Proper temperature for Elmax tempering?

[Alex Dron]

I'm trying to reach max hardness for my Elmax knives. So far I was doing it at home (Evenheat/dry ice). I know most makers would suggest to outsource Elmax heat threat, because the proper cryo is crucial etc.
I went into one such local shop which does that service and asked them about Elmax tempering. They mentioned it really depends on the blade hardness after quenching and cryo but overage would be 325F for 61-62RC. Which makes me worry about using their service. From Uddeholm data sheet the minimum temperature 350F is possible in _exceptional_ cases "where toughness is of less importance", and I would not think knife blade is that case.

Does anybody know, what is the proper way to reach 61 and above for Elmax? I would think it means hardening at 2100F or above, LN cryo and tempering around 900F-1000F. These numbers were mentioned in older Uddeholm datasheets but now replaced with max 2010 austenitizing... I know going to higher end of tempering curve is not good for corrosion resistance, but I can't see how else possible to reach 62RC. 325F sounds really scary to me, I'm afraid to give my blades into such shop.

 

[Larrin]

What the minimum tempering temper is is not well defined, without seeing the results of whatever toughness testing that Uddeholm has performed it is hard to say if that recommendation is iron clad or an extrapolation. The fact that you have a local shop that has heard of Elmax and also has heat treated it in the past is somewhat of a surprise by itself. You might want to ask yourself why you are targeting 62 Rc and what you are trying to achieve. If you are trying to max out hardness you are also limiting your toughness. Without testing the knife with the intended application you won't know what sufficient toughness is.

Here are two references that may help you, however. One recommended heat treatments for Elmax along with the corresponding toughness results direct from Bohler-Uddeholm:
http://www.bucorp.com/media/KNIFEHEATTREATSUM2.pdf
http://www.bucorp.com/media/Toughness_testing_for_knife_steels.pdf

Another is to show that a high temperature temper does not necessarily lead to higher toughness than a low temper. There is a "new" minimum tempering temperature with an upper temper and I would be no more confident in a temper at 950°F than 325°F. Either may be fine or either may not be:

 

[Alex Dron]

Thanks Larrin. I'm personally fine with 59-60RC and this is where I keep most of my blades at, but for kitchen knives I often asked by customer to go with higher possible values.
So basically there is no hard written rule, it is all try-out. Good, will have some fun then.

 

[kuraki]

I get 59-60 RC with 1950F aus temp, LN and 400F temper.