heat treat

I am a rookie machinist so forgive me, but this hasbeen bugging me for some time. When machining an oddly shaped part that would be difficult to grind post heat treat, what is the best way to stay within tolerance? Or is it going to grow/shrink that much? Hypothetical .003 +/- 8620 steel carborized 1.5 x 1.5 x 3.0

I just did a carburize to a batch of 9310
.900 diameter and 1.4" linear didn't seem to move one bit on a single sample I checked
+/-.003 should be no problem to finish before HT @3" overall.

I agree with what mkd shared. Without knowing the shape of your parts, etc., I will add this. It has been my experience that the heat treat person doing the job has some influence on the outcome. What part of the process, I'm unsure of, it could do with a lot of factors. Furnace choice, atmosphere, arrangement and support of parts and on and on.
Once you do find 'that' guy, you will want to stick with him. And he just might be a small time operation.

The part will grow due to the change from pearlite and ferrite to martensite (quenching produces a martensitic structure in steels). The size change is affected by the alloy carbon content. Lower carbon steel expands less than high carbon. As stated above, you will not likely see much change in 8620.

I am assuming you are case hardening the part. 8620 does not through harden like 4140.

it doesn't?

There's some ambiguity in Post#5. 8620 will through harden in smaller cross sections, but due to the lower carbon it's still relatively soft even after through hardening. So while the center will be martensitic, you can still machine it once you get through case.

Dave G. said:

The part will grow due to the change from pearlite and ferrite to martensite (quenching produces a martensitic structure in steels). The size change is affected by the alloy carbon content. Lower carbon steel expands less than high carbon. As stated above, you will not likely see much change in 8620.

Click to expand...

I agree with this 100%, but since the OP is a newbie, he needs to know don't count on this in every situation.
I have made thousands of small (.125 to 1" OD ) 4340 parts for nearly 20 years that get quenched, then tempered to 30 HRC +/- 2. They are short, sold and cylindrical with tolerances ranging from + /-.0005 to +0/-.0005. They used to be done in oil when i first started, in a vacuum for the last 12 or 13 years. Both ways, and done by several different heat treaters, all of the parts have shrunk .0002" every time over hundreds of batches. So much so we count on it and shoot for .0002" over nominal.

Hollow or thin parts are a different story. Very unpredictable and varies with design and heat treater based on the process, furnace, support or fixturing, etc., as another poster advised.

Either way, if you have to be exact, either grind or machine post heat treat, or make a small sample batch, see what happens, plan accordingly.

I also can't stress the importance of finding a good heat treater, as was previously posted. They are not all equal and you may have different needs in different situations. I have a cheap guy that i use when it isn't precision size or hardness and has some margin for error. i have another when minimal distortion and close hardness is crucial. It took me years to find a cheap guy who wasn't terrible and a top notch guy who really is what he's cracked up to be, not just all hype and exaggerated promises.

Bottom line, if you have a few thousandths to play with, heat treat is rarely a big deal.

Sorry to get preachy and long winded, i do it a lot because i wish more people had been more detailed and offered more advice than i knew to ask for when i was new.

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