Effect of microstructure on mechanical properties in 0.5 carbon-steel processed by high temperature thermomechanical treatment

The effect of microstructure on the slow bending stress and fracture energy in 0.5C-steel processed by high temperature thermomechanical treatment (processed by forging) (HTMT) was studied to understand which microstructural factors contribute to the strength and toughness of a HTMT steel. Significant improvement was achieved in the slow bending fracture energy, with moderate increase in the slow bending stress when the steel was deformed by 50% at 1473K followed by direct water quenching and subsequent tempering at 453 K. When the steel was deformed by 50% reduction at 1173 K followed by direct water quenching and subsequent tempering at 423 K, the slow bending stress significantly increased though the increase in the fracture energy was not as great as that of the 1473 K forged steel. However, an abrupt reduction occurred in the fracture energy above suitable tempering temperatures, so above these temperatures, there was little difference between the properties of the HTMT and conventional heat-treated steels. Microstructural factors contributing to the mechanical properties are discussed in terms of thin-foil transmission electron microscopy, non-isothermal dilatometry, and X-ray measurements.