Microstructure and mechanical properties of sulphur‐alloyed spray‐formed steels

Abstract

Small amounts of sulphur improve cutting properties of conventionally produced steels. In combination with manganese, sulphur forms manganese sulphide inclusions in the steel matrix which reduce the tool wear during machining and improve machinability. However, manganese sulphides may have a detrimental effect on the mechanical properties. Hot forming operations result in an elongation of sulphides in rolling direction and may decrease the fatigue properties under stresses in the transverse direction. In comparison to conventionally produced steels, spray-formed steels, due to the rapid solidification, have smaller precipitations which are homogeneously distributed. The existence of sulphides in these steels assures a good machinability whereas their small size limits the strength decay. The strength of spray-formed and sulphur-alloyed variants of the steels SAE5120, SAE4140H and SAE52100 has been investigated. The steels SAE5120 and SAE4140H were investigated after hardening and tempering while the strength of the steel SAE52100 was determined in a bainitic state. Further characterisations of the materials were performed on the basis of density measurements and metallographic investigations. The manganese sulphides of all steel variants are homogeneously distributed and their size depends on the spray-forming parameters. After hot rolling the elongated sulphides caused anisotropic fatigue properties. Specimens loaded in rolling direction primarily fractured at the surface. By contrast, loading in transverse direction mainly caused fracture by manganese sulphides at lower stress levels. Based on the weakest-link theory, a calculation method is introduced which enables the prediction of the endurance limit and the fracture sites if crack initiation at sulphides is the dominant mechanism of failure. The theoretical and experimental results are compared and discussed. nach dem Warmwalzen gestreckten Sulfide führten zu anisotropen Ermüdungseigenschaften. Der Rissinitiierung der Proben, die in Walzrichtung belastet wurden, lag vornehmlich an der Oberfläche. Im Gegensatz hierzu lag bei den senkrecht zur Walzrichtung belasteten Proben der Rissausgang bei geringeren Spannungsamplituden an Mangansulfiden. Unter der Annahme, dass Sulfide die dominante Versagensursache sind, wird eine auf dem Fehlstellenmodell basierende Berechnungsmethode vorgestellt, mit der die Dauerfestigkeit sowie die Rissausgangsorte auf Basis der Sulfidgrößenverteilung und der örtlichen Spannungsamplituden vorhergesagt werden können. Die theoretischen und experimentellen Ergebnisse werden verglichen und diskutiert.Schlüsselwörter: Sprühkompaktieren / Stahl / Dauerfestigkeit / statische Festigkeit / Schwefel / i

Comparative Study on the Microstructure Characteristics and Enhanced Mechanical Properties of Spray Formed GCr15 Bearing Steel

“…Besides, Tinscher et al revealed that the homogeneous and isotropic microstructure of spray formed bearing steel resulted in an isotropic mechanical properties including high fatigue resistance perpendicular to the rolling direction. Schumacher et al attempted to increase the amount of sulfur element of the bearing steel by spray forming in order to improve the machinability without reducing fatigue strength.…”