WELDING By ANIL KUMAR S (ASST PROFESSOR) Introduction :- • The joining of metals is an important part of the work in many industries. • Most of the metal fabrication industries adopt welding technique. • Different welding methods are developed to ensure the strength of weld part. • It is fastly replacing other process Welding Defination:- “Welding is the process of making a permanent joint by establishing inter atomic bonds between two or more pieces of metal by using heat or heat & pressure.” Applications :- • Boilers, pressure vessels, ships, bridges, storage tanks, pipelines, railway coaches, missiles & rockets, nuclear reactors, chemical plants, automobile parts, press frames & water turbines etc. For Ideal welding :- • To achieve good atomic bond between metals there must be sufficient activity between atoms of the pieces. 1) Perfectly smooth, flat. 2) Clean surfaces, free from oxides, absorbed gases, grease & from other 3) Metal with no impurities WELDING CLASSIFICATION PRESSURE WELDING FUSION WELDING FORGE RESISTANCE PRESSURE FUSION ARC GAS THERMIT THERMIT SPOT SEAM BUTT METAL CARBON INERT GAS OXY-ACETYLENE Differences between pressure & fusion welding :- • In Pressure welding the joint area of the base metal is heated to plastic state & forced together by external pressure. • In fusion welding the joint area of the part is heated to fusion state (molten) & a joint is formed as a result of solidification Principle of arc welding :- • Arc welding is developed in the mid 1800’s. • The source of heat in this process is electric arc. • An arc is a sustained electric discharge • Accelerated electrons – strike anode at high velocity – conversion of KE to heat –temperature generated is about 3500 °C • The temperature of this arc at the centre about 6500oC ELECTRIC ARC WELDING Ammeter + Resistance Voltmeter Battery Simple electrical circuit Welding electrical circuit (ac or dc power supply) I Welding Electrode machine - + Arc V Work • The heat of the arc is concentrated at the portion of welding as a result it melts the electrode & base metal. • Weld metal is allowed to cool gradually & slowly. • After cooling (solidification) a sound joint is formed. ELECTRODES :- • Arc welding is done with metal electrodes ELECTRODES CONSUMABLE NON CONSUMABLE BARE COATED ELECTRODE • Arc welding can be performed by (bare electrode or coated electrode) • If we perform welding with bare electrode it result’s • When molten metal is exposed to air, it absorbs oxygen and nitrogen, and becomes brittle or is otherwise adversely affected. • Contamination due to exposure to the weld. • Due to this , the use of arc welding process is very little until after world war I • Around 1920, shielded metal electrodes were developed. • These electrodes provide gas shield which protects the arc & weld pool from the atmosphere. • A slag cover is needed to protect molten or solidifying weld metal from the atmosphere. This cover can be obtained from the electrode coating. Coated electrode Bare electrode • The coating protects the metal from damage, stabilizes the arc, and improves the weld in other ways, which include: (1) Smooth weld metal surface with even edges. (2) Minimum spatter adjacent to the weld. (3) A stable welding arc. (4) Penetration control. (5) A strong, tough coating. (6) Improved deposition rate Section view of arc welding with a coated electrode Electrode Coating Protective gas cover Slag covering Base metal Arc crater Solidified weld metal Metal arc welding :- • It is the most common type of arc welding & its normally manual operation. • Manual metal arc (MMA) welding, also known as shielded metal arc welding (SMAW), stick, and electric arc welding . • In manual metal arc welding the heat source is an electric arc, • which is formed between a consumable electrode and the parent plate. • The arc is formed by momentarily touching the tip of the electrode onto the plate. • Then lifting the electrode to give a gap of 3 mm – 6 mm between the tip and the plate. • When the electrode touches the plate, current commences to flow. • As it is withdrawn the current continues to flow in the form of a small spark across the gap. • which will cause the air in the gap to become ionized, or made conductive. • As a result of this the current continues to flow even when the gap is quite large. • The heat generated is sufficient to melt the parent plate and also melt the end of the electrode . • the molten metal so formed is transferred as small globules across the arc into the molten pool. • Arc melts parent plate and electrode to form a weld pool that is protected by the flux cover. • Operator adjusts the electrode feed rate, i.e. hand movement, to keep the arc length constant. Sequence of operations :- • Preparation of edges • Holding the work piece • Striking the arc • Welding the joint Arc welding equipment :- • Based on current source: AC machines : Transformer DC machines : Transformer with DC rectifier • Electrodes • Electrode holder • Cables • Safety devices • Tools Joints, Welds & Positions Arc Welding Positions Flat Horizontal Vertical Up Vertical Down Overhead Gas welding :- • Gas welding is classified under the fusion welding process. • Heat source is a gas flame • Different gas combination can be used for producing the flame. • Most common combination is oxygen & acetylene. THE FIRE TRIANGLE TO PRODUCE FIRE, THREE THINGS MUST BE PRESENT AT THE SAME TIME The basic process that allows the oxy-acetylene equipment to work. OXYGEN HEAT FUEL Chemicals Used • Oxygen – Colorless, odorless, tasteless gas – Supports combustions & increase heat – Produce by cooling air to a low temperature and turning it into liquid where the oxygen is separate out. • Acetylene – Colorless, has a very distinctive odor – Highly flammable – Produce by mixing calcium carbide (coke + limestone burnt together) and water yields acetylene and calcium hydroxide. oxy-acetylene gas welding :- • It develops in 1900’s • It utilizes the heat generated by the combustion of oxy-acetylene • Burning acetylene in the presence of oxygen. • At the tip of the nozzle. • The temperature of the oxy-acetylene flame is 3250oC • Heat is to melt parent metal to form a weld pool. • Filler is added separately. • Torch moved to achieve a required length. • Molten metal is protected by the gaseous products of the flame. The principle of the oxyfuel-gas welding operation. Oxyacetylene Flame Types :- Three basic types of oxyacetylene flames used in oxyfuel-gas welding and cutting operations: (a) neutral flame; (b) oxidizing flame; (c) carburizing, or reducing, flame. The gas mixture in (a) is basically equal volumes of oxygen and acetylene. . Oxyacetylene Torch (a) General view of and (b) cross- section of a torch used in oxyacetylene welding. The acetylene valve is opened first; the gas is lit with a spark lighter or a pilot light; then the oxygen valve is opened and the flame adjusted. (c) Basic equipment used in oxyfuel-gas welding. To ensure correct connections, all threads on acetylene fittings are left- handed, whereas those for oxygen are right-handed. Oxygen regulators are usually painted green, and acetylene regulators red. Oxy-acetylene gas welding equipment :- Oxygen cylinder:- • pure oxygen produced by liquefaction process. • Made up of steel • Painted in black colour • It contains oxygen at a pressure of 175 bar • It can store 7m3 of gas. Acetylene cylinder :- • It contains acetylene product of carbon carbide & water. • Made up of steel • Painted in maroon colour. • Cylinder contains acetylene at 15 bar • The capacity of cylinder is 6m3 of gas. Type of acetylene flame :- Depending on the relative amount of oxygen & acetylene , the gas flame is classified into three types. • Oxidising flame • Neutral flame • Reducing (carburising) flame Oxyacetylene Flame Types :- Three basic types of oxyacetylene flames used in oxyfuel-gas welding and cutting operations: (a) neutral flame; (b) oxidizing flame; (c) carburizing, or reducing, flame. The gas mixture in (a) is basically equal volumes of oxygen and acetylene. (d) The principle of the oxyfuel-gas welding operation. ADVANTAGES :- • The relative cost of the equipment is low. • No electricity is required for this process. • Can be used for welding in all positions. • Can be used on both thick and thin materials, which makes it a very versatile process. • Very clean, producing no slag or spatter that must be removed from the weld. • Produces high quality welds when done properly. LIMITATIONS :- • The materials that can be welded are limited primarily to ferrous materials. • Can create a “Hot Zone”, fire hazard, because of the sparks and flame generated in the welding process. • Requires the handling of high pressure gases. • The process can often be slow when compared to other types of welding processes. • high temperature flame only with oxy-acetylene . Oxy-fuel gas flame cutting :- • It is oxygen cutting process • It is used for separating or cutting the a part from the whole. • It uses cutting torch to generate flame. • Oxy-acetylene flame preheats the metal to be cut. • After a spot area along the line of cut is heated to ignition temp (900oC). • Thin jet of high purity of oxygen is then directed or shot at this heated spot. • The jet quickly penetrates through the steel by cutting it. • The torch moves progressively forward over the metal surface. • Cutting a narrow slot along the desired line of severance. Flame Cutting :- (a) Flame cutting of steel plate with an oxyacetylene torch, and a cross-section of the torch nozzle. (b) Cross-section of a flame-cut plate, showing drag lines. Differences in torch tips for gas welding and gas cutting For pre-heating Oxygen Jet Position of cutting torch in oxy-fuel gas cutting Direction of travel pre-heating flames Drag Kerf Slag + Molten metal Advantages :- (i) Shapes and sizes difficult to be machined by mechanical methods can be easily cut by flame cutting. (ii) The process is faster than mechanical cutting methods. (iii) The cost of flame cutting is low as compared to that on a machine tool, i.e. mechanical cutting machine. (iv) Flame cutting equipment being portable also, can be used for the field work. (v) Multitorch machines can cut a number of pieces simultaneously Disadvantages of Flame Cutting :- • (i) Flame cutting is limited to the cutting of steels and cast iron. • (ii) As compared to mechanical cutting, the dimensional tolerances are poor. • (iii) The place of cutting needs adequate ventilation and proper fume control. • (iv) The expelled red hot slag and other particles present fire and burn hazards to plant and workers. Uses :- • (i) To prepare edges of plates for bevel and groove weld joint designs. • (ii) To cut small sized work pieces from bigger plates for further processing. • (iii) To cut rivets, gates and risers from castings. • (iv) To cut many layers of thin sheets at the same time (stack cutting) to reduce both time and cost for production work. • (v) To pierce holes and slots in steel plates. • (vi) For salvage work Resistance welding :- • In resistance welding both heat & pressure are used to effect coalescence. • Heat is the consequence of the resistance of the work piece. • A certain amount of pressure is applied initially to hold the work piece in contact. • Joint occurs at lower temperature than required for gas & arc welding. • Melting of base metal does not occur. • It is considered as solid state welding. • Joint achieved in few seconds. • Very rapid & economical suitable for automated manufacturing. Air Pneumatic and Air hydraulic valve cylinder Resistance welding (spot welding) Factors effecting the resistance welding :- • Heat • Resistance • Temperature • Pressure • Current control Brazing :- • Low melting point material is melted & drawn into the space between two solid surfaces. • Joining of metals by heat & filler material. • Melting point of filler material above the 450o C & below metals being joined. Comparison with welding :- • Composition of brazing alloy is different from the base metal. • Melting point of brazing alloy is lower than of the base metal. • Strength of the brazing alloy is substantially lower than the base metal. Strength of brazing :- The bonding is enhanced by cleanly surfaces, proper clearance, good wetting,& good fluidity. • Bond strength • Wettability • Fluidity Applications :- • Brazing is applicable to cast and wrought irons, steels, Cu and Cu alloys, Al and Al alloys, Mg and Mg alloys and to so many other materials. • 2. Brazing is used in place of welding where special metallurgical characteristics of metals have to be preserved after joining. • 3. Brazing can join (a) Cast metals to wrought metals. (b) Non-metals to metals. (c) Dissimilar metals. (d) Porous metal components Frequently used brazing materials :- • Copper • Silver • Copper zinc alloys • Aluminum –silicon alloys Soldering :- • Soldering is similar to brazing operation. • Filler material has a low melting temperature below than the 450o C • Bond strength relatively low • Bonding being the result of adhesion between the solder & the parent metal. • Most solder material are alloy of lead & tin,tin- antimony,tin-zinc.