Sigma phase in 2205 duplex

[bekks]

Anyone with experience looking at sigma phase able to give an opinion on whether or not this is it starting at the grain boundaries? Midwall of a 0.5" thick plate; around the edge of the HAZ. Etched with 20% oxalic for about 20 seconds at 5V. 250X magnification. I'm wondering about the "growths" along the ferrite-ferrite GBs. I'll tell you the thermal history after I hear a couple opinions.

 

[MagBen]

i agreed that sigma started to form inside ferrite grains, and majority of them at GBs. I didnot see any sigma at ferrite/austenite phase boundaries. no evidence showing additional secondary austenite indicates there is no eutectoid reaction. Material seemed to stay at 850-1000C temp range for a while (slow cool or aging)?

 

[bekks]

Thanks for the reply MagBen. This sample was PWHT at 620°C for 2 hours, so right at the bottom end of potential sigma formation.

 

[EdStainless]

1. Why are you doing PWHT on a duplex SS?
2. I presume that your weld procedure qualification included corrosion and impact testing on coupons after PWHT?
3. You may have avoided Sigma formation, but that isn't the only intermetallic that can ruin your day...
Alpha prime and carbides will form at temps below Sigma.

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P.E. Metallurgy, consulting work welcomed

 

[bekks]

I should have further explained. I had a sample lying around that I wanted to see if I could force sigma, so I threw the sample in with another heat treatment (carbon steel). That said, I do somewhat regularly talk to clients that insist that they need to PWHT austenitic SS parts. I try to talk them out of it and if it proceeds, then we do a thorough examination of the microstructure.

 

[EdStainless]

You can get curves for intermetallic formation in publications from NIDI or IMOA.
I have the publications but I don't have any scanned to post.
With duplex anything over 300C is dangerous.
Some austenitic alloys will tolerate high PWHT (such as 310), but any Mo bearing alloy will not survive.

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P.E. Metallurgy, consulting work welcomed

 

[MagBen]

have you check micros after welding and before 620C/2h aging? I suspect the sigma at ferrite GB might be formed at cool from welding. higher temp/short time (slow cool) often leads to such microstructure, while lower temp/long time leads to formation of secondary austenite via eutectoid reaction. I didnot see additional austenite phase.

ED comment "With duplex anything over 300C is dangerous" is related to the infamous "475C embrittlement”. For sigma, 500C and below is very unlikely high enough for this intermetallic phase to form.

 

[bekks]

I'll look into those curves Ed.

MagBen, the sample was evaluated prior to heat treatment and looked clean - no notable carbides nor the grain boundary sigma. There was a fair bit of carbides after heat treating, in other regions of the cross-section.

 

[EdStainless]

I have seen samples loaded with Alpha Prime, and with very poor toughness.
This will form below 350C in the right conditions.
The published curves are for solution annealed and water quenched material.
When you have welds you by nature have nonuniform chemistry and the initiation of many secondary phases.
These phases will later start growing much faster in these samples than in annealed material as there is no initiation stage to go through.
this is a similar situation to sensitization in austenitic alloys. In a well annealed sample it may take 10 hours, but if I anneal a sample just enough to get rid of visible carbides but not enough to erase the concentration gradients it will reform in less than 60sec.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed