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How I Heat Treat stainless for optimal performance.

Discussion in 'Tristone Bladesmithing' started by Chadd Smith, Oct 2, 2015.

  1. I'm assuming you mean Heat Treatments? Assuming this is what you meant I will answer it as such;

    I have worked with a number of steels over the course of my making experiences, and after all the research I had conducted on the subject both from a real life use reports and from a metallurgical point of view determined Heat treatment was the most important aspect of a knife. Once heat treatment is achieved to the highest level, edge geometry follows and then it is all aesthetics and balance.

    As a result from day dot my single most focused goal was perfect heat treatment, or the closest thing to it. Due to this I ended up with a Paragon Kiln, which initially did not function within the degrees of accuracy I required. After significant testing and data analysis I sent an email containing my findings and verbally spoke to my supplier about the issues and provided recommendations as to the faults and design of the kiln. As a result, from what I have been told by the CEO of the company, those findings directly resulted in testing being conducted on the old kiln model.

    Consequently this testing resulted in Paragon re designing their entire product line into the current model. I know this is a big statement to make but I can provide the email correspondence confirming this. I now have the new designed kiln and am happy as a pig in ********, and my HT has been dialed in. To confirm what kind of readings I get in the kiln I have used independent thermal couples at different points to identify hot and cold spots.

    With the heating element of heat treating determined, I then needed to work out the best way to reduce RA, increase hardness and form (depending on the steel) Eta carbides. This was a no brainer, with stainless steel if you are not using LN2 cryogenic treatment, you simply cannot attain the lowest level of RA in a steel, nor can you precipitate Eta carbides during tempering. To make sure my heat treatment is premium, I have an eleven stage quenching and cryogenic treatment process for stainless steel.

    This involves:
    1. Quench from kiln into thick aluminium plates in a bench vice,
    2. Stabilising at room temperature for a designated time,
    3. Doing a first step cryo down to a designated temp and stabilising
    4. Second stage cryo into LN2 and hold for an extended period of time dependent on the steel type.
    5. Allow to warm to room temp, stabilise for short time
    6. Temper the blades to a specific temp, below the level RA begins to stabilise for the particular steel type.
    7. Pull from temper and air cool then repeat cryo process.
    8. Rinse and repeat steps 6 and 7.
    9. Temper to desired HRC range
    10. Repeat Step 7.
    11. Final grind blade.

    Due to the amount of steels I have tested and the time it took me to learn what I needed to, I am more than confident in my HT process. My work is already out there as I have heat treated some of Tansu's AEB-L. If you look at the reports on those knives I believe one specifically says, "I have had a couple other knives in AEB-L and this one has the best retention."

    I have made the same approach to heat treating the other stainless steels I currently have in my rotation, including Niolox, AEBL, CPM S35VN, CPM154cm, RWL34. I have tested these steels from a maker and a users point of view. I have also snapped the tips off those blades to inspect the grain structure which was very fine. The fine grain is also a by product of the PM process, something which is not attainable when using traditional smelting processes. Consequently I see no point in using an inferior steel when I have access to the best current steels.

    I achieved just under 45 degrees deflection, 30mm from the tip on a 62HRC AEBL gyuto. I cant remember what the others were but I was surprised by their toughness at +60HRC ratings. I have conducted numerous sharpness and toughness tests which involved shaving, sisal rope, phone book paper, normal and sweet potato and some other foods. Basically I found the kinves took too long to deteriorate at the edge cutting paper and cardboard so I moved on to 12.5mm natural sisal rope to abrade the edge quicker.

    I understand this is not a "real world" test in the kitchen, however to make it more "kitchen like" I stopped my testing when the blades could no longer shave hair or cleanly cut phone book paper. I also concede that a minor difference in edge geometry on a microscopic level can easily throw out the readings along with the small differences in cutting however no one ever cuts the exact same in the kitchen. I repeated these tests numerous times as well and tabulated that data for future comparison against new steel batches.

    Yes I was as smooth as a baby due to this type of testing. For a zero grind on a thin kitchen knife, when you are looking at the 80-100 cut mark through sisal rope in a specific push cut section until you can no longer shave, the edge retention is... adequate.

    I sent a knife blade to another well known chef and maker who was impressed with the heat treatment/edge geometry, exceeded expectations especially in regards to the ability for sharpening.

    As per the above, I have to HT in batches as liquid nitrogen and other items are not cheap to get sent to my house and I will not HT without it.

    My next purchases will be a 500x microscope and Nital etchant so I can inspect and measure the grain size of steel properly.

    I would love to hear thoughts on my processes and will answer any questions you may have.

    Cheers,

    Chadd
     
  2. MotoMike

    MotoMike Founding Member

    Thanks Chadd. Far more complex than I'd imagined. Not for the easily intimidated.
     
  3. butch

    butch Professional Craftsman Founding Member

    its not all that bad think about it this way if you can cook you have a good chances you could HT (jsut need the gear and follow directions (then tweek a bit here or there to get what you need the steel to do)
     
  4. I had no idea there were so many steps! Also making me even more excited to get my hands on that blade from you!
     
  5. If i mis answered your question about the choil, i round the choil and spine out completely and generally mirror polish this area so that is is super comfortable on the middle finger.

    The recurve on the choil is the perfect size for my finger circumference. It swings back towards the tip at the edge to prevent cutting your fingers or stabbing them on a sharp point. The recurve also does not come back as far as the ferrule so compared to any other standard knife design, it should be in the same place.

    I also included a picture of some blades pre hand polishingredients, showing distal taper at the tip. I have since the added in a full distal taper along the length of the blades. These blades are convex ground too.

    Cheers,

    Chadd

    20150907_184739.jpg 20150907_184759.jpg 20150908_145911.jpg
     
  6. CrisAnderson27

    CrisAnderson27 Professional Craftsman

    Most end users understand how critical heat treating is, but still tend to take it for granted. Even simple steels such as my W2 take me 6hrs worth of prep, before quenching. If the initial attempt goes right, it's still hours of tempering etc after that. I don't have to mess with liquid nitrogen thankfully, as there's no benefit to it for W2...but the process is still quite time consuming and complicated.

    By the way, excellent job on the grind and finish work!
     
  7. butch

    butch Professional Craftsman Founding Member

    yeh simple steels get to skip LN and high temp asust. temps but are far less forgiving on grain size if you over shoot a temp. more and more the cost "savings" on carbon steel vs SS steel are more down to LN cost adn grinding belts. i buy the best HC steels i can when i do and ultra clean grades are not that much cheaper then PM ss steels
     

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