LAUSR

Abstract The present study was aimed at microstructure quenched around Ac3 point and its effect on the damage behavior. Thermo-Calc was used to ensure the A3, while the Ac3 was identified by means of heat treatments on DIL805A thermal dilatometer, and the quenching temperatures were determined according to the above parameters. The scanning electron microscopy (SEM) and electron back scattering diffraction (EBSD) were carried out to observe the microstructure. In-situ tensile tests with SEM were performed and the development of microcracks were recorded. The results illustrated that the microstructure depends on the quenching temperature: a dual phase microstructure of martensite islands with ferrite around them is observed when the material is quenched below A3, approximately 842 ℃. Ferrite near martensite island and interface between two martensite islands with different misorientation are generally the weak spots in microstructure. When the material is quenched from austenite region around or above Ac3, approximately 880 ℃, martensite with lath submicrostructure is obtained. The location with large misorientation between two packets is usually the weak spot in microstructure and leads to increased possibility of microcracks.