Effect of Strain Rates on Mechanical Properties of a Duplex Stainless-Steel Sheet Charged in Hydrogen Plasma

Abstract

Tensile properties of a JIS SUS329J4L duplex stainless steel with and without hydrogen plasma charging at various strain rates ranging from 1.38 × 10−7 to 1.38 × 10−3 s−1 in ambient atmosphere were investigated. Fracture surface was evaluated by scanning electron microscope (SEM) and the trapping state of hydrogen was assessed by thermal desorption spectroscopy (TDS). It was confirmed that ultimate tensile strength and elongation to failure were decreased with increasing charging time for testing at 1.38 × 10−3 s−1, while the ultimate tensile strength was affected less markedly by hydrogen charging than the elongation to failure. The extent of hydrogen embrittlement took a maximum at a strain rate of 1.38 × 10−6 s−1, where the proportion of smooth area (area without dimples) in relation to the total fracture area reached a maximum. This conformity between the extent of the degradation and the area without dimple was in accord with that reported in the authors’ previous paper using electrolytical hydrogen charging. On the other hand, the strain rate where the extent of degradation became maximum for the current tests was two magnitudes lower than that in the electrolytically charged test piece. It was assumed that the trapping site for plasma-charged hydrogen is more stable than that for electrolytically charged hydrogen. This was confirmed by TDS results that the desorption spectrum of the 24 h plasma-charged specimen had a shoulder at about 200°C and the peak temperature significantly higher than that of the 24 h electrolytically charged specimen.