Study on the aging embrittlement mechanism of Si in 30Cr2Ni4MoV rotor steel during long-term aging

The 30Cr2Ni4MoV rotor steel shows serious aging embrittlement in the high temperature service environment above 350 ℃. This study investigates the aging embrittlement mechanism of Si during aging at 500 ℃ for 0–10,000 h by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atom probe tomography (APT). There are differences between 30Cr2Ni4MoV steels containing different Mn and Si, such as, the degree of aging embrittlement in 30Cr2Ni4MoV steel containing only Si increases slightly with increasing aging time. After aging for 5000 h, there is no Si partitioning at GBs, and the carbides contain no Si element. But the element enrichment including Si, Mo and Ni at GBs promotes the precipitation of G-phase. As increasing aging time to 10,000 h, the number and length of G-phase increase significantly. Compared with Mn-containing G-phase, the longer incubation period of Mo-containing G-phase is due to the small diffusion coefficient of Mo and high formation energy of Mo-containing G-phase. The G-phase is conducive to crack initiation and propagation, which is the main reason for aging embrittlement.