Effect of Tempering Temperature on the Precipitation Behavior and Mechanical Properties of a Martensite Ferritic Steel

In this investigation, the precipitation of boron-containing M23C6 in the CB2 steel during tempering was investigated. Scanning electron microscopy, energy-dispersive x-ray spectroscopy, transmission electron microscopy and differential scanning calorimetry were used to analyze the influence of temperature on the microstructure and properties of CB2-tempered martensite ferritic steel. The steel was normalized at 1100 °C for 2 h and then tempered at six different temperatures between 690 and 790 °C for 2 h. During air-cooling following austenitization, the precipitation of M3C particles occurred in flakes forming with their diameter and thickness of 100 and 20 nm, respectively. The boron-containing M23C6 was formed on the lath boundary after the dissolution of M3C within the boundary. When the tempering temperature increased from 690 to 790 °C, the average diameter of boron-containing M23C6 increased from 132 to 198 nm. Owing to the rapid growth of the precipitates, the phenomenon of lath widening as well as the recovery of microstructure would take place at high temperature. With an increase in the temperature, the tensile and yield strengths decreased gradually, while the plasticity increased and the boron-containing M23C6 grew rapidly. Furthermore, when the tempering temperature exceeded 750 °C, the plasticity of CB2 steel was significantly improved.