An evaluation method for hydrogen embrittlement of high strength steel sheets using U-bend specimens

Abstract

An evaluate method for hydrogen embrittlement property of high strength steel sheet has been proposed in this study. To take into consideration of the effect of plastic strain in addition to the effects of applied/residual stress and diffusible hydrogen, U-bend specimens have been adopted because steel sheets for automobiles are usually used after press forming into various parts. After U-shape bending, the specimen was loaded using a bolt. The proposed evaluation method is based on the measurement of critical hydrogen content or critical hydrogen charging condition for hydrogen embrittlement fracture at given stress and strain conditions. The hydrogen charging current density was increased in step-wise manner until cracking was observed, and cracking was detected by optical observation and by monitoring voltage between the sample and a counter electrode. The critical hydrogen contents for specimens with varied applied stress were obtained by means of thermal desorption spectroscopy. For the critical hydrogen content, both the hydrogen contents in strained portion of the specimen and no-strained portion were measured. The former is affected by introduced dislocations caused by straining and the latter is thought to be proportional to the hydrogen fugacity. Both critical hydrogen contents tended to be decreased slightly when the applied stress was relatively high.