EBSD and AFM observations of the microstructural changes induced by low temperature plasma carburising on AISI 316
Highlights
► The microstructure of AISI 316 samples was observed before and after plasma carburising treatments. ► The changes in the microstructure were analysed by OM, AFM, and SEM with EBSD. ► The residual stresses developed during plasma carburising introduced microstructural changes in the treated layer. ► The effects of these changes on mechanical properties, tribological performance and corrosion resistance are discussed.
Introduction
Low temperature diffusion treatments of austenitic stainless steels with carbon and/or nitrogen have been studied at length. The diffusion layers produced by these processes typically consist of supersaturated austenite with carbon and/or nitrogen, and it is usually referred to as expanded austenite or S-phase [1]. The interest in expanded austenite is mainly the result of the significant improvement in hardness, tribological properties [2], combined with excellent corrosion resistance [3], and increased fatigue limit [4]. Due to this attractive combination of properties, expanded austenite or S-phase has been recognised as one the most significant recent developments in stainless steels [5].
The outstanding tribological and corrosion performance of expanded austenite has drawn the attention of many researchers. They have characterised the novel S-phase layers using various techniques, including X-ray diffraction [6], transmission electron microscopy [7], glow discharge optical emission spectroscopy [8], as well as atomic force microscopy (AFM) [9] and backscattered electron diffraction (EBSD) [10].
EBSD has the capability to provide statistically valid crystallographic information in a mesoscale, including crystal orientation maps, phase distribution, grain size, crystallographic texture, grain boundary analysis, etc. These microstructural features have a significant impact on the properties of materials and the evidence produced by EBSD have greatly contributed to the studies on corrosion [11], wear [12] and anisotropic diffusion [13].
Recent EBSD studies have provided valuable evidence on the lattice rotation induced by low temperature plasma nitriding on austenitic stainless steel [14], [15]. In addition, AFM has been successfully used to characterise deformation morphologies in AISI 316 [16]. It is also expected that the use of EBSD in conjunction with AFM can unveil complimentary microstructural evidence for advancing scientific understanding of the formation of S-phase during LTPC of austenitic stainless steel. Therefore, the intention of this work was to investigate the microstructural changes induced by low temperature plasma carburising (LTPC) on austenitic stainless steel substrates by means of both EBSD and AFM, and to discuss their effects on the properties of the S-phase layers.
Section snippets
Material and methods
Coupon samples, 10 mm in diameter and 2 mm in thickness, were cut from a hot rolled bar of AISI 316 of composition as listed in Table 1. The samples were wet ground with SiC emery paper up to grit #1200 and polished with diamond paste to 1 μm, followed by a final polishing step with colloidal silica. A set of Vickers micro hardness indentations, made on one side of the sample, were used to align the sample and relocate the position for subsequent EBSD and AFM observations on the same area.
Polished
Results
LTPC produced evident changes in the microstructure of austenitic stainless steel specimens, as it is illustrated by the optical micrographs shown in Fig. 1. The microstructure of the untreated material was revealed by chemical etching, whereas the picture after plasma carburising was taken in the as-treated condition, i.e. as the specimen came out of the plasma furnace.
The carburised sample shows typical signs of a deformation microstructure (Fig. 1b). Slip bands completely cross grains in
Plastic deformation and anisotropic hardening
One clear consequence of the supersaturation of austenite with carbon is the expansion of the unit cell, and the consequent development of high compressive residual stresses. For example, the residual stress in gas carburised AISI 316 has been reported to be higher than 2 GPa [19]. The stress clearly exceeds the yield strength of the material, leading to plastic deformation in the form of slip bands.
It has been found that the local misorientation obtained from EBSD COMs is sensitive to the
Conclusions
From the present study on the microstructural changes produced by low temperature plasma carburising on AISI 316, the following conclusions are drawn:
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The microstructure of AISI 316 evolves during LTPC through plastic deformation in favourably oriented grains, while unfavourably oriented grains undergo lattice rotation and severe distortion.
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Deformation studies from the pattern quality of EBSD COMs provided new evidence on the anisotropic hardening of LTPC AISI 316 through a strain hardening
Acknowledgements
The financial contribution made by Roberto Rocca Education Program and The University of Birmingham to this project are acknowledged.
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