Stainless steel 347H and PWHT

”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?

Another discussion:

“Generally you do not need to perform any post weld heat treatment, (PWHT) because the Nb added to the base metal and filler will tend to stabilize the weld and HAZ. As with all Austenitic alloys, you just need to weld it relatively cold, with a relatively low heat input.

Generally the H grades are used where high temperature strength properties are required. You will therefore find that often these alloys operate within the sensitization range in any case, so a PWHT will not deliver any lasting benefits, even if it did reduce the weld sensitization.

This sort of information is not really given in “standards”, but I may refer to ASME 8 Div 1, that shows that PWHT is not a requirement for any of the P8 materials. (347H is a P8 Gr1 material)” 

“I am concerned that you intended to use a standard E347 type electrode on 347H base material! The standard electrodes are now formulated with low C levels and consequently lower Nb is required; this will degrade the creep properties of the weld. Additionally the ferrite level ~ 6FN will be prone to sigma formation at the typical operating temperature that the 347H base material will operate at.

This is a classic case of not understanding the operating environment and utilising a corrosion resistant material for a high temperature application.

You should be using a 347 electrode that is formulated specifically for high temperature applications i.e. C ~ 0.05 and ferrite controlled at the low end ~ 4FN.

There is also history of HAZ cracking in >12.5mm thick joints due the lower ductility of the modified electrode. In this case the E16.8.2 type should be used which was developed to combat this problem.”

“I guess the heater is in a hydrocracker or hydrotreater unit. A thermal stabilization treatment at 899 -900 deg C for two hours is specified for increasing the corrosion resistance to intergranular corrosion and polythionic acid stress corrosion cracking. The ASTM standard for A-376 and A 312 gives this treatment for the stabilzed grade austenitic stainless steels as a supplementary requirement without specifiying the temperature and time. Chevron and UOP specifications generally specify such requirements for both the parent pipe and welds.”

‘”The ferrite content of deposit weld was measured after stabilising PWHT. The ferrite content in some welds is above 10%. In some welds to lower ferrite content, they performed in my opinion wrongly, more than one stabilizing PWHT. Again in my opinion wrongly the PWHT was only applied with electrical resistances in the weld and HAZ and not as it should be in all piece of pipe in a furnace.”

“It is LICENSOR of heater understanding that the elevated temperature heat treatment would need to be performed in the temperature range of 1065ºC to 1121ºC; and will convert the sigma phase.  The higher temperature is required to ensure the carbides can be restabilized after the heat treatments.  After the elevated heat treatment the tubes will then require the stabilizing heat treatment at 899ºC.  During the stabilizing heat treatment there is the potential to form sigma phase again.  Then during operation there is the potential to form sigma, if the tubes are exposed to temperatures above 537ºC.  The basic issues are the chemistry of the welds is not correct and this has resulted in the formation of higher levels of delta ferrite, which has the potential to convert to sigma during heat treatments and operation.  The only solution we are aware of to address this issue is to remove the welds.

In my opinion is that there was a mistake to specify a stabilising PWHT for TP347H in the operating conditions of this particular furnace tubes and the solution to remove the welds doesn’t solve the problem.”‘

“As regarding PWHT of SS TP 347H furnace tubes after welding and fabrication, NACE RP-170,-Protection of Austenitic S.S against Polythionic Acid Corrosion during Refinery Shutdowns- recommneds stabilizing anneal at the above temperature for typically 2-4 hrs.

Stabilizing anneal dissolves Cr-carbide back to solution and preferentially precipitates Ti or Nb-Carbides, thus protecting the intergragular corrosion resistance of 347H S.S.
This is a very standard requirement for oil refinery  design specifications.”

 

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