Need help: stainless steel and carbides 1

[kingnero]

I'm busy with some testing for temp repairs.
While I'm pretty versed in high-C steel, I don't know a whole lot about austenitic stainless steels.

I've got a specific question, and an open one.

1): If you encounter sensitization (depletion of Chrome) in stainless, I know this enables corrosion to develop faster than normal. But does it also enable mechanical malfunctions (cracking, flaking, ...) ? For a temp repair, corrosion resistance is not important (we're talking days/weeks here, not months/years). Mechanical strength however, is.
The repair gets quite hot, and stays for some time in the critical zone where carbides are formed. And there's no lack of carbon here.

2): Attached, you'll find some micros from a weld made with 18-8-6 Cr-Ni-Mn (AISI 307) on high-C steel (pearlitic structure).
What does this micro tell you, or what info do you need in order to evaluate these pictures?

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[metengr]

Typically, sesitization effects corrosion response of the material. Beyond that the mechanical properties will not be affected and there should be no increased risk of other damage mechanisms.

For the dissimilar metal weld I assume the fillee meral used is 309. The microstructure reveals nothing out of the ordinary.

 

[kingnero]

It's 307 (18-8-6).
Thx for your reply!

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[EdStainless]

Grain boundary carbides can impact fatigue properties, but they have to be very severe to have an impact on other properties. It isn't the carbides that lead to intergranuar corrosion, it is the Cr depleted zones along them. If they form or are later exposed to high temps there will be enough Cr diffusion to minimize the corrosion risks.

What are you welding to? I would also use 309 instead of 307 if possible, Mn is a very weak austenite stabilizer.

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P.E. Metallurgy, Plymouth Tube

 

[kingnero]

Welding to low alloy, 0.7% C steel.
Choice of 307 is made due to availibility. Our welders always carry 307 with them, and hardfacing rods that are not suitable for joining. This repair needs to be executed fast if it happens (a once in 10 years event), so this is the only practical solution.

Would etching with a particular agent be more helpful or would that tell you more? At this point, everything is still possible.

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[EdStainless]

What do you want to know? You have a weld, it has better corrosion resistance than the steel.
Other than that there isn't much to say.

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P.E. Metallurgy, Plymouth Tube

 

[kingnero]

I don't know what I'm looking at, hence my question.
I was only afraid that I'd have severe sensitization here, because of the large weld (time), and high hemperatures. I would've thought that this would be shown by a large amount on carbides as one comes from the other.
So Ideally, I was hoping for someone to comment particularly on this.

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[EdStainless]

re-polish and etch the stainless with 10% oxalic acid (electrolytic).
You should be able to clearly see grain boundaries, and if there is pronounced (and nearly continuous) ditching then it is sensitized.

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P.E. Metallurgy, Plymouth Tube

 

[metengr]

If you used proper welding technique where minimal preheat was used and low interpass temperature all you have left is to ensure a defect free weld. Senitization and carbides in this DMW is the least of your concerns considering the materials being joined.

 

[kingnero]

minimal preheat and low interpass temperature

The focus of this repair is to ensure the high-C steel does not crack any further, so these two premises are not followed.

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[metengr]

Any reason why you did not use a Ni-base filler versus 307? Especially for the high carbon steel material.

 

[EdStainless]

With the high Mn in the 307 you will not only have Cr carbides but also Mn sulfides.
It might work, but it is the wrong filler.

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P.E. Metallurgy, Plymouth Tube

 

[kingnero]

As for the choice of the filler, it's either this or nothing.
It is not feasable to hand out other rods to all of our welding teams (field welders, not plant based welders) for an event that might happen again in 10 or 20 years, and expect that they will still even find those rods, and/or that they will be in good shape.

If the repair holds for a few days, awaiting proper replacement, that's already a succes.

Thanks for your insights.
A picture of the stainless steel, etched as suggested will arrive soon.

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[kingnero]

As promised a few posts above, here are the results from the lab that did the oxalic acid etch.

Could someone please post a picture with sensitization, for my education and to compare the (non-)severity of what I should be able to see here?

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[EdStainless]

The continuous dark grain boundaries are indicative of sensitization.
The concern with sensitization is not the formation of the carbides, but the Cr depleted region adjacent to them, this area would then have relatively low corrosion resistance.
When welding to steel the corrosion resistance of the welds is not a real concern, since the steel will corrode first anyway.

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P.E. Metallurgy, Plymouth Tube

 

[kingnero]

If I understand your remark correctly, sensitiation has occured on this sample.

Corrosion resistance does not really matter for this application, but thanks for confirming this.

Could sensitization also have mechanical consequences, like a greater likelyhood for cracking or similar failure modes?

Thanks for your continued remarks!

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[metengr]

What are the potential failure modes under your expected service condition? For example cyclic loading or impact loading? With the observed microstructure I would not be concerned with static service stresses. Even cyclic loading or fatigue may not be affected with the,extent of sensitization. I would be more concerned about the grain aize, if anything.

 

[kingnero]

Cyclic loading, with a very fast rise from zero to max strain and back to zero, but as it's a temp repair, we're talking about 3000 cycles per day for max. 5 days.

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[metengr]

If the weld is determined as being sound by UT or other NDT volumetric method, I would not be concerned with sensitization under your stated service conditions.

 

[kingnero]

Thank you.

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