INDIVIDUAL AND COMBINED EFFECTS OF SULFUR AND PHOSPHORUS ON THE TOUGHNESS OF HIGH STRENGTH STEELS

This investigation was conducted to determine the individual and combined effects of the sulfur and phosphorus content on the toughness of a medium and high strength steel. A matrix of compositions of a 4340 cast steel was produced that contained sixteen combinations of low, medium and high sulfur and phosphorus contents. These steels were through quenched to martensite and tempered to provide a medium and a high strength level. The tensile, Charpy V-notch impact and static and dynamic toughness properties of these steels were measured. This data was analyzed to indicate the feasibility of specifying a maximum sum of phosphorus plus sulfur contents rather than a maximum for each element for steels where good toughness is required.;The computer analysis of data in the literature on the influence of phosphorus and sulfur on the toughness showed the individual and collective effects of these elements clearly. These results were confirmed by the experiments.;Both phosphorus and sulfur lower the toughness of steel significantly. However, the mechanisms by which these elements influence the toughness were proved to be sufficiently different. Phosphorus, present in supersaturated solid solution in steel, increases the lattice friction stress for dislocation motion. As the phosphorus content increases, the primary effect is to decrease the cleavage fracture strength and raise the yield strength, temperature dependence of yield strength and transition temperature. The embrittlement produced by phosphorus is more marked at higher strength levels and in the presence of sharp notches. Sulfur exerts its primary effect by forming sulfide inclusions that reduce the cohesive strength with the iron matrix. Sulfur primarily lowers the fracture energy for fibrous fracture but exerts little influence on the transition temperature in both strength steels. The behavior of the two elements is sufficiently different so that the substitution of a maximum for the sum of the elements in place of a maximum for each in specifications is not feasible.