While A515 Gr.65 steel plate is specified for intermediate-temperature pressure vessels, its performance degrades under sustained high-temperature exposure. Understanding these long-term degradation risks is essential for safe operation, life assessment, and maintenance planning of critical equipment like boilers and reactors operating near their design limits.
Primary Degradation Mechanism: Graphitization
The foremost risk is graphitization – the transformation of iron carbide into soft graphite and ferrite, particularly in weld heat-affected zones. This process occurs slowly over thousands of hours at temperatures above 800°F (425°C). Graphitization dramatically reduces tensile strength, creep resistance, and fracture toughness, potentially leading to sudden brittle failure under stress. This microstructural change is a primary reason why ASME codes impose lower allowable stress values on A515 Gr.65 at elevated temperatures.
Graphitization initiates or exacerbates other failure risks:
Creep Damage: With reduced microstructural stability, the steel becomes more susceptible to creep deformation and cavitation under constant load, especially at temperatures approaching 900°F (480°C).
Hydrogen Attack: In hydrogen-containing environments at elevated temperatures, atomic hydrogen can diffuse into the steel, reacting with carbides to form methane bubbles at grain boundaries, causing decarburization and severe embrittlement.
Oxidation and Scaling: Prolonged exposure leads to gradual surface oxidation and scale formation, reducing load-bearing cross-section.
To manage these risks:
Design within Code Limits: Strictly adhere to ASME-mandated temperature and stress limits, which incorporate safety margins for long-term degradation.
Material Upgrade for Critical Service: For temperatures consistently above 800°F (425°C) or in hydrogen service, specify more stable alloys like A387 chrome-moly steel.
Implement In-Service Inspection: Regular ultrasonic testing (UT) and replication metallography can detect microstructural changes, graphitization, and incipient cracking before failure occurs.
