Study On Phase Transformation And Mechnical Properties Of 55CrMo Steel During Induction Hardening

55CrMo is a medium carbon and low alloy steel,which is widely used in ball screw.Compared with the technical quality of foreign ball screw,it is prone to crack,uneven hardness,poor wear resistance,or other problems during the induction hardening of the domestic ball screw.The production quality and working efficiency of ball screw are affected.To improve the surface hardness and hardness uniformity of a ball screw after induction hardening,the process parameters should be carefully selected for 55CrMo steel to obtain the desired microstructures and properties to enhance the wear-resistance performance for prolonging work life.Firstly,the experiments were carried out using a Gleeble-1500D thermo-mechanical simulator equipped with a dilatometry system.By testing the expansion curves of different process parameters,the effects of process parameters(heating rate,austenitizing temperature,holding time,cooling rate)on the phase transformation(austenite,ferrite and pearlite,bainite,martensite)of 55CrMo steel during heating and cooling were studied.Microstructural characterization was performed using an optical microscope and a scanning electron microscope.Volume fraction of retained austenite was measured by X-ray diffractometer.The phase transformation characteristics were analyzed by means of expansion curve,microstructure and microhardness.Secondly,based on the results of the thermal simulation experiment,induction heating power supply and the multiturn ratio intermediate frequency quenching transformer were used for heating the samples in the experiments.The microstructure analysis,tensile and hardness test were carried out.The tensile strength and fracture morphology were analyzed,and the mechanical properties and failure mechanism were evaluated.Finally,as a contrast,the microstructure and properties of medium carbon S55C steel were studied.The results show that,the rapid heating and quenching process was different from the conventional quenching,which was beneficial to the refinement of the microstructure of 55CrMo steel and the obtaining of the cryptocrystalline martensite microstructure.Compared with the S55C steel,55CrMo steel had better hardenability,microstructure and properties after heat treatment.(1)The experimental results of thermal simulation test show that,during rapid heating,the austenitizing temperature of 55CrMo steel increases with the increase of heating rate.During the cooling of 55CrMo steel,the phase transformation includes ferrite pearlite transformation at high temperature,bainite transformation at medium temperature,martensite transformation at low temperature.Different process parameters affect the final phase transformation type.With the increase of heating temperature and holding time,the dissolution of original microstructure was promoted,the stability of undercooled austenite was improved,and the volume fraction of martensite in quenched samples increased.When the temperature was above 900?,only martensite transformation occurs,and the content of retained austenite was lower.When the heating temperature is higher than 1000?,the volume fraction of retained austenite increased and the grain size became coarse,resulting in cracks.(2)The experimental results of induction hardening of 55CrMo steel show that,when the heating temperature was 800?,the structure was a non-uniform austenite,and a mixed microstructure consisting of ferrite pearlite,bainite and martensite was formed during the subsequent cooling.It was low hardness and uneven tensile strength.When the heating temperature was in the range of 850-950?,a complete cryptocrystalline martensite with high hardness could be obtained.The volume fraction of retained austenite was lower.The tensile strength was high,and the fracture mechanism was the mixed mechanism of quasi-cleavage fracture,cleavage and micropore polymerization.However,with the increase of heating temperature,the tensile strength of the samples decreased.It appeared brittle fracture.The austenitization temperature in the range of 850-950? was the better parameters of induction hardening for the parts of 55CrMo steel required with higher hardness,strength and wear resistance.(3)The experimental results of induction hardening of 55CrMo steel show that,the microstructure of S55C steel during induction heating with oil quenching was martensite and bainite.The microstructure of S55C steel quenched by water during induction heating was more uniform than that by oil quenching,but the water was too cold,and intergranular crack was easy to occur.Therefore,the quenchant with cooling rate between water and oil could be chosen during induction hardening of S55C steel.Although the microstructure was fine martensite at lower temperature,the fracture morphology of the oil-quenched sample was brittle fracture at room temperature.When the heating temperature was 900?,the cleavage morphology contained more cleavage steps,and the tensile strength was higher.Therefore,the induction heating temperature was in the range of 900-1000? during induction heating of S55C steel by oil quenching.Therefore,the microstructure of 55CrMo steel was more uniform,the hardness was more uniform and the tensile strength was higher than that of S55C steel.