Delayed Fracture Enhanced by Martensite Transformed from Retained Austenite in Ultra-high Strength Steel Sheet

Abstract

Delayed fracture of ultra-high strength complex phase (CP) steel sheets containing either a small amount of retained austenite or substantially no retained austenite was investigated by a sustained tensile-loading test during hydrogen charging. The microstructures of both specimens are composed of martensite and bainitic ferrite. Most of the retained austenite exists as a secondary phase in the bainitic ferrite or near prior austenite grain boundaries. In comparison with the specimen containing substantially no retained austenite, the amount of absorbed hydrogen in the specimen containing a small amount of retained austenite increases under the same charging conditions, and the time to fracture increases under the same applied stress lower than the yield stress. Upon pre-deformation before the sustained tensile-loading test, the retained austenite transforms to martensite, and the time to fracture decreases significantly. Moreover, the morphology of the fracture initiation area also changes from a mixture of quasi-cleavage and intergranular to quasi-cleavage. In the fractured specimens without pre-deformation, many cracks initiate in martensite and at prior austenite grain boundaries, although a small number of cracks are also observed in the bainitic ferrite and at the interface between different phases. Upon pre-deformation, the number of cracks in martensite increases, particularly in the specimens containing martensite transformed from retained austenite (transformed martensite). The probable reason for the decrease in the time to fracture in the specimens subjected to pre-deformation is that cracks readily nucleate in martensite and transformed martensite. The results of the present study indicate that pre-transformed martensite substantially enhances delayed fracture.