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Heat Treating and Metallurgy Discussion of heat treatment and metallurgy in knife making.

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  #1  
Old 04-08-2018, 11:22 PM
Michael Grieb Michael Grieb is offline
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52100 heat treatment in a gas forge

If I am just trying to do a heat treat on 52100 can i take it to 1475 and quench, getting decent results or do i need to go through a more complicated song and dance? I have seen more random crap about this steel than any other and i have some coming in the mail.
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Old 04-09-2018, 08:58 AM
Kevin R. Cashen Kevin R. Cashen is offline
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Hey Michael, I hear what you are saying, I have joked before that I am scared of working with 52100 because it appears to have some sort of toxic elements in it that burn out when you heat it and gets into smith’s brains. It seems that every time I hear some convoluted fantastic ritual involved in heat treating a steel, I will stop the person and say “don’t’ tell me …52100?”, and the answer will be “yes, how did you know?”

Normally the reason is because people are coming at 52100 like it is a simple carbon steel that can be done with just any equipment and any temperatures. But 52100 is not a beginner’s steel that will tolerate much deviation in what it needs to respond to heat treatment. The more folks mess around with deviating from what it needs, the more hoops they start having to jump through to bring it back on track, and new whacky 52100 heat treatment is born.

Yes 1475°F is the trick, folks will heat threat this steel with a 6,000°F torch flame or an unmeasured forge fire and then the fun begins. Above 1475°F you will lose hardness to retained austenite by putting too much of that 1% carbon into solution, industry says 1550°F.
but this steel was designed and intended for bearings, not knife blades, so industry gives you the correct methods to make a bearing not a knife, the bearing process deals with retained austenite in different ways.

If you have forged and normalized the steel, 1475°F and a quench into a medium speed oil, (1.4% Chrome means this is not 10XX series or W2 that needs a fast quench) should get you to 65HRC with no problem, I have gotten 67.5-68HRC at times using this method. If you find your HRC locked down around 63, you may need to free up some heavily spheroidized carbon by backing off your annealing cycles a bit and going for hotter normalizing before some follow up refinement cycles.

Last edited by Kevin R. Cashen; 04-09-2018 at 09:18 AM.
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Old 04-09-2018, 02:07 PM
Doug Lester Doug Lester is offline
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When I read your heading I just said to myself "now there's his problem". As Kevin said 52100 is not a beginner's steel; it's also not a steel for heat treating with a forge. Don't get me wrong. It can be done by looking for decalesance and recalesance and shortening your soak times a bit it's just my opinion that you can't do it consistently. You may think that you are soaking the steel at 1475° F but in reality with out a regulated high temperature oven you don't know if it's 1500° or even 1600°. A moderately complex steel like 52100 just isn't designed to heat treat with a forge.

Doug


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Old 04-10-2018, 02:38 AM
Michael Grieb Michael Grieb is offline
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Ok, so if I am using stock removal I shouldn't need to normalize? Or is the way it comes from the factory not good for going straight into heat treat? Also, if retained austenite is the problem, could I do a cryo treatment to solve it? Once again, I am obviously not asking for the optimum solution, I am just trying to figure out a solution that I can perform with the tools I have, as I ordered about $100 of the stuff before I realized what I was getting myself into and kinda want to experiment with it now that I have it coming.

Thanks for all your help, Kevin and Doug.

Ps: for reference, I have only ever heat treated 1084 before, so this could get real ugly or real interesting real quick. 😂😂
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Old 04-10-2018, 03:49 AM
Doug Lester Doug Lester is offline
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Your stock should be coming already normalized and ready for profiling, grinding the bevels, and drilling the holes. You will still probably want to heat cycle the blades to relieve any stresses, or at least even them out, that occurred during grinding and to refine the grain. The problem is going to be with putting the optimal amount of carbon into solution to hit that sweet spot that will give you maximum hardness without creating retained austenite. As I said, this is a little hard with just trying to spot decalesence and holding at that place for a few minutes. With 1084 you can put all the carbon into solution with the iron and create very little retained austenite. There's just not that much carbon in the alloy.

Yes, you can try to get around the retained austenite problem by soaking the blade in liquid nitrogen. The problem here is that the dewars for the liquid nitrogen are not cheap. I don't know about the liquid nitrogen. People have different opinions if you can get 52100 cold enough to get the retained austenite to convert to martensite with a dry ice/acetone bath. If you try that do it outside and away from flames.

Doug


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Old 04-10-2018, 09:10 AM
Kevin R. Cashen Kevin R. Cashen is offline
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Much of this depends on which 52100 you buy. Much of it on the market today is heavily spheroidized. By heavily spheroidized I mean that the carbon in the steel is locked up in larger spheroidal carbides, which are very stable, and slow to dissolve. This will necessitate a longer soak at precise temperatures and in some instances that is not enough. Forging at proper temperatures is your normalizing and will undo much of this, it is also beneficial to follow up with proper normalizing. If you are stock removing this steel there is no forging to undo the spheroidization so normalizing becomes even more necessary if it is heavily spheroidized.

Liquid nitrogen for retained austenite on a low alloyed steel such as 52100 is merely putting a band-aid on the problem rather than avoiding it to begin with. If one sees a huge jump in HRC from freezing 52100 they overheated it in hardening. You may encounter this problem when heating in a forge if your eye isn’t really good at holding the temp you want.

If you have the ability to dial your gas forge down and have access to a pyrometer, this may still be not much problem. Do all of your heavy cutting and machining (especially drilling) then normalize the blade with a little final grinding left to do. Then you will be going from a very fine pearlitic/upper bainitic microstructure into the heat treatment, this will eliminate much of the need for demanding soak times. If you keep your steel just above decalescence (the shadows have just left the blade) and then quench, you should be able to get pretty acceptable results.

There are different levels of results, terrible, better, acceptable, real good and optimum. As we have already discussed, with all the wackadoodle heat treatments people are using on this steel and still liking the results, it is safe to say that there is a very wide latitude of results that will still make a serviceable blade for most people. We sweat over one or two percent because that is what drives us to get better, but for the most part is beyond what most will notice.
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  #7  
Old 04-10-2018, 12:53 PM
Michael Grieb Michael Grieb is offline
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Thanks again for the help, guys. Its hugely appreciated. Is steel i get from Aldo likely to be spheroidized? (thanks for explaining this term. I had no idea what people were talking about before) If i want to normalize it, what kind of a normalization cycle should I be shooting for and how much should I be worried about target temps for that? If i decided to get a pyrometer or make one, where in the forge should I be temping and how can I guarantee that I am hitting that temp in the blade metal?
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Old 04-10-2018, 01:18 PM
Michael Grieb Michael Grieb is offline
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I had a thought that might be a really really dumb idea, but might work out. I know that salt baths are a decently common method of heat treating. The melting point of salt is 1474 farenheit. I have access to an oxypropane/oxyacetylene torch. If i made a box and melted salt in it, could i reasonably control the temp by giving it a bit more juice from the oxypropane whenever i see parts of the salt bath starting to solidify?
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Old 04-10-2018, 02:00 PM
Doug Lester Doug Lester is offline
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Don't go simple on molten salt baths, those things can kill you. Find plans to build one from someone who has built one. First of all I know that you don't want to weld together a steel box from simple steel. You need stainless steel pipe because the salt will eat the plain steel for breakfast, lunch, and dinner. Be aware that if you introduce ANY water into the system you could be facing a volcano of molten salts.

The strong points is 1) you can hold a precise temperature for a long period of time if it's built with a PID regulator that cuts the gas off and on and 2) it prevents oxidation by blocking out atmospheric air from the blade.

Doug


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Old 04-10-2018, 02:27 PM
Michael Grieb Michael Grieb is offline
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Lol. Good call on the stainless. I did not immediately think of that but it makes a lot of sense. Is the phase change from solid to liquid readily apparent so i can manually turn it on and off or is electric control wholly necessary?
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Old 04-11-2018, 03:04 AM
Doug Lester Doug Lester is offline
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You can turn it off and on and regulate the temperature with a pyrometer inserted into a stainless steel tube sealed off on one end. After all we are just talking about keeping the 52100 at temperature for about 10 minutes instead of the 30 minutes to an hour that some of the "super steels" take. Just be aware that there is a lot that goes into making a molten salt pot that is safe to use. I understand that you will need a tapered stainless steel rod inserted into the molten salt as it solidifies so that when you heat it the next time the molten salt in the bottom of the pipe does blow out due to pressure against the solid salts above it.

I repeat myself: Learn about molten salt pots and how to build and operate them before you attempt one.

Doug


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Old 04-11-2018, 08:21 AM
Kevin R. Cashen Kevin R. Cashen is offline
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Aldo’s 52100 is very good steel, but it is very heavily spheroidized for easy machining, you will want to normalize it at 1650°F to increase its responsiveness to heat treatment.

Please forget any plans of improvising a salt bath. I have been working with salts since the mid 1990’s, there are only a handful of us knifemakers who have that many years getting to know them. My setup has always been fairly state of the art and I have almost been killed more than once using them. I really believe that salt baths are a bit on the overkill side for the vast majority of knifemaking applications. With the capital you would expend on a safe and reliable salt bath, you could have a nice oven and a good atmosphere purged oven will get you to most the places a salt bath will, and without the threat of violent, ugly death.
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Old 04-11-2018, 10:27 AM
Michael Grieb Michael Grieb is offline
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Ok. So if we put the idea of the salt bath out of mind, how long should I have the steel at 1650 to normalize and do I need to let it come all the way back down to room temp before i austenitize or can I just take it below critical then heat it right back up? Will taking it up to 1650 cause enlarged grain structure or is the final austenitization temp the only thing that matters? Should I be measuring the temp near the burner of the forge or further away? (It's a 2 brick forge with a 3 inch cylindrical opening 9 inches deep)
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Old 04-12-2018, 12:37 PM
samuraistuart samuraistuart is offline
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Thanks Kevin for taking the time to discuss the "How to heat treat Aldo's 52100 steel". I get tired of doing it. I first learned it from you, and discussed it with a few others just to make sure (not that I doubted you, I just don't want to be guilty of only listening to one version of how to do it). Since I followed the recommended method you showed us, and showed us WHY, I have gotten consistent results with Aldo's 52100. 66+hrc post quench, every time. I do the 1650f normalizing for 20 minutes, just to make sure that carbon is freed from it's spheroid bond, and then air cool it to room temp. Then I thermal cycle it around critical temp to help make sure aus grain is as fine as I can reasonably get it, and will usually quench on the final thermal cycle, around 1425f. I then austenitize at 1475 for 10 minutes, and quench in 130f canola (a commercial medium speed oil would be nice, at least I have P50 for W2 and 1095). I do put it in the 0°f freezer for 30 minutes, not for a bump in HRC (that isn't going to happen), but just "peace of mind" in that I am getting the most retained austenite I can convert. The way I see it, 0°f is better than 75°f room temp. I bet I am only converting 1% of RA, if that! But it is for peace of mind, nothing more. LN2 certainly isn't needed, and dry ice/acetone is a hassle to go get, and isn't needed either, really. My post quench HRC is around 66-67, so I have to temper much hotter than the industry charts show.

Interestingly, the 52100 from Chuck at AKS indeed is finely spheroidized. I can skip the normalizing and cycling and go straight to hardening. Same results, and quite a bit of time and energy saved.
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Old 04-13-2018, 09:46 AM
Kevin R. Cashen Kevin R. Cashen is offline
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The reason I have the insight that I do in Aldo’s 52100 is when he started carrying this particular product, he started getting phone calls from some folks about the lower hardness. This bothered him, Aldo wants to offer a good product and so he wanted to know what was going on, so he called me. We discussed it for a bit and I asked him for the certified chemistry, and then asked him to send me a sample. I also inquired about some of the folks that were having trouble, it is a small business and we tend to know each other and our particular approaches to working steel. I now had a pretty good picture of what was going on.

When the bar of steel arrived, I sliced a sample off and prepped it for metallography, it was between 95% and 98 % spheroidized with larger spheroids. I then heat treated like normal and could not push it above a 63HRC with standard heat treatment regardless of how far out I pushed the soak time. But samples of this under the scope showed lots of lower carbon lath martensite littered with undissolved spheroidal carbides; umm hmm, I said to myself.

I did another series of tests and gave Aldo a call. “How does 65.5-66 HRC sound to you?” I asked him. Obviously, he was very eager to hear what I had to say. I assured him that his 52100 was of excellent quality but was prepared for the easiest possible machining. All it needed was a good normalization at 1650°F to dissolve those large spheroids and homogenize the solution, with a subsequent heat treatment it would then respond as well as any other steel.

After normalization one can carefully refine grain if they want but they need to leave the carbides alone as much as possible, so they should avoid over doing it at lower temperatures. You see, one of the clues I was working with, before I even got the steel, was the folks who were having troubles, who happened to be bladesmiths. Forging should have the same effects as normalizing, but I knew some of the folks really like low temp cycles, in forging and normalizing. By not homogenizing with a good temp, the low temperature cycles were only making the problem worst. The divorced eutectoid reaction relies on particles to grow the spheroids on, existing spheroids will gladly volunteer, and grow quite fat in the process.

Last edited by Kevin R. Cashen; 04-13-2018 at 10:54 AM.
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