Welding, in engineering, covers a number of processes in which two or more pieces of metal are joined together by the application of heat, pressure, or a combination of both. The main processes are the pressure-welding processes and heat welding.
The welding process used to join two pieces of metal depends on the physical properties of the materials to be joined, the specific use to which they are applied and the production facilities available. With the development of new techniques during the first half of the 20th century, welding supplanted bolting and riveting in the construction of many types of structures including bridges, buildings and ships.
Welding processes are generally classified according to the sources of heat and pressure used. The original pressure process was forge welding but now the two processes most commonly employed are oxyacetylene gas and electric arc welding.
Arc-welding processes are the most important welding processes, particularly when joining steel beams and rods. This is performed using an electric arc which is formed between the weld materials and a consumable stick electrode which is used to deposit the weld metal. Sometimes these electrodes are coated with a flux to aid the weld process. The flux aims to prevent oxidisation of the weld metal as it is deposited, so with less oxidisation, the stronger the weld will be.
It requires a continuous supply of either direct or alternating electrical current. The electricity is continuously fed through the welding torch and deposited as the weld metal, where no flux is used, an inert gas such as argon, helium or carbon dioxide is used to prevent oxidisation. They have several advantages over other welding methods in certain applications. The welding speed is faster because of high heat concentration which also tends to reduce distortion. Also, in certain methods of arc welding, the use of fluxes may be eliminated.
There are a number of hazards associated with arc welding:-
Gases – Toxic and / or asphyxiating;
Radiation – Ultraviolet, visible and infra red;
Spatter and hot components;
Fire and explosion; and
The arc generates three types of radiation that may be directed or reflected from bright surfaces. It can cause injuries in the following ways:
Ultraviolet radiation – causes damage to skin., in the form of severe sunburn, and eyes (inflammation of the cornea and cataracts);
Visible light – dazzles eyes and can impair vision; and
Infrared (heat) radiation – damages skin and eyes (cataracts and corneal lesions).
Use protection against welding radiation for: The face and eyes by using a suitable welding shield equipped with eye protection filter.
The body by wearing suitable clothing. People in the vicinity of the arc need to be protected by means of suitable positions, non- reflective curtains or screens.
Incorrect welding parameters can generate excessive spatter. However, it will always be present so always wear appropriate personal protective equipment. Welded components will remain hot for some time so avoid handling them.
Arc welding generates sparks and hot spatter that can ignite combustible materials such as wood, paper, cardboard, textiles, rubber and plastics. The following precaution will help to prevent fire:
Move the work piece to a safe location; Remove combustible material from the welding area and cover remaining flammable material with fire-resistant material;
Use covers to prevent spatter passing through openings in floors and walls; Keep appropriate fire-fighting equipment nearby; and
After welding, observe surrounding work area for at least an hour afterwards to ensure no fire is developing.
There is a danger of explosion when welding tanks or drums that previously contained flammable substances. Even ‘empty’ containers may still have residues in the bottom enough to cause an explosion.
Never weld a container that has held a flammable substance unless it is known to have been made safe. Carry out thorough cleaning or inerting before welding – see Guidance Note CS15 The Cleaning and Gas Freeing of Tanks Containing Flammable residues.