From thermodynamic calculations, all niobium precipitates should have been dissolved at 1250ºC. The accurate study of the effect of solute Nb atoms on austenite grain growth is quite important for both scientific and industrial purposes.
However, it is difficult to characterise the austenite grain size at 1250ºC. Therefore, samples were quenched to room temperature at 100ºCs-1 after an austenitisation heat treatment, and the samples directly transformed from austenite to martensite, and thus the prior austenite grain boundary can be seen at room temperature. Even so, the prior austenite grain boundaries are difficult to reveal using chemical etching, therefore, thermal etching was utilised in this research.
There are a lot of methods for grain size measurement, e.g. the linear intercept images of steel 1 after 5 seconds, 30 seconds and 300 seconds holding at 1250ºC were obtained to get a clear overview of the grain size distribution, and a typical image for each sample is shown in Figure 5.7. The grain size distribution of steel 1 after different austenitisation times at 1250°C is plotted in Figure 5.8. From Figures 5.7 and 5.8, it can be found that the prior austenite grain size distribution becomes wider with increasing austenitisation time. After 300 seconds austenitisation, the grain size distribution becomes much wider than that with 5 seconds austenitisation, and the average grain size is significantly larger. Typical coloured grain size images of steels 2-4 with various austenitisation times at 1250°C are shown in Figure 5.9, Figure 5.11 and Figure 5.13, respectively. Grain size distribution curves of steels 2-4 with various austenitisation times are plotted in Figure 5.10, Figure 5.12 and Figure 5.14, respectively. The average prior austenite grain sizes for steels 1-4 as a
function of austenitisation time at 1250°C are plotted in Figure 5.15 and listed in Table 5.1.
From Figures 5.7 to 5.14, it can be found that the austenite grains become larger with increasing holding time at 1250°C for all these steels. For each steel, the sample with the longest austenitisation time has the widest grain size distribution.
From Figure 5.15, it can be seen that steel 3 has the smallest average grain size when the austenitisation time is longer than 1 minute. In addition, steel 3 also has the slowest prior austenite grain growth rate in steels 1-4. Steels 1-3 have similar initial conditions, and the Nb content is the only difference, therefore it can be concluded that Nb has a retardation effect on the prior austenite grain growth at 1250°C. From thermodynamic calculations, all niobium-rich precipitates are expected to be dissolved above 1150ºC. This has been investigated by TEM observation using carbon extraction replicas, which showed that there were no Nb(C,N) precipitates in the samples quenched from 1250ºC. Therefore, it is proposed that the observed difference in prior austenite grain size is caused by solute niobium atoms. From Figure 5.15, steel 4 has a faster prior austenite grain growth than steel 3, and thus it can be found that carbon has an accelerated effect on prior austenite grain growth.
From Figure 5.15, it can also be found that 5 seconds austenitisation at 1250°C for steel 1, 60 seconds austenitisation for steel 2, 300 seconds austenitisation for steel 3, and 150 seconds austenitisation for steel 4 all result in an average prior austenite grain size of ~80µm. Therefore, the prior austenite grain size can be adjusted to the same value by careful choice of the initial heat treatment to avoid its effect on the subsequent transformation kinetics.
5 seconds 30 seconds
300 seconds
Figure 5.7: Grain size characterisation for steel 1 after 5 seconds, 30 seconds, and 300 seconds holding at 1250ºC. Different colours of grains indicate the grain size ranges
(unit: µm).
Figure 5.8: Prior austenite grain size distribution of steel 1 with different
austenitisation times at 1250°C and subsequent quenching to room temperature, with prior austenite grain boundaries revealed by thermal etching.
0 0.05 0.1 0.15 0.2 0.25 0.3
10 30 50 70 90 110 130 150 170 190 210 230 250 270 290 355 370
Number fraction of grains
Grain size (µm)
1-1250°C5s 1-1250°C30s 1-1250°C300s
30 seconds 60 seconds
300 seconds
Figure 5.9: Grain size characterisation for steel 2 after 30 seconds, 60 seconds, and 300 seconds holding at 1250ºC. Different colours of grains indicate the grain size
ranges (unit: µm).
Figure 5.10: Prior austenite grain size distribution of steel 2 with different austenitisation times at 1250°C and subsequent quenching to room temperature, with
prior austenite grain boundaries revealed by thermal etching.
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
10 30 50 70 90 110 130 150 170 190 210 240 250 270
Number fraction of grains
Grain size (µm)
2-1250°C30s 2-1250°C60s 2-1250°C300s
30 seconds 60 seconds
300 seconds
Figure 5.11: Grain size characterisation for steel 3 after 30 seconds, 60 seconds, and 300 seconds holding at 1250ºC. Different colours of grains indicate the grain size
ranges (unit: µm).
Figure 5.12: Prior austenite grain size distribution of steel 3 with different austenitisation times at 1250°C and subsequent quenching to room temperature, with
prior austenite grain boundaries revealed by thermal etching.
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45
10 30 50 70 90 110 130 150 170 190 210
Number fraction of grains
Grain size (µm)
3-1250°C30s 3-1250°C60s 3-1250°C300s
60 seconds 150 seconds
300 seconds
Figure 5.13: Grain size characterisation for steel 4 after 60 seconds, 150 seconds, and 300 seconds holding at 1250ºC. Different colours of grains indicate the grain size
ranges (unit: µm).
Figure 5.14: Prior austenite grain size distribution of steel 4 with different austenitisation times at 1250°C and subsequent quenching to room temperature, with
prior austenite grain boundaries revealed by thermal etching.
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
10 40 50 70 90 110 130 150 170 190 210 230
Number fraction of grains
Grain size (µm)
4-1250°C60s 4-1250°C150s 4-1250°C300s
Figure 5.15: Average prior austenite grain size for steels 1-4 as a function of austenitisation time at 1250°C. The error bar indicates the standard deviation of all the
measured grains in the sample.
Table 5.1: Average prior austenite grain size after various austenitisation times at 1250°C for steels 1-4 (μm). The error bar indicates the standard deviation of all the measured grains in the sample.
Holding times at
1250°C (s) Steel 1 Steel 2 Steel 3 Steel 4
30 102.6+41.4 60.9+22.3 54.6+21.6 47.8+21.2
60 110.2+48.6 76.7+30.4 61.9+23.9 65.1+26.1
300 137.6+60.3 100.8+38.7 80.5+33.8 91.4+33.3