| Heat treatment,as an important step in thermal processing of metallic materials,plays an important role in the improvement to their properties.However,heat treatment is a complex process with the involvement of multiple physical fields,rendering some problems intractable by mere analytic solution or by mere experimental observation.For instance,the nonuniform temperature distribution and the non-simultaneous phase transformations during quenching may lead to various microstructure transformation and stress distribution,which consequently affects the properties of the processed parts.Moreover,it may result in cracks and even a failure of the components due to quenching stress.An in-depth understanding and precise control of procedure is of great important to the quality enhancement of the mechanical parts or products.The advances in computer technology and progress inheat treatment related disciplines lay a solid foundation for the numerical simulation of heat treatment technology.On the basis of heat transfer and phase transformation kinetics,a mathematical-physical model is established to describe the quenching process of shaft shaped part in this thesis.The temperature distribution field,microstructure transformation,stress field and hardness change of the 42CrMo and 42CrMo+Ni steels during the course of the heat treatment are studied thoroughly by employing the finite element(FEM)modeling software Deform-3D.And a comparison of the heat treatment process of these two kinds of steels is also conducted upon the modeling and experimental results.The reason for the selection of 42CrMo and 42CrMo+Ni steels is listed as follows.Given its relatively complete database of heat treatment,42CrMo steel is chosen to validate the model.42CrMo+Ni steel,which is a variation of the 42CrMo steel by adding more Ni element,is a new kind of ferrous material for manufacturing large-size forged shaft components such as main shaft of wind-driven generator.On account of a deficiency of heat treatment database and simulation work of the 42CrMo steel,it is urgent to establish a heat treatment database followed by carrying out the necessary simulation of the heat treatment of this steel.The aforementioned work will have great significance for the heat treatment technology optimization and property improvement of the 42CrMo+Ni steel.By the application of the software JMatPro,the supercooled austenite isothermal transformation cures(TTT diagram),martensite starting temperature(Ms),some thermal physical parameters such as Young's modulus,Poisson 's ratio and thermal expansion coefficient of the steel are calculated and verified with values in the reference books.The simulation results agree well with the experimental ones.Furthermore,TTT diagram,Ms,Young's modulus,Poisson's ratio and thermal expansion coefficient of the 42CrMo+Ni steel are also estimated by using the same software.A heat treatment database of the 42CrMo+Ni steel is built up.The Jominy end quench test of 42CrMo and 42CrMo+Ni steels is simulated and a Jominy hardness-distance curves are also calculated by using the software of Deform-3D.It is shown that they are in good agreement with the experimental results.Thus,the Jominy hardness-distance curve of the 42CrMo+Ni steel is also established.Further modeling or experimental results indicate that the water quenched end has the most rapid cooling rate.With the increase of distance to the water quenched end,the martensitic transformation is retarded and the martensitic volume fraction is reduced.At the start stage prior to any phase transformation,the area close to the water quenched end is subjected to a tensile stress.Once the martensite transformation is triggered at the water quenched end,the stress of this part immediately turns to a compression state due to phase transformation induced expansion followed by a tensile state as a result of further cooling and transformation.The combined action of phase transformation and thermal effect leads to a gradually reduced tensile stress at the water quenched end,which turns to a compressive state at the last stage.Under the circumstance of same quenching conditions and same position,the hardness of 42CrMo+Ni sample is higher than that of the 42CrMo steel.The water quench and oil quench process of the forged shaft is also simulated.The modeling results agree well with results of metallographic observation and hardness test.A microstructure-hardness cure is established.The modeling and experimental results show that the cooling rates are significantly different between the surface and the interior of the workpiece.The larger the heat transfer coefficient is,the faster the workpiece is cooled.In the case of a cylinder specimen with a diameter smaller than 60mm made from 42CrMo+Ni steel,no ferrite and pearlite are observed after the quench.The structure of the specimen is composed of a majority of martensite and a minority of bainite.The volume fraction of bainite produced by oil quenching is higher than that by water quenching.At the beginning stage prior to any phase transformation,the surface is subjected to a tensile stress while the core is subjected to a compressive stress.Later,the martensitic transformation is initiated on the surface,the stress of the surface rapidly changes from a tensile state to a compressive one.With lowering of the temperature,the transformation zone moves inside.When the near surface also reaches the martensite start temperature,the transformation at this part causes a decrease in the compressive stress in the surface layer.Meanwhile,the core is subjected a rapidly increased tensile stress due to thephase transformation induced expansion in the near surface layer.With further decline of the temperature,the core of the specimen also undergoes martensitic transformation,resulting in a change of the stress state from a tensile one to a compressive one in the core,and a tensile state in the surface layer.After thetransformation of the phase transformation,the change of stress is only affected by the temperature,and the stress value is changed. |
PDF Full Text Request