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Differential heat treating by accident...

Discussion in 'Shop Talk - BladeSmith Questions and Answers' started by Fortress Mark, Jun 14, 2019.

  1. Fortress Mark

    Fortress Mark Gold Member Gold Member

    Aug 7, 2018
    I have been using 5160 for my knives, but recently started trying some 1095 in hopes of getting a little more hardness out of my knives. Today's knife is 3/16" x 1 1/2" x 9" long. I finished all the grinding, sanded to 220 throughout and coated with ATP-641 prior to normalizing at 1650, 1550 & 1450 for 10 mins each. After normalizing the blade was badly carbonized. I attributed this to not waiting long enough for the ATP-641 to fully cure.

    So, then I cleaned up the blade again and this time I heated the blade to 150 degrees and applied the ATP-641 again and put it into a 400 degree oven for 20 minutes to cure. Then I heat treated the blade at 1525 for 15 minutes and quenched in McMaster oil. Then I tempered the blade at 300 degrees for 1 hour 2 times. The blade measured RC 40 throughout.

    This was a fail, so I did a second heat treat as below:

    I cleaned up the blade from the last fail, sanding to 220 grit throughout. I heated the blade to 150 degrees and applied a watered down coat (approx 15%) of ATP-641 on the the whole blade. I then let it dry outside for 1 hour and then placed into 1525 oven for 15 minutes. Quenched in McMaster Slow speed oil.
    RC (Pre-temper) measured 64 on the spine, but only 43 on the handle. I am actually ok with this outcome, but I didn't do anything different on the handle then on the spine. Everything was coated with ATP-641.

    The blade tested non-magnetic at the quench (I tested it first, then put it back in the oven for 3 minutes before I took it out for the actual quench). I got it in the oil in less than 1 second and agitated properly.

    I don't understand why the differential hardening?
  2. dirc


    Jan 31, 2018
    I would guess that the reason your handle didn't get as hard as your spine is likely due to temperature differences? ... is your oven open near the handle? that might explain it

    in any case, part of the issue might be that the temp range for atp-641 is 1000-2400F so maybe that is why your 400F cure caused problems? just let it cure normally before aust

    also, fwiw, try some condursal z1100 - people often report much better results with it
  3. Larrin

    Larrin Gold Member Gold Member

    Jan 17, 2004
    You need fast oil to fully harden 1095. It would be very sensitive to thickness variation with slow oil.
    dirc likes this.
  4. Stacy E. Apelt - Bladesmith

    Stacy E. Apelt - Bladesmith ilmarinen - MODERATOR Moderator Knifemaker / Craftsman / Service Provider

    Aug 20, 2004
    To slow oil is the issue. You need Parks #50 for 3/16" 1095.

    Also, the blade was not "Badly Carbonized". The blade was covered in scale ... which is a very different thing.
    Carbonizing is adding carbon, as is done in case hardening.

    De-carbonized steel has lost carbon. Scale is the oxidized and decarbonized layer of steel heated to a high temperature and exposed to air.
    dirc likes this.
  5. Natlek


    Jun 9, 2015
    That was not point on that one second ...to got it in the oil in less then 1 second . . . .Point is to cool down the blade in LESS then ONE second . You get hardness on spine because bevels are thin under spine and have no mass to hold heat and that oil cool it down enough fast .Tang was thick and have more mass to cool down and because of that oil don t cool it enough fast ......because oil is slow , Temp. from core of the steel returns back to the surface .......and you have 43 hrc.....
    PS.I think it was clear what I mean ? Slow transfer of temperature from steel to oil is reason why you get soft tang but hard spine . In tang , steel faster transfer temperature from core to surface then surface of steel can transfer to oil ......
    Last edited: Jun 14, 2019
    dirc and Ken H> like this.
  6. daizee

    daizee KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Dec 30, 2009
    It also looks like you had that blade in the oven a long time (and many times) before quench and hardness test. Did you sand off the decarb layer before testing?
  7. Fortress Mark

    Fortress Mark Gold Member Gold Member

    Aug 7, 2018
    Thanks everyone for this great information. I have also been doing some on line research and agree that I am using too slow of quench oil. I am currently using 29 second oil. It seems that 1095 needs a much faster oil. I read a very good post from Kevin Cashen about how to harden 1095. There were many details that I won't repeat, but he ONLY uses Park 50 when quenching 1095. I think Natlek has it exactly right. It's actually very interesting because with the slower quench, the blade (the thinest part) is hardening well and the rest of the knife remains soft. I think this is actually a good outcome.

    However, I did order a 5 gal bucket of Park 50, (Now called 50 Quench Oil) from a company called MaximOil in Ft. Worth, TX 817-293-4645. Cost was $102 plus shipping. BTW I also found the same product at USA Knifemaker.com for $198.85 plus shipping.
  8. Fortress Mark

    Fortress Mark Gold Member Gold Member

    Aug 7, 2018

    Thank you Stacy for this clarification. I need to bone up on my terminology. Do I understand that scale consists of oxidized carbon? If so, is that removing carbon from the outer surface of the steel? Does this effect the hardness of the blade? I also read somewhere that it can be sanded down back to normal (BTW, I've seen your comments on other threads and I think it was you who said this.) You're good! Thanks.
  9. Stacy E. Apelt - Bladesmith

    Stacy E. Apelt - Bladesmith ilmarinen - MODERATOR Moderator Knifemaker / Craftsman / Service Provider

    Aug 20, 2004
    Scale is black iron oxide - Fe3O4. Decarb is steel that has lost its carbon to some degree, but hasn't "burned" due to excessive heat and oxygen.

    When the surface of the steel is above 1400F for any length of time, the carbon on the exposed surface will oxidize, leaving the oxidized iron behind. This is what the stuff that falls of the blade on every heat when forging is. The higher the temperature and the longer it is held at that temp, the more amount of scale form. This looks like a huge amount of lost steel and carbon, but it is actually of no concern in most cases. The carbon content immediately below the scale and its associated decarb is the same as it was. Keep forging heats in the proper range ( 1650-1950 normally) and adjust your forge burner to create a neutral or reducing atmosphere.

    Scale is very hard, and hardness cannot be determined on steel that has scale on it. Also, grinding a blade with scale on it will kill your belts fast.

    Scale is easily removed by soaking in a pail of sodium bisulfate (Ph Down) solution. A half hour is enough to get it softened a bit, and an overnight soak is what many folks do. Brush the billet/blade off in running water and it will be clean and ready for the next procedure.

    A trick to avoid severe scale issues when forging ( especially making damascus) is to never let the blade drop below red heat. When the color drops to a medium red ... stick it back in the forge. In making damascus, do the same. If the blade does not drop below around 1500F, the hard scale does not form. You can hot cut - fold over - set the weld - draw out - hot cut - fold over - etc. and never get a scale inclusion or bad weld. All you need to do is brush it off every time it goes in and comes out of the forge ... and never let it drop below medium red ( 1500-1600F)
    dirc likes this.
  10. Fortress Mark

    Fortress Mark Gold Member Gold Member

    Aug 7, 2018
    Thanks Stacy. This is very helpful. For the sodium bisulfate, what is dilution formula when using the granular form?
  11. Fortress Mark

    Fortress Mark Gold Member Gold Member

    Aug 7, 2018
    Stacy. Nevermind. I found your post from 2008. Unless your steel ratio is different, here it is.

    Sodium Bisulfate is used as a jewelers pickle for precious metals. It will remove oxides, scale, and fluxes. Be aware that it will also dissolve steel and iron if left too long or too concentrated. The way we remove a broken drill bit or tap from gold is to leave it in the pickle pot for a day or two - no more tap. As with all acids, use safe chemical practices, and have a plenty of the anti-agent handy ( in this case water and Sodium Bicarbonate).
    I mix my acid from 7:1 to 10:1 (H2O:SB) for gold and silver.

    Stacy E.Apelt

    July 23, 2008
  12. Natlek


    Jun 9, 2015
    If it is 64 on spine , most probably close to edge is harder then that .I don t see anything wrong with that blade as it is now .If it was mine , I will finish it and use it ....
  13. Fortress Mark

    Fortress Mark Gold Member Gold Member

    Aug 7, 2018
    I agree. That's exactly what I'm going to do with them. Will post pictures when completed!

    Natlek likes this.

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