Mechanical Stress Effects on 550 °C Hot Corrosion Propagation Rates in Precipitation Hardened Ni-Base Superalloys: CMSX-4, CM247LC DS and IN6203DS

Oxidation of Metals https://doi.org/10.1007/s11085-021-10089-w ORIGINAL PAPER Mechanical Stress Effects on 550 °C Hot Corrosion Propagation Rates in Precipitation Hardened Ni‑Base Superalloys: CMSX‑4, CM247LC DS and IN6203DS Neil Chapman1,2 · Simon Gray2 · Joy Sumner2 · John Nicholls2 Received: 26 May 2021 / Revised: 16 November 2021 / Accepted: 18 November 2021 © The Author(s) 2021 Abstract Combinations of temperature, stress and hot corrosion may cause environmentallyassisted 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. Keywords High-temperature mechanical properties · Strengthening mechanisms * Neil Chapman 1 Siemens Energy Industrial Turbomachinery Limited, Ruston House, Waterside South, PO Box 1, Lincoln LN5 7FD, UK 2 Cranfield University, College Road, Cranfield, Wharley End, Bedfordshire MK43 0AL, UK 13 Vol.:(0123456789) Oxidation of Metals Introduction Critical rotor blades of an industrial gas turbine (IGT) experience high temperatures and stresses during routine operation. The rotor blades must therefore 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 investment casting technique is used in the manufacture of the rotor blades [3]. This is followed by a suitable heat treatment which ensures a microstructure consisting of minimal gamma-prime eutectic [1] and a homogeneous distribution of gamma-prime precipitates within a gamma matrix [1, 3–5]. 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 [3–5]. Other elements associated with the gamma prime are Ta, Ti and Nb which may substitute with the Al to strengthen this phase [1]. In addition, the unit cell of the gamma-prime generally has a smaller lattice parameter to that of the gamma [7], which creates coherency strains [5]. Between the temperatures of 400 to 650 ˚C, the magnitude of these strains increases [6] due to the different thermal expansion rates of the gamma-prime and gamma [7]. Thus, the strengthening mechanisms of the precipitation-hardened Ni-base superalloys include both solid solution and coherency strengthening. The amount of alloy strengthening tends to increase with the gamma-prime precipitate volume fraction [5] and development of the chemistries [1, 4] has resulted in gamma-prime precipitate volume fractions of up to 70% [3, 6, 7]. This has been achieved by increasing the proportion of elements within the superalloys that are associated with the gamma-prime [6]. The additions of Al and Cr to the chemistries also provide the superalloys with oxidation resistance. That is, a slow-growing protective scale of either alumina or chromia may form, initially alongside transient oxides of all alloying additions. Once the protective scale has formed a continuous layer though, the transient oxides will stop growing since the protective scale acts as a barrier. This prevents O2 reacting any further with the elements responsible for the transient oxides [8]. If damaged by cracks or spallation during service [8] though, the protective scales may self-repair [9] providing enough of the respective element (Al or Cr) remains in the superalloy. This self-repair period is known as steady-state oxidation, and during this stage, the rates of oxidation attack are low [9]. Eventually, though, the respective element in the superalloy may become so depleted that the protective scale will no longer be able to self-repair. The superalloy will then enter a breakaway stage and increased rates of oxidation attack will be experienced [9]. In hot corrosion, the self-repair period of the protective scale is known as incubation and may provide limited protection against attack [8, 9]. That is, when molten deposits such as Na2SO4 accumulate on the surfaces, the protective scale may be damaged [2] by dissolution [8] and thus shorten the incubation stage as 13 Oxidation of Metals the protective scale is forced to self-repair. The Na2SO4 deposits are in a molten state at a temperature of around 900 ˚C allowing so-called Type I hot corrosion to occur which, having passed through incubation, enters a propagation stage, and causes accelerated internal damage and sulphidation [2, 9, 10]. At temperatures around 700 °C, the N a2SO4 deposits are in a solid-state but, providing S O3 is present in the gas phase, may interact with a transient NiO on the surface of the superalloy and produce a molten Na2SO4:NiSO4 system [2]. This is known as Type II hot corrosion and, during the propagation stage, causes accelerated attack of the superalloy which is characterised by pitting [2, 9, 10]. Hot corrosion may also occur with the deposits remaining in a solid-state [11–13]. An example of this was provided by Kistler et al. [14] After performing hot corrosion exposures on a Ni-base superalloy and repeated on 99.98% pure Ni. These exposures were conducted over various durations (up to 20 h) and used Na2SO4 deposits (with a surface loading of 2.5 mg cm−2) in a gaseous environment of SO2 in O2 at a temperature of 550 °C. At this temperature, the deposits are not expected to melt when applied to the pure Ni since the N a2SO4:NiSO4 system has the lowest melting point of 671 °C [14]. Sim (...truncated)