CONCLUSIONES Y RECOMENDACIONES

Austenitising Temperature

The addition of molybdenum to a base vanadium steel

(alloy 640) resulted in an increase in the ideal critical 124

diameter value, see table 19 and figure 32. Considering the amounts of microalloying elements in solution, see table 20, the vanadium and nitrogen in solution for the two alloys were very similar and could be ignored.

Consequently, the increase in hardenability must have been directly related to the amount of molybdenum in

solution. Calculation of the hardenability multiplying factor for molybdenum resulted in a figure of 1.15 for a 0.10% molybdenum addition. The average factor for

molybdenum, according to Mangonon (9) is 1.12 when added singly but increases to 1.22 for a 0.10% molybdenum

addition when in combination with vanadium. Therefore, there is good agreement between the present study and other published work.

The addition of molybdenum and extra nitrogen to a base vanadium steel (alloy 641) resulted in an increase in the ideal critical diameter value when compared with the base vanadium alloy (612) but a decrease when compared with the molybdenum-low nitrogen alloy (640), see table 19 and figure 32. Considering the microalloying elements in solution the observed increase in hardenability when compared with the base vanadium alloy must be associated with the molybdenum in solution at the austenitising temperature. It can be seen that all the molybdenum was taken into solution and was therefore available to

increase the hardenability of the alloy. However, the extra nitrogen caused increased vanadium nitride

precipitation and thus less vanadium in solution to influence the ideal critical diameter value. Both the increased precipitation and the reduction in the vanadium in solution would tend to reduce the hardenability rather than cause the observed increase.

The decrease in the ideal critical diameter when compared with the molybdenum-low nitrogen alloy was associated with the combined effect of a reduced quantity of vanadium in solution and an increase in the amount of precipitation. It can be seen from table 20 that there was no change in the amount of molybdenum in solution, and only a slight increase in nitrogen in solution, which could therefore be ignored.

5.1.2.3.2 The Effect of Extended Times at a 950°C Austenitising Temperature

To investigate the kinetic effects in vanadium-molybdenum alloys, additional jominy end quench tests were performed on the vanadium-molybdenum-high nitrogen alloy (641)

after austenitising at 950°C for 4 and 8 hours

respectively. The results presented in table 21 and figure 33 show a high initial hardenability at short austenitising periods which gradually increased with extended times at temperature until a maximum

hardenability was achieved. The high hardenability

observed at the short austenitising periods was caused by 126

the molybdenum rapidly entering solution and therefore being available to inhibit the transformation. Also it has been suggested (123) that the presence of molybdenum increases the solubility of vanadium in an alloy. The resulting extra microalloying elements in solution would also tend to increase the hardenability of the alloy. The gradual increase in the ideal critical diameter value with extended austenitising times was believed to be due to the kinetics of solution of the vanadium nitride

precipitates. Comparison of the curves for the vanadium- molybdenum-high nitrogen (641) and the vanadium-high

nitrogen alloy (619) shows that the rate of increase of hardenability of the molybdenum alloy was higher than that of the base alloy. This indicates that the

solubility of the vanadium nitride precipitates, or possibly the rate of segregation of the microalloying elements to the grain boundaries, is enhanced by the presence of molybdenum in solution and would therefore support the observations of Watanabe (123).

5.1.2.3.3 Hardenability Results Using a 1200°C Austenitising Temperature

The addition of molybdenum to a base vanadium steel (alloy 640) caused an increase in the ideal critical diameter value obtained, see table 22 and figure 34. At an austenitising temperature of 1200°C all the

available to influence the hardenability of the alloy. It can be seen from table 23 that compared with the base vanadium alloy there was a slight reduction in the

amounts of both vanadium and nitrogen in solution but a significant additional quantity of molybdenum in

solution. The reduction in vanadium in solution would tend to form a vanadium-carbon-nitrogen cluster that was able to diffuse more readily and would therefore be more easily thermally dispersed from the grain boundaries. However, if the grain boundaries migrated the clusters would more easily move with the boundaries and maintain a high hardenability. Similarly, a reduction in nitrogen in solution would also form vanadium-carbon-nitrogen clusters which could diffuse more easily and therefore increase the hardenability. However, it is considered that the dominant effect would be the large quantity of molybdenum taken into solution, which indicates that even at 1200°C the presence of molybdenum increases the ideal critical diameter.

The addition of molybdenum and extra nitrogen to a base vanadium steel (alloy 641) resulted in an increase in the ideal critical diameter when compared with both the base vanadium alloy (612) and the molybdenum-low nitrogen

alloy (640), see table 22 and figure 34. The increase in hardenability when compared with the base vanadium alloy was associated with the combined effects of the decreased

vanadium in solution, the increased nitrogen and the additional molybdenum in solution, see table 23.

However, again it would be expected that the dominant factor on the hardenability would be the large quantity of molybdenum.

Comparing the theoretical quantities of microalloying elements in solution for the two molybdenum alloys, it can be seen from table 23 that the steels contained equivalent quantities of molybdenum in solution, which could therefore be ignored. Consequently, the observed increase in hardenability was associated with the

decrease in vanadium in solution and the increase in nitrogen in solution. Since no precipitates were

retained at the austenitising temperature of 1200°C the grain boundaries would be free to migrate. Therefore a decrease in vanadium in solution would form a vanadium- carbon-nitrogen cluster which could diffuse more easily. These clusters would be more able to migrate with the moving grain boundaries and would therefore be expected to increase the hardenability, whereas, the increase in the nitrogen in solution would slow down the diffusion rate of the vanadium-carbon-nitrogen clusters and would tend to decrease the ideal critical diameter.

5.1.2.4 Low Carbon-Vanadium-Niobium Alloys