Study On Technology And Mechanism Of Brazing YG12Cemented Carbide To W18Cr4V Steel

W18Cr4V possesses the properties of excellent red hardness, wear-resistanceand thermostability. A reliable joint of W18Cr4V and YG12can be used inproducing cutting tools which require great stiffness to guarantee their cuttingprecision. It can be also applied to manufacturing cold-working dies that used inpoor working conditions. However, a quenching process is a must after normalbrazing method in order to guarantee the hardening of W18Cr4V. High quenchingtemperature may soften or remelt the brazing filler which will probably weaken thejoint. The author took integrated brazing-quenching method to join the two materialsand analyzed the feasibility of common brazing fillers in connecting W18Cr4V andYG12. Cu-Ni and Ni-Co-Si were suitable fillers. The influence of Ni content to theCu-Ni filler and Co content to the Ni-Co-Si filler was revealed. The optimumparameters were selected by studying the parameters influence on the shear strengthand the microstructure.Fine copper evaporated and flowed away heavily at a high brazing temperatureunder vacuum. Thus there were many holes found in the joint brazed with finecopper. The addition of Ni to copper obviously weakened the phenomenon offlowing and contributed to the diffusion between the filler and base metals, whichsignificantly helped to reduce the number of Kirkendall voids. Face centered cubichigh-entropy alloy formed along W18Cr4V interface and YG12interface when theNi content was10wt.%. The atomic ratio of the elements in FCC phase was：Cu:Ni:Fe:Co=1:1:1:1. A penetration layer formed in the YG12base metal near the interfacewhen the Ni content was up to20wt.%. The maximum room temperature shearstrength of the joint brazed with Cu-20Ni was369.8MPa at the brazing temperatureof1210℃holding for10minutes and the typical microstructure of the joint was:W18Cr4V/(Cu,Ni)/penetration layer/YG12. Particularly worth mentioning was thatthe shear strength of the same joints tested at400℃was only51.3MPa. FCCformed along the interfaces at longer holding time or higher brazing temperature andthe microstructure of the joint turned to be: W18Cr4V/FCC/(Cu,Ni)/FCC/penetration layer/YG12.Ti-Ni and NiCrNb with outstanding high-temperature property were not suitedfor brazing W18Cr4V to YG12at a temperature above1200℃. Carbide formingelements, Ti and Nb, reacted with C and formed brittle compounds like TiC or Nb2C.Meanwhile M6C or M12C formed along the interface of YG12. The element B led tosevere corrosion and intergranular penetration and then Fe3W3C formed at the interfaces when BNi2was used as brazing filler. Brittle Compounds like M6C、Cr23C6、(Cr,Fe)23C6were also found in the joints brazed with NiCrSi.W18Cr4V and YG12brazed with Ni-Co-Si showed high shear strength both atroom temperature and400℃. The optimum Co content in Ni-Co-10Si was10wt.%.The typical microstructure of W18Cr4V and YG12brazed with Ni-10Co-10Si at1220℃holding for10minutes was: W18Cr4V/Ni(s.s)/penetration layer/YG12.Ni74Si26and Co2Si were formed at the surface of WC in the penetration layer. Thewidth of the matrix phase Ni(s.s) in the penetration layer gradually decreased fromthe brazing seam to YG12, which effectively reduced the residual stress.Themaximum room temperature shear strength of the joint was415.8MPa at the brazingtemperature of1220℃holding for10minutes. The shear strength at400℃was231.7MPa.