Study On Intergranular Dilution Of Phosphorus At High Temperature And Low Compressive Stress And Intergranular Brittleness In Steel 2.25Cr1Mo

The segregation of elements, especially the trace elements at grain boundaries, has animportant influence on the mechanical behavior of engineering materials. So it has basilicpractical meaning to study the segregation of elements at the grain boundaries. The segregationof solute atoms at grain boundaries may be classified into equilibrium segregation and nonequilibriumsegregation. In the present work, a 2.25Cr-1Mo steel was selected as theexperimental material. The non-equilibrium grain boundaries dilution of phosphorus at grainboundaries and the influence of phosphorus segregation on the ductile-to-brittle transitiontemperature (DBTT) were investigated in virtue of Gleeble-1500 thermal simulation machine,AES, SEM and impact machine, et al.The non-equilibrium dilution of phosphorous at the grain boundaries occurs obviously byAES after experimental materials ageing different times at 520℃and 40MPa compressivestress. And the critical time for this non-equilibrium segregation is 30 min. At this time, thesegregation concentration of phosphorus achieves minimal value 14.18at.%. Calculated by Xu'smodel, the order of diffusion coefficients of phosphorous-vacancy complexes are 10-21 m²Â·s^-1 indilution process. The order of diffusion coefficients of phosphorous are 10-15 m²Â·s^-1 in dedilutionprocess. Compared with the diffusion coefficients under no external stress, it is foundthat the compressive stress decreases the diffusion coefficient of phosphorous-vacancycomplexes by 5 order of magnitude and increases the diffusion coefficient of phosphorous by 4orders of magnitude. And the fracture appearances of AES samples correspond to the grainboundaries segregation results of phosphorus. Xu's model can effectively explain the nonequilibriumgrain boundaries dilution of phosphorus.Series of Charpy impact tests are carried after samples respectively ageing at 650℃, 560℃and 480℃, and ductile-to-brittle transition temperature is known. By the values of impact testand the analysis on SEM pictures of impact fracture, the DBTT of 2.25Cr1Mo increases withthe reduce of ageing temperature. According to equilibrium grain boundary segregation theory,the segregation concentration of phosphorus increases with the descending of ageingtemperature. So we can conclude that the lower the aging temperature, the greater theequilibrium segregation concentration of phosphorus and the brittleness of grain boundaries is,and the DBTT of materials is higher.