Direct observation of austenitisation in 1005 C–Mn steel during continuous heating using in situ synchrotron X-ray diffraction

Abstract

The austenitisation (α→γ) transformation in a 1005 C–Mn steel is monitored in real time at continuous heating rates between 1 and 10°C s⁻¹ using in situ synchrotron X-ray diffraction and validated using dilatometry. Experimental validation is provided for austenitisation models that predict that the austenitisation transformation proceeds through multiple mechanisms. At temperatures below the A1 transformation temperature, the starting microstructure undergoes recovery and recrystallisation to relieve stress imparted during the initial thermomechanical processing of the steel. The austenitisation transformation follows, beginning at the A1 temperature, with the initial transformation proceeding as the pearlite in the microstructure is dissolved and high carbon concentration austenite is formed. Since the carbon is localised near the original pearlite colonies, there is a pronounced heating rate dependent delay before the remaining low C ferrite grains begin to transform. The transformation reaches completion at temperatures above the A3 temperature, and the last ferrite to be transformed is nearly pure iron.