Thermal stabilization of SS TP321 and SS TP347

[0707]

For service temperatures above 850ºF (454ºC) type 321 and 347SS can sensitize. Some specs recommend furnace tubes (and other SS equipment that will operate above 850ºF (454ºC) be
thermally stabilized to prevent sensitization. Thermal stabilization consists of heating the entire piece of equipment after welding is complete, at 1650ºF (899ºC) for 4 hours. Is this practical and worthwhile?


Thanks

LM

 

[metengr]

Yes, it can be quite beneficial in service applications where long term sensitization can affect corrosion resistance.

 

[0707]

In the field apart of the weld seam how do you heat the entire piece of equipment? For instance an isometric can have several field welds and so it is not practical to heat treat all the entire line. The stabilization heat treatment, done only at the weld seam and HAZ can originate irregularities on grain boundaries of electric resistances.

In a furnace, sometimes there happens temperature peaks much above of 454ºC, if a temperature peak goes above 510ºC the stabilization heat treat at (899ºC), I am no sure, but I think, it can originate faster sensibilization or sensitization.

LM

 

[metengr]

In the field apart of the weld seam how do you heat the entire piece of equipment? For instance an isometric can have several field welds and so it is not practical to heat treat all the entire line....

You don't. You try to minimize the number of field welds with shop welds, where thermal stablization can be performed in the shop, and take advantage of low stress locations in the field.

 

[EdStainless]

The risk is with applications that are in the 850-1150 range as a general estimate. Hotter than this and the material should stabilize in service. Below this and sensitization is not an issue.
In that range the sensitization will happen faster than the stabilization.
Shop weld and stabilize all that you can.

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Plymouth Tube

 

[0707]

To reduce residual stresses from cold working the stress relief is benefitial to prevent SCC. To avoid sensitization the heat treated SS use shall be limited to operating services bellow 510ºC.


"Stress Relief: Optimal time and temperature for stress relief are reported between 1550° F. and 1650° F. (843° C. and 899° C.) for about 2 hours. Commonly, stress relief PWHT is performed on TP 347 stainless steel piping between 1550° F. and 1650° F. (843° C. and 899° C.) to reduce residual stresses from cold working and/or joint restraints, and to further reduce the susceptibility to chloride stress corrosion cracking.

Unfortunately, the stabilization heat treatment can also lead to substantial degradation of mechanical and corrosion properties because of complex physical-chemical interactions. For example, currently practiced stabilization heat treatment at 1650° F. (899° C.) frequently maximizes the rate of fine niobium carbide formation and allows for sigmatization of most remaining ferrite, often leading to substantial loss of ductility and elevated-temperature creep strength. Therefore, to prevent failure during high temperature service, heat treated stainless steel use is generally limited to uses with operating temperatures below 950° F. (510° C.) to ensure immunity to sensitization"


Thank you for your replies

LM

 

[EdStainless]

If you stay near the upper limit of 1650F you get better results. You may actually get some stress relief (any will help) and you will get a little Cr diffusion to improve chemistry uniformity.
Ferrite control is welds is important.

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Plymouth Tube