Welding PDF

Welding Process of joining two or more parts to make a single part termed as a Fabrication process. A fairly large number of industrial components are made by fabrication processes. Common examples are aircraft and ship bodies, building trusses, welded machine frames, sheet metal parts etc....

Welding Process of joining two or more parts to make a single part termed as a Fabrication process. A fairly large number of industrial components are made by fabrication processes. Common examples are aircraft and ship bodies, building trusses, welded machine frames, sheet metal parts etc. Fabrication Process Method of Joining Type of Assembly Mechanical Joining by Temporary Joining Process: connected means of bolts and nuts , parts can be separated easily with out any screws,rivets. damage. Example: - Joints obtained by brazing, soldering, bolts and nuts. Studs and nuts, screws etc Adhesive Joining by employing synthetic glues such as epoxy resins. Permanent Joining Process: connected parts have to be broken in order to separate them. Example: - Joints obtained by riveting, welding etc. Metallurgical Fusion process like Welding, Soldering and Brazing Welding: Welding is the process of joining similar metals by application of heat, with or without application of pressure or filler material. Applications of welding: It is used in the manufacture of automobile bodies, aircraft frames, railway wagons, machine frames, structural works, pressure vessels, furnaces, tanks, refrigerators, furniture, boilers, and shipbuilding. Repair and maintenance work. Example: joining broken parts, rebuilding worn out components etc. Classification of welding process: Plastic welding In plastic welding, the metal parts to be joined are heated to the plastic state (or above) and then fused together by applying external pressure. No filler metal is used in this process plastic welding is also called ‘pressure welding processes. Example: -Forge welding, resistance welding. Fusion welding In fusion welding process, the parts to be joined are heated above their melting temperature and then allowed to solidify without application of pressure. A filler material is used during welding process. Example: - arc welding and gas welding The various welding processes are: 1. Electric arcwelding, 2. Gaswelding 3. Thermalwelding 4. Electrical Resistance weldingand 5. Frictionwelding Electric arc welding Arc welding is the welding process, in which heat is generated by an electric arc struck between an electrode and the work piece. Electric arc is luminous electrical discharge between two electrodesthrough ionized gas. Any arc welding method is based on an electric circuit consisting of the following parts: a. Power supply (AC orDC); b. Weldingelectrode; c. Workpiece; d. Welding leads (electric cables) connecting the electrode and work piece to the powersupply. Fig:1 Arc welding set up Electric arc between the electrode and work piece closes the electric circuit. The arc temperature may reach 10000°F (5500°C), which is sufficient for fusion the work piece edges and joining them. When a long joint is required the arc is moved along the joint line. The front edge of the weld pool melts the welded surfaces when the rear edge of the weld pool solidifies forming the joint. Transformers, motor generators and rectifiers’ sets are used as arc welding machines.These machines supply high electric currents at low voltage and an electrode is used to produce the necessary arc. The electrode serves as the filler rod and the arc melts the surface so that, the metals to be joined are actually fixed together. Sizes of welding machines are rated according to their approximate amperage capacity at 60% duty cycle, such as 150,200,250,300,400,500 and 600 amperes. This amperage is the rated current output at the working terminal. Transformers The transformers type of welding machine produces A.C current and is considered to be the least expensive. It takes power directly from power supply line and transforms it to the voltage required for welding. Transformers are available in single phase and three phases in the market. Motor generators These are D.C generators sets, in which electric motor and alternator are mounted on the same shaft to produce D.C power as pert the requirement for welding. These are designed to produce D.C current in either straight or reversed polarity. The polarity selected for welding depends upon the kind of electrode used and the material to be welded. Rectifiers These are essentially transformers, containing an electrical device which changes A.C into D.C by virtue of which the operator can use both types of power (A.C or D.C, but only one at a time). In addition to the welding machine, certain accessories are needed for carrying out the welding work. Welding cables Two welding cables are required, one from machine to the electrode holder and the other, from the machine to the ground clamp. Cables are specified by their current carrying capacity, say 300 A, 400 A, etc. Electrodes Filler rods are used in arc welding are called electrodes. These are made of metallic wire called core wire, having approximately the same composition as the metal to be welded. These are coated uniformly with a protective coating called flux. While fluxing an electrode; about 20mm of length is left at one end for holding it with the electrode holder. It helps in transmitting full current from electrode holder to the front end of the electrode coating. Flux acts as an insulator of electricity. In general, electrodes are classified into five main groups; mild steel, carbon steel, special alloy steel, cast iron and non‐ferrous. The greatest range of arc welding is done with electrodes in the mild steel group. Various constituents like titanium oxide, potassium oxide, cellulose, iron or manganese, Ferro silicates, carbonates, gums, clays, asbestos, etc., are used as coatings on electrodes. While welding, the coating or flux vaporizes and provides a gaseous shield to prevent atmospheric attack. The size of electrode is measured and designated by the diameter of the core wire in SWG and length, apart from the brand and code names; indicating the purpose for which there are most suitable. Electrodes may be classified on the basis of thickness of the coated flux. As 1. Dust coated or light coated 2. Semi or medium coated 3. Heavily coated or shielded Electrodes are also classified on the basis of materials, as 1. Metallic 2. Non‐metallic or carbon Metallic arc electrodes are further sub‐divided into 1. Ferrous metal arc electrode (mild steel, low/medium/high carbon steel, cast iron, stainless steel) 2. Non‐ferrous metal arc electrodes (copper, brass, bronze, aluminum, etc). In case of non‐metallic arc electrodes, mainly carbon and graphite are used to make the electrodes WELDING TOOLS Electrode holder The electrode holder is connected to the end of the welding cable and holds the electrode. It should be light, strong and easy to handle and should not become hot while in operation. Figure shows one type of electrode holder. The jaws of the holder are insulated, offering protection from electric shock. Ground clamp It is connected to the end of the ground cable and is clamped to the work or welding table to complete the electric circuit. It should be strong and durable and give a low resistance connection. Wire brush and chipping hammer A wire brush is used for cleaning and preparing the work for welding. A chipping hammer is used for removing slag formation on welds. One end of the head is sharpened like a cold chisel and the other, to a blunt, round point. It is generally made of tool steel. Molten metal dispersed around the welding heads, in the form of small drops, is known as spatter. When a flux coated electrode is used in welding process, then a layer of flux material is formed over the welding bead which contains the impurities of weld material. This layer is known as slag. Removing the spatter and slag formed on and around the welding beads on the metal surface is known as chipping. Welding table and cabin It is made of steel plate and pipes. It is used for positioning the parts to be welded properly. Welding cabin is made‐up by any suitable thermal resistance material, which can isolate the surrounding by the heat and light emitted during the welding process. A suitable draught should also be provided for exhausting the gas produced during welding. Face shield A face shield is used to protect the eyes and face from the rays of the arc and from spatter or flying particles of hot metal. It is available either in hand or helmet type. The hand type is convenient to use wherever the work can be done with one hand. The helmet type though not comfortable to wear, leaves both hands free for the work. Shields are made of light weight non‐ reflecting fiber and fitted with dark glasses to filter out the Harmful rays of the arc. In some designs, a cover glass is fitted in front of the dark lens to protect it from spatter. Hand gloves These are used to protect the hands from electric shocks and hot spatters  Fig :3 Electrode holder Fig :4 Ground Clamp Fig:5 Wire brush Fig :6 Chipping hammer Fig:7 Hand gloves Fig :8 Face shield TECHNIQUES OF WELDING Preparation of work Before welding, the work pieces must be thoroughly cleaned of rust, scale and other foreign material. The piece for metal generally welded without beveling the edges, however, thick work pieces hould be beveled or veed out to ensure adequate penetration and fusion of all parts of the weld. But, in either case, the parts to be welded must be separated slightly to allow better penetration of the weld. Before commencing the welding process, the following must be considered 1. Ensure that the welding cables are connected to proper power source. 2. Set the electrode, as per the thickness of the plate to be welded. 3. Set the welding current, as per the size of the electrode to be used. WELDING POSITIONS Depending upon the location of the welding joints, appropriate position of the electrode and hand movement is selected. The figure shows different welding positions. Flat position welding In this position, the welding is performed from the upper side of the joint, and the face of the weld is approximately horizontal. Flat welding is the preferred term; however, the same position is sometimes called down hand. Horizontal position welding In this position, welding is performed on the upper side of an approximately horizontal surface and against an approximately vertical surface. Vertical position welding In this position, the axis of the weld is approximately vertical as shown in figure. Overhead position welding In this welding position, the welding is performed from the underside of a joint Fig :9 Weld positions  V-BUTT JOINT EXPERIMENTNo:01 DATE: Aim: Preparation of butt joint as shown in figure using Shielded Metal Arc Welding process. Material required: M.S Plates – (75 x 50 x 5) 𝑚𝑚3 −2 Nos Tools required 1. Welding transformer 2. Connecting cables 3. Electrode holder 4. Ground clamp 5. Electrodes 6. Chipping hammer 7. Welding shield Procedure 1. The given metallic pieces filled to the desired size. 2. On both pieces beveled in order to have V groove. 3. The metallic pieces are thoroughly cleaned from rust grease, oil. 4. The metallic pieces are connected to terminals of Transformer. 5. Select electrode diameter based on thickness of work piece and hold it on the electrode holder. Select suitable range of current for selected dia. 6. Switch on the power supply and initiates the arc by either striking arc method or touch and drag method. 7. Take welding to be done before full welding. 8. In full welding process after completion one part before going to second part. Slag is removed from the weld bed. With the metal wire brush or chipping hammer. 9. Then the above process will be repeated until to fill the groove with weld bed or weld metal. Precautions Use goggles, gloves in order to protect the human body. Maintain the constant arclength. Result A single V-butt joint is prepared as shown in figure by using arc-welding process.