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Thread: Another HT question

  1. #11
    member Bruno's Avatar
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    Hi Neels,

    I believe the stepped heating curve that is used commercially is due to the volume of product to be treated.
    Depending on the position in the furnace, the temp of pieces could differ. In order to ensure equal heat throughout the furnace and that pieces reach the desired critical temperature at roughly the same time (in order to ensure accurate soak time) the temperature is taken up in steps below the critical temp, with enough soak time at each step.
    At least this is how I understand it.

  2. #12
    member NeelsRoos's Avatar
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    Quote Originally Posted by Bruno View Post
    Hi Neels,

    I believe the stepped heating curve that is used commercially is due to the volume of product to be treated.
    Depending on the position in the furnace, the temp of pieces could differ. In order to ensure equal heat throughout the furnace and that pieces reach the desired critical temperature at roughly the same time (in order to ensure accurate soak time) the temperature is taken up in steps below the critical temp, with enough soak time at each step.
    At least this is how I understand it.
    Bruno, is it better and more accurate then to treat your own knives instead of Bohler and avoid this long cycle as stated in threat #8 ? That is too much retained austenite causing reduced hardness and edge holding ability.

  3. #13
    member Bruno's Avatar
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    Neels, that is difficult to answer. I do not have enough experience with HT to give you a confident reply.
    You can only determine the amount of retained Austenite through Metallurgical testing in a lab.
    So how do you know you have a problem in the first place?
    Are we talking about 1 Rc in hardness difference, or are we comparing 50 Rc with 61Rc?

    The commercial HT people do it for a living - so they should have equipment, processes an knowledge that surpasses my garage setup.

    That said, I like to do the HT myself, because I can control it and play around with it to find what works for me. And I get my blade "returned" that same day
    Up to date I have treated only about 7 knives, and the hardnesses are not what I wanted (54 to 57 Rc) - I am in the process of experimenting to find out why.

    If your focus is on your craftsmanship, then only you can decide if a couple of points in harness will be detracting from the quality of your product.
    If hardness is a problem, you should find out why first - this will most likely mean sacrificing some blades in order to get detail about the micro-structure of the material after you have put it through all the steps.
    Only then can you decide if you will have better control over this aspect than the HT professional that you are using.

    I hope that I have answered your question.

  4. #14
    member NeelsRoos's Avatar
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    Quote Originally Posted by Bruno View Post
    Neels, that is difficult to answer. I do not have enough experience with HT to give you a confident reply.
    You can only determine the amount of retained Austenite through Metallurgical testing in a lab.
    So how do you know you have a problem in the first place?
    Are we talking about 1 Rc in hardness difference, or are we comparing 50 Rc with 61Rc?

    The commercial HT people do it for a living - so they should have equipment, processes an knowledge that surpasses my garage setup.

    That said, I like to do the HT myself, because I can control it and play around with it to find what works for me. And I get my blade "returned" that same day
    Up to date I have treated only about 7 knives, and the hardnesses are not what I wanted (54 to 57 Rc) - I am in the process of experimenting to find out why.

    If your focus is on your craftsmanship, then only you can decide if a couple of points in harness will be detracting from the quality of your product.
    If hardness is a problem, you should find out why first - this will most likely mean sacrificing some blades in order to get detail about the micro-structure of the material after you have put it through all the steps.
    Only then can you decide if you will have better control over this aspect than the HT professional that you are using.

    I hope that I have answered your question.
    Bruno, thank you for your answer.
    I have heat treated myself over 200 blades. The lowest Rc achieved on Martensitic Stainless was 59Rc. On normal heat treating of N690 and Damasteel (stainless damascus) I achieve 60Rc constantly. Using M390 on a good day I have achieved 62Rc. The nice thing about doing your own heat treating is the short window you have after quenching to straighten a blade.
    My main concern here is the facts. I ramp my kiln up slowly and evenly to final temperature (about 1 hour), soak for the required time (15 minutes to 30 minutes) depending on the steel. I quench in oil with the blade wrapped in Knu-foil all the way until after quenching.
    I used to do pottery and with clay especially your first bisc fire must be very slow as the clay will distort and crack. But we are not working with clay here you might say. Right. Then why is it bad to fire up slowly when you achieve the required hardness based on facts?

  5. #15
    member Bruno's Avatar
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    Neels,

    It seems I have the least amount of experience on this thread, but I am trying to answer the questions - that makes me either brave or stupid

    I am not saying heating the blade up with the furnace is wrong - I have done it that way as well. I just prefer to do it the other way around and it cannot hurt the blade to be placed into the furnace when it is @ temp.
    Logically there should be little difference in heating up with the furnace, or placing the blade into the furnace once it has stabilized at temp. I say this, because in as far as I know, things start happening above the critical (austenising) temperature - so how you get there should have little effect on the result.
    The time spent @ Austenising temp determines the amount of C and Cr dissolved in the Austenite, and this has an effect on the amount of retained Austenite, along with decreasing cooling rate.
    (thus the time spent in the bar, more-so affects if you will get home, rather than how you got to the bar in the first place....)

    I do not have 40 years experience in HT, but as far as I know it is the time @ Austenising temp that is the issue, not how you get there.

  6. #16
    member NeelsRoos's Avatar
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    Quote Originally Posted by Bruno View Post
    Neels,

    It seems I have the least amount of experience on this thread, but I am trying to answer the questions - that makes me either brave or stupid

    I am not saying heating the blade up with the furnace is wrong - I have done it that way as well. I just prefer to do it the other way around and it cannot hurt the blade to be placed into the furnace when it is @ temp.
    Logically there should be little difference in heating up with the furnace, or placing the blade into the furnace once it has stabilized at temp. I say this, because in as far as I know, things start happening above the critical (austenising) temperature - so how you get there should have little effect on the result.
    The time spent @ Austenising temp determines the amount of C and Cr dissolved in the Austenite, and this has an effect on the amount of retained Austenite, along with decreasing cooling rate.
    (thus the time spent in the bar, more-so affects if you will get home, rather than how you got to the bar in the first place....)

    I do not have 40 years experience in HT, but as far as I know it is the time @ Austenising temp that is the issue, not how you get there.
    Hi Bruno, Thank you for participating in this thread. You are definitely not stupid. We all learn from each other and makes you think. Some of us are passionate about correct heat treating and want to improve our results. Another person that is also passionate about correct heat treating started the thread below and from the results of that competition very few people knew what they were doing. It is a case of I think my heat treating is spot-on but is it? The blade being the heart of the knife, it is the most important part and heat treating the most important step.
    Here is the thread.
    http://www.sablade.com/forums/showth...ighlight=14c28

  7. #17
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    Clay is not quite the same .When I was in school one of the other Met E students , during lunch, asked if anyone knew about clay. He had just put a piece into the furnace for his nephew.I said , you should have asked first !! The water is chemically bound and each step-up in temperature removes some water .If you try to do it all at once you create steam which breaks the ceramic ! First remove water , then bake the ceramic. !

  8. #18
    member Bruno's Avatar
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    Hi Neels,
    Thank you for the information - I have read the thread you posted. It seems there has been no massive response to the request for "what worked and what did not" except for JD Ellis and Connie.
    I too have a passion to get this right. What use is it if I can make the knife but it does not do the job it was intended for.
    I will not be making knives for friends / family until I am sure of my process.
    I have just returned from the metallurgical lab where I tested 4 blanks (pre-ground) of 14C28N.
    2 Of them was after tempering and 2 before tempering (I wanted to see if my tempering operation could be a possible reason for the low hardnesses I was getting).
    In both cases the results were poor (44Rc after tempering and 55 Rc un-tempered)
    I have followed the process as set out by Sandvic but still did not come close to what I was supposed to.
    I have now requested a micro to be polished up on one of the blanks in order to see why I did not achieve the expected hardness, I will post the results once I have it.
    Next step is to have my furnace checked for accuracy at those high temperatures.

    Thank you for sharing you knowledge.

  9. #19
    Knifemaker Member Des Horn's Avatar
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    Hi Bruno,
    from here, I say the first thing you check is the furnace temperatures!
    With the thin blades we make (2mm to 6mm thick) I have never had a problem placing them in the furnace that is at the required temperature.
    For RWL-34 and Damsteel , being an air quench steel , the best way , and the cleanest, is to quench between two steel plates.
    This way there is no warping and the blades are up to specification at between 61 and 63 HRC after undergoing a double temper and cryogenic quench in Liquid Nitrogen.
    The tempering quench is simply done in air after the pieces have been in the oven for 2 hours.

  10. #20
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    Hi Neels, The reason Bohler do their HT over a long period is the method they use. First the blades are placed in the furnace and a vacuum used to remove the air, nitrogen is then used to fill the furnace to a positive pressure of 2 bar and the furnace is switched on, ramping to different temperatures to relieve stress etc. This process cannot be applied if the furnace is pre heated and dont think it is possible to draw a vacuum on a furnace at 1060 degrees as no equipment exists. The longer time they use is also due to the amount of steel they treat.

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