Mod 5 - High-Temp Damage

Module from the Materials and Corrosion Curriculum curriculum.

HIGH-TEMPERATURE DAMAGE MECHANISMS · Learning Objectives · 1. Understand creep and creep-rupture as life-limiting damage at elevated temperatures 2. Identify carburization, metal dusting, and sigma phase embrittlement 3. Apply damage rate vs temperature relationships 4. Specify inspection methods for high-temp damage detection · High-Temperature Damage Mechanisms

Mechanism	Cause	Susceptible Materials	Temperature Range	Indicator	Prevention / Mitigation
Creep	Long-term stress above ~40% of melting point	CS >750°F, 1.25Cr >850°F, 2.25Cr >900°F, austenitic SS >1000°F	Varies by material	Dimensional change, voids, tertiary creep	Limit stress, proper alloy, Larson-Miller curves for design life
Graphitization	Pearlite → graphite nodules (CS, Cr-Mo <1.25%)	CS and low-Cr-Mo (P1, P11)	800-1100°F	Carbon nodules at grain boundaries, reduced strength	Use P22 or higher Cr, replicate inspection
Carburization	Carbon diffusion into metal at high temp	Austenitic SS, HP/HK alloys, Ni alloys	>1500°F	Surface hardening, loss of ductility, brittle failure	Control carbon activity, select alloy (HP-Mod), clean furnace atmosphere
Metal Dusting	Catastrophic carburization with metal loss	Ni-rich alloys (incl. 800H, 617), SS in CO-rich	900-1500°F	Pitting, powder formation, rapid metal loss	Sulfide additions, Al-coated tubes, cleaner atmosphere
Sigma Phase Embrittlement	Fe-Cr intermetallic at grain boundaries	Austenitic SS with high Cr (>18%), duplex, HP/HK	1000-1700°F long exposure	Ductility loss, impact toughness drop, brittle failure	Solution anneal if sigma forms; avoid prolonged temp range
High-Temperature H2 Attack (HTHA)	H2 decarburizes CS/Cr-Mo at high temperature + H2 PP	CS, 0.5Mo, 1Cr-0.5Mo	500-1000°F (per API 941 Nelson curves)	Surface decarb, micro-fissures, loss of strength	Follow Nelson Curves API 941; use 1.25Cr or higher if outside
Thermal Fatigue	Cyclic thermal stresses from temperature changes	Any alloy, worse for high-expansion materials	Cycling through wide temperature range	Cracks at fillets, welds, nozzles	Minimize transients, design for thermal expansion, fatigue analysis
Oxidation / Scaling	O2 + metal → metal oxide at high temp	Any; worse above scaling limit of alloy	Alloy-dependent	Scale, metal loss, scale spalling	Higher Cr alloys (304→316→321→310→800H→617)
Source: API 571 Damage Mechanisms, API 941 (Nelson Curves), FOS Chief Files — Metallurgy folder, Hydroprocessing Design Manual

Source: Materials_and_Corrosion_Curriculum_v1.xlsx · Sheet: Mod 5 - High-Temp Damage

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