Comparison of Fracture Morphologies and Hydrogen States Present in the Vicinity of Fracture Surface Obtained by Different Methods of Evaluating Hydrogen Embrittlement of DP and TRIP Steels

Fracture morphologies and hydrogen states present in the vicinity of the fracture surface of dual-phase (DP) and transformation-induced plasticity (TRIP) steels have been investigated. The fracture surfaces were obtained by the slow strain rate technique (SSRT) at strain rates of 8.33 × 10⁻⁶ s⁻¹ and 1.66 × 10⁻⁶ s⁻¹ and by constant load testing (CLT) under the same hydrogen charging condition. Fracture surfaces were observed using scanning electron microscopy (SEM). The hydrogen states present in the vicinity of the fracture surface were analyzed by thermal desorption analysis just after the specimens fractured. The results showed that the critical fracture strength was in the order of SSRT (8.33 × 10⁻⁶ s⁻¹) > CLT > SSRT (1.66 × 10⁻⁶ s⁻¹) in both types of specimens. A DP specimen fractured by SSRT at a higher strain rate had only a lower-temperature peak in the hydrogen desorption profile, but the results for SSRT at a lower strain rate and CLT had not only a lower-temperature peak but also a higher-temperature peak. Furthermore, the fracture surfaces changed from intergranular to quasi-cleavage fracture corresponding to the decrease in fracture strength in both types of specimens.