52100 thermal cycling

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elementfe

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May 3, 2008
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I *think* this is a pretty straightforward question, here goes: I usually do thermal cycling in my forge, but decided that since I have a programmable oven, why not do it with better control?
New purchase Aldo's 52100 at about .010" thickness
First heat 1650, air cool to black
Second to 1475, air cool to black
third, to 1275, air cool. (or as we call in in blacksmithing, "floor quench")
No quenches.
When I went to drill holes for handle hardware, I was quite surprised to find that I couldn't...consistently got about a third through the blade and hit hard steel.
Do I have a case of air hardening due to cooling rate, or what?
I've not seen this with cycling in the forge.

Also noticed much more scaling- perhaps that nice fuel rich atmosphere in the forge keeps it cleaner, or perhaps I'm not going as high on that first heat...
 
Why are you forging .010 52100? If you are not forging, then drill it before you do the thermal cycling. That is why it comes in the spheroid annealed state. ;) Also, anything that I put in my Paragon typically is protected by foil, clay or anti=scale compound.
 
Nope, not forging it. I usually drill before doing the cycling, but somehow neglected to do it on this batch.
 
I think you're going about it backwards.
Profile, drill, grind
Heat to 1200 for stress relief
Heat to 1650, cool to black. This is where your major scale is formed due to temp and time in the oven. A forge's atmosphere does protect it from forming scale.
Heat to 1500, cool to black
Heat to 1475 and quench.

I have tried to drill/grind 52100 once after the thermal cycles and it is super tough to do.
 
You are doing a decent job other than the floor quench at 1275F. I think that is too damn close to critical for such thin stock and the inconsistencies of an electric kiln. I would heat it to 1200F and put it in pre-warmed perlite, ash or vermiculite.

Also, you don't need to cool to black in between cycles... just cool down to magnetic(and a little past for good measure). All you are looking for is the austenite to turn into something else... which happens at recalescence... and seems to have a relationship(from what I can tell) with magnetism.

My question is what type of knife are you making that is .010" thick? To me, that is shim stock not blade stock. Aldo's stuff come spheroidal, does it not?
 
Oh, duh...I make cooking knives, and of course meant .110!
Thanks, Rick, lots to ponder in your reply!
For some reason I was under the mistaken impression that cycled 52100 would still be soft, since it turns out that way when I do it in my forge, and my curiosity was piqued by the difference. I'll just assume it was air hardened and do another 12hundredandsomething with a slow cool. Or carbide bits...it's just a few blades I was doing to have them on hand when I felt like finishing them, all ground to 240, just had a brain fart and forgot to drill some of the holes.
 
It is the thin cross-section that is difficult to slow cool in air... let alone laying it on a heat sink like concrete or an anvil. No doubt you'd get martensite if you tossed it on a shop floor. L6 is even worse for that.
 
Just a few metallurgical notes to add to Rick's advice:

Air cooling in thin sections...like knives.....can cause partial hardening that is not desired. Even if you get pearlite, slow cooling will create fine pearlite, which is harder than one might think. To avoid getting any martensite or fine pearlite, once the steel has cooled to about 900°F ( black heat), water quench to cool rapidly. That locks in softer coarse pearlite.

Cycling above 1500°F is to dissolve the alloying ingredients and get them distributed to form carbides and such.
Cycling between 1350°F and 1500°F is to refine the grain.
Any cycle below 1350°F should be at 1200°F-1250°F to soften the steel.

Holding the steel at 1200°F-1250°F for between 10 and 30 minutes will create spheroidite, which will be the easiest structure to drill and shape. Spheroidite is when the iron carbides get locked up in balls ( spheres) in a matrix of coarse perlite. It is the softest and most machinable structure you can create in steel. ( note - in industry to full spheroidize, the steel is held at 1200-1250F for 20-30 hours. Knives don't really need that degree of spheroidization, and 10-30 minutes will usually suffice.)

Again, the best procedure for cooling from any non-hardening cycle is to air cool to black and water quench.
This will NOT harden or crack a blade.
 
I think you've gotten good advise to consider, and there's some who may hold at an annealing temp closer to a 1000*. Since you get part way through the material, I'd also watch your drilling technique. If you start to slow the drilling rate down and cook the bit, that can be enough heat to spot harden your blank. Just a thought, best of luck with it.
 
This sounds like a job for....CARBIDE MAN.
Thank you for taking the time to reply, sometimes the answer is pretty obvious, but having someone who's farther along go over it again can be a huge help for one who's working on the basics.
 
Stacy E. Apelt - Bladesmith said:
Again, the best procedure for cooling from any non-hardening cycle is to air cool to black and water quench.
This will NOT harden or crack a blade.
Click to expand...
I agree with this IF you intend and need to take the blade down to room temperature... like the final cool down of your thermal cycling. Otherwise, you are just wasting time and energy for zero return. I know that Stacy is trying not to complicate things. His metallurgical knowledge is beyond mine. I think he is giving us a dummy-proof procedure that works but for those who have a basic grasp of the science, you can streamline your method a bit. There is no need to cool to black(let alone room temperature) in between cycles. This is when a magnet can be best utilized for bladesmithing.
 
Rick is correct. In the cycles above 1350F, there is no need to do more than cool to around 1200F before the next cycle. All you are trying to do is have a phase change. The magnet will tell you that has happened. The resultant structure isn't important, as you are going to reverse it into austenite in the next cycle. On the last cycle above critical ( usually around 1400F), cool to about 900F and quench to bring down to room temp quickly, then do the sub-critical last step.



To expand on the rapid cooling, even in oven annealing at a controlled rate, the softening changes that you desire happen between 1350F and 900F. Once there, the slow cooling rate is no longer necessary. Cooling at a rate of 50 degrees per hour between 1250F and 900F ( roughly six to eight hours) will produce wonderful spheroidite. Once at 900F, just cool it off and get to work shaping it into a knife.
 
Follow up-
Heated to 1250, it was in there about ten minutes, cooled about 20 sec. in still air, quenched in water, drilled no problem with standard HS bit. I also had a piece of Kelly's 52100 that had a hard spot in this batch, and it drilled just fine, too.
Thanks to all who responded!
 
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