Superalloy 718 and Derivatives: Environmental Behavior & Protection II
Program Organizers: Joel Andersson, University West; Chantal Sudbrack, National Energy Technology Laboratory; Eric Ott, GE Additive; Zhongnan Bi, Central Iron and Steel Research Institute
Tuesday 10:25 AM
May 16, 2023
Room: Admiral
Location: Sheraton Pittsburgh Hotel at Station Square
Session Chair: Michael Fahrmann, Haynes International; Chantal Sudbrack, National Energy Technology Laboratory
10:25 AM Invited
Effects of High-temperature Oxidation on Fatigue Life of Additive-manufactured Alloy 625
: Grace de Leon Nope1; Guofeng Wang1; Juan Manuel Alvarado Orozco2; Brian Gleeson1; 1University of Pittsburgh; 2CIDESI (Centro de Ingenieria y Desarrollo Industrial)
The effect of thermal oxidation on fatigue performance of additive-manufactured Alloy 625 was studied. Uniaxial fatigue tests at room temperature were performed after exposures at 800°C for 24h, 300h, and 1000h in either air or argon. Exposures in air resulted in chromia-scale formation, internal attack, and subsurface precipitates. Fatigue results indicated a life reduction for the oxidized additive manufactured samples. By contrast, any of the prior high-temperature exposures were not detrimental to the performance of the wrought Alloy 625. Fractographic analysis indicated that crack initiation occurred in the subsurface, closer to the edge, for exposures to oxidation compared to those exposed to argon. The observed behavior in AM-processed samples is attributed to the exacerbation of subsurface defects during oxidation (i.e., internal attack, decohesion in the scale, and subsurface precipitates). These defects act as preferential crack-initiation sites, leading to reduce a reduction in fatigue life.
10:55 AM
Subcritical Crack Growth of 718 Alloy in Marine Exposure Conditions and Microstructural Modeling: Attilio Arcari1; Derek Horton1; Mohammed Zikry2; M. Chen2; 1Naval Research Laboratory; 2North Carolina State University
UNS N07718 is widely used in marine service applications and under a variety of conditions: alternate immersion, different levels of cathodic protection, freely corroding galvanic couples. Environmentally assisted cracking can significantly affect the performance of this alloy and constraint design as it needs to account for subcritical crack growth in service. We measured subcritical crack growth rates and thresholds in different environmental conditions for two different heat treatments of UNS 07718. The first heat treatment, following AMS 5664 is typically used in the aircraft industry, and the second, following API 6A, is used in the marine and oil and gas industry. Other materials studied are UNS R30035 and UNS S66286. The material environmentally assisted cracking was studied under alternate immersion to natural seawater, and under cathodic protection in natural seawater.Microstructural modeling of simultaneous fracture nucleation and propagation modes is presented to further understand and predict how precipitates, their volume fraction, morphology, and properties, on the evolution and accumulation of dislocation-densities within the microstructure affect the fracture process at different physical scales. Furthermore, a dislocation-density crystalline plasticity formulation is coupled to a hydrogen diffusion approach and the nonlinear fracture framework to further understand how cracks and dislocation-densities diffuse and interact.
11:15 AM
Hot Corrosion Behavior of a GH4720Li Disk Superalloy at 700℃: Teng An1; Fangzhen Duan2; Yu Gu2; Yuting Shi2; Di Wang2; Jinglong Qu1; Zhongnan Bi1; Jinhui Du1; 1Central Iron & Steel Research Institute; 2Beijing GAONA Materials & Technology Co., LTD
The nickel-based superalloy disk components in the turbine sections were subjected to Type II hot corrosion damage in the sulfur-containing salt contaminants at 650℃–750℃. The alloy GH4720Li with different grain sizes was corroded in a mixture of sulfates (25% NaCl + 75% Na2SO4) at 700℃ for 200 h, and the microstructure evolution was investigated. The experimental results showed that when the grain size increased from 15.9 to 127 μm, the mass loss decreased by 96%, and the corrosion layer thickness decreased by 44%. The hot corrosion resistance increased with the increasing grain size, and the corrosion failure mechanisms changed from pitting corrosion to uniform corrosion. The corrosion layer comprised NiCr2O4, Al2O3, CoO, TiO, Ni3S2, and CoS2. The oxide layer, Ni/Co-rich layer, and S-rich layer were stratified and sequentially located on the alloy GH4720Li surface. The corrosion behavior was accelerated by the triangular grain boundaries (GBs) and γ´ phase, the segregation behavior of Cr elements in the GBs, as well as the γ´ phase formation promoted the tendencies for pit nucleation in the fine-grained structure. In contrast, the Ni/Co-rich layer provided better resistance to hot corrosion and was easier to form on the surface of the coarse-grained structure. The sulfide-oxidation cycle mechanism could well describe the hot corrosion behavior of the disk superalloy.
11:35 AM Conference Luncheon