Research On Strengthening And Toughening Mechanism Of Carburized Gear Steel C61 Under Different Heat Treatment Processes

In this paper,high temperature carburizing alloy C61 gear steel designed by Questek Co.,Ltd.in the United States is taken as the reference object.On the basis of adjusting the alloy composition,C61 gear steel is smelted and forged by our own.By means of mechanical properties test（hardness test,tensile test and impact test）,metallographic microscope（OM）,scanning electron microscope（SEM）,X-ray diffraction（XRD）and transmission electron microscope（TEM）,the evolution of microstructure and mechanical properties of C61 gear steel under different heat treatment conditions were investigated.The results are as follows:（1）When quenched at 850℃～1050℃,the hardness value of C61 gear steel gradually decreases with the increase of quenching temperature.Among them,at 850℃,the hardness value is the largest with a value of44.25 HRC.When the temperature reaches 1050℃,the hardness value is the lowest,with a value of 39.97 HRC.The tensile strength（R_m）and yield strength(Rp_0.2)decrease with the increase of quenching temperature,and the elongation and reduction of area reach their peaks at 950℃.Through comparison,it is found that the best quenching temperature of C61 gear steel is 950℃,and after heat treatment,a fine and uniform lath martensite structure can be obtained.At this time,the tensile strength of the steel is 1193MPa,the yield strength is 1151 MPa,the impact absorption energy is 174.5J,and the hardness is 41.97 HRC,which can achieve the best matching of strength and toughness;（2）When tempering at 360℃～600℃,the hardness of C61 gear steel gradually increases with the increase of tempering temperature.At 482℃,the hardness is 41.97 HRC.When the temperature is increased to 540℃,the hardness reaches the highest The value is 42.3 HRC,and there is not much difference between the two.After that,as the temperature increased,the hardness of C61 gear steel began to decrease,and the hardness appeared to the lowest value at 600℃,which was only 29.88 HRC.The tensile strength and yield strength of the test steel showed a trend of increasing first and then decreasing,reaching a peak at 420℃and decreasing to a minimum at 600℃,respectively 837 MPa and 834 MPa.Elongation and reduction of area have different trends in strength,and both increase gradually with the increase of tempering temperature;（3）For C61 gear steels with different tempering times,the length of the tempering time has little effect on the microstructure of the test steel.When tempered for 4-20 h,the matrix structure remains as a tempered lath.Stenite+retained austenite+a small amount of carbides.The hardness of the test steel increased first and then decreased with the increase of tempering time.The strength gradually decreased,while the toughness gradually increased.At the same time,the coherent relationship between the M2C carbide and the matrix was destroyed due to the extension of the tempering time,and the dispersion strengthening effect was weakened.,Resulting in a decrease in the strength and hardness of the test steel.（4）After hot rolling and water cooling,the tensile strength and reduction of area of C61 gear steel are 1450 MPa and 62.7%,respectively,which are increased by 21.5%and 23.3%compared to the forged state.XRD analysis shows that the retained austenite content of the steel increases after hot rolling,and the face-centered cubic austenite is beneficial to improve the toughness of the test steel.It can be seen that C61 gear steel can help to improve the mechanical properties after hot rolling.