heat treatment 4140 info req

[litherland]

could somebody please tell me the harness of a 4 inch block of 4140 with 2 inch hole through it after it has been quenched and not tempered. we have some cracked blocks that have a hardness of 60/61 hrc on the outside and 60/58 hrc on the inside ,is this normal prior to tempering

 

[swall]

First of all, a 4" block of 4140 will not harden fully, even with a 2" hole in it. What hardness are you looking for?

 

[unclesyd]

Comeback with your heat treating procedure.

Your as quenched harnesses are too high unless you quenched in water.

Your shape is causing "die cracking" due to thermal stresses.

 

[litherland]

the final hardness require was to be 52/54hrc

 

[Metalguy]

litherland

"we have some cracked blocks that have a hardness of 60/61 hrc on the outside and 60/58 hrc on the inside ,is this normal prior to tempering"

First, "normal" 4140 will not get to Rc60 regardless of how its quenched-even in ice cold brine. If your Rc is accurate I'd suspect that you're adding carbon to the surfaces during HT. You need 0.50% C to reach Rc60.

Various heats of 4140 and other steels react differently to HT. Most heats are "average", but a heat that's lean in C, Mn, Cr and Mo won't harden much at all. But find a heat that's rich in all 4 elements and it will harden like some heats of 4340.

"I'm that dog who saw a rainbow, only none of the other dogs believed me." from "Kate and Leopold"

 

[swall]

I made some assumptions and ran the numbers. I assumed the 2" hole was in the center of the 4" section. This gives a max "wall thickness" of 1.4" at the corners. The equivalent round for a "tube" with wall thickness of t= 2t or 2.8". This is a Jominy of about 17 and you can only expect to quench out at Rc 35; a little better if you get a high hardenability heat. With 4340, you would quench out at Rc 51 min. Then, of course, you have to temper back and you will loose some hardness.

 

[tripleZ]

52 HRc seems about right out of heat. Your block IS capable of hitting 50 HRc with a good soak time and a good oil quench. Metalguy is spot on. If you're getting 60 HRc out of heat, you need to have your tester recalibrated.

My 4140 die cases were typically hardened and tempered to 40-45 HRc or 45-50 HRc depending upon the customer. The cycle (using stainless wrap and a conventional furnace) had me preheat the block at 1200 F for about 1.5 hours, then pull from preheat and transfer to high heat at 1600 F. Without seeing the block, I'd guess a 1.75 hour soak at 1600 F would be sufficient. Pull it out and get it into the oil immediately. Make sure to keep it moving in the oil. I'd say 5 minutes ought to do it. You'll probably be around 50 HRc out of heat. Draw/temper immediately. I don't have a wheel chart handy, but I seem to remember the draw temp being in the range of 500-700 F.

As to cracking, one other thing that "might" be happening is if you are failing to temper the part and are letting it sit over the weekend after the oil quench. I've seen die cases crack out of the tapped lift out holes if they are allowed to sit for an extended period of time without tempering.

 

[unclesyd]

If you need something with a hihger hardness than what 4140 is capable of you could a little higher hardness by going to 4340 or 4150. Depending on the end use I would stongly look at the air hardening tool steel like A4 or D2. Your part would be an ideal canidate for a air hardening tool steel.

We had thousands of parts fabricated/machined from 4140 all with a maximum hardness of Rc44 with the majority being in the 30's.

In quenching 4140 or any material you need to have a good quench oil and have a volume of oil to part of at least 50:1 or have some method of cooling the quench pot.