Simon Gray

Influence of shot peening on high-temperature corrosion and corrosion-fatigue of nickel based superalloy 720Li

High-temperature corrosion fatigue, a combination of corrosion with a fatigue cycle, is an emerging generic issue affecting power generation and aero gas turbine engines and has the potential to limit component life. Historically, surface treatments, such as shot peening have been used to improve component life and have been optimised for fatigue response. Research into optimisation of shot peening techniques for hot corrosion and high-temperature corrosion fatigue has shown 6–8A 230H 200% coverage to provide overall optimum performance for nickel-based superalloy 720Li based on the limited data within this study. Utilisation of electron backscatter diffraction techniques, in combination with detailed assessment of corrosion products have been undertaken as part of this work. The resultant cold-work visualisation technique provides a novel method of determining the variation in material properties due to the shot peening process and the interaction with hot corrosion. Through this work it has been shown that all three shot peening outputs must be considered to minimise the effect of corrosion fatigue, the cold work, residual stress and surface roughness. Further opportunity for optimisation has also been identified based on this work.

Impact of Deposit Recoat Cycle Length on Hot Corrosion of CMSX-4

Hot corrosion causes significant problems for both aerospace and power generation industries, where the combination of high temperature, corrosive gases, and contaminants severely limits component operating lifetimes in gas turbine hot gas streams. Multiple laboratory testing methodologies exist to study this hot corrosion, and these can be affected by a range of variables. This paper investigated the impact of varying deposit recoat cycle length when using the ‘deposit recoat’ testing method. CMSX-4 samples were exposed to simulated type II (pitting) hot corrosion conditions, with the same overall deposit load (averaged across the total exposure run), but different deposit recoat cycles. Post-exposure, samples underwent dimensional metrology analysis to compare metal loss resulting from different deposit recoat cycle lengths. Results for CMSX-4 suggest very small differences in corrosion losses, indicating CMSX-4 hot corrosion datasets obtained from deposit recoat experiments with different deposit recoat cycle lengths can be compared with confidence.

Oxide Growth Stresses in an Austenitic Stainless Steel Determined by Creep Extension

The development of growth stresses during isothermal high-temperature oxidation can affect the mechanical integrity of protective oxide layers. The ability to measure the magnitude of these stresses is then important not only from the point of view of developing more robust degradation models, but also from the need to understand better the nature of these stresses and the role of alloy mechanical properties in their relaxation. Growth stresses within the chromia layer grown on 20Cr25Ni steels have been estimated in this paper from the creep extension observed during oxidation in air at 900 o C. Two variants of the steel have been used, one containing 0% Si and the other 0.76% Si, sufficient to form a silica interlayer between chromia and steel. In each case, growth stresses were compressive but, at any given exposure time, those in the Si-free alloy were up to a third smaller than in the Si-bearing alloy. However, oxidation rates in the absence of silicon were higher than in the alloy containing silicon and when growth stresses were compared at a given oxide thickness there was much less difference between the alloys. This indicates that the silica interlayer has no large direct effect on the chromia growth stress. Both alloys show that the growth stress decreases with oxide thickness from approximately -1.6 GPa at 1.2 μm to approximately -0.3 GPa at 4-μm thick. Introduction A potentially important parameter affecting the mechanical integrity of a protective oxide layer is the stress developed within it during the oxidation process. The existence of such growth stresses has been known for many years an overview is provided in [1]. A plausible model [2] envisages that growth stresses develop as a consequence of the formation of new oxide in a mechanically constrained environment within the oxide layer. The in-plane stresses will then be compressive and this accords with the majority of experimental observations [3-8]. The majority of recent determinations of growth stresses has been on alumina-forming alloys although Goedjen et al. [5], using high-temperature XRD, have reported a compressive stress of around 350 MPa in a chromia layer on chromium. The emphasis on alumina-forming alloys not only reflects the generic importance of such materials but also their increasing use at high temperatures and in thin sections. In such cases, the oxide growth stresses may be determined from the creep deformation of the alloy substrate. For the usual case of compressive growth stresses, the substrate is placed into tension and can extend by creep at the oxidation temperature. Measurement of the specimen extension and its rate provides a method [1] for evaluating the oxide growth stresses provided that the creep properties of the alloy substrate are known. One of the earliest studies of extension due to oxidation was undertaken by Noden et al. [3] on various austenitic steels in both oxygen and CO2-based environments. They found extensions of up to approximately 1% after about 1000 hours exposure at 900 o C. These alloys were expected to form a dense adherent chromia layer but tended to form mixed chromia/spinel scales. Subsequent estimates [1] of growth stress based on a force balance argument (see below) with estimated values Materials Science Forum Online: 2004-08-15 ISSN: 1662-9752, Vols. 461-464, pp 755-764 doi:10.4028/www.scientific.net/MSF.461-464.755

Stress corrosion of Ni-based superalloys

Abstract The development of gas turbines to increase fuel efficiency is resulting in progressively higher operating temperatures in the under platform regions of the blades. These regions have traditionally been considered low risk areas. However, higher metal temperatures combined with stresses and the deposition of contaminants from the cooling air system may result in complex degradation mechanisms. Static stress corrosion testing has been conducted on C-ring specimens at a range of stresses in a hot corrosion environment. Cracks were observed in C-rings after exposure times greater than 100 h. Scanning electron microscopy (SEM) systems were used to image cracks and characterise deposits to improve understanding of the mechanism. Finite element analysis (FEA) has been used to model the stress intensity under test conditions. CMSX-4 specimens subject to static stresses combined with hot corrosion demonstrated significant material degradation (crack initiation and propagation) suggesting a combined stress corrosion mechanism resulting in cracking.

Corrosion fatigue testing: the combined effect of stress and high temperature corrosion

Abstract A corrosive environment can have a detrimental effect on the fatigue life of a material due to a change in failure mechanism. Attempts have been made to replicate this change on nickel-base superalloy CMSX-4 cast in the <001> orientation. Fatigue testing in air, of this material typically produces a fracture on an angle of approximately 55° which is consistent with the fracture having propagated on a {111} slip plane. The aim of the research was to fatigue test in a corrosive environment with the purpose of producing a crack/fracture which deviated from the typical angle and thus confirm that the corrosive environment had affected the fatigue mechanism. It was concluded that the change in mechanism to high temperature corrosion fatigue was associated with a reduced load application rate together with precorroding the test specimens to trigger the initiation of the corrosion fatigue mechanism.

Optimisation of a salt deposition technique for the corrosion-fatigue testing of nickel based superalloys

ABSTRACT This paper summarises the work undertaken to develop a salt deposition technique that can be utilised for hot corrosion and high temperature corrosion-fatigue testing of high performance alloys for gas turbine application. The optimisation process has yielded a new method of application of a sodium sulphate based solution to various test piece geometries. The technique employs the use of a micro-pipette to deposit a pre-calculated solution, consisting of methanol, water, Na2SO4–NaCl onto preheated specimens, in preparation for corrosion testing. This optimised salting technique has reduced the variability in the salting of test specimens, when compared with existing salt spray methods and has therefore enabled test repeatability along with an associated significant reduction in scatter within high temperature corrosion-fatigue results.

Low Temperature Hot Corrosion Screening of Single Crystal Superalloys

Single crystal superalloys were screened in Type II molten (Na,K)-sulfate hot corrosion re-coat tests in air +300 ppm SO2 at 700 °C. They exhibited large 20–40 mg/cm2 weight changes, repeated spallation, and non-protective, 25–50 μm thick corrosion layers after 300 h of testing. Scale cross sections revealed dual outer Ni(Co)O and inner Al(Cr)S-rich corrosion layers. This chemical differentiation was partially consistent with previous models of oxide fluxing, alloy sulfidation, NiO micro-channel diffusion, and synergistic dissolution mechanisms. Broad shallow pits or uniform attack morphologies were consistent with prior studies performed in high >100 ppm pSO2 environments. Higher Mo experimental alloys trended toward more degradation, producing 100 μm thick scales with distinct Al(Cr)S-rich inner layers or 500 μm thick NiO. The aggressive behavior in these environments supports the need for LTHC-resistant coatings for single crystal superalloys.