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Thread: Taking the "Black Magic" out of Heat Treatment

  1. #11
    member NeelsRoos's Avatar
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    Mete
    Thank you. In the Knufoil users manual they specifically mentions oil and water quenching. I have never heard of any coatings that is effective on the Martensitic stainless steels. (1050degrees C 1920degrees F). This is interresting. Can you please name a product that I can try.
    The two steels I am using is 12c27 and N690 (similar to 440c) both recommended by their manufacturers to be oil quenched.
    Regards

  2. #12
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    Neels , now I understand your confusion . I'd have to check to see if there is a high temperature coating.
    Please understand that much of the manufacturer's HT recipes are based on 1" or thicker sections ! Many steels used for knives were never designed for that. But don't worry plate quenching the air hardening steels is fast and they will be fully hardened. I assume that all your quenching involves a quick trip from furnace to quench.
    I don't remember any complaints about using plate quenching on steels recommended for oil quench. Try it you'll like it !

  3. #13
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    Des, if I may be 'picky' in metals there are atoms, crystals and grains -- but no molecules !
    I was always wary of the claim that cryo 'stress relieved ' the steel .When you look up stress relieving in a metallurgy book you see the comment 'stress relieving is done by HEATING to ...'
    Anyway the latest research shows that cryo [ not sub-zero] causes some realignment of the crystal structure to permit formation of eta carbides on tempering. So definitely something is happening when you get down to liquid nitrogen temperatures. BTW I searched for a long time to find exactly what happens in cryo and now there's finally an answer.

  4. #14
    member JD Ellis's Avatar
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    Quote Originally Posted by NeelsRoos View Post
    Mete
    Thank you. In the Knufoil users manual they specifically mentions oil and water quenching. I have never heard of any coatings that is effective on the Martensitic stainless steels. (1050degrees C 1920degrees F). This is interresting. Can you please name a product that I can try.
    The two steels I am using is 12c27 and N690 (similar to 440c) both recommended by their manufacturers to be oil quenched.
    Regards
    Neels, I have tried the plate method with the blade in Knufoil. Des suggested that I use it on the Dammasteel so I gave it a try on the 19C27. Johan Oosthuisen tested it to be 61 R. The advantage is that it cools down fast and the steel stays straight. What I do is to keep the air-hose handy and blow it, when clamped between the 2 slabs, with cold air to cool it down faster.
    Regards, Johan Ellis

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  5. #15
    Quote Originally Posted by mete View Post
    Please understand that much of the manufacturer's HT recipes are based on 1" or thicker sections ! Many steels used for knives were never designed for that. But don't worry plate quenching the air hardening steels is fast and they will be fully hardened.
    mete, I understand where you are comming from with the above rule of thumb of 1hr/inch thickness. However, the steels that the SA knife makers are using are specifically supplied in the form of 6mm - 3.2mm. Being an ex-Bohler man (supplying the N690/K460/K110/K340 etc grade to the market) we specifically provided specifications to the knife makers market which supplied information pertaining to holding time for these thinner sections. I am sure other suppliers of knife strip would have done the same.

    Also please note that the plate quenching method does not work on all of the grades of stainless that the knife makers use. the lower C / lower Cr grades if not quenched agressivly will begin to form alpha-carbides (even in the thinner sections). Please keep this in mind. The rate of quench can be found on the materials specification TTT diagrams.

  6. #16
    member NeelsRoos's Avatar
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    Graham thank you .
    I will stay with the recommendations of Bohler and Sandvik and do oil quenching.
    Constant heating and equal distance from kiln elements and up and down quenching (not sideways) in oil result in no warping.
    I have done this for some time and results are constant and am satisfied. By using foil very little scale is encountered on my blades which is sometimes engraved prior to heat treating. I can achieve a delicate cleanup this way. I temper to 58 - 59 Rc which is what I want and not harder.

  7. #17
    member Tiaan Burger's Avatar
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    Graham and Mete, thanks for a very interesting thread!
    I make multiblade folding knives. I think it is the most critical use of springs a knifemaker can encounter. (Two blades with one spring, two blades with two springs, three blades with two springs, etc..)
    For the springs I recycle vehicle coil springs I get from a junkyard. To get consistency I don't mix steel from different coils on the same folder. I forge them down to about 3mm (1/8") and anneal using vermiculite. After annealling I grind the pieces clean, shape and drill the necessary hole/s.
    Hardening is done using a gas fired forge, using a magnet to determine the critical temp. The springs are immediately oil quenched, then tempered to a straw yellow. After grinding flat together with the blades for that folder, the springs are tempered to a peacock blue, sanded clean and bright, then tempered again.
    Thus far my results have been very consistent, the springs don't break and my knives have a good lock-up, and walks and talks like a sliplock should.


    My questions:
    1. Should I do a stress relieve somewhere in this process and at which point?
    2. Will these springs work-harden and fail due to metal fatigue? If so, how do I delay this for as long as possible?

    3. How does one anneal 52100?

    Thanks again!
    Last edited by Tiaan Burger; 03-01-2011 at 07:27 PM. Reason: Question added

  8. #18
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    Neels, www.advancedtechnicalproducts.com makes a high temperature anti-scale compound they say is good to 2400 F [1316 C ] . It's water based and available from www.brownells.com.

    Graham, Thanks for the comment. I often tell blade makers to treat each steel on it's own merits . Have you done specific tests on the oil quench steels using plate quench ? I'd like to see that info.
    I do not make knives or collect them but I use them and have a great interest in them. I became involved with the blade forums because I realized than many who forge and HT knives every day knew nothing about metallurgy .So after about 6 years on the forums , here I am and still teaching and learning !!

  9. #19
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    Tiaan, steels above the eutectoid ,especially the 1.00 % carbon grades present a problem that you must understand . The slow cooling from the austenitizing range gives lots of time for the carbon to diffuse to the grain boundaries .You can get continuous grain boundary carbide and that is extremely brittle !!! So no cooling in the furnace or sand . Air cool to give a pearlitic structure or quench and temper at 1200-1300 F to give a fine spheroidized structure .

    Springs - how are you tempering the springs ? A proper two hour temper in a furnace is much better than a quick heat to the color.Tempering will stress relieve to a degree.

    Fatigue failures of springs often start at surface problems such as scratches so polish carefully.

  10. #20
    Quote Originally Posted by NeelsRoos View Post
    Graham thank you .
    I will stay with the recommendations of Bohler and Sandvik and do oil quenching.
    Constant heating and equal distance from kiln elements and up and down quenching (not sideways) in oil result in no warping.
    I have done this for some time and results are constant and am satisfied. By using foil very little scale is encountered on my blades which is sometimes engraved prior to heat treating. I can achieve a delicate cleanup this way. I temper to 58 - 59 Rc which is what I want and not harder.
    Neels, by choosing to stick with specifications is probably winning more than half of the fight. In my industry many companies get fancy and begin to "play" with the specification and unlimatly end up with a product that is inferior (90% of the time). I am not ashamed to say that Metallurgical Engineers, who are allot better and more knowledgeable than I, sit in a comitee and draw up specifications which are designed to meet customer requirements and needs. Because of this I am a seriouse fan of Keep it simple and stick to spec.

    As for your methods of heat treatment, from your comments above it seems that you have a good understanding of heat treatment. it also seems that you have already gone through some learning curves and at the same time leant a great deal in the process. I believe that you hardness requirements are also where they should be as the consumable that you use are designed for this hardness range.

    An interesting tit-bit of information that you will not find in any spec (I picked this up from world class knife makers). After quenching (immediatly after) your blade will still be maluable for a short period of time, so if you do have some distortion (12C27 is a dancer), place the heat treated blade between 2 fairly heavy pieces of steel, not as a means of cooling it but as an effective way of straightening.

    Graham

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