PSI - Issue 13

Galina Maier et al. / Procedia Structural Integrity 13 (2018) 1053–1058 Galina G. Maier et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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1. Introduction New class of high-nitrogen nickel-free or low-nickel austenitic stainless steels has been developed for variety of applications. These Cr-Mn and Cr-Mn-Ni steels possess a unique complex of strength and plastic properties, which depend on the chemical and phase composition of the steel. Because of solid-solution hardening, nitrogen increases strength properties of austenitic steels without significant loss in ductility and toughness. Both elements, Mn and N, stabilize the austenitic structure of steels (Berns et al., 2013; Gavrilyuk et al., 1999). High-nitrogen nickel-free steels show a good corrosion resistance but their cost is much lower compared to conventional Cr-Ni stainless steels. High-nitrogen steels (HNS) suffer from hydrogen environment embrittlement, which is mainly related to their planar dislocation microstructure, microlocalization, twinning and instability against martensitic transformation (Michler et al., 2012; Phaniraj et al., 2015; Igata et al., 1991; Astafurova et al., 2017; Maier et al., 2016) The role of precipitates in HNS structure in hydrogen embrittlement (HE) is unclear. The susceptibility of steels to HE is strongly related to the interaction between traps (grain boundaries, defects, particles) and hydrogen. Vanadium carbides has been proved as effective hydrogen traps by many researchers (Cheng et al., 2018; Wei et al. 2011; Turk et al., 2018; Asahi et al., 2003). Cheng et al. (2018) postulated that V-addition decreases the diffusion coefficient of hydrogen in tempered martensitic Fe-Mn-Cr Ni-Mo-Si-C-xV steel. For tempered martensitic Fe-Ni-V-C (wt.%) steel, Wei et al. (2011) showed that coherent NbC and VC particles are more effective traps in comparison with the incoherent precipitates. They also found the following sequence of hydrogen trapping capacity in particle-strengthened martensitic steel: NbC-containing steel > TiC-containing steel ≥ VC -containing steel. Turk et al. (2018) investigated hydrogen trapping in V-alloyed ferritic steel and suggested that VC traps relatively little hydrogen in the absence of dislocated matrix. According to Asahi et al. (2003) VC carbides increase the strength of martensitic steels by precipitation strengthening mechanism and, at the same time, introduce strong hydrogen traps that immobilize hydrogen atoms. The aim of this paper is to establish the effect of electrolytic hydrogen-charging on tensile behaviour and fracture micro- and macromechanisms of two V-free and V-containing HNSs. Two HNSs were selected as objects of investigation (Tabl. 1). Steel specimens were cold-rolled and solution treated at the temperature 1200°C for 1 h following by water-quenching to obtain austenitic structure. Table 1. Chemical compositions of the steels (wt.%), the lattice parameters ( a ) and grain size ( d ) of austenite after solution-treatment Notation Mn Cr V C N Fe a , nm d , μm 0V-HNS 17 23 0 0.1 0.6 balanced 0.36299 50 1.5V-HNS 22 19 1.5 0.3 0.9 balanced 0.36315 10 Before hydrogen-charging (H-charging) the specimens were cut dumb-bell shaped flat tensile specimens with nominal dimensions of 18 x 2.7 x 0.7 mm in the gauge section. Mechanical grinding and a final electrochemical polishing (50 g CrO 3 , in 200 g H 3 PO 4 ) were employed to remove the entire processing-affected surface layer. The final thickness of tensile specimens was 0.5 mm. Electrochemical hydrogen-charging was performed at a current density of 50 mA/cm 2 for 100 hours at room temperature in 3% NaCl water solution containing 3 g l -1 of NH 4 SCN as a recombination poison. Immediately after hydrogen-charging, the tensile tests were carried out at room temperature and a strain rate of 1×10 -4 s -1 using an electromechanical machine (LFM-125 by Walter+Bai AG). The hydrogen embrittlement index ( HEI ), representative of percentage loss in uniform elongation due to hydrogen alloying, was calculated = [( 0 − ) 0 ⁄ ] × 100 %, here ε 0 and ε H are values of uniform elongation for H-free and H-charged specimens respectively. The fracture and lateral surfaces of the specimens were examined using a LEO EVO 50 (Zeiss, Germany) scanning electron microscope (SEM). The X-ray diffraction (XRD) analysis was done using DRON 7 diffractometer with Co-K  radiation. The light microscopy (LM) and transmission electron microscopy (TEM) were used to evaluate the mean grain size of austenite and size of precipitates. 3. Results and discussion For both solution-treated steels, the lattice parameters have similar values, a =0.363 nm, as it was revealed by XRD analysis (Table 1). This implies the similar and rather high concentration of interstitial atoms (nitrogen and carbon) in 2. Materials and Methods