Three-dimensional morphology of grain boundary Widmanstätten ferrite in a low carbon low alloy steel
Introduction
Widmanstätten ferrite grains are precipitates which grow directly from a prior austenite grain boundary and/or from allotriomorphic ferrite at the grain boundaries or from inclusions. Nearly fifty years ago, from many observations of microstructures using optical microscopy, Dubé [1] developed a classification system for the different morphologies of precipitates in steels. Aaronson [2] subsequently modified this classification system to distinguish between primary Widmanstätten precipitates and secondary Widmanstätten precipitates. In the past decades, many studies on the morphologies of Widmanstätten ferrite are conducted by means of two-dimensional (2D) observations.
With the development of computer science and automatic quantitative polishing, serial sectioning and computer-aided reconstruction and visualization is becoming an important characterization approach for microstructures. These techniques allow one to determine the overall shape, size and connectivity of microstructures in three dimensions [3]. The advantages of 3-D visualization of microstructures are reviewed recently by Kral and Spanos [3], [4]. The morphologies of precipitates, including cementite [3], [4], ferrite [5], [6], [9], [10] and Widmanstätten ferrite [5], [6] have been an actively pursued research topic in the last decades [1], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15].
The microstructural evolution and mechanical properties are strongly related with the nucleation sites, growth kinetics and morphologies of grain boundary ferrite precipitates. Although some progress on the three-dimensional morphologies of Widmanstätten ferrite has been made, further study still needs to clarify the microstructural features and connectivities. In the present work, three-dimensional observations and analyses are undertaken to further understand Widmanstätten ferrite morphological features in a low carbon low alloy steel.
Section snippets
Experimental
A low-carbon low alloy steel sample was prepared by vacuum induction melting utilizing high purity electrolytic iron, graphite and manganese. The chemical composition of the steel sample is shown in Table 1. A 50 kg ingot of the steel was hot rolled to a plate of 50 mm thickness at 1000 °C and subsequently, the slab was cold rolled by a reduction ratio of 70%. Steel specimens were then homogenized at 1250 °C for 24 h in a vacuum quartz capsule. Specimens of 10 × 10 × 0.35 mm3 were austenitized at 1250 °C
Primary Widmanstätten Sideplates
Fig. 1 shows optical micrographs and 3D-reconstructed images of Widmanstätten primary sideplates in the specimen isothermally reacted at 690 °C for 7 s. It is seen that primary Widmanstätten sideplates (see Fig. 1a) grow directly from the austenite grain boundary faces. They appear to be more like two primary sawteeth on another planar section (see Fig. 1b), as serial sectioning proceeded. Three-dimensional reconstruction indicates that primary sideplate 1 seems to be a wedge (Fig. 1c–d). It is
Microstructral Features of Grain Boundary Widmanstätten Ferrite
A grain boundary allotriomorph tends to nucleate on the austenite grain surfaces and to cover the surfaces, forming layers that follow the grain boundary contours. Unlike grain boundary allotriomorph, Widmanstätten ferrite does not grow along grain boundaries, it grows along well-defined planes of the austenite and towards austenite grain interiors. The primary Widmanstätten sideplate or sawtooth grows directly from the austenite boundary faces. It tends to widen its base along grain boundary
Summary
Three-dimensional morphology of ferrite grains nucleated at austenite grain boundaries were revealed by serial sectioning and computer-aided 3D reconstruction techniques in a low carbon low alloy steel. Conclusions drawn from the results are as follows:
- 1.
Three-dimensional shape and connectivity of grain boundary Widmanstätten ferrite are different from those deduced on random planar sections by two-dimensional microscopy.
- 2.
Primary and secondary Widmanstätten sideplates are better described to be
Acknowledgements
The authors express their thanks to Professor M. Enomoto, Ibaraki Univerity, for providing them with alloy specimen. The authors gratefully acknowledge the support from NSFC (National Natural Science Foundation of China) under the Grant No. 50471107 and No. 50734004.
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