Recommendation? Heat Treating 5160 - a question for the metal heads

Discussion in 'Shop Talk - BladeSmith Questions and Answers' started by teclis22, May 7, 2017.

  1. teclis22


    Jun 10, 2009
    Hi guys,

    this post is going to be a tad longer so i hope you bear with me. The basic question behind it is, that i want to validate my understanding of heat treatment and the processes that happen in the steel. Furthermore i like to squeeze out every last drop of a steels performance. Makes my.... 11th finger grow. Really.

    I had mail contact with a smith (names dont matter), regarding a large knife 8 mm thick, 60cm overall length. A very large chopper. The steel of choice is 5160. I send him a heat treatment protocol. This is not because i don't trust him but i want to feel like i am part of the process and put my theoretical knowledge to use. Plus that way i know what happened to the knife and i can adjust as necessary with the next order.

    The reply i got that i was wrong with my concept. But unfortunately not very detailed. So i will repost my suggestion here together with the reasons/thoughts behind it.

    I do appreciate any and all feedback. Here it goes:

    Assumption: full heat treatment, no differential hardening

    The outline below describes the process after the forging itself, meaning the blade got the form it needs.

    1) Tripple normalizing.
    The grain of the steel can have grown during the forging process. Normalizing helps resetting the structure and ensures you have a fine grain in the steel matrix to work from. Basically it resets any errors that can have happened during the forming process.

    2) Stress relieving
    Theoretically a piece of steel laying on a work bench has no inert stress. Actually it means the sum of the various stresses inside the steel are 0. By working in a stress relieve cycle you can release them. This means the blade can warp and might need to be straightened out before the actual heat treatment.

    3) The austenizing temperature is approx 830-840°C (between bright red and orange red in the color spectrum). The hold time is 8 mins, which equalls 1 minute per mm thickness of the blade. The temperature is a tad higher then necessary and carries the risk of grain growth but the time frame is so short that it should not occur. The advantage is that you gain a little bit of increased hardness in the finished product. Since the hardening is done in the forge i did not include a heat ramp.

    4) quenching is in warm oil (room temperature is just fine). insert the blade spine first. This prevents the thick part of the blade from radiating heat into the thinner part, hence prolonging its cooling cycle. After the blade is submerged it can be moved around for an increased cooling rate.

    alternative suggestion: broken quench. Starting with water and moving over to oil. But that would be entirely optional, as the cooling rate of water is very high it would run the risk of cracking the blade if the time window isnt exactly right.

    5) A steel with a carbon content of 0.6% has a Martensite finish temperature below ambient. That means a cryo treatment is useful as it allows to reach close to 100% martensite with next to 0% remaining austenite.
    As cooling medium i suggested acetone and dry ice or ethanol and dry ice. Both should be sufficient to reach Mf.
    Hold time about 3 hours. While the "switch" of the structure from austenite to martensite happens at the speed of sound, it doesnt happen in every cell at the same time. The long time period ensures that the entire still matrix has reacted.
    Cryo needs to be done immediately after quench to precent the remaining austenite from stabilizing. The blade can be stored in boiling water if an immediate treatment is not possible.

    6) following the cryo treatment the blade needs to be tempered. I suggested a kitchen oven at 200°C. (going too high risks the chance of blue-brittleness) Preheated with an oven thermometer as the dials are not accurate enough, and every electric oven has immense temperature swings during its heating phase (thats why you pre-heat when you do a cake). Hold time is 2 hours. While the function of the temper is not linear (the main effect happens in the first few minutes) 2 hours gives ample time for the entire blade to even out in temperature and for the steel structure to react.
    Directly after the first temper the blade is to be quenched in water (as cold as possible). This shifts the temperature range of the physical attributes of the steel to lower temperatures (not sure of the english term, in german its "Kerbschlagbiegezähigkeit"). This has an advantage if the knife is used during winter and temperatures below 50°C . So.. pretty much always.

    7a) Cryo treatment if available same as above

    7b) There is a chance that there is still some left over austenite at this point. Called secondary austenite. It will have changed to secondary martensite after the 2nd cryo treatment. That needs to be tempered as well with the same process as detailed in Step 6. In other words: a 2nd Temper cycle.

    8) after the cold water quench following the 2nd temper the blade benefits from another cycle of stress relief. this is to be done at 50°C below the last temper, so 150°C. This does not lead to a matrix change but releases build up stress in the steel . Any form of matrix change carries the risk of inducting stress into the blade. Hold time 2 hours. Afterwards let it air cool.

    Expected results:
    approx. 58-59HRc.
    a very fine grain
    close to 100% tempered martensite in the structure.

    That is my thinking behind my suggestions. Please correct me where i am wrong or why this wouldnt work.

    Thanks a lot in advance.

    yours sincerely,

    Last edited: May 7, 2017
  2. Forgentro


    Apr 6, 2019
    Better response on IFI...
  3. DevinT


    Jan 29, 2010
    What is the maker’s normal heat treat for 5160?

    JG Custom Metal Works likes this.
  4. tkroenlein


    Dec 10, 2016
    Seems pretty complicated for a steel that known for its toughness with much simpler heat treating.

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