Study On Microstructure And Property Of Laser Remelted Fe-based Coating Deposited By HVOF

A Fe-based alloy coating containing Fe, Cr, Mo, Ni, P, Si, B was deposited onto Q235 matrix by HVOF technology. The re-melting research of as-sprayed and thermal insulation coating was done. The micro-structure and property of the coating were surveyed by the Microhardness Tester, the Optical Metallographic Microscopy (OM), the Scanning Electron Microscopy (SEM), the X-ray Diffraction (XRD) and the Electrochemical System.The single-way laser re-melting showed that the coating consisted of alloy area, heat affected zone and matrix area. The orthogonal-experiment was arranged to decide the effect law of the re-melting parameter according to the height, micro hardness and large hole diameter greater than 50μm of the alloy area. It was discovered that coating with a porosity of 1.82%, a thickness of 340μm has an optimal parameter as follow: the set power of laser was 700W; the scanning rate was 2mm/s; the light diameter was 3mm. The micro hardness of alloy area was promoted from 600HV0.1 to 900HV0.1 due to a dense microstructure and fine grains. The result of multi-way re-melting experiment with a step of 1.5 mm showed that the alloy area consisted of white and square grains. The grains'shape and size of the tap area were different and its microhardness was lower. The coating thermal insulation at 300℃for 1 hour was re-melted and showed white square grains and gray grains in the alloy area with no change of the crack quantity.It was discovered that convection and mass transfer were co-existed in the pot. The height of the pot has a proximately linear relativity with the laser power density. The hot stress during the re-melting caused the cracks which originated from the pore and the interface of the re-melting and coating area. The metal liquid preferentially crystallized inhomogeneously on the solid-liquid interface, the grains grew vertically to the isothermal surface and a faster speed at a larger temperature gradient. The pot had a dense and homogeneous solidification microstructure with a weak lamellar structure and little pores.The as-sprayed coating and the feed powder had the same crystals asα-Fe,Cr3P,Ni3P and Cr₃Ni₅Si₂. The re-melting coating was fully crystallized and contained Fe₃Si, Ni₃P, SiO₂. The dynamic potential polarization test showed that the re-melting coating and thermal insulation coating both had a wider passive region in the 10% NaOH solution than the as-sprayed coating. The re-melting coating had a lower icorr than the as-sprayed coating and the thermal insulation coating. The Electrochemical Impedance Spectroscopy (EIS) test showed that the maximum impedance of the re-melting coating , the thermal insulation coating and the as-sprayed coating were 1005?, 888? and 562?. It was concluded that the sequence of corrosion-resistance property from the lower to the higher was the re-melting coating, the thermal insulation coating and the as-sprayed coating. The promotion of corrosion-resistance was attributed to the dense and fine-grain microstructure, less defects and homogeneous phase distribution.