关键词: 高温合金, 螺栓, 沿晶脆性断裂, 混晶, Ni₅Al₃相, β相, 预紧力

Abstract: Early failure and fracture of GH783 superalloy bolts have occurred frequently since their extensive use in coal-fired power generating units. In order to clarify the failure nature and cause of the GH783 alloy bolt, SEM, OM and TEM were used to analyze the fracture morphology, microstructure and precipitated phases. Specimens were taken near the fracture for tests of tensile, hardness and impact properties. The results show that the early failure of the GH783 alloy bolt is a typical intergranular brittle fracture. The mixed grain microstructure produced by improper hot processing of the bolt is an important factor that leads to the early failure of the bolts. The significantly degenerated microstructure, coarsened grain boundary and the coarse needle-like Ni₅Al₃ precipitates of the bolt weaken the positive effect of β phase on the comprehensive properties of the material. The β phase is the main source of crack initiation, which gradually breaks and falls off and leads to formation of some cavities under the action of multi-factor coupled external loads, and leads to cracks by connecting the multiple cavities. Such cracks propagate along the grain boundaries and eventually lead to early failure of the bolt. Therefore, the microstructure of the GH783 alloy bolts should be inspected and evaluated before installation of the bolts, especially the grain size, the distribution and morphology of β phase. Additionally, the pre-tightening force of the bolt installation should be optimized and adjusted under the premise of meeting the equipment sealing requirements.

Key words: superalloy, bolt, intergranular brittle fracture, mixed grain, Ni₅Al₃ phase, β phase, pre-tightening force