Assessing the microstructural stability of A515 Gr.65 steel plate under sustained high-temperature conditions is critical for determining its safe service life in applications like boiler shells and pressure vessels. While designed for intermediate temperatures, long-term exposure near its upper limit can initiate microstructural changes that degrade mechanical properties.
Primary Degradation Mechanism: Graphitization
The most significant microstructural concern for A515 Gr.65 in long-term high-temperature service (particularly above 800°F / 425°C) is graphitization. This process involves the gradual decomposition of metastable iron carbides (cementite) into stable, soft graphite nodules and ferrite. Graphitization occurs preferentially along grain boundaries or in the Heat-Affected Zone (HAZ) of welds, significantly reducing strength and, critically, impact toughness and creep resistance. This embrittlement poses a major risk to pressure boundary integrity.
Concurrent with graphitization, other changes can occur:
Carbide Coarsening: Existing carbides may grow and spheroidize over time, reducing dispersion strengthening and softening the steel.
Potential Temper Embrittlement: If specific impurity elements (e.g., phosphorus, tin) are present at elevated concentrations, exposure within certain temperature ranges can lead to segregation at grain boundaries, further reducing toughness.
These changes have direct consequences:
Reduced Allowable Stress: Design codes account for strength loss over time. The ASME Boiler and Pressure Vessel Code significantly reduces the maximum allowable stress for A515 Gr.65 as temperature increases and service time extends.
Inspection and Monitoring: Components in high-temperature service require regular non-destructive inspection (NDE). Ultrasonic testing (UT) is vital for detecting cracks or flaws that may initiate more easily in embrittled material. Replication metallography can sometimes be used to monitor microstructural changes directly.
