Grain Size characterisation of a steel sample using the AZtecHKL EBSD system

Introduction

Grain size is an important characteristic used in understanding the development, engineering and potential failure of steels. The mechanical and physical properties of metallic materials are often related to grain size e.g. via the Hall-Petch relationship where strength is inversely dependent on the square root of grain size [1]. Electron backscatter diffraction (EBSD) is an ideal technique for determining grain size; it offers microstructural characterisation including grain size, grain boundary characterisation and texture quantification.

In this example a galvanized high carbon steel wire is examined. This wire is used for manufacturing bridge cables, and therefore it requires high strength, plasticity and toughness coupled with good torsion performance. Improving torsion performance and achieving higher strength is a challenge and research has focused on how the microstructure of the raw steel rod materials influences the performance of the final cable.

To accurately measure grain size, it is imperative that all of the grain boundaries are detected. Therefore the technique used must produce the highest degree of grain boundary delineation. Traditionally, steel grain size is measured using light optical microscopy (LOM). This optical technique requires a chemical etching of the steel surface in order to highlight the grain boundaries. However, this etching is influenced by the existing pearlitic microstructure in the steel wire rod. This has a very fine lamellar microstructure with complicated grain boundaries, which make it difficult to clearly reveal and identify all the grain boundaries by etching.

In addition, as the trend is towards nano scale materials, there is a limit to the grain size which can be detected by LOM. Therefore, EBSD becomes the only viable alternative to measuring grain size in this steel wire rod.

EBSD measures crystallographic orientations. A grain is defined as a region that differs in crystallography from its surroundings but internally has little variation. A grain boundary is defined by the angle between the two regions on either side of the boundary segment. In this way the system creates a grain structure map, which identifies and characterises all grain boundaries accurately.

EBSD provides a fast and automated solution to directly measure grain size in the Scanning Electron Microscope (SEM). In addition, Oxford Instruments AZtec® platform enables data acquisition over large areas, so that a representative analysis can be achieved.

Data Acquisition

Two hot rolled steel wire rod samples were investigated. The in-use performance of steel cable generated from Sample 2 was superior to that of Sample 1; it had both higher strength and higher torsion. The aim of this work was to characterise the raw material, such that this difference in performance was both understood and predictable.

Sample cross-sections with thickness of 2.2 mm and diameter of 13 mm were cut, mounted and mechanically polished using standard metallographic techniques. These specimens were examined with AZtecHKL coupled with the NordlysMax² EBSD detector attached to a FEGSEM.

Initially, automated AZtec large area mapping was applied to collect low resolution EBSD data over a quarter circle (r = 6.5 mm) with 132 continuous fields, shown in Fig. 1. During large area mapping the individual fields are acquired and aligned automatically creating a montage data set which can be interrogated as a single site of interest.

The resulting large area image was registered in AZtec and used as a reference to relocate and define regions for higher resolution, more detailed mapping, at specific locations in the sample.

These higher resolution EBSD maps with step size of 0.2 µm were analysed to identify microstructural differences, both within the samples and between the samples.

Results

A grain map showing the grains randomly coloured is an effective way to illustrate the grain distribution. Fig. 2 shows a grain map of the quarter circle of Sample 1. This illustrates the microstructure from the centre to the edge. The grain size at the sample edge is smaller than in the centre. Therefore, regions from centre and edge were further examined at higher resolution, so that the grain size could be compared.

Fig. 1. Cross-section of steel wire rod with diameter of 13mm, sectioned surface was prepared for EBSD analysis. Large area mapping covered a quarter circle (r=6.5mm) with 132 individual fields. Montaged FSD image is overlaid on sample surface.

Fig. 3 shows these high resolution grain maps and corresponding grain size distribution histograms. The grain size was determined using a grain boundary threshold angle of 10°. Grains with equivalent circle diameter greater than 1 µm are measured and over 3000 grains were detected in each map. The average grain size is 5.9 µm from centre and 4.5 µm from edge; the average grain size in the centre is larger than that at the edge.

A comparable data set was collected from Sample 2, and a similar trend was identified (Fig. 4). Here the average grain size of 3.9 µm at sample edge is smaller than the average grain size of 4.7 µm in the centre. Overall, Sample 2 has a finer grain size than Sample 1. Although the difference in grain size measured between the two samples is relatively small, this may still in part account for the difference in performance.

These results help in understanding and controlling grain refinement processes which can be applied to improve the performance of these materials.

Fig. 2. A grain map of a quarter circle of Sample 1. White line indicates 2 mm.

Fig. 3. Grain maps and corresponding size distribution histogram from Sample 1. Average grain size is indicated at the top right corner.

Fig. 4. Grain maps and corresponding size distribution histogram from Sample 2.

Conclusion

Characterising and controlling grain size is important in controlling steel properties. However, as the material develops, the traditional methods for grain size measurements are not always suitable. The combination of SEM with EBSD is an ideal solution.

Oxford Instruments AZtecHKL EBSD system includes a Grain Analysis mode for fast and reliable analysis of grain size directly after data collection. In this example, the EBSD data offers an insight into the microstructural differences between two samples which exhibit different levels of mechanical performance. The material can then be better understood and optimised for the final application.

Reference

[1] Smith & Hashemi 2006, p. 242