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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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Thanks for the reply Jerrod, that all makes sense. I didn't realize that's what the actual "line" of a hamon was, but it makes sense. Is it essentially a region where the cooling rate is right on the "nose" and the austenite becomes martensite in some places and cementite/ferrite in others? I've wanted to do metallography on differentially hardened samples but it was one of those projects I never got around to. When you say a lack of brittleness, does that essentially mean that there is at least some yielding before fracture? What being "tough" or "brittle" means for a knife is still something I'm trying to get a better grasp on, especially since it seems to me that those things are not constant for a given material and depend on geometry, strain rate, and temperature. It seems something like un-notched impact toughness tests would be a comparative metric to differentiate materials but maybe not the full story. 

 

One thing I will say for knives with ductile cladding/spines is that they are much easier to heat treat for thin geometries IMO because you can straighten any warps more easily and consistently. I've heard the same argument about straightening san mai knives after they bend in use, but it seems kind of circular in that instance (I wonder why they always end up bent...). 

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The hamon is a combination of phases.  I would bet there is a bit of bainite in there.  Martensite edge, transitions to some bainite then to pearlite at the spine, with blends of two phases each time. IIRC someone has polished, etched, and taken micrographs of what is inside a hamon, but I can't remember who, and can't find them with a quick search.  I seem to recall it being on my some-day to-do list, but then taking it off the list because someone else already did it.  

There are 2 things to consider before (non-impact) failure:  Elastic deformation and plastic deformation.  Elastic deformation is that which will be completely undone when the load is removed.  Like when a spring is loaded it compresses (or stretches, depending on load), but returns to exactly the same length when the load is removed.  Plastic deformation is bending past the limit of elastic deformation to the point where it stays permanently deformed.  When the load is removed then the steel will recover its elastic portion as normal, but the plastic portion will remain.  A brittle material has little-to-no plastic range.  Martensite has virtually no plastic deformation. tempered martensite has a little, pearlite has a lot more.  Impact toughness is a different beast altogether.  

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Thanks Jerrod. I'll take a look around for hamon micrographs, that seems interesting. I took these micrographs a few years ago of a piece of 5160 ish steel that been "quenched" by aggressive forging and seems to have a mixed phase microstructure. Is this something like what you would see in the transition region with intermediate cooling rates?

 

Large Piece 10x scaled.pngLarge Piece 50x Scaled.png

Apologies for the blurry images, my polishing wasn't the best so these were not as flat as they could have been. Outer surface is down in the first photo.

 

That makes sense about being brittle/not. I've been reading some of Larrin Thomas's articles at knife steel nerds which has been helpful. It seems like he often uses subsize notchless specimens for impact toughness tests to compare steels, which makes sense for materials with a high strength/low toughness that all expect a primarily brittle failure without changes to geometry.

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That is indeed the kind of thing to expect.  Another thing that this just reminded me of is case hardened material.  Here are some pictures I took about 17-18 years ago in my lab classes.  

 

Center of part.

Quenched Middle - upload.jpg

 

Mixed microstructure

Quenched Mixed - upload.jpg

 

Hardened case (surface)

Quenched Case - upload.jpg

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That study you're thinking about was from 2004, prepared by R.K. Nichols, AKA Quenchcrack, another metallurgist, at the request of Don Fogg here on this forum.  I'll see if I can find the original post later, but here's the report.

 

The Nichols report.pdf

 

Edit: here's the original post:

 

Edited by Alan Longmire
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