Hardness increase after temper

R

RedCrow

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I had some blades professionally heat treated, and the hardness increased after temper. Is that possible? They were a2 steel, they were ramped to 1200, held for an hour, ramped to 1775-1800, held for an hour, and then air quenched in a partial nitrogen vacuum. That was followed by a liquid nitrogen cryo treatment, and then a double temper. They checlked the hardness, and as quenched hardness was 63, but after cryo and tempering they were at 64 and 64.5. I double checked to make sure they didn't transpose those numbers, but they said those were definitely the correct numbers, and it wasn't at all unusual for the hardness to increase multiple point after cryo and the hardness stabilizes. If I have ever seen or heard that anywhere, I must have forgotten it, because doesn't sound right to me.
 
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What Happens During Tempering of Steel? - Knife Steel Nerds

Thanks to Shawn Houston and Tyler Christian for becoming Knife Steel Nerds Patreon supporters. We have reached our first goal and therefore I will be interviewing Devin Thomas and posting it as an article to the website. In a previous post I wrote about what happens during rapid quenching of...
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What temp was tempering done at?
 
They gave me a "process certification" , but it doesn't list what they were tempered at. If they truly are 64.5 HRC, it had to have been a pretty low temp. 400 degrees should be somewhere in the ballpark of 60 HRC,.
 
That austenization temperature is fairly high. The gain in hardness from quench through cryo is normal. Some steels will gain in hardness when tempered at low temperatures. 1095 has been know to gain a point or so when tempered around 325 F. Tempering carbides are responsible. Higher alloy steels have more of a tendency to harden during high temperature tempering too.
 
me2 said:
That austenization temperature is fairly high. The gain in hardness from quench through cryo is normal. Some steels will gain in hardness when tempered at low temperatures. 1095 has been know to gain a point or so when tempered around 325 F. Tempering carbides are responsible. Higher alloy steels have more of a tendency to harden during high temperature tempering t
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The austenitizing temperature is the only part I wasn't questioning, but now you have me second guessing, lol. Seems like most recommendations are fairly universal in suggesting somewhere around 1775 degrees.
 
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That may be true. I’ve had A2 treated but I think it was at 1750 so I could get a batch with some S7. 1750 F, no cryo, temper at 400 F for both gives 62-63 and 56-57 hardness. If one isn’t cryo or cold treating, a lower austenizing temp can give a little higher hardness after tempering.
 
And to add, there is no need for a 1200°F soak, especially an hour. Blades are thin and simple in cross section, they should go right to the austenitizing temperature. The faster the rate of heating to the aust temp is generally better for grain size. Plus, an hour hold at the austenitizing temperature is way way too long.

I heat treat A2 by getting my heat treat oven up to 1775°F, soak for 20 minutes, plate quench, "cryo", 350°F tempers, ~63HRC
 
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