Microstructure evolution and crack formation mechanism of high-nitrogen bearing steel subjected to warm deformation

Abstract

In this work, the microstructure evolution and crack formation mechanism of the X30CrMoN15-1 high-nitrogen steel under warm deformation (700 ℃ and 800 ℃) have been investigated. The microstructure observation indicates that the warm deformation (both at 700 °C and 800 ℃) facilitates the ferrite transition and the carbonitrides precipitation, which leads to the reduction of retained austenite by 40% in the specimen deformed 700 °C and 18% for the specimen deformed 800 °C. In addition, cracks are likely to generate under the low strain rates of 0.005 s−1. In the specimen deformed at 700 °C, the lamellar carbonitrides are precipitated along the interface between ferrite and M/A islands, while the spherical carbonitrides are precipitated inside the ferrite. The volume expansion generated by ferrite transformation and the growth of recrystallized grain are hindered by carbonitrides, resulting in the significant stress concentration and even crack initiation. For the specimen deformed at 800 ℃, the spherical carbonitrides are mostly precipitated from the recrystallized ferrite grain boundary. The significant dislocation aggregation around the austenite /ferrite interface and strength mismatch between ferrite and austenite together result in the interfacial debonding and even cracks propagation at phase boundary.