Morphology and three-dimensional structure of ferrite formed below the bay in an Fe–C–W alloy
Introduction
Widmanstatten ferrite morphologies termed “degenerate” were first found in plain carbon steels, as reported by Aaronson [1]. Similarly-named morphologies have since been observed in alloy steels, as studies of Fe–C–X alloys (where X=Mo, Cr, Mn, and W) have shown [2], [3], [4], [5], [6]. In general, the term “degenerate” has been given to morphologies which are irregularly shaped counterparts of those normally encountered. Hence, “degenerate” encompasses not a single morphology but instead a group of morphologies that can vary widely depending on alloy and heat treatment. Overall, degenerate ferrite connotes an image of a random and ill-defined microstructure with no obvious deference to crystallographic constraints.
Explanations have been proposed to account for these morphologies from both the diffusional [7], [8] and displacive [9], [10] viewpoints. Prior studies in this regard have used 2D imaging techniques such as optical microscopy [11] and TEM [2], [5], [6], [9]. Given the apparent complexity of such morphologies, 3D microstructural analysis techniques would be important tools for analyzing this, but so far, no studies have been reported on degenerate structures. This paper reports the 3D morphology of apparently degenerate ferrite formed in a well-characterized Fe–C–W alloy [12]. Of specific interest are the connectivity and preferred growth directions of ferrite (if any).
Section snippets
Alloy and heat treatment
A high-purity ternary Fe–C–W steel was vacuum induction-melted and hot-rolled to 2.54 cm thick plate. A square bar was cut and homogenized 3 days at 1200 °C while sealed in a quartz tube under a partial pressure of argon. The alloy composition was found to be Fe–0.30C–6.3W (wt.%) by chemical analysis. A specimen was cut from this and re-encapsulated under argon prior to heat treating as follows: 1200 °C-30 min austenitization, followed by 590 °C-10 min isothermal treatment in a lead bath, ended
Serial sectioning and three-dimensional reconstruction
The reconstruction of the 3D microstructure in a computer workstation is a modification of the technique developed by Shiflet and Coworkers [13], [14], [15] that allows 3D reconstruction of randomly curved boundaries. A total of 81 fine-scale serial sections were obtained using an automatic polisher with 1.0 μm alumina. Vickers microhardness indents were placed in the martensitic regions near the degenerate ferrite area of interest; these would later be used to register the slices [13]. The
Results and discussion
Immediately upon crossing the bay temperature at 600 °C, the morphology of bainite changes drastically from grain boundary bainite and twin boundary bainite [12] to a form of `degenerate' bainite similar to that seen in Fe–C–Mo [3]. Fig. 1 documents the size, shape and distribution of the bainite product which formed at this heat treatment. The lower magnification view (Fig. 1a) shows that the bainite is no longer confined to the grain boundary region (as it is at and above the bay); in fact,
Conclusions
The technique of 3D reconstruction of 2D serial sections of so-called “degenerate” ferrite formed below the bay in Fe–0.3C–6.3W demonstrated the misconceptions that can be associated with microstructural interpretation from 2D images. Ferrite subunits that appear roughly equiaxed or `popcorn' like on 2D sections are actually arranged in groups of branching ferrite rods, which show much more deference to crystallographic constraints than previously thought.
Acknowledgements
Funding was provided by the National Science Foundation (grant DMR-9904034) and further support for the preparation of this paper for REH was provided by the US Department of Energy (LDRD grant funded under contract W-7405-ENG-36). Bethlehem Steel is thanked for providing the alloy used in this study.
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Present address: Materials Science and Technology Division, MS G770, Los Alamos National Laboratory, Los Alamos, NM 87545, USA.