1. Welding Metallurgy 2

2. Welding Metallurgy 2 Objectives
The region of the weld where liquid does not form
Mechanisms of structure and property changes associated with these regions

3. Heat Affected Zone Welding Concerns
Let us now start to investigate concerns in the true heat affected zone.

4. Heat Affected Zone Welding Concerns
Changes in Structure Resulting
in Changes in Properties
Cold Cracking Due to Hydrogen
Two major concerns occur in the heat affected zone which effect weldability these are, a.) changes in structure as a result of the thermal cycle experienced by the passage of the weld and the resulting changes in mechanical properties coincident with these structural changes, and b.) the occurrence of cold or delayed cracking due to the absorption of hydrogen during welding.

5. First let’s 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.

6. Look At Two Types of Alloy Systems
There 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.

7. Note 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.

8. Cold Worked Alloy Without Allotropic Transformation
One 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.
Introductory Welding Metallurgy,
AWS, 1979

9. Solution Annealed Condition
Welding
Precipitation
Hardened Alloys
Without Allotropic
Phase Changes
Welded In:
Full Hard Condition
Solution Annealed Condition
A 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.

10. Annealed upon Cooling
When 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.

11. Precipitation Hardened Alloy Welded in Full Hard Condition
Here 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, 1979

12. Precipitation Hardened Alloys Welded in Solutioned Condition
On 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.
Introductory Welding Metallurgy,
AWS, 1979

13. Let 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.

14. Steel Alloys With Allotropic Transformation
This 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.
Introductory Welding Metallurgy,
AWS, 1979

15. We 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.

16. Introductory Welding Metallurgy,
As 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.
Introductory Welding Metallurgy,
AWS, 1979

17. Hydrogen Cracking
Hydrogen cracking, also called cold cracking, requires all three of these factors
Hydrogen
Stress
Susceptible microstructure (high hardness)
Occurs below 300°C
Prevention by
Preheat slows down the cooling rate; this can help avoid martensite formation and supplies heat to diffuse hydrogen out of the material
Low-hydrogen welding procedure
As 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. You will see these inspection methods in NDT session. Small cracking areas may be cut out and repair welded. Extensive cracking may result in scrapped parts.

18. How 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, it 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.

19. During cooling, the cooler material tries to push hydrogen out while at the same time the solidifying weld metal tries to push hydrogen in. 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.

20. The hydrogen then slowly diffuses to any location where is can relieve the stress of being stuck in the lattice in the supersaturated condition. 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.
The 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.

21. The above diagram summarizes the discussions about delayed cracking
The 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.

22. Why Preheat?
Preheat reduces the temperature differential between the weld region and the base metal:
Reduces the cooling rate, which reduces the chance of forming martensite in steels
Reduces distortion and shrinkage stress
Reduces the danger of weld cracking
Allows hydrogen to escape
If 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.

23. Using Preheat to Avoid Hydrogen Cracking
If the base material is preheated, heat flows more slowly out of the weld region
Slower cooling rates avoid martensite formation
Preheat allows hydrogen to diffuse from the metal
T base
Cooling rate µ (T - Tbase)
If 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 500°F 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.
Cooling rate µ (T - Tbase)
T base

24. Interaction of Preheat and Composition
CE = %C + %Mn/6 + %(Cr+Mo+V)/5 + %(Si+Ni+Cu)/15
Carbon equivalent (CE) measures ability to form martensite, which is necessary for hydrogen cracking
CE < no preheat or postweld heat treatment
0.35 < CE < preheat
0.55 < CE preheat and postweld heat treatment
Preheat temp. ­ depends on CE and plate thickness ­
Which 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.

25. 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 hardness
Reduces strength
Increases ductility
Increases toughness
Residual stress is also reduced by the postweld heat treatment
If 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.

26. Postweld Heat Treatment and Hydrogen Cracking
Postweld heat treatment (~ 1200°F) tempers any martensite that may have formed
Increase in ductility and toughness
Reduction 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 minutes
If 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.

27. Base Metal Welding Concerns
Lets 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.

28. Lamellar Tearing
Occurs in thick plate subjected to high transverse welding stress
Related to elongated non-metallic inclusions, sulfides and silicates, lying parallel to plate surface and producing regions of reduced ductility
Prevention by
Low sulfur steel
Specify minimum ductility levels in transverse direction
Avoid designs with heavy through-thickness direction stress
In 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.
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.

29. Multipass Welds
Heat from subsequent passes affects the structure and properties of previous passes
Tempering
Reheating to form austenite
Transformation from austenite upon cooling
Complex Microstructure
In 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.

30. Multipass Welds Exhibit a range of microstructures
Variation of mechanical properties across joint
Postweld heat treatment tempers the structure
Reduces property variations across the joint
In 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.