Dont drop your freshly queched 1095 blade!!
- Thread starter millejn3
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Kevin R. Cashen
Super Moderator
Believe it or not, this can avoided, even when dropping an as-quenched blade (no temper) onto concrete. How do you quench your blades?
Kevin,
This is the first blade I have drop in 10 years. It was completely my fault as I was in a hurry last night due to my 5 yr old daughter having a christmas concert and didnt follow my regular proccess of put it into a cooling rack that I use so there is no chance of it getting knocked off the bench onto the floor.
O Well.... Live and learn.....
This is the first blade I have drop in 10 years. It was completely my fault as I was in a hurry last night due to my 5 yr old daughter having a christmas concert and didnt follow my regular proccess of put it into a cooling rack that I use so there is no chance of it getting knocked off the bench onto the floor.
O Well.... Live and learn.....
Kevin R. Cashen
Super Moderator
Beyond a doubt the quickest way to avoid this "arrggg" moment is to see to it that we don't drop them, but I could share a tip that would make it so dropping them isn't a problem at all. I asked about your quenching methods because if you were aware of marquenching I wouldn't want to tell you something you already know. With an interrupted quench the auto-tempering effect will give you enough advantage in toughness that you could drop all the blades you want with no worry, except that they could chip up your floor. One of my favorite demos at the Ashokan seminar is to quench a blade (many times 1095) and then when it is totally cool, I drop I repeatedly from head height onto the concrete, often point first. These are blades that are marquenched using an interrupt at around 400F and we eventually have to lean them against the wall and stomp on them to get them to snap. The auto-tempering effect will also allow you the freedom to let your oven come down to tempering temperature with no worry of what could be happening to the fully quenched steel in the interim. When I first started full and actual martempering/marquenching of my blades using the salts I was working on a damascus dagger that I had hardened, I went to put it in a vice vertically and slipped. As it fell to the floor, point first, my heart went with it and I just sat there with my eyes closed. When I looked down I saw it standing upright and stuck in the concrete floor. Very puzzled I reached down and pulled it out to find the tip in perfect condition but my floor had a chip. I don't think I have quenched all the way to room temp, uninterrupted, since.
This would be my question as well
Kevin R. Cashen
Super Moderator
Any oil or water hardening steel can be marquenched, air hardening steels don’t really offer themselves to the technique for the obvious reasons. Be aware that actual marquenching or martempering is done using special baths of quenchants designed for those temps, but the effect can be approximated by an interrupted quench. The trick is to learn the cooling curve, and thus timing of your oil. Some guys have used the infrared pyrometers to quick check the temperature of the blade to find the right count to interrupt, this is great but I just do it old school. I have learned to count off the seconds when properly agitating the blade in the oil to interrupt at around 400F. Of course this will change a little with blade size and clay coatings will really throw me off, but if you interrupt at the correct time the blade should be wet with a light coating of oil that is giving off light wispy vapors. If it is dry and smoking like crazy it is too hot, if it is dripping wet with no vapors it is probably too cool. Ideally you want to get at close to 400F to 420F as you can. If it is 500F or better there are some issues with auto-tempering and possible unwanted transformations, if it is too cool there is little benefit.
If done correctly as much as 30-40% of the hardened phase of steel can form at temperatures where the blades own thermal mass can cause it to get a head start on the tempering operation, and that is huge in terms of avoiding brittleness. Also with steels of certain carbon contents it heads off a strain generating impingement of the hard structures components before it can happen, with quenching to room temp these impingements occur unfettered, by limiting them gains as high as 20% more toughness can be had. Also since the blade is still not entirely hardened you can put on gloves to protect your hands from the hot blade and gently straighten any warps as they occur.
Many people make two big mistakes with the technique however. Some feel they need to put the blade back into the quench. This is not only not necessary it defeats the whole purpose of marquenching, if you quenched fast enough from 1200F to 450F, the blade has no choice but to harden and there is no reason to fast cool it any longer. The other mistake comes from mistakenly thinking they can improve on the effect by cooling slower than air all this does is cause retained austenite and other undesirable mixed phases. Also remember that if you Rockwell these as-quenched blades they will read 1.5 to 2 points lower than normal, this is not because they didn’t fully harden it is because they are already partially tempered. It is also important to remember that they are only partially tempered, they still absolutely need a full tempering as well.
The next time you do a quench give it a shot. Use a practice bar of steel and interrupt it to see how easily it will bend, (by the way a magnet also will not stick to it until around 375F.) and cut with a file. Then after it reaches room temp it will just dull the same file. Another demonstration that I like to do is bring bent blades that I straighten after the quench before they fully harden, and then toss them on the concrete.
If done correctly as much as 30-40% of the hardened phase of steel can form at temperatures where the blades own thermal mass can cause it to get a head start on the tempering operation, and that is huge in terms of avoiding brittleness. Also with steels of certain carbon contents it heads off a strain generating impingement of the hard structures components before it can happen, with quenching to room temp these impingements occur unfettered, by limiting them gains as high as 20% more toughness can be had. Also since the blade is still not entirely hardened you can put on gloves to protect your hands from the hot blade and gently straighten any warps as they occur.
Many people make two big mistakes with the technique however. Some feel they need to put the blade back into the quench. This is not only not necessary it defeats the whole purpose of marquenching, if you quenched fast enough from 1200F to 450F, the blade has no choice but to harden and there is no reason to fast cool it any longer. The other mistake comes from mistakenly thinking they can improve on the effect by cooling slower than air all this does is cause retained austenite and other undesirable mixed phases. Also remember that if you Rockwell these as-quenched blades they will read 1.5 to 2 points lower than normal, this is not because they didn’t fully harden it is because they are already partially tempered. It is also important to remember that they are only partially tempered, they still absolutely need a full tempering as well.
The next time you do a quench give it a shot. Use a practice bar of steel and interrupt it to see how easily it will bend, (by the way a magnet also will not stick to it until around 375F.) and cut with a file. Then after it reaches room temp it will just dull the same file. Another demonstration that I like to do is bring bent blades that I straighten after the quench before they fully harden, and then toss them on the concrete.
Robert Dark
Well-Known Member
My, my........ just when I think I have things figured out, along comes Kevin with a simple explanation of something I never understood.
Although I have tried this many times over the years, I never really knew why it was supposed to be a better method of quenching. Now I see the light.
Thank you so much for this.
Robert
Although I have tried this many times over the years, I never really knew why it was supposed to be a better method of quenching. Now I see the light.
Thank you so much for this.
Robert
KCorn
Well-Known Member
That is very well written Kevin. I will have to give that a shot and see what happens. My 10 year old son is always shooting things around the shop with my non contact thermometer anyway. Now I can put him to work during an interrupted quench. Thanks for your insight on the subject!
Shawn Hatcher
Well-Known Member
Agreed. Thank you so much, Kevin. You help us more than you know!
Knifemaker.ca
Dealer - Purveyor
So does Marquenching result in higher RA? I recall reading that cryo - when and if it is used - has it's best effect if done promptly so as to be part of the quench rather than an afterthought. You can see where my feeble mind is going with this. If cryo has a positive effect on conversion, does marquenching inhibit it somewhat?
Kevin R. Cashen
Super Moderator
Once again, it is a matter of simple steels vs. complex alloys. If there are RA problems with simpler steels, as most water or oil quenching are, it is mostly a matter of overheating during austenitization. Air hardening alloys are a different beast and by their very nature tend to have more stable austenite, but if a steel requires a liquid quench, almost by definition the austenite is not even stable enough to avoid upper bainite or pearlite without assistance in cooling, unless you put more than the necessary carbon into solution. Once again I will leave claims of the possibility of exotic precipitates or lattice reordering via cryo until it is definitively established by objective research, but retained austenite we pretty much have figured out and if one has enough problems with it in simple alloys to realize gains with colder treatments the problem was in the heating rather than the cooling.
An air hardening alloy has enough alloying elements added to stabilize the austenite enough to avoid any of the diffusion based phases, thus it also has the stability to resist the critical strain necessary for complete martensite conversion as well. These steels benefit for the cold treatment providing the extra strain needed to finish the job of the quench below room temperature. In these alloys tempering before this is done will result in precipitation of tempering carbide products with the best to hope for is an anemic martensite when the conversion occurs. Freezing without the precipitation these products will result in conversion of full blown martensite followed up with a more homogenous distribution the tempering carbides, a better deal all around. The fear however is that freezing all out alpha (body centered tetragonal) martensite could be too much for the steel, so often the “snap temper” is utilized rather than a full temper, before cryo. The snap temper is much lower (around 300F to 350F) and only relaxes the body centered tetragonal stacking somewhat, but does not result in significant tempering carbide precipitation nor austenite break down, which happens at the next higher temperature phase of tempering.
It is worth noting however that a total arrest of the cooling during the martensite transformation range (below Ms), has been shown to stabilize the austenite in some steels, and that is why I strongly suggest that you just allow the steel to air cool and not try to slow it any more. It would appear that so long as the cooling is continuous, you have the momentum to maintain the Bain strain driving the transformation, stopping and holding at around say M50% could get you RA but probably some lower bainite as well in these simper steels.
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Kevin R. Cashen
Super Moderator
I have seen industrial/academic studies that claimed even greater but I am a bit dubious and rather cautious with those numbers, the 20% comes from that information more conservatively backed up with my testing and a colleagues who worked with an independent lab for the impact results.
Mark Andrews
Well-Known Member
Interesting topic. Glad I stopped by for a look. Thanks Kevin.
Eddie Mullins
Well-Known Member
Kevin - Thanks for sharing this info.