Grain-boundary Segregation Of Phosphorus Induced By High Temperature And Low Tensile Stress In 2.25Cr1Mo Steel

The segregation of solute atoms in grain boundaries may be classified into equilibrium and non-equilibrium grain boundaries segregation. Solute atoms segregation in grain boundaries has an important influence to the mechanical behavior of engineering materials. Many serious accidents were caused by intergranular brittle fracture in engineering. Therefore, it is the most challenge in the world to control the grain boundaries segregation. Two kinds of P-doped 2.25Cr1Mo steels were investigated by AES, SEM and RC—0950 high temperature creep machine.The diffusion coefficients of phosphorus and solute atom-vacancy complex for equilibrium grain boundaries segregation and stress-induced non-equilibrium grain boundaries segregation were account base on AES data. The result shows that, there is a very evident equilibrium segregation on grain boundaries when the material was tempered on 480℃, 520℃and 560℃respectively. When the aging temperature become higher, the grain boundaries segregation level fall down, and the time that reach the equilibrium decrease.While the materials were aged at 520℃with a low tensile stress 40MPa, non-equilibrium grain boundaries segregation occurred. This is not ordinary grain boundaries segregation, but a kind of stress-induced non-equilibrium grain boundaries segregation. On the dynamics curve, the grain boundaries segregation level reaches the peak point at critical time. After that, the grain boundaries segregation level decrease slowly before it gets to equilibrium boundaries segregation.The SEM pictures of low temperature fracture show that the fracture appearances correspond to their grain boundaries segregation fairly well. The critical time is 30 minutes in this experimentation, so the material is the most brittle at this time. After accounting the diffusion coefficients of phosphorus and complex, it is clear that alloy elements and microstructure both play an important role in diffusion process of solute atoms. The diffusion coefficient of phosphorus in bainite is much lower than which in ferrite.Solute atoms acting on each other and their segregation on grain boundaries both influence the phosphorus atoms grain boundaries segregation greatly.