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

M Zorgani,C Garcia-Mateo,M Jahazi

doi:10.1016/j.matdes.2021.110082

Abstract

The influence of tempering at 400 and 500 °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 microscopy, X-ray diffraction, and microhardness measurements were conducted. The softening due to tempering at 400 °C was negligible for both microstructures. Tempering at 500 °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 °C was the decomposition of as-ausformed highly dislocated bainitic ferrite plates and/or film-like retained austenite into cementite and ferrite as a result of enhanced carbon diffusion. While tempering at 400 °C did not result in changes in the size of the bainitic plates, the higher tempering temperature of 500 °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 precipitates, which moderate the tempering effects on hardness reduction.

Highlights

It is imperative to reduce the bainitic steel carbon content to a level that overcomes the disadvantages of high carbon content and retaining the benefits gained from the superior nano-scale microstructure
They found that nanosize cementite precipitates were formed after tempering at 400C and 500C for 30 min, which prevented the coarsening of bainitic ferrite plates
The thickness distribution of the bainitic plates were calculated for each condition, and the results are reported in Fig. 2c and 2d

Summary

Introduction

In the later stages of tempering, the same C-depleted austenite decomposes into a mixture of ferrite and cementite, while from supersaturated bainitic ferrite (abÞ precipitation of cementite occurs, changing its lattice from body-centred tetragonal (BCT) to its body-centred cubic (BCC) equilibrium state. Santajuana et al [11] reported no change in the microstructure when two high-C-nanostructured bainite steel with carbon contents of 0.7 wt% and 1 wt% were tempered in the temperature range of 200–400 °C. Peet et al [12] studied the effect of tempering temperature on the microstructure of a high-carbon nanostructured CFB steel They found that nanosize cementite precipitates were formed after tempering at 400C and 500C for 30 min, which prevented the coarsening of bainitic ferrite plates. Ruiz-Jimenez et al [10] noted that tempering at 450 °C did not produce noticeable microstructural changes in 0.6 wt% C carbide-free bainitic steel and the overall hardness remained unchanged

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