Effect of Spherical θ Precipitation in 1.5 GPa Grade Tempered Martensitic Steel on the Occurrence of Delayed Fracture

The objective of this study is to clarify the effect of spherical cementite (θ) precipitation on the occurrence of delayed fracture in 1.5 GPa grade tempered martensitic steels. Constant load tests were performed with a cathodically charged specimen. A 1GPa-load was applied to the specimen, and cathodic charging was performed in 3% NaCl + 3 g/L NH4SCN solution. The specimen of steel without spherical θ did not fracture at the current density of 5 A·m−2 or even by increasing to 50 A·m−2. On the other hand, the specimen of steel with spherical θ fractured after 0.2 h at 5 A·m−2. The strain around the spherical θ after 30%-rolling observed by transmission electron backscatter diffraction showed that the local deformation around the spherical θ was larger than that in the whole measurement field by 3.05 × 1014 m−2 in terms of geometrically necessary dislocation density. In the hydrogen desorption curve by thermal desorption analysis, steel with spherical θ after 30%-rolling showed a larger hydrogen desorption peak around 250 °C than steel without spherical θ. The value of the activation energy of the 250 °C-peak was 109.2 kJ·mol−1. From these results, the 250 °C-peak is inferred to be hydrogen at the disordered interface of θ/tempered martensite. Transmission electron microscopy observation showed cracks and voids on the spherical θ near the delayed fracture surface. These results indicate that the precipitation of spherical θ facilitates the occurrence of delayed fracture. Cracks appear to form around spherical θ.