Welding introduction

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WELDING

Introduction :
Welding is a process of joining two similar or dissimilar metals by the application of heat with or without application of pressure and addition of filler material. The welding is extensively used in manufacturing industry, construction industry and maintenance work, replacing riveting and bolting, to a greater extent. 
The various welding processes are
1. Electric arc welding
2. Gas welding
3. Thermit welding
4. Resistance welding and
5. Friction welding
However only electric arc welding is discussed here. In this process, the work pieces are melted along a common edge, to their melting point and then a filler metal is introduced to form the joint on solidification. The materials to be welded must be free from rust, scale oil or other impurities, so as to obtain a sound weld.

Arc Welding:
In arc welding, the heat required for joining the metals is obtained from an electric arc. Transformers or motor generator sets are used as arc welding machines. These machines supply high electric currents at low voltages and an electrode is used to produce the necessary arc. The coated electrode serves as the filler rod and arc melts the surfaces so that the metals to be joined are actually fused together. In addition to the welding machine, the following accessories and tools are needed for carrying out the welding work. The given figure shows the principle of Arc Welding using a transformer.

Welding Cables: Two welding cables are required, one from the machine to the electrode holder and the other, from the machine to the ground clamp. Cables are specified by their current carrying capacity say 300A, 400A etc.
Electrodes: Filler rods 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 bare 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 and protects from oxidization.

Electrode Holder: The electrode holder is connected to the end of the welding cable and holds the electrode. It should be light, strong, easy to handle and should not become hot while in operation. 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.
Bench Vice: It is commonly used for holding the work piece. It has to harden Steel jaws having serrations to ensure good grip. When the vice handle is turned to a clockwise direction, the sliding jaw forces the work piece against the fixed jaw and is fixed on the working table.
Hack Saw: The hacksaw is used for sawing all metals except hardened steel. A hand hacksaw consists of frame, handle blade, prongs, tightening screw and nut.
Steel Rule: The steel rule is one of the most useful tools in the workshop for taking linear measurements of blanks and articles to an accuracy of 1.0 to 0.5mm
Scriber: A scriber is a slender steel tool, used to scribe or mark lines on metal work pieces.

Try Square: Try square is used for making and testing angles of 90 degrees and is used for checking the squareness of many types of small works, when extreme accuracy is not required.
Flat File: Flat file is used to remove small amount of material from the surface of the metal part. It is also used to remove sharp edges from the metal pieces.
Wire Brush or File Card: A wire brush is used for cleaning and preparing the work piece before and after the welding.
Welding Table: It is made of steel plate and pipes. It is used for positioning the parts to be welded properly.
Welding Screen: When people are around where welding is done, they may be protected from the rays of the arc by means of a protected screen.
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 in hand or helmet type. The hand type is preferred to use and they are made of light weight, non reflecting fiber fitted with dark glass.
Hand Gloves: Hand gloves are used for protecting the arms while welding form sparks and from current.
Flat Tong: A flat tong is used for holding works of rectangular shape during welding and after welding.
Chipping Hammer: It is made of tool steel and is used for removing slag formation on welds. One end of the head is sharpened like a cold chisel and the other end to a blunt, round
point.
Ball Peen Hammer: Ball peen hammers are used to stride a job or a tool. It consists of hour parts namely peen, head, eye and face. Ball Peen hammer is chiefly used for chipping and riveting.
Welded Joints:
The relative positions of the two pieces being joined determine the type of joint. The following joints are commonly used in fusion welding.
The Butt Joint : It is used to join the ends or edges of two plates or surfaces located approximately in the same plane with each other.
The Lap Joint: It is used to join two overlapping plates so that the edge of each plate is welded to the surface of the other.
The T-Joint: It is used to weld two plates or sections whose surfaces are at approximately right angles to each other.
The Corner Joint: It is used to join the edges of two sheets or plates whose surfaces are at the angle of approximately 90 deg to each other. It is common in the construction of boxes, tanks, frames etc.

Preparation of Work:
Before welding, the work pieces must be thoroughly cleaned of rust, scale or other foreign material. Thin pieces of metal are generally welded without beveling the edges. However, thick work pieces should be beveled to ensure adequate penetration and fusion of all parts of the weld. But, in either case, the parts to be welded must separated slightly to allow better penetration of the weld.
Different edge preparation is particularly used in fusion welding process for welding butt joints.
These are: square Single-V, Double-V, Single-U and Double-U. The preparation of edges depends upon the thickness of metal being welded.

Instructions:
General:

1. Always weld in a well-ventilated place. Fumes given off from welding are unpleasant and in some cases may be injurious, particularly from galvanized or zinc coated parts
2. Do not weld around combustible or inflammable materials, where sparks may cause a fire.
3. Never weld, containers, which have been used for storing gasoline, oil or similar materials, without first having them thoroughly cleaned

Arc welding:

1. Check the welding machine to make sure that it is properly grounded and that all leads are properly insulated.
2. Never look at the arc with a naked eye. The arc can burn your eyes severely. Always use a face shield while welding.
3. Prevent welding cables from coming in contact with hot metal, water oil or grease.Avoid dragging the cables around sharp corners.
4. Ensure proper insulation of the cables and check for openings.
5. Always wear the safety hand gloves, apron and leather shoes.
6. Always turn-off the machine when leaving the work
7. Apply eye drops after welding is over for the day to relieve the strain on the eyes.
8. While welding, stand on dry footing and keep the body insulated from the electrode, any bare parts of the electrode holder and the work

Safety Precautions:

1. Use welding screen, leather apron and leather hand gloves while welding
2. Use flat tong and hand gloves for handling of work pieces during and after welding.