Functions of microstructures in delayed fracture of martensitic steels

The susceptibility of a Mo-V martensitic steel to delayed fracture has been examined with respect to tempered microstructures of a similar strength level. Tempering at 650degreesC improved the delayed fracture characteristics compared with tempering at 550degreesC. By means of thermal and room temperature desorption analysis of hydrogen, it was revealed that fine acicular precipitates acted as strong hydrogen traps in tempering at 650degreesC. Concurrent hydrogen-charging/loading further increased the density of the strong traps, prominently in the presence of the precipitates. The amount of weakly trapped hydrogen in vacancies acting as traps also increased due to loading, and again the presence of fine precipitates made the increment more pronounced. The increase in the total amount or strain-induced increment of diffusive hydrogen associated with reduced susceptibility ran counter to previous concepts. The tendency for loading to create vacancies was then tentatively examined by means of a stress relaxation test. At a stress level as low as 0.6 of the tensile strength, the presence of fine precipitates reduced the amount of relaxed stress. Hydrogen accelerated the relaxation process and increased the amount of relaxed stress, but the effect of hydrogen was reduced in the presence of precipitates. The stability of microstructures under loading is thought to play an essential role in delayed fracture susceptibility through the formation of vacancy-originated defects.