Cooling Transformations in the in Changes in Properties Cold - - PowerPoint PPT Presentation

Slide 1

Cooling Transformations in the Weld

Slide 2 Heat Affected Zone Welding Concerns

Let us now start to investigate concerns in the true heat affected zone. This is the region where melting does not occur, but temperatures reach high enough values for phase changes or changes in structure and properties to occur. Before we look at the exact changes in structure and properties, and weld deficiencies which might result from these changes, we need to review the thermal cycles which resulted in these changes, and categorize the types of deficiencies which might

• ccur.

Slide 3 Heat Affected Zone Welding Concerns

• Changes in Structure Resulting

• 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. A separate section is presented below for each of these occurrences.

Slide 4

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. We have seen similar thermal cycles in the heat transfer section above. As the welding arc passes by the plane of reference and heat from the molten pool is conducted

• utward from the weld into the Heat Affected Zone, the temperature increases to

a maximum temperature until the arc is past, and then heat continues to flow

• utward cooling each location. Points closest to the weld fusion line reach the

hottest maximum temperatures while points removed from the fusion line do not reach as high a temperature and the maximum temperature occurs at a slightly later time than that near the fusion line. 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. 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 5 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. There are several ways that materials without any allotropic phase changes can be strengthened. Two typical methods are cold working and precipitation strengthening (review the section on material strengthening if you are not familiar with these types). We will first consider that this material has been cold worked (note 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.

Slide 6

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 recovery, 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.

Slide 7

One of the factors that occur when cold worked grains recrystalize and grain growth 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 (we will discuss this more later) is used to determine how much impact energy a structure will absorb over various temperature ranges. This is illustrated in the figure where the large grains illustrated by curve 4 will only absorb high energy at very high temperatures above about 100 degrees

• Fahrenheit. At lower temperature like at freezing, they absorb very little energy.

Contrast this with the cold worked grains as illustrated by curve 1 where even 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 8

• Full Hard

• Solution

A second way of strengthening materials without allotropic phase changes is by precipitation strengthening. 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.

Slide 9

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.

Slide 10

Here are presented hardness traverses of welds made in the pre‐weld full hard

• material. Curves for the as welded condition and a subsequent hardening heat

treatment after welding are presented, and also curves for low heat input and high heat input conditions are presented. 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.

Slide 11

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 aging occuring 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.

Slide 12

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

• ne 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.

Slide 13

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 are expected. Closer to the weld some dual phase ferrite austenite will

• ccur 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.

Slide 14

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.

Slide 15

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.

Slide 16

Knife‐Line Attack in the HAZ

• Cr23C6 precipitate in

• Can occur even in

Finally, a defect called knife‐line corrosion attack can occur in some stainless steel heat affected zones. A discrete band in the heat affected zone of the austenitic stainless steel welds experiences peak temperatures in the 800°‐1600°F 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.