Effects of Q&T Parameters on Phase Transformation, Microstructure, Precipitation and Mechanical Properties in an Oil Casing Steel

The heat treatments with different quenching and tempering temperatures were conducted to clarify the effects of Q&T (quenching and tempering) parameters on phase transformation, microstructure, precipitation, mechanical properties in an oil casing steel. Results reveal that the optimum property of tensile strength of 1071 MPa and elongation of 16.62% was obtained at quenching temperature of 920°C and tempering temperature of 715°C. The volume fraction of carbides decreased with quenching temperature and increased with the tempering temperature. The increased volume fraction of carbides at a higher tempering temperature contributes to the higher strength by precipitation strengthening. The decreased strength at a higher quenching tempering temperature originates from the larger prior austenite grains and concomitantly decreased volume fraction of carbides. Moreover, with the increase of tempering temperature, the carbides transformed from needle or rod-like shapes to almost spherical. And the apparent yield point appeared in the specimen quenched at 920°C and tempered at 715°C due to interaction between dislocations and carbide particles during deformation.