Episode 50 - Dynamic Transformation w/ Tensile Tests Using a Gleeble and In-Situ X-Ray Diffraction скачать в хорошем качестве

Episode 50 - Dynamic Transformation w/ Tensile Tests Using a Gleeble and In-Situ X-Ray Diffraction

Presenter: Prof. Samuel Filgueiras Rodrigues, PhD. Federal Institute of Maranhao, Brasil. Abstract: When austenite is deformed in the single austenite phase field, a partial amount of this phase transforms dynamically into ferrite by a displacive mechanism. After releasing the applied loading, some amount of this induced formed ferrite retransforms back into the stable austenite by a diffusional process. This influences the machine loads reflecting in its decreasing under isothermal conditions and does not increase as rapidly as expected when continuous cooling thermomechanical processes are employed. Physical process simulations by means of tensile tests in a customized Gleeble machine simulation system equipped with a synchrotron light X-ray beam (12 keV) under isothermal conditions were carried out on an Nb X-70 steel. The flow stress-strain behavior indicates that dynamic transformation was taking place during straining. The presence of ferrite peaks obtained the X-ray diffraction results was evident during deformation and vanished after isothermal holding. Previous researches have been shown that ferrite can be formed due to the applied strain and increases its volume fraction as the deformation continuous to be increased. It is only possible to form such a softer phase when the driving force for dynamic transformation is higher than the total free energy barrier which prevents its formation. This barrier consists of the free energy difference between austenite and ferrite summed to the lattice dilatation work and shear accommodation work. The presence of Nb retards the retransformation of the ferrite into austenite due to pinning and/or solute drag effects. Moreover, holding times after deformation increases the amount of austenite available for microstructure control during rolling. This phenomenon mechanism can be used as an advantage to design improved manufacturing schedules.