Welding A515 Gr.65 steel plate for pressure vessel fabrication can lead to a localized reduction in strength and hardness within the Heat-Affected Zone (HAZ), a phenomenon known as HAZ softening. This occurs due to the tempering or overtempering of the steel's normalized microstructure by the welding heat cycle. While less critical than cracking, managing this softened zone is essential for ensuring the structural integrity of highly stressed components.
A515 Gr.65 derives its strength from its normalized microstructure. During welding, the area adjacent to the weld bead is heated to a temperature range below the critical transformation point (Ac1), typically between 1100°F and 1330°F (600°C and 720°C). This subcritical heating can over-temper the existing microstructure, causing carbide coarsening and a measurable drop in yield and tensile strength compared to the base metal. The width and degree of softening depend on heat input and preheat temperature.
Optimize Welding Heat Input: Use the lowest practical heat input that still ensures proper fusion and defect-free welds. Higher heat input increases the width and severity of the softened zone. Employ techniques like multi-pass welding with stringer beads to better control the thermal cycle.
Implement Proper Post-Weld Heat Treatment (PWHT): A correctly executed PWHT (Stress Relieving) cycle in the range of 1100°F–1200°F (595°C–650°C) can help. While its primary purpose is stress relief, it also homogenizes the HAZ microstructure. Crucially, the entire component is heated uniformly, preventing the localized overtempering caused by welding and restoring a more consistent strength profile.
Design and Procedure Considerations: In critical applications, design the weld joint to place it in a lower-stress region of the assembly. Qualify the Welding Procedure Specification (WPS) to demonstrate that the mechanical properties across the welded joint, including the softened HAZ, still meet the minimum required design strength.
