H-1 is a very unique steel. To really do it justice, here's an
excerpt from a book that I wrote about the Spyderco Warrior knife,
which is made from H-1:
H-1 is an extraordinary material developed by Myodo Corporation in Japan and used exclusively on Spyderco knives. H-1 is unique among blade materials because it is completely rustproof, it is an austenitic steel, and it is exceptionally tough.
Traditional steels are produced by adding carbon to iron. The addition of the carbon changes the molecular matrix of the metal, giving it the ability to be altered through a variety of heat-based processes to produce hard, extremely durable things like knife blades. Unfortunately, traditional carbon-based steels also have a significant flaw: they are vulnerable to rust.
Rust occurs when the carbon in steel reacts with chloride. To avoid this, H-1 uses 0.10% nitrogen instead of carbon to give the material its steel-like qualities. Nitrogen does not react with chloride, making H-1 blades completely impervious to rust, even when submerged in salt water for extended periods of time. Obviously, this quality is ideal for a knife that will be subjected to harsh battlefield environments.
H-1 is also an austenitic steel. 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 temperature 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
crystalline structure called martensite. It is this hardness that allows a knife 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 significantly, 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 achieve the optimal balance between the properties of hardness and toughness.
One unique thing that bladesmiths (knife makers who forge their blades) and some other custom makers do to get the best of both worlds is called 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, which is also a laborious hand process, achieves differential heat treatment in a different way. It is done by applying a coating of clay to the spine of the blade before heating it. When the blade reaches critical temperature and is quenched, the clay retains heat, allowing the edge to cool quickly to maximum hardness while the spine cools more slowly and achieves a tough, springy hardness. This is the process that gives traditional Samurai swords their amazing combination of flexibility and cutting power.
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.
As an austenitic steel, H-1 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. The areas “worked” the most therefore become the hardest. As such, in the process of shaping a blade, the spine is worked the least, the primary blade bevels are worked significantly more, 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. This, along with is corrosion-proof quality, make is the perfect material for a state-of-the-art military knife.
Stay safe,
Mike
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