Research On Microstructure And Properties Of Stellite 6 Coatings By Laser Cladding On 2Cr12MoV

The 2Cr12 MoV stainless steels are widely used as the materials of last stage blade of steam turbine. However, it undergoes severe water erosion and fatigue fracture owing to the vibration force during the period of the service. Stellite alloy brazing is usually employed to improve the property of the water erosion resistance, which results in a weak bonding with the substrate and a large deformation when it involves the large area of brazing. Therefore, the Stellite 6 alloy coating is prepared on the surface of 2Cr12 Mo V stainless steels by laser cladding in this paper.The cladded coating with good shape, no flaws, low dilution rate and good metallurgical bonding with the substrate was fabricated using high power diode laser cladding system with the process parameters which were laser power P=1.4 kW, scanning speed Vs=8 mm/s, powder feeding speed Vp=13.56 g/min, makeup gas flow rate 15 L/min, shielding gas flow rate 5 L/min. The microstructure and hardness of the multi-layer under different scanning path were mainly studied. The results show that the as-deposited microstructure of the single direction raster scanning(SDRS) specimen grows epitaxially along the deposition direction while the continuous directional growth of the microstructure in the cross direction raster scanning(CDRS) path is inhibited. The distribution of hardness in each layer of the CDRS specimen is more homogeneous than that of the SDRS specimen. Furthermore, the hardness has a significant decrease in the overlapping area between layer and layer due to the tempering.The tribological performance and the water erosion resistant performance have an obvious improvement after the substrate 2Cr12 MoV stainless steels are cladded with the Stellite 6 alloy. Furthermore, the fatigue strength of the as-cladded specimens is 380 MPa which is close to the fatigue strength of the substrate with 400 MPa. After the heat treatment(550 ℃ for 6 h),the fatigue property of the cladded specimens has a increase, even better than that of the substrate. The fatigue surface of substrate specimen shows that the fatigue crack initiates at the surface, then propagates forward with the secondary fatigue steps at the early stage and with the secondary crack and the tyre pattern at the late stage. Deep dimple is observed in the fast fracture region. The fatigue crack of cladded specimens initiates at the corners in the side of the substrate, then extends forward from the substrate to the coating. In the fast fracture region, the substrate shows the shallow dimple while the coating shows the interdentritic fracture mixed with the quasi-cleavage fracture.The effect of the laser scanning path on tensile properties and the fatigue crack growth rate of the laser solid formed Stellite 6 alloy is studied. The results show that the tensile properties of the longitudinal specimens which is parallel to the scanning direction is better than that of the cross specimens which is perpendicular to the scanning direction with the SDRS path, while the tensile properties of the CDRS specimens are between the above. This is similar to the effect of the laser scanning path on the fatigue crack growth rate. The fatigue crack growth of CDRS specimens is faster than the longitudinal specimens with SDRS path, but slower than the cross specimens with SDRS path.