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O-1 Toolsteel Hardening, To Prheat or Not?

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  • O-1 Toolsteel Hardening, To Prheat or Not?

    I'm in the middle of a 3 way argument.

    Pete insists it's OK to place a O-1 Part into the cold oven and heat to 1450-1500 F.

    Bob insists (quite red faced adamantly) to wait and place the part in the oven, only when it has reached the 1450 F.

    For some dumb reason, I've always waited till the oven has reached 500-900 F to start heating the Part, or preheat it in another oven and transfer it to the 1450 F Oven.

    Tom M.

  • #2
    The only important issue is that one needs to bring the entire piece to critical temp. The standard rule of thumb is one hour of soak time per inch of cross section.

    Waiting to put the work in the oven does not accomplish anything. I've put parts in a cold oven and brought them to temp and put parts in an already hot oven. It does not matter.

    Personally I prefer to put parts in a cold oven. It has nothing to do with the heat treating process, but it cuts down on the number of times I get blasted in the face with 1400+ degree air.


    • #3
      I don't know much about metallurgy, so someone here will shoot me down but if you where going to harden it with a torch you heat it up quickly, you don't bring the temp up slow like putting it in a cold oven. That's not to say that putting it in a cold oven and bringing the temp up slowly isn't a better idea, one that is absolutely necessary with other types of steel.
      The shortest distance between two points is a circle of infinite diameter.

      Bluewater Model Engineering Society at


      • #4
        If it is so at all, it is to reduce or eliminate warpage of thin sections if thrown into a hot "at temperature" oven. I would think bringing the part up to temperature as slowly as possible would cause less stress warpage. I don't think it has anything to do with hardness of the part once it has reached critical temperature, allowed to soak required time per cross section of area and is then quenched.

        The reverse of the scenario is true of course for drawing any semblance of temper from steel if it cools slowly from it's critical temp.
        Last edited by Your Old Dog; 05-11-2010, 01:15 PM.
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        • #5
          I am no expert,however I did work for 3 years in a production heat treat shop.
          The ovens we used were ran 24-7 and all product was put into hot ovens.
          product (98%) was 4140 ranging from .10 to .75 thick, allso we ran some 3500lb drop forge dies I am not sure of the metal type on those.thay were allso put into hot ovens.(no pre heat on any of the product.)


          • #6
            I start cold,run to critical and soak,then quench.After the quench I set the furnace control to the draw temp I want and let the furnace coast down to draw and then draw.No since wasting juice bringing a cold furnace up to draw temps when your already above it.
            I just need one more tool,just one!


            • #7
              Where in the hell is Forrest when you need him?
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              Thank you to our families of soldiers, many of whom have given so much more then the rest of us for the Freedom we enjoy.

              It is true, there is nothing free about freedom, don't be so quick to give it away.


              • #8
                I believe the technical term is the 'heating curve'. while the cooling curve matters a great deal, the ASM vol 4, 2100 pages of everything you ever wanted to know about heat treating, says:

                "In most practical cases, it is not necessary to expend much effort for the accurate calculation of the heating curve"

                kind of reminds of a Douglas Adams summation, 2100 pages and that's what they say about heating curves


                • #9
                  Heat Treat

                  I really don't think it matters ether way. Over the years I have done it both ways many times with the same results.
                  As for preheating I don't preheat before heat treating but preheating before welding broken die steels is required.
                  Last edited by Machinist-Guide; 05-11-2010, 05:01 PM.
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                  • #10
                    Very large masses (large die blocks, heavy forgings etc weighing tons need to be started in a cold furnace. When the outside of olid massive materal expands it's possible to indroduce cracks in the center of the mass. For masses like this (visualize solid steel car sized and larger) you often see see HT specs read: " work in cold furnace, raise the temperature not more than 25 (50, 100) degrees F per hour..." etc.

                    Small stuff 200 Lb or less it doesn't make much difference cold or hot unles the work in long and skinny, has thin and thick sections, or funny sharp inside corners, or the material is heat senstive. There' always exceptions. People who get red-faced arguing the exceptions by inflating them into rules probably are more interested in dominance than information exchange.
                    Last edited by Forrest Addy; 05-11-2010, 05:23 PM.


                    • #11
                      If you were starting out with a cold oven, then putting the workpiece in from the start means it would be at or near temperature at the same time as the oven. If you pre-heat the oven before adding the workpiece, it won't start coming up to temperature till it's put in the already hot oven.


                      • #12
                        Forrest nailed usual
                        I believe some of this controversy occurs due to oven types .
                        With electric ovens there should be no difference..

                        With Gas furnaces , some folks might be concerned with the atmosphere
                        and its effect on the materials surface , like carburizing or oxidation, which occurs with longer exposures
                        Wrapping your metal with SS wrap is a good idea.
                        I have a nitrogen environment I can also use ( but on the electric unit)

                        With Induction furnaces, all materials start cold !

                        I like to throw the material in a cold furnace because it removes a variable.
                        For thin materials, I always use a thick steel "backer" ( or sandwich) to prevent burn
                        A poster mentioned "Soak" time, which is very important.
                        By having the part heat with the furnace, I "feel" you have better control of soak. Just my opinion
                        Last edited by Rich Carlstedt; 05-12-2010, 12:13 AM.