Effect of tempering upon the tensile properties of a nanostructured bainitic steel
Introduction
Strong steels with a nanostructure of bainitic ferrite and austenite can be manufactured by isothermal transformation from austenite at temperatures around 200–250 °C [1]. In these steels the strengthening due to the small size of the bainite plates dominates other mechanisms [1], [2]. The transformation by shear [3] at temperatures around 200 °C takes more than one week to reach an asymptotic limiting fraction, or hours at higher temperatures. Careful alloy design is the only demonstrated method that can be used to accelerate the transformation while retaining the fine structure [4], [5].
In the earliest work on these steels it was observed that the hardness is relatively insensitive to quite severe tempering [1], when compared to martensitic steels of similar composition. This is because the latter derive most of their strength from interstitial carbon, which on precipitation leads to a large decrease in hardness. In the case of nanostructured bainite, intense precipitation of carbides due to the decomposition of carbon-enriched retained austenite occurs at lath boundaries – preventing the ferrite from coarsening and thus preserving the hardness and strength [6].
The fact that the austenite can decompose on tempering might be a cause for concern if the steel is to be used at elevated temperatures. Early work demonstrated that when carbide-free microstructures of bainitic ferrite and carbon-enriched austenite are tempered, the decomposition of the austenite, and associated carbide precipitation, leads to a decrease in toughness [7], [8]. The general philosophy is that the austenite helps improve ductility via the classical TRIP effect, in which the plasticity associated with martensitic transformation helps delay the onset of plastic instabilities during tensile testing [9], [10], [11]. The motivation for the present work was, therefore, to study the effect of severe tempering on the tensile properties of nanostructured bainite.
Section snippets
Methodology
After homogenisation for 48 h at 1200 °C, steel of composition was transformed at a range of temperatures under vacuum in the Thermecmastor-Z thermomechanical simulator, with which cylindrical samples of 8 mm diameter and 12 mm in length can be induction heated under vacuum, and then rapidly cooled using inert gas. The temperature is controlled by feedback from an R-type thermocouple, and dimensional changes due to thermal expansion and solid
Results
Fig. 1 shows the change in hardness with tempering time for samples transformed at different transformation temperatures (200, 220 and 250 °C) and then tempered at 500 °C for different times. Based on previous experience, tempering for 24 h was estimated to be sufficient to ensure that retained austenite had decomposed fully [6], [12], [13], [14]. X-ray diffraction was used to investigate the austenite fraction after tempering, finding none present. As seen in the figure, most of the hardness
Discussion
In order to better understand the change in mechanical properties upon tempering, calculations were performed to evaluate the strength contributions to the bainitic ferrite in the material transformed at 250 °C in the as-transformed and tempered condition. The strength in MPa has been expressed in previous work by Eq. (2) [26], [27] for 0.4C wt% alloys. Although this formulation is not directly applicable to the total strength in this case, especially as it neglects composite effects due to a
Conclusions
The present work shows that tempering of hard nanostructured bainitic steels to remove austenite can result in steels which maintain or have improved elongation in spite of an intense precipitation of carbides at the plate boundaries. This comes at the cost of lowering the strength of the steel, just as occurs classically in the tempering of quenched martensitic steels.
After tempering the strength contribution from the scale of the bainite plates becomes increasingly dominant. Quite severe
Acknowledgements
The authors would like to thank Professor A. L. Greer for the provision of laboratory facilities at the University of Cambridge and also express their gratitude to the Scholars Rescue Fund of International Institute of Education in Washington DC, for supporting Hala Salman Hasan׳s work in Cambridge. We also thank Dr. Yan Pei and Dr. Lucy Fielding for helpful discussion.
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