Investigation on deformation induced ferrite (a kind of martensite) transformation above Ae3 temperature in a low carbon steel

A series of unidirectional compression tests of a low carbon steel Q235 were performed on a Gleeble 3500 thermal simulator, and the influences of strain, strain rate and deformation temperature (above the isothermal transformation temperature A(e3) from austenite to ferrite) on deformation induced ferrite transformation (DIFT) have been examined. The microstructure, nanoindentation hardness and the elastic modulus of deformation induced ferrite (DIF) and proeutectoid ferrtie were determined by optical microscope, scanning electron microscope (SEM), X-ray diffraction (XRD) and nanoindentation techniques. The results show that DIFT can take place above A(e3) temperature, the higher the strain rate and the strain, the more favorable for DIFT. When epsilon=80% and (epsilon) over dot=20 s(-1), the upper limit temperature of DIFT can be elevated to 945. degrees C (A(e3)+98 degrees C). An interesting phenomenon is found that when the deformation temperature is between 870-920 degrees C, the total stress is increased with deformation temperature decreasing. However, when deformed between 830-870 degrees C, the total stress is decreased with deformation temperature decreasing. Compared with the diffraction peak of proeutectoid ferrite, the diffraction peak of DIF was shift to a low angle in XRD analysis, and both the nanoindentation hardness and elastic modulus of DIF are much higher than those of proeutectoid ferrite, which proved DIF to be a kind of martensite.