| The application of heat energy storage(TES)system in the concentrating solar thermal power(CSP)plant can solve the problem that the solar energy cannot be supplied continuously due to alternating seasons.321 stainless steel,as a Ti-stabilized austenitic stainless steel,is widely used in critical parts such as and heat exchange pipes in CSP systems.AlSi12 alloy is one of the most ideal high-temperature phase change materials due to excellent thermal conductivity and heat storage.The cyclic load induced by the self-weight and thermal expansion of AlSi12 alloy will cause fatigue damage of the heat exchange tube in the TES system,and the corrosions will also occur due to the molten AlSi12 alloy and accelerate the fracture failure of the tube.Generally,aluminized coating can effectively improve the corrosion resistance of the material against molten aluminum,but only aluminizing treatment cannot meet the requirement of mechanical properties.Laser shocking process(LSP)is an effective surface modification method for strengthening mechanical properties of materials.Consequently,321 stainless steels were surface treated by packed aluminizing with LSP.The tensile behavior,high-cycle fatigue behavior and the effect of molten AISi12 environment on high-cycle fatigue interaction behavior of 321 stainless steel were investigated.Considering surface characteristics and fracture analysis,the effects of surface treatment on high-cycle fatigue mechanism under different environments and loading modes were discussed.The main conclusions are as follows:(1)The microstructure,microhardness,XRD and surface roughness of aluminized steel were investigated,and the results showed that the outer layer Al2O3 film,the transition layer Fe-Al intermetallic compounds(FeAl,FeA12,Fe3Al)and the infiltrated layer A1(Fe,Cr)solid solution are distributed uniformly from the surface of aluminizing coating to the substrate.The microhardness of aluminized coating is significantly higher than that of 321 stainless steel,and slight softening occurs in the matrix.The thickness of infiltrated layer increases and the overall hardness increases after annealing treatment.The surface roughness of the material significantly improved after aluminizing and annealing.As the laser power density and the number of impacts increase,the surface roughness and microhardness of the aluminized 321 stainless steel increase.(2)The results of high-temperature tensile behavior showed that both the process of aluminizing and annealing resulted in a decrease in yield strength and tensile strength.And the elongation decreases after aluminizing but increases greatly after annealing.The yield strength,tensile strength and elongation of the aluminized steel increase after LSP treatment,and the aluminized steel with laser energy density of 6.59 GW/cm2 and three-times impact possesses the best tensile performance.The surface grain refinement,high dislocation intensity and residual compressive stress are the main strengthening mechanism of LSP.(3)The aluminizing and annealing process decreases the high-cycle fatigue life of stainless steel by orders of magnitude.The high-cycle fatigue life of aluminized 321 stainless steel increases as impact times and power density of LSP increase.The three-impact aluminized 321 stainless steel with power density of 6.59 GW/cm2 possesses the best high cycle fatigue performance.The trans-granular cleavage fracture of Fe3Al and intergranular fracture of FeAl were the primary fracture mode of Fe-Al intermetallic compound,and the fracture mode of the Al(Fe,Cr)infiltration layer is trans-granular cleavage fracture.Annealing process increases the grain size of Fe-Al intermetallic compound and the thickness of A1(Fe,Cr)infiltration layer,which leads to a decrease in material fatigue life.(4)The corrosion action between molten AlSi12 alloy and 321 stainless steel is significant,which results in fatigue life reduction.The corrosion action of aluminized 321 stainless steel and molten AlSi12 alloy is less evident,and the fracture mechanism is similar with that in the high-temperature air environment.The aluminized steel with laser energy density of 6.59 GW/cm2 and three-times impact possesses the best high-cycle fatigue performance in the molten AlSi12 alloy. |
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