Study On Microstructure Control And Strengthening-Toughening Mechanism Of 42CrMo4 Steel With Large Cross-section

42CrMo4 medium-carbon bearing steel,which has high strength,toughness,and fatigue performance,is frequently used in the manufacture of large cross-section components(LCSCs).In the field of LCSC manufacturing,a large gap exists between advanced domestic and international levels in the control of inclusions,grain size,and microstructure uniformity.As a result,the low-temperature impact toughness of materials cannot meet the requirements of high-end products,such as the main shafts and bearings of 5MW and above specifications in wind turbine and the main bearing rings of 3m and above specifications in shield machines.Consequently,related products are heavily dependent on imports.In response to these problems,domestic LCSCs were subjected to physical anatomy and full-section microstructure analysis in this study.On this basis,the formation mechanism and damage mechanism of toughness hazard phases at different positions were analyzed,and the control methods of microstructure uniformity,grain size and inclusion were explored.According to the research results,the whole process manufacturing process development and pilot manufacture of the slewing ring with a diameter of 3 m were carried out.The main research contents and conclusions are summarized as follows:(1)The evolution of the microstructure and mechanical properties of 42CrMo4 steel from the surface to the core was studied.The main toughness hazard microstructures at different locations were also clarified.The results indicated that surface microstructure was full lath martensite.As the sampling position deepened,the content of bainite microstructure increased rapidly.The bainite content reached about 80%at 50 mm below the surface and remained stable afterward.In addition,a white block microstructure(Abbreviated as BK)began to appear 20 mm under the surface.The problems of low impact toughness of LCSCs were manifested in two aspects.The long strip-shaped and net-like distribution of the boundary precipitates led to an unsatisfactory impact performance of the surface microstructure.The bainite content substantially increased,and the formation of the white block microstructure,resulting in substandard impact toughness in the center of the component.In addition,large inclusions were also present in all the positions of the component,seriously harming the impact toughness.(2)The typical characteristics,formation mechanism,and toughness hazard mechanism of BK microstructure were clarified.The results showed that the BK formed in the carbon-poor zone.It had irregular contours and could grow across the prior austenite grain boundaries.Fine dispersed cementite distributed on BK had a specific orientation relationship with the matrix:[001]?//[011]M3C,(111)M3c 5±1 deg from(100)?,and(111)M3C 3±1 deg from(010)?.The BK microstructure was the tempered product of massive ferrite,and its formation involved three stages:diffusion at high temperatures,massive transformation at moderate temperatures,and tempering at low temperatures.The carbon-poor region produced by the fluctuation and diffusion were transformed into the BK matrix by the massive phase transformation mechanism under the high stress inside the LCSCs.In the subsequent cooling process,the supersaturated carbon in the BK matrix was precipitated into the special cementite.Under the action of impact load,stress was concentrated around large and irregular carbides in the boundaries and internal part of the BK.The BK microstructure as the soft phase initially reached the ultimate tensile strength,subsequently generated microcracks,and expanded into the surrounding microstructures.Thus,the crack initiation energy of the material greatly reduced.(3)The formation mechanism and influencing factors of the long strip-shaped and net-like distribution of the tempered precipitates in the surface microstructure were investigated.On the basis of the results,a method for controlling the secondary precipitates was proposed.The results showed that austenitizing temperature could significantly affect the matrix carbon content and grain boundary density.Consequently,the nucleation rate of M3C during tempering was affected,and its morphology and distribution were altered.On the basis of these findings,a new quenching process was designed to optimize the tempered precipitates.Thus,the impact toughness of the material greatly improved without reducing the strength.(4)The effect of vanadium microalloying on the grain size,hardenability,and toughness of 42CrMo4 steel was systematically studied.Under the austenitizing condition of 880?,the hardenability of the material initially increased,subsequently decreased,and increased again as the vanadium content increased.When the vanadium content was high,a large amount of vanadium segregated around the grain boundary,leading to the superhardenability effect.As the vanadium content increased,the amount of undissolved MC carbides increased,and their size decreased,resulting in a significant refinement of the grain size.In terms of the strengthening mechanism,the dislocation strengthening effect significantly weakened under high vanadium conditions.As a result,the strength of the as-quenched microstructure decreased.By comparison,the strong precipitation strengthening effect of nano-scale MC carbides led to an increase in the strength of the as-tempered microstructure.In terms of the toughening mechanism,high vanadium contents decreased the up-platform energy.However,it reduced the ductile-brittle transition temperature of 42CrMo4 steel.The former was mainly due to the reduction of plastic deformation energy caused by the strong pinning effect of nano-sized MC carbides on dislocations.Conversely,the latter is attributed to the increase in the crack propagation energy caused by grain size refinement.(5)The influence of rare earth(RE)content on the evolution behavior of inclusions in 42CrMo4 steel was systematically investigated.The results showed that the combination sequence of RE and impurity elements in steel was S?O?As?P?C as the RE content increased.The proper addition of RE was beneficial to the modification of MnS and Al2O3 into well-spheroidized fine RE sulfides.(6)The slewing ring with a diameter of 3m were manufactured on the basis of the results.Through the purification of RE,the microalloying of vanadium,and the control of quenching and tempering,the prior austenite grain size of the produced bearing ring reached level-8.0.The inclusions and BK were fine and dispersed,and the tempered precipitates in the surface microstructure were well spheroidized.The difference in the section hardness was within ± 15HB,and the mechanical properties could meet the following index requirements:Re?720MPa,Rm?840MPa,A?15%,Z?60%,AKv(-20?)?40J.