Understanding 300, 400 & 500 Series Steels: Composition, Properties & Applications

Steel, an alloy of iron and carbon, is the backbone of modern industry. Its exceptional strength and durability make it ideal for everything from automotive engines to towering bridges.

To meet specific performance needs, steel is categorized by its alloy composition. Fasteners, for example, are typically produced from 300, 400 or 500 series steels—each with distinct properties that suit different applications.

300 Series Steels: Austenitic, Corrosion‑Resistant, Cold‑Workable

300 series steels are austenitic alloys rich in chromium (18–20 %) and nickel (8–10 %). Because of this composition, they can be hardened only by cold working, not by heat treatment. Their high chromium and nickel content provides outstanding resistance to rust and oxidation, making them popular for kitchen equipment, medical devices, and exterior structural elements.

400 Series Steels: Ferritic/Martensitic, Heat‑Hardenable, Stronger

400 series steels contain higher levels of chromium (12–15 %) and manganese (1–3 %). Unlike the 300 series, they can be hardened through heat treatment, which increases their tensile strength and wear resistance. However, the increased chromium also means they are more susceptible to corrosion in moist environments, so protective coatings are often applied. These steels are common in automotive components, heavy‑duty tools, and machinery housings.

500 Series Steels: High‑Chromium Alloys, Exceptional Heat Resistance

500 series steels boast even higher chromium content (50–70 %) and are engineered to withstand temperatures above 1,000 °F (540 °C). They are frequently used in high‑heat applications such as exhaust manifolds, furnace components, and turbine blades. While they excel at resisting thermal deformation, their strength and ductility are lower than those of 300 or 400 series steels.

Other series—such as 100, 200 and 600—also exist, but most fasteners on the market are still made from 300, 400 or 500 series alloys.

Regardless of series, all steels are fundamentally composed of iron and carbon, with trace amounts of other elements added to tailor their mechanical and chemical characteristics.