Study On Co-based Hard-facing Process Of Pressurized Water Reactor Internals

As an important strategy in the development of Chinese energy,the safe and efficient development of nuclear power helps improve the environment and economy.However,the application and development of nuclear power are restrained by the limitations of welding,material and surface treatment in the manufacturing process.In this paper,manual GTAW and laser cladding process are performed on cobalt-based alloy surfacing of 304 austenitic stainless steel used for reactor internals which is a main equipment of PWR nuclear plants,and their process and structure property are studied.With manual GTAW,a series of cladding tests are performed on 304 austenitic stainless steel through varying of different welding parameters such as welding current,preheat,slow cooling,PWHT,and the microstructure,phase composition,micro-hardness,wear behavior of the surfacing are studied.Optimized parameters are obtained with sound metallurgical bonding between the coating and the base metal;the best cladding structure and property are achieved;and defects such as crack,air hole and uneven hardness are reduced.Meanwhile,the orthogonal experiments are conducted in the laser cladding process.The optimized parameters are obtained based on the evaluation indexes including the surface quality,dilution rate and hardness of the coating.The microstructure,phase composition,micro-hardness,wear behavior of the coating are analyzed and compared with those of manual GTAW.The results show that the coating is well bonded with base metal without any defect,and the comprehensive mechanical properties are best when the welding parameters of GTAW are as follows: preheated at 150? before welding,146 A current is used in the first surfacing layer,165 A current in the second to fourth layers,and mica powder for slow cooling without post heat treatment.The coating is mainly composed of ?-Co and ?-Co solid-soluted Cr,W,Fe elements.At the same time,there are precipitated phase and eutectic structures between the crystals.The Vickers hardness of the coating is 400 HV to 684.8HV;the friction coefficient of the coating is 0.24;the wear loss is 0.2986g;and the base metal intergranular corrosion structure is step structure.For laser cladding process,when the laser cladding parameters are as follows: laser power 5500 W,scanning speed is 5mm/s,and powder feeding rate 10g/min,the coating is well formed with low dilution rate,the cladding layer rich in Cr,W,Fe and other elements of ?-Co solid solution and FeNi,and exist intergranular precipitates and crystalline phases.Average Vickers hardness is 411.5 HV,and hardness is less volatile,satisfying the requirement of engineering technology;the friction coefficient of the coating is 0.13,and the wear loss is 0.3597 g.As the first application of laser cladding technique to the equipment of the reactor internals in nuclear island,this test explores the laser cladding in engineering application prospect in the field of nuclear power equipment manufacturing.