Effect of alternately time quenching process on microstructure and mechanical properties of low alloy steels

The fraction and form of retained austenite could have an effective effect on mechanical properties of a Fe-0.38 C-2.03 Mn-1.66 Si-0.65 Cr (wt%) bainitic steel. Based on the method of altering microstructure of low alloy steel by Alternately timed quenching (ATQ), the relationship between quenching time and the change rule of retained austenite was discussed. The microstructure was investigated and observed through TEM, SEM and XRD. The performance was determined by tensile and impact test. The results showed that the microstructure composed of bainite, martensite and retained austenite, and the values of the tensile strength, impact toughness and elongation were about 1100 MPa, 74 J cm(-2) and 4.8%, respectively. Compared with quenching and partitioning (Q and P) process, with prolonging of water cooling time, the strength gradually increased and reached the maximum value of 1184 MPa at 25 s, while the elongation and toughness enhance first and then decrease, and reached the excellent values of 5.90% and 97 J cm(-2) at 20 s respectively. In addition, the maximum product of strength and elongation (PSE, 6.77 GPa%) at 20 s of water quenching time could be obtained during ATQ processes. Due to the formation of film-like retained austenite under different cooling time, the ATQ process could dramatically elevate the combination mechanical properties of steel.