Measurement of Steel Phase Transformation Kinetics by Dilatometry and in-Situ Neutron Diffraction – a Comparative Study

Solid-state phase transformation kinetics between ferrite (α) and austenite (γ) phases in a C-Mn steel were investigated by the dilatometry and in-situ neutron diffraction methods, respectively. Dilatometry technique is commonly used to study the phase transformation kinetics. It infers the allotropic phase transformation from the overall dimensional changes, e.g., the diameter changes of the specimen, during heating and/or cooling. In this regard, it is an indirect measurement technique. On the other hand, neutron diffraction directly detects and measures the individual phases as they present during different stages of phase transformation.

In this work, a Gleeble thermal-physical simulator was used to measure the phase transformation kinetics by the dilatometry method. The VULCAN beamline at the Spallation Neutron Source was used for the diffraction measurement. A newly developed asynchronous stroboscopic neutron diffraction technique made it possible to perform in-situ measurement of the phase transformation during fast heating and cooling. As an example, Figure 1 shows the comparison of the austenite phase volume fraction as a function of temperature obtained by the two techniques and the equilibrium volume fraction from Thermo-Calc calculations. A considerable discrepancy between the two techniques was observed at the low temperature end. The dilatometry measurement showed much higher austenite formation temperature and much lower volume fraction between 750-875C. Such discrepancy is attributed to the micro-heterogeneity of alloying elements in different phases as revealed by the neutron diffraction results. The potential shortcomings of the dilatometry method under certain circumstances are discussed.

Figure 1. Comparison of phase transformation (PT) kinetics obtained by neutron diffraction and Gleeble dilatometry at a heating rate of 3C/s. Superimposed is the equilibrium phase fraction of austenite vs. temperature calculated using Thermo-Calc for the specific steel chemistry.