Heat Treatment Process And Numerical Simulation Of Marine Crankshaft Journal S34MnV Steel

Marine crankshafts journal is the key component of large marine engine,and its manufacturing level determines the national shipbuilding capacity.S34MnV,as an advanced material for marine crankshafts,has been widely used in crankshafts manufacturing.Heat treatment is an important part of crankshafts production process.Nowdays,computer technology,testing technology and control theory develop by leaps and bounds,the emerging heat treatment simulation technology solves the problems of traditional heat treatment,such as time-consuming,laborious and unable to quantitative analysis.The microstructure transformation rules,transformation kinetics and grain growth of S34MnV steel during heating and cooling were discussed by means of DIL-805AD/T dynamic thermal expansion phase transformation instrument,vacuum atmosphere furnace,OM optical microscope,SEM scanning electron microscope,energy spectrum composition scanning and XRD phase detection.The end quenching and tempering experiments were carried out by using the self-made end quenching test equipment,and the numerical simulation of end quenching was verified and optimized by using the experimental data.The mathematical model is used to numerically simulate the S34MnV steel transmission shaft forgings,and a beneficial exploration is made for the heat treatment of S34MnV steel.Firstly,the austenization and austenite grain growth of S34MnV steel were studied:by using DIL-805AD/T dynamic expansion phase transformation instrument,the austenization heat simulation test of S34MnV was carried out at different heating rates.The expansion curves of austenite at different heating rates were measured,and the temperature points of Ac₁ and Ac₃ were determined by tangent method.The function relationship between Ac₁,Ac₃ and heating rate and the kinetic parameters of austenitizing transformation of S34MnV steel are fitted.The transformation curve calculated by J-M-A kinetic model is in good agreement with the transformation time curve measured by experiment;the austenization heat simulation test of S34MnV steel was carried out by DIL-805AD/T dynamic expansion transformation instrument at different heating temperature and holding time.Using grain boundary corrosion and critical point method to measure the average austenite grain size of S34MnV,and the austenite grain growth of the steel was analyzed.The kinetic model of austenite grain growth of S34MnV steel was established by optimizing the parameters of Sellars model.The calculated results obtained by using Sellars model are in good agreement with the measured data.Secondly,the cooling decomposition process of undercooled austenite in S34MnV steel was studied:the isothermal cooling thermal simulation test of S34MnV steel was carried out by DIL-805AD/T dynamic expansion phase change instrument at different temperatures.The isothermal transformation expansion curve of 380-700°C temperature range was measured.The analysis of expansion curve and microstructure showed that the transformation of ferrite and pearlite occurred at 600-700°C and bainite transformation at 380-530°C.The transformation was determined by fitting the expansion curve with tangent method and lever law,and the isothermal transformation curve(TTT)of S34MnV steel was obtained.The temperature of nose tip was about 400°C.The parameters k and n of J-M-A equation of pearlite and bainite phase transition are fitted.Utilizing the fitting equation to calculated the fraction time curve of phase variable,which is consistent with the experimental results.Making use of DIL-805AD/T dynamic expansion phase change instrument to test the continuous cooling transformation of S34MnV steel at different cooling rates.The phase change expansion of S34MnV steel at different cooling rates was measured.It can be known that ferrite and pearlite transformation occurs when the cooling rate is less than 0.08°C/s,pearlite and bainite transformation occurs when the cooling rate is between 0.8 and 2°C/s,bainite and martensite transition occurs when the cooling rate is 8°C/s to15°C/s,and martensite transition occurs at a cooling rate of 30-100°C/s.The phase transition points Bs=580°C,Ms=354°C and Mf=229°C are determined by tangent fitting method.The CCT of S34MnV steel is obtained.Li model was modified by CCT curve and K-M equation was fitted.The kinetic model of austenite decomposition of S34MnV steel during continuous cooling was obtained,and the CCT curve calculated by the model was in good agreement with the experimental resultsNext,the end quenching and tempering experiments of S34MnV steel are carried out:the end quenching experiments of S34MnV steel are carried out by using self-made end quenching test device and vacuum atmosphere furnace.The relationship between hardness,microstructure and distance from quenching end of S34MnV steel are deeply analyzed.The results show that the hardness of the quenched end is the highest(about HRC55.2),and the hardness gradually decreases to the average value of 29HRC with the increase of the distance from the quenched end.It is found that the microstructure of the end quenched sample of S34MnV steel changes continuously from Martensite to Bainite to Pearlite to Ferrite.The combined hardness shows that there is a semi Martensite zone at 15mm,that is,the hardened layer is 15mm.The tempering test of S34MnV steel after end quenching was carried out by using self-made end quenching test device and vacuum atmosphere furnace.The microstructure type,morphology and distribution of S34MnV steel tempered after end quenching and the relationship between microstructure and properties were analyzed.The microstructure of S34MnV steel after tempering at 500°C is TM+UB+TT.Then,the numerical simulation of S34MnV steel end quenching test is carried out:the mathematical models needed in the heat treatment simulation of S34MnV steel are systematically introduced.The thermal and physical properties of S34MnV steel are calculated by using JMat Pro material performance calculation software,such as density,specific heat capacity,thermal conductivity,latent heat of phase change,etc.ABAQUS subroutine was used for secondary development.The calculation models of temperature field,austenite grain growth,microstructure field and hardness in the process of heating and cooling were embedded into ABAQUS finite element software with Fortran language code for numerical simulation.The simulation results of temperature,microstructure and hardness in the end quenching process of S34MnV were obtained,and the reliability of the model was verified by the end quenching experimental results,which provided the basis for the next forging simulation.Finally,the numerical simulation of heat treatment process for marine S34MnV steel transmission shaft forgings is carried out:the established mathematical model was used to simulate the large forging of S34MnV steel marine transmission shaft,and the temperature change and austenite grain growth of S34MnV steel transmission shaft forging during heating and heat preservation were predicted.The results show that the simulation results of temperature distribution and grain size of the whole forging are more uniform and the grain size is more appropriate when holding for 9000s.The temperature change,microstructure change and hardness distribution of S34MnV steel transmission shaft forging during water quenching were predicted.It is found that the martensite layer is mainly on the surface and near the surface,with a hardness of 582HV(54.2HRC).The Bainite layer is far away from the surface,and the Pearlite+Ferrite zone is in the center.The hardness value decreases gradually,with a hardness of 194HV(13.6HRC)in the center.