Multiple knife heat treat

scott.livesey

Dealer - Purveyor
Hello,
I just bought a front loading kiln(Paragon E9A) that is big enough for several blades. So, I put in 3 blades, heat to temperature(1500F), soak for recommended time, open door, remove blade 1, close door with left hand while putting knife in quench with right. Thermocouple says temp is down to 1400F, heat back to 1500F. How long do I wait before removing next blade? Is my soak clock at zero or should I just give it a minute or two once kiln is back to temperature? Would O1 be different from 1084 or 52100?
thanks for the help
scott
 
I do much the same as you describe, but with an EvenHeat oven. I only lose 15 to 20 degrees between blades. I allow the temp to come back up, then proceed to the next after a couple minutes or so. If you are using 01, you really need to lower your temp to around 1475°. At much higher of temp, your carbides are robbed of some of their carbon. It basically goes into solution where it does no good.
 
In another recent discussion here I mentioned the effect of hysteresis on the Ac temperatures versus the Ar temperatures and that these are two different sets of temperatures, this is what applies here. If you achieve proper solution, even with a slight temperature dip, you will still have that solution until you approach the Ar1 line. In hypereutectoids there will be some carbide precipitation before that point but it will not be as difficult to reengage as if you were starting from a static system. I would make sure the temps are back to where you want them and then proceed but I would not worry too much about more than that. 1084 has a very wide range with no proeutectoid precipitation, so your temperature could drop more than almost any other steel and you would still be fine, it is yet another reason that I cannot recommend this steel enough for beginners. O-1 and 52100 both should be heated 25 degrees cooler at around 1475F., as LRB mentioned not only do they have more carbon in carbide than should be used for solution, but the real problem is retained austenite if those carbides are dissolved. There will still be plenty of carbide left for abrasion resistance but Rockwell will plummet for every degree over 1480F for the 52100 and 1500F for the O-1. These are two steels that tell me a lot about a makers temperature control if they claim to be making gains in Rockwell by chilling or freezing. Bumping up Rockwell by freezing with these steels is merely putting a band-aid on the symptom instead of fixing the cause, which is overheating during the soak. Any carbon over the eutectoid point in austenite solution is not only unnecessary for full hardness, it can prevent it. The ideal situation is around .8% in solution for maximum martensite hardness with the rest in the form of very fine an evenly dispersed carbides, thus you obtain the full strength and greatest abrasion resistance.
 
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I've always done blades that require a soak one at a time. Its good to know that I can soak several blades and not "lose" the soak time. I can't seem to find a graph with Ar1. Could someone provide a link or give an approximate temp. for Ar1.
 
I've always done blades that require a soak one at a time. Its good to know that I can soak several blades and not "lose" the soak time. I can't seem to find a graph with Ar1. Could someone provide a link or give an approximate temp. for Ar1.

You can probably find them but it is not essential that you follow them to the letter since the Fe- Fe3C diagrams will not account for any alloying which virtually all modern steels will have to some extent. Most Fe - Fe3C phase diagrams will often only show A1, A2, A3 or Acm for simplicity sake, since the other limitation of the diagram is that it only applies under equilibrium conditions. The Ac (arete chauffant) and Ar (arête refroidissement) are normally included mostly to illustrate the differences that non equilibrium conditions can make.
 
Please correct me if I'm wrong but from what I can gather as long as the steel has been properly soaked it can safely fall to approx. 1350-1375 before "losing" any effect the soak has attained. So all that is necessary is to bring the steel back to the proper Austenitizing temp., equalize for a couple of minutes, and you're ready to quench the next blade.
 
Please correct me if I'm wrong but from what I can gather as long as the steel has been properly soaked it can safely fall to approx. 1350-1375 before "losing" any effect the soak has attained. So all that is necessary is to bring the steel back to the proper Austenitizing temp., equalize for a couple of minutes, and you're ready to quench the next blade.


Not exactly. The small point of a quick reheat back to proper temp was also part of the equation in this thread. The closer to the eutectoid that you get, the safer what you suggest is Darrin. But the more hypoeutectoid or particularly the more hypereutectoid you get, the more you will have proeutectoid (extra Carbide or ferrite constituents) phases precipitating out as you lower the temperature slowly before you begin making pearlite. But before anybody panics we must once again emphasize the equilibrium conditions of the Fe - Fe3C diagram. In air cooling the amount of carbon movement will not be as fast or as far so in operations such as normalizing most the proeutectoid phases will be incorporated in the pearlite (i.e. more carbide of ferrite lamellae). In the oven the conditions are a bit closer to equilibrium and there could be some precipitation but it will be limited and will be easily dealt with on the rebound. Also remember that just because the cool air of the room replaced some of the heated oven atmosphere and was read by the thermocouple, this does not mean the blades precisely followed suit.

With hypereutectoids this should be less of a problem if your soak was accurate enough to only put the required amount of carbon into solution, then all of your extra carbide will already be in the carbide form that you want anyhow. But if you are overheating there will be a greater carbon concentration to give you problems with retained austenite so the precipitation of the proeutectoid phase may be doing you a favor.
 
Ok thanks, I guess I'll just keep doing them one at a time to be sure I do it right.

That's a good solution. Even with the open ended muffle, where those types of fluctuations aren't a problem, I do one at a time just so I can give each blade the attention it deserves. To me, custom heat treating is a significant part of what makes a knife "custom". :)

I think it's only natural to try and save time, cut a few corners when ever practical,... but at some point it may start to diminish the quality of the end product.

Quality, as it pertains to handmade knives requires patience, attention/concentration and a certain amount of time. It's something I surrendered to a long time ago and haven't regretted it. Getting things right the first time, saves time.
 
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For what it is worth, I could fit several blades in my salts at one time and there would be zero fluctuation of temperature as I took them out, but I still do them one at a time. The main reason for this is that want to give individual attention to each blade as is forms martensite in the marquenching technique. I guide each blade to full hardness, often while holding it in my hands, watching for any problems that I can nip in the bud. I could also quench several blades from high temp salts to low temp salts at the same time but since the marquench to M90% can take several minutes this would leave the other blades in either the high temp bath or low temp bath for an increasingly longer time with each quench and that would eliminate the precise consistency in the process that I insist upon. If left in the high temp the austenite solution would be inconsistent, and of left in the low temp there could be the danger of bainitic precipitation, which not only don't I want, it makes straightening any distortion much more difficult and defeats the whole point of marquenching. I also wouldn't have time to observe each blade through the martensite formation, especially if one did have some problems. I do suspend multiple metallurgical test samples at the same time and quench them together in order to guarantee that they are all exactly the same, but blades are individual creations to me. For my OCD to do blades any other way would be like the doctor who is to deliver a baby leaving just as the head is crowning; I prefer to be there for the full birthing process of each blade.

This is not a criticism in any way of Scott's approach, it is just one of those quirks that are presented by the marquenching process for me.
 
I rarely HT more than half a dozen blades at any given time (usually just 2 to 4), so it's not a super big deal to me if I only do one heat and quench at a time with approximately 12 to 15 minutes of oven time per small 1095 blade anyhow.

That said, I've hesitated in the past to do 2 or 3 blades in a single heat, simply because I was afraid of encouraging some decarburation each time I opened and then closed the door.

I don't have a compltely air tight oven, but I guess I've convinced myself that every little bit helps.

Am I completely off base here?
 
Anyone know if opening/closing your oven encourages decarb in any way? Or is this negligible?


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not trying to do short cuts, just trying to be efficient. if doing 10 small knives of O1 or 52100, would take almost 5 hours to do one at a time. two batches of 5 would be about 90 minutes. i am a part timer, so every second in the "knife shop" (10'x15' section of basement) is precious. if the office is calm this afternoon, will try to get temp controller and SSR wired and ready to connect to kiln.
O-1 question: numerous sources say don't cool to room temperature before tempering. how critical is this? should i have tempering oven right next to furnace? is keeping quenched blades in the hot water(about 150F) used to control quench oil temp(maker says 125 to 140F) ok?
the old sailor
 
It really just depends what you are trying to do.

If warping, decarb etc., are not a big concern, you should be alright with multiple HT. Personally, I put a lot of handwork, time and energy into each blade I make,... so, I don't want to take any unnecessary chances or risks at HT. I need my success rate to be as high as possible, given all the variables and circumstances. I think Kevin is coming from a similar place and shares in a bit of OCD.

Without knowing the overall process, blade geometry, finishing process etc., it is not a judgment call I or anyone else could logically make, for you or anyone else.

If multiple HT fit's your overall knife concept, tooling, work ethic, experience level etc., I don't see any thing wrong with it. The important thing is that we understand the principals involved and, as a figure of speech,... "do the best we can".

Shop commandment #1. "I am The lord and master here."
(It's your shop and your knife.) :)
 
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The problem with not cooling the O1 to where you can at least hold it without burning yourself is that if you don't cool it enough to pass through the Mf point you will be leaving some of the austinite unconverted to martensite when you put it into the tempering oven. Kevin would know better but I would thing that, especially with a 2 hour temper cycle, that you will be getting some bainite formation from some of the retained ausinite.

Doug
 
Just my experience, and only using test files, I have found 01 harder after 30 minutes than after 10 minutes, out of quench.
 
Someone on this board or another said that he found the same thing with 1095. Again, I'm wondering if this has to do with not giving the steel enough time to cool enough to complete conversion from austinite to martensite.

Doug
 
My 01 blades are cool enough to touch bare fingered when they come out of quench, although would be uncomfortable to fully grip. They are well below 400°, and at that point are going to cool faster in air than in the heated oil.
 
Below 400° doesn't count for much being that this is close to the Ms point. Even well below may not be enough. Conversion from austinite to martensite happens at the speed of sound but not all at once. It progresses as the steel cools. If you put the blade into the tempering oven before maximum conversion to martensite has taken place then you risk forming products of austinite conversion that you don't want in the blade. I would rather let the steel cool an extra half hour to make sure that maximum austenite conversion has taken place.

Doug
 
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