1. Field of the Invention
The invention is related to the field of hardening the bores of rifle barrels and in particular to forming nitrided and nitrocarburized surfaces in the bores of rifle barrels using a fluidized bed furnace.
2. Description of the Prior Art
The hardening of the internal surfaces or bores of rifle barrels, gun barrels and cannons is well known in the art. These hardened surfaces reduce friction and wear of the bore increasing the accuracy and life of the rifles and gun barrels. The bores of the rifles may be hardened by heat treatment followed by a rapid quenching as taught by Somes in U.S. Pat. No. 2,541,114 or Polcha in U.S. Pat. No. 3,780,465. Alternatively, the bores of the rifles or guns may be hardened by nitriding as taught by Chenault et al in U.S. Pat. No. 2,596,981 and Osborn in U.S. Pat. No. 2,799,959. Chenault et al teach nitriding at a temperature of approximately 1000.degree. F. at a pressure of 100 atmospheres for approximately 15 hours while Osborn teaches nitriding at a temperature of 950.degree. F. to 975.degree. F. for 38 hours. Siemers et al, in U.S. Pat. No. 4,577,431, and Gstettner et al, in U.S. Pat. No. 4,747,225, disclose the application of a hard material over the internal surface of the rifle's bore. Siemers et al disclose coating the bore with a layer of refractory metal such as tantalum alloy by means of a vacuum plasma spray while Gstettner et al teach sintering of a thin heat resistant nickel based alloy on the surface of the bore. The use of a fluidized bed furnace for nitriding or nitrocarburizing various metals is taught by Ross in U.S. Pat. No. 4,461,656 and Staffin et al in U.S. Pat. No. 4,512,821. Ross teaches the treatment of ferrous metal components in a particulate medium fluidized with ammonia gas, a hydrocarbon gas, and nitrogen gas while Staffin et al teach the use of an atmosphere precursor, such as methanol or ethyl acetate in the fluidized bed furnace to produce the desired atmosphere.
In their paper "Nitriding of Titanium with Ammonia" presented before the Thirty-fifth Annual Convention of the American Society of Metallurgy, held Oct. 17 through 23, 1953, and published in the Transactions of ASM, Volume 46, 1954, pp. 191 through 218, James L. Wyatt and Nicholas J. Grant presented a detailed process for nitriding titanium and titanium alloys by the decomposition of ammonia at elevated temperatures.
The direct application of the methods taught by Ross and Wyatt et al to steel and aluminum alloy rifle barrels using a fluidized bed furnace failed to produce satisfactory nitrided or nitrocarburized surfaces within the bores of these rifle barrels. The invention is a solution to this problem.