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Subjecting 304L to PWHT temperatures

Would a socket welded 304L tee be damaged if it were subjected to PWHT. This tee is to be cut in on a caustic line and from the tee on out would be 304L stainless components to get away from PWHT. Caustic lines that are carbon require PWHT.


Ron Redler
Maintenance at chemical plant - Decatur, Alabama
December 5, 2008

simultaneous replies

Have I got it right? You will weld a 304L fitting to a carbon steel pipe. You are required to apply PWHT to the carbon steel (which will of course heat the 304L the same way).

Check the 304L fitting before welding, to ensure that is not attracted to a hand magnet (304L would not be expected to be attracted, but best test it). No attraction means no ferrite phase in the microstructure and therefore no transformation of ferrite to embrittling sigma phase at PWHT temperature. In turn, no attraction means go ahead and weld, and apply PWHT as necessary, with confidence that the 304L will not be affected.

You describe the liquid in the pipe as "caustic", presumably an electrically conductive liquid. There is a big gap between carbon steel and passive 304 on the galvanic series, so I would be concerned about the possibility of galvanic corrosion of the steel pipe near the stainless.

Bill Reynolds
Bill Reynolds [deceased]
consultant metallurgist - Ballarat, Victoria, Australia
We sadly relate the news that Bill passed away on Jan. 29, 2010.

December 5, 2008

In general, I don't believe you'd have a problem. The one and only concern you'd have with post weld heat treating of 300 series stainless steels is that you have a chance to sensitize them- the carbon diffuses to the grain boundaries preferentially in the range of 800 to 1300 F, so spending much time at those temperatures is generally forbidden. (Carbon at GB's creates chrome carbides at the GB's, and that means vastly reduced corrosion resistance.) Yet 304L is especially formulated, with its low carbon content, to resist sensitization. Unless you are somehow adding carbon to the 304L, I wouldn't worry about it.

lee gearhart
Lee Gearhart
metallurgist - E. Aurora, New York
December 6, 2008

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