Toughness dependence on the microstructural parameters for an ultrahigh carbon steel (1.3 wt.% C)

An ultrahigh-carbon steel containing 1.3 wt.% C (UHCS-1.3C) was processed to obtain spheroidized and pearlitic microstructures. Spheroidized microstructures exhibit carbide particle size and ferrite grain size distributions which are invariable with austenitizing temperature below 870 °C. Pearlitic microstructures exhibit prior austenite grain sizes and pearlite colony sizes which increase and interlamellar spacings which decrease with increasing austenitizing temperatures above 870 °C. Plane-strain fracture toughness, KIv, was measured at room temperature for all heat-treated materials. In the case of spheroidized UHCS-1.3C, fracture toughness does not change significantly with austenitizing temperature. The fracture toughness of UHCS-1.3C processed for pearlitic microstructures decreases with increasing austenitizing temperature. Austenite grain size, pearlite colony size, and interlamellar spacing are evaluated for influence on the fracture toughnesses associated with pearlitic microstructures. It is found that the primary microstructural parameter controlling fracture toughness is the cleavage facet size, which typically spans several pearlite colonies. The size of cleavage facet size is primarily controlled by the austenite grain size. The influences of pearlite colony size and interlamellar spacing are minor.