Stainless Steel Families: Austenitic, Ferritic, and Martensitic

## What Makes Steel Stainless All stainless steels contain at least 10.5% chromium by weight. Chromium reacts with oxygen to form a thin, adherent chromium oxide (Cr₂O₃) passive film on the surface that acts as a barrier to further oxidation. This film is self-repairing: if scratched, it reforms in air or water within seconds. Most grades also contain nickel, molybdenum, manganese, or other elements to improve corrosion resistance, mechanical properties, or processability. The three main microstructural families—austenitic, ferritic, and martensitic—differ fundamentally in their crystal structure, magnetic behavior, heat treatability, and corrosion performance. ## Austenitic Stainless Steels (200 and 300 Series) Austenitic stainless steels have a face-centered cubic (FCC) crystal structure stabilized by nickel and/or manganese. They are the most widely used stainless family, accounting for roughly 70% of stainless production. **Key characteristics:** - Non-magnetic in the annealed condition (cold work can induce some magnetism in 300 series through martensitic transformation) - Cannot be hardened by heat treatment; cold work is the only way to increase strength - Excellent weldability in most grades - Good toughness at cryogenic temperatures - Superior corrosion resistance compared to ferritic and martensitic grades **AISI 304 (S30400, 18-8 stainless)**: 18% Cr, 8% Ni. The most common stainless steel in the world. Resists most oxidizing acids, food products, and atmospheric exposure. Not suitable for marine environments or chloride-rich conditions without careful consideration of crevice and pitting corrosion risk. Tensile strength (annealed): 515 MPa minimum. **AISI 316 (S31600)**: 16-18% Cr, 10-14% Ni, 2-3% Mo. The molybdenum addition moves the pitting resistance equivalent number (PREN = %Cr + 3.3 × %Mo + 16 × %N) from ~18 for 304 to ~26 for 316, significantly improving resistance to chloride pitting. Widely specified for marine hardware, chemical processing, and pharmaceutical equipment. **AISI 316L**: Like 316 but with carbon limited to 0.03% maximum, reducing the risk of sensitization (carbide precipitation at grain boundaries that depletes the chromium adjacent to grain boundaries) during welding. Used for welded assemblies exposed to corrosive environments. ## Ferritic Stainless Steels (400 Series) Ferritic stainless steels have a body-centered cubic (BCC) crystal structure similar to plain carbon steel. They contain chromium but little or no nickel, which keeps their cost lower than austenitic grades. **Key characteristics:** - Magnetic - Cannot be hardened by heat treatment - Lower nickel content means lower cost than austenitic grades - Good resistance to stress corrosion cracking (SCC)—a significant advantage over austenitic grades in chloride environments - Lower toughness than austenitic grades, especially at low temperatures - Limited weldability in thick sections due to grain growth in the HAZ **AISI 430 (S43000)**: 16-18% Cr, no nickel. The most common ferritic grade. Used for automotive trim, appliances, kitchen equipment, and architectural panels where mild corrosion resistance is sufficient and cost matters. **AISI 444 (S44400)**: 17-19.5% Cr, 1.75-2.5% Mo, stabilized with Ti and Nb. Higher corrosion resistance than 430 and lower SCC susceptibility than 316. Used for solar water heater tanks and hot water systems where 304 suffers SCC and 316 is cost-prohibitive. ## Martensitic Stainless Steels (400 Series) Martensitic stainless steels can be hardened by quenching and tempering because their carbon content is high enough to form martensite. They typically contain 12-18% Cr and higher carbon than ferritics. **Key characteristics:** - Magnetic - Heat-treatable to high hardness (up to ~60 HRC for 440C) - Lowest corrosion resistance among the common stainless families - Good strength but lower ductility and toughness than austenitic grades **AISI 410 (S41000)**: 11.5-13.5% Cr, 0.08-0.15% C. The base martensitic grade. Hardened and tempered: 620–755 MPa tensile strength, 25–30 HRC. Used for cutlery, fasteners, pump shafts, and valve parts. **AISI 420 (S42000)**: Higher carbon (0.15-0.40%) for greater hardness potential. Hardened and tempered to 50–52 HRC; used for surgical instruments, knife blades, and molds. **AISI 440C (S44004)**: Highest carbon of the standard martensitic grades (0.95-1.20% C). Reaches 58–60 HRC when properly hardened. Superior wear resistance and hardness make it the choice for high-quality knife blades, bearings, and valve balls in non-aggressive environments. ## Duplex Stainless Steels Duplex stainless steels (e.g., 2205, S32205) have a two-phase microstructure of roughly equal amounts of austenite and ferrite. This gives them: - Higher yield strength than austenitic grades (typically 450–550 MPa minimum vs. 170–205 MPa for 304/316) - Better resistance to chloride SCC than austenitic grades - Good weldability - PREN above 35 for standard duplex, above 40 for superduplex grades like 2507 Duplex grades are the preferred choice for seawater piping, offshore structures, and chemical plant equipment where austenitic grades suffer SCC failures.