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Dual Phase Heat Treatment and Knives?


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It might be kind of an odd question, and maybe I've been reading too much about high strength sheet steel as of late, but is there any kind of utility for dual phase heat steel (i.e. a blend of martensite and ferrite) in knife making? The edge of a knife needs the high yield strength and hardness you get from a quenched and tempered martensitic steel, but with differential hardening/tempering and composite blades using several types of steel there is certainly effort to maintain some ductility in the bulk of knives in some situations. Because the temperature to get full austenization varies with carbon content it seems like you could theoretically have a heat treatment and composite structure that yields a fully martensitic edge with something like 1075 (to be close to the eutectoid composition) and have the rest be dual phase, with a good strength-toughness combination.

 

What made me think of this is that, surprisingly, 1018 is pretty close to the chemistry of some dual phase steels in the 800-1000MPa strength level, minus a bit of Mn (and actually with more C) and perhaps a few traces of other things. Despite this potential, it seems like it's always treated as a non-participant in the hardening of sanmai knives. Likewise, if 1075 was put through a "dual phase" heat treatment for 1018, from what I can tell it would come out entirely as tempered martensite.

 

I did a back of the envelope 1018 dual phase heat (quenched from 900 C in water, soak at 750 C for 15 minutes, quench in water, temper at 150 C 60 minutes) treatment and admittedly it was not particularly impressive. The as received, DP, and quench and tempered pieces were all more than ductile enough for a knife and my (crude) bending strength comparison showed them to be roughly the same (QT and DP roughly tied, maybe a hair stronger than as received). I may try something with a bit more carbon (like 1045 which I have on hand), but that definitely would be further from the composition of "proper" DP steels. Does this seem to anyone like something that could work, or at least be a way to make a modified heat treatment for a blade that would already have some low carbon portion?

 

It seems like there's been a lot of advancements in high strength steels in the past 50 or so years trying to defeat the strength-ductility tradeoff, and this is one thing it seems like might be able to apply to knives. Or maybe a properly quenched and tempered blade is still the best 99.9% of the time. Thanks for reading!

 

 

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I'd say you are missing a few 9s there.  What you are talking about is basically what is going on with differential hardening, and is visible as a hamon or hardening line.  One thing to keep in mind is that 800-1000 MPa (116-145 ksi) is not actually all that high.  These steels are useful because it is pretty high for the given alloy content and ductility/impact properties.  For a point of reference, we make 4340 (still fairly low alloy content) and can get nearly 200 ksi (1379MPa) with just a fan cool and temper at 1000F.  If we were to liquid quench it (hot water) we would certainly be able to get well over that.  The ductile-brittle transition temperature (DBTT) is a bit higher though.  Tempered martensite is a fantastic thing.  Think about what a knife needs, basically just hardness and lack of brittleness (elasticity more that ductility).  Think about all the ABS bend tests that I am sure you have seen.  Do you need any more bending than that?  

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