Effect of tempering temperatures on the mechanical properties and microstructures of HSLA-100 type copper-bearing steels

Two copper-bearing high-strength low-alloy (HSLA) steels with chemistry similar to HSLA-100, were made on a laboratory scale, one in an air induction (100 kg) furnace and the other in a vacuum induction (50 kg) furnace. The ingots cast were hot-rolled to 25 mm thick plates which were subsequently austenitized and tempered at different temperatures (400–700°C) for 1 h. Evaluation of mechanical properties and microstructure of as-quenched and tempered plates revealed that substantial improvement in strength (YS-1024 and 1025 MPa; UTS-1079 and 1111 MPa for steels 1 and 2) occurred at the expense of impact toughness on tempering at 500°C owing to profuse Cu precipitation in the matrix. With increase in tempering temperature however, the notch toughness improved considerably, reaching peak values of 53 and 123 Joules (J) at −85°C for steels 1 and 2 at 650 and 700°C tempering temperatures, respectively. The partially recovered matrix and the coarsened Cu precipitates in this temperature range presumably enhanced dislocation movement and notch toughness.