Non-Equilibrium Grain Boundary Segregation Of Phosphorus Under A Tensile Stress In A 2.25Cr1Mo Steel

There are some impurities in steel, such as phosphorus, sulfur, tin, arsenic and antimony. The mechanical properties can be changed result from grain boundary segregation, which will easily lead to brittle fracture. In these impurities, phosphorus is a major one in steel. The addition of phosphorus has a contribution to the strength of steel, but it also lead to the grain boundary segregation,giving rise to an increase in ductile-brittle transition temperature and thus deteriorating the toughness of steel. This is the main reason for non-hardening embrittlement. So it is of importance to control the amount of phosphorus. The theory of grain boundary segregation can be classified into two types: equilibrium segregation and non-equilibrium segregation. Equilibrium grain boundary segregation has been studied for a long time, and its theory has approached mature Non-equilibrium grain boundary segregation can be caused by thermal cycling, stress, and radiation, and there is so much unknown in this field, especially for the stress-induced segregation. Since most of the engineering structures have to withstand an applied stress during service, investigation into non-hardening embrittlement of a structural steel under an applied stress is more important as compared with the case without application of an external stress.As is well known, 2.25Cr1Mo steel is widely used in the power and petrochemical industries because of its good high-temperature mechanical properties. In the present work, this steel was used to investigate the grain boundary segregation of phosphorus induced by a tensile stress. In the experiment, the samples were normalized at 920℃for 50min, and then austenitized at 980℃for 30min and then oil quenched to room temperature. The samples heat-treated above were tempered at 650℃for 2h, followed by water quenching. They were then aged at 520℃for 1000h to enable phosphorus segregation to reach thermal equilibrium, followed by stress ageing under a tensile stress of 100MPa for different times, and subsequently the grain boundary concentration of phosphorus in the stress-aged samples was measured by virtue of Auger electron spectroscopy.The results show that there is only a depletion trough of phosphorus below its equilibrium value. This phenomenon is much different from that under 40MPa/200MPa/350MPa tensile stress. Meanwhile, there is creep during stress ageing, which exhibits initial elastic deformation, primary creep and part of steady state creep, but it is not so obvious. Comprehensive analysis demonstrates that the depletion trough of phosphorus is mainly induced by creep deformation inhomogeneity effect. During the stress ageing, creep deformation inhomogeneity effect will lead to the increase of vacancy concentration on grain boundary. Oversaturated vacancies combine with solute atoms to form complex, which will diffuse to grain interior, causing grain boundary depletion of phosphorus. Finally, the depletion trough emerges.