.PennathurHeat Affected Zone Welding ConcernsChanges in structure resulting in changes in propertiesCold cracking due to hydrogen

Mailam India Ltd.Mr. R.D.Pennathur3Let us now start to investigate concerns in the true heat affected zone.

Mailam India Ltd.Mr. R.D.Pennathur4First lets review the thermal cycles experienced in the heat affected zone as a result of the passage of the weld. The figure illustrated here shows the temperature vs time curve at various distances from the weld metal. Note that almost every thermal cycle imaginable occurs over this short distance of the heat affected zone. Thus a variety of structural and property variations are expected.

Two Types Of Alloy Systems:Mailam India Ltd.Mr. R.D.Pennathur5There are two types of alloy systems which we will consider, those which do not have an allotropic phase change during heating like copper, and those which have an allotropic phase change on heating like steel. We will first consider those materials which do not have an allotropic phase change. The top schematic illustrates this type of material. We will however consider that this material has been cold worked (not the elongated cold worked grains present in the base material (region A). The weld metal is represented by region C, and the heat affected zone is region B.

Mailam India Ltd.Mr. R.D.Pennathur6Note that the heat of welding has effected the structure of this material even though there are no allotropic transformations. Recall that cold worked structures undergo recover, recrystalization and grain growth when heated to ever increasing temperatures. So it is in this material. As we traverse from the cold worked elongated grains in the unaffected base metal, we come to a region where the cold worked grains undergo recovery and then shortly there after they recrystalize into fine equiaxed new grains. Traversing still closer to the weld region we note grain growth where the more favorably oriented grains consume neighboring grains and grain growth occurs. The grains within the weld epitaxially nucleate from the grains in the heat affected zone at the fusion boundary, and grain growth continues into the solidifying weld metal making very large grains.

Introductory Welding Metallurgy,AWS, 1979Cold Worked Alloy Without Allotropic TransformationMailam India Ltd.Mr. R.D.Pennathur7One of the factors that occur when cold worked grains recrystalize and grain grow occurs we have already discussed, and that is the material softens. Thus the heat affected zone and weld metal will not hold the same strength level as the cold worked base metal. Another consequence of increased grain size is perhaps equally important and that is that the larger grains are more brittle. A Charpy impact test is used to determine how much impact energy a structure will absorb over various temperature ranges. Note that the larger grain size material will become brittle and not absorb much of an impact load even at temperatures around room temperature and above.

Mailam India Ltd.Mr. R.D.Pennathur

Welding precipitation hardened alloys without allotropic phase changes

Welded In: Full Hard ConditionSolution Annealed ConditionIntroductory Welding Metallurgy,AWS, 1979Mailam India Ltd.Mr. R.D.Pennathur8A second way of strengthening materials without allotropic phase changes is by precipitation strengthening. (The first we just discussed was cold working). Recall that in precipitation strengthening, the base metal is solutionized, rapidly cooled and then aged at some moderately elevated temperature to promote precipitate formation. There are two ways that precipitation hardened material can be welded. One is to weld on the full hard, that is the already aged base metal. The second is to weld on material which has been solution annealed and rapidly cooled, but not yet given the ageing heat treatment. In either case, when welding, the heat affected zone will see some additional time at temperature (varied temperature over the distance of the HAZ) as illustrated above, and this will effect the aged or overaged condition of the precipitates.

Annealed upon CoolingMailam India Ltd.Mr. R.D.Pennathur9When welding on the already aged (full hard) material, the unaffected base metal will have aged precipitates that are just the right size for strengthening. The heat affected zone, on the other hand, will experience some additional heating. In the region farthest from the weld the heat will be sufficient to overage the precipitates with the resulting loss in strength. In regions closer to the weld, the heat will be so excessive that the temperature will exceed the two phase region and the single phase solutionizing region on the phase diagram will be entered. Again, a loss in strength will occur, but this region at least might be able to be re-aged to recover some strength.

Introductory Welding Metallurgy,AWS, 1979Precipitation Hardened Alloy Welded in Full Hard ConditionMailam India Ltd.Mr. R.D.Pennathur10Here are presented hardness traverses of welds made in the pre-weld full hard material. Note the softening as mentioned previously in the as-welded condition. Note that heat input also has an effect on the extent of softening in the as welded condition. In some cases, a post-weld aging treatment can restore hardness in some of the regions of this weld, but it never fully erases the effect of the weld overaging.

Introductory Welding Metallurgy,AWS, 1979Precipitation Hardened Alloys Welded in Solutioned ConditionMailam India Ltd.Mr. R.D.Pennathur11On the other hand, welding precipitation hardened material in the solution condition with a low heat input, only slightly ages the material in the heat affected zone. Subsequent post-weld ageing strengthens the entire weld region (only a slight overaging occurs in the slightly ages regions from the weld). With high heat input, however, the case is somewhat different as moderate again occurs on welding and post-weld treatment only serve to accentuate the overaging process. So care must be exercised when establishing a welding procedure for welding the precipitation hardened alloys.

Mailam India Ltd.Mr. R.D.PennathurDiscuss !!Turn to the person sitting next to you and discuss (1 min):

Precipitation hardened austenitic stainless steel is used for high strength applications like rocket components etc. Reviewing the various procedures for welding precipitation hardened steels, what procedure would you recommend? Does it make any difference that this is austenitic stainless steel and not just plain carbon steel?Mailam India Ltd.Mr. R.D.Pennathur

Mailam India Ltd.Mr. R.D.Pennathur13Let us now turn our attention to the materials which do have an allotropic phase change during heating. A typical material like steel is ferrite at low temperatures and transforms to austenite when heated. Each time the material goes through one of these phase changes, new finer equiaxed grains grow starting from the grain boundaries of the previous grains present. So in the case of cold worked steels in the base metal, the elongated cold worked grains will undergo recovery, recrystalization and grain growth just as discussed above. But now the recrystallized grains at higher temperature will undergo the allotropic phase change, reducing the grain size again which then is followed by grain growth at still higher temperature (nearer the weld). This variation in grain structure is schematically shown in the lower figure above.

Introductory Welding Metallurgy,AWS, 1979Steel Alloys With Allotropic TransformationMailam India Ltd.Mr. R.D.Pennathur14This illustration shows the various regions in the heat effected zone and what microstructure would be predicted as related to the iron-carbon phase diagram. Note that at the far extent of the weldment in the base metal, ferrite and cementite arte expected. Closer to the weld some dual phase ferrite austenite will occur at temperature of welding. Closer yet we would expect single phase austenite, and then maybe some austenite of delta ferrite and liquid mixtures until at the maximum temperature the liquid phase would be present as the welding arc traverses. These are the structures at temperature, but we now must consider what happens during cooling.

Mailam India Ltd.Mr. R.D.Pennathur15We have already seen that the cooling rate from welding can vary depending upon a number of weld variables. The two most important are preheat and heat input. The cooling rate is fastest when no preheat and low heat input are used to make the weld. On the other hand, the cooling rate is slowest when high preheat and high heat input are employed.

Introductory Welding Metallurgy,AWS, 1979Mailam India Ltd.Mr. R.D.Pennathur16As we have learned before, the cooing rate from austenite can effect the room temperature structure as defined by the continuous cooling transformation diagram. Rapid cooling results in non-equilibrium hard brittle martensite. Slow cooling results in some higher temperature transformation products such as bainite, ferrite and pearlite which tend to be softer. Examining two welding procedures here, one with no preheat (number 1) and the other with preheat (number 2) we find some differences in structure. The no preheat weld has a narrower HAZ and rapid cooling and the austenite transforms to martensite on cooling giving a hard martensite peak near the fusion line. The weld with preheat has a wider HAZ, a slower cooling rate producing ferrite pearlite and bainite and the fusion line peak is softer. There is also more outer HAZ region grain growth and overaging so that the softening in the HAZ is greater. Thus, once again, welding procedures have to be carefully tailored for the material being welded.

Mailam India Ltd.Mr. R.D.PennathurQuestions?Turn to the person sitting next to you and discuss (1 min.): As we saw, the cooling rate can depend upon the preheat and the heat input. Many codes actually specify the range of heat inputs that can be used to weld certain materials. We had an equation to determine the heat input before. What is it? What processes have the highest Heat Inputs? The lowest?Mailam India Ltd.Mr. R.D.PennathurHydrogen CrackingHydrogen cracking, also called cold cracking, requires all three of these factorsHydrogenStressSusceptible microstructure (high hardness)Occurs below 300CPrevention by Preheat slows down the cooling rate; this can help avoid martensite formation and supplies heat to diffuse hydrogen out of the materialLow-hydrogen welding procedureCracking in Welds0.1.1.5.2.T12.95.12

Mailam India Ltd.Mr. R.D.Pennathur18As has been discussed, the cooling rate in the HAZ of steel may be sufficiently high as to produce martensite. In combination with hydrogen and stress, a high-hardness martensitic microstructure can result in hydrogen cracking. Hydrogen-induced cracking may be avoided by elimination one of its three requisites: hydrogen, stress, high hardness microstructure. Hydrogen can come from moisture; therefore, electrodes and flux must be dry. Paint, oil, or heavy oxide layers may also be sources of hydrogen.Hydrogen cracking can be detected by ultrasonic methods; surface cracks may be detected visually or with penetrant methods. Small cracking areas may be cut out and repair welded. Extensive cracking may result in scrapped parts.

Mailam India Ltd.Mr. R.D.Pennathur19How does the hydrogen get into the heat effected zone where the cold cracking is often observed? Liquid metal can absorb more hydrogen than solid austenite, and austenite more than ferrite. When welds are made on wet material or with wet electrodes, the hydrogen is absorbed into the liquid. As the liquid solidifies, if forces some of the hydrogen which it is trying to get rid of into the surrounding hot austenite. If there is still too much to be absorbed even in a supersaturated solid, some hydrogen porosity may form in the weld metal, a sure sign that poor procedures were followed.

Mailam India Ltd.Mr. R.D.Pennathur20During cooling, the cooler material tries to push hydrogen out while at the same time the solidifying weld metal tries to push hydrogen out. Note that the large grained region of the HAZ which just may have the hardest most susceptible martensitic microstructure thus acquired hydrogen from both directions and a supersaturated condition exists there.

Mailam India Ltd.Mr. R.D.Pennathur21The hydrogen then slowly diffuses to any location where is can relieve the stress of being stuck in the lattice in the supersaturated condition. The hydrogen atoms are often carried by dislocation and the preferred site for collection is often inclusions. At this point, they can either weaken the surrounding structure or the hydrogen atoms can recombine and form molecular hydrogen gas and exert an internal pressure. As this pressure grows, the crack slowly expands until a critical size is reached and catastrophic failure occurs. This takes time at low temperature , thus the common name of cold cracking or delayed cracking applies.

Mailam India Ltd.Mr. R.D.Pennathur22The time after welding has an effect. As time proceeds, the hydrogen diffuses away from the high concentration in the most critical portion of the heat affected zone. If hydrogen diffuses away before the critical crack length is reach, the weld has occurrence of some microcracks but catastrophic failure does not occur. On the other hand, if hydrogen diffusion is slower than that failure may occur. Elevated temperature post weld treatment will allow fast hydrogen diffusion and may help in the reduction of cold cracking.

DickinsonMailam India Ltd.Mr. R.D.Pennathur23The above diagram summarizes the discussions about delayed cracking. The red regions are crack sensitive regions while the blue represents the safe region. Materials with high hardenabilty will promote the formation of martensite, and materials with high carbon content will produce a harder martensite. Increases in heat input and preheat and stress reliving practices increases the safety against hydrogen delayed cracking. And the decrease in hydrogen in the welding process likewise increases the safety region.WHY PREHEAT?Preheat 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 escapeCarbon and Low-Alloy Steels0.1.1.5.1.T9.95.12Mailam India Ltd.Mr. R.D.Pennathur24If the cooling rate is slowed sufficiently, martensite is not formed. The application of preheat before welding slows the cooling rate after welding. Preheat, as the name implies, involves heating up the plate to be welded to a specified temperature prior to welding. Thus, after welding, the temperature differential between the weld and the surrounding plate is less. This acts to slow the cooling rate and avoid the formation of martensite. By reducing the temperature differential between the weld and the surrounding plate, preheat also helps to reduce shrinkage stress and distortion. Since steels are susceptible to hydrogen cracking, the preheat also provides energy for hydrogen to escape from the metal. Hydrogen is introduced into the metal from several sources: moisture in the shielding gas or flux, degreasing agents that were not properly removed prior to welding, moisture in the air.Using Preheat to Avoid Hydrogen CrackingIf the base material is preheated, heat flows more slowly out of the weld regionSlower cooling rates avoid martensite formation

Preheat allows hydrogen to diffuse from the metalCooling rate (T - Tbase)2SteelCooling rate (T - Tbase)3T baseT baseMailam India Ltd.Mr. R.D.Pennathur25If the cooling rate is slowed sufficiently, martensite is not formed. The application of preheat before welding slows the cooling rate after welding. Preheat, as the name implies, involves heating up the plate to be welded to a specified temperature prior to welding. Thus the temperature differential between the weld and the surrounding plate is less. This acts to slow the cooling rate and avoid the formation of martensite. Preheat has the added benefit of reducing residual stress and distortion by reducing the temperature differential between the weld and the surrounding plate.In a manufacturing operation, the time, equipment, and energy costs associated with preheat detract from the overall productivity of the welding operation. Also, in confined spaces, high preheat temperatures, as high as 500F for some steels, are a major source of discomfort for the welder. Nonetheless, based on composition and other factors, preheat is required for many steels. Ensuring its proper application and control can be a daunting task; however, the alternative to proper preheat is clear - scrapping parts after welding. Interaction of Preheat and CompositionCarbon equivalent (CE) measures ability to form martensite, which is necessary for hydrogen cracking

CE < 0.35no preheat or postweld heat treatment0.35 < CE < 0.55preheat0.55 < CEpreheat and postweld heat treatment

Preheat temp. as CE and plate thickness CE = %C + %Mn/6 + %(Cr+Mo+V)/5 + %(Si+Ni+Cu)/15SteelMailam India Ltd.Mr. R.D.Pennathur26Which formula to use? There are a number of carbon equivalent formulae and charts and graphs. Check to ensure that you are using a method suited to your steel. Software packages are also available that examine joint geometry, process variables, and steel composition to compute the proper preheat and postweld heat treatment. Procedure selection for avoiding hydrogen cracking is often made based on the chemical composition of a material. Unfortunately, records that contain chemical composition information for older, existing structures, such as pipelines and piping systems, are often difficult or impossible to locate. In these cases, estimates of chemical composition can be made based on the maximum allowable limits of the specification to which the materials were produced. This usually results in an overestimation of the tendency for unacceptably high hardness levels to result and can, therefore, be restrictively over-conservative.The chemical composition of an in-service component can be determined by removal of samples for laboratory analysis, provided that care is taken in removing the samples and that they are properly analyzed. For example, a flat area can be machined on the pipe provided that the remaining wall thickness is greater than the specified nominal wall thickness minus the pipe mill tolerance. The amount of material required for direct spectrographic analysis of remelted machining chips is 10 to 20 grams. Care should be taken to prevent oxidation (i.e., overheating) of the machining chips. Low-cost portable milling machines that are used primarily to cut keyways in shafts can be used for this purpose.

Why Post-Weld Heat Treat?The fast cooling rates associated with welding often produce martensite

During postweld heat treatment, martensite is tempered (transforms to ferrite and carbides)Reduces hardnessReduces strengthIncreases ductilityIncreases toughness

Residual stress is also reduced by the postweld heat treatmentCarbon and Low-Alloy Steels0.1.1.5.1.T10.95.12Mailam India Ltd.Mr. R.D.Pennathur27If martensite is produced in the HAZ, its poor mechanical properties can be remedied through a post-weld heat treatment. Heating the martensite to an elevated temperature, but not high enough to change it back to austenite, allows some of the carbon to form iron carbide. This process is referred to as tempering. It reduces the hardness and increases the ductility of the martensite. Although the strength may be somewhat reduced, the toughness increases. Post-weld heat treatment also helps to reduce any residual stress left behind from the welding process.Some compositions of steel were designed to always form martensite on cooling in order to take advantage of its high strength. These steels are generally postweld heat treated in order to increase the ductility of the martensite.Postweld Heat Treatment & Hydrogen CrackingPostweld heat treatment (~ 1200F) tempers any martensite that may have formedIncrease in ductility and toughnessReduction in strength and hardness

Residual stress is decreased by postweld heat treatment

Rule of thumb: hold at temperature for 1 hour per inch of plate thickness; minimum hold of 30 minutesSteelMailam India Ltd.Mr. R.D.Pennathur28If martensite is produced in the HAZ, its poor mechanical properties can be remedied through a post-weld heat treatment. Heating the martensite to an elevated temperature, but not high enough to transform it back to austenite, allows some of the carbon to form iron carbide. This process is referred to as tempering. It reduces the hardness and increases the ductility of the martensite. Although the strength may be somewhat reduced, the toughness increases. Post-weld heat treatment also helps to reduce any residual stress left behind from the welding process.As with the preheat process, the time, equipment, and energy costs associated with postweld heat treatment detract from the overall productivity of the welding operation. Temperbead or controlled deposition welding sequences have been designed that are self-tempering. In these processes, the heat from subsequent welding passes tempers the martensite produced by prior passes. Such processes have been used successfully for many years, particularly for weld repair. These processes do require special welder training and have limited use with some of the more hardenable materials (higher alloyed steels such as 2.25Cr-1Mo) when it comes to meeting code requirements for harsh service environments.

Mailam India Ltd.Mr. R.D.PennathurQuestions?Mailam India Ltd.Mr. R.D.Pennathur

Base Metal Welding ConcernsMailam India Ltd.Mr. R.D.Pennathur30Lets end our discussion of weld problems by considering difficulties which may occur in the base metal. Ordinarily, one would not consider that there could be any problems occurring in the base metal as it does not see the effect of any weld heat. But let us not forget that the weld solidification does introduce some stresses into the weldment because of the solidification and variable expansion and contraction of heated materials.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 stressCracking in Welds0.1.1.5.2.T14.95.12Mailam India Ltd.Mr. R.D.Pennathur31In the processing of steel, sulfur combines with manganese to form MnS inclusions in the ingot. When the ingot is rolled, these inclusions elongate into what are referred to as stringers. The strength of the steel in the direction transverse to these stringers is reduced. The stress produced by welding can cause cracks if the weld is made in the rolling direction, parallel to the stringers. Small regions can be cut out and replaced with weld metal. Large regions may result in scrapping the part.

Mailam India Ltd.Mr. R.D.Pennathur

Improve CleanlinessImprove through thickness propertiesButteringMailam India Ltd.Mr. R.D.Pennathur32This illustrates how the rolled out inclusions (mainly MnS) can de-bond from the base metal matrix and under the action of short transverse (through thickness) stresses they can actually link to form a stepped like fracture. Improving cleanliness of the steel during steel processing, and improving through thickness properties by steel making processed line calcium or rare earth treatment which produces inclusions which to not roll out a long stringer during plate processing can help. Also laying a weld bead on top of the plate which has lower strength and improved ductility before welding the attachment can help by letting the weld bead take the shrinkage stresses rather than transmitting them into the base plate.MULTIPASS WELDSHeat from subsequent passes affects the structure and properties of previous passes

TemperingReheating to form austeniteTransformation from austenite upon cooling

Complex MicrostructureCarbon and Low-Alloy Steels0.1.1.5.1.T11.95.12Mailam India Ltd.Mr. R.D.Pennathur33In a multi-pass weld, the heating and cooling cycles of one pass are superimposed upon those of previous passes. Portions of previous passes are heated high enough to form austenite again, and upon cooling this austenite once again can transform to ferrite and pearlite or to martensite. Some portions of previous weld passes will not transform to austenite but will be tempered by the heat from subsequent passes. All in all, this leads to a rather complicated structure in multi-pass welds.MULTIPASS WELDSExhibit a range of microstructures

Variation of mechanical properties across joint

Postweld heat treatment tempers the structureReduces property variations across the joint

SteelMailam India Ltd.Mr. R.D.Pennathur34In a multi-pass weld, the heating and cooling cycles of one pass are superimposed upon those of previous passes. Portions of previous passes are heated high enough to form austenite again, and upon cooling this austenite once again can transform to ferrite and pearlite or to martensite. Some portions of previous weld passes will not transform to austenite but will be tempered by the heat from subsequent passes. All in all, this leads to a rather complicated structure in multi-pass welds.REHEAT CRACKINGMo-V and Mo-B steels susceptible

Due to high temperature embrittlement of the heat-affected zone and the presence of residual stress

Coarse-grained region near fusion line most susceptible

Prevention 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 concentrationCracking in Welds0.1.1.5.2.T15.95.12Mailam India Ltd.Mr. R.D.Pennathur35Steels containing molybdenum or vanadium resist creep at elevated-temperature. These steels, along with thick sections of high-strength, low-alloy steels, are subject to reheat cracking in combination with residual stress and low creep-ductility in the HAZ. During postweld heat treatment, cracks form along the grain boundaries in the HAZ, particularly in the coarse-grained region near the fusion line. Defects at the weld toe can promote reheat cracking; therefore, grinding or peening the weld toe can help prevent this cracking. The cracked area must be heat treated to restore ductility prior to repair. Then it can be cut out beyond the ends of the cracks and rewelded.KNIFE-LINE ATTACK IN THE HAZCr23C6 precipitate in HAZBand where peak temperature is 800-1600FCan occur even in stabilized gradesPeak temperature dissolves titanium carbidesCooling rate doesnt allow them to form againWeldHAZKnife-line attackStainless SteelMailam India Ltd.Mr. R.D.Pennathur36A discrete band in the heat affected zone of the austenitic stainless steel welds experiences peak temperatures in the 800-1600F temperature range associated with sensitization. Chromium carbide precipitation in this region can lower the chromium content near the grain boundaries to less than 12%, thereby causing sensitization.Stabilized grades can also suffer from knife-line attack. Elevated temperatures in the heat-affected zone can dissolve titanium and niobium carbides. The fast cooling rates in the welded joint do not allow these carbides to reform. This leaves excess free carbon, which can then form chromium carbides.We are one of the renowned manufacturers of various grades of welding consumables which are second to none in terms of quality. We have the most modern manufacturing facility equipped with latest sophisticated machinery at Pondicherry, IndiaWe are manufacturing a vast range of Shielded Metal Arc Welding Electrodes and Flux Cored Arc Welding Electrodes. Wealso supply tested GMAW and GTAW welding consumables for all the applications.Our hardworking team would always be interested in any opportunity to cater your requirement of welding consumables.Currently we are supplying massive quantity of welding consumables to many world class EPC companies in various range of welding consumables including Carbon steel, Low Temperature Carbon Steel, Stainless Steel, Low alloy steel etc. If at all the need arises for a special consumable which is not in our arsenal, our R&D team is fully equipped in developing special electrodes to meet Service Requirements, Impact Test Requirements with strict chemistry controls and as welded hardness test criteria.We built quality and consistency in our consumables. Our manufacturing facility always had the market pulse to meet its demands and fast track delivery requirements without compromising quality and consistency.Willing to be a part of your esteemed organization, we took this privilege to approach and request your kind consideration for providing opportunity to MAILAM INDIA LTD for the following consumables.

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