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Hardening and Quenching

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  • Hardening and Quenching

    As I sit here, I have the Forged In Fire program on the History Channel on the TV. I watch it because there is nothing better on the TV (the news is interesting, but I tire of it after a while) and I may actually pick up some actual knowledge.

    One thing that I have noticed is that when they harden their blades, they quench them for a fairly short time. In the oil and then back out after only a couple of seconds. Often they are still so hot that the remaining oil on them will catch fire. And often, their blades are not hardened when they test them so they need to heat and quench them again. I am far from being knowledgeable in the art of hardening steel, but I was under the impression that a quench would be for a longer time. Swish it around for a while and let it completely cool.

    I know that these guys on the show are often working with mystery metal and that could explain their problems with getting it hard. But, I am curious. Exactly what is the best practice when quenching? Is only two or three seconds enough? Or should it be somewhat longer than that, perhaps 10 to 30 seconds or even a minute or more?
    Paul A.
    SE Texas

    And if you look REAL close at an analog signal,
    You will find that it has discrete steps.

  • #2
    Paul,

    The traditional (Roman, I believe) way to harden a long blade is to get it red hot, and then thrust it into the thigh muscle of a slave. This both quenches and nitrides the blade. Everyone but the slave find this to be a very satisfying method.

    In the absence of a handy slave, quenching is done in oil or water. Water has a much higher specific heat capacity than oil, so gives a more rapid quench.

    The way my metalwork teacher taught me to harden things like chisels was to quench them for a couple of seconds in water, and then quickly polish up a bit of the cutting tip on emery cloth. Heat still inside the chisel reheats the just-quenched surface. When it reaches the right colour (dark straw for a chisel), plunge it back into the water until it goes cold. This hardens and tempers the chisel in one go.

    If the knife being hardened has sufficient mass, briefly quenching and then leaving it will probably completely draw the temper, leaving it soft again.

    Ian
    All of the gear, no idea...

    Comment


    • #3
      Originally posted by Ian B View Post
      Paul,

      The traditional (Roman, I believe) way to harden a long blade is to get it red hot, and then thrust it into the thigh muscle of a slave. This both quenches and nitrides the blade. Everyone but the slave find this to be a very satisfying method.

      In the absence of a handy slave, quenching is done in oil or water. Water has a much higher specific heat capacity than oil, so gives a more rapid quench.

      The way my metalwork teacher taught me to harden things like chisels was to quench them for a couple of seconds in water, and then quickly polish up a bit of the cutting tip on emery cloth. Heat still inside the chisel reheats the just-quenched surface. When it reaches the right colour (dark straw for a chisel), plunge it back into the water until it goes cold. This hardens and tempers the chisel in one go.

      If the knife being hardened has sufficient mass, briefly quenching and then leaving it will probably completely draw the temper, leaving it soft again.

      Ian
      Thats a fantastic way to both waste a slave and ruin a blade. Nailing a bone can wreck a finished blade, stabbing a red-hot piece of metal into a femur will give you a piece of spaghetti, and not even a hard one since there's not enough there to adequately quench the blade.

      Humor aside, there's a few reasons for the process you're describing (interrupted quench, if you're curious about the name). The big one is stress, taking a piece of steel from red hot to room temperature in less than a second puts a lot of stress on the metal, depending on the steel type and quenchant it's not unlikely to snap a piece in half. Quenching the piece until it's no longer glowing, then bringing it out of the oil slows the cooling process and helps to reduce the stress. This is also particularly handy for steel that require being quenched in water to get hard. The speed water cools steel leads to some incredibly violent forces trying to rip the steel to pieces, but a dunk in water to drop the steel below about 900f, followed by removing it from water and placing it in oil to finish cooling will make the entire process more gentle. End result is a lot less blades cracked in half. DAMHIK

      Another handy effect is more related to the material properties of steel. Quenching a piece will result in a piece that's extremely hard once it's cooled. In the middle portion there though, after the blade goes into the oil and cools to black but before it sets up all glass hard, the steel is actually extremely malleable, about 600-400f. So, if you take a sword or what have you out of the oil after it's cooled to black but before it's cool, to touch, you have a bit of a period where you can bend the blade back into straightness to correct any warp. Pretty bloody handy on longer blades, straightening them later ranger from "impossible" to "f*** this".

      Ian mentioned another handy effect, the auto-tempering effect. If you partially quench a piece, on a knife heat the entire thing then only dunk the blade end in the quenchant leaving the tang (handle) as hot as it was out of the fire, the heat from the tang will leech back to the blade section, drawing the temper back from that glass hard state to a more usable hardness. Course, its a bit of a crap-shoot as to how much it draws the temper back since you can't do much to precisely control the heat and therefore end hardness. Cant say I'm a fan of the method, but it is handy. However, if the entire blade is cooled in the quench to the 600-400f range, then taken out, the residual heat in the piece isn't usually enough to mess with the temper, see that previous stuff about the superplastic phase. Something about the carbon lattice crystalline structure being established but not fully set, the metallurgical reason flies over my head to be honest.

      There is a risk of not getting a piece of steel fully hard when you interrupt the quench, pull it out too soon and the steel doesn't drop below the pearlite curve fast enough and it doesn't hardened. It's also possible that some of the pieces you're seeing in the show that aren't hardening are made of steel that don't hardened in oil. Some steel have to be cooled faster than oil can physically manage.

      Oh, and the oil on the blades can catch fire because the stage your pulling the steel out is usually in the 400-600f range, and no oil I know of has a flash point higher than 600f, with a lot being lower. Temperature won't hurt the knife, but it will light up the oil. Can't say I like the smell though, I leave my blades submerged until they're cool to touch. The flames are also a fire hazard

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      • #4
        Being a reality show maybe its just for the visual effect. A big fire is more visually appealing than alot of boring smoke.

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        • #5
          The blades are relatively thin so the heat dissipates pretty fast.

          JL................

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          • #6
            Pages 26 to 29 of this link show the TTT ( Temperature-Time-Transformation ) curves for formation of martensite, and other info about martensite.
            http://web.utk.edu/%7Eprack/MSE%20300/FeC.pdf

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            • #7
              Some where or time I saw a program where the blade maker insisted the quench was done with oil and the quench tank (a horizontal pan) was oriented to North and South. Something to do with aligning the metals molecules with the north pole, some thing like a compass, probably an old wife's tail.
              _____________________________________________

              I would rather have tools that I never use, than not have a tool I need.
              Oregon Coast

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              • #8
                Originally posted by plunger View Post
                Being a reality show maybe its just for the visual effect. A big fire is more visually appealing than alot of boring smoke.
                Being a NON-reality show you mean, right? There's very little that is real in these so called reality shows. I'd be willing to bet that they are directed to do this on purpose for the showmanship aspect.
                Chilliwack BC, Canada

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                • #9
                  Originally posted by BCRider View Post
                  Being a NON-reality show you mean, right? There's very little that is real in these so called reality shows. I'd be willing to bet that they are directed to do this on purpose for the showmanship aspect.
                  Really? It's a competition. While they do have a few lesser Smiths start on the show, most are well known and quite skilled. If you won place on the show and was told "take a dive" and be eliminated, wouldn't you sing like a canary afterwards, having been cheated out of the $10,000?

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                  • #10
                    Originally posted by lugnut View Post
                    Some where or time I saw a program where the blade maker insisted the quench was done with oil and the quench tank (a horizontal pan) was oriented to North and South. Something to do with aligning the metals molecules with the north pole, some thing like a compass, probably an old wife's tail.
                    Not as crazy as it sounds, even solidifying rock shows the alignment of the magnetic feild it solidified under
                    Mark

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                    • #11
                      Originally posted by lugnut View Post
                      Some where or time I saw a program where the blade maker insisted the quench was done with oil and the quench tank (a horizontal pan) was oriented to North and South. Something to do with aligning the metals molecules with the north pole, some thing like a compass, probably an old wife's tail.
                      Must be a new wise tale from a modern day sword maker trying to copy centuries old methods. In those days what sword maker knew about molecules or any alignment with the earths poles????
                      They didn't know what a molecule was, the word wasn't even in any language back then.

                      JL...................

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                      • #12
                        I'm not a knife maker and don't play one on TV but they may just be relieving stress in the piece with a short cool. I've seen them do that then go back to heat and banging some more. The hardening and tempering step is done near the end and before grinding or edging. At least that's the way I've seen a few do it.

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                        • #13
                          They didn't need to know about molecules to know that quenching along the field lines meant they were magnetised end to end, reducing the tendency of the edge to pick up metal dust from the subsequent grinding.
                          Richard - SW London, UK, EU.

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                          • #14
                            Humm, no wonder the art of metal work was considered a bit on the magic side in days of old. But I think that slave quench thing was a Hollywood thing. Slaves cost money.

                            I don't know, but do have to wonder if the extremely weak magnetic field of the earth would have much of an effect on the molecules that are red hot. Any real research on that?

                            I wouldn't think that the show would ask them to take a dive, it is a contest and there were some serious consequences back in the 50s or 60s about fixed contests. But I would not be surprised if they supply a flammable oil for the quench to liven up things. They seem to do some other questionable things like having the furnaces back to front so the guys have to stand between them.
                            Last edited by Paul Alciatore; 07-24-2017, 01:45 AM.
                            Paul A.
                            SE Texas

                            And if you look REAL close at an analog signal,
                            You will find that it has discrete steps.

                            Comment


                            • #15
                              I have no clue about steels and hardening so please excuse my ignorance but...........is there anything to do with the quenching process that would leave the center material less brittle so as to help with a sword not breaking or shattering?
                              Location: The Black Forest in Germany

                              How to become a millionaire: Start out with 10 million and take up machining as a hobby!

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