型壳保温方式对一种等轴晶镍基铸造高温合金组织和力学性能的影响

Effect of shell insulation methods on microstructure and mechanical properties of an equiaxed Nickel-based cast superalloy

摘要:

通过设计型壳保温方式改变合金凝固冷却速率,分析了凝固冷却速率对一种等轴晶镍基铸造高温合金K4750显微组织和室温拉伸性能的影响。设计的四种型壳保温方式包括填砂、试棒不包保温毡、包单层保温毡和包双层保温毡,型壳试棒处的凝固冷却速率大小顺序为：试棒不包保温毡≥填砂＞＞包单层保温毡≥包双层保温毡。标准热处理后,型壳填砂的试棒室温抗拉强度最高(1115MPa),试棒不包保温毡次之(1095MPa),包单层保温毡与包双层保温毡最差(分别为950MPa和952.5MPa)。室温屈服强度、延伸率和断面收缩率变化规律与室温抗拉强度相同。采用OM、SEM以及EBSD等方法表征合金显微组织,发现型壳填砂、试棒不包保温毡、包单层保温毡和包双层保温毡的试棒晶粒均为等轴晶,平均晶粒尺寸分别为176μm、167μm、325μm、315μm。填砂和试棒不包保温毡合金凝固冷速较快,形成的细小等轴晶在应力条件下易于协调变形,凝固过程形成的MC型一次碳化物更细小且主要呈块状,有利于抑制变形时微孔洞、裂纹的扩展,显著提高了合金室温拉伸性能。相反,型壳包单层和双层保温毡的合金凝固冷速较慢,形成的大尺寸等轴晶不利于晶粒间的协调变形,析出的大尺寸长条状MC型一次碳化物促进孔洞、微裂纹的萌生与扩展,使合金室温强度和塑性均显著下降。

Abstract:

The effect of solidification cooling rate on microstructure and room temperature tensile properties of equiaxed nickel-based cast superalloy K4750 were studied by designing shell insulation methods. Four types of insulation methods were designed, including: sand-filled, test bars without insulation felt, wrapped single-layer insulation felt and wrapped double-layer insulation felt, and the order of solidification cooling rate at the test bar of the shell is: test bars without insulation felt ≥ sand-filled > > wrapped single-layer insulation felt ≥ wrapped double-layer insulation felt. After standard heat treatment, the test bar with sand-filled shell has the highest room temperature tensile strength (1115MPa), the test bar without insulation felt is the seconed highest (1095MPa), and the single-layer insulation felt and the double-layer insulation felt are the worst (950MP and 952.5MPa respectively). OM, SEM and EBSD were used to characterise the microstructure of the alloy. The results show that the grains of sand-filled, test bars without insulation felt, single-layer insulation felt and double-layer insulation felt are equiaxed crystals, and the average grain sizes were 176μm, 167μm, 325μm and 315μm, respectively. Sand-filled and test bars without insulation felt alloys solidification cooling rate is faster, the formation of small equiaxed crystals more easily coordinated deformation in the stress conditions, solidification process to form the MC type primary carbide is finer and mainly in the form of blocks, which is conducive to inhibit the expansion of microvoids and cracks during deformation, significantly improving the tensile properties of the alloy at room temperature. On the contrary, single-layer insulation felt and double-layer insulation felt alloys solidification cooling rate is slower, the formation of large-size equiaxed crystals are not conducive to the coordinated deformation between the grains, and the precipitation of MC type primary carbide with large size and long strip shape promotes the emergence and expansion of holes and microcracks, which significantly reduces the strength and plasticity of the alloy at room temperature.