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  • Hardening

    I have never had to harden any pieces but have been doing a bit of reading. I am curious, when you harden a piece of steel (tool steel or whatever) by heat treating does it generally change the final dimensions? I guess what I am getting at is if you machine a piece in its soft state to final dimensions and then have it heat treated will you generally receive it back within the same tolerances it went out or will there be warpage and/or dimensional growth?

  • #2
    You will almost always get some measure of warping or distortion, but I've never noticed volumetric or dimensional changes.

    Different materials react differently to heat treat. A2 is terrible for warping, for instance

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    • #3
      My experience is that the dimensional change on a typical heat treated small part is neglegable, maybe in the 0.0001" range if any. Generally, if you have a precision part that you are heat treating, warpage and oxidation/scaling is what might be noticeable.
      Warpage really depends on the slenderness of the part and/or how asymmetrical it is. Highly asymmetrical parts will pull in the direction of the thinnest section do to uneven cooling. Generally I deal with warpage by leaving the part a little oversize for finish machining, grinding or polishing.
      Unless you have an inert chamber oven you will almost always get a little oxidation so again, if you need a real clean finish always leave some material for finish grinding or polishing.
      Surface oxidation/scaling depends on many factors including soak time, and quench medium (air/water/oil).

      In my opinion its really warpage and oxidation that will have the greatest effect on the final overall dimensions.

      As an example of how much warpage and oxidation much one might expect, I recently created a series of form cutting tools from A2 (air quench) tool steel. The nominal dimensions were 1.50 x 0.75 x 0.25 with oblique ellipses cut from one side that varied per piece from about 0.125 to 0.375 in major radius. (Think of a rectangle with a bite out of one side) The form tools with the smaller radii warped about 0.010 TIR from end to end and the large radii pieces warped about 0.012 TIR end to end. I wrapped the pieces prior to heat treating in stainless foil with a strip of brown paper bag in each foil packet to burn off the sealed in oxygen and thus the oxidation was negligible. The thickness of the finished piece was not critical so I surface ground both sides and called it done.
      Last edited by jungle_geo; 01-20-2011, 01:24 AM.

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      • #4
        Yes, the usual is warping due to the part shape and/or the way it is cooled after the oven. For professional hardening plant we use about 0.3-0.5 mm extra material on the parts and when they come back we machine them to final dimensions. Usually hot tool steel called Orvar (for die casting molds) and MC 212, a really good surface hardenable steel for general machine work.

        The last warpage I saw was with Orvar mold insert, the piece was like a banana from end to end when I measured it. Showed 0.2 mm of deflection on the needle along 300 mm length.

        The usual is that holes tend to shrink a little when hardened and outer dimensions warp or bulge out a little, but this depends heavily on the material and hardening technique used.

        Can't say anything about other tool steels as the best place to find information is to look at the steel manufacturers website or ask them for hardening information. They are more than happy to give it to you and it usually says how some specimens react to hardening regarding the outer dimensions.
        Amount of experience is in direct proportion to the value of broken equipment.

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        • #5
          Originally posted by rp designs
          I have never had to harden any pieces but have been doing a bit of reading. I am curious, when you harden a piece of steel (tool steel or whatever) by heat treating does it generally change the final dimensions? I guess what I am getting at is if you machine a piece in its soft state to final dimensions and then have it heat treated will you generally receive it back within the same tolerances it went out or will there be warpage and/or dimensional growth?

          To make certain there is as little dimensional change as possible, heat the steel up to bright red BEFORE doing any work on it, allow to cool as slowly as possible - a bucket of lime can slow cooling to several hours, and only then start your machining and hardening!

          How do I Know this? During my apprenticeship, I was instructed to make a hole drilling jig, out of oil quenching tool steel, As far as I remember, six holes 3/16", on about 1½" p.c.d, with a locating hole of about 1" dia, in the middle. Even tho' I made the locating hole about 25 thou under, before hardening, when I set it up to grind the locating hole to size, I found that even with a 'touch' to the inside surface, the hole was already slightly oversize!! My foreman was all set to berate me until we measured the p.c.d. of the drill holes and found it to be over 25 thou oversize - a lesson learnt!!

          The next one was heated and slow cooled, before any subsequent machining and hardening and checked out very thoroughly before being put into use

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          • #6
            For any of several reasons, heat treating will change dimensions. This is why plus 0.005" taps are readily available, they are used so the tapped hole will end up on size after heat treat.
            Jim H.

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            • #7
              Originally posted by RobbieKnobbie
              You will almost always get some measure of warping or distortion, but I've never noticed volumetric or dimensional changes.

              Different materials react differently to heat treat. A2 is terrible for warping, for instance
              How large was the A2 piece? Dimensions?

              I keep hearing "it will warp" yet haven't seen warping with A2.

              I keep wondering if the person was talking about something like 0.100" x 2" x 6" knife blade that warped?

              I'm just making up small tooling (typically 1" x 2" x 3" size things). Little tooling fixtures.

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              • #8
                There is a mechanism by which steel once hardened can grow slightly. The crystal structure of the iron molecules changes at high temperatures and the different structures have a different size. The quenching is to cool the steel at a rate greater than the crystals can reform into their original structures and hence 'harden'. However it is possible for some molecules to have not changed during quench (called retained austenite), but they will re-form themselves more slowly over time resulting in some small growth.

                Some tool steels are more susceptible to this than others. It's a very small amount and in most cases you'd never notice it but it shows up in a few instances. In molds we would sometimes see that large hardened inserts would not go back into the pockets and when measured were actually longer than spec. They were fine when first made. This phenomenon was known and noticed when Johanssen first developed gauge blocks. He would put his blanks through repeated cycles of boiling water and ice water to try to get the retained austenite to precipitate out so the blocks were finally stable.
                .
                "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

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                • #9
                  generally its a volume reduction, called a pritectic tranformation
                  regards
                  mark

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                  • #10
                    HT

                    In your post you said your parts were 1X2X3 and you also mentioned "receiving them back" this leads me to believe you are planing to have them professionally heat treated. If this be the case and you are using A2 or S7 and not working in tolerances of less then .0005 I think you will be ok. As far as scale on the surface if heat treated right there wont be any.
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                    • #11
                      Originally posted by Machinist-Guide
                      In your post you said your parts were 1X2X3 and you also mentioned "receiving them back" this leads me to believe you are planing to have them professionally heat treated. If this be the case and you are using A2 or S7 and not working in tolerances of less then .0005 I think you will be ok. As far as scale on the surface if heat treated right there wont be any.
                      Thanks. Yes, A2 steel.

                      Long term I'm planning on buying a surface grinder, but if "as is" is less than 0.000 5" that is fine for what I am doing.

                      Oh, humor of the weekend. Cutting up some reclaimed A2 - it got to the end of the cut on the horizontal bandsaw and I didn't hear it drop. Turned out the end of the bar had been cut off with a torch. Yes, nice and hard the last 0.020" or so. Gave me a good idea for some home made strain gages.

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                      • #12
                        I found no humor when I learned that lesson. Trashed a perfectly good bi-metal 10 TPI blade just about the same distance from the torch cut that you had. Never gave it a thought before. $20 later, I give it a lot of thought.
                        Wayne

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                        • #13
                          Originally posted by atty
                          I found no humor when I learned that lesson. Trashed a perfectly good bi-metal 10 TPI blade just about the same distance from the torch cut that you had. Never gave it a thought before. $20 later, I give it a lot of thought.
                          Yep. About then I was thinking, maybe I should have used some 4140 for this project. It was a fine 32 tooth blade (thin wall 4130 tubing). Thankfully I still had some more Starrett 10-14 blades from the last Enco sale (glad I stocked up... ). I like those blades, but on 0.049 and smaller tubing needed the finer ones.

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