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Looking for a site with good tempering info for 4340

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  • Looking for a site with good tempering info for 4340

    I have a machinist handbook and I'm looking for tempering temperature to get
    42 rc. All it says in the book is it can be tempered from 400-1200 degrees. Any have a link to a site that has a chart telling what tempering temperature will yeild what rockwell?


  • #2
    Tempering 4340 Steel

    041x... Where I work we machine a large amount of 4340 for military weapons or commercial fire arms. Almost all of our specs call for a finished hardness of RHC 41-45. Now the difference in our parts are; some are forged, some cast, and some are machined straight from bar stock. Each seems to have properties or structures that requirer the temperature to be adjusted for the same results in hardness.

    Based on the thickness and type of manufactured means used to produce the part the tempering will vary. If your part lets say is a wrought bar machined then quenched in oil. Your tempering temperature will be between 900 - 1100 f depending on your thickness of your part and the time. We use a 2 hour minium for plate and bar, and 2 hours 40 minutes on cast material parts. My boss has an AMS Chart from their Metals Handbook and he also uses their link to check heat treat procedures. I don't know if this is a pay for use site or if it is there for the public to view.

    4340, Oil quenched I would start at 900 f temper for 2 hours. If you do not achieve your hardness results they will be on the high side and you may re-temper at a higher level.

    Call your local heat treater and ask for their metallurgist, I have found them very helpful over the years. Most of them want to teach people their trade and do not mind talking to the novice.

    You may want to note that if there is any de-carbonization on the surface you will need to carbon restore before quenching or you will run the risk on quench cracking. The high Ni content in 4340 makes this prone to crack in quenching if the core Carbon is .08 or higher than the surface.

    I hope this helps

    The Worm

    Ex mea sententia


    • #3
      Thanks for the info about tempering. I'm also making rifle parts. The 4340 I used is normalized 2" bar stock. Section thickness is about .5" to .75"