Effects of Nb carbide on hydrogen embrittlement in tempered martensite steel

Abstract

The issue of weight reduction for automobiles has been discussed continuously in order to improve fuel efficiency and reduce the exhaust gas. Since the wire rod products in automobiles is mainly used for power transmission, it makes friction torque with other parts continuously which means power loss and less fuel efficiency. Because of these factors, weight reduction of transmission part is more effective than others to improve fuel efficiency. Weight reduction in a limited space needs strength improvement but based on previous research of high strength product, the hydrogen embrittlement resistance drastically decreases when the strength is more than certain point. Several ways have been used to improve hydrogen embrittlement resistance. One of the method is addition of alloying element to make carbides which traps hydrogen effectively. In this study, hydrogen embrittlement resistance in tempered martensite steel is evaluated on different types of Nb carbide which are Undissolved Nb carbide and precipitated Nb carbide. It was difficult to make an accurate comparison of the precipitated form and undissolved form due to the grain size difference. However, in the case of the precipitated carbide, it has poor hydrogen embrittlement resistance even though it traps the more amount of hydrogen than undissolved carbide. It showed if hydrogen trapping energy is not strong enough in reversible region, it made worse effect of hydrogen embrittlement resistance.