Introduction to Oxy-Acetylene Gas Welding
The majority of the metals can be joined by welding.While some metals, present little difficulty to the welder. These metals need extreme care and specialised treatment while welding. For welding different metals, different job preparation and welding technique is followed. After welding, treatment is necessary to have successful results.
Approved drawing is provided where specific requirements are laid down; which contain the preparation of the parts, methods of welding, heat treatment and other factors. These instructions must be followed. Such drawing should always be supplied when the welding of airframe or aero-engine parts are to be undertaken.
Welding of Ferrous Metals
Welding of steel offer few difficulties. Experience shows that practically all categories of steel are weldable. But as the series of steel depend upon the percentage of carbon and it ranges from 0.15 to 1.5%, it is necessary to know the exact quality of steel being welded as the increase in carbon content leads to weld brittleness. The principal quality of silicon is to deoxidise and clean the steel. Sulphurlowers the ductility and strength of material. Phosphorus produces brittleness and weakness in the metal, particularly in regard to the stress due to shock and vibration. It also produces cold shortness. The effect of manganese depends on the carbon content, but generally increases the tensile strength of steel when present up to 4%.
Welding of Mild Steel
Steel melts at a temperature of 1400-1530°C but is not very fluid. When heated it begins to oxidize rapidly at dull red heat. The oxide melts at a temperature lower than the melting point of the metal and so no flux is necessary. The filler rod should be of pure iron or good mild steel. The flame should be strictly neutral. Due to the ductility of the metal, unequal expansion or contraction do not occur. Preheating will ensure that warping and buckling do not occur. The mechanical properties of the metal depend on the heat treatment it undergoes. Therefore, it is essential that the metal is properly heat treated after welding i.e. normalized.
All low carbon steels (mild steels) are very easily weldable. The carbon content of mild steel is 0.15 to 0.3 %. No flux is necessary when welding these metals. The steel melts at 1430-1530°C and the oxide melt below the melting temperature of mild steel. The welding filler rodmust be pure iron. A strict neutral flame must be used at all costs. Heat treatment of mild steel after welding is not necessary unless specifically required. However, the job must not be quenched after welding if the parts are subjected to machining. After welding, a stress reliving treatment is recommended i.e. the job is heated to 650°C and allowed to cool down in air.
Medium and High Carbon Steels
The carbon percentage of medium carbon steel varies between 0.3 to 0.8%, while high carbon steel may contain up to 1.5% carbon. Both classes of steel may cause considerably more difficulty to the welder than mild steel. As the amount of carbon increase, the tendency of the weld to become hard and brittle also increases. Besides this the steels and their oxides melt at almost the same temperature and therefore, great care is necessary to prevent oxides being embedded in the weld. A neutral or slightly carburising flame is used for welding these metals. The welding rod should be of the same grade of steel.
Heat treatment after welding is necessary to modify the tendency of the weld to become hard and brittle. High carbon steel job must be preheated to a red heat and then welded. After welding, all the steels are subjected to a stress relieving treatment in accordance with the approved drawing, and in the absence of such drawings, it is heated to 600°C and allowed to cool down in still air.
From the point of view of carbon content, this metal can be considered to be the same as mild steel. However due to its fibrous structure, it is more difficult to weld.
These steels possess distinctive properties due to the presence of such elements as manganese, nickel, chromium, vanadium, tungsten and titanium. Heat treatment is employed to these alloy steels to control their mechanical properties. Therefore, welding should be undertaken only in accordance with approved repair schemes or detailed recommendations for the steel concerned. A point of special importance is that a welding rod or wire suitable for the particular alloy to be welded should be used to ensure continuity of mechanical properties throughout metal. Preheating is generally advised, and heat treatment after welding may be required.