Elsevier

Materials & Design

Volume 86, 5 December 2015, Pages 603-609
Materials & Design

Effects of isothermal transformation conditions on the microstructure and hardness values of a high-carbon Al–Si alloyed steel

https://doi.org/10.1016/j.matdes.2015.07.151Get rights and content

Highlights

  • Carbide precipitation was retarded during austempering of a low-alloy high-carbon steel with 0.80 wt.% Si and 0.84 wt.% Al

  • A yield strength of 1170 MPa and an ultimate strength of 1370 MPa were obtained by austempering at 360°C for 4 h

  • An extraordinary high hardness of about 660HV30 was achieved after austempering at 300°C

Abstract

In this investigation, the microstructure and hardness of a high-strength high-carbon steel containing 0.80 wt.% silicon and 0.84 wt.% aluminum have been evaluated under different austempering conditions. Austempering was performed at temperatures of 300, 330 and, 360 °C for different times from 1 to 8 h. Observations carried out by light optical microscopy (LOM) and scanning electron microscopy (SEM) revealed that the austempered samples have a microstructure consisting mostly of bainitic ferrite and retained austenite, and this was confirmed by X-ray diffraction (XRD) analysis and microhardness measurements. This simply indicates that it is possible to partially substitute aluminum for silicon and still retard the formation of carbides. There were no significant changes in ferrite plate thickness with increasing transformation temperature. The fraction of retained austenite, however, increased with temperature. A yield strength of 1170 MPa, an ultimate tensile strength of 1370 MPa, and a total elongation of 9.1% were obtained after isothermal transformation at 360 °C for 4 h. Depending on the transformation conditions, hardness values of about 500–660 HV30 were obtained. The hardness increased as the transformation temperature decreased.

Introduction

Notable mechanical properties of austempered steels make them ideal replacement materials for quenched and tempered (QT) steels and austempered ductile cast irons (ADI) in an increasingly large number of applications, especially in railway and automotive industries [1], [2], [3], [4], [5], [6], [7]. The microstructural features of austempered steels, especially plate size of bainitic ferrite and volume fraction of retained austenite, are most seriously affected by the austempering variables, i.e., austempering temperature and time [8], [9], [10]. Owing to the strong dependence of the mechanical behavior of steels on the microstructure, the high state of performance and reliability of austempered steel products is a direct consequence of the proper selection of austempering variables.

Speich and Cohen [11], in a study of the growth kinetics of bainite plates, reported that the growth rate of bainite was time-independent at a given temperature and that edgewise and sidewise growth rates increased with temperature. Chang and Bhadeshia [12] reported that the thickness of bainitic ferrite plates was increased by increasing transformation temperature. A few years later, Singh and Bhadeshia [13] showed that temperature rarely contributes independently to the determination of the plate thickness. They also indicated that the largest effect on the plate thickness comes from the driving force available for nucleation of bainite and the yield strength of austenite at the transformation temperature. These both factors increase as the transformation temperature decreases, leading to thinner bainite plates [13].

Lee et al. [14] showed that the volume fraction of retained austenite initially increases with temperature, reaches a maximum, and then drastically decreases. A similar trend was reported by Putatunda [2]. During austempering process, austenite (γ) initially undergoes a transformation into bainitic ferrite (αB) and carbon-enriched austenite (γHC) (Eq. (1)). At longer holding times, this carbon-enriched austenite further decomposes into a mixture of bainitic ferrite and carbide (ε) (Eq. (2)) [15].γα+γHCγHCα+ε

The undercooling below the bainite start temperature (BS) is quite large at very low transformation temperatures, thereby providing a higher driving force for nucleation of bainitic ferrite. This in turn leads to the formation of a greater amount of bainitic ferrite. This is why only a little amount of austenite remains in microstructures obtained following transformation at lower temperatures. The degree of undercooling and hence the driving force for nucleation of bainite decrease as the temperature rises. Therefore, the amount of retained austenite increases gradually by increasing the transformation temperature. At very high transformation temperatures, carbon-enriched austenite decomposes into ferrite and carbide in accordance with Eq. (2), leading to a significant reduction in the amount of retained austenite [2], [14], [16].

The precipitation of carbides from austenite during bainite transformation leads to a remarkable deterioration in toughness of austempered steels [8]. About 1.5–2 wt.% silicon is generally added to these steels to retard carbide precipitation during austempering [17], [18]. However, it has been demonstrated that silicon retards bainite formation [19], [20]. Silicon also forms an oxide scale that impairs the response of steel to hot rolling [21]. Moreover, high silicon content has been shown [22] to deteriorate the galvanizability of steel. Majority of previous investigations related to carbide-free bainitic steels [2], [10], [23] have been conducted on alloys containing more than 1 wt.% silicon and only limited work [19], [24] has been devoted to alloys with a silicon content below 1 wt.%. The present study was aimed to explore the possibility of formation of the carbide-free bainitic microstructure in a high-carbon steel co-alloyed with 0.8 wt.% Si and 0.84 wt.% Al and examine the effects of austempering time and temperature on the microstructure and hardness of this steel.

Section snippets

Materials and methods

The alloy investigated in this work was prepared in a 20 kg medium frequency induction furnace with charge materials of clean mild steel scrap, petroleum coke, Fe–Cr, Fe–Si, Fe–Mn and Fe–Mo ferro-alloys and commercially pure aluminum (99.7%) and cobalt (99.3%). The steel was cast as a cylinder bar of 50 mm in diameter and 600 mm in length. The cast cylinder was electro slag remelted (ESR) to get clean steel. The chemical composition of the manufactured steel, measured after ESR process, is given

Phase characterization

The microstructure of the fully annealed steel is shown in Fig. 3. As it can be seen, a coarse-grained ferrite-pearlite microstructure has been obtained through full annealing.

Typical optical micrographs of samples austempered at different conditions are shown in Fig. 4. One can see that the microstructures consist of bainite (dark-etched areas) and retained austenite (white areas).

In order to insure that the white matrix in Fig. 4 was correctly identified as retained austenite, micro-hardness

Conclusions

Isothermal transformation of austenite to bainite in a high-carbon Al–Si alloyed steel was investigated and the following conclusions were drawn:

  • 1.

    The precipitation of carbides was retarded, and a mixed microstructure of bainitic ferrite and retained austenite was obtained. This indicates the potential of aluminum to suppress carbide formation during bainite transformation.

  • 2.

    No obvious variation in ferrite plate thickness was detected with changes in transformation temperature, while the amount of

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