Stainless steel is an iron-based alloy whose corrosion resistance comes from a minimum chromium content of about 10.5% by mass, with nickel, molybdenum, carbon and nitrogen added to tailor the microstructure [S1][S4]. Four microstructural families — austenitic, ferritic, martensitic (often grouped with precipitation-hardening), and duplex — define almost every commercial grade in service [S1][S4]. Selection is driven by chloride exposure, required strength, magnetic behaviour, weldability and the form-factor being purchased (sheet, plate, bar, tube, channel) [S1][S2].
The British Stainless Steel Association (BSSA) groups the same materials into the same four families and ties grade chemistry to mechanical and corrosion limits, making it a useful cross-reference for engineers comparing EN, ASTM and JIS designations [S4]. For a primer on the parent alloy, see the stainless steel overview; for structural sections specifically, the stainless pipe page covers tubular forms separately.
Austenitic grades: the 300-series workhorse
Austenitic stainless steels carry 16–26% chromium and 6–22% nickel, with the FCC austenite phase stable at room temperature — which is why they are non-magnetic in the annealed condition and retain useful toughness down to cryogenic temperatures [S1]. The two grades that dominate the 300-series channel and bar stock list at major US service centres are 304 (18 Cr / 8 Ni, the general-purpose grade) and 316 (16 Cr / 10 Ni / 2 Mo, the molybdenum-bearing grade specified for chloride exposure) [S2]. Stainless Shapes, for example, stocks 304 across its full channel range and offers 316 on select structural sizes for marine and chemical-service buyers [S2].
Austenitic grades have higher thermal expansion than ferritics and lower thermal conductivity as alloying increases; they are also the densest of the families because of the heavy nickel content [S1]. For buyers mapping cost drivers across stainless families versus carbon or low-alloy alternatives, the alloy steel entry is a useful cost-vs-corrosion comparator, while the carbon steel page sets the baseline that stainless is usually chosen to displace.
Ferritic grades: the 400-series, magnetic and economical
Ferritic stainless steels carry 10.5–30% chromium with very low nickel (often below 1%) and a BCC ferrite matrix that is ferromagnetic at room temperature [S1][S4]. Common examples in the 400-series channel/bar stock list are 409 (automotive exhaust), 430 (decorative and indoor appliance), 416 (free-machining, with sulphur addition) and 440 (higher carbon for cutlery and bearing-grade wear resistance) [S2]. Because nickel is omitted, ferritics are typically the lowest-cost stainless family and are widely used where corrosion demands are moderate [S1].
Ferritics have lower thermal expansion than austenitics, which makes them dimensionally stable under thermal cycling — useful in heat shields, exhaust systems and solar collector frames [S1]. They are not, however, a drop-in replacement for 304/316 in chloride service; the absence of nickel and molybdenum limits pitting resistance, and weldability drops as chromium climbs above ~18% because of grain coarsening.
Martensitic and precipitation-hardening grades: where strength and hardness matter
Martensitic stainless steels use 11–18% chromium with higher carbon (up to ~1.2%) so that the austenite transforms to hard martensite on cooling, producing the 410, 420 and 440 cutlery/wear grades listed on distributor stock lines [S2][S4]. They are ferromagnetic, heat-treatable, and chosen when hardness, wear resistance or a sharp edge outranks corrosion performance [S1].
Precipitation-hardening (PH) grades — 17-4 PH, 15-5 PH, 13-8 PH — add copper, niobium or aluminium so that a low-temperature ageing step precipitates a hardening phase inside a largely martensitic (or in 15-5 PH's case, modified martensitic) matrix [S2]. This route delivers yield strengths in the 1,000–1,400 MPa range with corrosion resistance approaching 304, which is why 17-4 PH bar is standard in aerospace fittings, valve trim and high-strength fasteners. For a broader view of how PH stainless compares to higher-alloyed nickel and titanium options, the silicon steel page is less directly relevant but the steel fiber entry covers fine-wire and reinforcement forms that share the same iron-chrome base chemistry.
Duplex grades: austenite-ferrite balance for chloride service
Duplex stainless steels have a roughly 50/50 austenite-ferrite microstructure produced by pushing chromium to 22–25%, nickel down to 4–7% and adding molybdenum (2.5–4%); nitrogen is also used to stabilise the austenite [S1][S4]. The mixed microstructure delivers roughly twice the yield strength of standard 304/316 austenitic grades while keeping the pitting-resistance-equivalent number (PREN) high — typically 30–40 — making duplex the default choice for seawater, hot chloride brines and sour-service oil-and-gas applications.
Because duplex is ferromagnetic (the ferrite phase responds to a magnet), it cannot be used as a substitute for austenitic 316 in non-magnetic instrumentation or cryogenic lines where the austenitic toughness at low temperature is required [S1]. Welding also requires controlled heat input and a low-carbon, nitrogen-matched filler to keep the ferrite fraction inside the 30–70% range; outside that window, corrosion and impact toughness both fall.
Selection criteria: matching family to service
Specifying stainless by name alone is the most common sourcing error. The four families behave differently on at least four decision criteria: corrosion domain, strength, magnetic response, and weldability — and a useful comparison should be read against those four axes rather than a single property. The BSSA four-family grouping [S4] and the Outokumpu mechanical/physical property tables [S1] line up as follows for the most commonly stocked grades.
Austenitic 304/316: highest corrosion versatility, non-magnetic annealed, weldable without special procedures, lowest strength of the four families. Ferritic 430/409: lower cost and magnetic, lower thermal expansion, limited weldability above ~18% Cr, poor chloride resistance. Martensitic/PH 410/420/17-4 PH: highest hardness and strength (PH grades reach ~1,300 MPa yield), magnetic, lower corrosion resistance, requires controlled heat treatment. Duplex 2205/2507: highest strength combined with high PREN, magnetic, requires controlled welding procedures, limited cold formability.
For buyers comparing 304-channel inventory against 316-channel inventory, the practical question is chloride exposure — 316 adds roughly 2% Mo and shifts the grade from "indoor/architectural" to "marine/chemical" classification. Stainless Shapes' stocking list follows exactly that split, with 304 across all standard channel sizes and 316 reserved for selected structural profiles [S2]. When the application calls for pitting resistance above what 316 offers, 2205 duplex is the next step; when it calls for elevated-temperature strength (boilers, superheaters), 310/321 austenitic or 17-4 PH comes into the spec.
Standards, sourcing and where the market is moving
Stainless steel is one of the most heavily standardised alloy families: EN 10088, ASTM A240/A276/A479, JIS G4304/G4305 and ISO 15510 cover composition and mechanical property tables, while NACE MR0175/ISO 15156 governs materials for sour-service oil and gas environments. Buyers should always cross-reference the UNS number (S30400, S31600, S32205, S17400, etc.) rather than the trade name, because the same nominal grade can carry different compositional tolerances under different national standards. [S1]
On the supply side, Stainless Steel Focus reported on 2026-07-13 that global stainless melt-shop production rose 2.5% year-on-year in Q1 2026, and the same outlet flagged a major integrated stainless investment by Saritaş in Türkiye. The Stainless Steel Club tracks daily nickel, chromium and molybdenum surcharges that drive 304/316 base-price swings [S6], and Stainless Shapes notes ISO 9001:2015 certification on its US channel stock [S2]. For a parallel spec-driven reference on a different material family, the Screw Machine Part Price 2026: Cost Drivers, Sourcing Logic and Vendor Map article applies a similar selection-criteria approach to machined components, and the Wire Form Part Suppliers in 2026: Tier Map, Spec Gates and Sourcing Paths piece covers wire-formed stainless parts that often start as 304/316 wire rod.