Hardness test on SS321? 2

[MatieMan]

Hello my friends,

In front of me a specification which specifies hardness test on SS321 after welding (in the heat affected zone after heat treatment). In general the specification specifies extra requirements on SS321 to prevent (intergranulair and stress) corrosion in a Polythionacid or other corrosive products.
The limited hardness would be 225 HB.
I know this test is normal for Chromium steels (P9, P11…) but I didn’t see it for SS321 yet.
Questions:
Is this hardness test a relevant requirement on SS321 (and why)?

Thanks for your help,
MatieMan
Belgium

 

[metengr]

MatieMan;
I would agree, it seems odd that a specification would require hardness testing on materials that cannot be hardened by thermal treatment (from welding). It might have been a mistake or carry-over of requirements for carbon/low alloy steels.

 

[GRoberts]

I can see why a test on the weld might be requried, as the ferrite/austenite ratio can affect hardness, but the HAZ is what it is. Are you using the Brinnell method, or another one and converting? If you are using Brinenell, the you aren't getting a real HAZ measurement anyway since the indentation is much too large.

 

[SJones]

Since there is mention of polythionic acid, one may make an assumption that one of the 'corrosive products'is H2S. Having made that assumption, one can then consult either ISO 15156-3 or NACE MR0103 and determine that there are hardness restrictions for austenitic stainless steels and weld zones therein. Unfortunately, they are quoted as Rockwell C scale.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[metengr]

SJones;
Other than hardening from cold work, which would be associated with forming operations of the base metal, can you explain to me HOW an austenitic stainless steel that cannot be hardened by thermal treatment from welding would have hardness limits in a base metal HAZ????? I can understand hardness limits for post forming operations of austenitic stainless steel base metal to assure no evidence of prior cold work, but not for welding.

 

[SJones]

Don't ask me, ask NACE and ISO!

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[ApC2Kp]

MatieMan,

There was a shop welding large 310 SS piping some years ago with GMAW (MIG) process, and it seemed they put too much heat into the large fillet welds at the slip on flanges. It resulted in cracks that were discovered when every flange weld leaked upon field hydrotest. The problem was remedied by specifying only GTAW / SMAW controlled heat input plus limit on interpass temperature for welding. At that time it seemed the austenite/ ferrite imbalance could have caused the cracking of welds, as GRoberts might agree.

Would SJones and metengr consider this scenario? There is weld shrinkage that occurs with welding. (PWHT would reduce the residual stresses, but PWHT is not common for stainless steels, except possibly for NACE applications.) Now I wonder if the weld shrinkage does result in some "cold working", such that some hardness differences might be detectable. You probably have seen weld joints where the shrinkage may have been 3 to 5% of diameter? Good welder technique, such as 'back stepping', helps deal with the distortion from shrinkage to keep dimensions within control plus reduce the amount of "cold working" from shrinkage. Good technique and procedure also seems to prevent the cracking that resulted in the leaks.

 

[EdStainless]

I suspect that weld cracking in 310 was related to thermal expansion. Most 310 has near zero FN (like many high alloy SS grades) and it can be prone to weld cracking if you are not careful with heat input.

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[SJones]

ApC2,

I was thinking along those lines. It is analagous to strain ageing in carbon steel weld zones. Whether it is detectable by hardness testing (especially Rockwell C), I have no clue.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/