Cleavage fracture and microstructural effects on the toughness of a medium carbon pearlitic steel for high-speed railway wheel

Abstract

The fracture behavior and microstructural effects on the toughness of a medium carbon wheel steel were systematically investigated by using instrumented impact tests. The microstructure of the steel consisted of lamellar pearlite and a small amount of proeutectoid ferrite. The results showed that with the refinement of prior austenite grain size, the pearlite block size decreased and the ferrite content increased, while the pearlite interlamellar spacing kept almost unchanged. The impact toughness was improved in the temperature range from −80 °C to room temperature. Observation of fracture surfaces revealed that the fracture occurred in a brittle manner, which was characterized by cleavage and a small proportion of ductile fracture. However, the toughness was positively correlated to the ductile fracture area. Cleavage cracks were found to be initiated at the sites of proeutectoid ferrite, pearlite colony or pearlite/ferrite interface. The values of macroscopic cleavage fracture stress measured were similar (~1833 MPa) for the specimens with different pearlite block size and ferrite content but the same pearlite interlamellar spacing. The toughness improvement by microstructural refinement in the transition temperature region was attributed to the increase in cleavage crack initiation energy. The cleavage crack propagation energy was, however, relatively low and mainly unchanged. With decreasing the test temperature, the difference in crack initiation energy was decreased, causing the difference in total impact absorbed energy to decrease. The results also suggested that in the lower shelf temperature region where complete cleavage fracture occurred, the toughness was insensitive to the microstructural parameters.