THE INFLUENCE OF CHROMIUM UPON THE OXIDIZING PROPERTY OF TUNGSTEN AT HIGH TEMPERATURES.

Abstract

As is well known, tungsten has the highest melting point among the metals known at present and is one of the metals having the highest tensile strength at high temperatures. It is, however, one of the most oxidizing metals at high temperatures, in the presence of air. The authors attempt here to investigate the influence of chromium added to tungsten upon the improvement of the properties of this material and the removal of the defect.
The preparation of the W-Cr alloys was the most difficult task. Many attempts for the preparation of samples by sintering powdered tungsten and chromium had failed, but finally it was succeeded with the thermit process.
In Table 2, the compositions of the samples by chemical analysis are given. It will be seen that a certain amount of aluminium, say 5 percent, is always present as a result of the preparation by the thermit process. In this paper, the W-Cr equilibrium diagram is first dealt with. In Sargent's paper, the existence of W₇Cr₃, WCr₃ and WCr₇ is proposed, but it appears to the authors that there is no other literature regarding the W-Cr equilibrium diagram.
In the present research on the diagram, the melting temperatures, microscopic structures, X ray analyses and hardness were investigded in order.
The melting temperatures of the samples were too high to be determined by the ordinary laboratory method. As a result of the observation of the microscopic structures of the samples in the state as cast and annealed at 1200°C for a long duration, the aggregation of the small particles are seen. These structures differ considerably from those which appear in the samples prepared by the ordinary melting process, but they are nearly the same as in the samples containing chromium of from zero to 400/8 in atomic percentage. The X-ray analyses were made by the reflection method, and as is shown in Fig. 3, and Fig. 5, it was found that a solid solution of chromium exists in tungsten in a range from zero to about 26% of Cr, and a solution of tungsten exists in chromium from zero to about 32% of W. The result of the hardness tests is shown in Fig. 4. This curve indicates that the hardness of the samples rapidly increases as the percentage of chromium in tungsten increases, but the maximum point of the hardness in the curve is somewhat shifted towards the lower chromium side from the limit of the solid solution on chromium in tungsten obtained by the X-ray analyses. In this paper, however, the result of the X-ray analyses is adopted for the solid solution range of chromium in tungsten. Anyhow, the existence of some solid solution ranges in the W-Cr binary alloys is certain notwithstanding Sargent's proposition refered above. The authors consider that Fig. 5 is a portion of the supposed W-Cr equilibrium diagram. From the above investigations, the fact of the existence of the solid solution of chromium in the W-Cr alloys is presumable, and also the high resistance to oxidation at high temperatures in the presence of air of the solid solution containing chromium. Therefore, oxidizing tests at 900°C were carried out for the prepared samples, the results being shown in Fig. 6. From this curve, the oxidizing property of tungsten at high temperatures is found to be greatly improved by adding about 13 percent of chromium to tungsten.