Effect of phosphorus on the ductility of high strength spring steels

Phosphorus as a tramp element deteriorates the toughness and enhances the brittle fracture in high strength steels. Even very low bulk concentrations can cause severe enrichment on the prior austenite grain boundaries. This seriously restricts the applicability of highly demanded vehicle components. Three approaches were chosen to reduce the detrimental effect of phosphorus in quenched and tempered high strength CrV-steels for high power leaf springs: Change in the chemistry (addition of boron for repelling phosphorus from the grain boundaries), application of thermomechanical treatment (TMT) instead of conventional heat treatment (CHT) (producing lattice defects as intragranular traps for phosphorus atoms) and optimizing the tempering process (balancing grain boundary segregation and lattice restoration). Whereas no effect of boron was determined, TMT reduces the sensitivity to phosphorus and leads to better ductility, especially for the highest strength Rm > 2000 MPa. The best strength - ductility combination was found for the range of tempering temperatures ϑtemp between 280 and 330 °C even for phosphorus contaminated steels. Outside of this range of ϑtemp, there is a significant deterioration of ductility for P ≥ 0.02 %. Independent of P-content, there is a dramatic decrease in 0.2 % proof strength for ϑtemp decreasing below 280 °C due to residual internal stresses.