Influence of nonmetallic inclusion characteristics on the mechanical properties of rail steel

An extensive investigation has been carried out on six commercial heats of pearlitic rail steel to study the influence of nonmetallic inclusion charac teristics on the tensile, fatigue, and fracture toughness properties. The s teels investigated were made through the basic oxygen furnace (BOF)-continu ous casting route and rolled in the rail and structural mill into 90 kg/mm( 2) ultimate tensile strength (UTS) grade rails. While tensile properties (y ield strength [YS], UTS, and elongation) of the rail steels investigated we re found to be insensitive to inclusion type and volume fraction at their p resent level (0.23 to 0.45%), the fracture toughness and high-cycle fatigue properties were found to be inclusion sensitive, The fracture toughness va lues of the steels were found to range between 42.33 and 49.88 MPa rootm; h igher values, in general, were obtained in heats exhibiting lower volume fr actions (0.15 to 0.19%) of sulfide inclusions. The high-cycle fatigue limit , ie., stress corresponding to 10(7) cycles, was found to be higher in clea ner steels, particularly in those with lower volume fractions of oxide incl usions, This phenomenon was corroborated by scanning electron microscopy (S EM) observations of fracture surfaces, where oxide inclusions in particular were found to be instrumental in crack initiation. Although fatigue life d id not show any direct correlation with the volume fraction of sulfides, el ongated MnS inclusions were sometimes observed at crack initiation sites of fatigue-tested specimens.