Magnetic stainless steel grades in the 400 series, plus duplex 329

Grades 410, 420, 431 and 440 contain enough carbon that heating and quenching produces martensite, the same hard phase found in tool steel. Tempering then trades some hardness back for toughness. Carbon sets the ceiling. SS 410 holds 0.08–0.15% carbon and suits valve trim, pump parts and bolting. 420 starts at 0.15% minimum carbon and goes into cutlery, surgical tools and plastic moulds. 440 runs from 0.60% in 440A up to 0.95–1.20% in 440C, which gives the highest hardness of any stainless and is used for bearings, valve seats and blade steel.

431 is the odd one. It adds 1.25–2.50% nickel and 15–17% chromium, so it resists corrosion better than 410 or 420 while still hardening to high strength. Marine shafts and pump shafts are its usual home.

All four must be ordered with the heat treatment condition stated. ASTM A276 uses Condition A for annealed bar, Condition H for bar hardened and tempered at a relatively low temperature, and Condition T for bar tempered at a higher temperature, which is tougher but not as hard. Annealed bar machines easily; hardened and tempered bar is ready for service but harder on tools. Many shops buy annealed, rough-machine, then send parts for hardening.

Ferritic steels hold their body-centred cubic structure from room temperature right up to melting, so quenching does nothing to them. 430, with 16.0–18.0% chromium, is the workhorse for appliance panels, kitchen equipment, trim and anything decorative that does not see chlorides. Being low in nickel, it costs less than 304.

446 pushes chromium to 23.0–27.0%. That much chromium forms a tight oxide scale at high temperature, which is why it appears in furnace fixtures, burner parts, kiln components and annealing boxes. It is less ductile at room temperature, so heavy cold bending needs care.

Grade 329 is not a 400 series steel at all. It is an early duplex grade with 23.0–28.0% chromium, 2.0–5.0% nickel and 1.00–2.00% molybdenum, and its structure is part austenite, part ferrite. The ferrite makes it magnetic. The mix gives it roughly double the yield strength of 304 (485 MPa min against 205 MPa in ASTM A240) and much better resistance to chloride stress corrosion cracking, which is the failure that splits 304 and 316 tubes in hot brine. For modern duplex and super duplex grades, see our duplex and super duplex page.

No, but it is less corrosion resistant than 304 in most environments, because it carries less nickel and, except for 329, no molybdenum. Martensitic grades in particular need a clean, smooth finish and the right temper to resist rust. Treat these grades as engineering materials chosen for hardness, heat or cost, and keep 304 or 316 for wet, corrosive service. Our stainless steel vs carbon steel guide is a useful reminder that even a 12% chromium grade outlasts mild steel by a wide margin.