The microstructure of rolled and annealed tungsten rod
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I.
INTRODUCTION
O N E of the most common uses of tungsten is for lamp filaments. The production of these fdaments requires that tungsten be processed from an ingot made by powder metallurgy methods to a fine wire that can be coiled to form a filament. Although this process must end by wire drawing, the initial reduction in area of the ingot is often carried out by hot rolling. Thus, there is a need to understand the effects of this process on the microstructure of tungsten. Lamp quality tungsten is purposely doped with potassium. This doping makes the resulting microstructure quite interesting, because the potassium is virtually insoluble in tungsten. In a filament that has been operated at high temperatures, the potassium is found in fine bubbles, as shown in Figure l(a). These bubbles pin the migrating grain boundaries and produce the interlocking transverse grain boundary structure that is shown in Figure l(b). Ill In the initial ingot, the potassium is usually contained in pores, as shown in Figure 1(c), that are as much as two orders of magnitude larger than those found in the recrystallized filament. Therefore, one of the important functions of rolling, as well as of all of the deformation that the tungsten may receive, is to lengthen these pores into ellipsoids so that in the final wire, they can break up into the small bubbles shown in Figure l(a). Although hot rolling is one of the most common methods of processing tungsten, there has never been a careful study of the microstructure of the material that develops during this process or that develops when hot-rolled tungsten is annealed. In this paper, we present such a study. One of the main goals of this work is to help understand the processing of tungsten for use in the lighting industry. However, we can also use an inves-
C.L. BRIANT and E.L. H A L L , Technical Staff Members, are with the Research and Development Center, General Electric C o m p a n y , P.O. Box 8, Schenectady, NY 12301. Manuscript submitted September 22, 1988. METALLURGICAL TRANSACTIONS A
tigation of this applied problem as a starting point for a more fundamental study of the microstructural changes that occur during hot rolling and subsequent annealing of a metal containing a deformable second phase.
II.
EXPERIMENTAL
All of the rods that were used in this study were rolled during an experimental program that was carried out at the General Electric Lighting Division in Cleveland, OH. The material was rolled on a Kocks mill. This type of rolling works in the following way. A series of stands are set up, each of which contains three rolls arrayed at points 120 deg apart, as shown in Figure 2. At each stand, the opening for the rod decreases, and the positions of the rolls are rotated by 60 deg. The material is fed continuously from stand to stand, and the resulting rolled rod has a distorted hexagonal shape. One can vary the amount of deformation by changing the number of rolling stands. In the results reported below, we will compare the microstructures of materials that had been
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