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4340 hardening from full anneal

4340 hardening from full anneal

4340 hardening from full anneal

I am having problems achieving full hardness from 4340 bar stock from full anneal.  I can achieve full hardness from a part as machined (normalized and tempered-orinally 30 HRC) to 55 HRC. However, I then fully annealed the part (to about 10 HRC)and when I went to re-harden I could not achieve higher than 35 HRC, even after 3 tries, 2 at 30 minutes and again at 60 minutes. My hardening is at 1525F and oil quenching.  Any ideas?

RE: 4340 hardening from full anneal

Possibly the long annealing process has resulted in decarburizing of the surface layer. If you can remove this layer before hardening it might help.

Also what is the purpose of softening initially and then hardening.

RE: 4340 hardening from full anneal

We do have an argon atmosphere during all processes, so there should not be significant decarburization as far as I know. Here is the rest of the story. We are trying to weld repair a part, and I am doing testing to try and establish the process before I perform the work on a customer part. My original thought was to anneal the part fully before welding, and then stress relieve, and then harden and temper again. I was trying to test all the steps, and I found that after annealing I can no longer achieve full hardness.

RE: 4340 hardening from full anneal

What is the part mass?  What is the quenchant mass?  What is the quenchant temperature?  Do you agitate?



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RE: 4340 hardening from full anneal

Part weighs approximately 13 oz, we have an agitated container with about 3 gallons of AAA quench oil. We are thinking that the part has to be normalized prior to austenitizing and quenching. This is the only difference we know of between the initial part and the annealed part we are trying to harden.

RE: 4340 hardening from full anneal

How are you checking hardness?

RE: 4340 hardening from full anneal

Rockwell hardness tester, calibrated.

RE: 4340 hardening from full anneal

The full anneal may have resulted in a microstructure that is difficult to austenitize.  A normalize cycle can help (mostly because you'd normalize at or around 1700).  Try austenitizing at 1700 30 minutes, furnace cool to 1525 and oil quench.  1700 for half an hour isn't going to cause any grain growth, and cooing to 1525 will help out your oil quench.  You may have to hit it that hard to get it to fully austenitize.

As an alternative, modify your process.  Instead of a full anneal, use a sub-critical anneal (temper at 1275 F for an hour).  While perhaps not as soft as a full anneal, the toughness will be better (I'm guessing you are annealing to prevent cracking during the welding), so it should do what you want.  With the sub-critical anneal, it should harden up just fine from 1525.


RE: 4340 hardening from full anneal

Thanks for all your help, decarburization seems to be the culprit.  We did a normalization, as you mentioned, then did an austenitize at 1525 and oil quench. The surface hardness was again exactly the same. We then machined off about 0.020" and re-tested, and there was the desired 55 HRC we were expecting. It seems that the long anneal cycle resulted in significant decarburization from the surface. I will take your suggestion and do a sub-critical anneal in place of the full anneal. Thanks again to arunmrao and redpicker especially.

RE: 4340 hardening from full anneal


My original thought was to anneal the part fully before welding, and then stress relieve, ...

A full anneal is not required for this material. Prior to welding, a subcritical anneal is the preferred method.

RE: 4340 hardening from full anneal

A star to you for having come back and solved the mystery. I have been thinking the whole night of this strange observation. Though I was certain about decarb theory. I discussed with ASM expert and he too was  of the same opinion of decarb layer .

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