Microstructural Evolution and Mechanical Performance of Martempered Medium Carbon Steel

Abstract

The mechanical properties and microstructure of medium carbon steel (0.367 wt.% C) were studied following martempering treatments in oil and water baths maintained at 25 °C and 100 °C. Specimens were initially austenitised at 800 °C before controlled quenching. Oil-martempered samples at 100 °C showed superior impact toughness (59.21 J) and tensile strength (1.875 MPa), while water-martempered samples at 100 °C exhibited the highest hardness (180.9 BHN). Microstructural analysis via SEM revealed tempered martensite with uniformly distributed spheroidised carbides (0.3–0.5 µm) in oil-quenched samples, whereas water-quenched specimens displayed lath martensite with noticeable interlath microcracks. XRD analysis confirmed the presence of significantly higher retained austenite in oil-treated specimens (4.2 ± 0.5 vol.%) compared to water-quenched steel (0.9 ± 0.2 vol.%). These findings highlight that martempering medium carbon steel in oil at elevated temperatures provides an optimal combination of strength and ductility, emphasising the critical influence of quenchant type and temperature on final material performance.