Comparative study on the effects of Cr, V, and Mo carbides for hydrogen-embrittlement resistance of tempered martensitic steel.

In this study, the ideal alloying element (among Cr, V, and Mo carbides) to enhance the resistance to hydrogen embrittlement (HE) in a tempered martensitic steel was investigated. Four types of steels were designed to contain cementites, Cr-rich M7C3 carbides, V carbides, and Mo carbides, respectively. These steels were tailored to possess a comparable tensile strength (~1.6 GPa). The HE resistances of these steels were evaluated through the slow strain rate test and cyclic corrosion test. The results showed an enhanced HE resistance, characterized by a high notch fracture strength after hydrogen charging, in the samples containing V carbides and Mo carbides. In particular, Mo carbide was regarded as the most ideal alloying element for HE resistance because of the high resistivity parameter, inhibited hydrogen penetration, and suppressed strength loss by internal hydrogen.

Effect of Heat Treatment on Microstructures and Mechanical Properties of Severe Plastically Deformed Hypo- and Hyper-Eutectoid Steels by Caliber Rolling Process.

This study investigated the effect of post-heat treatment on the microstructures and mechanical properties of severe plastically deformed hypo- and hyper-eutectoid steels that underwent a caliber rolling process. First, 28 passes of caliber rolling were performed on both the hypo-eutectoid steel with Fe-0.47% C (wt%) composition and the hyper-eutectoid steel with Fe-1.02%C (wt%) composition. Then, the caliber rolled materials underwent heat treatment at 500 degrees C for 1, 3, 5, 10, 30 and 60 minutes. The caliber rolled steel possessed a 300-400 nm-sized oval cementite structure created through elongating and segmentation regardless of the C composition. The observation of heat-treated microstructures showed that cementite structure became globular and ferrite size increased as heat treatment temperature increased. In the hardness measurement, the initial caliber rolled samples showed 372.8 Hv (hypoeutectoid) and 480.1 Hv (hyper-eutectoid). However, hardness dramatically decreased up to 10 min. heat treatments, and then showed a constant or small reduction with time. The yield strengths (compression) of caliber rolled hypo- and hypereutectoid steels obtained were 1097 MPa and 1426 MPa, respectively, and the yield strengths of the same steels after heat treatment (500 degrees C, 60 min.) were identified to be 868 MPa and 1316 MPa, respectively.