As part of my new duties here at Spyderco, I've been brushing up on the full product line and filling in the blanks in my knowledge base (to be very honest, even while I was working for my previous employer, two of my three EDC knives were Spydercos and I still have more Spydies in my personal collection than any other brand--so I have been "keeping up").
Although I was aware of the non-corrosive properties of H1, I did not know the "full story" regarding the properties of this amazing steel. As Ed explained, it is an Austenitic steel. That makes it very special. The vast majority of steels used in the knife industry are Martensitic steels. They start out in an annealed (soft) form with an austenite structure, but to become usable as knives, they must be heat treated. The heat treating process raises the steel to critical temperature before it is quenched (rapidly cooled). The exact temperatures and quenching media for this process vary according to the steel alloy, but the basic goal is to transform the austenite structure of the steel to hard martensite so it will have the hardness to take an edge and be an effective cutting tool.
After the initial hardening portion of the heat treating process, the resulting knife is very hard, but also quite brittle and prone to breaking. To increase its toughness without compromising its cutting ability too much, the blade is tempered. This process re-heats the steel to a prescribed temperature to "draw" it to a lower hardness. In many cases, the tempering process is repeated more than once. Again, the idea is to hit the best balance between the properties of hardness and toughness.
One thing that bladesmiths and some other custom knifemakers do to get the best of both worlds is differential heat treating. As the name implies, the goal is to achieve "different" hardnesses in different parts of the blade. This is done by carefully heating the spine of the hardened blade with a torch to soften it without affecting the hardness of the edge. The result is a blade that has an extremely tough structure, while maintaining maximum hardness and edge retention at the cutting edge.
Japanese clay tempering--also a laborious hand process--also achieves differential heat treatment. The clay applied to the spine of the blade retains heat, allowing the edge to cool quickly to maximum hardness while the spine cools slower and achieves a tough, springy hardness.
Differentially heat treated blades are the perfect knives because they offer the very best compromise between edge hardness and toughness. The problem is that differential heat treating is very difficult to do as a manufacturing process--at least with conventional martensitic steels.
The "other" amazing thing about H1 (besides the fact that it is impervious to rust, which is already pretty amazing) is that, as an Austenitic steel, it does not go through the traditional heat treating process of other steels. Instead, it is a "work hardened" steel. The mechanical processes of shaping and grinding the steel selectively increase the hardness of the affected surfaces. If you think about that, you'll realize that, in the process of shaping a blade, the spine is "worked" the least. The primary bevels are "worked" significantly more in the grinding process. And the edge is "worked" the most. More work equals more hardness, which means that H1 has the exact same properties of differential hardness as expensive hand-forged blades and Japanese clay-tempered blades--all in a steel that is completely and totally immune to rust.
How cool is that?
To be very honest, I have never been a "steel geek." Although I certainly acknowledged the differences in performance of various steels, I have always focused more on edge geometry than metallurgy--particularly in production knives. What DID always fascinate me, however, were processes like clay tempering that allowed hand-crafted blades to do things that production knives couldn't. The more I learn about H1, the more jazzed I am about it. I will continue to share my knowledge and experiences with it as we go.
I hope this helps.
Spyderco Special Projects Coordinator
Founder and Lead Instructor, Martial Blade Concepts