What happened? Tempering ATS-34

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godogs57

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Apr 16, 2004
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I HT'd a batch of ATS-34 knives this weekend and now when I temper them I cannot get the hardness to drop into a range I would feel comfortable with. In short, my recipe is 1920 degrees for 20 minutes. Temper @ 375-390 for two hours. That usually gets me about 58 Rc. Hardness out of the oven ran 63 on my blades, 62 on a few. So far, this is about normal for my oven.

Later on, I placed them in the oven @ 380 for two hours and some dropped a point, others nothing. (It is a habit of mine to write on the handle portion of the blade the initial hardness with a Sharpie marker, so when I go back after tempering, I can see how I did) After the first temper, I again wrote whatever the hardness was on the handle. Not satisfied, I tempered 'em all again - this time @390 for two hours. The lowest Rc on a blade out of the batch now is 61...thats it. Some have yet to drop a dern point yet after both attempts at tempering.

Any suggestions would be greatly appreciated! I am making sure I clean the blades off in order to get a true reading on the hardness tester, and am taking several tests on each blade, also to get a true reading...got me wondering what I did wrong. Thanks in advance.

Hank Hammond
 
By cleaning the blades off, do you mean cleaning off dirt, or polishing to remove any scale or other such stuff that might affect the reading? In Wayne Goddard's book, "The Wonder of Knifemaking," one recipe calls for 2 950 F temper of 2 hours. Of course this is preceded by a -220 6-8 hour soak. A lower temp recipe from the same book has 3, 1 hour tempers at 350-400. You may also be experiencing secondary hardening from precipitation during the temper cycles. Finally, you may want to check the thermostat on the oven and check with your supplier and make sure you indeed have ATS-34. Sounds silly, but steels get mixed up and ,without markings, there is no way to tell w/o expensive tests. Kinda like the time some friends threw some 304 stainless into the SEM to get chemical readings, and iron and chromium disappeared, but copper and nickel went through the roof. Funny how Monels and stainless steels look alike when polished.
 
You will not get secondary hardening unless you temper at about 900-1000F and probably not unless you harden from 2000F. There isn't going to be any difference between 390 and 380F ,nor any difference in tempering for longer times .Cryo will give you a point or two higher hardness. IIRC makers on this forum found it best to harden from 1950F, cryo, double temper at 400F. There is no benefit to tempering higher . ...Perhaps you have a batch of steel that is higher in the alloying range.
 
So...any suggestions for this batch, with respect to getting the hardness on down another notch or two? I tested the hardness only after I removed the scale, etc...it was "clean" metal where I tested it. Also, I have checked the thermocouple, et. al., and everything checks out temperature wise. I just don't want to make the same mistake again...your suggesitons are appreciated.

Many thanks!
 
I use lots of ATS34. Always wrapped in Ticronic heat treat foil. My recipe is 1920 for 20 to 30 minutes, lightly sand the tang for hardness check, check hardness, which is usually around 63Rc.

Flash temper in my Mom's kitchen oven at 350 for two hours while the furnace cools.

Temper at 400 for two hrs., check hardness.

If the hardness is over 60Rc, then I raise the last temper to 425 or 450 degrees.

I keep a file card on the first blade from each new bar. The recipe for the first blade will work the same for all blades from the same bar.

Steel mills have a tolerance range for the chemistry in each batch. Since the chemistry varies a little between batches, then the hardness obtained from a bar from one batch may be a little different that the hardness from another batch. That's why commercial heat treaters guarantee hardness within a range of 3 points. They don't check the hardness of each piece of steel in a batch furnace.

I suspect your problem with varying hardnesses between several blades may be related to making the blades from different bars, as explained above.

The second, and most common, problem may be that you had all the blades in the furnace at the same time. This would cause varying rates of cooling. If you have several blades wrapped together, one may cool faster than another.

The blade that cools (air quenches) faster will check harder.

Cryo treating after flash quenching will raise the hardness a point or two, so your tempering temperatures have to be raised accordingly. In order to be accurate, it's necessary to check hardness before each tempering cycle, then adjust the temperature to suit.

Hope this helps. :)

Just raise the next tempering temp. to suit each blade and you'll be fine.
 
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