Controlling the size of S7 parts during heat treating

[karthur]

When I have my parts heat treated, they always come back either undersized or oversized. Is there something that will tell me how much the dimensions will change. If I had something that would tell how much per inch of length that would be great. I am mostly concerned with S7 material.

Would this change as a function of the tempering tempurature?

Thanks

 

[tripleZ]

http://www.bucorp.com/files/compax_supreme-am_030413.pdf

See page #4 of the PDF file. You also need to consider distortion and how much stress you're introducing into the part. What sort of dimensional control are you looking for? Note that conventional quenching is done in oil. If your heat treater does not have vacuum or atmospheric capabilities, you're likely going to see more distortion and size change when using oil.

 

[karthur]

Here is a view of the part. I want to finish the Ø2.438 hole and have it on size when it comes back from heat treating. This is S7 material that will be 48-52 Rc.

I will have to find out exactly how the heat treater does the temper.

Thanks.

 

[tripleZ]

If he's oil quenching it, forget ever holding that tolerance. Leave yourself stock to finish the part after heat treating.

If he's doing this in a vacuum, and this is going to be a production part for you, I'd encourage you to run about 30-50 parts and do a small capability analysis on size change at heat treat. I personally wouldn't try and hold that tolerance through heat treatment...especially as I'm assuming there are some roundness or concentricity requirements implied.

 

[NickE]

Thats basically a jig grind tolerance. I just dont think you can get what you want.

from your info and the Timken Latrobe data sheet:

Hrc 48-52 should be aroudn 1100F temper when quenched from 1725F, the section is small so a slower quench wont be too bad, air would work.

the size change during hardening is given as aprox 0% at this tempering temp. Although I'd expect some sort of scaled surface, unless a vacumn or hydrogen furnace is used.

Are you cryo-treating after quench? that cna minimize retained austenite, RA can cause a dimensional change over time as it "rolls down the hill".


Nick
I love materials science!