| 2.25Cr1Mo steel is an important low-alloy structural steel which mainly contains the elements of Cr and Mo. This steel has high strength and oxidation-resistance in the high temperature, and can be well welded. So it is widely used for pressure vessels in the power, nuclear energy and petrochemical inductries, especially used in main steam and heat steam pipes in the power industry. Unfortunately, when subjected to extended exposure to intermediate service temperature(350℃~600℃) and high pressure(0~28Mpa), the steels become embrittled because of segregation of some impurity elements to grain boundaries.It is well known that temper embrittlement in 2.25Cr1Mo steel is caused by grain boundary segregation of impurity elements like P, Sn, Sb, and As, which decrease the grain boundary cohesion. Studies indicate that temper embrittlement is mainly caused by grain bondary segregation of P.Temper embrittlement results in a loss of material toughness. This phenomenon is best represented by a shift of the ductile-to-brittle transition temperature(DBTT) to higher temperatures. The DBTT is usually defined as the temperature corresponding to the mean value of the upper-shelf and lower-shelf energies or the temperature at which the fracture surface contains 50% shear fracture characteristics(the fracture appearance transiton temperature(FATT)).In the present work, we examined the equilibrium grain boundary segregation behavior of phosphorus in a 2.25Cr1Mo steel. The steel was quenched from 980℃and tempered at 650℃for 2h, followed by ageing at 560℃, 520℃, 480℃for different times. By means of Auger Electron Spectroscopy(AES), the equilibrium grain boundary segregation of phosporus was examined. By the computer software VB with the equilibrium segregation kinetic equation, we simulate the kinetic curve of the specimens aged at 480℃, which accord with the experimental results well if consider the standard deviation. In the end, the DBTTs are tested by Charpy V-notch impact test.
|
PDF Full Text Request