PHOSPHORUS AND CARBON SEGREGATION - EFFECTS ON FATIGUE AND FRACTURE OF GAS-CARBURIZED MODIFIED 4320 STEEL

Phosphorus and carbon segregation to austenite grain boundaries and it s effects on fatigue and fracture were studied in carburized modified 4320 steel with systematic variations, 0.005, 0.017, and 0.031 wt pct, in alloy phosphorus concentration. Specimens subjected to bending fat igue were characterized by light metallography, X-ray analyses for ret ained austenite and residual stress measurements, and scanning electro n microscopy (SEM) of fracture surfaces. Scanning Auger electron spect roscopy (AES) was used to determine intergranular concentrations of ph osphorus and carbon. The degree of phosphorus segregation is directly dependent on alloy phosphorus and carbon content. The degree of carbon segregation, in the form of cementite, at austenite grain boundaries was found to be a function of alloy phosphorus concentration. The endu rance limit and fracture toughness decreased slightly when alloy phosp horus concentration was increased from 0.005 to 0.017 wt pct. Between 0.017 and 0.031 wt pct phosphorus, the endurance limit and fracture to ughness decreased substantially. Other effects related to increasing a lloy phosphorus concentration include increased case carbon concentrat ion, decreased case retained austenite, increased case compressive res idual stresses, and increased case hardness. All of these results are consistent with the phosphorus-enhanced formation of intergranular cem entite and a decrease in carbon solubility in intragranular austenite with increasing phosphorus concentration. Differences in fatigue and f racture correlate with the degree of cementite coverage on the austeni te grain boundaries and the buildup of phosphorus at cementite/matrix interfaces because of the insolubility of phosphorus in cementite.