Research On The Hardenability And Phase Transformation Behavior Of Heavy Transmission Gear Steel

In this thesis,the effects of chemical composition fluctuations of heavy-duty transmission gear steel 20Cr2Ni4 on its hardenability,phase transformation,mechanical properties and carburizing distortion were investigated by using end-quenching,dilatometry and heat treatment distortion simulation tests.The main conclusions are as follows:The results of hardenability experiment show that C exhibited the most significant influence on the hardenability among those elements investigated,i.e.,with the increase of C content,both the hardness and hardenability increased,and the turning point of hardenability curve moved to the right side.The effect of alloying elements is mainly realized through the end-quenching characteristic value Eb;with the increase of alloy elements contents,the turning point of the hardenability curve also moved to the right side.The hardenability prediction model obtained through optimizing the interaction values among alloying elements showed higher precision with error limit less than 5%.The measurement of the continuous cooling transformation(CCT)diagram of the tested steel revealed that CCT diagram moved to the right,and the martensite transformation region was enlarged,the bainite transformation region was shifted to the right side of the diagram while the pearlite transformation region gradually disappeared.The critical cooling rate for martensite transformation is-4.3 ?/s,while when the cooling rate was lower than-0.86 ?/s,a small amount of acicular bainite began to appear.The experimental results of carburizing distortion simulation test showed that both the total distortion value and distortion rate increased with the increase in the contents of C,Cr and Ni,while the effect of C was the most significant.It was found that hardenability and martensite transformation point(Ms)are the main factors that affect the distortion of carburizing heat treatment,that is to say,both the total distortion value and distortion rate increased with the increase in hardenability and the decrease in Ms point.