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blade steel hardening and edge retention

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  • blade steel hardening and edge retention

    I bought some 56si7 steel to make a cutting blade for my leather splitter. It was received at 28rc hardness and they say to harden it I need to heat it to 850C and dunk in oil and then in an oven at 220c for one hour. I have made the blade and used it unhardened and it cuts great. Would it have a better cutting edge if I hardened it or would the edge just hold up longer?
    Location: The Black Forest in Germany

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

  • #2
    Originally posted by Black Forest View Post
    ..... Would it have a better cutting edge if I hardened it or would the edge just hold up longer?
    To some degree "yes".

    Harder material takes a sharper edge, because it deforms less in sharpening.

    And it obviously wears less, although it can fracture.
    1601

    Keep eye on ball.
    Hashim Khan

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    • #3
      oh boy, this is like the "which coffee grinder is better" topic Knife making seems to be up there in the "get them out of the woodwork and start fighting" category.

      as for the question, only good way to tell in your hands is to try it both ways and see which works better for you. I don't have enough experience (!) with knives and sharpening to give an authorative opinion. One thing I will say though is that the tempering step is very important. it's amazing how brittle hardened steel can be without it.

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      • #4
        A properly hardened and tempered edge can be sharper and hold an edge longer, BUT, if it works now...... Trying to harden something, especially a blade (or any long thin piece of steel), can make you wish you just left it alone. Cracking and warpage being the main headaches that can arise.

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        • #5
          Originally posted by Dan Dubeau View Post
          A properly hardened and tempered edge can be sharper and hold an edge longer, BUT, if it works now...... Trying to harden something, especially a blade (or any long thin piece of steel), can make you wish you just left it alone. Cracking and warpage being the main headaches that can arise.
          The blade is 60mm wide, 7mm thick and 140mm long. The grind is 24mm wide. Meaning it goes from knife edge back to 7mm thick in 24mm. Warping would not be good.
          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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          • #6
            Hard is a mixed blessing. Contrary to belief, hard gives off to soft. 309 SS over a 30 year period erodes completely wearing against a mild steel chain. I have experienced it on ID bracelets.
            In the case of a leather splitter that already works I wouldn't harden it.

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            • #7
              It's working great now. See how long it works before it needs re-sharpening.

              From the leather work I do it seems like leather is only slightly abrasive to edge tools. They seem to stay sharper for longer than tools used on wood or other materials. So if it's working well you may find you can live with the apparently shorter life since that edge life will be easily long enough to make you happy. And if not? There's always hardening and tempering as the option.
              Chilliwack BC, Canada

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              • #8
                You might be better off just leaving it as is, and resigned to the fact that over it's lifespan you may have to sharpen it more often than you would if it was hardened. Depending on how much you use it you might come out ahead in the time it would take to harden and temper, then possibly straighten, vs the extra few shapenings you'd have to do over it's lifespan. If you're the only one that will ever use it you can keep that in mind while you use it too.

                On the other hand, if you DO decide to try and harden it, you could just try and flame harden the edge portion (heat the whole blade up evenly with a torch, but only bring the edge part up to non-magnetic temperature), and be careful during the quench. Drop the blade straight down in a chopping motion into oil and don't start waving it around like they do on forged in fire . Only move it forward and back in a slicing direction, or up and down slightly. I torch harden O1 flat ground regularly (not blades, but have done a few) and found that most warpage is caused by uneven heating, and uneven quenching. If you think it through and don't heat one side more than another, or drop it in the quench tank haphazardly on one side first, it wont turn out like a banana. Steel moves, it moves more when heated. You'd move too if somebody held a torch to your backside

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                • #9
                  There is always the case hardening option at the edge leaving the back of the blade soft.

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                  • #10
                    Originally posted by Franz© View Post
                    There is always the case hardening option at the edge leaving the back of the blade soft.
                    But that still involves heating up to the soak temperature and commonly a quench which would cause the warping anyway. I have never heard of any case hardening options that don't involve this sort of heating and quenching.
                    Chilliwack BC, Canada

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                    • #11
                      I am getting the impression that most of the replies here don't seem to reflect much familiarity with knife blade hardness. 28 Rc is not adequate hardness for much of anything in a knife. Even cheap junk ebay knives are considerably harder than that. It may be tolerable in a stainless kitchen knife, but that's a little different since the edge of those is supposed to be sharpened frequently with a steel, which work hardens the edge.

                      So yes, you will get a much better edge and much longer life after you harden the blade. Blade steels do need to be hardened, and then tempered back from full hard for toughness. If you're concerned about making the blade too brittle (i.e. maybe you want to use it as a large chopping blade rather than a smaller slicing blade, or maybe you want to pry on stuff with it), then raise the tempering temperature from 220 to something higher. Typically you wouldn't go higher than ~315C in tempering. You can also temper twice for a little better result.

                      One consideration though - typically the blade should not be sharpened prior to heat treatment. That fine edge degrades at high temp; usually you want to leave it about a dime's thickness and then sharpen after tempering. In your case, you'll need to grind back the sharp edge and re-sharpen.

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                      • #12
                        Originally posted by BCRider View Post

                        But that still involves heating up to the soak temperature and commonly a quench which would cause the warping anyway. I have never heard of any case hardening options that don't involve this sort of heating and quenching.
                        Well, yer partly right. Back before all the silly OSHA came along so dullards could be leafed into the workforce we had a wonderful product called Cyanide. I sure touched up a knife or 7 that cut wax paper 60 times a minute as it wrapped around a loaf of bread cause somebody refused to put a drop of fingernail polish on the adjusting screw or somebody was sure they could set the machine without their glasses.
                        Pretty much just fill the chip with O/A sitting on a firebrick shape on the grinder and then Cyanide the edge. No need to go back far and not enough heat to warp anything.

                        Cyanide is pretty much history, but I think Casinite is stiall available, and there is hard surfacing powder for torch application by those adorable hopper torch contraptions.

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                        • #13
                          Yes, you should harden it (source: I make knives)

                          Hardened steel will take a finer edge, hold that edge far, far longer, be more resistant to the edge rolling over, and funnily enough actually resist rust better, least in my experience. That last one isn't as important as the first few. If you don't fully harden your steel, you'll get something that cuts sure, but not nearly as well as it could, and you'll spend more time sharpening than you will using it

                          It's like trying to use the burr on a piece of sheet metal to cut your steak. Sure, it works, once. Proper cutting tools are worth the small investment of time it takes to heat treat them, which isn't much. For a lot of steels, including the alloy you're using, you can get away with heating the entire blade with a propane torch until it's non-magnetic (more accurate than saying 'cherry red') and dunking it in a pot of warm canola oil. Total investment is minimal, maybe $10 for the oil assuming you have a torch already. Can substitute a charcoal fire and a hair dryer for a heat source too

                          All that said, I looked up the specs for the steel you posted and frankly it's not the best choice for a slicing knife to begin with, so while hardening should still be done, it's still not going to be optimal for leatherwork. Reason is the carbon content, at .5% it's not quite high enough to be in what I consider the acceptable range for an A grade slicing tool. Good for swords, chopping knives, axes, stuff where springy and resilient is what you want, but less ideal for a keen edge

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                          • #14
                            Originally posted by Black Forest View Post
                            I bought some 56si7 steel to make a cutting blade for my leather splitter. It was received at 28rc hardness and they say to harden it I need to heat it to 850C and dunk in oil and then in an oven at 220c for one hour. I have made the blade and used it unhardened and it cuts great. Would it have a better cutting edge if I hardened it or would the edge just hold up longer?
                            Thanks for the info. What steel would you recommend for such a blade? This is a blade mounted in a frame above a roller that is adjustable. The leather gets pulled against the blade.
                            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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                            • #15
                              I'd use AEB-L for a project like that, although you'd have to send it out for HT

                              There are high carbon steels you could work with and HT yourself (with a simple fire brick forge)

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