Equilibrium And Stress-induced Non-equilibrium Grain Boundary Segregations Of Phosphorus In A 2.25Cr1Mo Steel

Solute (impurity or alloying elements) segregation at grain boundaries has an important effect on the mechanical behavior of engineering materials. It has been an interesting topic to metallurgists and material engineers. Solute segregation at grain boundaries may be clarified into equilibrium segregation and non-equilibrium segregation. Studies of equilibrium grain boundary segregation have started long before, and its theory has approached almost perfect. However, there are many things unknown about the non-equilibrium grain boundary segregation, especially the stress-induced non-equilibrium segregation. The segregation behaviors studied before were mainly in a non-stress state. However, the materials in service are usually subject to an applied stress, and thus the research into stress-induced non-equilibrium grain boundary segregation is more important in engineering practice.Due to their excellent high-temperature mechanical properties, Cr-Mo low alloy structural steels are widely used in the power and petrochemical industries. Nevertheless, when the materials serve in a high temperature and pressure environment, impurities, such as phosphorus, sulfur, tin and antimony, would segregate to the grain boundary, making it embrittled. The fracture toughness of the materials is lowered, and the ductile-to-brittle transition temperature (DBTT) is increased. Grain boundary embrittlement deteriorates the service performance, making the materials fracture intergranularly, which may cause disasters. Since phosphorus is a typical grain boundary embrittling element in steel, investigation into the grain boundary segregation of phosphorus in a high temperature and pressure condition can not only guide the engineering practice, but also improve the grain boundary segregation theory. Owing to the fact that 2.25Cr1Mo steel is one of the most widely used Cr-Mo steels, equilibrium and stress-induced non-equilibrium grain boundary segregations of phosphorus in this steel were examined.For equilibrium grain boundary segregation, the samples were quenched at 980oC, tempered at 650oC, and subsequently aged at 480, 520 and 560oC for different times, followed by Auger electron spectroscopy measurements of phosphorus and molybdenum grain boundary concentrations. With the use of Seah's model, the thermodynamics of phosphorus and molybdenum were analyzed. The free energies of segregation of phosphorus and molybdenum were approximately 38 and 17 kJ/mol, respectively, and the interaction between them was very weak in segregation.The relationship between grain boundary concentration of phosphorus and DBTT was explored. The samples in different ageing conditions were impact fractured and the resulting fracture surfaces were analysed using scanning electron microscopy. The DBTT was obtained, characterized by fracture appearance transition temperature. It was found that there is a linear relationship between DBTT and phosphorus boundary concentration. By use of this relationship in conjunction with the kinetic model of equilibrium grain boundary segregation, a temperature-time embrittlement diagram was established. From the diagram, the DBTT of the sample aged for any time at any temperature can be predicted. The effect of phosphorus boundary segregation on intergranular fracture was explored, indicating that phosphorus boundary segregation facilitated the intergranular fracture and the fracture modes were ductile fracture, intergranular fracture and cleavage fracture when the test temperature goes from high to low levels.For stress-induced non-equilibrium grain boundary segregation, after quenching at 980oC and tempering at 650oC, the samples were aged for 1000h at 520oC without stress to enable the boundary concentration to reach themal equilibrium. Subsequently, the samples were stress aged for different periods of time and the stress levels were 40, 200 and 350 MPa, respectively. The phosphorus boundary concentration was measured using Auger electron spectroscopy so as to obtain the segregation kinetics. For the 40 MPa stress ageing, there was one segregation peak over its equilibrium segregation level. For the 350 MPa stress ageing, there were two segregation peaks over the equilibrium segregation level. For the 200 MPa stress ageing, there were two segregation peaks over the equilibrium level, and between them there was a depletion trough below the equilibrium level. It was proposed that elastic deformation and creep deformation could both affect the phosphorus segregation kinetics during stress ageing.Based on the fluxes of vacancies and complexes, a kinetic model of stress induced non-equilibrium grain boundary segregation was established. The segregation kinetics of phosphorus during stress ageing were predicted under different conditions such as stress, temperature, grain size, and migration energies of vacancies and complexes. The phosphorus segregation kinetics under 40 MPa tensile stress was simulated, and model predictions were well consistent with the experimental results.The outcomes of the present work may promote understanding of grain boundary segregation and embrittlement, and improve the segregation theory. Accodingly, it is of importance to engineering practice.