PRESSURIZED WELD CHAMBER

Abstract: A novel weld chamber 12 and process for using such weld chamber 12 are disclosed. A flow of shielding gas through the weld chamber 12 is regulated to provide a constant pressure in the weld chamber 12 while the shield­ ing gas flow sweeps away metal vapors.

Description: Pressurized welding processes, and pressurized tungsten inert gas (TIG) welding processes in particular, are conducted in a sealed pressure chamber (US-A-4 003 788). Flooding the weld chamber with a shielding gas creates pressure in the weld chamber. After the desired pressure is achieved the weld chamber is sealed. There is thus no flow of shielding gas through the weld chamber to sweep metal vapors formed in the welding zone away from the electrode tips. In this environment, often blobs of metal form on the electrode tips, causing them to become unusable. Typically, electrodes must be replaced after about five to ten welds. Metal buildup or blob formation at the electrode tip causes welds of unacceptable quality. Furthermore, metal buildup often falls from the electrode tip into the weld puddle, thus contaminating the weld and making the object being welded unacceptable. A further problem with such pressurized welding processes is the rapid buildup of metal oxide in the pressure chamber which necessitates frequent cleaning of the weld chamber. Furthermore, the heat of the welding operation causes a pressure buildup in the weld chamber so that the desired atmosphere cannot be provided properly. It is the object of the present invention to provide an improved welding process and weld chamber so as overcome the above shortcomings. Accordingly, the present invention resides in a welding process as defined in claim 1 and in a welding chamber as defined in claim 5. The process of the present invention provides a flow of shielding gas through the weld chamber so as to sweep metal vapours formed in the welding zone away from the electrode tips and constant pressure and temperature conditions can be obtained within the welding chamber. To this end, the weld chamber may include valves and pressure regulators provided in pipes leading to and from the weld chamber. The invention will become more readily apparent from the following description of a preferred embodiment thereof shown in the accompanying drawing, of a schematic diagram of a pressurized weld chamber according to the present invention. The drawing shows an exemplary embodiment of a weld chamber 12 in accordance with the invention. A gas inlet 14 is equipped with a pressure regulator 16 and a valve 18. The gas inlet 14 opens into pressure chamber 20 in which is located grounded support 22, on which work piece 24 is placed for performance of a welding operation. An electrode 26 is disposed such that its tip 28 is located within pressure chamber 20 in sufficiently close proximity to the work piece 24 to permit the welding operation. Also opening into pressure chamber 20 is gas outlet 30 which is equipped with a normally closed valve 32. Outlet port 36 intersects gas outlet 30 at T-juncture 34. Outlet port 36 is equipped with a series connected valve 38, pressure regulator 40, flow meter 42 and needle valve 44. The outlet port 36 permits a purging or flow of the shielding gas and metal vapors from pressure chamber 20 when valve 38 is open. The purge results in a flow of shielding gas from gas inlet 14 through pressure chamber 20, then out through outlet port 36. The typical gas flow path is illustrated by the arrows of the drawing. Pressure regulators 16 and 40 control the flow of the shielding gas and permit the control of the internal pressure of pressure chamber 20. Flow meter 42 and needle valve 44 are used to monitor and control the gas flow such that a constant internal pressure is maintained. During welding, metal vapors form in and about a welding zone 46. As the shielding gas flows through pressure chamber 20, it sweeps away metal vapors created by the welding operation from the welding zone 46. The concentration of metal contaminants in the weld chamber 12 is substantially decreased by the flow of shielding gas therethrough. This gas flow not only sweeps away contaminating metal vapors, but also provides a clean, fresh atmosphere in which to perform the welding operation. A further advantage offered by the flow of shielding gas through the weld chamber 12 is that it acts to chill welding zone 46. In conventional processes performed with conventional apparatus, the heat of the welding operation causes a pressure buildup in the weld chamber 12. Freshly supplied, relatively cool shielding gas nullifies any pressure buildup by preventing a heat buildup in welding zone 46. Sweeping of the metal vapors by the gas flow decreases metal buildup at electrode tip 28, thereby allowing the electrode 26 to be used for more welds before being replaced. Electrodes used in conventional pressurized welding apparatus and processes usually must be changed after every five to ten welds. In contrast, electrodes used in the present invention typically last for more than 100 welds before requiring replacement. The sweeping of the metal vapors from the pressure chamber also allows the pressure chamber to be used for more welds before cleaning is required. Metal oxides deposit on the interior walls of conventional pressurized weld chambers. In the present invention, the metal vapors are swept out of the chamber 20 before such metal oxide formation and buildup occurs. Therefore, it is necessary to clean the apparatus less frequently. Before using the weld chamber 12 of the present invention, the system is first purged with a shielding gas. After the purge, valves 18 and 38 are normally opened and valve 32 is normally closed. Pressure regulators 16 and 40 are then adjusted such that the desired pressure is established in pressure chamber 20. The flow of gas to achieve the desired pressure is monitored on flow meter 42 and any necessary adjustment is made with needle valve 44. Once the desired internal pressure has been achieved, the welding operation may begin. The internal pressure within the weld chamber 12 may be maintained at any desired level for the welding operation to be performed. Typically, the pressure chamber 20 is maintained at a pressure above atmospheric pressure between 0 bar (0 psig) and about 68 bar (1,000 psig), but more commonly the internal pressure ranges from about 6,8 bar (100 psig) to about 47,6 bar (700 psig). The internal atmosphere of the weld chamber 12 is a controlled atmosphere, as is conventional in the art. The controlled atmosphere is composed of the shielding gas used to flood the weld chamber 12. Controlled atmospheres are used to enhance the welding operation by decreasing the amount of metal oxides produced during the welding operation and to prevent oxidation of the workpiece 24. Any type of welding under pressure may be performed using the apparatus and process of the invention. Typically, seal welding of nuclear fuel rods is performed under pressure, and may be performed in the apparatus of the invention. Circumferential girth welding is a further welding operation which may be performed using the invention. The most common type of welding operation performed with the welding chamber 12 of the invention is tungsten inert gas (TIG) welding, which is also sometimes referred to as gas tungsten arc welding. This type of welding makes use of direct current. Metal inert gas (MIG) welding, in which the electrode comprises the filler material, may also be performed using the chamber of the invention. Examples of pressurized welding techniques which can be used with the present invention are disclosed in U.S. Patents 4,003,788 and 4,075,454, which are incorporated herein by reference. The shielding gases which can be used to pressurize the weld chamber are those conventional in the art, and include helium, argon, helium-argon mixtures, and helium-xenon mixtures. Additionally, some shielding gases contain small amounts of oxygen, and examples of such shielding gases include helium-oxygen mixtures and argon-oxygen mixtures.

Claims: A process for welding a workpiece under a controlled atmosphere within a weld chamber (12) adapted to contain a shielding gas therein, characterized in that a continuous flow of shielding gas is created through said weld chamber and said flow of shielding gas is regulated for continuously sweeping the welding zone (46) within said weld chamber from stray contaminating metal vapours produced during the welding operation and maintaining the weld chamber (12) at a constant internal pressure and temperature. The process of claim 1, characterized in that said constant pressure is a pressure between 0 and 68 bar (1000 psig) above atmospheric pressure. The process of claim 2, characterized in that said constant pressure is a pressure between 6,8 bar (100 psig) and 47,6 bar (700 psig) above atmospheric pressure. The process of any claims 1 to 3, characterized in that said shielding gas is introduced in a relatively low temperature for cooling the welding zone (46). A weld chamber (12) for welding therein a workpiece (24) in a welding zone (46) maintained under a controlled atmosphere of a shielding gas within said weld chamber, the weld chamber having a gas inlet (14) and a gas outlet (30) for delivering and removing the shielding gas to and from the weld chamber, characterized in that it comprises means (16, 18, 40, 42, 44) for creating a regulated continuous flow of the shielding gas through the weld chamber (12) for sweeping said welding zone (46) of contaminating metal vapours produced during the welding operation, said shielding gas being delivered through said gas inlet (14) and shielding gas and metal vapours being drawn from said gas outlet (30) in a controlled manner. The weld chamber of claim 5, characterized in that said means comprise a pressure regulator (16) associated with said gas inlet (14) for regulating the pressure of delivery of the shielding gas to said weld chamber (12) and removal therefrom to maintain a constant pressure therein. The weld chamber of claims 5 for 6, characterized in that said means comprise a pressure regulator (40), a flow meter (42) and a needle valve (44) associated to an outlet port (36) in said gas outlet (30) for regulating shielding gas pressure and for controlling the flow of shielding gas and withdrawn metal vapours therethrough. The weld chamber of any of claims 5 to 7, characterized in that said shielding gas pressure within said weld chamber (12) is maintained at a pressure between 6,8 bar (100 psig) and 47,6 bar (700 psig) above atmospheric pressure.

Inventors: DUNCAN, ROBERT, KLAPPER, KENNETH KANE, YEO, DENIS
Applicants: WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1988
Main classification: B23K9
IPC codes: B23K9