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Thread: Question for blade smiths and metallurgists about honyakis and retained Austenite

  1. #1
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    Question for blade smiths and metallurgists about honyakis and retained Austenite

    I've been reading John D Verhoeven's 2005 treatise on metallurgy for bladesmiths (I'm not planning to make blades but I'm fascinated by the process). It's a fascinating read that's fairly easy to understand given the complexity of the subject matter.

    There is one thing that I can't quite get my head around:

    Please tell me if & where I've got it wrong:

    My understanding is that when austenitic steel is quenched rapidly to room temp (I assume that traditionally this was done with water?), it will rapidly reach the Ms (martensite start) point (temp), and will begin to transform into Martensite. Because the transformation to martensite is so rapid (in comparison to pearlite and bainite), once Ms is reached, the formation of pearlite and bainite are circumvented.

    The Mf point (temp at which austenite is fully converted to martensite) is lower the higher the C content. If the carbon content is under 0.3-0.4%, it will reach Mf (will be fully converted to martensite) at room temp but if the carbon content is more, then the Mf point will be sub-zero and a cryo quench is needed to fully martensise (if that's even a word?) the steel.

    So after a room temp quench , steels over 0.4% C or so will have retained austenite (which will soften the steel compared to a fully martensitic steel).

    The hardness of the martensite is dependant on the amount of carbon in it (more carbon 'stretches' the Fe lattice more which puts the bonds in the lattice under more stress, giving them less leeway to move- I kind of think of it like being a taught rope)

    In steels up to 0.8% C, the harder martensite (created by the increased carbon content of the martensite) is more important than the retained austenite and the (as quenched) steel will harder than a 0.4% C steel. However, beyond 0.8% C, the retained austenite becomes more important and the overall hardness drops.

    This can be improved with a cryo quench (bringing the higher %C steels closer to their (sub-zero) Mf temp, giving less retained austenite and more hardened martensite). So a higher C steel would be harder than a 0.8% steel with a cryo quench.

    I'm assuming that honyakis have been around long before we had cryo quenching?

    A simple 0.8% carbon steel like 1080 should be able to get to about HRC 65 before tempering with a room temp quench. This would be reduced after tempering. Because of retained austenite, higher carbon steels should have a lower HRC with a room temp quench.

    So how do honyakis made from a steel like shirogami 1 (~1.3% C) and quenched at room temp get to HRC 64-65 after tempering?

    Is there any benefit of a 1.3% steel over a 0.8% steel with a room temp quench? If not, why do shirogami and similar knife steels have so much carbon?

    Thanks for explaining.

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    Increasing the amount of carbides increase hardness, the carbon content above 0.8% is intended to become iron carbide. But I don't know why razors and similar thin blades aren't made at 0.8% carbon, the eutectic point should offer the toughest lattice.

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    Great content, I remember looking at his essay years ago. Hope you get the answer you're looking for.

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    Quote Originally Posted by natto View Post
    Increasing the amount of carbides increase hardness, the carbon content above 0.8% is intended to become iron carbide. But I don't know why razors and similar thin blades aren't made at 0.8% carbon, the eutectic point should offer the toughest lattice.
    Thanks natto. When you say iron carbide, do you mean cementite? My understanding is that cementite has a hrc of 70 and will form in thr old austenite grain boundries or as a part of pealite (which has a much lower hrc).

    Does that mean that the trick with a honyaki is to get the cementite to form in the grain boundries (and in so doing, soak up most of the carbon in excess of 0.8%) but to trigger the formation of martensite before there is significant formation of pearlite?
    "My greatest fear is that when I die, my wife will sell my knives for what I told her they are worth"

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    Quote Originally Posted by Jacob_x View Post
    Great content, I remember looking at his essay years ago. Hope you get the answer you're looking for.
    Thanks Jacob. I'm fascinated by the complexity that is possible just by combining two elements (Fe abd C) in different ways.
    "My greatest fear is that when I die, my wife will sell my knives for what I told her they are worth"

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    Hi Nemo, you remind me to read my copy of Verhoeven, and I hope some more knowledgeable people chime in.

    If I got it right Fe3C means iron carbide and cementite. As carbide they are smaller than tungsten carbides, but I can't remember the structure.

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    Quote Originally Posted by natto View Post
    Hi Nemo, you remind me to read my copy of Verhoeven, and I hope some more knowledgeable people chime in.

    If I got it right Fe3C means iron carbide and cementite. As carbide they are smaller than tungsten carbides, but I can't remember the structure.
    Hi natto. I think on page 154 he's saying that cementite has the formula (Fe2Mg)3.C, but it could be interpreted as (FeMg)3.C.

    My understanding is that it can be present in 4 forms:
    1) Small granules throughout the grain (often done deliberately to improve the machinability of hypereutectiod steels prior to heat treat).
    2) Cementite on the grain boundry (formed when the temp of austenite falls below the Cm line). I'm not sure what effect this has on the overall hardness and toughness of the steel.
    3) In pearlite (a complex of cementite and ferrite) which significantly reduces the hardness of the steel.
    4) In bainite (a different microstructure of cementite and ferrite that can get almost as hard as tempered austenite and is a little tougher).
    "My greatest fear is that when I die, my wife will sell my knives for what I told her they are worth"

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    Quote Originally Posted by Nemo View Post
    Hi natto. I think on page 154 he's saying that cementite has the formula (Fe2Mg)3.C, but it could be interpreted as (FeMg)3.C
    Sorry, I meant Mn rather than Mg, so it should read (FeMn)3.C
    "My greatest fear is that when I die, my wife will sell my knives for what I told her they are worth"

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    Any one else have some knowledge to share?
    "My greatest fear is that when I die, my wife will sell my knives for what I told her they are worth"

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    Quote Originally Posted by Nemo View Post
    So how do honyakis made from a steel like shirogami 1 (~1.3% C) and quenched at room temp get to HRC 64-65 after tempering?
    One of the important parts of heat treating is controlling where the carbon goes.

    Carbon is locked up in steel and doesn't really move at room temperature.
    As you've probably heard, you can ruin a knife's heat treatment if you overheat it. What this really means is that you don't want to let the temperature get high enough where the carbon can start moving. As we increase the heat, more and more carbon begins to move.
    You can take a 1.3% carbon tool steel and, with the correct time and temperature, effectively have 0.8% carbon moving around inside the iron lattice ready for the martensitic transformation (the quench). This would result in a standard 65-66HRC untempered martensite.
    Where are the other 0.5% carbon atoms you ask? They're still locked up in the carbides and can be released with the addition of more heat. This extra heat is actually something we do before-hand, during normalising. By the time we're ready to quench, we should already have the extra carbon in the places where we want it to be. These carbides effect the properties of the steel all the way to the end product, and can add extra hardness among other things.

    Let's say I have accidentally overheated the steel before the quench. Now I have more then 0.8% carbon moving around in the iron lattice, which is more then it can hold during the martensitic transformation. I quench the overheated blade and a large amount of the austenite transforms into martensite, which spits out the extra carbon that it cannot hold on to. This carbon quickly bunches up in and around the austenite that hasn't yet transformed and grid-locks it in it's current state. This is one way of causing retained austinite (RA).

    There is always some RA and we deal with it as best we can. You'll notice on TTT graphs theres a Ms (martensite start point), then a M50 (50% conversion), M90 and so on. Some graphs don't show a M100/Mf (martensite finish point) because it's not realistic. The ones that do show Mf place it where the calculations work it out to be.
    95% conversion is great for bladesmiths. Other industries can take advantage of RA and keep it for it's toughness (e.g. 52100 and the crazy world of ball bearing steels).

    I hope this helps.

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