Inspection for carburization on 3XX Series stainless boiler tubes 3

[metalhoosier]

Folks,

Anyone know a quick visual method for determining carburization on 3XX SS boiler tubes (waiting for coal ash corrosion is too long). I anticipate we will sandblast several strips across the pendants and then use ??? (etchant) to determine the severity of carburization the pendant is experiencing. What would that etchant be? Is there another good visual method to determine the extent of carburization?

Thank you,
Metallhoo

 

[metengr]

metalhoosier;
We had developed on one of our SH assemblies a carburization problem with 304H caused by carbon carry-over from LOI. The obvious feature for this was a high density of pock marks on the gas touched tube surface and local bands of wastage along the sides of the vertical pendant tubing.

The only way I could reliably confirm carburization was to remove several damaged tubes and have the met lab confirm by metallographic examination. So the take away from my experience is the following; high density of surface pock marks, and bands of external wastage.

If you observed these features on SH and RH boiler tubing it is probably surface carburization. As the carbon is removed from the matrix to form chromium carbides, the loss of chromium results in lower corrosion resistance and sulfidation attack - pock marks and wastage.

 

[metalhoosier]

metengr;

I have observed both the bands of wastage and pock marks and those tubes will be removed from this particular SH. I am looking to go a step further, if possible, with a proactive, safe, macroetch approach that could identify areas where the marks and wastage had not yet initiated, but the surface matrix is ripe for it to begin. I don't think Marbles or hot HCl is the answer, or safe enough to use hanging on a skypic. Any other thoughts?

Metalhoo

 

[metengr]

metalhoosier;
I don't believe field macroetching is going to provide any benefit. Again, what I found is this is an all or nothing damage mechanism and is driven by location of carbon carry-over. Visual inspection is your best bet. We had to re-tune our low Nox burners to reduce carbon carry-over. The other problem with this is we burn high sulfur eastern coal which promotes sulfidation attack in lower chromium alloys in a reducing atmosphere. Use visual inspection as your guide.

 

[EdStainless]

Remove some of the surface scale before you blast. If you are getting much carburization damage there will be Fe and Cr carbide in the scale that was against the tube surfaces.
Caburization causes the surface falls off as gray dust in a clean environment.

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Plymouth Tube

 

[metalhoosier]

Ed,

Thank you for your thoughts as well. We aren't experiencing much scale on the surface and the scale that is attached is very tenacious as observed in the attached photos. Looks as if a thorough visual and UT inspection is my only safety net besides tuning. With management extending run times, this does not give me a good feeling I won't see this type of failure in my lab again. This tube only saw 6 years of operation.

Thank you,
Metlahoo

 

[metengr]

metalhoosier;
Thanks for the picture. This is exactly what we expierenced in our SC boilers. Our wastage rates as a result of selective carburization were near 90 mils/year, so for SC tubing 0.40" thick, you get the picture. Look at carbon carry-over from your burners. Lower the carbon, you extend the tube life.