低合金鋼SCM440Hの長周期変動および静応力下の水素に助長されたき裂進展挙動

Abstract

Crack propagation behavior of SCM440H low alloy steel enhanced by absorbed hydrogen was investigated. A continuous hydrogen charging method was designed, in which the crack tip was isolated from the electrolyte and kept dry. Six materials which were tempered at different temperatures were used. Effects of stress ratio, loading frequency, hold time and material hardness on the crack propagation rate were examined under long term varying load and static load. An acceleration of crack propagation rate about six times compared to the uncharged material was commonly found in all materials. In addition to this, however, unexpected acceleration of crack propagation up to 1000 times was experienced in certain condition. In materials with Vickers hardness higher than 280 tested at low frequency, the marked acceleration was experienced. The crack surface morphology was quasi cleavage. This critical hardness (HV=280) is a little lower than the usually accepted critical hardness for delayed failure (HV=350). In material with Vickers hardness lower than 268, however, such a marked acceleration was not experienced. INTRODUCTION It has been pointed that absorbed hydrogen in metal has detrimental effect such as hydrogen embrittlement [1] and hydrogen enhanced fatigue crack propagation [2,3] and so on. Delayed failure of high strength steel under static loading is a typical example of hydrogen embrittlement. It has been recognized that low alloy steel whose Vickers hardness is higher than 350 is prone to delayed failure. The design of hydrogen utilization machine sometimes requires the use of high strength steels. It is important to prevent the hydrogen embrittlement for the safety in hydrogen economy. Hydrogen utilization machine experiences varying loading as well as static loading in service. Therefore the effect of material hardness on the crack propagation behavior of low alloy steel enhanced by hydrogen under long-term varying load and static load was studied.