Effect of Different Austempering Heat Treatments on Corrosion Properties of High Silicon Steel.

Mattia Franceschi,Luca Pezzato,Alessio Giorgio Settimi,Katya Brunelli,Dmitry Konstantinov,Marina Polyakova,Manuele Dabalà,Claudio Gennari,Mirko Pigato

doi:10.3390/ma14020288

Mattia Franceschi, Luca Pezzato + Show 7 more

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https://doi.org/10.3390/ma14020288

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Abstract

A novel high silicon austempered (AHS) steel has been studied in this work. The effect of different austenitizing temperatures, in full austenitic and biphasic regime, on the final microstructure was investigated. Specimens were austenitized at 780 °C, 830 °C, 850 °C and 900 °C for 30 min and held isothermally at 350 °C for 30 min. A second heat treatment route was performed which consisted of austenitizing at 900 °C for 30 min and austempering at 300 °C, 350 °C and 400 °C for 30 min. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) have been used to evaluate the microstructural evolution. These techniques revealed that the microstructures were composed of carbide-free bainite, ferrite, martensite and retained austenite (RA) in different volume fractions (Vγ). An aqueous borate buffer solution with 0.3 M H3BO3 and 0.075 M Na2B4O7∂10H2O (pH = 8.4) was used for corrosion tests in order to evaluate the influence of the different volume fractions of retained austenite on the corrosion properties of the specimens. The results showed that when increasing the austenitization temperatures, the volume fractions of retained austenite reached a maximum value at 850 °C, and decrease at higher temperatures. The corrosion properties were investigated after 30 min and 24 h immersion by means of potentiodynamic polarization (after 30 min) and electrochemical impedance spectroscopy (after both 30 min and 24 h) tests. The corrosion resistance of the samples increased with increases in the volume fraction of retained austenite due to lower amounts of residual stresses.

Highlights

In the latest years, significant interest has been dedicated to austempered high-silicon steels (AHS) [1,2]
Palaksha et al [18] observed that when increasing the austempering temperature from 275 to 350 ◦ C, the final volume fraction of retained austenite increases, a further temperature increase leads to a decrease in RA stabilization
When the material was austenitized at 900 ◦ C, the volume fraction of when the material was austenitized at 900 °C, the volume fraction of retained austenite increased from 300 ◦ C to 350 ◦ C of austempering, and decreased retained austenite increased from 300 °C to 350 °C of austempering, and decreased (Figure 9b)

Summary

Introduction

Significant interest has been dedicated to austempered high-silicon steels (AHS) [1,2]. Palaksha et al [18] observed that when increasing the austempering temperature from 275 to 350 ◦ C, the final volume fraction of retained austenite increases, a further temperature increase leads to a decrease in RA stabilization Such an effect has been observed by Putatunda [2], who demonstrated that the same trend can be observed for carbon content in RA and material fracture toughness. The authors in a previous work [17] reported results for the effect of different austenitizing conditions; as the austenitization temperature increased, a grain coarsening was Materials 2021, 14, 288 observed This led to the presence of bainitic ferrite (BF), promoted by element diffusion [23]. Given the lack of data in the literature regarding the corrosion resistance of these steel grades, the effect of the microstructural constituents and RA content on the corrosion resistance of the material was investigated in borate buffer solution [31]

Materials and Methods

In all the the patterns patternsofare are visible BCC

Corrosion Resistance

Findings

Conclusions