Studies On Processing Of Laser Cladded Fe-based Coatings With Grain Boundaries Tonghening

High hardness coating were developed and applied to industries of mining engineering,tool and mould,machinery and steel,etc.Manufacturing techniques of high hardness coating include plasma powder surfacing,thermal spray technology,arc welding and laser cladding,etc.These technologies can meet industry needs in specific fields.However,due to high hardness coating includes a lot of hard and brittle phase,grain boundry has been a weak part of the high hardness coating,and under the combined effects of thermal stress,morphology stress and residual stress,cracks occur easily.In this paper,"grain boundary toughening"thinking is applied.It is desirable to avoid the occurrence of cracks in the coating by controlling the microstructure of the coating.To this end,a high hardness iron-based coating containing retained austenite along the grain boundaries was obtained by laser cladding.The average hardness of the coating is about 775HV.No crack was formed in the multi-pass overlapping laser cladding coatings.The microstructure and phases of the cladding layer were investigated by colored metallography method,scanning electron microscopy(SEM),energy dispersive spectrometry(EDS),X-ray diffractometry(XRD)and transmission electron microscopy(TEM).Results showed that the coating was consisted of uniform and fine cellular dendrites.Martensite was distributed in primary dendrites.Reticulated retained austenite was precipitated in the interdendritic region.Carbides were distributed uniformly in the reticulated retained austenite.Molybdenum,chromium,wolfram and niobium were distributed at grain boundaries of the coating,and carbon was distributed uniformly in the cladding layer,which is beneficial to avoid the formation of large lump carbides at grain boundaries of the coating.Many stacking faults were observed in the retained austenite at grain boundaries,and matensite was observed in the stacking faults intensive area.Besides,with preheating the substrate in room temperature,150?,200?,250?,300?and 350?,coatings were obtained by laser cladding.The effects of preheating temperature of substrate on the macrostructure,cooling rate,microstructure and mechanical properties of the coating were studied.The mechanism of preheating temperature of substrate affecting on the microstructure and mechanical properties of the coatings were analyzed.The results suggest,with preheat temperature of substrate increasing,the dilution rates of coatings increase.Besides,the cooling rates of coatings decrease and the SDASs(secondary dendrite arm spacing)of dendrite in coatings increase.Through calculate,the solidification rates of coatings decrease when preheat temperature of substrate increased.The decrease of solidification rate makes the effective partition coefficient ke of the solute in the coating decrease.So,in the interdendritic region,the degree of segregation of carbon increased significantly,and the positive segregation degree of Cr,Mo,W and Nb also increased.Conversely,the concentration of positive segregating solute elements in dendrites decreased,making the degree of supersaturation of the martensite in dendrites decrease,and the lattice parameter of martensite decrease.With preheat temperature increasing,the concentrations of C,Cr,Mo W and Nb in dendrites decreased,making martensite transformation starting temperature(M_s)in dendrites increase,and the degree of martensite transformation in dendrites increase.So,the volume fraction of retained austenite in the coating decreased accordingly,and the volume fraction of martensite increased,which make the average microhardness of coating increase.Besides,with preheat temperature increasing,the positive segregation degree of Cr,Mo,W and Nb in the interdendritic region increased,the actual concentration product of carbide increased,the precipitation of carbides in the interdendritic region increased,and eutectic structure formed in the interdendritic region.These studies provide experimental evidence for controlling the microstructure and mechanical properties of Fe-based high hardness coatings by controlling the fabrication process.