Evidence of dynamic ferrite transformation during thermomechanical simulation of an X70 microalloyed steel above Ae3temperature

It is known that dynamic transformation can occur in the austenite field above the Ae3temperature in a rapidly displacive way and after diffusionally under stress. This phenomenon influences industrial thermomechanical processes by changing the expected applied loads during forging or rolling. In this investigation, laboratory-scale physical simulation of tensile test in a customized Gleeble thermomechanical simulator system equipped with a synchrotron light X-ray beam (12 keV) was carried out on a linepipe steel at 50 °C above the Ae3. The obtained stress-strain curves indicate that dynamic transformation took place during deformation. The X-ray diffraction patterns confirmed the formation of ferrite inside the austenite field during strain. The results confirm the direct evidence of the onset of ferrite formation due to the applied strain and its increase in volume fraction as the deformation continues. The main ferrite-phase peaks detected by the in-situ X-ray diffraction were the ones related to the (110) and (221) planes. Both peaks increased their intensity with the increasing strain, leading to a decrease in the gamma-phase peak intensities of the (111) and (220) planes. The total volume fraction of all ferrite peaks increased to approximately 50% when the strain reached a maximum value, while the fraction of all austenite peaks decreased proportionally. Microscopic measurements of the presented phases have shown excellent agreement with the results obtained from direct X-ray diffraction results. A comprehensive understanding of the dynamic transformation phenomenon mechanism is important to the industry sector in designing improved manufacturing parameters for products with good quality.