Hot Work Processes
Welding involves joining two or more pieces of metal together to form a single piece. Molten metal is generated through an intense heat source, such as oxygen and fuel gas or an electrical arc. Common welding processes using an electrical arc include Shielded Metal Arc Welding, Gas Metal Arc Welding, and Gas Tungsten Arc Welding.
Unlike welding processes which join two pieces of metal, cutting processes involve separating or severing a piece of metal through intense heat generated to melt the metal. Cutting processes include oxygen and fuel gas and electrical arc gouging.
Gas welding, or oxy/fuel welding as it is commonly referred to, is slower and easier to control than arc welding. This method unites metals by heating - the heat source being a flame produced by the combustion of a fuel gas, such as acetylene, methylacetylene (MAPP gas), or hydrogen. Temperatures can reach up to 6,000 deg. F. This process sometimes includes the use of pressure and filler material. Gases commonly used are oxygen and either acetylene, hydrogen, propane, or propylene. These gases are commonly supplied in compressed gas cylinders, which can pose additional handling and transport hazards.
Review the OSHA standard on gas welding and cutting.
In arc welding, the intense heat needed to melt metal is produced by an electric arc. The arc is formed between the actual work piece and an electrode (stick or wire) that is manually or mechanically guided along the joint. The electrode can either be a rod, with the purpose of simply carrying the current between the tip and the work, or it may be a specially prepared rod or wire that not only conducts current but also melts and supplies filler metal to the joint. Power sources for arc welding can be either alternating (AC) or direct (DC) current. The work cable connects to the work piece and the electrode cable creates an arc across the gap when the energized circuit and the electrode tip touches the workpiece and is withdrawn (yet still in close contact). The arc produces a temperature of about 6,500 deg. F at the tip. This heat melts both the base metal and the electrode, producing a pool of molten metal. Metals at high temperatures can react chemically with elements in the air (oxygen and nitrogen). Oxides and nitrides form, which destroy the strength of the weld. A protective shield of gas, vapor, or slag is used to cover the arc and molten pool to prevent or minimize contact or molten metal with air.
For more information, review this article from the National Ag Safety Database on Arc Welding Safety.
Review the OSHA standard for arc welding and cutting.
Shielded Metal Arc Welding (SMAW) is commonly known as "stick" welding. A flux-covered electrode is used to form a gas shield around the molten weld pool. The flux coating quickly forms a protective "slag" during welding, which produces a gas shield that decreases exposure to oxygen. The electrode is consumed as it moves down the length of the weld joint and the "slag" must cool and later be chipped away.
Gas Metal Arc Welding (GMAW) is commonly known as "MIG" welding. A continuous-feed electrode (i.e. wire) from a spool is used to supply filler metal directly from the torch tip to the weld. As arcing occurs, the electrode instantly melts and a shielding gas, such as argon, carbon dioxide, or helium, is supplied through the torch tip.
Gas Tungsten Arc Welding (GTAW) is commonly known as "TIG" welding. An electric arc between a tungsten electrode and the base metal is created. A separate filler rod is fed into the molten base metal if needed. A shielding gas (i.e. commonly argon, helium, or carbon dioxide) also flows around the arc to minimize atmospheric interactions. Water is often used to cool the torch and cables.
Plasma Arc Welding (PAW) is similar to TIG welding in which an arc, shielded by an inert gas, creates the necessary heat to melt the metals involved. The electrode is not consumed in PAW; however, the primary means of transferring heat to the workpiece is by a hot ionized gas (i.e. "plasma"). Temperatures can reach up to 30,000 deg. F, which is substantially hotter than those produced by an arc only. Commonly, PAW is a fully automatic process. Filler metal may be used, and plasma and shielding gases include argon, argon/helium, and argon/hydrogen.
Brazing is a process similar to welding in that a liquid filler metal is heated and flows between two or more metal surfaces to be joined. It is very flexible in that any number of metals may be joined. However, brazing occurs at lower temperatures than welding, typically around 840 degrees F. A braze metal is heated to a liquified state and is spread over the surface to be joined, rather than both the base metal and filler metal being heated to a molten state as in welding.
Parts to be joined must be very clean, often using mechanical methods such as sanding, grinding, abrasive blasting, or the use of chemical solvents. There are several types of brazing based upon the source of heat. Brazing is commonly used to seal or join pipes and the associated hazards are similar to those of welding. Reviewing Safety Data Sheets for the metals, cleaning agents, fluxes, and filler metals is very important in identifying associated health hazards and implementing appropriate hazard controls.
Soldering is similar to welding in that both the base metal and the filler metals are heated to melting and then solidify to form a joint; however, soldering temperatures are typically 840 deg. F or less. Soldering typically involves smaller components to be joined and "softer" metals such as lead/tin or silver. Manual soldering processes use a hand-held iron to heat the components to be joined and the filler metals. Often the filler metal is in the form of a flux-cored wire with additional flux added to assist with wetting (i.e. flow). Always review the Safety Data Sheet for the materials involved. Where possible, lead-free solder should be used to avoid potential exposure to lead.
There are several relatively new heat sources for welding and cutting, such as friction, ultrasonics, and lasers. Each of these special heat sources requires guarding and safe practices, and is beyond the scope of this program. Follow the manufacturer's recommendations for safe and proper use of equipment.