Carbon and hardness in 400 series stainless
Carbon and hardness in 400 series stainless
(OP)
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.
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.





RE: Carbon and hardness in 400 series stainless
RE: Carbon and hardness in 400 series stainless
I have not failed. I've just found 10,000 ways that won't work." — Thomas Edison
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RE: Carbon and hardness in 400 series stainless
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Plymouth Tube
RE: Carbon and hardness in 400 series stainless
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.
RE: Carbon and hardness in 400 series stainless
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.
RE: Carbon and hardness in 400 series stainless
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
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RE: Carbon and hardness in 400 series stainless
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.
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RE: Carbon and hardness in 400 series stainless
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.