Microstructural evolution during tempering of an ausformed carbide-free low temperature bainitic steel

The influence of tempering at 400 and 500 degrees C on two bainitic ferrite microstructures obtained via pure isothermal transformation and ausforming followed by the isothermal transformation was studied in a medium C-Si rich bainitic steel. To understand the dilatometric behaviour and mechanisms of carbide-free bainite decomposition, extensive microstructural characterization using scanning electron micro-scopy, X-ray diffraction, and microhardness measurements were conducted. The softening due to tem-pering at 400 degrees C was negligible for both microstructures. Tempering at 500 degrees C led to a remarkable contraction in dilatometric signal in an ausformed sample, while a pure isothermal sample showed slight contraction. The main mechanism governing microstructural changes during tempering at 500 degrees C was the decomposition of as-ausformed highly dislocated bainitic ferrite plates and/or film-like retained austen-ite into cementite and ferrite as a result of enhanced carbon diffusion. While tempering at 400 degrees C did not result in changes in the size of the bainitic plates, the higher tempering temperature of 500 degrees C led to a thickening of the bainitic ferrite plates for both testing conditions. However, the thickening was restricted in the ausformed bainite, which was related to the pinning effect of very small cementite pre-cipitates, which moderate the tempering effects on hardness reduction. (c) 2021 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).