99 Diseases of Pressure Equipment: Stabilization Heat Treatment of Austenitic Stainless Steel Weldments

In the welded condition many stainless steels are susceptible to rapid intergranular corrosion or stress corrosion cracking. This is because the heat from welding sensitizes the base metal heat affected zone (HAZ) and the weld. Sensitization is the condition where chromium carbide precipitation at the grain boundaries (from a heating process, e.g., welding, hot forming, hot bending, service temperature, etc.) reduces the amount of chromium in solution in the stainless steel. The temperature range for sensitization to occur for austenitic stainless steels is approximately 700 °F to 1500 °F. Since the carbides precipitate in the HAZ or weld deposit at the grain boundaries, the chromium depletion is at the grain boundaries, and this significantly reduces the steels grain boundary corrosion resistance. Typically the first line of protection is to use an L grade (low carbon) stainless steel and/or filler metal (e.g., Types 304L or 316L stainless steel). Lower carbon, means fewer carbides to deplete the chromium. However, with enough heat or time sensitization can still occur in the L grades. 

The next level of defense is to use chemically stabilized grades of stainless steel like Types 321 and 347. For these stainless steels Ti (Type 321) or Cb (Type 347) is added. The intent is for titanium or columbium carbides to form leaving chromium in solution. However, some chromium can still be precipitated when the alloy is heated in the 700 °F to 1500 °F range. The next level of defense is to thermally stabilize the chemically stabilized alloy. Thermal stabilization occurs by heating the chemically stabilized stainless steel to a temperature where titanium or columbium carbides form preferentially and chromium carbides do not form. This heat treatment is conducted at approximately at 1625 °F for chemically stabilized stainless steels and the hold time is typically 4 hours. The temperature is different for other alloys, e.g., Alloy 20 Cb-3, Alloy 825, Alloy 625, etc. Note that this heat treatment is generally not beneficial for alloys not already chemically stabilized, because the purpose of the thermal stabilization is to preferentially form other types of carbides over chromium carbides. 

This level of defense is common and the stainless steel base metals e.g. Types 321 and 347 are generally purchased in the thermally stabilized condition and placed in service in the as welded/fabricated condition, depending on severity of service, i.e., no further stabilization heat treatment is needed after the welding process. The base metal is usually stabilized enough to resist much of the sensitization to occur from the welding process. 

For the most severe operating conditions or for the highest level of defense (that is the highest level one can achieve using 300 series stainless steels) the component is fabricated from a chemically and thermally stabilized 

stainless steel, and then the weldment is thermally stabilized. While this is not a frequent need, under some corrosive circumstances it may be advisable. The post weld thermal stabilization heat treatment would be the same heat treatment as previously discussed. On those few occasions when heavy wall austenitic stainless steel piping needs to be stress relieved (e.g. heavy wall, large bore piping) then the stress relief PWHT should be this stabilization PWHT to prevent sensitizing the weldment. In any case adequate QA/QC is needed during the thermal stabilization heat treatment (e.g., heating and cooling rates, hold times, temperatures) to assure that the weldment is stabilized. This is especially true when the heat treatment occurs in the field.

Types 321 and 347 stainless steels are the alloys most commonly thermally stabilized, but these alloys are also susceptible to weld and HAZ cracking problems during these heat treatment procedures, i.e., reheat cracking and grain boundary liquation cracking (mechanism covered in a previous article). This susceptibility increases with increasing component thickness. Therefore, another part of the QA/QC program should include some type of surface (PT) and volumetric (UT) inspection. Remember that these cracking mechanisms can be subsurface only. 

Do you have process services where you may need to thermally stabilize your austenitic stainless steel welds? Are the base metals chemically stabilized stainless steels? Do you have adequate QA/ QC practices in place for the whole project to assure that your heat treatment is achieving the stabilization benefit you’re seeking and will not cause other forms of cracking?