stainless-steel-grades

name	stainless-steel-grades
description	Stainless steel grades — austenitic (304/316/317), ferritic, martensitic, duplex, PH grades, mechanical properties, corrosion resistance, weldability, applications.
metadata	{"priority":7,"promptSignals":{"phrases":["stainless steel grade","304 stainless","316 stainless","duplex stainless","austenitic stainless","martensitic stainless","17-4 PH"],"minScore":3}}

Stainless Steel Grades — Complete Skill

Classification

Stainless steels contain ≥ 10.5% Cr (minimum for passive Cr₂O₃ layer)

Austenitic (300 Series) — Most Common

FCC structure; non-magnetic; not heat-treatable for strength (cold work only) 304 (18-8): 18Cr-8Ni; general purpose; S_y = 205 MPa, S_u = 515 MPa; excellent weldability 304L: Low carbon (0.03% max); better weld sensitization resistance; S_y = 170 MPa 316: 18Cr-10Ni-2Mo; better pitting (Mo); seawater, acids; S_y = 205 MPa 316L: Low carbon version of 316; welded vessels, pharmaceutical 317L: 19Cr-13Ni-3Mo; higher Mo → better pitting resistance than 316 321: Ti-stabilized; high temperature; prevents sensitization (furnace parts) 347: Nb-stabilized; welded structures; elevated temperature

Sensitization: carbide precipitation at 425–815°C → Cr-depleted zones → intergranular corrosion Prevent: use L grades (low C); stabilized grades (321, 347); solution anneal after welding

Ferritic (400 Series)

BCC structure; magnetic; lower cost; not hardenable; limited toughness at low T 409: 11Cr; automotive exhaust; limited corrosion resistance; cheapest 430: 17Cr; appliances, automotive trim; moderate corrosion; weldable but grain growth 444: 18Cr-2Mo; better pitting than 430; water heaters; lower thermal expansion than 316

Martensitic (400 Series)

Heat-treatable; magnetic; high strength but lower corrosion resistance 410: 12Cr; S_y = 275 MPa (annealed); up to 1100 MPa (hardened); cutlery, pumps, valves 420: 13Cr+C; higher hardness (HRC 50+); surgical instruments, razor blades 440C: 17Cr-1C; highest hardness (HRC 58–60); bearing races, knife blades

Duplex (Ferritic-Austenitic)

~50/50 ferrite/austenite microstructure; higher strength + better SCC than austenitic 2205 (22Cr-5Ni-3Mo-0.15N): S_y = 450 MPa; pitting equivalent PREN = 35; oil/gas, seawater 2507 Super Duplex (25Cr-7Ni-4Mo-0.27N): S_y = 550 MPa; PREN = 42; offshore, chemical Lean duplex (LDX 2101, 2304): lower Mo/Ni; cost-effective; S_y = 380–480 MPa

Precipitation Hardening (PH)

Heat treatable to very high strength while maintaining corrosion resistance 17-4 PH (630): 17Cr-4Ni-4Cu; H900: S_y = 1170 MPa, S_u = 1310 MPa; H1150: S_y = 790 MPa Higher H-number = higher aging temp = lower strength, better toughness 15-5 PH: similar to 17-4; improved toughness; aerospace fasteners 17-7 PH: cold work + age; highest strength; spring/strip applications PH 13-8 Mo: excellent H-bearing corrosion resistance; highest strength PH grade

Pitting Resistance Equivalent Number (PREN)

PREN = %Cr + 3.3×%Mo + 16×%N

Grade	PREN	Environment
304	~18	Indoor; non-chloride
316	~25	Mild chloride; coastal
2205 duplex	~35	Seawater splash zone
2507 super duplex	~42	Full seawater immersion
904L	~34	Dilute H₂SO₄ and HCl

Critical pitting temperature (CPT): temperature at which pitting initiates (ASTM G150) Higher PREN → higher CPT → better pitting resistance

Welding Stainless Steels

Austenitic (304/316): GTAW with 308/308L, 316/316L filler; no preheat needed; interpass T < 150°C Low heat input reduces sensitization Avoid Cl contamination during fabrication (stress corrosion risk)

Duplex: GTAW with 2209 filler; heat input 1–2.5 kJ/mm; interpass T ≤ 250°C Post-weld solution anneal if required (1020–1080°C); maintain 50/50 microstructure

Martensitic (410): Preheat 200–300°C; post-weld PWHT at 650–750°C to temper martensite GTAW with 309L or matching filler

Ferritic (430): Preheat 150–230°C; minimize heat input; grain growth in HAZ → may need 5-15%Ni filler

Corrosion Types

Pitting: local penetration; starts at surface defects; Cl⁻ accelerates; use PREN > 40 for seawater Crevice corrosion: at gaskets, flanges, threaded joints; eliminated by design or higher alloy SCC (stress corrosion cracking): austenitic stainless + Cl + tensile stress + elevated T → cracks Prevention: use duplex or nickel alloys above 60°C in Cl environment

Mechanical Properties Summary

Grade	S_y [MPa]	S_u [MPa]	El [%]	Hardness	Notes
304 annealed	205	515	40	82 HRB	General
316 annealed	205	515	40	80 HRB	Marine
2205 annealed	450	620	25	31 HRC	Duplex
17-4 H900	1170	1310	10	38 HRC	High strength
410 Q&T	760	900	20	28 HRC	Martensitic
440C hardened	—	—	2	58 HRC	Bearing grade

Output

Provide: grade recommendation, PREN (for corrosion environment), S_y and S_u [MPa], weld filler specification, heat treatment if applicable, SCC risk assessment, CPT vs. service temperature, corrosion test specification (ASTM G48, G36, G61 as applicable).