Effect of Heat Treatment Process on Microstructure and Properties of 80 kg Hydropower Station Steel

The influence of heat treatment process on the microstructure and properties of 80 kg hydropower station steel was studied by optical microscope,tensile testing machine and low-temperature impact toughness testing machine.The microstructure changes of steel at different quenching and tempering temperature were analyzed.The mechanical properties changes of test steel under different heat treatment conditions wer also discussed.The results show that test steels under the low temperature quenching produces granular bainite and under high temperature quenching forms lath bainite.The dislocations formed during quenching merge and recombine and disappear,and the stress in quenching is released when the test steel is tempered at different tempering temperatures.With the increase of tempering temperature,the lamellar structure appeared merging and growing,and the ferrite volume fraction increased,and gradually transformed into polygonal ferrite.The yield strength firstly increases and then decreases and reaching its maximum value at 630 ℃ tempering temperature.Tensile strength decreases unilaterally and fails to meet requirement when the tempering temperature reaches 650 ℃.The percentage elongation and-40 ℃ low temperature impact toughness increases significantly at tempering stage.Thus quenching temperature has been proposed at 900～930 ℃ and tempering temperature at 600～630 ℃ for the best performance.