Due to the limited availability of test material it was decided to restrict this study to Alloys 4 and 5, with approximately 50% cold work, and aged at 7 00°C, only. This
ageing temperature was chosen because of the commercially important thermal treatment (16 hours at 700°C) following mill annealing.
Table 30 presents the tensile results for material solution treated, cold worked by 50% and thermally aged at 700°C for 1 to 100 hours. It can be seen that both alloys undergo a significant reduction in tensile properties during ageing at 700°C. As discussed in section 4.5.1 these alloys are undergoing recrystallisation and carbide precipitation. The results show that alloy 5 begins and completes recrystallisation before alloy 4 for reasons that will be discussed later. These tensile results support the hardness data previously discussed and clearly reveal the effect of recrystallisation on mechanical property levels. 4.5.3 Recrystallisation Kinetics
Thermal ageing caused all four conditions, namely alloys 4 and 5 cold rolled to approximately 25 and 50% reduction in thickness, to recrystallise. The combination of ageing time and temperature which leads to recrystallisation depends upon the level of internal strain (from the cold work) as well as the grain size and microstructure of the material. Hence it is to be expected that material with
50% cold work will recrystallise before that with 25% cold work. The different rate of recrystallisation between alloy 4 and alloy 5, for the same level of cold work, is a function of the grain size, microstructure and composition.
Examination of figures 56 to 61 reveal an order of recrystallisation, with the alloy condition which recrystallises most readily, as; alloy 5-50% cold reduction, alloy 4-50% cold reduction, alloy 5-25% cold reduction and alloy 4-25% cold reduction. The values of cold reduction are approximate, but the increased tendency for alloy 5 to recrystallise before alloy 4 is evident. Table 20 gives the actual % cold reductions for each alloy and the as-rolled hardness values. Tables 3 and 5 give the grain sizes for each alloy prior to cold rolling, with alloy 4 at 81 microns and alloy 5 at 94 microns M.L.I.. These grain sizes are very similar and indicate that this would not be a significant factor in determining the order of recrystallisation. Figures 54 and 55 show that the two alloys work harden at slightly different rates, with alloy 4 attaining a higher final hardness value. Section 4.5.1 indicates that this is related to the alloy composition with the higher hardness occurring for the higher carbon
content.
It can be surmised that alloy 4 shows a greater resistance to recrystallisation than alloy 5 for equivalent levels of cold reduction. It is suggested that this effect may be partly due to the titanium containing alloy (alloy 5) having a distribution of stable TiC/TiN precipitates which generate dislocations during cold working and accelerate subsequent recrystallisation during ageing. These effects will be discussed later.
and 5 in the fully recrystallised condition following ageing. The presence of distinct precipitates marking ’ghost’ boundaries of deformed grains overlaid on a fully recrystallised structure can be seen. There is little evidence of grain boundary precipitation on the
recrystallised boundaries, which indicates that
recrystallisation occurs below the carbide solvus temperature. This observation is important because of the effect of precipitate location and distribution on stress corrosion cracking. Such a structure has previously been observed as offering a low resistance to SCC (61). The relationship between cold work, thermal ageing and SCC will be discussed later.
4.5.4 Precipitation of Carbides
As for section 4.3.2 a study of the onset of precipitation can only effectively occur if the alloy is initially free from precipitates prior to ageing. This condition was observed for alloys 4 and 5 with both levels of cold reduction. The onset of precipitation was studied by optical and electron microscopy, with the latter technique being used to determine the crystal structure of the precipitates.
Figures 63 and 64 show the time-temperature-precipitation C-Curves for the cold rolled and aged alloys 4 and 5. In all cases the precipitates were identified as M23C6 chromium carbides with the onset of precipitation occurring at grain boundaries.
Figure 63 for alloy 4 shows only a slightly increased tendency for precipitation as the level of cold reduction increases from 25 to 50%. Similarly figure 64 for alloy 5 indicates very little change in the precipitation rate or domain as a result of increasing the level of cold reduction. However, comparison with the C-curves for no cold work indicates significant increases in the precipitation rate and domain as the level of cold work is increased from 0 to 50%.
These results suggest that precipitation is occurring both during and after recrystallisation. If full recrystallisation occurred before precipitation then cold work would not affect its rate or temperature domain. However it is clear that cold work does influence precipitation and so both reactions must be occurring
simultaneously. This effect will be discussed later. 4.6 Corrosion Testing of Experimental Alloys
To conclude the experimental studies a corrosion test programme was undertaken to study the effects of various thermal and mechanical treatments on the corrosion behaviour. Analysis of the corrosion test results reveals trends and effects related to all the preceding treatments and additionally to the effect of localised post ageing cold deformation (ie bent samples).