Microstructural Evolution of a Continuously Cooled Air Hardening Steel

This study studied the phase transformations occurring at different continuous cooling rates in an air hardening steel used in the rock-crushing industry. Samples of this steel were submitted for calorimetric testing using the differential scanning calorimetry (DSC) technique. In the experimental run, the samples were heated at the rate of 0.33 C/s from 50 to 1050 C and equilibrated at this temperature for 900 s, then cooled at seven different cooling rates between 0.05 and 0.5 C/s. For all the cooling rates, the DSC traces of the samples showed a first exothermic peak at about 500 C and for samples cooled at rates higher than 0.15 C/s, a second exothermic peak at about 295 C was observed. From the microstructural investigations carried out by light microscopy (LM) and scanning electron microscopy (SEM), it was observed that all the samples after DSC treatment were characterized by the presence of bainite. In the samples cooled at rates higher than 0.15 C/s, martensite was also detected. Comparing the results of DSC and SEM, it was concluded that the first peak at 500 C corresponds to the austenite - bainite transformation, while the second peak at 295 C corresponds to the austenite - martensite transformation. The experimentally determined bainite and martensite start temperatures were compared to the values derived from a number of well-known empirical equations.