Effect of cooling time on HAZ toughness and microstructure: Simulated HAZ toughness of low‐alloy SQV‐2A pressure vessel steel (1st report)

Summary This paper describes an investigation of the microstructure and Charpy impact energy of the simulated weld HAZ of low‐alloy SQV‐2A steel with particular reference to the effect of the formation of M‐A constituent and its morphology. Weld HAZ thermal cycles having cooling times ?t8/5 from 6 to 1000 sec, which reflect various welding heat inputs, were simulated with a Gleeble 1500 thermal/mechanical simulator. The formation of M‐A constituent and its morphology were examined by TEM and SEM. The M‐A constituent of this steel forms at cooling times ?t8/5 above 20 sec. With an increasing cooling time, the M‐A constituent forms not only near the grain boundaries, but also in the grain interiors (transgranularly) and changes from an elongated to a massive shape. Both elongated and massive M‐A constituents appear to impair the HAZ toughness of SQV‐2A steel. The M‐A constituent acts as a microcrack initiation site inducing toughness loss, but the toughness appears to be affected not only by the M‐A constituent, but also by the matrix, which is embrittled with an increasing cooling time.