Ageing Tests of Alloy 617 to Simulate Service Embrittlement

Abstract

Abstract Alloy 617 is a solid solution-strengthened, wrought, nickel base superalloy. It is extensively used in the chemical industries, whenever superior corrosion resistance is required, and in piping applications of the most highly advanced thermal power plants, where creep resistance is important for piping and steam turbine disk applications, and in hot going casings of gas turbine engines, both heavy-duty and industrial scale. It is known from numerous studies that the relatively high chromium and carbon contents of this alloy gives rise to chromium carbide precipitation at well determined temperature ranges. This applies to both the precipitation of primary carbides on solidification, which may be rich in both chromium and molybdenum and are mostly of the MC and M6C types, and the precipitation of secondary chromium carbides of the Cr23C6 type, which are precipitated preferably at grain boundaries, twin boundaries, and slip planes. This precipitation might embrittle the alloy, which could manifest itself in a marked drop of ductility and toughness. In long-term high-temperature applications, this may lead to a failure mechanism called stress relaxation cracking, which may occur when high residual or service stresses cannot be relieved by plastic deformation, because of simultaneous precipitation of secondary carbide precipitates. The onset of this embrittling mechanism is reported to begin as early as after only 500 service hours, if affected components are operated within the carbide precipitation range of approximately 750 – 875 °C. In a more recent development, there were concerns in the fabrication department of the authors' organization that tooling, used for heat treatment of such components, could be affected not only by embrittlement, but also by a drop in strength levels, this after long-time service in industrial furnaces at heat treatment temperatures in the order of 850 °C. Because this is somewhat counter-intuitive, a long term study was launched to test the sensitivity of this alloy for a decrease in strength after service. This study was performed for ageing times between 500 and 3000 h, at the relevant temperature of 850 °C. The findings suggest, as expected, that the onset of embrittlement by secondary carbide precipitates occurs as early as after 500 h in service, as also reported in the literature. This is evidenced by a marked drop in both tensile ductility and Charpy V-notch (CVN) impact toughness, and can also be seen microfractographically by an increase of the fraction of intergranular cleavage fracture. Also as expected, a drop in strength could not be determined. On the contrary, the marked drop in ductility and toughness goes along with a marked increase in ultimate tensile strength (UTS) and yield strength (YS).