Microstructural Development and Fracture Behavior after Rapid Tempering in the TME Regime in 1045 Steel

Quenching and tempering (Q&T) allows for a wide range of combinations of strength and toughness to be produced. Tempering is generally done to increase toughness and lower strength, however embrittling mechanisms result in temperature ranges where both strength and toughness may decrease. Tempered martensite embrittlement (TME) represents one such mechanism, associated with the decomposition of retained austenite and precipitation of cementite during tempering, usually between 250-450 °C. The use of induction heating allows for time-temperature combinations, previously unobtainable by conventional methods, that have been shown to improve properties. This work shows a beneficial effect of rapid tempering in alloy 1045, with an increase in energy absorbion of about 50% when measured via a three-point bending fracture test in the TME regime. Phase fraction measurements by Mössbauer spectroscopy showed that increased energy absorption was obtained despite essentially complete decomposition of retained austenite during tempering. Scanning electron microscopy (SEM) investigation of the carbide distribution showed refinement of the average carbide size of approximately 15% in the rapid tempered conditions. SEM characterization of the fracture surfaces of the three-point bend samples showed that, despite an increase in energy absorption in the TME regime, increased ductile fracture appearance was observed only at the highest test temperature.