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Soak time for heat treating steel

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  • Soak time for heat treating steel

    The text book will tell you about calculating the soak time required for heat treating based on the cross section dimension of the material. I've never bothered with that much, and the heat treating has always worked out. Mind you the parts and tools I've heat treated have been small, non critical and usually only used a couple times.

    Is the time factor to ensure complete heating throughout, or does the crystalline structure move at a snails pace and need a lot of time to change. And if that's the case, why wouldn't they all, small and large, need about the same time? i don't think time matters much, its the temp....but maybe I've doing it wrong all these years.

    caught by click bait, I was watching this video today , at 1:32 part is induction heated and immediately quenched. We don't know a lot of details, but it does suggest someone thought getting it to temp is all that matters.

    What say you guys, is the emphasis on soak time a bit off the mark provide its heated thoroughly?
    Last edited by Mcgyver; 03-05-2021, 12:36 PM.
    in Toronto Ontario - where are you?

  • #2
    I think the answer is a little bit of both. Heat needs time to penetrate to the core, *and* the structure needs time to move around. I have an old paperback copy of the Crucible catalog,

    and have read it front to back multiple times. The one overall trend I notice is that, the higher the amounts of carbon and alloys, the longer it takes at higher temps. Makes sense to me anyway. They do a pretty good job of explaining things.
    25 miles north of Buffalo NY, USA


    • #3
      Most transformation reactions in steel occur quickly at the stated temperature. Soak time is generally to assure that the entire part is at temperature.


      • #4
        Steel is actually a pretty poor conductor for heat. So I think you'll find that the reason for soaking is for bulkier items that might be up to the cherry red on the outside while still below that on the inside.

        Like you my heat treating has primarily been with stuff up to around 3/4 round. The smaller items I can trust to be the same temp inside as out. Even there though I get to the right redness and hold it on the 1/2" stuff for a few added seconds. And on the 3/4" diameter item when it looked right I left it in my soup can forge for an extra 30 seconds to be sure.

        I'm going to be making a cute little 2 to 3 oz chasing hammer head soon from some 7/8" drill rod. That I'll hold at temp for a little longer to ensure full depth heat soak.
        Chilliwack BC, Canada


        • #5
          I sold induction equipment for years.
          Induction is fast and a great production tool. It's fast due to power density where a lot of heating power is put into the part in a small surface area. FWIW, 12-20 kW per square inch of power is used for many hardening operations. So if you are heat treating a part say a zone on a shaft to be hardened, the heat cycle can be a short as a couple seconds. Get the part up to austenitizing/transformation temperature ~1550-1600F then quench it immediately. You'll have a good austenitic grain structure. You also will typically induction harden to provide a hardened case from .030 to .100 inch deep leaving the rest of the part in an annealed condition. With induction you can turn the power down to slow heat to get the core to temperature, or blast it and melt the skin off of it if you're not careful. You can also secondary cycle the part to slow heat it to perform tempering.

          For furnace processing, soak times are published to be much longer to insure the core of a larger chunk of material is up to temperature. A basket of parts are thru heated then withdrawn and dumped into a quench bath of polymer quenchant or oil.

          You can download the heat treaters guide 'bible' here should you want more knowledge.

          Last edited by I make chips; 03-05-2021, 02:39 PM.


          • #6

            Plain carbon steel don't need much time but alloyed tool steels and HSS especially benefit from soak time. Carbides need some time to dissolve in the steel.
            But this is also a strong function of temperature so needed soak time varies from maybe 5 minutes to 30 seconds.

            Similarly tempering is time-dependant.
            Location: Helsinki, Finland, Europe


            • #7
              Depends on the specific steels. Some just need to get up to temperature and not spend any significant length of time at that temperature, so soaking is only necessary to make sure that the core of the piece gets up to said temperature. A lot of simple carbon steels are like that. Other steels, generally the more alloyed stuff, have to spend a specific length of time at a certain temperature, in addition to the time it takes to heat the piece all the way through.

              In short, the answer depends on the specific alloy


              • #8
                so what is the specific alloy that "only needs to get up to temperature"?

                most phase changes in steel alloys are diffusion based (austenite-martensite transition is not). therefore the transformation rate is a three dimensionel function with time and temp. as independent variables.

                homogenisation: purpose is to get "pure" austenite with other ellements in solution. primarily you want carbides to dissolve, not only cementite but also stubborn alloy-carbides. temperature is around 1200°c and holding time often several days. result is huge austenite grains (bad).

                refining: procedure above aust. temp. resulting in reduction of size and uniform distribution of grains. typically several hours.

                austenisation: heating to very precise temp. and holding for very precise time. depending on the desired effect. if you skipped the two steps above typically 30-600 minutes. e.g. 1045 as received (normalized) requres 840°c and 30 min. minimum.

                perlitic and bainitic ferrit: hold at temp. for 1 hour (approximate order of magnitude) until transformation is complete

                annealing and tempering: hold at temp. for typically 10-100 min.

                getting tired of typing. holding time above is from the point where the part or section in question has reached temp.

                yes, you can stick some steel in the drill press, coat it with borax, heat it with torch to what you happen to be percieving as the color in some chart you printed out and drop it in oil. will it harden? yes. how much? impossible to say. what other properties (usually more important than hardness) will it have? impossible to say, but probably so bad that you dont want to know about it.

                edit: the first step might have another name, solution treating maybe?
                Last edited by dian; 03-08-2021, 01:54 AM.


                • #9
                  Yeah, I have read all of that/ The time table VS the make up of the steel.

                  I read every Book, name one? might have read it.


                  PS:> Do not over heat the metal. It will gas out and become brittle. . JR
                  Last edited by JRouche; 03-06-2021, 03:58 AM.


                  • #10
                    oh, i completely forgot induction hardening.

                    sometimes things happen that actually "should" not happen. its a special case with heating rates up to 1000°c/second and extremely fast quenching (cycle times can be as short as 5 seconds). not too many steels get induction hardened (4140, 4340, 1541 among others) and im led to believe that they somehow (!) lend themselves particularly well to this prozess.

                    three things happen:

                    - austenizing temp. increases by 100-300°c which considerably accelerates diffusion
                    - extremely fine austenite grains form under these cicumstances facilitating transformation to martensite
                    - retained austenite dereases dramaticaly at these cooling rates.

                    which result in a hardness even several hrc points higher than conventional heat treat.

                    besides, induction hardening is not really hardening, its rather heat treat, in the sence that the parts usually dont get tempered. you dont expect hrc 60 from the process, so that martensite transformation doesnt have to be "complete". its also important in which state the steel procesed: pherodized, annealed, normalized or hadened&tempered austenize/harden differently in ascending order.

                    maybe somebody can explain better why it works.
                    Last edited by dian; 03-06-2021, 08:15 AM.


                    • #11
                      Just some thoughts,
                      Hardening and heat treating is a well published process but if you ask a metalligist you'll always hear the 'depends on' response.
                      Any steel alloy with over 25 points of carbon can be hardened. How hard it gets is dependent on that carbon content and other constituents. Some alloys can be hardened well over rC 60 and some with lower carbon won't provide much over rC 30-40. 'It all depends on' ! Overheating you can get cementites and other undesirables. Underheating you have undissolved carbides and a poor martinsitic transformation. Developing a heat treated part ideally requires some lab work with cutting, mounting, polishing and etching specimens to observe the grain structure under the microscope.It's a long process.

                      Higher carbon steels such as 4140 provide good hardenability. But, especially in the case of induction hardening a shaft or gear, there are tremendous retained compressive stresses in the part. Therefore 4140 must be immediately tempered upon hardening. Failing this, the part will crack. Larger parts say over 6 inches diameter will actually explode the case hardened portion from the part with dangerous force if they are not immediately tempered. Been there done that with 30 inch diameter wheels that had the outer circumference hardened.

                      In any case an alloy should be hardened to the maximum it can provide then be tempered back to a particular hardness if it is required.


                      • #12
                        Originally posted by I make chips View Post
                        if you ask a metalligist you'll always hear the 'depends on' response.
                        If you call them a metallurgist, you might get a politer response.
                        Just sayin.


                        • #13
                          ^ Oops sorry. I spent a lot of time in England and worked for an English company. You'll see it in my spelling periodically as I'll sometimes toggle between English spelling vs NA spellings if I don't proof read what I key in.

                          Whilst in the main ima yank.


                          • #14
                            I recall watching axle shafts being made at one of the local GMC factories, they must have used induction hardening. The rough machined axles were put into a machine, the switch was thrown, and within seconds the splined end was the color of boiled carrots. The worker then pulled the shaft out and threw it into a barrel of oil.

                            At the time, GMC was using 1045/C45 in their truck axles. Today they are using 4140. They timed their process to achieve about 45-50 hardness rc. I use the old shafts sometimes for a cheap source of good steel, and its difficult if not impossible to turn them without annealing or using carbide tooling.
                            25 miles north of Buffalo NY, USA


                            • #15
                              Nickel, that may have been on one of the machines I sold to GM Detroit forge. A couple machines were used for forging where we heat about 8 inches of a piece of stock and it then gets whacked in an upsetting press to form the flange.

                              Then there were several machines to scan harden the flange and bearing areas. The part then moved to harden the spline end. These were quite simple jobs for induction actually. Quenching isn't using oil anymore due to fire, stink, smoke and health reasons. A polymer quenchant is mixed with water at a particular concentration and sprayed at high pressure on the part within the machine cycle.
                              Last edited by I make chips; 03-06-2021, 02:43 PM.