440c heat treat problem

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PugKnifeCompany

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Apr 25, 2017
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so I am working on a batch of kitchen knives in 440c.
My initial test knife hardened properly at 1850F, plate quenched with an AQ hardness of 60rc..then cryo and tempered down to 58 RC.

I then decided to treat the next 4 knives together, and repeated the above process, giving the knives a little extra time to soak (30min)
They only hardened to 53-54 rc.
I assume because of the the added knives in the oven at the same time, they didn’t have enough soak time?

My question

Do I need to normalize / anneal now before I retry hardening?
If so, what is the best process?
 
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Well not knowing exactly what transpired during your heat treating, I'll go out on a limb and make a few comments. I'm sure someone will come back and dissagree with what I say so then you'll have to figure out which of us is correct.

If you didn't use a foil wrap, you may have a fairly thick layer of decarb on the blades, grind a spot clean on the tang and recheck hardness. If still soft, check your hardness tester against the first blade to see if it still reads the same.

If the hardness tester isn't the culprit and the four blades are still coming up soft, re-heat them one at a time. You should be fine going back into the kiln without any additional thermal cycling. Preheat the kiln to 1400°f, insert the blade, lrt sit for two minutes, then ramp up to the 1850°f. Let soak for 3 minutes then plate quench with a fan blowing air on the outside of both plates. Test the first blade bfore proceeding with the other three.
 
Thanks for the reply.
I DID use foil wrap, and made sure surfaces were smooth prior to testing.

I’m pretty sure I didn’t let them soak long enough, BUT, my bigger question is what I need to do now prior to heat treating again d/t the blades now being hard-ish. Normalize? Anneal? Or just go for it?
 
I guess I’m just worried about grain structure...if it was a test piece I’d try it and abuse it a little and look at the grain structure.but these are finished knives...so ide like to be sure that even if they get hard, they aren’t too brittle.
 
I guess I’m just worried about grain structure...if it was a test piece I’d try it and abuse it a little and look at the grain structure.but these are to be sold...so ide like to be sure that even if they get hard, they aren’t too brittle.
 
Grain refinement is accomplished by thermal cycling above the austenizing temp then dropping it below to let everything recrystalize.
This can be accomplished by normalizing, thermal cycling at slitly lower temps than normalizing or by multiple quenches.
Your blades would fall into this last category. Heat, quench, then temper.
 
I’ve been doing some research, and see that some makers will cycle treat stainless multiple times, bumping up the Aust. Temp until max hardness is reached...usually “hot” tempering in b/w cycles....will I benefit from a 2 hr temper prior to putting them back in the heat treat oven for a soak at 1900?
 
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