Process And Mechanism Research On High-Frequency Induction Brazing Of YG8Cemented Carbide/20#Steel Composite Tappet

The wear failure of tappet in engine is often caused by the CAM friction. TheYG8cemented carbide possesses the properties of high hardness and good wearresistance. Its joining with20#steel can be used as YG8cemented carbide/20#steelcomposite tappet, which can improve the wearability and prolong its service life. Inthis paper, the BNi2solder was used as the filler metal to join the YG8cementedcarbide with20#steel by the high frequency induction brazing. The microstructureof joint was analyzed, and the influences of process parameters on joint interfaceorganization and mechanical properties were expounded. And the YG8cementedcarbide/20#steel composite tappet was finished.Under the high frequency induction heating, argon gas could improve thewetting of BNi2solder on the parent metal, prevent oxidation and burning loss ofsolder alloy elements. When the joining temperature was1030℃and the holdingtime was180s, perfect joint was got without any cracks and pores. According to theback scattering and metallographic images, Fe-Ni(s,s) formed on the20#steel sidein the seam. Meanwhile, as a result of C atoms infiltration, the carburized layer andreticular secondary cementite generated in the20#steel near the interface. On thecemented carbide side, the WC grain grew up and the brittle η phase produced inblock shape.The change of joining temperature and holding time had significant influenceon the joint microstructure. As the increase of joining temperature and the extensionof holding time, the diffusion of alloying elements and the dissolve of parent metalenhanced gradually. The Fe-Ni(s,s) layer became thicker obviously. As the increaseof the Fe element content in the brazing seam, the η phase transformed from theNi2W4C to the Fe3W3C. The content of η phase increased and was diffuselydistributed in the brazing seam as the increase of joining temperature and theextension of holding time. The thickness of carburized layer and the content ofreticular secondary cementite kept increasing and caused the high joint brittlenessand low property. When the joining temperature was1030℃and the holding timewas180s, the joint shear strength reached the maximum of441MPa.The joint fracture started from the micro cracks in large block η phase in thebrazing seam. The cracks extended along the brittle η phase and cemented carbidebase metal. A certain plastic deformation occurred in the solid solution, which madethe fracture show a mixture of brittle and ductile fracture characteristics. The jointmicrohardness value increased from the20#steel to the YG8cemented carbide side and had a sharp rise in the YG8cemented carbide side. The welding of YG8cemented carbide/20#steel composite tappet was finished under the optimumtechnological parameters with perfect appearance and no welding defects. Thecoaxiality of two parts was within0.3mm, which was completely meet the productstandards.