Low Temperature Temper for Toughness?

[Fancier]

I've heard some discussion about low temperature tempers so I thought I'd come to you with a question since you used the phrase in a recent post.
Doesn't a lower temperature temper leave the steel harder? Isn't hardness a tradeoff with toughness? Doesn't a higher temperature temper give lower hardness but more toughness?
The only way this makes sense to me is if the quench is also modified to achieve a lower hardness through less conversion to martensite. That way you'd be tempering from a less hard condition and you could use a lower tempering temperature. Doesn't this also give more retained austenite? Can you cryogenically treat that retained austenite and recover the desired hardness?
Thanks!

 

[Big Chris]

I assume you are asking about 3V. 3V has a dual hardening hump meaning you can get the same hardness from a low temperature temper as you can from a high temperature temper. The low temp temper does not work with all steels though. The 4V knife I low tempered had chipping issues when pushed hard where the high temper 4V knife had no such issues. Both knives were within a point of each other at 63 Rc and had identical edge geometry.
The low temper on 3V is not what you want to use if edge holing is your goal. Edge holding will be slightly less, but the knife does exhibit added toughness. Again this is with 2 knives at similar hardness with similar edge geometry. The ideal use for the low temper, IMO, is for knives that are going to be seeing very hard use. Choppers, swords, hatchets, and combat knives are where I see this being the most advantageous. Another benefit of the low temper is going to be added stain resistance since chromium is not being used to make carbides as with a high temper; not that 3V has that much chromium to start with.

 

[Fancier]

Thanks Chris! What I'm hearing is that on some of these more complex alloys you can take advantage of the shape of the tempering curve to obtain different properties while hitting similar hardness values. Interesting, and this is a much clearer explanation than what I've heard from others.

 

[marthinus]

Thanks Chris! What I'm hearing is that on some of these more complex alloys you can take advantage of the shape of the tempering curve to obtain different properties while hitting similar hardness values. Interesting, and this is a much clearer explanation than what I've heard from others.

I do not hope Chris minds me posting this here but here is some information for you if you want to learn some more.

http://www.uddeholm.com/files/heattreatment-english.pdf

http://www.edge-1.com/Edge/stores/afri-thipa/articleimages/SandvikHardening Guide.pdf

 

[waterstoneblades]

I would like to add that I've bean testing a 3V knife with the low temper. The hardness is 62 rc and about .013" at the edge. Aggressive carving on hard padauk wood with no chipping or rolling. I used it in the kitchen to prep a big dinner and there weren't signs of staining after being wet for a long period of time. Like Chris stated above it doesn't work on all steels but does on 3V. If you get a good quench hardness and get it into sub zero sooner then later it makes a good blade.