H1 Steel Differential Hardness: What is the Molecular Mechanism?

[SpyderEdgeForever]

This is a question for those here who are into the nitrogen-based H1 steel knives such as the Spyderco Salt series that use H1: What is the actual molecular level mechanism that causes this "differential hardness" as explained in the Spyderco Catalog/Product Guide? How does it work at the metal-grain and metal-crystal level to achieve this factor?

And this may be a bit of a Stretch, Too, but how can this be achieved in other types of steel, or is it unique to H1 and related alloys?

[AndrewM]

Sorry SpyderEdgeForever, I have absolutely no idea.. I am interested in reading any replies though.

I would like to know more about the work hardening properties of H1. I have a Ladybug H1 serrated which is an awesome little EDC knife. It is far to small for many tasks but fine for what I need EDC to do (I work in office environment and do not want to pull a PM2 out in the lunchroom to open a meal). I also have a Rock Salt. Love the ergonomics, and is so great for camping. I use it instead of a machette for clearing scrub and to process / split wood when I don't have my Gransfors with me. The concave shape to the blade close to the handle/scales is probably the best blade I have used for making feathersticks, and the blade geometry closer to the tip makes light work of slashing.

In saying this, I cannot get a fine razor edge on it. I can get a great work edge for hacking and chopping, and with a big swing can cut some pretty thick rope. But put the blade to a piece of paper and it is far inferior to any other Spyderco I have.

I would like to know if this hard use would actually harden the steel at all, or is such abuse on wood not enough to work harden the steel?

I'd love to get the steel a little harder to see if that improves the ability to take a razor edge?

In hindsight I am feeling that perhaps I should have bought the version in VG10, but I love the idea of an H1 steel for a camping knife due to no maintenance ie oiling, corrosion etc, and can take it on my kayak in salt water.

[Evil D]

I've followed the discussion about this over the years and it's still really hard to swallow. For years it was said that the steel's edge retention would improve with repeated sharpening due to the work hardening. Then it was said that work hardening only happens during hard grinding, as with cutting serrations and grinding the main blade grind. I've asked if a custom regrind on a plainedge blade would give the same results as cutting serrations, even if you have the blade a shallow chisel grind similar in bevel size as serrations are cut, but I've never gotten a reply. If this could be true it would be a serious game changer because you could make a Scandi grind H1 blade that would have the edge retention of SE H1 in PE form and it would most likely be an amazing blade. I can't really understand why this topic hasn't been explored more.

[bearfacedkiller]

At 10:50 ish Mr. Schempp states that it is rolled out cold from about 7mm down to 3mm and that is how it is hardened.

https://youtu.be/EUaA0xkygws

Another one where he shares his thoughts on H1. He believes sharpening increases edge retention. Seems like some do and some don't.

https://youtu.be/ZNulyy1dn90

This is some mysterious stuff.......

[Evil D]

But if rolling it out was all it takes then PE would have the same edge hardness as SE. There has to be something going on when grinding it.

[tonijedi]

I'd love to get the steel a little harder to see if that improves the ability to take a razor edge?

According to my experience H1 sharpens easy to a razor edge. Probably the difficulty is related with the blade size and thickness rather than the steel itself.

[curlyhairedboy]

I recently did some literature review of nitrogen stainless steels, as I was curious about the mechanisms leading to its corrosion resistance. H1 is one of these.

There's a lot about Fermi surface state density calculations in some of the early 2000s papers, but what that boils down to is this: Both nitrogen and carbon act as strengthening elements when distributed interstitially in the iron lattice. However, nitrogen tends to be much more finely distributed in the lattice, while carbon tends to clump together in regions of high concentration.

This is where H1's toughness comes in - nitrogen as an interstitial encourages a more metallic nature in the lattice, and thus more ductility along with the strength.

As for H1's superior corrosion resistance - there are several ideas about how the nitrogen helps. First, most of the corrosion resistance is due to the high chromium content (14% if my memory serves). The chromium oxidizes on exposed surfaces and forms a protective layer - a durable, invisible patina.

One theory has the nitrogen in the lattice concentrating near the surface, aiding the chromium oxide layer
One theory has the nitrogen converting to NH4 ion when exposed in salty solutions, changing the chemistry near the surface to be less favorable to corrosion
One theory has the nitrogen - being more evenly distributed in the lattice - preventing the kind of intergranular corrosion common with carbide formation at grain boundaries.

There are more ideas out there, and the reality is probably one or more of them.

Regarding the differential hardness - this is probably one of the more transparent mechanisms. Iron has a number of crystalline forms, but the lattices we're most interested in are Austenite and Martensite. Austenite is iron's most stable form above a certain temperature. Below that temperature, it switches to a slightly different form, Martensite. However, you can add in other elements into the lattice that will stabilize the Austenite and allow it to exist at room temperature and even at cryogenic temperatures. Austenite is a lot tougher and more ductile than Martensite. H1 is mostly Austenite. However, when deformed and worked at room temperature, the additional mechanical energy is enough to force the transition to Martensite close to the areas where the force was applied. Furthermore, "The carbon levels of austenitic stainless steels are always relatively low, so strain-induced martensite is self-tempering and not brittle."

H1 is a win-win.

[ZMW]

But LC200n is a N based alloy, why does it not experience the same work hardening with SE? If LC200n has better PE retention, would the SE work hardened be even better then H1?

Different questions the OP, but along the same line. Sorry! This stuff is super interesting

Edit - I looked at the Z knives steel chart, and H1 has WAY more Nickel and Silicon then LC200n. C and N are similar, but I guess these are two different steels

[curlyhairedboy]

The high nickel content in H1 acts as an austenite stabilizer. The lower nickel content in lc200n probably contributes to its martensitic nature - it can be conventionally heat treated, and this means the bulk of the material is martensite.