Carbon and hardness in 400 series stainless 1

[smslab]

Does anyone know of a diagram showing the effect of carbon content in martensitic stainless steels with approx 12 % Cr? Ideally I'd like to find diagrams for a series of tempering temperatures but at this point I'd be delighted to find anything.

In case anyone is interested, the background involves loss of carbon at the surface of 410 steel castings which I believe is resulting in hardness requirements not being met.

 

[TVP]

I'm not immediately familiar with any such diagrams, but it is possible that one of the ASM reference books like ASM HANDBOOK Volume 4 Heat Treating or the Ferrous Heat Treater's Guide may have something. This sounds like a fairly easy thing to document if you have actual castings to test. Cross-section the part, grind & polish, then obtain microhardness from surface to core. After this, etch the pieces using Villela's reagent of one of the other suitable martensitic etchants to see if there is decarburization. ASM HANDBOOK Volume 9 or the technical information section of Buehler's website are good sources of info on sample prep, etchants, etc.

 

[arunmrao]

If you could let us know more about the heat treatment practice and furnace used, a better response could be obtained. As these are castings,are you allowing the castings to cool in the mold or are you providing a hot shake out. perhaps this may also contribute to decarb layer on the skin. Finally are the castings machined before heat treatment?

I have not failed. I've just found 10,000 ways that won't work." — Thomas Edison
_____________________________________

 

[EdStainless]

This data should be available since 410 is made with a wide range of C. The low C grades are not marensitic but ferritic.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[smslab]

ASM Handbook Vol 4 does not have it. So far I've found something that references the Welding Handbook Part 4 1972 that shows a Brinell hardness vs C (goes from 0.07 to 0.15 %) and Cr (goes from 12-13%) but only for oil quench from 980 C.

I do not have detail on the casting process but following casting, the parts are put through molten salt and acid, then 1850 F 1-1/2 hrs, GFQ, 1350 2 hr in vacuum, then 1850 F 1-1/2 hrs GFQ and tempered 1100 F 3 hrs. Target hardness is 26-24 HRC.

There is also a 300 F temper that gives 40-45 HRC.

And there IS decarb, that is not the issue. We've done the metallography and I have some OES results to back it up with numbers. What I am trying to do is quantify it - ie at 0.15 C after the 1100 F temper you would expect 28 HRC, at 0.10 C you would expect 22 HRC or whatever it actually is. This is one of those intermittent problems - most of the time the hardness requirements are met. Frankly I would not have expected the process to work that well as 410 has a maximum of 0.15 C and any loss at all I think would make it impossible to reach the target.

Good thoughts as always arunmrao I'll see if I can get reliable answers.

 

[smslab]

Quicker than expected I got answers:
These are investment castings with no hot shake out, they cool in the mold. They are not machined prior to heat treat.

Aside from a long-term fix I am wondering if the current batch can be saved or if they are a lost cause.

Thanks in advance for words of wisdom.

 

[arunmrao]

Interestingly last year, a company in US was faced with the same problem. They were getting these 410 investment castings from China and there was a variance in the hardness value as mentioned in the certificate and the value actually measured. I hope smslab, your source is not the same.

On investigation I found that the certificates were fake. These were critical parts, the source continued to get orders due to commercial constraints,but with better vigilance.



I have not failed. I've just found 10,000 ways that won't work." — Thomas Edison
_____________________________________

 

[metengr]

Does anyone know of a diagram showing the effect of carbon content in martensitic stainless steels with approx 12 % Cr

Martensite is martensite regardless of Cr content. In the presentation below is a chart showing the hardness of martensite as a function of carbon content.

http://www.nd.edu/~manufact/MET pdf_files/MET_Ch4.pdf

 

[smslab]

arunmao I hear ya. I've seen issues with suface polishing in 316 (ok CF-8M) from a Chinese source where the issue was a cooling rate. They'd rather fix it by using an anneal than monitor their cooling rate.

But anyway...these castings are made in the US - the raw material is the same for parts that meet the requirements as well as those that do not. And they do check with their own OES.

So far it is looking like the slightly larger parts are the ones that are running into the problem with decrease in hardness at the surface, which would support the theory that the cooling rate may be too slow. They are measuring Rockwell on the thickest section. There is a region on this part that is thin-walled and I've suggested to them to prepare metallographic cross-sections from this region to see if the same issue crops up there.

The diagram I found for hardness vs C in the narrow range of 0.07-0.15 % does have similar values as the one for plain iron. If the same similarity holds for Q&T hardness then I'm home free.