Mod 2 - Stainless Steels

Module from the Materials and Corrosion Curriculum curriculum.

STAINLESS STEELS · Learning Objectives · 1. Distinguish between austenitic, ferritic, martensitic, duplex, and precipitation-hardening stainless steels 2. Understand the role of Cr, Ni, Mo, and N in corrosion resistance 3. Select stainless grade based on service conditions and corrosion requirements 4. Recognize sensitization, stress corrosion cracking, and other SS-specific damage · Stainless Steel Families

Family	Example Grades	Chemistry	Strength	Corrosion Resistance	Weldability	Typical Application
Austenitic	304, 316, 316L, 321, 347	18Cr-8Ni to 18Cr-14Ni-3Mo	Moderate (Sy ~30 ksi)	Excellent general	Excellent	Piping, vessels, food/chemical
Ferritic	405, 409, 430	11-17Cr, low Ni	Moderate (Sy ~40 ksi)	Good general, poor pitting	Good	Automotive exhaust, tubes (stabilized)
Martensitic	410, 420, 440	12-17Cr, 0.1-1.2%C	High (Sy ~80-160 ksi)	Moderate	Difficult	Knife blades, pump shafts, valve trim
Duplex	2205, 2304, 2507	22-25Cr, 5-7Ni, 3-4Mo, N	High (Sy ~65-80 ksi)	Excellent (esp. Cl-SCC)	Good with care	Chemical tanks, sour service, seawater
Super Duplex	2507, 255	25Cr, 7Ni, 4Mo, N	Very High (Sy ~80 ksi)	Outstanding (PREN >40)	Good with care	Seawater, offshore, aggressive service
Precipitation-Hardening	17-4 PH, 15-5 PH	17Cr-4Ni-4Cu	Very High (Sy ~150 ksi)	Good	Good	High-strength pump/compressor shafts, bolts
Stainless Steel Damage Mechanisms
Mechanism	Cause	Susceptible Grades	Temperature	Environment	Prevention
Sensitization	Cr carbide precipitation at grain boundaries	304, 316 (non-L grades)	800-1500°F	After welding or slow cooling	Use L-grades (low carbon) or stabilized (Ti/Nb)
Chloride SCC	Cl⁻ + tensile stress + moisture	304, 316 (austenitic)	>140°F	Chlorides >50 ppm + moisture	Avoid chlorides, use duplex SS, insulate properly
Caustic SCC	NaOH + tensile stress	All austenitic, ferritic	>150°F	Caustic solutions	Alloy 600/625 for caustic service, low stress
Polythionic Acid SCC	Sulfur + moisture during downtime	Sensitized austenitic SS	Ambient (during shutdown)	Sulfur compounds + H2O + O2	N2 purge during shutdown, soda ash wash, L-grades
Sigma Phase	Long-term exposure forms brittle sigma	Duplex, 309, 310 ferritic/austenitic	1000-1700°F	Prolonged time in range	Solution anneal if exposed, limit temperature
Pitting / Crevice	Chloride attack on passive film	Austenitic (especially 304)	Any	Chlorides, seawater, stagnant conditions	Higher PREN (316L or duplex), design to avoid crevices
Source: FOS Chief Files — Metallurgy folder (SS Composition, SS Grades, SS Physical Properties), IPE-EP-10 series

Source: Materials_and_Corrosion_Curriculum_v1.xlsx · Sheet: Mod 2 - Stainless Steels

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