Microstructure and Mechanical Properties of X80 Pipeline Steels after QLT Process Heat-Treatment

A complex heat treatment,simply QLT process for brevity,consisting of sequential processes of quenching at the temperature of 920℃,intercritical quenching at the temperature of 830℃ and tempering at different temperatures was carried on X80 pipeline steel,and the microstructure and mechanical properties of the tested steel after heat treetment were studied.The results indicate that a mixed microstructure of ferrite,bainite and M/A island formed in the tested steel after QLT treatment.With the increase of tempering temperature,the quantity and size of ferrite increased and the number of M/A island reduced significantly,at the same time,the yield strength and tensile strength of the tested steel all decreased and the yield ratio increased.With the increase of the tempering temperature,M/A island further decomposed,which led to the low temperature impact energy of the tested steel at -20℃ greatly improved.The tested steel could obtain good combination of the strength and toughness when the tempering temperature was 430-490℃.