Estimation of phase fraction in dual phase steel using microscopic characterizations and dilatometric analysis

We made a quantitative comparison of three different methods, optical microscopy by tint etchant, EBSD phase mapping and dilatometry for analysis of phase fraction in steel subjected to intercritical annealing and isothermal treatment. While the results from optical microscopy and EBSD technique showed quantitative agreement, the dilatometry gave rather higher martensite fraction compared to the microscopic analyses. Nevertheless, all three methods showed qualitative agreement in the variation of martensitic fraction depending on the processing conditions. The analyses revealed that the martensitic fraction in final microstructure decreased as intercritical annealing temperature increased because it deteriorated the hardenability of austenite. Raising the isothermal treatment temperature increases the martensite fraction due to the increases of austenite fraction, which transformed into martensite afterward.