Effect of Cooling Time on the Weld HAZ Toughness and Microstructure

Weld HAZ Toughness and Its Improvement of Low Alloy Steel SQV-2A for Pressure Vessels (Report 1)

Abstract

The microstructure and Charpy impact energy of simulated weld HAZs of low alloy steel SQV-2A have been investigated with particular reference to the effect of the formation of M-A constituent and its morphology. Weld HAZ thermal cycles having cooling times Δt_8/5, from 6 to 1000 s, which reflect various welding heat inputs, were simulated with Gleeble 1500 thermal/mechanical simulator. The formation of M-A constituent and its morphology were examined with TEM and SEM. For this, steel, the M-A constituent formed at cooling times Δt_8/5 more than 20s. With increasing cooling time, the M-A constituent formed not only near the grain boundary but also in bulk region, and changed from elongated shape to massive shape. Both enlongated and massive M-A constituents seem to impair the HAZ toughness of SQV-2A steel. The M-A constituent acts as the initiation site of microcracking to induce the toughness loss, but the toughness may be affected by not only M-A constituent but also the matrix, because the matrix itself seems to become brittle with increasing cooling time.