Study Of The Organization And Properties Of NiCoCrAlY Coatings On 17-4PH Stainless Steel Surface By Laser Melting Under The Action Of High-current Pulsed Electron Beam

The pursuit of high efficiency of coal-fired generating units has always been the eternal goal of the power industry.To improve the thermal efficiency of gas turbines,ultra-supercritical power generation technology has been proposed,that is,the water vapor pressure and temperature are increased to supercritical parameters,which puts forward higher requirements for the key heat-resistant stainless-steel components of gas turbines.Conventional heat-resistant stainless-steel parts tend to embrittlement and age during long-term use in high temperature environments,which will greatly reduce the performance and life of gas turbines and cannot achieve the efficiency improvement of gas turbines.Therefore,it is urgent to improve the high temperature performance of heat-resistant stainless steel.Laser cladding technology is widely used in the field of material surface modification because it allows the preparation of coatings with excellent properties directly on the surface of the substrate material and has a low impact on the substrate.Therefore,in this paper,laser cladding technology was used to improve the performance of 17-4PH stainless steel.A NiCoCrAlY coating with excellent properties was prepared on its surface,and the prepared coating was irradiated with high-current pulsed electron beam(HCPEB)to clean up the coating defects and also to enable the subsequent comprehensive performance of the coating.In order to study the changes in microstructure and properties of NiCoCrAlY coating before and after irradiation,the microstructure,mechanical properties,and hot corrosion properties of the unirradiated coating,5 times irradiation,10 times irradiation,20 times irradiation,and 30 times irradiation were studied respectively.The modification mechanism of the pulsed electron beam on the coating was revealed.The main research contents and results are as follows:(1)The evolution of microstructure and phase composition of NiCoCrAlY coating before and after HCPEB irradiation was analyzed.The results show that the NiCoCrAlY coating before and after irradiation is composed of γ/γ′ phase andβ-Ni Al phase.After irradiation,not only the surface of the coating is purified,but the cracks and holes disappear,the content of the Al element increases,but also the diffraction peak shifts.With the increase in pulse irradiation times,the craters and gullies on the surface gradually decrease,and the purification effect of the coating is more obvious.In addition,the surface grains of the irradiated coating were refined,and a uniform remelting layer with a thickness of 3-4μm was formed on the surface.Many Al2O3 bubbles and Al-Y enrichment particles were formed on the surface of the remelting layer.(2)The microhardness,room temperature,and high temperature friction and wear properties of NiCoCrAlY coatings before and after HCPEB irradiation were analyzed.The results show that the microhardness of the irradiated coating is improved.The maximum hardness is about 209.4HV0.2 after 10 times irradiation,and the minimum hardness is about 170.3HV0.2 for the unirradiated coating.The hardness of the coating is increased by 23 %.The main wear form of the coating before and after irradiation is adhesive wear at room temperature,and adhesive wear and oxidation wear at high temperature.Among them,the smallest friction coefficient and wear amount are the coatings after 20 irradiations,and the largest is the unirradiated coatings.The wear amount of the unirradiated coatings at room temperature and high temperature is 3.5 times and 3.6 times that of 20 irradiations,respectively.Irradiation improves the wear resistance of the coating at room temperature and high temperature.(3)The hot corrosion properties of the unirradiated coating and the coating after 30 irradiations at different periods were analyzed.The results show that the hot corrosion performance of the unirradiated coating is relatively poor,and the corrosion weight gain is large.During the hot corrosion process,three stages of rapid oxidation stage,slow oxidation stage,and accelerated oxidation stage are formed.The thermally grown oxides(TGO)formed during the whole corrosion process of the coating were massively exfoliated,resulting in a large number of spinel oxides and severe internal oxidation.The hot corrosion performance of the coating after 30 times of irradiation is significantly improved,and the corrosion weight gain is relatively small.There are only a rapid oxidation stage and a slow oxidation stage in the whole hot corrosion process.The layered TGO structure was formed during the hot corrosion process,and the Al2O3 layer was always maintained at the interface between the coating and TGO,which effectively improved the hot corrosion performance of the coating.