Tham khảo tài liệu 'sổ tay tiêu chuẩn thiết kế máy p18', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | STANDARD HANDBOOK OF MACHINE DESIGN cesses that require less skill and are less costly it is still preferred in some manual operations where close control of heat input is required. In the atomic-hydrogen process an arc is established between two tungsten electrodes in a stream of hydrogen gas using alternating current. As the gas passes through the arc molecular hydrogen is dissociated into atomic hydrogen under the intense heat. When the stream of hydrogen atoms strikes the workpiece the environmental temperature is then at a level where recombining into molecules is possible. As a result of the recombining the heat of dissociation absorbed in the arc is liberated supplying the heat needed for fusing the base metal and any filler metal that may be introduced. The atomic-hydrogen process depends on an arc but is really a heating torch. The arc supplies the heat through the intermediate of the molecular-dissociation atomrecombination mechanism. The hydrogen gas however does more than provide the mechanism for heat transfer. Before entering the arc it acts as a shield and a coolant to keep the tungsten electrodes from overheating. At the weld puddle the gas acts as a shield. Since hydrogen is a powerful reducing agent any rust in the weld area is reduced to iron and no oxide can form or exist in the hydrogen atmosphere. Weld metal however can absorb hydrogen with unfavorable metallurgical effects. For this reason the process gives difficulties with steels containing sulfur or selenium since hydrogen reacts with these elements to form hydrogen sulfide or hydrogen selenide gases. These are almost insoluble in molten metal and either bubble out of the weld pool vigorously or become entrapped in the solidifying metal resulting in porosity. ARC-WELDING CONSUMABLES Arc-welding consumables are the materials used up during welding such as electrodes filler rods fluxes and externally applied shielding gases. With the exception of the gases all the commonly used .
