Study On Heat Treatment Process And Properties Of 42CrMo Steel Marine Crank

This article focuses on the formulation of the heat treatment process for 42 CrMo steel marine crankshafts.First,the grain growth law of 42 CrMo steel under different heating conditions is studied,and the microstructure evolution under different cooling conditions is discussed.Then the numerical simulation of the end quenching process verifies the accuracy of the microstructure transformation model.Finally,the temperature,grain size,organization and hardness distribution of marine crankshafts during the heat treatment were simulated.In order to guide the development of the heating process of marine crankshafts,thermal simulation tests of austenitization of 42 CrMo steel under different heating temperatures and holding conditions were carried out using the DIL-805-ADT dynamic quenching/expansion phase transducer.And the evolution of austenite grain size of 42 CrMo steel under different austenitization conditions was investigated using the the truncated line method.The results reveal that the steel is fully austenitized when heated to these temperatures and the grain growth is normal during the holding time.Temperature has an important effect on the grain size.Effect of holding time is minor.Grain growth is slow as holding time prolongs.Grain sizes possess an exponential relation with holding time.Experimental data were fitted to determine parameters in the Beck model,Sellars model and Anelli model.Anelli model is considered the most suitable thus is recommended for the prediction of austenite grain size of 42 CrMo steel in the determination of heat treatment temperature.In order to guide the development of the cooling process of marine crankshafts,the continuous cooling experiments and isothermal cooling experiments of 42 CrMo steel were carried out using the DIL-805-ADT dynamic quenching/expansion phase change instrument.And the microstructure transformation of 42 CrMo steel under different forms of cooling was studied,and the CCT curve and TTT curve of 42 CrMo steel were plotted.The result of continuous cooling transformation shows that when the cooling rate is 0.1°C/s,the cooling product is pearlite and ferrite.when the cooling rate is between 0.2-0.5°C/s,the cooling product is pearlite,ferrite,bainite.when the cooling rate is between 1-4.3°C/s,the cooling products are bainite and martensite,and the martensite content gradually increases with the increase of the cooling rate;When the cooling rate is greater than 5°C/s,the cooling product is martensite.Therefore,it can be inferred that the critical cooling rate of martensite transformation is between 4.3-5°C/s.In addition.The result of isothermal transformation shows that when the holding temperature is 320-540°C,the transformation product is bainite,and the bainite transformation is still incomplete.when the holding temperature is 560-720°C,the transformation product is pearlite and ferrite.The heat transfer model,phase change model,and hardness model used in the numerical simulation of heat treatment for marine crankshafts were established.Then,the basic parameters of 42 CrMo steel,such as density,specific heat capacity,thermal conductivity,and austenite decomposition parameters in equilibrium,were calculated using material performance software JMat Pro.The end quenching process of 42 CrMo steel is simulated and verified by experiments,which can be used to verify the correctness of the numerical simulation model.The temperature results show that after the sample is end-quenched for 600 s,the temperature at the top of the sample is230°C,while the temperature at the bottom of the sample is 81°C,and the temperature difference between the up and bottom of samples is nearly 150°C.The farther away from the bottom of the sample,the slower the temperature decrease.The microstructure results show that the microstructure of the end-quenched specimen after the simulated end quenching for 600 s is mainly a mixed microstructure of martensite,bainite and ferrite.The hardness results show that the hardness distribution is determined by the microstructure distribution.The hardness at the bottom of the sample is the largest,reaching 663 HV,and the hardness at the top of the sample is the smallest,only 330 HV.The simulation results are basically consistent with the experimental results.The temperature and grain size changes of 42 CrMo steel marine crankshafts during the heating process were predicted through computer numerical simulation,and the temperature,microstructure and hardness changes during the cooling process were predicted.The results show that the temperature and the grain size difference at each position of the crankshafts are very small when the heating speed is kept low during the heating process.During the cooling process,the temperature of the crankshafts at the ring changes quickly,so there are only two microstructures of martensite and bainite;the temperature of the core at the crankshafts arm decreases slowly,so There is some ferrite microstructure.The calculated surface hardness of the crankshafts can reach672 HV,while the core hardness is only 271 HV.