The Alphabet Soup of Tool Steel
Q:I see various references to the steels used in woodworking tools such as O1, A2, etc. Is there a decoder ring that can help me figure out what these designations mean and which type is best?
A: Tool steel, also known as high-carbon steel, is a generic term used to describe steel alloyed with 0.5% to 1.5% carbon. The addition of carbon allows the steel to be hardened so that it will better hold an edge. Tool steels may also contain small amounts of other alloying elements, such as vanadium, chromium, manganese, or molybdenum. These elements impart different properties such as increased hardness, greater toughness (making the metal less brittle), or more resistance to abrasion or rust.
O1, A2, D2, PM-V11, and M2 are the tool steels most commonly encountered by woodworkers. There really is no "best" here; each steel type has slightly different properties and what works in one application may not work as well in another.
O1 is one of the most basic tool steels, containing only carbon and manganese as its major alloying elements. The "O" in O1 stands for oil-hardening. This steel is hardened by heating to red hot and then quenching in oil. Oil quenches the steel more slowly than water, reducing the chance of cracks forming in the steel from thermal shock. O1 steel sharpens relatively easily and takes an extremely keen edge. On the downside, O1 steel doesn't hold its edge as long as some of the other alloys.
A2 steel hardens in air (which is what the "A" stands for). Containing molybdenum as well as up to 5% chromium, A2 steel is tougher and more resistant to abrasion than O1 steel, so it holds an edge longer. This edge retention ability makes it a preferred choice for many woodworkers, particularly if you combine hand-tool work with a lot of dense hardwoods. The trade-off with A2 steel is that it takes longer to sharpen and it can't be sharpened to as keen an edge as O1 steel.
D2 steel, which you'll encounter less frequently, can be hardened in oil or air and contains 12% chromium. This high percentage of chromium gives the steel some rust-resistant properties. D2 steel resists wear and abrasion better than A2 or O1, but is not as tough as either. Increased wear-resistance makes D2 is more difficult to sharpen than O1 or A2 steel.
O1, A2, and D2 steel are made in traditional ways by melting and mixing various alloys. Powdered metal alloys use a different process where the melted metal is atomized into tiny powder-like particles. The particles are screened then fused into a billet using heat and pressure. Lee Valley uses a form of powdered metal known as PM-V11 to make many of their plane irons and chisels. The advantage of PM-V11 is that it has better edge-holding ability than 01 steel and sharpens faster than A2 or D2.
All of these steels are used for chisels, plane irons, and other woodworking hand tools. But for powered cutting tools, you're likely to encounter a subset of tool steel known as high-speed steel. High-speed steel contains alloys that allow edge retention even at high temperatures. This makes it a good choice for powered cutting tools where the cutting edge is subjected to high heat from friction. M2 high-speed steel is one of the most common types and is used for drill bits, planer and jointer knives, lathe tools, and jigsaw and reciprocating saw blades. More often though, high-speed steel is listed without any letter or number designation.