The influences of Nb on both austenite grain growth and ferrite grain size are quite important for the study of the effects of Nb on phase transformation behaviour. The austenite grain growth as a function of temperature or time in steels with different Nb contents was studied, and the ferrite grain size after transformations was also studied. There are currently many grain size measurement techniques, e.g. linear intercept method, ASTM grain size chart comparison method, and EBSD. The linear intercept method is commonly used and easily operated, but it is not accurate enough, and it is inappropriate for multiphase samples. The ASTM chart comparison is also a commonly used method, but it is not accurate enough, and cannot show the actual grain size distribution. EBSD is accurate and works well for multiphase samples, however, each EBSD scanning can take quite a long time, and the scanning area is typically smaller than optical microscopy images.
In order to accurately study grain size, especially in a multiphase sample, a new method using a ‘grain boundary tracing technique’ has been developed in this research. It can be used to measure phase fraction, average grain size of a particular phase, and the grain size distribution from optical microscopy images. In this method, the scale bar from an optical microscopy image was firstly input in the Image Tool software (step 1). Then, phase boundaries of martensite or pearlite were drawn by black lines in Adobe Photoshop software, using a pen tablet called
“Wacom Bamboo Pen & Touch” (step 2), and all the martensite and pearlite regions were painted to black (step 3). After that, ferrite was selected using a function called
“threshold” in the Image Tool software (step 4), and the ferrite area fraction was measured using a function called “counting black/white pixels” (step 5). For grain size measurement, ferrite grain boundaries were drawn by black lines (step 6), and all ferrite grains except those which were at image edges and not fully displayed, were painted in white (step 7). Then, the white grains were highlighted using
“threshold” (step 8) and automatically selected in the Image Tool software (step 9).
The average grain size was then calculated by analysing all the selected objects (step 10). The ferrite grain size distribution was obtained by classification of all of the selected objects with different ranges of feret diameters (step 11). After that, grains
within different size ranges were automatically painted with different colours, and the number of grains within each size range was automatically counted (step 12). The flowchart of the procedure used is illustrated in Figure 3.11. Images illustrating the phase fraction measurement procedures, which were steps 1-5, are shown in Figure 3.12. Images illustrating the average grain size measurement, which were steps 6-10, are shown in Figure 3.13. Images illustrating the ferrite grain size distribution measurement, which were steps 11-12, are shown in Figure 3.14. From the Image Tool software, the feret diameter of a grain is calculated by:
( ) Equation 3.1 where A is the area of a grain measured from the image analysis software.
In this method, all ferrite grains in an image are involved in measurement, and other phases e.g. pearlite and martensite would not affect the ferrite grain size measurement. However, grains at the edge of an image were not counted, because their grain sizes are not fully measured. If these grains are involved in the grain size measurement, the resulted average grain size will be smaller than the true value.
This is an image analysis method based on typical optical microscopy images, and thus the sample preparation is quite easy and fast. Since the grain size of every ferrite grain can be obtained, the ferrite grain size distribution of a sample can also be plotted. Grains with different ranges of size are displayed by different colours, and thus the grain size distribution can be clearly observed. Since different phases in an image can be easily selected, the phase fractions of ferrite, pearlite, and martensite can also be measured simultaneously. Using the phase fraction measurement, the transformation progress can be accurately studied after interrupted isothermal transformations.
It is very difficult to measure the prior austenite grain size, because austenite does not exist at room temperature in these steels. If the steels are quenched to room temperature just after austenitisation treatment, austenite will transform to martensite, and the boundaries of martensitic regions will indicate the prior austenite grain
boundaries. However, it is very difficult to observe martensite grain boundaries of the steels after chemical etching. Neither 2% nital nor picral can clearly highlight martensite grain boundaries. In order to clearly observe prior austenite grain boundaries, thermal etching was utilised instead of chemical etching. Samples were firstly polished to get a flat surface. Then the samples were welded to a thermocouple and heated to the austenitisation temperature in the dilatometer. It is important that the polished surface is not touched. After a few minutes austenitisation, the samples were directly quenched to room temperature. Using the thermal etching, the prior austenite grain boundaries could be clearly observed on the polished surface, as shown in Figure 3.15.
Figure 3.11: Flowchart of the grain boundary tracing method to quantify phase
a b
c d
e f
Figure 3.12: Images illustrating the procedures to measure the phase fraction: (a) inputting the scale bar of an image into the software, which is step 1; (b) using the
pen tablet to draw grain boundaries, which is step 2; (c) and (d) painting of a particular phase, which is step 3; (e) using the “threshold” function to select the painted phase, which is step 4; and (f) calculating the phase fraction, which is step 5.
a b
c d
e
Figure 3.13: Images illustrating the procedures to measure the average ferrite grain size: (a) drawing black lines on ferrite grain boundaries, which is step 6; (b) painting of ferrite grains using a different colour, which is step 7; (c) highlighting of all ferrite grains except those at the edge of the image, which is step 8; (d) Selection of all the
highlighted ferrite grains, which is step 9; and (e) results of ferrite grain size measurements using the Image Tool software, which is step 10.
a
b
Figure 3.14: Images illustrating the procedures to measure the ferrite grain size distribution: (a) classification of grains by the range of feret diameter, which is step 11;
and (b) coloured grain image and the number of grains in each range, which is step 12.
Figure 3.15: A typical optical microscopy image for a thermally etched sample. The polished sample was austenitised at 1250°C for 30 seconds, and then quenched to room temperature. The prior austenite grain boundaries can be clearly observed.
4 Thermodynamic calculations
4.1 Introduction
Before doing heat treatments, the temperatures, times and cooling rates should be determined in order to determine the influence of the Nb on the transformation kinetics. In this research, thermodynamic calculations were utilised to determine phase boundary temperatures, and then heat treatment temperatures were determined. Both MTDATA and ThermoCalc are powerful tools for thermodynamic calculations. They are based on Gibbs free energy minimisation, thus they are typically used for calculation of the equilibrium state. However, with some additional codes, metastable equilibrium phases can also be analysed, and then CCT and TTT diagrams can be studied. CamModel which was developed by H.K.D.H. Bhadeshia, S.V. Parker et. al from Cambridge University around 15 years ago and has been modified many times during the last 10 years, was also utilised to predict the CCT and TTT diagrams of steels.