Slide 1 Slide 2 Slide 3 Heat Affected Zone Welding Heat Affected Zone Welding Concerns Concerns • Changes in Structure Resulting Cooling Transformations in the in Changes in Properties • Cold Cracking Due to Hydrogen Weld Let us now start to investigate concerns in the true heat affected zone. This is the Two major concerns occur in the heat affected zone which effect weldability these region where melting does not occur, but temperatures reach high enough values are, a.) changes in structure as a result of the thermal cycle experienced by the for phase changes or changes in structure and properties to occur. Before we look passage of the weld and the resulting changes in mechanical properties coincident at the exact changes in structure and properties, and weld deficiencies which with these structural changes, and b.) the occurrence of cold or delayed cracking might result from these changes, we need to review the thermal cycles which due to the absorption of hydrogen during welding. A separate section is resulted in these changes, and categorize the types of deficiencies which might presented below for each of these occurrences. occur. Slide 4 Slide 5 Slide 6 Look At Two Types of Alloy Systems First let’s review the thermal cycles experienced in the heat affected zone as a There are two types of alloy systems which we will consider, those which do not Note that the heat of welding has effected the structure of this material even result of the passage of the weld. The figure illustrated here shows the have an allotropic phase change during heating like copper, and those which have though there are no allotropic transformations. Recall that cold worked structures temperature vs time curve at various distances from the weld metal. We have an allotropic phase change on heating like steel. We will first consider those undergo recovery, recrystalization and grain growth when heated to ever seen similar thermal cycles in the heat transfer section above. As the welding arc materials which do not have an allotropic phase change. The top schematic increasing temperatures. So it is in this material. As we traverse from the cold passes by the plane of reference and heat from the molten pool is conducted illustrates this type of material. There are several ways that materials without any worked elongated grains in the unaffected base metal, we come to a region where outward from the weld into the Heat Affected Zone, the temperature increases to allotropic phase changes can be strengthened. Two typical methods are cold the cold worked grains undergo recovery and then shortly there after they a maximum temperature until the arc is past, and then heat continues to flow working and precipitation strengthening (review the section on material recrystalize into fine equiaxed new grains. Traversing still closer to the weld region outward cooling each location. Points closest to the weld fusion line reach the strengthening if you are not familiar with these types). We will first consider that we note grain growth where the more favorably oriented grains consume hottest maximum temperatures while points removed from the fusion line do not this material has been cold worked (note the elongated cold worked grains neighboring grains and grain growth occurs. The grains within the weld epitaxially reach as high a temperature and the maximum temperature occurs at a slightly present in the base material (region A)). The weld metal is represented by region nucleate from the grains in the heat affected zone at the fusion boundary, and later time than that near the fusion line. Note that almost every thermal cycle C, and the heat affected zone is region B. grain growth continues into the solidifying weld metal making very large grains. imaginable occurs over this short distance of the heat affected zone. Thus a variety of structural and property variations are expected. In the next section we will examine some of the structural changes expected (they are dependent upon the type of material welded and the prior processing of the material) and the resulting property changes.
Slide 7 Slide 8 Slide 9 Cold Worked Alloy Without Allotropic Transformation Welding Precipitation Hardened Alloys Without Allotropic Annealed upon Cooling Phase Changes Welded In: • Full Hard Condition • Solution Annealed Condition Introductory Welding Metallurgy, Introductory Welding Metallurgy, AWS, 1979 AWS, 1979 One of the factors that occur when cold worked grains recrystalize and grain A second way of strengthening materials without allotropic phase changes is by When welding on the already aged (full hard) material, the unaffected base metal growth occurs we have already discussed, and that is the material softens. Thus precipitation strengthening. Recall that in precipitation strengthening, the base will have aged precipitates that are just the right size for strengthening. The heat the heat affected zone and weld metal will not hold the same strength level as the metal is solutionized, rapidly cooled and then aged at some moderately elevated affected zone, on the other hand, will experience some additional heating. In the cold worked base metal. Another consequence of increased grain size is perhaps temperature to promote precipitate formation. There are two ways that region farthest from the weld the heat will be sufficient to overage the equally important and that is that the larger grains are more brittle. A “Charpy” precipitation hardened material can be welded. One is to weld on the full hard, precipitates with the resulting loss in strength. In regions closer to the weld, the impact test (we will discuss this more later) is used to determine how much that is the already aged base metal. The second is to weld on material which has heat will be so excessive that the temperature will exceed the two phase region impact energy a structure will absorb over various temperature ranges. This is been solution annealed and rapidly cooled, but not yet given the ageing heat and the single phase solutionizing region on the phase diagram will be entered. illustrated in the figure where the large grains illustrated by curve 4 will only treatment. In either case, when welding, the heat affected zone will see some Again, a loss in strength will occur, but this region at least might be able to be re ‐ absorb high energy at very high temperatures above about 100 degrees additional time at temperature (varied temperature over the distance of the HAZ) aged to recover some strength. Fahrenheit. At lower temperature like at freezing, they absorb very little energy. as illustrated above, and this will effect the aged or overaged condition of the Contrast this with the cold worked grains as illustrated by curve 1 where even precipitates. down to minus 100 degrees a large amount of energy is still absorbed. Materials that do not absorb large amounts of energy are said to be brittle and they can fracture with only slight impacts by foreign objects. Thus the weld region is subject to impact fracture with these type materials. Slide 10 Slide 11 Slide 12 Precipitation Hardened Alloy Welded in Full Hard Condition Precipitation Hardened Alloys Welded in Solutioned Condition Introductory Welding Metallurgy, Introductory Welding Metallurgy, AWS, 1979 AWS, 1979 Here are presented hardness traverses of welds made in the pre ‐ weld full hard On the other hand, welding precipitation hardened material in the solution Let us now turn our attention to the materials which do have an allotropic phase material. Curves for the as welded condition and a subsequent hardening heat condition with a low heat input, only slightly ages the material in the heat affected change during heating. A typical material like steel is ferrite at low temperatures treatment after welding are presented, and also curves for low heat input and zone. Subsequent post ‐ weld ageing strengthens the entire weld region (only a and transforms to austenite when heated. Each time the material goes through high heat input conditions are presented. Note the softening as mentioned slight overaging occurs in the slightly ages regions from the weld). With high heat one of these phase changes, new finer equiaxed grains grow starting from the previously in the as ‐ welded condition. Note that heat input also has an effect on input, however, the case is somewhat different as moderate aging occuring on grain boundaries of the previous grains present. So in the case of cold worked the extent of softening in the as welded condition. In some cases, a post ‐ weld welding and post ‐ weld treatment only serve to accentuate the overaging process. steels in the base metal, the elongated cold worked grains will undergo recovery, aging treatment can restore hardness in some of the regions of this weld, but it So care must be exercised when establishing a welding procedure for welding the recrystalization and grain growth just as discussed above. But now the never fully erases the effect of the weld overaging. precipitation hardened alloys. 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.
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