GB468449A - Improvements in or relating to the manufacture of double or multiple helical tungsten incandescent bodies - Google Patents

Abstract

In a process for manufacturing double or multiple helical tungsten incandescent bodies for use in gas-filled vessels containing nitrogen, particularly gas-filled electric incandescent lamps, by subjecting to at least one heating a tungsten wire wound on at least two cores made of metal of high wetting point, and subsequently removing the cores, the incandescent body, before being mounted in the vessel which receives the gas filling, is heated to such a temperature between 1450 DEG C. and the re-crystallization temperature of tungsten, and for such a time that, not only are the internal stresses eliminated but, under continued heating or re-heating, the content in the incandescent body of material taken up from the core, is less than 0,4 per cent, and preferably less than 0,2 per cent, of the weight of the incandescent body. The heat treatment to which the incandescent body, with the core, is subjected may be such that it takes up less than 0,4 per cent of core material, or it may take up material in excess of 0,4 per cent of its weight, whereupon, after the removal of the core, the body is heated to a white heat in a space free from nitrogen, for instance in a vacuum or in a hydrogen atmosphere to reduce its content of core material to less than 0,4 per cent. In an example, a tungsten wire which assumes a macro-crystalline structure during heat treatment is wound upon primary and secondary cores of molybdenum and drawn for a few seconds through a hydrogen atmosphere in a furnace at a temperature of 1500 DEG C. The secondary helix is then cut into pieces of the required length which are incandesced for one minute at a temperature of 1600 DEG C. in a tungsten tube furnace through which a mixture of 25 per cent hydrogen and 75 per cent nitrogen is passed. The cores are then dissolved out. In another example using a heavier wire which is wound upon primary and secondary cores of molybdenum, the coiled coil is passed for a few seconds through hydrogen in a furnace at 1500 DEG C., cut into lengths, and then incandesced for three minutes at 1700 DEG C. in a tungsten tube furnace through which hydrogen is passed.