Cooling from Temper

Stacy E. Apelt - Bladesmith

Stacy E. Apelt - Bladesmith

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I have had some requests for info on why I say to cool the blade between temper cycles with a water quench or in running water at the faucet. I thought I would post one of those replies and toss this out for discussion.

I am not saying that blades cooled slowly or annealed overnight are ruined or inferior, just explaining what the metallurgical reasons are for a more rapid cooling during temper and during annealing and normalization.

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Roman Landes did some research on this, and his results showed that a quick cooling from temper was better. From what I got out of his info, the type of structure is slightly finer. There are certain "precipitations" that happen during a slow cool. The main effect is most noticed when the blade is exposed to extreme cold in use ( winter and Arctic hunting/working). The edge will be chippier than a blade made with a faster cooling from temper. In normal temperature use ( room temp) blades, the difference is less noticeable, but still there.

Lets start with what is happening at temper;
The brittle martensite is "eased" and forms a structure that is somewhat pearlite like. In truth, there is a little pearlite structure being made from the martensite during temper. If the cooling is slow, the structure becomes a bit coarser, similar to coarse pearlite. If it is fast, it stays finer. In a long slow oven cooling, you have allowed the structures to change as they wish with far more time, often eight to ten hours. That is because things can change pretty easily until about 200F. So, if you cool from 400F to 200F in two or three seconds ( water quench) the martensite stays the same ....... fine structure. If you slow cool in the air over ten minutes, or in the oven over hours, the structure coarsens.

How much difference will it make ..... probably none that the average maker could detect. But, if there is a gain in edge retention, it is one that should be taken. So, the cool from temper should be as quick as practical. It won't ruin a blade if done slow, though.

This is also the reason you should not normally completely oven cool a blade when annealing. Once below 800F, it should be quenched to lock in the fine pearlite. I know, I know,.... all the masters have taught it the "Heat to bright red and slow cool" way for years, and the box of vermiculite or kaowool wrap anneal will not go away ( yes, I have one ,too), ...... but in metallurgical view, a quick cool after a slow cool to 800F would be better. The reason the slow method is the standard, is that most folks have no idea where 800F is, or how long it should sit in the annealing box until it drops to 800F. Sticking it in the annealing box and going to bed .... seems foolproof .... and it is. Stick it in the box and take out when it is 800F and then quench sounds fraught with problems ...... and it is.
However, with modern metallurgical tools, like HT ovens, you can program the oven to heat the blade to 1250F and cool at 25° per hour to 800F and then hold at 800F. You stick the blade in the oven at about 5PM, go eat dinner, watch a movie with the kids, go to bed, and get up the next morning then pull the blade after the 18 hour slow cool, and quench in water....no problems no worries ..... perfectly annealed blade.

Hope this helps somewhat - Stacy
 
Interesting stuff there Stacy. Like many others, I've always heard NOT to quench the blade between temper cycles a s this "wasn't good for the steel", though I never really heard a reasonable explanation why. Secretly, if I was in a hurry, I'd stick my blades in front of a fan for a few minutes until they were cold enough to touch for a second or two, and then I'd give them a quick dunk and hurry them into the next cycle. I always felt just a little guilty "expediting" the process like that, but as it turns out, it appears that I was actualy helping things.

Great news. hahah

I always hated have to wait for my blades to cool.
 
And, just to dispel the "It will crack the blade":
There is no change of phase or structure in the cooling from 400F to 80F. Thus, you can't crack the blade as is possible in a water quench from 1450F to 80F. The expedited cooling of a tempered blade will neither warp or crack it. Any warps or cracks found after temper were there during the temper, not after the cooling.

I do any straightening needed as I take the blades out of the temper oven, and if needed cool with water while over bending in the reverse direction to the warp.
I doubt this "sets" the bend straighter than just bending it straight and dunking it in the slack tub, but it makes me feel like I'm doing more :)
Bill Moran taught me to do the bend and cool with water thing. Old habits die hard ... especially when they are tied to fond memories.
 
Thanks for posting this, Stacy.
I did send you a PM after my message from you, but maybe you can post the reply here for everyone's benefit:
Would it be detrimental, in any way, to speed up the cooling from 1250 at, say, 50*/hr, so as to avoid the oven running for 18 hours, or is 25*/hr just pretty much recommended like you say?
And, for the record - how long at 1250? An hour? Hour and a 1/2?
 
So are longer times at tempering temperature bad? If you slow cool it isn't that very similar to just leaving it in the oven an extra 15 min and then quenching? I haven't seen many hard rules on how long the temper should last.
 
Karl, Thanks for asking that.

In the 25°/Hr example, I was mainly pointing out the possibilities of using a good oven. 50°/Hr would be fine, and slow cooling in air will also anneal the steel. The long holds and very slow cooling rates are more important in high alloy steels, like the super stainless steels.

In a sub-critical annealing, it is far more important to have precise control of where you start than how long it takes to cool.

For a stress relief, or basic sub-critical anneal, the time at 1250°F should be long enough to allow all structures to equalize and prepare to convert to pearlite. An hour should be much more than long enough. Probably 10 minutes would be enough time, but it is better to err on the safe side.

For a spheroidal annealing you do things a bit different:
Heat the steel to the austenitizing point, around 1450-1500°F for most carbon steels, and soak to equalize.
Oven cool at 50°/hr to 1200°F. (This is the critical cooling step)
Cool in air to black heat ( around 800-900°F) and then quench in water.

Now, I know that many ( most) of us just do something more along these lines:
Heat the blade to a good bit bit above non-magnetic, quench .
Heat to just above non-magnetic, quench or air cool to black .
Heat to just below non-magnetic, Air cool to black, quench or let it sit until cool enough to hold.

That will also get a soft and fine grained structure, but the oven controlled method will give you the max results.
 
Nebulae,
Another good question - Lets look at what happens during quench and temper.

The structure starts out as austenite at 1450-1475°. We need to hold the steel at this temperature long enough for all the steel alloy ingredients to form up in the proper ways. This is from a few minutes for the simplest steels to nearly and hour for complex stainless steels.
1) As the temperature rapidly drops in the quench, the ferrite ( iron) and cementite ( iron carbide) separate as it goes below 1350°F and then drops into the pearlitic zone around 1000°F.
2) Because we always use the proper quenchant ( wink-wink) the cooling rate is fast enough to continue this drop down into the Martensitic range at 400°F, where the ferrite and cementite mix into the structure of martensite. This stabilizes at around 200°F. Because it had no time to align evenly, it is extremely brittle and under great internal stress. The cooing rate between 400°F and 200°F is where all the cracking and warping occur. It has to be just right. This is where the engineered quenchants show their stuff.
3) We then temper IMMEDIATELY at around the conversion point - 400-450°F - to allow these structures to move around a bit. The martensite breaks up a bit and forms some C-carbides. The structures also align better. Some of the martensite converts into fine pearlite, and the retained austenite tries to convert into Bainite. A little of the ferrite also picks up some carbon, and becomes martensite. This all starts when the blade is re-heated above 200°F and continues to the temper point around 400-450°F. It might seem that staying in the lower part of this tempering range will yield a better blade ( and many good makers do this). However, the drop in hardness is very small until you reach 450°F. If the tempering is done with good control ( no overheating), a blade tempered in the 400-450°F range will be much tougher than the 300-350°F blade, and will be within a point or two of the same hardness.
4) This is not an instantaneous process like the martensitic conversion ( which happens at the speed of sound), but takes a couple of hours. Two hours is the standard time at temper. Less cuts the process down and may allow some remnant stresses, more ( within reason) won't hurt of help.
5) As the blade cools from temper, the C-carbides will try and go back to cementite if allowed enough time. Also, the bainite trying to form will stabilize if it has enough time. If it cools quickly, it will convert back to martensite. That is why we should rapidly cool from temper by quenching in water. Now, what would be the problems if we cooled form temper slowly - very little, but cementite is brittle, and the edge would be a bit chippier. The difference is minute, but it isn't like sticking the blade in water is hard to do, so why not get all you can from your blade.
6) We now have tempered martensite, some funny named structures, and a little new martensite. We need to temper again for two hours to align all these structures into a tough and hard arrangement of primarily martensite - behold the properly heat treated blade.

Can you do all this in a forge by eye, and temper with the kitchen or toaster oven - sure. Will it get every last drop of properly hardened and tempered martensite squeezed out of the steel - NO.
That is the main reason for an engineered quenchant and well controlled HT oven .... Consistent, Reliable, Repeatable quality in your blades.

I think the problems with austenitizing with a torch, quenching with whatever is on the shelf in the garage, and tempering by heating the spine with the same torch for less than a minute are obvious, and should be the subject of another thread.
 
Good stuff, Stacy.
Thanks for taking the time.
I'll keep doing what I'm doing and knowing I'm getting the best from my steel.
 
I've always water quenched between tempering cycles. Figured if I didn't, it'd be like a 2 hour temper instead of two at one hour each. I usually do an oil quench between normalizing cycles, to speed up the process, but not always.

Good stuff Stacy!
 
Just for my clarification, are you quenching after the last/final temper cycle as well? I am assuming so, but just wanting to make sure.
 
Every temper cycle, the steel needs to cool down below 200F, you should quench the blade is some coolant ( water is best).
In the quenches from 800F, oil is probably best.
 
bladsmth said:
Heat the blade to a good bit bit above non-magnetic, quench .
Heat to just above non-magnetic, quench or air cool to black .
Heat to just below non-magnetic, Air cool to black, quench or let it sit until cool enough to hold.
Click to expand...

I've been doing that before grinding my 52100 kitchen knives- two normalizing cycles, three sub criticals. Makes it easier to grind and saves belts. Then I normalize one more time before heating to quench, for a total of three normalize cycles and just to relieve any grinding induced stress. Works well for me. I got the rough sub critical regimen from you a couple years ago, works great for me.

I do think that when shimming and tempering a warped blade cooling it in the jig helps to set the correction. I've done it quite a few times both ways and I tend to always quench a warped blade still in the shim jig now. Works better IME.

Short answer would be- I agree with all of the above and thanks Stacy for originally cluing me into a better working anneal for hypo steels. Also, Rick Marchand brought shimmed tempering and quenching from temper heat to my attention, so thanks to him. With all the big thin blades I do now for chef knives it would be hard to get by with out the above methods.
 
Karl B. Andersen said:
Stacy, when you say "at temper" are you suggesting 2 hours TOTAL, or 2 hours twice?
Click to expand...

Two hours twice.

Salem,
The best thing these procedures give you is repeatability. Maybe it is overkill, but you will always be sure that the steel was right before HT. If there is a problem, a quick look at the HT parameters will usually quickly disclose the problem.

One big difference between a professional attitude and a hobbyist attitude is the hobbyist may look at his procedures and say, "That's good enough this time." The professional will say, " How good can I make it.....every time."
 
I just wanted to let everyone know that I'm in the process of heat treating and tempering 7 blades, and at the end of the first temper cycle I dunked each of them in water straight out of the oven.

The result? A nice satisfying TSsshh.....

No explosions, no broken blades, the lights didn't flicker, black clouds didn't roll in from the horizon....

And it saved me having to wait an addition 15 or 20 minutes for my blades to cool before my second cycle.
 
Stacy,

Why only 2 tempering cycles. I seem to remember 3 due to retained austenite going to new, untempered martensite after the second temper as well. Is my memory failing me? Or bad facts?
 
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