Fe-P alloys containing up to 0.67wt% phosphorus show intergranular fracture (IGF) by an impact test after quenching from 1073K. Additions of up to about 0.01wt% of carbon to the alloys prevents the intergranular fracture caused by the grain boundary segregation of phosphorus, and lowers the ductile-brittle transition temperature (DBTT). This effect is due to the segregation of carbon at grain boundaries. Aging at 873K of the quenched specimen with 0.52% P and 0.0008% C lowers DBTT and increases the degree of grain boundary segregation of carbon while it does not affect the degree of grain boundary segregation of phosphorus. The grain boundary segregation of carbon strengthens the grain boundary cohesion and prevents intergranular fracture without removing phosphorus from grain boundaries.
DBTT of the Fe-P-C alloys are analyzed in terms of the degree of grain boundary segregation of carbon and phosphorus and the solution hardening by phosphorus (the bulk concentration of phosphorus). The site-competition between carbon and phosphorus is taken into account in the calculation of the degree of segregation. It is found that the increase in the boundary cohesion by the carbon segregation is an important factor in preventing IGF.