Effect of intergranular ferrite on hydrogen delayed fracture resistance of ultrahigh strength boron-added steel

Abstract

The effect of intergranular ferrite (IGF) on hydrogen delayed fracture of an ultrahigh strength boron added steel with yield strength over 1 GPa was investigated. For this purpose, a series of constant loading tests and thermal desorptlon analyses was conducted on the hydrogen préchargea steel having different volume fractions of IGF, 0%, 6%, and 10%. Tensile strength of unnotched and notched specimens slightly decreased only by 4% with increasing the volume fraction of IGF up to 10%. Time to failure of the steel In the constant loading tests became prolonged with increasing the volume fraction of IGF. Microstructural observation informed that the presence of IGF at prior austenite grain boundaries suppressed the precipitation of film-like carbides, which act as the susceptible crack nucleation sites, at these boundaries. In addition, IGF was very effective on retarding crack propagation. The thermal desorption analyses revealed that hydrogen was trapped mainly at grain boundaries and that the critical hydrogen content causing delayed fracture Increased with increasing the volume fraction of IGF, resulting in better hydrogen delayed fracture resistance. The present results shed light on the fact that the presence of IGF is beneficial for improving hydrogen delayed fracture resistance of the steel with little degradation of strength. © 2007 ISIJ.