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

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.

Effect of electrolysis parameters on corrosion resistance of extra-low carbon high silicon iron-based alloy

Purpose High silicon iron-based alloys possess excellent corrosion resistance in certain specific media, but the effects of electrolysis parameters on corrosion resistance remain unknown. This study aims to guide the development and application of an extra-low carbon high silicon iron-based alloy (ECHSIA) in electrode plates. Design/methodology/approach The corrosion resistance of ECHSIA and a conventional high-silicon cast iron (CHSCI) was analyzed through experimental characterizations. The morphology was observed by scanning electron microscopy. The influence of electrolysis parameters on the corrosion resistance of ECHSIA was investigated through corrosion experiments. The relationship between the electrolysis parameters and the corrosion resistance of ECHSIA was statistically investigated using the grey correlation analysis method. Findings The corrosion resistance of the ECHSIA is better than that of the CHSCI. The corrosion rate showed an increasing tendency with the increase in the nitric acid concentration (CHNO3), electrolyte temperature and current density. The grey correlation analysis results showed that the CHNO3 was the main factor affecting the corrosion rate of the ECHSIA. Originality/value An ECHSIA with a single ferrite microstructure was prepared. This study provides a guideline for the future development and application of ECHSIAs as electrode plates.

Effect of Multi-Step Austempering Treatment on the Microstructure and Mechanical Properties of a High Silicon Carbide-Free Bainitic Steel with Bimodal Bainite Distribution

The effect of multi-step austempering treatments on the microstructure and mechanical properties of a novel medium carbon high silicon carbide-free bainitic steel was studied. Five different isothermal treatment processes were selected, including single-step isothermal treatments above martensite start temperature (at 350 °C and 370 °C, respectively), and three kinds of two-step routes (370 °C + 300 °C, 370 °C + 250 °C, and 350 °C + 250 °C). In comparison with single-step austempering treatment adopting a two-step process, a microstructure with a bimodal-size distribution of bainitic ferrite and without martensite was obtained. Bainitic transformation was studied using dilatometry both for single-step and two-step routes and the specimens were completely characterised by electron microscopy (SEM and TEM), X-ray diffraction (XRD) and standard tensile tests. The mechanical response of the samples subjected to two-step routes was superior to those treated at a single temperature.

Influence of Austempering of As-Cast Medium Carbon High-Silicon Steel on Wear Resistance

The aim of this study was to evaluate the effect of austempering compared to quenching and low-temperature tempering on wear resistance of an as-cast medium carbon high-silicon steel intended for rock breaking. Austempering was done by isothermal holding at 270, 300 and 350 °C in molten salt baths, while quenching was done in water. The austempering treatments resulted in microstructural combinations of bainite and martensite. The isothermal holding at 270 °C resulted in bainite and self-tempered martensite, while isothermal holdings at 300 and 350 °C resulted in bainite and untempered martensite. The two quench and temper treatments resulted in tempered martensite. In general austempering resulted in lower hardness values when compared to quenching and tempering but higher impact toughness. The wear resistance was best for quenching and low temperature tempering, followed by austempering at 270 °C, but at slightly lower hardness and 25% higher impact toughness. The other two austempering treatments resulted in worse wear resistance.