New insights into the austenitization process of low-alloyed hypereutectoid steels: Nucleation analysis of strain-induced austenite formation
Introduction
The topic of combined deformation and phase transformation requires a fundamental understanding of the influence of plastic deformation on microstructure, which eventually governs the properties of materials [1], [2], [3], [4]. In metallic systems, strain-induced phase transformation from austenite (γ) to ferrite (α) has been widely employed to refine ferrite grains in low carbon steels, which is considered as a promising approach for increasing strength without sacrificing toughness [5], [6], [7], [8], [9]. Prior to such strain-induced transformation, a reverse transformation process involving ferrite (α) and/or carbides to form austenite (γ), namely, austenitization is indispensable. The austenitization mechanism has been extensively studied, while emphasizing mainly the effects of the initial microstructures [10], [11], [12], [13], [14], [15], [16], [17] and alloying elements [12], [15], [18]. In our recent study [19], the strain-induced austenite formation below the equilibrium austenite–pearlite transformation temperature, Ae1, was observed during superplastic tensile testing of an Al-alloyed multicomponent Mn–Si–Cr–C steel with an initial microstructure comprising ferrite plus spherical carbides (generally austenitization occurs above Ae1). This result opens up a new microstructure design pathway, that is, relatively fine austenite grains can be obtained more easily at relatively lower annealing or ambient deformation temperatures (e.g. below Ae1). In this context, the mechanism of the strain-induced austenite formation below Ae1 needs to be better understood.
There is considerable evidence in the literature suggesting that the cementite particles could act as potential nucleation sites for austenite formation [11], [12], [13]. In highly alloyed steels, austenite nucleation was found to be sluggish due to the presence of stable complex carbides while the growth of austenite was rapid [20]. These results indicate a substantial effect of carbides, specifically their alloy-dependent stability and dissolution rates on austenite formation. Apart from carbides, other microstructural factors such as grain size and defect density may also affect austenite formation. Therefore, the objective of this study is to understand the synergistic influences of carbide size, grain size of the ferrite matrix and defect density on austenite nucleation. To study these questions we design two different initial microstructures using a hypereutectoid steel, both consisting of ferrite and spherical carbides. The differences between these microstructures lie in the size distribution of the carbides, ferrite grain size and dislocation density. The presence of a sufficient volume fraction of carbides (∼10 vol.%) in these two microstructures allows us to investigate the influence of carbide on austenite nucleation. Specifically, we focus here on the austenite nucleation conditions during warm deformation below Ae1 by combining thermodynamic calculations and microscopic observations. Based on this, we further reveal the mutual influence of initial microstructure and warm deformation on austenite nucleation. However, we do not follow the growth process of austenite in detail since austenite growth is affected by multiple further factors which are not pursued here (e.g. pinning effect of carbides, diffusion of alloying elements etc.).
Section snippets
Materials and processing
The alloy used in this study is an Al-alloyed multicomponent hypereutectoid steel with a nominal composition of Fe–5.6C–2.2Mn–3.3Si–1.1Cr–1.9Al (at.%). The Ae1 temperature (1033 K) was calculated by ThermoCalc using the TCFE5 database [21], [22]. The cast steel was first hot-rolled at 1323 K, and subsequently air-cooled to room temperature. The hot-rolled plates were then annealed at 1173 K for 60 min followed by water quenching to room temperature. The final warm rolling was performed at two
Ferrite plus carbide initial microstructures
Fig. 1a and b shows the phase plus IQ EBSD maps of the two initial microstructures obtained after warm rolling at 1073 K and 923 K, consisting of both ferrite (green color) and spherical carbides (blue color). The ferrite grains in the coarse-grained microstructure (CG; Fig. 1a) are larger than those in the fine-grained variant (FG; Fig. 1b). The average ferrite grain size in CG and FG are ∼1.7 μm and 1.0 μm, respectively. From the corresponding SE images in Fig. 1c and d, it is evident that the FG
Accelerated growth of ferrite grains and Ostwald ripening of carbides
During isothermal annealing at 973 K, the ferrite grains grow very slowly. Even after 3 h, the average ferrite grain size of the CG material (∼1.7 μm, Fig. 2c) remains close to its initial state (∼1.7 μm, Fig. 2a). In contrast, the ferrite grain size increases up to ∼4.0 μm (Fig. 5b) after 250% strain during tensile testing at 973 K (even though the deformation period was only ∼20 min). It indicates that warm deformation promotes ferrite grain growth in the CG variant below Ae1. In addition, the size
Conclusions
Two ferrite plus carbide microstructures (fine-grained and coarse-grained) were used for isothermal annealing and tensile testing at 973 K (60 K below Ae1) and 2 × 10−3 s−1. During isothermal annealing, the dislocation density decreases and ferrite and carbide coarsen in both the materials. Austenite formation was not observed, even after 3 h annealing period. However, during tensile testing, strain-induced austenite formation unexpectedly occurs below Ae1 in the fine-grained material. The warm
Acknowledgments
The authors are grateful for the kind support of the Alexander von Humboldt Stiftung (AvH, Alexander von Humboldt Foundation, www.humboldtfoundation.de).
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