Abstract
A multiphase microstructure comprising of different volume fractions of prior martensite and ultra-fine bainite (bainitic ferrite and retained austenite) was obtained by quenching to certain temperatures, followed by isothermal bainitic transformation. The effect of the prior martensite transformation on the bainitic transformation behavior, microstructures, and mechanical properties were discussed. The results showed that the prior martensite accelerated the subsequent low-temperature bainite transformation, and the incubation period and completion time of the bainite reaction were significantly shortened. This phenomenon was attributed to the enhanced nucleation ratio caused by the introduced strain in austenite, due to the formation of prior martensite and a carbon partitioning between the prior martensite and retained austenite. Moreover, the prior martensite could influence the crystal growth direction of bainite ferrite, refine bainitic ferrite plates, and reduce the dimension of blocky retained austenite, all of which were responsible for improving the mechanical properties of the ultra-fine bainitic steel. When the content of the prior martensite reached 15%, the investigated steels had the best performance, which were 1800 MPa and 21% for the tensile strength and elongation, respectively. Unfortunately, the increased content of the prior martensite could lead to a worsening of the impact toughness.
Highlights
In the past decades, a kind of nanostructured high-carbon silicon-rich bainitic steel involving fine-scaled carbide-free bainitic ferrite plates and the uniform dispersion of carbon enriched austenite was developed by Caballero and Bhadeshia [1]
Volume fraction of bainite is generated is considered as the start and completion time of method
Charpy impact tests were conducted on 10 mm × 10 mm × 55 mm notched samplesthe at isothermal bainite transformation
Summary
A kind of nanostructured high-carbon silicon-rich bainitic steel involving fine-scaled carbide-free bainitic ferrite plates and the uniform dispersion of carbon enriched austenite was developed by Caballero and Bhadeshia [1]. These bainite steels exhibit a remarkable combination of ultrahigh strength and excellent toughness by isothermal treatment at a low temperature, which has incredible potential in the transport, construction, and offshore industries, as well as in defense applications [2,3]. The isothermal bainite transformation cycle is very time consuming, taking up to even a month, and the tediously lengthy heat treatment limits its industrial application. Many researchers have been striving to explore the transformation acceleration mechanism, which contains composition optimization [4,5,6,7], grain refinement [8,9], heating treating optimization [10,11,12], applying stress to austenite [13,14,15,16], and introduced a partial
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