Precipitation behavior of G-phase and its effect on mechanical properties of 30Cr2Ni4MoV steel

Abstract

Precipitation behavior of G-phase and its effect on mechanical properties of 30Cr2Ni4MoV steel during thermal aging at 350–600 °C for 0–10,000 h are investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), atom probe tomography (APT), hardness and impact tests. Aging at 500 °C, the martensitic matrix of 30Cr2Ni4MoV steel basically keeps stable. With the lath width almost remaining unchanged, carbides coarsen to a slight degree, but G-phase will precipitate at grain boundary (GB). The G-phase is Ni16Mn6Si7 and its “nose” temperature is around 500–550 °C. Due to relatively high enrichment level of forming elements including Ni, Mn and Si at GB, the G-phase can nucleate at triple junction or adjacent to M23C6 at GB. Subsequently, the G-phase grows along GB until contacts with M23C6, and then coarsens along the width direction, which results in alternating distribution morphology of G-phase and M23C6 at GB. The Vickers hardness results show that the hardness remains stable due to the relatively stable microstructure after aging. After aging for 10,000 h, the hardness undergoes little decrease due to the slight coarsening of M23C6 and G-phase. Interestingly, the impact energy drops obviously after the precipitation of G-phase. The reason is that the G-phase at GB promotes the initiation and propagation of cracks during the impact process, leading to intergranular fracture (IGF).