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

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  #1  
Old 02-28-2011, 07:31 AM
Ed Tipton Ed Tipton is offline
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Differential hardening or quenching vs. differential tempering

OK guys, I need some help in sorting out all of my information. I have just recently been introduced to 52100. Prior to this, I restricted myself to using only 1080 and 5160.

With 52100, I would like to produce a blade that is differentially treated...but I do have some questions.

If I use a "regulator plate" as does Mr. Caffrey during the quenching/hardening phase, this should produce a blade that is softer on the spine than on the edge. If I completely immerse the blade into the quenching oil, I will get a blade that is hardened throughout from spine to edge.

It is my understanding then, that by differentially tempering the blade that the hardness in the spine can be "backed down" to give a blade with greater toughness, while at the same time leaving the edge hardened to a greater extent..
My question is:
To get a differentially hardened blade, would it be better to differentially quench, differentially temper, or can such a blade be achieved equally well by either technique.
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Old 02-28-2011, 09:47 AM
The Tourist The Tourist is offline
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T o be honest, I know nothing about 'modern' hardening except for what my BIL--a metallurgist--tells me about the steel I polish. But I would turn your attention to history.

I always remark that "my future is in the past." Briefly, I like to study and consider why sword cutlers and polishers in feudal Japan took the course they did. We have 800 year old swords that rival edges we create now. This did not happen by accident.

If you look at the creation of a hamon (or also called a 'hamon line') then you'll know that an application of slurry was applied to the blade blank. This created a tough spine and a glass-hard edge. In effect, they did their hardening jointly and severally.

Admittedly, this process is never practical for an assembly line. But in crafting a custom blade it might be worth a bit of time to study.
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Old 02-28-2011, 02:27 PM
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Ed Caffrey Ed Caffrey is offline
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I have done it both ways, and I choose to differentially harden. The reason that I choose this method is that differential tempering is difficult to correctly accomplish. To explain: Everything to do with steel is about time AND temp.....a specific temp, or a specific period of time. Most often than not, when people try to accomplish differential tempering or "soft back draw", they wind up with a tempered "skin" of about .005 to .010", with the remainder of the blade staying "as tempered". With 52100 you also have to realize that the steel does exhibit some air hardening characteristics, and even when an edge quench is conducted, there is a small degree of hardening that takes place in the spine.

The key...and the ease of creating a blade using differential hardening is that you have the ability to dictate how limber or stiff the blade will be, simply adjusting the depth of the quench. This gives me much more control and predictability than trying to soft back draw a blade.

When I really started to notice this was during testing folks for their JS.....those who chose to differentially harden generally had no problems getting through the tests. Those who fully hardened their blade(s), and then tried to soft back draw, often failed. After questioning those individual who had failed, the common theme was that they would heat the spine of the fully hardened blade "until it was blue", and when their blades broke, the tempered "skin" that I mentioned previously was evident.
IF a person can conduct the differential tempering is such a manner that they can achieve full tempering of the spine, it will work....but that requires at least an hour of soak time, and generally more, at the desired tempering temp.


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Old 02-28-2011, 02:43 PM
Doug Lester Doug Lester is offline
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Ok, Tourist, to start with the anchient sword smiths of Japan, or anywhere else, had no idea of why what they did worked. What they knew about iron was only on the macro level. They didn't know about carbon; it was just charcoal to them. They only knew what was achieved through centuries of trial and error and included a lot of superstition. As far as blades that rival modern products, that's something that just doesn't stand up. Yes, they produced very good blades, they also produced mediocre blades. The old steels just do not stand up to modern alloys. If the old steels were better we'd still be using them. These methods are not lost, they are still being practiced by a few masters of the art and what goes on with their processes are well understood.

Ed was asking about 52100 which is a deep hardening steel totally different from the tamahagane steel of medieval Japan or modern simple steel, for that matter. Painting a slurry of clay on the blade will not retard cooling long enough for pearlite or bainite to start to form. The steel will still go right past the nose of the cooling curve to the Ms point.

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Last edited by Doug Lester; 02-28-2011 at 02:46 PM.
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Old 02-28-2011, 03:15 PM
The Tourist The Tourist is offline
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I understand. However modern swords are made with modern steel. It is still hammered and folded on a forge. They stil use slurry. In fact, the best hammered and folded kitchen knives I sell have a core of VG-10 stainless steel.

I will comment on one factor in sword making. I do not know where it is, but there is still a riverbed where they "wash out" the iron deposits in the mud on the banks to make "traditional" swords. Supposedly this area has 'nature treasure' status and the refining of ore is closely monitored.

My point was a lot simpler. I'm not suggesting that we all stir up some slurry, but the ideology might be worth a look. After all, I don't sit on a low stool and have my clients address me as a sensei togishi. But I did steal the idea of using waterstones, paste and glass to polish modern kitchen knives and a lot of folders.

My comments addressed an overall study.
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Old 03-01-2011, 10:49 AM
Ed Tipton Ed Tipton is offline
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Thanks for all responses.
Ed, that was exactly the type of info I was looking for...and your answer did validate my understanding of the process as well. I was not aware of the "skin" effect you mentioned and in my attempt at tempering with a torch, I had a failure that may well have been the result of the steel not having been completely through-tempered...and remaining more brittle than I had thought it should have been.
When my blade broke, it revealed excellent grain, and had it been completely through-tempered, I don't think it would have broken. It's all part of the learning process, and the lesson was not lost. The learning never stops, and just when you think you've got it figured out...something jumps up and bites you in the a$$...but that's what makes it all worthwhile.
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Old 03-01-2011, 03:07 PM
Doug Lester Doug Lester is offline
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Ed, reading your reply about trying to do a soft draw on the temper, would it be better to put the edge of the blade in water or wet sand for a heat sink and then applying heat to the spine?

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Old 03-01-2011, 03:33 PM
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Ed Caffrey Ed Caffrey is offline
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Doug,

Actually, if you want to get the spine to temper all the way through, that's about the only way you can do it...otherwise with the duration of heat required, you wind up softening the cutting edge.
This is not any kind of hard and fast rule, but for me to successfully get a spine to tempered all the way through, its generally 30-40 mins with the heat applied to the spine. It's difficult to do, which is why I favor using the depth of an edge quench... just more predictable and controllable.


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Old 03-01-2011, 05:07 PM
Doug Lester Doug Lester is offline
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Thanks, Ed. It looks like when I start playing with being able to get a 90 degree bend in the blade without breaking it I'll go back to edge quenching.

Doug Lester


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Old 03-01-2011, 06:45 PM
Ed Tipton Ed Tipton is offline
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OK then...If I understand correctly, the best,most controllable and easiest way to get a thoroughly softened spine would be to do a differential quench where just the edge is hardened, followed by 2 hours in the toaster oven at say...350 degrees...and then follow that up with the edge submerged in water and a torch applied to the spine? Does that sound correct...or am I beating a dead horse here? I don't want to do unnecessary work and I really don't even need a thoroughly softened spine....but I would like to know how to get there. Does thei discussion apply to basically all types of steel...or are we just talking about 1080,5160, and 52100.
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Old 03-01-2011, 09:21 PM
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If you do an edge quench, the spine remains relatively soft, and no soft back draw is necessary. My process is to anneal a blade after forging, then do my rough grind, and then, when I'm heating the blade for edge quenching, I try to heat ONLY that portion of the blade that I intend to quench to critical.....then the only portion of the blade that hardens is that portion that gets submerged in the quenchant. Depending on the steel type I then temper for 3, 2 hour cycles, allowing the blade to cool to room temp between tempering cycles. You mentioned "toaster oven", which throws up a red flag for me. Toaster ovens are notoriously inaccurate....often swinging 50 or more degrees as they cycle. Every 25F will produce a difference in the steel's matrix...so if you're using a toaster oven to temper, NEVER believe the dial setting unless you verify it by other means.
This applies to MOST "forgable" steels....once exception that comes to mind is O1. All of the steels you mentioned...the information would apply to.


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Old 03-01-2011, 10:47 PM
Doug Lester Doug Lester is offline
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I've worked with 52100 and I would recommend that you start out tempering at 400 degrees. I austemper my 52100 blades at 430 degrees for three hours. If I can overide the stop on my eletric roaster, I plan to change that to quench to 425-430 degrees for a few seconds and then up quench it in oil to 510 degrees for an hour. Both ways should produce a mixture of martinsite and lower bainite, hopefully with an HRC of about 59-60 but the data on the second method is more solid. I have a data sheet from ASM International on order. It only cost $20 and, hopefully, will give more information than I have now without paying $255 for the Heat Treater's Guide.

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Old 03-02-2011, 07:59 AM
Ed Tipton Ed Tipton is offline
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This discussion leads to another question: I do know that by applying clay to the spine you can limit the amount of heat that is applied directly to the spine and thereby achieve a level of differential hardening. Aside from using a torch with a directed flame, is there any other accepted metheod of applying enough heat directly to the edge while at the same time limiting the amount of heat getting to the spine...for example, could you clamp the spine between two "plates" of heavier steel in a vise leaving only the edge exposed and apply heat directly to the edge using a torch while allowing the plates to act as a heat sink on the spine? I can see where this metheod could work, but I am concerned that the edge might not be heated equally throughout it's length....but perhaps with enough "back and forth motion" with the torch it could be done. Comments please.
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Old 03-02-2011, 10:14 AM
The Tourist The Tourist is offline
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Quote:
Originally Posted by Ed Tipton View Post
I do know that by applying clay to the spine you can limit the amount of heat that is applied directly to the spine...I am concerned that the edge might not be heated equally throughout it's length.
You used the term 'clay' and I used the term 'slurry.' Same principle.

Perhaps a study on how folded steel was hardened might offer suggestions on how cutlers can apply this uniform HT. After all, a katana is over 30 inches long.
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Old 03-05-2011, 02:24 AM
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The clay does not stop the spine from getting hot. The clay slows the cooling of the spine. This is very similar to leaving the spine of a uniformely heated blade in air while quenching the edge.


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