Materials Glossary — Start Here
Steel is the workhorse material of the entire machining world, and at its core the idea is simple: iron, with a controlled amount of carbon mixed in. Everything else — carbon steel, alloy steel, tool steel, stainless steel — is a variation on that one basic recipe. This page is the map before you dive into any of those family pages.
Steel is, at its core, an alloy of iron and carbon. As covered on the Iron and Carbon pages, pure iron on its own is relatively soft. Add a small, controlled amount of carbon — typically up to about 2.1% by weight — and the result is steel: a material that can be tuned across a huge range of hardness, strength, and ductility just by adjusting that one number. Go above roughly 2.1% carbon and the material is no longer steel at all; it becomes cast iron, a different family with its own machining behavior and its own page on this site.
Within the steel range, carbon content is the single biggest lever on mechanical properties. More carbon means higher hardness and strength but lower ductility — the ability to bend or stretch without cracking. That's why a low-carbon steel bracket can be cold-formed in a press brake without issue, while a high-carbon tool steel blank has to be heat treated with real care. This carbon-driven tradeoff is exactly what separates carbon steel into its own low/medium/high bands, and it's covered in full detail on that page.
Plain carbon steel is just iron and carbon (plus small residual amounts of manganese, phosphorus, and sulfur). Once a steelmaker deliberately adds other elements in meaningful amounts, the material becomes alloy steel. Each element is added for a specific reason: chromium improves hardenability and, at higher levels, corrosion resistance; nickel adds toughness and impact strength; molybdenum improves hardenability and high-temperature strength; vanadium refines grain structure and boosts wear resistance. Tool steel pushes this further, combining high carbon with heavy alloying to produce grades built specifically to be hardened for cutting, forming, and shaping other materials. Push chromium content past roughly 12% and the steel crosses into stainless steel territory, gaining serious corrosion resistance — split further into martensitic, ferritic, and austenitic families depending on the balance of chromium, nickel, and carbon.
Composition sets the ceiling; heat treatment decides how close a given part gets to it. Quenching — rapid cooling from a high temperature — locks steel into a hard, brittle state. Tempering — reheating to a lower temperature afterward — trades away some of that hardness for toughness, landing the part at a usable balance point. Heat treatment can't add properties a composition doesn't support (a low-carbon steel will never harden the way a tool steel does), but within what the chemistry allows, it's a powerful second lever.
Carbon content sets the baseline. Alloying elements each contribute a specific property. Heat treatment fine-tunes the result within the limits the composition allows. That's the whole map. From here, the Carbon Steel, Alloy Steel, Tool Steel, and stainless steel (austenitic / martensitic) pages go deep on each family's specific composition, properties, and machining behavior.