Studies On Microstructure And Properties In PTA Weld Coatings Of Nickel-base Hardfacings On Austenitic Stainless Steel

Rapid development of mining, petrochemical, chemical industry and nuclear power have been the driving force for the vibrant research on hardfacing materials. Ni-Cr-B-Si alloys are the preferred hardfacing alloy coatings on components susceptible to galling, high temperature oxidation and corrosion. Plasma transferred arc welding (PTAW) is widely appied in the hardfacings industry since it offers lower dilution, excellent bonding with substrate and dense and defect-free deposits.The nickel-base hardfacings were deposited on austenitic stainless steel substrate by plasma transferred arc welding (PTAW). The optical microscope (OM), scanning electron microscope (SEM) with Energy Disperse Spectroscopy (EDS), electron probe microanalyzer (EPMA), X-ray diffraction (XRD), microhardness tester and sliding wear tester were employed to determine the influence of welding parameters, original alloy chemistry and the content of spherical tungsten carbides on the microstructure and properties of the coatings.The results indicate that:(1) Adherent, defect-free nickel-base alloy coatings of maximum3-6mm thickness could be obtained on austenitic steel substrate by suitable selection of process parameters and pre-heat and post-heat treatment using plasma transferred arc welding.(2) Obvious microstructure gradient was found in Ni60coatings. While current increased20%, large fraction of irregular floret-like eutectic structure were observed and the average hardness decreased about20%. While powder feed rate decreased40%, large fraction of strip-like and separated bulk-like borides was observed clearly and the average hardness of the coating decreases about19.8%.(3) The microstructure of Ni-Cr-B-Si coatings mainly consisted of y-Ni dendrites and interdendritic constituents. The width of fusion area decreased and the volume of soft primary dendritic phase drastically decreased and the proportion of chromium borides and carbides in the interdendritic region increased with an increase of C, B, Si, and Cr content. The average hardness of coatings increased with higher content of C, B, Si and Cr because of lower ratio of soft primary dendrites and higher fraction of Cr-particle phases in interdendritic region.(4) In the process of PTAW, tungsten carbides sank in the region near interface. The primary tungsten carbides particles dissolved and reacted with the elements of nickel-base alloys to form low melting eutectics, which precipitated in the shape of bulk and long flake. With the increasing content of tungsten carbides, the average microhardness of coatings increased. The hardness of coatings was not affected by the herringbone and block structures near the surface. Rapid glazed surface oxide layer formation prevented the direct metallic interactions and furthermore the cleavage of primary tungsten carbides enhanced the wear resistance.(5) Welding currents strongly affected the dilution with the base material and furthermore the formation of transition zones where tungsten carbides are completely melted near the interface. Concerning hardness it was found that higher currents contributed to lower hardness of the coating because of the intermixing of ferrum during the hardfacing, the dissolution of the tungsten and the re-precipitation. However the hardness in the transition zone with higher currents was higher than that with lower ones, which was related to the predominant effect of melted tungsten carbides.