Effect of prior austenite grain size and pearlite interlamellar spacing on strength and fracture toughness of a eutectoid rail steel

Abstract

The influence of prior austenite grain size dγ, and true interlamellar spacing of pearlite St on the strength and fracture toughness of a eutectoid rail steel has been investigated. Specimens were machined from rail sections and heat treated to produce a wide variation in dγ and st. Mechanical properties studied included 0·2% proof stress σ0·2, ultimate tensile strength σu, tensile ductility δ, cleavage fracture stress σf, and plane strain fracture toughness K1c. All tests were performed at a temperature of −80°C. The values of σ0·2 and σu increase as st decreases. The proof stress is related to the mean free distance λin the pearlitic ferrite by a Hall–Petch equation. A microstructural dependence similar to that of σ0·2 is shown by σf and for all but the finest pearlites σf is interpreted as a shear stress controlled cleavage nucleation stress. The value of K1c first decreases with decreasing st and then increases for the finest spacings. This behaviour is attributed to a change in the micromechanism of cleavage nucleation as the pearlite spacing changes from coarse to fine. The value of dy has very little effect on K1c, but δ decreases progressively as dγ increases. The effect of dγ on K1c is negligible because the fracture process zone is much smaller than the grain size and therefore the grain boundaries cannot influence the fracture processes occurring at the crack tip. The tensile ductility is interpreted as the strain necessary to develop an internal microcrack which then propagates as a quasibrittle fracture. The size of the microcrack is shown to be related to the pearlite nodule size which in turn is related to dγ.MST/396