核级IN718高温合金中混晶原因及其对力学性能的影响

Causes of mixed grains in nuclear grade IN718 high temperature alloy and its effect on mechanical properties

摘要:

718合金因其在高温下优异的机械性能和良好的加工性能,已成为核电、航空等领域的关键材料。核级718合金需满足抗辐照能力强、热中子吸收截面小等特殊要求,因此,合金中Ni、Nb、C 等元素含量一般控制在较低水平,然而对这种低Ni、Nb、C的核级718合金热变形组织性能的研究较少。本工作通过设计不同初轧温度及变形量,研究了不同热轧工艺参数及轧后热处理对核级718合金组织和拉伸性能的影响。结果表明,在初轧温度低于1150℃且热变形量低于90%的条件下,组织中出现大量拉长晶,并降低合金的延伸率；当初轧温度达到1150℃或热变形量达到90%时,可以得到均匀的等轴晶组织。轧后板材进行990℃固溶处理后,δ相析出并对晶界产生钉扎作用,形成了混晶显微组织且在标准时效处理后315℃屈服、抗拉强度均值可达到1050MPa和1238MPa,断后伸长率高达26%；当进行1020℃固溶处理后可得长大的等轴晶粒,标准时效处理后315℃屈服、抗拉强度均值可达到985MPa和1175MPa,断后伸长率高达29%,显示出良好的强韧性。

关键词:核级718合金;拉长晶;混晶;拉伸性能

Abstract:

718 alloys have become key materials in nuclear power and aerospace fields because of their excellent mechanical properties and good high-temperature processing performance. Nuclear grade 718 alloys need to meet special requirements such as strong irradiation resistance and small thermal neutron absorption cross-section, therefore, the elemental content of Ni, Nb, and C in alloys is generally controlled at low levels, however, there is less research on the thermal deformation structures and properties of such low Ni, Nb, and C nuclear grade 718 alloys. This work investigated the effects of different hot rolling process parameters and post-rolling heat treatment on the structures and tensile properties of nuclear grade 718 alloys by designing different initial rolling temperatures and deformation amounts. The results show that, under the condition that the initial rolling temperature is below 1150℃ and the heat deformation 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 achieves 1150℃ or the heat deformation reaches 90%, a uniform equiaxed grain structure can be obtained. After the solid solution treatment of the rolled plate at 990℃, δ phases are precipitated and pinned to grain boundaries, forming a mixed grain microstructure, as well as the average values of yield and tensile strength at 315°C after standard aging treatment can reach 1050MPa and 1238MPa, accompanied by an elongation after the fracture as high as 26%; Upon solid solution treatment at 1020°C, the grown equiaxed grains are obtained, whereas the average yield and tensile strength at 315°C can attain 985MPa and 1175MPa after standard aging treatment, respectively, and the elongation after the break is up to 29%, showing good strength and toughness.

Key words:Nuclear grade 718 alloy; elongated grain; mixed grains; tensile properties