Influence of heat treatments & tempering time on the mechanical properties of SA 387 steel

Pressure vessels are subjected to various adverse working conditions during their service period. Hence material used for pressure vessels must possess excellent mechanical and chemical properties. Low alloy steels like SA 387, possess very good mechanical properties and corrosion resistance. Additionally, mechanical properties of these steels can be easily manipulated by using appropriate heat treatments. The selection of tempering temperature and tempering time is crucial to achieve the desired mechanical properties. Tempering temperature for SA 387 steel is specified in ASME codes for pressure vessel design, however, no information about tempering time is provided. In the present investigation, the effect of tempering time on the mechanical properties of SA 387 pressure vessel steel has been studied. Two types of heat treatments were performed, Set A (Quenching-Tempering) & Set B (Quenching-Shallow Cryogenic Treatment-Tempering). Hardening, shallow cryogenic treatment and tempering were carried out at 920^oC, -80^oC and 650^oC respectively. For tempering, specimens were hold at 650^oC temperature for three different tempering times i.e., 2.5 h, 5 h and 10 h respectively. Significant changes were observed in microstructure at different tempering times. The influence of tempering time on mechanical properties has been attributed towards the changes in microstructure. These observations were verified using the images obtained by scanning electron microscope (SEM).