Crack propagation behavior of SCM440H low alloy steel enhanced by hydrogen under long-term varying load and static load

Crack growth 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 solution environment and kept dry. Six materials which were tempered at different temperature were used. Effects of stress ratio, loading frequency, hold time and material hardness on the crack growth rate were examined under long term varying load and static load. An acceleration of crack growth rate about ten times compared to the uncharged material was commonly found in all materials. In addition to this, however, unexpected acceleration of crack growth up to 1000 times was experienced in certain condition. In materials with Vickers hardness higher than 280 tested at low frequency, this 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. The use of low strength material is desirable to prevent the cracking enhanced by hydrogen.