The world of steel is as fluid as molten metal; it is ever-evolving. Steel as a matter of opinion is very subjective as it relates to knives and knife nuts. There is no clear cut answer as to which is the best steel. We all have different requirements and preferences.
At a very simplified level, making steel is like baking a cake. You follow a precise recipe to achieve the type of cake (steel) that you desire. You begin with flour (iron) and from there you add various ingredients (elements). These additional ingredients will determine what type of cake (steel) you end up with. Once you have added all of the additional ingredients (elements) you are left with a batter that is ready to bake (heat treat). Baking (heat treating) is just as much a part of the "recipe" as the ingredients (elements). If not done properly, several properties can suffer. Once baked, you have a new – completely different – finished product. Your cake will forever be a cake, it can never go back to being batter. Of course steel can be re-melted to a molten state, but that simply is the beginning of becoming a new type of steel.
Steel is an alloy of iron and carbon; just as bronze is an alloy of copper and tin. Historically, steels have been prepared by mixing the molten materials. Alloying elements are melted and dissolved into molten iron to make a steel. The molten steel is cast into an ingot, which is then rolled out (while it is still hot) and shaped much like you would roll out cookie dough. As the steel begins to slowly cool below the critical temperature, things start to happen inside the steel. At these elevated temperatures, alloying elements are able to move around in the steel, or diff use. Different elements diff use at different rates, (typically the larger the atom, the slower it diff uses). If the alloying contents are too high for some elements to assimilate with, the excess will separate or segregate out of the steel and form inclusions or possibly combine with another element to form large undesirable carbides. These diffusional processes are also controlled by the austenite grain size of the steel – grains are little packets of specifically oriented crystals. Grain boundaries act as barriers to diffusion, the smaller the grains, the more boundaries, and the slower the steel. This limits the performance capabilities of the steel both in corrosion resistance, and in wear resistant carbide formation.
More recently, Powder Metallurgy has become the chosen method of preparation. The difference in the processing of a powdered metal allows for steel chemistries not possible with traditional steelmaking practices. The process starts out the same as wrought steels – alloying elements are added and dissolved into molten iron. Then comes the main difference. The molten steel is atomized (misted into microscopic droplets) into liquid nitrogen where the steel is instantly frozen, leaving no time for diffusional processes. The chemistry of the resulting powder is identical to that in the molten vat. Additionally, there are no inclusions or large carbides that form. The austenite grain size is the size of the powder at the very largest, which is small. The powder is then cleaned and sorted by size and then the remaining ideal powder is sintered in a hot isostatic press to solidify the steel. Sintering is heating the steel to a temperature just below its melting point, and then pressing it together at high pressures to solidify or remove the voids between powder spheres. This allows for drastic changes in the steel chemistry namely in carbon and vanadium. A larger volume of the highly wear resistant vanadium carbides form upon heat-treating. Since Vanadium has a greater propensity to interact with carbon and form carbides than it does with Chromium, most of the excess carbon is utilized in the formation of vanadium carbides. These leave the Chromium free to help keep the steel corrosion resistant. The result is a premium steel product with properties of exceptional wear-resistance and good corrosion-resistance.
Heat treating the steel to its critical temperature allows the carbon atoms to enter into the crystalline molecules of the iron which have expanded due to the heating. Quenching the steel at this point causes the molecules to contract, trapping the carbon atoms inside. More specifically, the process of hardening steel by heat treatment consists of heating steel to a temperature at which austenite is formed. Austenite has the property of dissolving all the free carbon present in the steel. Quenching is then used to "freeze" the austenite changing it to martensite. These treatments set up large internal strains in the steel; these are relieved by tempering (further heating the steel at lower temperatures). Tempering the steel decreases the hardness, strength and brittleness. It however, increases the ductility and toughness.
52100: a ball-bearing steel used in the first run of the Mule project.
154CM: an American stainless premium cutlery steel
8Cr13MoV: a Chinese stainless steel tempered at the RC56 to RC58 range and used in the Tenacious, Persistence, Ambitious, Resilience, Grasshopper, Kiwi3 and Byrd lines of knives. Often compared to AUS-8, but with slightly more Carbon.
9Cr18Mo: a higher level Chinese stainless steel used mostly in high-end barbering scissors and surgical tools.
440C: a stainless steel, known for corrosion resistance and ease of sharpening.
Aogami Super: a Japanese exotic, high-end steel made by Hitachi. The "Blue" refers not to the color of the steel itself, but the color of the paper in which the raw steel comes wrapped.
ATS-55: a performance stainless steel similar to ATS-34 with the molybdenum reduced, used only by Spyderco for knife steels until the early 2000s.
AUS-6: similar in quality to 440a, used as a "budget" steel in early Spyderco models.
AUS-8: a frequently used Japanese steel, which is known for taking a very fine edge, due to the inclusion of vanadium. Sharpens easily, and has moderate edge holding and corrosion resistance.
AUS-10: a Japanese stainless steel series made by Aichi with the same carbon content as 440C but with slightly less Chromium.
BG-42: a high performance stainless steel formulated for ball bearings, similar to ATS-34 (same composition, but with added Vanadium), which has similar properties.
CTS-XHP: made by Carpenter Technology. Often referred to as a stainless version of D2, which has similar properties.
CTS-20CP: Carpenter Technology's version of S90V, with slightly reduced Chromium. Features incredible wear-resistance and edge-holding, hardened to about 60 RC.
CTS-BD1: Carpenter's versions of Gin-1 with improved chemistry. Originally feature in a Mule Team fixed blade.
D2: a high performance tool steel that has 1 percent less Chromium than required to classify as stainless steel. Spyderco uses Crucible's version of D2, which is a particle metallurgy ("powdered") version, not wrought. CPM-D2 is found in a sprint run version of the Military model.
G2, aka GIN-1: A Hitachi-made low cost stainless steel comparable to, but softer than, AUS-8. Generally hardened in the mid to high RC 50s. A tough, corrosion-resistant steel.
H-1: is ideal for marine applications, because it substitutes nitrogen for carbon and thus is nearly rust-proof in any normal environment such as salt water exposure though can still oxidize if exposed to extreme heat and chemical attack. It grinds, scratches and has edge retention similar to the low carbide steels such as AUS-6. It is a precipitation hardening steel, which is a particular type of heat treatment in which the hardness and microstructure is formed through an extended soak.
MBS-26: A Japanese stainless steel, very fine grained with high corrosion resistance used in the Catcherman and in most kitchen knives by Spyderco.
N690CO: an Austrian stainless steel hardened to the high RC50 range. Currently found in the Squeak and previously used in Spydercos manufactured by Fox Cutlery.
CPM-M4 [aka AISI M4]: Has a usual RC of 62-65, used in special purposes, high-speed steel with combination of high Carbon, Molybdenum, Vanadium, and Tungsten for excellent wear resistance and toughness; a powder-metal, non-stainless steel.
CPM S30V: an American powder-metallurgy, high-carbide steel developed for the cutlery market.
CPM-S60V (aka 440V, aka CPMT440V): a modern American super-steel that is wear resistant, but difficult to sharpen. Unfortunately the low level of toughness means that it can only be hardened to around 56 RC, causing the edge-holding performance to be diminished.
CPM-S90V (aka 420V): similar to Crucible's S60V but designed to be more wear resistant with a very high carbide volume and high vanadium content. Appreciated for extreme edge-holding. S90V was featured in a sprint run of Spyderco's Military in 2008. Since then it has been used in several sprint runs in knives like the Manix 2 and Paramilitary 2. While S90V holds an edge significantly better than S30V, both are usually hardened to about 59-61 RC.
VG-10: a Japanese steel developed for the horticulture industry by Takefu, often hardened around the RC60 range. Reported to have better corrosion resistance but slightly less edge retention than S30V. Appreciated for taking an extremely fine edge, and being extremely easy to sharpen, while still holding an edge well. Used in most of Spyderco's Japanese-made knives.
ZDP-189: a premium Japanese powdered super-steel made by Hitachi, hardened to RC 62-67, with very high carbide volume. Along with S90V, it boasts some of the best edge-holding of any steel on the market.
Alloy: A material that is dissolved in another metal in a solid solution; a material that results when two or more elements combine in a solid solution.
Alloy Steels: Have a specifi ed composition, containing certain percentages of vanadium, molybdenum, or other elements, as well as larger amounts of manganese, silicon, and copper than do regular carbon steels.
Austenetized: The basic steel structure state in which an alloying is uniformly dissolved into iron.