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

Originally Posted by Vasilis

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.
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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.

Originally Posted by bluesman7

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.

Originally Posted by Vasilis

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.

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.

To my understanding, aside from inclusions and cracks in steel, the reason blades get damaged on quenching isn't the reduction on size of the martensite crystals, while a millimeter next to it a different form of iron-carbon combination exists, like ferrite, that doesn't change shape? Steel gets literary stretched or pressed, until it snaps. Is there some other serious reason a blade breaks on quenching?
What I'm purposing is to have the whole blade, or cubic centimeter/inch to drop its temperature uniformly whether it's the edge or center (of course, the center will be a bit harder since it's further away). The liquid alloy will get hotter as you dip the blade, ending up a few degrees colder than the steel in an instant.
This liquid alloy won't work like water, a polymer or salt or NaOH and any other combination but like the outer layer of a few molecules from a piece of steel that is being quenched since the heat transfer will be similar. Theoretically, the temperature difference between the hot blade and finished-tempered blade will not matter because of that heat transfer.

The same way heat travels inside the hot piece of steel that gets colder will also be transferred out of it is what I'm saying reducing the chance for failure.
So, a fast cooling rate between Ms and Mf won't matter since it will be happening to the whole blade, and, from the center of it, to the outer layer, there will be no deformation, aside from the one because it got cold. After that, increasing the volume of the alloy, the surface of the pot with the other pot that contains water or leaving it to cool down by itself as simple as posible is up to the blacksmith-scientist.

@ Vasilis:

“If the piece of steel cools uniformly, no matter its speed, wouldn't that reduce greatly the formation of these cracks?”

“To my understanding, aside from inclusions and cracks in steel, the reason blades get damaged on quenching isn't the reduction on size of the martensite crystals, while a millimeter next to it a different form of iron-carbon combination exists, like ferrite, that doesn't change shape? Steel gets literary stretched or pressed, until it snaps."

You are right, the cracks form due to the stress of the surface of the piece cooling faster than the centre of the piece, not due to (in most part) there being slightly different rates of cooling at different parts of the surface (due to the formation of vapor jacket, which with good technique can be considered negligible anyway).

There is no way possible (when quenching austenite straight to martensite) of dropping the temperature at the surface and in the centre of the piece at the same rate (the physics of heat transfer in steel does not allow for that). Different cooling rates means different structures present in the steel (which have different shapes/densities) which results in stress. Faster cooling rates (i.e. faster quench mediums) exacerbates the differences and leads to increased failure.

Marquenching is the solution you are looking for, as it insures that the entire piece is at the same temperature before it is cooled (relatively slowly) through the martensite transition zone. Therefore giving you your desired “uniform cooling”. Obviously there will still be a temperature gradient across the piece as it cools through the martensite transition zone, but as the cooling rate is so slow (i.e. in ambient air) the gradient can be considered negligible


“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.”

Try marquenching, alternatively:
More uniform grinding and the elimination of stress risers in the blank will also help, the former takes years of practice and the latter is a matter of simple design (and may be helped by doing more of the grinding post heat treatment so that the quenched blank is thicker and less intricate).
Quenching spine first may also help

However, blade failure is an occupational hazard for smiths using water as a quenching medium, and water is necessary (when quenching straight from austenite to martensite) to get low alloy/plain carbon steel razors to their maximum hardness (when using forms relevant to blades/razors). I recall seeing a YouTube video of the Iwasaki workshop that showed a pile of razor blanks that had failed during heat treatment.

@ Mike Blue:

“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.”

I agree (and I imagine anyone who understands metallurgy would do likewise) that brine is unnecessary for any steel when used in forms appropriate to razor/knife blade. However, Tim Zowadas post (http://straightrazorpalace.com/forge...elf-today.html) beautifully exemplifies a further set of (very limited) conditions where brine is appropriate.

“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.”

I found Superquech an interesting phenomenon that allows some hardening of “unhardenable” steel (an alternative to case hardening if you will), but you are wise in advising that it cannot in any way replace a good heat treatment of a “hardenable” steel”.

@ Vasilis:

“If I could reduce the failure rate, that would be a big bonus since grinding a blank, at least for me, takes days.”

The laminar construction and/or differential hardening techniques used (initially) by Japanese smiths may also help. By hardening only the cutting edge, the rest of the blank works as a stable support matrix during the quench to prevent (or at less lessen) the stress of hardening cracking the blade.

Originally Posted by Vasilis

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.

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.

But you are taking observations and learning from them. I am fascinated by the fact that so many "good" blades have been made without the makers having the benefits of the education available to us. Humans are keen observers and learn from their mistakes.