Effects of intergranular carbide precipitation on delayed fracture behavior in three TWinning Induced Plasticity (TWIP) steels

The delayed fracture behavior related with intergranular carbide precipitation of three TWinning Induced Plasticity (TWIP) steels was investigated. According to the microstructural analysis, nano-sized (Fe,Mn)3C cementites were precipitated along grain boundaries in the 0.6C–22Mn and 0.6C–18Mn steels, whereas their precipitation was hardly observed in the 0.6C–18Mn–2Al steel, which was confirmed by equilibrium phase diagrams calculated from a ThermoCalc program. When cup specimens were dipped in the boiled water, the 0.6C–22Mn, 0.6C–18Mn, and 0.6C–18Mn–2Al steel cups were cracked after 5.5, 15, and 169 h, respectively. The delayed fracture regions consisted of intergranular facets, and the tendency of intergranular facture decreased in the order of 0.6C–22Mn, 0.6C–18Mn, and 0.6C–18Mn–2Al steels. Thus, the delayed fracture behavior was closely related with the intergranular fracture mode caused by grain boundary cementites. The addition of Al remarkably increased the resistance to delayed fracture because it suppressed the formation of grain boundary cementites and reduced the residual stresses in the cup specimen.