Grain characterisation of a steel wire

Introduction

The mechanical and physical properties of metallic materials are closely related to grain size e.g. through the Hall-Petch relationship, where strength is inversely dependent to the square root of grain size [1]. Electron backscatter diffraction (EBSD) on a Scanning Electron Microscope (SEM) is the ideal technique for determining grain size, illustrated here with a galvanized steel wire. This steel is used to manufacture bridge cables, and the importance of grain size and microstructure has been recognised as it influences the critical performance of the final steel cable.

Method

To accurately measure grain size, all of the grain boundaries must be detected. EBSD detects grain boundaries by measuring crystallographic orientations, overcoming these limitations and enabling an accurate determination of grain size. In addition, the EBSD data set is mined to get a thorough overview of the steel microstructure.

Steel Wire Sample

A cross-section (diameter 13 mm) was examined with AZtecHKL coupled with the NordlysMax3 EBSD detector. Data were collected automatically from 132 continuous fields over a quarter circle (radius 6.5 mm). The individual fields were acquired and aligned automatically, creating a montaged data set which was interrogated as a single site of interest. More detailed analysis was also conducted from individual fields.

Results

AZtec Data Analysis was applied to characterise the full microstructure of the steel sample.

The figures below illustrate some of the possible data collected. An overview of the microstructure of the steel is shown in two montaged large area maps of the quarter sample. Fig. 1 shows the grain structure and Fig. 2 grain orientation. The grain structure is examined in more detail in the individual fields and size distribution histograms (Fig. 3 and 4) taken from the centre and edge of the sample respectively. In these grain maps there is a distinct decrease in grain size from the centre to the edge of the sample. Grain size was determined using a grain boundary threshold angle of 10° and over 3000 grains were detected in each map. The average grain size is 5.85 μm from centre and 4.53 μm from edge.

The difference in microstructure is further illustrated looking at the distribution of grain boundaries (Fig. 5 and 6), with a higher proportion of lower angle boundaries at the sample edge.

Fig 1. Montaged map of grain structure.

Fig. 2. Montaged orientation map.

Summary

Characterising and controlling the grain size is important in controlling material properties. The combination of SEM with AZtec EBSD offers an ideal solution for this application. Here a wealth of data is extracted and visualised with ease, and readily compared from more than one site of interest.

In this study it was found that samples with better torsion performance had a finer grain size. These results help to understand and control the grain refinement process which can be applied to improve the performance of these materials.

Reference [1] A. P. Day and T. E. Quested, Journal of Microscopy 195 (1999) 186.