Overview Of Welding Technology.docx

29.FUNDAMENTALS OF WELDING OVERVIEW OF WELDING TECHNOLOGY The term joining is generally used for welding, brazing, soldering, and adhesive bonding, which form a permanent joint between the parts–a joint that cannot easily be separated. The term assembly usually refers to mechanical methods of fastening parts together. Some of these methods allow for easy disassembly, while others do not. Welding is a materials joining process in which two or more parts are coalesced at their contacting surface by a suitable application of heat and/or pressure. Many welding processes are accomplished by heat alone, with no pressure applied; others by a combination of a heat and pressure; and still others by pressure alone, with no external heat applied. In some welding processes a filler material is added to facilitate coalescence. The assemblage of parts that are joined by welding is called weldment. Welding is most commonly associated with metal parts, but the process is also used for join plastics. Welding involves localized coalescence or joining together of two metallic parts at their faying surfaces. The faying surfaces are the part surfaces in contact or close proximity that are to be joined. Welding is usually performed on parts made of the same metal, but some welding operations can be used to join dissimilar materials. Application of Welding        

Automobile Industries Building Industries Railroad Industries Pressure Vessel and Storage Tank Manufacturing Aircraft Industries Petrochemical Industries Ship building Industries Pipe Line Industries

THE WELD JOINT A welding joint is a common point or edge where two or more workpieces of plastic or metal are joined. These workpieces are joined using different types of welding processes.

Types of Welding Joints 1. Butt Joint -The type of joint in which two metal pieces are joined in the same plane is called as a buttjoint. Butt joints are used to weld thin metal sheets which can be welded in a single pass Types of Butt Joints  Single welded butt joint  Double welded butt joint  Open welded butt joint  Closed welded butt joint (i)Single Welded Butt Joint The joint which has been welded from one side is called as a single welded joint. (ii)Double Welded Butt Joint The joint which has been welded from both sides is called as a double welded joint. (iii)Open Welded Butt Joint The joint which has a small gap between the workpieces when being joined is called an open welded joint. (iv)Closed Welded Butt Joint The type of joint in which the two workpiecesare touching during the welding is called as a closed welded joint. Welding styles used to create Butt Joints  Bevel Groove butt weld  Square groove butt weld  V groove butt weld  J groove butt weld  U groove butt weld  Flare bevel groove butt weld  Flare V groove butt weld 2. Corner Joint The type of joint that is formed by placing the corner of two parts at a right angle to each other is called a corner joint. As a result, an L shape is formed between the two parts that are being joined. This technique is one of the most popular to join metal sheets. This is used on the outer edge of the sheet.

Types of Corner Joints  Flush Corner Joint  Half Open Corner Joint  Full Open Corner Joint (i)Flush corner Joint Flush corner joint is primarily designed for welding metal sheets which are equal to 12 gauges or thinner than this. Deep penetration is difficult most of the times as a result design can afford moderate loads. (ii)Half Open corner Joint Half open corner joints are primarily used for welding materials which are heavier than 12 gauges. Penetration is better than a flushcorner joint but only used for moderate loads. (iii)Full Open Corner Joint Full open corner joint is primarily used for producing strong joints especially when welding is done on both sides of the materials. Therefore Mostly useful for welding plates of all thicknesses i.e. thinner or thicker. Welding styles used to create Corner Joints  Edge weld  Bevel Grooved weld  Corner Flange weld  Fillet Weld  J-Groove weld  Flare V groove weld  Spot weld  Square groove weld  Butt weld  U-groove weld  V-groove weld

3. Edge Joint -The type of joint that is formed by welding edges of two different parts together is called an edge joint. Welding styles used to create Edge Joints  U-Groove weld

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V- Groove weld J- Groove weld Bevel Groove Weld Edge-flange Weld Corner flange Weld Square-groove weld/butt weld

4. Lap Joint The type of joint in which two workpieces are placed at each other i.e. one above the other is called a lap joint. Lap weld joint is formed when pieces are placed at overlapping positions. Lap joint may be a) One sided b) Double sided. Lap joint welded on double sides provides strength as compared to one-sided weld because no material is removed from both of the sheets that are being joined.This welding joint is useful when metal sheets of different thicknesses are being used. Welding styles used to create Lap Joints  Fillet Weld  Bevel-groove Weld  Slot weld  Plug weld  Spot weld  Flare-bevel–groove weld  J-groove weld 5. Tee Joint -The type of joint in which the two parts intersect each other at right angle i.e. 90 degrees & one part is above the other at the center is called as T joint. The metal surface that is being joined is never on the same plane. An approximate shape of English letter “T” is obtained hence named as T joint. T joints are also considered as a type of fillet weld. Welding styles used to create Tee Joints  Fillet Weld  Flare-bevel groove  Plug weld  J-groove weld  Slot weld  Bevel-groove weld  Melt-through weld

PHYSICS OF WELDING Fusion is most common means of achieving coalescence in welding. To accomplish fusion, a source of high density heat energy must be supplied to the faying surfaces, so the resulting temperatures cause localized melting of base metals. For metallurgical reasons, it is desirable to melt the metal with minimum energy but high heat densities. Power Density Power transferred to work per unit surface area, W/mm2 • If power density is too low, heat is conducted into work, so melting never occurs. • If power density is too high, localized temperatures vaporize metal in affected region. There is a practical range of values for heat density within which welding can be performed. Comparisons Among Welding Processes • Oxyfuel Gas Welding develops large amounts of heat but heat density is relatively low because heat is spread over a large area Oxyacetylene gas, the hottest of the OFW fuels, burns at a top temperature of around 3500°C (6300°F). Arc Welding produces high energy over a smaller area, resulting in local temperatures of 5500° to 6600°C Unit Energy for Melting Quantity of heat required to melt a unit volume of metal depends upon: • Symbolized • It is the sum of:

Heat to raise temperature of solid metal to melting point Depends on volumetric specific heat Heat to transform metal from solid to liquid phase at melting point Depends on heat of fusion Heat Transfer Mechanisms in Welding • Not all of the input energy is used to melt the weld metal Heat transfer efficiency f1 – actual heat received by workpiece divided by total heat generated at source Melting efficiency f2 – proportion of heat received at work surface used melting; the rest is conducted into work metal. Heat Transfer Efficiency f1 Proportion of heat received at work surface relative to total heat generated at source • Depends on welding process and capacity to convert power source (e.g., electrical energy) into usable heat at work surface Oxyfuel gas welding processes are relatively inefficient Arc welding processes are relatively efficient Melting Efficiency f2 Proportion of heat received at work surface used for melting; the rest is conducted into the work • Depends on welding process but also influenced by thermal properties of metal, join configuration, and work thickness Metals with high thermal conductivity, such as aluminum and copper present a problem in welding because of the rapid dissipation of heat away from the heat contact area. Heat Available for Welding H = f1 f2 H where H = net heat available for welding; f1 = heat transfer efficiency; f2 = melting efficiency; and H= total heat generated by welding process Energy Balance Equation

• Net heat energy into welding operation equals heat energy required to melt the volume of metal welded H=UV Where H = net heat energy delivered to operation, J (Btu); U = unit energy required to melt the metal, J/mm3 (Btu/in3); and V = volume of metal melted, mm (in3)

FEATURES OF FUSION-WELDED JOINT 1. Fusion Zone • Consist of a mixture of filler metal and base metal • The section of material that is present after an object has been melted. 2. Weld Interface • A narrow boundary that seperates the fusion zone from heat affected zone. • It consists of a thin band metal that was melted or partially melted. 3. Heat Affected Zone • Metals has experienced temperatures below melting point, but high enough to cause microstructural changes in the solid metal. • Chemical composition same as base metal, but this region has been heat treated so that its properties and structure have been altered. • Effect on mechanical properties in HAZ is usually negative, and it is here that welding failures often occur. 4. Unaffected Base Metal Zone • Where no metallurgical change has occurred. • The base metal surrounding the heat affected zone (HAZ) is likely to be in a state of high residual stress, due to the shrinkage in the fusion zone.