Austenitising Temperature
The addition of niobium to the base vanadium steel (alloy 671) resulted in a slight increase in the ideal critical diameter, see table 24 and figure 35. The niobium, being a strong carbide/nitride former, preferentially combined with the carbon and nitrogen to precipitate niobium
carbide and niobium nitride. Consequently, more vanadium was taken into solution and influenced the hardenability, see table 25. The increased vanadium in solution
together with the additional niobium in solution would be expected to increase the hardenability despite the
reduced quantity of nitrogen in solution. However, the increase in hardenability was small due to the change in the precipitates present at the 950°C austenitising
temperature. The addition of niobium reduced the amount of vanadium nitride to a very low level but caused
additional amounts of both niobium carbide and niobium nitride. The removal of the grain boundary pinning
effects of the vanadium nitride precipitates would be to some extent compensated by the presence of niobium
nitride precipitates. However, the niobium carbide precipitates, being coarser, could act as preferred
nucleation sites for the transformation (2) and therefore result in a lower hardenability than expected.
The addition of niobium and extra nitrogen to the base vanadium steel (alloy 668) resulted in a slight decrease
in the hardenability when compared with both the base vanadium alloy (623) and the vanadium-niobium-low
nitrogen alloy (671), see table 24 and figure 35. The decrease in hardenability when compared with the base vanadium alloy was unexpected if the microalloying elements in solution are considered, see table 25. It would have been expected that the positive effects of the increased vanadium and niobium in solution would have more than offset the negative effects of the reduced quantity of nitrogen in solution. Consequently, the
observed slight reduction in hardenability could be associated with the precipitates present at the
austenitising temperature. The addition of niobium and extra nitrogen resulted in approximately the same
quantity of vanadium nitrides present but additional quantities of both niobium nitride and niobium carbide. The extra precipitation would have been expected to help pin the austenite grain boundaries and increase the
hardenability by allowing segregation of the
microalloying elements to the grain boundaries to become more effective. However, the larger precipitates,
especially the niobium carbides, would be expected to be less effective at pinning grain boundaries but more
effective at providing preferred nucleation sites for the transformation and could be responsible for the observed slight decrease in hardenability.
The decrease in hardenability at the higher nitrogen content when comparing the two niobium alloys can be explained in terms of the negative effect of the reduced vanadium in solution offsetting the positive effect of the slight increase in the nitrogen in solution, see
table 25. However, the hardenability must also have been effected by the change in the combination of precipitates present at the austenitising temperature. The extra
nitrogen in the vanadium-niobium-high nitrogen alloy (669) resulted in increased quantities of both vanadium
and niobium nitrides but a reduced quantity of niobium carbides. The wider dispersion of the precipitates would be expected to more effectively pin the austenite grain boundaries and increase the hardenability while the reduction in the number of large niobium carbide
precipitates present would tend to reduce the number of preferred nucleation sites for the transformation and also cause an increase in the hardenability. However, the vanadium and niobium nitrides would be larger in the high nitrogen alloy and would therefore be less effective at pinning grain boundaries and could therefore
contribute to the observed decrease in the ideal critical diameter.
5.1.3.4.2 Hardenability Results Using a 1200°C Austenitising Temperature
At the higher austenitising temperature of 1200°C the addition of niobium to the base vanadium steel (alloy 671) resulted in a decrease in the ideal critical diameter, see table 26 and figure 36. Comparing the theoretical quantities of microalloying elements in solution it can be seen from table 27 that the niobium alloy contained an increased amount of both vanadium and niobium in solution but a decreased quantity of nitrogen in solution. At 1200°C an increase in vanadium would be expected to decrease the hardenability of the alloy by forming a vanadium cluster with a reduced rate of
the moving austenite grain boundaries.
From the work of Eldis et al (7), and the results
obtained in section 5.1.2.4.2, it is possible that at low niobium levels the niobium in solution combines with the nitrogen and/or carbon to form extra grain boundary
clusters which increase the hardenability. However, at higher niobium concentrations the niobium clusters would diffuse at a slower rate, and may not be able to move with the migrating austenite grain boundaries resulting
in a reduced effect on the ideal critical diameter of the steel. The reduction in the quantity of nitrogen in
solution would tend to form clusters which were more carbon concentrated and would therefore result in the clusters having a higher rate of diffusion. Consequently the reduction in nitrogen would be expected to increase the hardenability of the alloy. However, even at 1200°C some precipitates would still be present and influence the ideal critical diameter. The precipitates present in the niobium steel (671) would mainly consist of large niobium carbides which would be relatively ineffective as a grain boundary pinning agent but would act as preferred nucleation sites for transformation and would therefore be expected to reduce the hardenability.
The addition of niobium and extra nitrogen to a base vanadium steel (alloy 668) resulted in a very slight
decrease in hardenability when compared with the base alloy (623) but an increase when compared with the
vanadium-niobium-low nitrogen alloy (671), see table 26 and figure 36. The decrease in hardenability when
compared with the base vanadium alloy was associated with an increase in the quantities of all the microalloying elements vanadium, niobium and nitrogen in solution, see table 27. Consequently, it would be expected that the increase in all the three microalloying elements would have caused a decrease in the hardenability since each would have resulted in an decrease in the rate of
diffusion of the clusters. However, the niobium alloy (668) contained undissolved niobium carbide and nitride precipitates even at the 1200°C austenitising
temperature. The finer niobium nitrides would tend to pin the grain boundaries but due to their wide dispersion they would be expected to be relatively ineffective.
Also the larger niobium carbides would tend to act as preferred sites for the nucleation of the transformation and could therefore contribute to the decrease in the ideal critical diameter.
Considering the two niobium alloys it can be seen from table 27, that the only change in the microalloying elements in solution was an increase in the quantity of nitrogen. Since, the extra nitrogen would be expected to form clusters which would diffuse more slowly it would be more difficult to migrate with the austenite grain
boundaries and would therefore be expected to result in a decrease in the hardenability. However, it must also be noted that the extra nitrogen produced an additional small quantity of niobium nitride. These precipitates would be smaller than the niobium carbides and therefore be more effective at pinning grain boundaries. Thus they could contribute to the observed increase in the
hardenability by allowing more clusters to segregate to the boundaries.
5.1.3.5 Medium Carbon-Vanadium-Titanium Alloys