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.

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Nb carbides have attracted significant attention to enhance the resistance of tempered martensitic (TM) steel to hydrogen embrittlement (HE). However, previous studies have elucidated the role of Nb carbides in HE resistance without categorizing their types (i.e., undissolved and newly precipitated). This study focuses on the effect of "undissolved" Nb carbides on the tensile and fatigue properties of hydrogen-precharged TM steels. It validated the following two factors for the HE resistance of the TM steels containing undissolved Nb carbides: hydrogen-trapping by the carbides and refinement of prior austenite grain. The former factor rarely affected the HE resistance owing to the interfacial incoherency between the undissolved carbides and ferritic matrix. Such results are distinguished from previous studies focusing on the newly precipitated carbides. In contrast, the latter factor contributed significantly to the HE resistance via the decrease in hydrogen contents per unit surface of prior austenite grain boundaries. Hydrogen embrittlement (HE) indicates the deterioration in mechanical properties owing to hydrogen atoms inside a ferrous alloy 1,2. HE is particularly important for the use of high-strength steels, such as a tempered martensitic (TM) steel. The TM structure can yield a high strength that exceeds 1.2 GPa after a simple heat treatment, rendering this steel important for various industries. Nevertheless, a high density of defects in the TM structure renders it highly vulnerable to HE 3. Consequently, this has resulted in increasing demands for an enhanced HE resistance of TM steels. The addition of Nb is an effective approach to increase the HE resistance of TM steels. It has been reported that HE resistance increases owing to the hydrogen trapping by Nb carbides and the refinement of prior austenite grains (PAGs). Such factors hindered the diffusion and concentration of the hydrogen present inside materials 4. A recent study 5 suggested a different mechanism for achieving an improvement in HE resistance, wherein Nb addition decreased the Σ3 boundary fraction in lath martensites. However, previous studies have primarily investigated Nb carbides, which are newly precipitated during a tempering process. The other type of Nb carbides (i.e., "undissolved" carbides) has attracted significantly less attention despite the fact that their resultant amounts are not negligibly small. Therefore, the effect of undissolved Nb carbides on the HE resistance of TM steels was investigated in this study. Four hydrogen-precharged TM steels were evaluated under uniaxial and cyclic loading conditions by considering the application environment of the material.