A515 Gr.65 is generally considered to have good weldability. However, as a medium-carbon steel, it has a propensity for hardening in the heat-affected zone (HAZ) and increased risk of cold cracking if proper procedures are not followed. The primary concerns are hydrogen-induced cracking and the formation of brittle microstructures due to rapid cooling.
Preheat and Interpass Temperature: Preheating is the most critical step. A minimum preheat of 150°F to 300°F (65°C to 150°C) is typically required, depending on plate thickness and restraint. Maintaining a proper interpass temperature within this range is equally vital to slow the cooling rate, allowing hydrogen to diffuse and reducing HAZ hardness.
Filler Metal Selection: Use low-hydrogen electrodes (stick welding) or fluxes (submerged arc). Common choices include:
SMAW (Stick): E7018 electrodes.
SAW (Submerged Arc): AWS F7A*-EM12K or similar neutral flux/wire combinations.
GMAW/FCAW (MIG/Flux-Cored): ER70S-6 or E71T-1 class wires with CO₂ or mixed gas shielding.
Crucially, all filler metals must be kept dry per manufacturer instructions to minimize hydrogen.
Heat Input Control: Use a medium heat input. Too low an input increases cooling speed and HAZ hardness; too high can degrade mechanical properties. Follow qualified Welding Procedure Specifications (WPS).
Post-Weld Heat Treatment (PWHT): For most pressure vessel codes (like ASME Section VIII), PWHT (stress relieving) is mandatory for A515 Gr.65 beyond a certain thickness. A typical cycle involves heating to 1100°F - 1200°F (595°C - 650°C), holding for 1 hour per inch of thickness, and controlled cooling. This relieves residual stresses and tempers the HAZ.
Best Practices & Conclusion
Clean all joint surfaces thoroughly of rust, oil, and moisture.
Use a qualified WPS and perform procedure qualification tests (PQT).
For critical applications, always consult the relevant construction code (ASME, EN) and material specifications.
By strictly adhering to these guidelines—proper preheat, low-hydrogen practices, and correct PWHT—you can reliably weld A515 Gr.65 steel plate, ensuring the structural integrity required for demanding pressure vessel and boiler service.
