Microscopic defects formed during crack incubation, initiation and propagation processes causing hydrogen-related fracture of dual-phase steels

摘要

Factors promoting hydrogen embrittlement of dual-phase (DP) steels fractured in the elastic region have been investigated from the viewpoint of the relationship between hydrogen states present in the vicinity of a fracture surface and hydrogen embrittlement susceptibility. Hydrogen embrittlement susceptibility was evaluated using slow strain rate testing (SSRT) and constant load testing (CLT). The hydrogen states present in the vicinity of the fracture surface were analyzed using thermal desorption analysis (TDA). The results indicated that fracture strength significantly decreased only at higher hydrogen contents as the crosshead speed was reduced. Additionally, the hydrogen desorption profiles showed that the hydrogen content corresponding to each temperature peak markedly increased with decreasing crosshead speed at higher hydrogen content. Only specimens fractured at a lower crosshead speed at higher hydrogen contents showed both a lower-temperature peak and also a higher-temperature peak. Specimens fractured by CLT also showed a higher-temperature peak, which was detected before fracturing at the applied load for 20 h. This indicates that microscopic defects might have formed and grown to cause damage even in the elastic region. In addition, it was confirmed that microscopic defect formation changed depending on the hydrogen content, crosshead speed and loading time, and the states of defect formation significantly affected hydrogen embrittlement susceptibility.