CX13VDW vacuum carburizing response

[Lyrl]

I work at a commercial heat treat company that has an opportunity to vacuum carburize CX13VDW material; we have never worked with this particular chemistry before.

We have established processes to carburize other stainless steels such as Pyrowear 675, and my first process was based on this history. It gave the samples way more case than they needed: targeting around 0.040" the samples ended up with 0.065" effective case depth measured to 50 HRC. Microstructure was martensite with lots of carbides, typical of our past experience with carbuized stainless.

So we get more samples from the potential customer and I dial the carburizing time back. The result is a surface layer of, I think, mostly retained austenite - it has hardness in the 30s HRC. A micrograph taken at 1000x is attached. These samples have been cryo-treated to -110°F.

I would think the retained austenite would be caused by too-high carbon concentrations and perhaps heating it back up and diffusing for a few more hours would allow the case to quench out to mostly martensite. I am really puzzled, though, that the earlier, longer carburizing cycle didn't result in anything like this.

Any thoughts on what the pictured microstructure is, whether a reheat for a few hours would help the surface quench to martensite, and why this shorter cycle turned out so much differently than the longer cycle would be much appreciated.

 

[EdStainless]

Has the material been Q&T? Annealed? Is it quenched from the carb and then tempered?
What temp are you using?
My first thought would be to drop the temp.
The V is going into carbides, and some Mo, but you really want to avoid the Cr carbides if you can.

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P.E. Metallurgy, Plymouth Tube

 

[metengr]

I would do exactly as you suggested, reheat the sample and see the response if the carbon content is reduced allowing for transformation to martensite.

 

[Lyrl]

EdStainless - the material is carburized at 1700°F, normalized at 1900°, stress relieved at 1300°, hardened from 1900°F, sub-zero treated at -110°F for 2.5 hours, then tempered at 350°F. As far as the Cr, the part is copper plated over all the surfaces that need to maintain corrosion resistance. The surfaces getting carburized are wear areas that need to be very hard but will not be exposed to the corrosive environment; losing the Cr to carbides is OK for this application.


An update - I was able to get in contact with our customer's metallurgist who has worked with this material for a while and it turns out the problem is too low of a carbon concentration. This is their explanation:

Raising the carbon content near the surface will eliminate the R.A: there is a limited range in the carbon content that produces this band of R.A.

The larger carbides formed in the case are rich in alloy, reducing the alloy content of the case during austenitizing. In those areas with lower carbon content (minimal carbides) the austenite during austenitizing is rich in alloy.

The high matrix alloy content (due to not being tied up in carbides) combined with carbon dissolved during austenitizing drives down the Ms temperature in this medium-carbon region producing the band of R.A. The austenite in the band is quite stable; even sub-zero cooling in liquid nitrogen will not eliminate this band of R.A.

 

[EdStainless]

considering that C is a very powerful austenite stabilizer, I wonder if the temp for normalize and the hardening are optimal, just thinking about HT on 440C and similar alloys.

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P.E. Metallurgy, Plymouth Tube

 

[metengr]

You might want to review the information below

http://sullivansteelservice.com/uploads/Brochure CX13VDW.pdf