Segregation of phosphorus in Cr-Mo-V steel and cast steel after long time service at elevated temperature

Steel and cast steel after long time service in the temperatures range 500-530oC have been investigated. In the areas close to the grain boundaries diffusion of phosphorus causes an occurrence the stable layers of ferrite enriched in phosphorus. The relation ship between the concentration of phosphorus in the alloy and the width of the enriched layers have been established.The width of the phosphorus enriched layers are independent of the grain size. The results suggest that diffusion of phosphorus is controlled by the bulk mechanism from the interior to the grain boundary and takes place only in the area near the grain boundary. Grain boundary etching method by using of picric acid is a useful as a technique analyzing grain boundary concentration of phosphorus. The good method of investigation is the measurement of the width of the etching grooves as the grain boundaries on the SEM microstructures. Segregation of phosphorus in the vicinity of grain boundaries drastically decreases the notch impact fracture toughness and increases the brittle fracture transition temperature of the alloy. Desegregation of phosphorus from the area close to the grain boundaries to the interior grains is long-time process and takes a few hours annealing in the temperature range 650-700oC. Desegregation of phosphorus from area close to the grain boundaries into the interior of grains there is observed an enrichment in the phosphorus of the ferrite around the interphase carbides/matrix boundaries. The notch impact fracture toughness of steel is related to the changes in time and temperature of phosphorus desegration process. (author)