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

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  #1  
Old 11-24-2013, 12:38 PM
Ed Tipton Ed Tipton is offline
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I should know this...but

I have been making knives successfully for some time now, and I have had good success with my HT, but I still have this nagging question in my mind.
Typically, I heat my blades up until they reach non-magnetic and then go just a touch beyond that prior to quenching. My blades do harden and they retain their keen edge through hard use. My question is why is it better to catch the correct temp on the increase as opposed to getting it even hotter and then quenching the blade as the temp backs off to the optimum temp.
I am wanting to get an infrared non contact thermometer and try using it to determine the correct point on decreasing temp to quench...but I have this nagging feeling that someone is going to tell me this approach isn't going to work. I do realize that I could also use it to determine the exact point on increasing temp, but it just seems like it would be easier, simpler, and more exact to overheat it and then while poised over the quench tank, watch it cool back to the optimum temp and then plunge it into the quench at the perfect temp. I don't have enough metallurgical knowledge to understand why this wouldn't be better than guessing at how far past non-magnetic I should be heating the blades.
As you've probably guessed, I am using 1080, 5160, CRU5v, and occasionally 52100 steels for my blades....all relatively simple steels to HT.
Something in my make-up tells me that if it ain't broke...don't fix it......but I am constantly looking for improvement...and I have this obvious hole in my knowledge base.

Last edited by Ed Tipton; 11-24-2013 at 12:45 PM.
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  #2  
Old 11-24-2013, 01:36 PM
WBE WBE is offline
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Hi Ed. It is best to catch it at the desired temp, then allow a short soak of a few minutes, but without good heat control that is difficult. As far as going above the desired heat, it depends on how much, and for how long. If you're looking for improvement I would suggest a HT oven in your future. Otherwise, if your results are working for you, I'm not sure if minor changes are going to be noticeable, but you might experiment a little and see.
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  #3  
Old 11-24-2013, 01:58 PM
jmccustomknives jmccustomknives is offline
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Why don't you test your theory? It's easy enough to do. Do once piece the way you've been doing and one the second way. Do the break test. I suspect you'll notice grain growth in the second piece.
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  #4  
Old 11-24-2013, 04:31 PM
WBE WBE is offline
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Quote:
Originally Posted by jmccustomknives View Post
Why don't you test your theory? It's easy enough to do. Do once piece the way you've been doing and one the second way. Do the break test. I suspect you'll notice grain growth in the second piece.
That would depend on how hot, and for how long, anyway you cut it. Pun intended.
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  #5  
Old 11-25-2013, 10:19 AM
Ed Tipton Ed Tipton is offline
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WBE...From your response, my take on this is that there is no inherent problem in catching the optimum temp as the temp is backing down provided that the overshoot is not extreme and that the time is not excessive. My set-up does not lend itself to prolonged soaking at a specific temp, and my financial situation pretty much rules out getting an oven at this time.
My thinking is to use an infrared thermometer to monitor the increasing temp....try and leave the blade in the fire for as long as I can without greatly overheating it...and then slightly overshooting the optimum temp and monitoring closely as the temp decreases...and then plunge the blade into the quench. To me, this seems better than simply guessing at what would seem like a good temp to quench at.
Prior to doing all of this, I would go through three thermal cycling events to try and reduce the grain size as much as possible. Maybe I'm making too much of all of this, but I am simply trying to gain more control of my process and eliminate guesswork as much as possible.
Also, is there any benefit to doing more than one hardening event? It seems to me that by re-heating the blade after it has been hardened would erase the initial hardening attempt. I have read entries from Mr. Doug Lester where he talks about "retained austenite", and by improving the blade by doing multiple hardening cycles to reduce the amount of retained austenite. I will be the first to admit that all of this is above my pay grade....so I am asking for some clarification and help in understanding all of this.
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  #6  
Old 11-25-2013, 03:23 PM
WBE WBE is offline
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With limited equipment, you might want to experiment and see how multiple quenching works for you. 52100 and 5160 may benefit some because of the high chrome content, but the 1080 will not. CRU5v, I know nothing about. You can reduce RA by water quenching from each temper heat. Not much, but some. In a perfect hardening set up, with the correct quench medium and controlled heat, one quench does the job, or something was not done right in the procedure, with the possible exception of 52100. In re-heating a quenched blade, you will do no harm other than adding a bit more decarb and adding more risk of cracking with some steels, but for most steels you will not gain much improvement, if any, if your procedure is right for the first quench. Those who obtain gains from multi-quenching simply did not get the first one right. Again, with "possible" exception of 52100. I will say, that multi-quenching in normalizing is helpful to gain finer grain, but not in the acuall hardening, and even then you add to the risk of cracking with some steels.

Last edited by WBE; 11-25-2013 at 03:39 PM.
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  #7  
Old 11-25-2013, 06:18 PM
Ed Tipton Ed Tipton is offline
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JMCustomknives...I do test my blades, and I do intend to continue with my testing, but If asking a question will save me time and material I will do that as well. I understand your comment about increased grain growth...and you may be correct. Obviously, if I overheat a blade by an excessive amount, then I also would expect to see grain growth...but it is my intention to overshoot by only a small amount. What I am trying to do may prove out to be more difficult than I expect...and if so...so be it...but this is something that I at least want to try. I will give an update on the results as well as any benefit or problems encountered. I guess the bottom line is nothing ventured...nothing gained.
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  #8  
Old 12-17-2013, 05:32 AM
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Crex Crex is offline
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I think the biggest trick will be actually getting the timing right and the "shot" from the IRT in the right place. You are working with distal tapers therefore the thinner parts will cool faster while you are holding for the plunge and watching the drop in temp. The IRT's that I have worked with in the past were not instantaneous but had a slight time delay, enough for a pretty substantial temp change in the thinner areas (which are most critical, unless you plan to grind off a lot of material later).
Haven't used an IRT in a few years and technology changes so..... may be a mute point.

How's your color discernment? Your brain/eye is a much more calibrated instrument than most realize. Just takes a good deal of practice and concentration. Limit variables like ambient light, air temp, air circulation, etc. and you can be pretty darn close. Most likely as close or closer than watching the IRT display.

Also, the overheating thing would concern me as the thin areas will get hotter quicker while you wait for the rest to catch up (not so in an oven). I'd worry about grain growth issues in the thinner areas.
It's a hard balance to maintain. How much control do you have over your forge's internal heat? That's where your IRT may come in more handy. Learn to adjust the forge to your desired heat if possible and you can do your temp soaks without worrying about the overheat issue. This will have a lot to do with you forge design. Some will adjust quite well some will not. That's where I'd play with changes first.


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5160, 52100, bee, blade, blades, design, easy, edge, fire, forge, harden, heat, hot, knife, knives, make, making, material, problem, quenched, temper


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