Proper heat treatment is fundamental to ensuring the structural integrity and long-term performance of A515 Gr.65 pressure vessel steel. This intermediate-temperature carbon steel relies on controlled thermal processes to achieve its specified mechanical properties and relieve fabrication stresses.
A515 Gr.65 plates are typically supplied in the normalized condition. This involves heating the steel to approximately 1650°F (900°C), holding until uniformly soaked, and then cooling in still air. Normalizing refines the grain structure, enhances uniformity, and achieves the required balance of strength and toughness as per the ASTM A515 specification. This condition is the baseline for all subsequent fabrication.
Post-Weld Heat Treatment (PWHT / Stress Relieving): This is mandatory after welding for most pressure vessel applications to prevent stress corrosion cracking and improve dimensional stability. The standard PWHT range for A515 Gr.65 is 1100°F to 1200°F (595°C to 650°C). Components are soaked at temperature (typically 1 hour per inch of thickness) and then cooled at a controlled rate.
Re-Normalization after Hot Forming: If the steel undergoes hot forming (e.g., head dishing) at temperatures that alter its microstructure, full re-normalization is required to restore the original grain structure and mechanical properties.
Strict adherence to qualified Heat Treatment Procedures (HTPs) is critical. Records must document heating rates, soak temperatures, holding times, and cooling rates. These parameters must align with the vessel's ASME Code construction requirements. The final material certification must reflect all performed heat treatments to ensure traceability and compliance.
