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Heat treatment of some small 4140 parts ~ 2" long and 3/4" section

I've made some small shaft parts of 4140. They were heated to 1575F, then q uenched in oil held at 150F, then I put them in another small oven at 650F for 2 hours then air cooled. The Rockwell came out at 45. The application of the short shafts is low speed but involves some shock load. I'm worried I tempered them at too low a temp. Thoughts? Should I re-temper to say 800F or leave it alone? The surface harness is not too critical but should be o ver 35.

Reply to
oldjag
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1000F, according to Bethlehem Steel's "Modern Steels and Their Properties."

pdfs of this excellent reference on carbon and alloy steels are available online.

Reply to
Ned Simmons

Ned;

Any chance to re-temper at 1000F?

Reply to
oldjag

Not to hi-jack your thread but one guy on a hand gun forum says to heat treat springs in the oven at 450 F.

I'm skeptical, but don't know for sure that that is bogus?

Reply to
Richard

Sounds like you're not shooting for an exact hardness, so I think it'd be fine.

Reply to
Ned Simmons

If he's talking about a stress relieve instead of what we usually think of as heat treating (hardening & tempering), then he's right, springs are stress relieved after winding. The proper temperature varies with the material.

Reply to
Ned Simmons

That's a reasonable light-yellow temperature for plain carbon steel.

Reply to
Jim Wilkins

Yes, the hardness is really not critical, Rc 35 or so is fine. Main worry with the these 4140 parts is that I tempered them at 650F which seems to be in the temp range best avoided for 4140 according to some literature. Seem s to be a range that causes crack sensitivity in 4140 and I was hoping to j ust re-temper at 1000F to solve that. But not sure if issue is reversible w ith just a temper. If req'd. I could normalize, and re heat to 1575, re-que nch and re-temper. However if I need to redo the whole heat treat, I'll nee d to get my small home high temperature oven better sealed. I've been purg ing with ~ 2LPM Argon to minimize scaling/oxidation but I'm still getting s ome scaling, so I'd rather avoid another trip to 1575F if possible.

Reply to
oldjag

The Izod impact value for 4140 is at a minimum when tempered around

600F, so the advice to avoid tempering at that temp makes sense. MIL-HDBK-5J for aerospace materials shows 4140 as tolerant to 625F short term exposure in use, so there's probably nothing going on at that temp that would interfere with your 2nd temper.
Reply to
Ned Simmons

Thanks for the follow up info Ned. Think I'll try some of these stainless foil bags I bought a while ago to enclose the parts for a re-temper at 1000 F. Should prevent oxidation/scaling and since I don't have to quench, no w orries about pulling them out of the bags hot.

Reply to
oldjag

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