Carburization of TP SS304H Austenitic Stainless Steel?

[replica]

Dear all,

Is it possible to carburise TP SS304H stainless steel convection tube at 450C with exhaust gas containing source of carbon? As far as I know the temperatrure is too low for the low temperature carburization to take place. Can the carbon potential influence the carburization and can happen at 450C?

The microstructure is attached ....any comment is highly appreciated.

 

[replica]

200X magnification

 

[replica]

at 100X magnification

 

[replica]

All the microstructure are from replica. This is again at 500x mag.

 

[metengr]

This is too low of a metal temperature for surface carburization in austenitic stainless steel tubes. I have seen it before with metal temperatures well above 450 deg C, more like 800 deg C.

What you have in the replicas are surface oxide or debris and slip lines from grinding which were revealed upon polishing.

 

[replica]

metengr

Thank you very much for the comments. That is what I am thinking right now. But at 500x magnification the region looks like carbides, but I believe that at 450C, the chromium carbides can hardly formed. Furthermore the average hardness is just 235 Hv which is just right for the effect of slip bands rather than carburization.

 

[EdStainless]

You can get significant carbon uptake at 450C, but you shouldn't form any Cr carbides.
There are some surface hardening methods that do this.
You should see a hardness gradient.
And if you heat a sample to 700C for a few minutes you should form significant Cr carbides near the surface.

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P.E. Metallurgy, Plymouth Tube

 

[btrueblood]

Why did you cut this sample, was there an issue with cracking? 450 C is within the carbide precipitation range (sensitization) for 304 stainless, depending on prior heat treatment history, see

http://corrosionjournal.org/doi/abs/10.5006/0010-9312-34.2.60?code=nace-prem-site

 

[metengr]

http://www.asminternational.org/doc...mple.pdf/3841f98e-2bce-4f5d-9c97-f42726201e4a

Keep in mind the above is a process and I seriously doubt that this would be duplicated in a convection pass boiler tube or reheater/superheater tube. The carburization problems I have seen are elevated temperature where you form carbides and reduce the chromium content along the surface rendering the 304H tube susceptible to wastage from fireside corrosion (sulfur).

 

[replica]

btrueblood,

There was no crack found. We did the replica during shut down because we think the convection tube of Reactivation Gas Heater was experiencing carburization. The internal operating temperature was at 320C but the external where the microstructure was taken by replication was subjected to exhaust gas at 450C. I think the dark patches that look like carbides was probably artifact due to insufficient grinding prior to replication. Anyway slip bands due to cold working can be clearly seen. I will try to grind further to see whether these artifact can be removed. Thank you everybody for the comments.


EdStainless , are you referring to kosterizing? But the lowest temperature report for this plasma carburizing process is 500C.

 

[EdStainless]

There are about 6 other processes that do a similar this, the Swagelok process being one that comes to mind.
They usually rely on halogen activation or plasma, but in a clean, non-oxidizing environment and given very long times it is easy to imagine some C uptake.
The only way that you should see carbide formation at this temp is if the material was under annealed in the first place.

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P.E. Metallurgy, Plymouth Tube

 

[replica]

Thank you EdStainless, We conclude that the particles that look like carbides or carburised layer is actually the artifact due to insufficient grinding. I would consider the hardness will increase should carburization or chromium carbides formation had taken place. The hardness value is only 235 Hv which is too low for carburization should it occur.