Abstract
Combinations of temperature, stress and hot corrosion may cause environmentally-assisted cracking in precipitation-hardened Ni-base superalloys, which is little understood. This research aims to increase current understanding by investigating the effects of mechanical stress on the hot corrosion propagation rate during corrosion-fatigue testing of CMSX-4, CM247LC DS and IN6203DS. The parameters used during the tests included a high R-ratio, high frequency, and a temperature of 550 °C. The results showed CMSX-4 experienced a predictable increase in the hot corrosion rate, CM247LC DS also experienced increased rates, but no obvious trend was apparent; whilst IN6203DS showed no evidence of an increased rate. These different behaviours appear to be a result of an interaction between the mechanical stress and microstructural features, which include gamma-prime volume fractions in both the matrix and eutectic regions, along with the distribution of the eutectic structure. The different behaviours in the hot corrosion propagation rate subsequently affected the respective corrosion fatigue results, with both CMSX-4 and CM247LC DS experiencing fracture but with significantly more scatter involved in the CM247LC DS results. All IN6203DS corrosion-fatigue specimens completed the respective tests without fracture and showed no evidence of cracking. It, therefore, appears that precipitation hardened Ni-base superalloys, which are susceptible to environmentally-assisted cracking, also experience increased hot corrosion propagation rates.
Highlights
Critical rotor blades of an industrial gas turbine (IGT) experience high temperatures and stresses during routine operation
energy dispersive X-ray (EDX) mapping, on the unetched longitudinal sections, of the hot corrosion products within the CMSX-4 and CM247LC DS cracks that were remote from the final fracture, revealed an O [21] and S [20] embrittled phase (Fig. 3 shows an example from CMSX-4)
The IN6203DS specimens successfully completed the 300 h of CF testing without showing any evidence of cracking
Summary
Critical rotor blades of an industrial gas turbine (IGT) experience high temperatures and stresses during routine operation. The rotor blades must be manufactured from materials, such as precipitation-hardened Ni-base superalloys, that have favourable high temperature mechanical and oxidation properties. Improvements in these properties have been attained through development of the superalloys [1], allowing the IGT to operate at higher temperatures [2] and efficiencies resulting in reduced CO2 emissions [3]. The gamma has a disordered fcc unit cell [4, 5] containing elements such as Cr, Co, Re and Mo [6] whilst the gamma-prime is a N i3Al based phase [1] [3] that has an ordered L 12 fcc unit cell [3,4,5]. The strengthening mechanisms of the precipitation-hardened Ni-base superalloys include both solid solution and coherency strengthening
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