Quantitative phase analysis of martensite-bainite steel using EBSD and its microstructure, tensile and high-cycle fatigue behaviors

This study conducted a quantitative analysis of the martensite/bainite (M/B; bainitic structure region) fraction of martensite-bainite steel using electron back-scatter diffraction (EBSD) analysis. The M/B fraction analyzed using the EBSD analysis method was then compared with phase fraction measurement results with an optical microscope (OM), field emission scanning electron microscope (FE-SEM), and field-emission transmission electron microscopy (FE-TEM). In addition, microstructure, tensile and high-cycle fatigue behaviors according to M/B phase fraction were investigated. Initial microstructural observation measured a prior austenite grain size (PAGS) of 24 mu m (alloy A) and 11 mu m (alloy B). Both alloys were observed to have martensite and bainite structures. XRD phase analysis of the two alloys identified an alpha-Fe peak expected to be martensite or bainite. Quantitative phase fraction of M/B using EBSD analysis measured M: 40.37% and B: 59.63% for alloy A, and M: 53.03% and B: 46.97% for alloy B. Tensile tests of the above materials confirmed that alloy B, which had finer PAGS and a higher martensite fraction, had greater yield strength (1423 MPa) and tensile strength (1826 MPa) that were approximately 200 MPa higher than alloy A. The yield strength was calculated based on the M/B phase fraction using EBSD and the measured microstructure factors, with a consideration of the prediction model. The calculation value was similar to the actual tested strength one. In the high-cycle fatigue test, alloy B, with its greater strength, had an approximately 200 MPa higher fatigue limit (1075 MPa) than that of alloy A. EBSD analysis of the fatigue crack initiation area confirmed that the M/B interface can act as a fatigue crack initiation site. Based on the above findings, tensile and fracture surface analyses were performed, and attempts were made to identify the tensile and deformation mechanism according to the M/B phase fraction.