Study On The Mechanism Of Hot Corrosion Resistance Of NiCoCrAlY Cladding Layer

Martensitic precipitation hardening stainless steel has good comprehensive properties such as corrosion resistance,strength,toughness and excellent machinability,and has been widely used in chemical,petroleum,aerospace,nuclear industry,energy and other fields.With the development of modern manufacturing and changes in the application environment of stainless steel,there are more and more problems.Surface modification treatment is one of the methods to effectively improve the surface hardness,wear resistance and fatigue resistance of the material,and plays an important role in modern manufacturing.This paper takes 17-4PH stainless steel as the research object,and uses laser cladding technology to prepare NiCoCrAlY coating on the stainless steel surface to improve its surface performance.The effect of different scanning velocitys on the geometrical characteristics of NiCoCrAlY coating on the surface of 17-4pH stainless steel.The results show that the increase of scanning velocity reduces the weld width,weld height and weld depth of the coating,and the dilution decreases first and then increases.The effects of different scanning velocitys on phase composition,microstructure and properties of coatings.NiCoCrAlY coatings are mainly composed of??Ni?main phase and?NiAl?auxiliary phase.With the increase of scanning velocity,the microstructure of the coating was gradually refined,forming uniform and regular cellular crystals.When the scanning velocity is 3mm/s,the microhardness is low due to the coarse microstructure of the coating.When the scanning velocity is 5mm/s,the microstructure is refined and dense,the grain is fine,the grain boundary strengthening and toughening were enhanced,and the hardness of the coating increased.When the scanning velocity was 7mm/s and 9mm/s,cracks and pores appeared in the coating tissue,leading to a decrease in hardness.The effect of different laser power on the geometric characteristics of NiCoCrAlY coating on on the surface of 17-4pH stainless steel.The results show that the increase of laser power increases the weld width and weld depth of the coating,but the increase in the weld width is much smaller than the increase in the the weld depth.With the increase of laser power,the melting height of coating can be increased or decreased.The dilution rate increases first and then decreases with the increase of laser power.The effects of different laser power on phase composition,microstructure and properties of coating.When the laser power was1500,2000 and 2500W respectively,planar crystals were formed in the bonding zone close to the matrix,and columnar crystals grew in an epitaxial way above the planar crystal region perpendicular to the solid-liquid interface during cooling.When the laser power was 1000,1500,2000 and 2500W,the average microhardness of the coating was 421HV0.1,436HV0.1,449HV0.1 and 411HV0.1 respectively.The laser power has little effect on the hardness of the coating.NiCoCrAlY coatings exhibit better wear properties at high temperatures than at room temperature.High temperature corrosion experiments were carried out on the substrate and NiCoCrAlY coating at 650?75wt.%Na₂SO₄+25wt.%NaCl molten salt,respectively.The microstructure morphology,corrosion dynamic curve,surface oxide film of the coating,the composition and phase composition of the surface after high temperature corrosion were preliminary discussed and studied.The results show that:the stainless steel substrate gains weight rapidly in the first 10 hours,and then quickly loses weight.The surface of the substrate cannot form an effective protective film,so the substrate has basically no protection under the 650?molten salt environment.Corrosion dynamic curve shows that the coating gains weight slowly,and a protective oxide film is gradually formed on the surface.The weight gain is only 0.08mg/cm²after 10h oxidation,but with the corrosion time reaching100h,the protective film on the surface of the coating is gradually destroyed.Internal oxidation and vulcanization reaction occurred inside the coating.