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 a 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.
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cable creates an arc across the gap when the energized circuit and the electrode tip touches the workpiece and is withdrawn (yet still in close contact).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.
– Shielded Metal Arc Welding
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
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
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
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 transfering 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.
Braze welding differs from gas welding in that the melting temperature of the filler metal being used is below that at which the base metal will melt, but at or above the melting temperature of the filler material. Braze welding differs from brazing in that the joint design for braze welding is similar or identical to that used in gas welding. Capillary action is not a factor in the formation of a bond.
While acetylene gas is always required as the fuel for gas welding, braze welding can be performed with the other fuel gases such as propane, natural gas, propylene, etc., as well as with acetylene. This is due to the fact that in braze welding there is addition of flux to perform the functions normally associated with pre-cleaning and the deoxidizing characteristic of the acetylene-oxygen flame.
Braze welding is most commonly associated with joining of steels. In general, the filler material used is a low fuming bronze – either a bare rod which is dipped into a flux material or a bare rod with a flux coating on the rod itself. In actual practice, the steel to be joined is heated to an approximate dull red color at which point the low fuming bronze filler material is applied with the flux and it flows against the steel to be joined, creating a tight, adherent bond.
In general, the strength level of a braze-welded joint is lower than that of a gas-welded joint. The advantages of braze welding are:
- Lower heat input which results in less distortion.
- The steels to be welded do not have to be melted in order to create the welded joint.
- Dissimilar materials can be readily joined where this might not be possible with gas welding
Brazing and Soldering Hazards and Precautions
Cadmium and Beryllium: Brazing and soldering with or on alloys containing cadmium or beryllium can be extremely hazardous because the fumes are extremely toxic and can cause death.
- Always avoid skin contact with cadmium and beryllium.
- Consult an expert in industrial hygiene whenever using cadmium or beryllium compounds or when performing repairs on parts containing the metals.
Fluoride Compounds: Fluxes containing fluoride compounds are also toxic. Good ventilation is essential when soldering or brazing and the operator should always observe good safety practices.
A common hazard when soldering is exposure of the skin, eyes, and clothing to acid fluxes. Be sure to observe these safety precautions when brazing or soldering:
- Always work in a way that flux will not be spilled on the skin or clothing.
- Always wear chemical splash-proof goggles, rubber gloves, and long sleeves when using cleaning solutions, pickling solutions, or acids.
- If you are exposed to any chemical solutions, acids, or fluxes, wash the affected area at once, and seek medical attention immediately.
- Remove or keep away all flammable material from the heating flames. Remember, heating soldering copper sometimes presents a fire hazard if an open flame is used.
- When performing hot work, make sure there are no flammable vapors present, such as gasoline, acetylene, or other flammable gases.
- Do NOT start a job until you have taken all safety precautions and the fire marshal notified, if applicable.