Formation of Mixed Grains in Nuclear Grade IN718 Superalloy and Its Effect on Mechanical Properties
The effects of different hot rolling parameters and post-rolling heat treatment on the structures and tensile properties of nuclear grade 718 alloys were investigated by designing different initial rolling temperatures and deformation amounts.The results show that when the initial rolling temperature is below 1150 ℃ and the heat deformation amount is lower than 90％,a large number of elongated grains appear in the structure and the elongation of the alloy is reduced;when the initial rolling temperature is 1150 ℃ or the heat deformation amount reaches 90％,a uniform equiaxed grain structure can be obtained.After the solid solution treatment at 990 ℃ of the rolled plate,δ phases are precipitated and pinned to grain boundaries,and a mixed grain microstructure forms.The average values of yield strength and tensile strength at 315 ℃ after standard aging treatment can reach 1050 MPa and 1238 MPa,respectively,and elongation is up to 26％;Upon solid solution treatment at 1020 ℃,the grown equiaxed grains are obtained,whereas the average yield strength and tensile strength at 315 ℃ can reach 985 MPa and 1175 MPa after standard aging treatment,respectively,and the elongation is up to 29％,and the alloy exhibits good strength and toughness.
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