Abstract: Effect of tempering time on the microstructure and properties of a low welding crack sensitivity pressure vessel steel with designed chemical composition was studied using metallographic microscope, scanning electron microscope, transmission electron microscope, tensile testing machine, impact testing machine and hardness tester, etc. The tested steel was quenched at 950 ℃ after rolling and tempered at 610 ℃ for 12, 23, 45 and 180 min, respectively. The results indicate that the microstructure after quenching and tempering is mainly composed of tempered sorbite. With the increase of tempering time, the intragranular dislocation density decreases, and lath grains with different orientations are generated, and carbides are precipitated along the grain boundary, which ensures that the steel plate has a good low welding crack sensitivity. As the tempering temperature increases, the strength decreases first and then increases due to the influence of intragranular dislocation density and carbides of Nb and V elements, while the impact property and hardness are basically decreasing due to the precipitation of carbides M₂₃C₆.

Key words: tempering time, pressure vessel steel, low welding crack sensitivity, tempered sorbite, mechanical properties