PSI - Issue 42

Guocai Chai et al. / Procedia Structural Integrity 42 (2022) 155–162 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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to be susceptible to hydrogen embrittlement (HE) in some environments and applications. Arisoy et al. (2003) have reported a failure of a sailboat propeller shaft, and Tavares et al. (2009) have reported two failures of the pressure safety valve (PSV) springs working in off-shore oil platforms. Material condition and microstructure are two important factors to susceptibility to hydrogen embrittlement in the steel, Chiang et al. (2003) and Alnajjar et al. (2020). It is generally accepted that hydrogen-enhanced localized plasticity (HELP), Beachem et al. (1972) and Birnbaum et al. (1994), hydrogen-enhanced decohesion (HEDE), Pfeil (1926) and Oriani and Josephic (1974) and adsorption induced dislocation emission (AIDE), Lynch (2011), are three principal mechanisms for HE in metals. The other mechanisms such as gas bubbles, surface energy decrease, hydride formation and cleavage, nano-void coalescence (NVC), hydrogen-enhanced and strain-induced vacancies (HESIV) in steels have been well reviewed by Djukic et al. (2019) and Li et al. (2020). Recently, several studies have shown coexistent and synergistic effects of different HE mechanisms. One example is the unified HELP + HEDA model on localized plastic deformation and decohesion for HE, Martin et al. (2012) and Djukic et al. (2016). HEDE mechanism is mainly correlated to intergranular fracture or matrix/inclusion interface decohesion, and HELP mechanism is related to hydrogen-assisted cracking by the intersection between slip bands. A combination of local stress and sufficient hydrogen concentration results in a reduction of the cohesive strength of the boundary/interface that favors hydrogen-induced intergranular failure, Martin et al. (2012). Nano-void coalescence mechanism is proposed to have simultaneous effects of HEDE, HELP and HESIV mechanisms, Li et al. (2020). Zhang et al. (2022) used a combination of Gurson model and cohesive zone model to simulate hydrogen embrittlement by a competition between fracture due to micro-void growth and coalescence and fracture in the cohesive zone. Hydrogen embrittlement occurs when a critical hydrogen concentration is reached. From a fractographic analysis, Möser et al. (2018) have reported that hydrogen induced crack initiation requires both the storage of hydrogen in cavities and plastic deformation. A local crack will occur at a cavity where an internal pressure formed by hydrogen diffusion is high enough. “Fisheye” type of small crack forms at the fracture surface by slip plane decohesion. In this study, a slow strain rate tensile test (SSRT) with an in-situ electrochemical hydrogen charging is used to evaluate susceptibility to hydrogen embrittlement and fracture mechanism in a 17-4PH stainless steel. The correlations between hydrogen embrittlement and the microstructures of the materials in the solution annealed condition and the tempered condition are investigated. This study increases the knowledge to minimize the susceptibility to hydrogen

embrittlement in 17-4PH stainless steel. 2. Material and experimental work

In this study, a 17-4PH bar material with a diameter of 100 mm was used. Table 1 shows the chemical composition of the alloy. It is in two conditions. One is in the solution annealed (1100  C for 30 minutes) condition. It is called as delivered material in this paper. The other condition is where the solution annealed material was further tempered at 510  C for 4 hours and then water quenched. It is hereafter called tempered material.

Table 1. Chemical composition of the 17-4PH steel (wt%). C_max Cr Ni Si Mn

Cu

Nb

Fe

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The specimens used in this study were taken in the longitudinal direction of the bar material. Fig. 1 shows a schematic drawing of the specimen with a diameter of 3 mm. A slow strain rate testing (SSRT) was done according to ISO 7539-7 (2005) at a strain rate of 10 -6 /s and a test solution of 3.5 % NaCl at ambient temperature. Hydrogen was introduced electrochemically using a Radiometer potentiostat with a Voltamaster software. The current applied was calculated by using an average surface area of 9 cm 2 . An Ag-AgCl reference electrode was applied to obtain potential during galvanostatic charging. Specimen acted as a working electrode and the auxiliary electrode was a platinum mesh. The experimental set-up used a plastic beaker as vessel for the electrolyte, approximately 500 ml solution. Specimen elongation during testing was measured using wire sensors. Nitrogen gas was bubbled through the solution during the testing to avoid any effects of solution stagnation or oxygen depletion. SSRT for reference specimens, no hydrogen charging, was run in air. The total SSRT time for each specimen was about 10 hours. For each specimen, the stress versus strain up to failure was recorded and evaluated. Area reduction of the fractured specimen was measured.