Characteristics and Kinetics of Bainite Transformation Behaviour in a High-Silicon Medium-Carbon Steel above and below the Ms Temperature

Shima Pashangeh,Seyed Sadegh Ghasemi Banadkouki,Mahesh Somani,Jukka Kömi

doi:10.3390/ma15020539

Shima Pashangeh, Seyed Sadegh Ghasemi Banadkouki + Show 2 more

Open Access

https://doi.org/10.3390/ma15020539

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Journal: Materials	Publication Date: Jan 11, 2022
Citations: 5	License type: CC BY 4.0

Affiliation: Yazd University, University of Oulu

Abstract

This work deals with the kinetic aspects of bainite formation during isothermal holding above and below the martensite start (Ms~275 °C) temperature using a low-alloy, high-silicon DIN 1.5025 steel in a range suitable for achieving ultrafine/nanostructured bainite. Dilatation measurements were conducted to study transformation behaviour and kinetics, while the microstructural features were examined using laser scanning confocal microscopy and electron backscatter diffraction (EBSD) techniques combined with hardness measurements. The results showed that for isothermal holding above the Ms temperature, the maximum bainitic transformation rate decreased with the decrease in isothermal holding temperature between 450 and 300 °C. On the other hand, for isothermal holding below the Ms temperature at 250 and 200 °C, the maximum rate of transformation was achieved corresponding to region I due to the partitioning of carbon and also possibly because of the ledged growth of isothermal martensite soon after the start of isothermal holding. In addition, a second peak was obvious at about 100 and 500 s, respectively, during holding at 250 and 200 °C due to the occurrence of bainitic transformation, marking the beginning of region II.

Highlights

Steels processed at a temperature close to Ms temperature via the quenching and partitioning (Q&P) route with essentially finely divided martensite–austenite–nanostructured bainite structures, as well as quenching and bainititizing (Q&bainite start temperature (Bs)) treatment with mainly ultrafine/nanostructured bainite–austenite structures, are two such groups of multiphase third-generation advanced high-strength steels (AHSSs) that show greatly improved mechanical property combinations, including good ductility imparted by transformation-induced plasticity (TRIP), an effect of the finely divided retained austenite (RA) in the steels [2,4,5]
Is several times greater than the dilatation that occurred in athermal martensite formation, revealing that the austenite-to-bainite transformation continues during isothermal holding
The dilatometer measurements and microto the adjacent austenitic areas, thereby enhancing bainite havepartitioning been investigated in a thermomechanical simulator

Summary

Introduction

Steels processed at a temperature close to Ms temperature via the quenching and partitioning (Q&P) route with essentially finely divided martensite–austenite–nanostructured bainite structures, as well as quenching and bainititizing (Q&B) treatment with mainly ultrafine/nanostructured bainite–austenite structures, are two such groups of multiphase third-generation AHSSs that show greatly improved mechanical property combinations, including good ductility imparted by transformation-induced plasticity (TRIP), an effect of the finely divided retained austenite (RA) in the steels [2,4,5]. In order to achieve a mixture of ultrafine bainite and finely distributed, carbon-enriched retained austenite, isothermal heat treatment procedures close to (both above and below) the Ms temperature have been suggested [6,7] These isothermal heat treatments have been shown to impart excellent mechanical properties to the steel, including ultrahigh strength, adequate toughness, reasonable ductility and good wear resistance [7,8].

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