Effect of trapping and temperature on the hydrogen embrittlement susceptibility of alloy 718

Abstract

International audienceNi-based alloy 718 is widely used to manufacture structural components in the aeronautic and nuclear industries. Numerous studies have shown that alloy 718 may be sensitive to hydrogen embrittlement. In the present study, the susceptibilities of three distinct metallurgical states of alloy 718 to hydrogen embrittlement were investigated to identify both the effect of hydrogen trapping on hydrogen embrittlement and the role of temperature in the hydrogen-trapping mechanism. Cathodic charging in a molten salt bath was used to saturate the different hydrogen traps of each metallurgical state. Tensile tests at different temperatures and different strain rates were carried out to study the effect of hydrogen on mechanical properties and failure modes, in combination with hydrogen content measurements. The results demonstrated that Ni-based superalloy 718 was strongly susceptible to hydrogen embrittlement between 25 °C and 300 °C, and highlighted the dominant roles played by the hydrogen solubility and the hydrogen trapping on mechanical behavior and fracture modes

Effect of temperature on hydrogen embrittlement susceptibility of alloy 718 in Light Water Reactor environment

“…Severa! in terpretations of this phenomenon consider that precipitates act as brittle inclusions on which dimples are initiated easily [19,20). Although these precipitates were regularly distributed in the matrix, with respect to thermal vacancies, for hydrogen content of 1000 ppm, leading to HE.…”

“…B, C and D samples presented a homogeneous grain size of ASTM 9 10. All the microscopie observations of these microstructures have already been presented in detail in a previous work [19). For all samples, surface preparation before experimental tests was realized using a 1200 grid SiC paper.…”

“…In order to emphasize the influence of y' and y• pre cipitates, further mechanical tests were conducted at 300 •c, but with a higher strain rate {5.10 2 s 1 ) for C samples. This aimed to promote strain localization around y' and y• pre cipitates [16,19). For this strain rate, fracture surface of H precharged samples exhibited several areas with nano scaled dimples that were not observed on H free samples.…”

“…To confirm the previous results, i.e. the strong influence of g 0 and gʺ precipitates on HE for samples A, additional mechani cal tests were performed on samples C. Hydrogen cathodically precharged C tensile samples were tested at 80 C after 30 min of desorption at 300 C. During the heat treatment at 300 C for 30 min, it was assumed that the most part of hydrogen had desorbed from the matrix and that hydrogen remained trapped around g 0 and gʺ precipitates [19]. Results showed that the presence of hydrogen did not affect YS or UTS whereas there was a significant loss of ductility, of about 22%.…”

Materials and Corrosion Trends in Offshore and Subsea Oil and Gas Production

“…1 Threshold stress (σ th ) of low alloy steels with different microstructures exposed to 0.5 wt% CH 3 COOH + 5 wt% NaCl in 1 atm H 2 S at 24°C, normalized to the actual yield strength (σ y Currently, no standard defines the maximum allowable temperature for seawater service of NA725 and NA716; nevertheless, NA625 is restricted by ISO 21457 to 30°C due to crevice corrosion concerns in chlorinated systems. While it is well established that the presence of δ-phase severely compromises NA718's HSC and SSC resistance, 56 PHNA have been, a priori, considered immune to hydrogen embrittlement in the age-hardened conditions used in O&G applications. 48 However, sudden cleavage failures of NA718, 57 NA716, 58 and NA725 59 subsea components in relatively benign environments have been reported during installation and operation, all associated with HE.…”