Heat treating 4140

Post by **GlennW** » Thu Dec 17, 2009 10:52 pm

I’ve been trying to find a little info on heat treating 4140 and having limited success.

What I did find sounds like it would be a three furnace process as it states to pre-heat it to around 1300°f and soak it for a duration depending on thickness. Next, transfer it to a 1600°f furnace for another pre-determined duration and then quench it. After quench it immediately goes into a drawing furnace for tempering.

Can 4140 be heat treated using a single furnace by soaking the parts at the lower temp and then just raising the furnace/part temp to the higher temp, and then after quench hold the parts in a toaster oven or similar at 300° or so until the furnace cools back to the proper tempering temperature?

A seperate drawing furnace is not out of the question, just not what I had in mind right now.

I would like to end up as close to RC50 as possible and the parts in question will be tools.

I would also like a bit more info as far as the process and drawing temps for differing hardness if possible.

This is what I found so far, other then some fairly technical info on a metallurgist forum.

http://tidewaterblacksmiths.net/2.html

Thanks!

jpfalt · Post by **jpfalt** » Fri Dec 18, 2009 2:08 pm

This comes from the Pacific Machinery and Tool Steel catalog:

Heat to 1550 deg F
Quench in agitated oil
Temper at ~600 deg F for Rc51

As far as hardness vs tempering temperature,

Deg F HRc
400 56
500 53.5
600 51
700 48.5
800 46
900 42
1000 37

This is for 1/2" round stock.
Larger diameter stock will come out softer.

For 2" dia stock, tempered at 1000 deg F, the hardness drops to 30 HRc, so depending on the thickness of the stock you are using, you will want to temper at something lower, check hardness and retemper if it needs to go softer.

Up to about 600 deg you can get in a kitchen oven. Send your wife out shopping first.

Anything over that, I heat in the furnace and quench. Then I turn off the heat and open the furnace to get the temp down, reheat to tempering temp and temper then.

4140 isn't very critical about getting it into temper as soon as it comes out of quench. You can let it sit a while before you temper it. In some applications, 4140 gets quenched and put directly into use without a temper. In any case, though, I would give it at least a 400 deg temper.

Post by **GlennW** » Sat Dec 19, 2009 7:16 am

I have, and was looking at the MIL Specs and they are quite detailed and specific. What I was looking for was just what you posted. More of an alternative "home shop" method. Now I can combie the two and hopefully end up with some good results.

Thank You!

Jay, Idaho · Post by **Jay, Idaho** » Sat Dec 19, 2009 10:33 am

Kitchen ovens often have temp swings as the heat is turned on and off. I'd put a piece of metal, or several, in the oven to act as a heat sink, reducing the temperature swings when it was cycling.

jpfalt · Post by **jpfalt** » Sat Dec 19, 2009 4:18 pm

Jay is right about the temperature swings. Another issue with the oven is that the stove element can radiate the part you are heat treating and heat it much hotter than the thermostat indicates.

When tempering in the oven, I put the part on an air-bake cookie sheet and set the thermostat about 25 degrees below target and creep up on it until the temper colors are right.

The Home Machinist!