INVESTIGATION OF PREPARATION PROCESSES AND CORROSION RESISTANCE OF NICRBSI COATINGS FABRICATED BY LASER-HIGH FREQUENCY INDUCTION HYBRID CLADDING

摘要

The surface of many key components used in petrochemical industry may be damaged by corrosion, wear and overheat during service. This damage can make a significant impact on the safe and reliable operation process of the device. Low alloy steel is widely used to manufacture pressure vessels and pipes because of the higher strength and good processing properties, but has poor corrosion resistance and wear resistance. Laser cladding is an advanced and effective surface modification technology. It can improve the surface properties of the matrix material at lower cost. But these claddings crack easily due to the high temperature gradient in the laser molten pool and physical properties differences between the cladding material and matrix This problem has affected the development of laser cladding. Laser-high frequency induction hybrid cladding is a novel technology which combines laser beam heat source and induction power. It can decrease the cracks in the claddings effectively and has been received significant attentions in recent years. Nickel base alloy powders have good corrosion resistance, wettability and high temperature lubricity. In this research, NiCrBSi composite claddings were fabricated on the surface of low alloy steel by the way of laser-high frequency induction hybrid cladding and coaxial powder feeding. The optimum cladding technology parameters to obtain the cladding layers that with good metallurgical combination, reasonable dilution rate and without cracks and defects were developed by optimizing the processing parameters such as laser power, powder feeding rate, laser scanning speed and induction heating temperature. The hardness distribution, microstructure, element distribution and phase of the cladding layers fabricated by the optimum parameters were systematically investigated by means of micro-hardness tester, optical microscopy (OM). Compared with the untreated material, experiment results show that the micro-hardness from substrate to NiCrBSi cladding layer exhibits step distribution, and the hardness of the NiCrBSi cladding layer is higher than that of the base metal. The microstructure showed good metallurgical bonding between NiCrBSi cladding and substrate had been achieved. In addition, directionally solidified microstructures were deposited. From the top to bottom of the cladding layer, the microstructures are, in order, equiaxed crystal, dendrite crystal, cellular crystal, columnar crystal and plane crystals. The corrosion resistance of the base metal and specimens manufactured by laser-high frequency induction hybrid cladding were evaluated in 3.5% NaCl solution by electrochemical testing. The experimental results demonstrate that the corrosion resistance of the NiCrBSi cladding layer fabricated by laser-high frequency induction hybrid cladding technology is better than the base metal.