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Thread: A question about quenching media, and the science behind quenching

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    Senior Member Vasilis's Avatar
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    Default A question about quenching media, and the science behind quenching

    A question I have about quenching blades is, since water boils at 100C or so, and oils at 400-500C along with most waxes, when the steel, above critical temperature comes in contact with the quenching media, ("special quenchant", oil or water), that blade doesn't come in contact with the liquid but with it's vapor, not any liquid since that media is boiling at a lower temperature.
    So, for that temperature range, between say 800 degrees C, and 400-500 degrees C, the heat transfer is a lot slower than below that temperature.
    I've been searching for organic materials that don't boil at temperatures below 800C, but nothing that can be found easily seems to appear, and it makes sense.
    On the other hand, there are some metal alloys (like Wood's alloy/"metal" and many other combinations) that are liquid at water's boiling temperature, boil at a higher temperature as well as not releasing fumes at critical temperatures for steels.
    So, I've been thinking, has anyone used such an alloy for quenching, and if done correctly (for example, a big pot of water that boils, with a smaller in it that contains that alloy, in which you dip the piece of steel; the bigger the surface of the pot containing the alloy that comes in contact with the water, the faster the cooling rate, same with the volume of liquid alloy, and, metals are great heat conductors by their nature, so, it sounds excellent. Too good maybe to be working?) what happens to the steel, hardness and rate of unsuccessful attempts?

    Has anyone tried it? I'm not referring to reaching hardening temperature using an alloy, nor tempering again, using some liquid alloy, the question is about quenching hot steel on low melting temperature alloys.

    Overall, I expect, the quenched piece of steel will reach a temperature around 100C/212F faster than it would with any other media used. Also, because of specific gravity, a lot of pounds/kilos will be needed, and they are expensive, but again, liquids specifically for quenching are expensive too. (of course we are careful with the fumes anyway, as well as splashing etc etc)
    If the metals were cheaper I wouldn't even ask and try it, but buying 20kg of them and being careful with lead/Thallium/Cadmium/Mercury is kind of important.
    So I would appreciate your help on the question, as I imagine we could achieve higher hardness that way with the faster cooling rate and possibly obtain the bainite crystalline structure more easily. Or it would end up being a failure.
    I've been searching on google for days and I didn't find anything serious.

    Thank you for your time.
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    I believe “lead quenching” is used in the wire industry.

    Liquid salt baths and liquid metal baths are also used for Marquenching/Martempering.

    Your concern about the formation of a “vapour jacket” around the hot metal is well founded and a known problem.

    That is why the hot metal should be continuously moved/agitated when quenching, as it insures that the metal remains in contact with the quench medium, as the vapour rises to the surface. Of course agitating the medium helps achieve the same result.

    The search for quicker quench mediums in hopes of achieving more hardness may be misguided.

    My understanding is that you only need to cool the steel just fast enough to get it through the transition zone in time, any faster does not give better results.
    In fact going too fast leads to the dreaded “ping” and the breakage of the steel. By way of example if you take an oil hardening steel such as O1 and quench it in a fast medium such as water you will end up with lots of broken blades.

    As for quenching speed, I don’t think there is anything faster than an agitated and saturated brine solution. Not to many steels can survive such a harsh quench, especially with the thin cross sections and stress prone shapes found in blades/razors.

    Additional: This reasonable discussion (http://www.iforgeiron.com/topic/25457-quenching-medium/) is interesting and suggests that “super quench” (as outlined here http://magichammer.freeservers.com/robb_gunter.htm) is faster then brine.
    Last edited by DrDalton; 12-07-2016 at 01:25 AM. Reason: additional info
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    You ask good questions Vasilis. And DrDalton has succinctly answered, and well. I'll buy you both a beer any time.

    Lead baths are still used by Dovo if I watched that manufacturing video attentively. I think I recall Thiers Issard having a similar setup. Molten lead will get hot enough to austenitize the steel but it will work for martempering. It was probably the original martempering system. Unfortunately lead has a reputation for affecting children's intelligence and so has a reputation for being dangerous. As if a hot molten bath is not dangerous enough. Probably ecologically unsound too. The stuff is really depleted radioactive ore anyway.

    The one item left off the list is a type of polymer. See here for examples, there have to be other suppliers. Quenchants My good friend Howard worked with polymer quenchants for a few years. The intention of this material was to have a water based solution act like water for a minimal critical period of time, then as the water vaporized, the blade would be coated with the polymer which slowed the quench and acted more like an oil. He may step in to correct this impression. It was messy and required particular attention to the dilution. That adds variables that all affect the results. Sometimes it's just hard enough to get from the right heat to the right cooler temperature without adding more stuff to think about.

    If my memory serves me well enough, this was about the time we both were beginning to switch to using high and low temperature salt baths to achieve better results. The low temperature salts are hygroscopic and will draw water from ambient humidity. There will always be water available in them to boil and create a vapor jacket but for practical purposes this is minimal. I'm sure that the vapor jacket could be measured and could probably be shown to have an effect on quenching. But, according to John Verhoeven, hardening occurs at the speed of sound in the material. I am no longer fast enough to put a blade into the quenchant and pull it out fast enough to interrupt the quench to hardening process. I quit worrying about that.

    I don't know that it would be practical to try when ultimately (the limiting factor for all razors is...) the hard blade then has to be tempered back from what is possible to achieve because the honemeisters begin to grumble about the blade eroding their stones. There is an upper limit to desirable hardness.

    In the end, you want to achieve the correct austenitizing temperature and hold there for the recommended interval and then quench to below the "nose of the curve" on the isothermal transformation diagram for that steel and allow a faster or slower reduction in temperature in the lower temperature quench to achieve the phases between austenite and martensite as you desire. This will always be a simple mathematical relationship between time at temperature. Fast is needed if the nose of the curve requires it. Bainite needs more time and controlled temperature during that time.

    A lot of decent blades have been made through history without any special equipment or attention to the modern metallurgical variables discussed here. There is no downside to years of quenching from a simple heat into a simple quench like oil or water. Whiskers won't complain if they are removed by a blade that is good enough, and honed well, versus the absolute best metallurgical performance known to modern science, and honed like crap.
    Last edited by Mike Blue; 12-07-2016 at 05:03 PM. Reason: comment about molten lead was wrong
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    Quote Originally Posted by DrDalton View Post
    I believe “lead quenching” is used in the wire industry.

    Liquid salt baths and liquid metal baths are also used for Marquenching/Martempering.

    Your concern about the formation of a “vapour jacket” around the hot metal is well founded and a known problem.

    That is why the hot metal should be continuously moved/agitated when quenching, as it insures that the metal remains in contact with the quench medium, as the vapour rises to the surface. Of course agitating the medium helps achieve the same result.

    The search for quicker quench mediums in hopes of achieving more hardness may be misguided.

    My understanding is that you only need to cool the steel just fast enough to get it through the transition zone in time, any faster does not give better results.
    In fact going too fast leads to the dreaded “ping” and the breakage of the steel. By way of example if you take an oil hardening steel such as O1 and quench it in a fast medium such as water you will end up with lots of broken blades.

    As for quenching speed, I don’t think there is anything faster than an agitated and saturated brine solution. Not to many steels can survive such a harsh quench, especially with the thin cross sections and stress prone shapes found in blades/razors.

    Additional: This reasonable discussion (Quenching Medium - Heat Treating, general discussion - I Forge Iron) is interesting and suggests that “super quench” (as outlined here Robb Gunter) is faster then brine.


    Thank you for your answer, you appear to know about the process.
    I've read about marquenching and salt baths. What I'm wondering about is, metals are excellent heat conductors by their nature, so, wouldn't that result in a more uniform temperature drop to the piece of steel we'll be quenching, unlike oil/water/salts? If the piece of steel cools uniformly, no matter its speed, wouldn't that reduce greatly the formation of these cracks?
    Theoretically, quenching in an alloy would be a smaller shock for the piece of steel; the liquid alloy will be a lot colder, but again, since it's an alloy, the heat transfer would be a lot faster, resulting in both a faster AND gentler quench for the whole piece. IF we achieve same or higher heat conductivity with that of iron (steel should be close to that) that would mean, we will be able to achieve maximum hardness with close to zero failure rate for our quenched edge no matter it's shape-size.

    Does that make sense?

    I would like to find a way to achieve hardness, for low alloy steel with carbon content close to 1%, of above 65 RC (like some Japanese razors that have showed a hardness rating of 67RC, how could they do that in an age where knowledge and technology was behind) and am searching for ways to do that. If I could reduce the failure rate, that would be a big bonus since grinding a blank, at least for me, takes days.

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    Quote Originally Posted by Vasilis View Post
    Theoretically, quenching in an alloy would be a smaller shock for the piece of steel; the liquid alloy will be a lot colder, but again, since it's an alloy, the heat transfer would be a lot faster, resulting in both a faster AND gentler quench for the whole piece.
    .
    I would think that the most gentle quench would be as slow as you could go and still stay left of the nose and the formation of pearlite and bainite in the edge region. Faster than that does not result in higher hardness AFAIK. The depth of the hardness would be affected, but for razors this is not a concern IMO.
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    Quote Originally Posted by bluesman7 View Post
    I would think that the most gentle quench would be as slow as you could go and still stay left of the nose and the formation of pearlite and bainite in the edge region. Faster than that does not result in higher hardness AFAIK. The depth of the hardness would be affected, but for razors this is not a concern IMO.
    By gentle, I mean the whole piece of steel would contain the same concentration of martensite/ferrite/perlite/bainite and any other formation. The damage on quench happens because the size of the martensitic crystals is different from the ones on ferrite/perlite; the one expands, the other stays the same in the end, and, on the weakest part of the blade/place where that formation was the harshest, a crack is formed.
    I think, the thermal conductivity is something really important aspect to that. Lead has about 35W/(m.K) and 1095 has 48 or so, where water has 0.591 BUT is faster quenchant. I think.

    Mike, once again thank you for your answer. On the Dovo video I remember that they were heating the razors on an alloy, lead as you said, but I thought they were quenching them in oil as the surface tension or viscosity was lower that that of any liquid metal, or so I remember observing, but I won't find it strange if I'm wrong.
    So, blacksmiths do think about wear resistance of their blades, and try not to reach the highest point. I would have never think about it, always striving for the highest possible.

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    Quote Originally Posted by Vasilis View Post
    By gentle, I mean the whole piece of steel would contain the same concentration of martensite/ferrite/perlite/bainite and any other formation. The damage on quench happens because the size of the martensitic crystals is different from the ones on ferrite/perlite; the one expands, the other stays the same in the end, and, on the weakest part of the blade/place where that formation was the harshest, a crack is formed.
    I think, the thermal conductivity is something really important aspect to that. Lead has about 35W/(m.K) and 1095 has 48 or so, where water has 0.591 BUT is faster quenchant. I think.

    Mike, once again thank you for your answer. On the Dovo video I remember that they were heating the razors on an alloy, lead as you said, but I thought they were quenching them in oil as the surface tension or viscosity was lower that that of any liquid metal, or so I remember observing, but I won't find it strange if I'm wrong.
    So, blacksmiths do think about wear resistance of their blades, and try not to reach the highest point. I would have never think about it, always striving for the highest possible.
    In martempering, martensite does not start forming until the piece is removed from the quench medium with the idea that the steel moves slowly through the Ms to Mf temperatures resulting in a more uniform rate though out the piece. According to Verhoeven the susceptibility to quench cracking depends strongly on the cooling rate through the Ms-Mf range.

    Maybe we are talking about different types of cracking.
    Last edited by bluesman7; 12-07-2016 at 04:39 PM.

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    I returned to the Dovo video and they are using molten lead to austenitize the blades and quench in oil. The oil is probably warmed and that means marquenching.

    The shock to the steel is reduced when you reduce the required drop in temperature to the least amount needed to achieve hardness. A drop from 800C to 200C is much less severe than a drop from 800C to 50C. Even so all the dynamic movement in the crystalline structure is going to occur at the speed of sound. Any stressors induced by temperature or the shape (e.g. avoid sharp corners and avoid large grain crystals - more fracture potentials) mean that the shock waves from transformation all work within the dimensions of the blade. Quenching also reduces grain size which translates into a collapse in dimension and movement of the steel. That movement is another factor in potential stressors that induce cracking.

    Blade preparation includes grain size uniformity before heat treatment and the least amount of thermal shock to the material during heat treatment.
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    Quote Originally Posted by Vasilis View Post
    By gentle, I mean the whole piece of steel would contain the same concentration of martensite/ferrite/perlite/bainite and any other formation. The damage on quench happens because the size of the martensitic crystals is different from the ones on ferrite/perlite; the one expands, the other stays the same in the end, and, on the weakest part of the blade/place where that formation was the harshest, a crack is formed.
    More or less, with some debate, this is true. Watch: https://www.youtube.com/watch?v=viqrOAG13Q0
    This is a much larger blade than a razor but the same forces are at work. There is a lot of movement happening in a very short time, eh? I have seen similar smiles in razors of only four inches length.

    Re: minor alloys with sometimes major effects. A low-hardenability steel, e.g. only iron and carbon, may not completely form martensite as the section thickness can determine how fast or slowly the steel will cool. The edge can get hard but the spine may not harden as much because in the half second it takes to cool, it's not fast enough to form martensite and falls back through to pearlite.

    Add manganese in a small amount and the same blade will through harden. The material composition will affect heat treatment performance as well.

    I will just stick this out there...I do not quench in brine. I see no reason to do so using a well-prepared known alloy of steel and do not recommend it unless you like to gamble on how many blades you will get to survive this violent process. It's just not needed but persists in the Mythology of How Things Should Really Be Done. I can identify a very limited set of conditions that might require brine. Mostly they are unknown steels from the junkyard that won't harden using normal techniques.

    And, Rob Gunther's Superquench is a soap solution that addresses the surface tension. It will leave a hardened thin skin on even low carbon steels. The first honing will remove anything good about using this on a cheap steel to achieve a quick result. It's best to use known materials of good quality and good heat treatment rituals.
    Last edited by Mike Blue; 12-07-2016 at 05:01 PM.
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    I've been interrupting my quench at somewhere around 450 F (a SWAG and a 480 degree temp stick) The last time I did this I was thinking that the steel, in theory, should still be austenite at this point. So directly out of the quench I checked the steel with a magnet and low and behold it was non-magnetic despite being almost a 1000 degrees below the curie temperature. Ain't science great!

    Just an interesting side note, mildly related to the topic.

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