Fatigue life of deposited repair welds on single-crystal high-temperature nickel alloy under cyclic oxidation

Abstract

In modern gas-turbine units increase of gas working temperature leads to shortening of blade service life. For this reason their repair becomes a priority. Retailoring of single-crystal blade tip by hardfacing is a rather complicated task. In order to select suitable filler material, thermal cycling tests of samples of deposited welds on CMSX-4 alloy with a single-crystal structure have been performed. Evolution of the structure under the conditions of high-temperature cyclic oxidation is considered. Selection of filler material, which ensures high temperature resistance and stability of weld metal structure, was optimized. These requirements are satisfied by Co-Ni-based material, which was earlier tried out and proved to be very good in complex technology of repair of a blade from ZMI-3U alloy. The technology includes airfoil tip retailoring by hardfacing with subsequent deposition on the item surface of a high-temperature metal coating by electron beam deposition to ensure the required service properties of the item. 18 Ref., 4 Tables, 10 Figures.

Crack‐Free Welding of a Co‐Base Superalloy with High γ' Precipitate Fraction

“…EDS investigations revealed an increased content of Ta and Hf in these carbides. The shape and composition of these carbides indicate an MC type, which was also reported by Belyavin et al [53] for the alloy PWA795, where the carbides are also mainly present in the interdendritic regions. The median grain size rises to 31 μm with the quartiles being Q 0.25 ¼ 18 μm and Table 1.…”

“…(Co, Ni)Al precipitates have been identified by EDS measurements corresponding with the literature. [ 53 ]…”

“…(Co, Ni)Al precipitates have been identified by EDS measurements corresponding with the literature. [53] The microstructure was characterized after a standard metallographic sample preparation using a SEM (Crossbeam 1540 EsB, Zeiss, Oberkochen/Germany) and the grain size was determined by electron backscatter diffraction (EBSD) (Nordlys, Oxford Instruments, Abingdon/UK). The chemical composition was probed with a different SEM (Crossbeam 540, Zeiss, Oberkochen/Germany) by energy dispersive X-ray spectroscopy (EDS) (X-Max N 150, Oxford Instruments, Abingdon/UK).…”

“…The composition of this alloy lies between the γ' strengthened Co-base wrought alloy CoWAlloy2 [34,52] and the conventional, mainly carbide hardened Co-base weld filler alloy PWA795 with excellent oxidation properties. [53] The alloy called CoWAlloy2Mod has in contrast to CoWAlloy2 no additions of Zr and Si as they form intermetallic precipitates, especially at grain boundaries, [54][55][56] which should greatly reduce the weldability. Furthermore, The Ni-and W-content was decreased and the Cr-content was increased since Ni and W increase the γ' solvus temperature while Cr decreases it.…”