| The main shaft bearings of aircraft engines work at high temperature,high rotation speed,and heavy load environment.High temperature bearing steel(Cr4Mo4V)with excellent red hardness,dimensional stability,and wear resistance is widely used in the manufacturing of main shaft bearings.However,large,irregular primary carbides will precipitate directly from the melt due to the segregation of alloying elements,such as C,Cr,Mo,and V.The coarse primary carbides cannot be completely refined during subsequent processing,which usually act as stress raisers under rolling contact fatigue,leading to fatigue crack initiations.It is essential to manufacture high temperature bearing steel with fine carbides and a homogeneous microstructure to ensure its broad applicability.Metal Additive Forging(MAF)is proposed to manufacture large homogeneous high temperature bearing steel with refined primary carbides.The key issues for MAF includes the manufacturing of additive forging elements with homogeneous microstructure and the interface metallurgical bonding.In this work,the effects of Cerium refinement,semi-solid forging,and high temperature isothermal treatment on the primary carbides in bearing steel were investigated for the preparation of homogeneous addivive forging elements.Subsequently,the effects of temperature,strain,holding time,and vacuum degree on the interfacial microstructure of hotcompression bonded joints were studied to reveal the healing mechanism of interface.Finally,A piece of Cr4Mo4V bearing steel with isotropic tensile strength was manufactured by bonding four stocks,which proves that the hot-compression bonding technology is promising for the manufacturing of large Cr4Mo4V steel with homogeneous microstructures.The main contents and results of this work are summaried as follows:(1)The addition of Rare Earth Ce(0.056 wt%)element can greatly refine the prioraustenite grains and primary carbides in Cr4Mo4V steel.The refinement of grains can be theoretically attributed to the Ce-inclusions acting as heterogeneous nucleation sites of austenite and Ce segregating at grain boundaries,resulting in solidification mode change from columnar dendrites to equiaxed ones.The refined austenite grains divide the remaining melt into small and dispersed pools where primary carbides nucleate and grow.Thus,the volume percentage of primary carbides decreases from 4.15%to 2.1%and their size mainly is in the range of 10～20 μm.The addition of Ce only changes the amount,morphology,and distribution of primary carbides.(2)Semi-solid forging breakes bulky dendrites and promotes the formation of globular grains.On the one hand,semi-solid forging promotes the mixture of residual liquid and the segregation of alloying elements in the residual liquid is reduced,which inhibites the nucleation of primary carbides.On the other hand,large amounts of grain boundaries,vacancies,and dislocation defects are introduced into the microstructure of Cr4Mo4V steel during semi-solid forging,which accelerates the diffusion rate of the alloying elements and restrains the growth of primary carbides.The volume fraction of primary carbides was reduced from 7.23%(as-unforged state)to 1.29%when forged in the end of solidification(1340℃).However,the carbides refinement efficiency at the semi-solid forging temperatures of 1380℃and 1320 ℃ is limited.Different from conventional remedial methods,primary carbides are eliminated by promoting the diffusion of alloying elements and alleviating liquid segregation during semi-solid forging.(3)The V-rich rod-like MC carbides possess great thermal stability.The dominant transformation mechanisms of MC carbides are disintegrating and coarsening during high-temperature soaking treatment.The Mo-rich rod-like M2C-R carbides are metastable,and their diffusion-controlled transformation process follows the paths:the separate precipitation of MC and M2C-R+γ→MC+M6C.The decomposition products are spherical MC carbides and M6C carbides,but the latter gradually dissolves into the matrix,leaving just MC carbides as the final decomposition product.The evolution of Fe-and Mo-rich lamellar M2C-L carbides consists of two stages:the partial dissolution of M2C-L carbides into the matrix and the transformation of M2C-L→MC+γ.The decomposition processes of M2C-R and M2C-L carbides are controlled by temperature,and they can be completely transformed into MC when held at 1150℃ for 6 h.(4)The hot-compressed Cr4Mo4V steel samples were bonded by the migration of newly-formed grain boundaries at the interface.The interfacial bonding ratio(δ)increased with the deformation temperature and strain.A tensile strength comparable to that of the base material was obtained at 1050 ℃/50%,1100℃/30%,1100℃/50%,1150 ℃/30%.and 1150 ℃/50%.Post-holding treatment eliminated the interfacial voids and promoted interfacial recrystallization and grain boundary migration,which led to a higher bonding ratio.The samples hot-compressed at 1050 ℃/30%.1100 ℃/10%,and 1150 ℃/10%were completely healed after holding for 12 h at 1150 ℃.At the appropriate bonding temperature(≤1150℃),the interfacial primary carbides bulged into the other side due to the great hardness.The newly-formed interface between the carbides and new matrix is similar to that between the carbides and initial matrix when the deformation strain increases to 30%,which does not affect the complete healing of interface.(5)At a high vacuum degree(10-3 torr),only nano-scale δ-Al2O3 oxides were formed at the interface,which is stable and does not deteriorate the tensile strength of bonded samples due to their small size and limited amount.At a low vacuum degree(10-1 torr),dispersed rod-like and granular δ-Al2O3 oxides,irregular Si-Al-O compounds,and spheroidal SiO2 particles were formed at the interfacial area in the heating and holding processes,which is due to the selective oxidation and internal oxidation of Al and Si.After the post-holding treatment at 1150 ℃,SiO2 particles and Si-Al-O compounds successively dissolved into the matrix due to the low pressure of O(1.61×10-37 atm)in the matrix,and δ-Al2O3 oxides were finally transformed into stable nanoscale α-Al2O3 oxides.The effect of intragranular residual α-Al2O3 oxides on the mechanical properties of joints was limited due to their small size and limited amount.The mechanical properties of samples containing initial interface are comparable to that of the base material. |
