PWHT of duplex stainless steels 6

[springwaterbr]

Hello, all,

I've read around these forums and have come across a seeminlgy consistent opinion that post-weld heat treatment is not a recommended pratice for duplex stainless steels on account of the possible formation of a deleterious phase in the material.

What is making me uneasy about this whole topic is how both ASTM A790 and A928 require welded pipe to be heat treated, unless otherwise explicity ordered.

What am I missing?

Our pipe spec calls for A790 pipe, and a vendor is offering to furnish welded pipe without the heat treatment. Given that the ASTM standards explicitly allow for the purchaser to make a requirement on the necessity of heat treatment, in what situations would it be necessary/recommended?

Thank you.

 

springwaterbr,

I consider ASTM system to be the gold standard for technical standards (sorry). However, they can be painfully uninformative in the heat treatment department (one example is A352, which I am currently dealing with, says basically nothing). That said, codes and standards can give useful guidance, but they are not meant to be prescriptive in all aspects, and they certainly are not design guides. Ditto for corrosion considerations.

If there are exceptions to my previous summary (and material producers will find ways to make cost-cutting exceptions), they must be addressed on a case-by-case basis and validated by relevant testing. The goal is to achieve the full potential corrosion resistance of the CRA grade in the finished product, over every square mm.

A thorough purchase specification for duplex SS fabrication will include a large number of items: ferrite balance (specific to each welding process); minimum purity of inert shield and purge gas; testing to ASTM A923 (including for each welder); and an elevated level of in-process QC scrutiny. (I've probably omitted a few others.)


"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

some of the publications on this page may be helpful to you.

https://www.imoa.info/molybdenum-uses/molybdenum-grade-stainless-steels/duplex-stainless-steel.php

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P.E. Metallurgy

 

[SJones]

@springwaterbr - re your excerpt from A815: you might want to look at the latest version where Section 6 text is way different.

It's still not really apparent where you are going with all of this. It started with A790 and has meandered through A928 and A815. If you are in oil & gas, the upshot is: everything will be heat treated except fabrication and installation welds. The thinking is to achieve as much microstructural uniformity as possible, and optimised corrosion resistance. That's not to say that heat treatment can't be screwed up, and that's why there are detailed, rigorous specifications. Field heat treatment of fabrication welds is totally impractical, and that's why they get a dispensation, but they get the crap tested out of them to prove their acceptability. Here are several more references that you may be interested to peruse:

https://www.iso.org/obp/ui/#iso:std:iso:17781:ed-1:v1:en

https://www.iso.org/obp/ui/#iso:std:iso:17782:ed-1:v1:en

https://www.onepetro.org/conference...een=&fromSearchResults=true&to_year=&rows=25#

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.