Growth And Refinement Mechanism Of Primary M₇C₃ Carbide In Hypereutectic Fe-Cr-C Alloy

The wear-resistance of hypereutectic Fe-Cr-C alloy is excellent,which makes it widely applied in surface hardfacing industry.It also aroused wide concern in metallic 3D-printing inductry.Primary M₇C₃ carbide is the main strenthening phase in hypereutectic Fe-Cr-C alloy,which determines its wear-resistance.However,because large cracks are easy to be formed between coarse primary M₇C₃ carbides and substrate,primary M₇C₃ carbide is easy to desquamate from hypereutectic Fe-Cr-C alloy,and reduces the service life of the alloy.Therefore,on the basis of knowing the growth mechanism of primary M₇C₃ carbide,it is an urgent issue to promote the service life of hypereutectic Fe-Cr-C alloy by refining primary M₇C₃ carbide.In this paper,hypereutectic Fe-Cr-C alloys were prepared firstly.The physical properties of primary M₇C₃ carbide such as crystal structure and electronic structure were studied.Its microstructure characteristics were analyzed and its grwoth mechanism was investigated.Then,the abrasive cracking behavior and tensile cracking behavior of hypereutectic Fe-Cr-C alloy were studied,and the relationship between its crack resistance and the size of primary M₇C₃ carbide was analyzed.Finally,primary M₇C₃ carbide was refined by Ti and Nb-alloying treatments.The homogeneous nucleation solid-liquid interfacial energy of primary M₇C₃ carbide was calculated by molecular dynamic simulation.The properties of TiC/M₇C₃ interface and NbC/M₇C₃ interface were calculated by first-principles.Thereby,the refining mechanisms by Ti and Nb-alloying treatments were analyzed.Primary M₇C₃ carbide is Fe₃Cr₄?A,C?C₃ type,whose bonding is a combination of metallic bonds,covalent bonds and ionic bonds.By observations,the as-welding primary M₇C₃ carbide is irregular polygon which contains several hollows in the center and gaps on the edge.Some primary M₇C₃ carbides are formed by layers of non-closed shell and some are composed of distinct parts.The bulky primary M₇C₃ carbide is a combination of multiple small carbides.The orientation of the small carbides are not absolutly same,so there is low-angle boundary between adjacent small carbides.By in-situ observation,the coalescence growth mechanism of primary M₇C₃ carbide is put forward.During the growth process,primary M₇C₃ carbide is easy to form protrusion along{011?0}plane,which caused by its stacking faults and diversity of defects.The growing and revolving protrusion forms carbide shell and some small carbides may be enclosed within it.Then they coalesce into a bulky primary M₇C₃ carbide.In addition,The growth of the protrusions can also cause the coalescence of adjacent carbides.Based on the worn surfaces of hypereutectic Fe-Cr-C hardfacing alloy,the abrasive cracks mainly appear on the interface between primary M₇C₃ carbide and eutectic austenite.Abrasive cracks are reduced when primary M₇C₃ carbides are refined.Based on the tensile fractures of hypereutectic Fe-Cr-C hardfacing alloy,when tensing along axial direction of primary M₇C₃ carbide,primary M₇C₃ carbide is cleavage fractured on?0001?plane and eutectic austenite is ductile fractured.In addition,intergranular fracture is occurred between primary M₇C₃ carbide and eutectic austenite.When tensing along radial direction of primary M₇C₃ carbide,the tensile fracture is mainly composed of the intergranular fracture between primary M₇C₃ carbide and eutectic austenite.The axial tensile strength is enhanced while the radial tensile strength is reduced when primary M₇C₃ carbides are refined.The abrasive crack and tensile fracture of hypereutectic Fe-Cr-C hardfacing alloy are both related to the interface between primary M₇C₃ carbide and eutectic austenite.Plastic deformation band is formed between primary M₇C₃ carbide and eutectic austenite because of the large thermal mismatch stress.The coarser primary M₇C₃ carbide is,the larger thermal mismatch stress is and the wider plastic deformation band is.When primary M₇C₃ carbides are refined,the reduced abrasive cracks is due to the decreased shaking amplitude of primary M₇C₃ carbide and the reduced thermal mismatch stress between primary M₇C₃ carbide and eutectic austenite.Based on the equilibrium phase diagram,TiC precipitates firstly during the solidification of hypereutectic Fe-Cr-C alloy when Ti content reaches 0.6wt.%,and then primary M₇C₃ carbide precipitates.According to equilibrium phase diagram,hypereutectic Fe-Cr-C alloy with 0.63wt.%Ti is prepared.The XRD spectrum shows the alloy mainly consists of M₇C₃ carbide,austenite and TiC.The microstructural observation shows the primary M₇C₃ carbides in the alloy are obviously refined.In addition,TiC and primary M₇C₃ carbide are tightly combined.By molecular dynamic simulation,the homogeneous nucleation solid-liquid interfacial energy of primary M₇C₃ carbide is 3.312J/m².By first-principles calculation,the work of cohesion for M₇C₃?0001?/TiC?111?interface is3.48J/m².Its interfacial energy is lower than the homogeneous nucleation solid-liquid interfacial energy of primary M₇C₃ carbide.Its interfacial bonding is a combination of polar covalent/ionic bond and metallic bond.Therefore,the refinement of primary M₇C₃carbide by Ti-alloying treatment is ascribed to the heterogeneous nuclus effect of TiC.Based on the equilibrium phase diagram,NbC precipitates firstly during the solidification of hypereutectic Fe-Cr-C alloy when Nb content reaches 1.2wt.%,and then primary M₇C₃ carbide precipitates.According to equilibrium phase diagram,hypereutectic Fe-Cr-C alloy with 1.2wt.%Nb is prepared.The XRD spectrum shows the alloy mainly consists of M₇C₃ carbide,austenite and NbC.The microstructural observation shows the primary M₇C₃ carbides in the alloy are obviously refined.In addition,NbC and primary M₇C₃ carbide are tightly combined.According to atomic stacking modes,two M₇C₃?0001?/NbC?111?interface models are built.By first-principles calculation,the interfacial bonding of interface I is a combination of polar covalent/ionic bond and metallic bond and the work of cohesion is 1.23J/m²,while that of interface II is a combination of ionic bond and metallic bond and the work of cohesion is 2.24J/m².There exists a region where interfacial energy of interface II is lower than the homogeneous nucleation solid-liquid interfacial energy of primary M₇C₃ carbide.Therefore,the refinement of primary M₇C₃ carbide by Nb-alloying treatment is ascribed to the heterogeneous nucleation effect of NbC.