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Differences Between Arc Welding Processes

Arc welding processes are based on fusion. Fusion requires closeness and cleanliness at the atomic level, both of which can be achieved by shielding the molten puddle with gas or slag. There are several types of arc welding processes as follows:

Shielded Metal Arc Welding (SMAW)

An electric arc is produced between the end of a coated metal electrode and the steel components to be welded (Figure 1). The electrode is a filler metal covered with a coating. The electrode’s coating has two purposes:

1. It forms a gas shield to prevent impurities in the atmosphere from getting into the weld, and 
2. It contains a flux that purifies the molten metal.

SMAW is almost exclusively a manual arc welding process. Because of its versatility and simplicity, it is particularly dominant in the maintenance and repair industry. The most common quality problems associated with SMAW include weld spatter, porosity, poor fusion, shallow penetration and cracking.

Figure 1: Shielded Metal Arc Welding (SMAW) 

Gas Metal Arc Welding (GMAW)

Gas Metal Arc Welding (GMAW) is fast and economical. As shown in Figure 2, a continuous wire is fed into the welding gun. The wire melts and combines with the base metal to form the weld. The molten weld metal is protected from the atmosphere by a gas shield that is fed through a conduit to the tip of the welding gun. The process may be semi- automatic or automated. It cannot be used in a windy environment as the loss of the shielding gas from air flow will produce porosity in the weld.

Figure 2: Gas Metal Arc Welding (GMAW)

Flux Cored Arc Welding (FCAW)

Flux Cored Arc Welding (FCAW) is similar to the GMAW process and is usually performed by semi/full automatic methods. The difference is that the filler wire has a center core that contains flux (see Figure 3). With this process it is possible to weld with or without a shielding gas, which makes it useful for exposed conditions where a shielding gas may be affected by the wind.

Figure 2: Flux Cored Arc Welding (FCAW)

Submerged Arc Welding (SAW)

Submerged Arc Welding (SAW) is usually performed by semi/full automatic or handheld methods. As shown in Figure 4, it uses a continuously fed filler metal electrode. The weld pool is protected from the surrounding atmosphere by a blanket of granular flux fed at the welding gun. It results in a deeper weld penetration than the other processes. However, only flat or horizontal positions may be used.

Figure 4: Submerged Arc Welding (SAW)

Process Selection

Selection of the welding process is typically left to the contractor. The characteristics of the various processes are:

• SAW: long, big, semi/full automatic or handheld methods.
• FCAW: semi/full automatic methods.
• SMAW: small, miscellaneous, repair, tack welds and handheld method.
• GMAW: semi/full automatic methods in shop.

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Reprinted from the Michigan Department of Transportation Field Manual for Structural Welding.

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Welding Terminology

• Arc Welding – A welding process that produces a coalescence of metals by heating them with an electric arc.
• Arc Welding Electrode – A part of the welding system that conducts current and ends at the arc.
• Back Gouging – The removal of weld and base metal from the other side of a partially welded joint. The objective is to assure complete penetration upon subsequent welding from that side.
• Backing – A piece of material placed at the root of a weld joint for the purpose of supporting molten weld metal.
• Base Metal – The metal to be welded.
• Bevel – An angled edge preparation.
• Butt Joint – A joint between two members lying in the same plane.
• Complete Fusion – Fusion over the entire fusion faces and between all adjoining weld beads.
• Complete Joint Penetration – A groove weld condition in which weld metal extends through the joint thickness.
• Defect – A discontinuity or discontinuities that, by nature or accumulated effect (for example, total crack length or amount of porosity), renders a part or product unable to meet minimum applicable acceptance standards or specifications.
• Depth of Fusion – The distance that fusion extends into the base metal or previous bead from the surface melted during welding.
• Facing Surface – The surfaces of materials in contact with each other and joined or about to be joined together.
• Filler Material – The material to be added in making a welded, brazed, or soldered joint.
• Fillet Weld – A weld of approximately triangular cross section that joins two surfaces approximately at right angles to each other in a lap joint, T-joint, or corner joint.
• Flat Welding Position – A welding position where the weld axis is approximately horizontal and the weld face lies in an approximately horizontal plane.
• Flux – Material used to prevent, dissolve, or facilitate removal of oxides and other undesirable surface substances.
• Flux Cored Arc Welding (FCAW) – An arc welding process that produces coalescence of metals by means of a tubular electrode. Shielding gas may or may not be used.
• Fusion – The melting together of filler metal and base metal, or of base metal only, to produce a weld.
• Gas Metal Arc Welding (GMAW) – An arc welding process where the arc is between a continuous filler metal electrode and the weld pool. Shielding from an externally supplied gas source is required.
• Gas Tungsten Arc Welding (GTAW) – An arc welding process where the arc is between a tungsten electrode (non-consumable) and the weld pool. The process is used with an externally supplied shielding gas.
• Groove Weld – A weld made in a groove between two members.
• Heat-Affected Zone (HAZ) – That section of the base metal, generally adjacent to the weld zone, whose mechanical properties or microstructure have been altered by the heat of welding.
• Hot Crack – A crack formed at temperatures near the completion of weld solidification.
• Incomplete Fusion – A weld discontinuity where fusion did not occur between weld metal and the joint or adjoining weld beads.
• Incomplete Joint Penetration – A condition in a groove weld where weld metal does not extend through the joint thickness.
• Interpass Temperature – In a multi-pass weld, the temperature of the weld area between passes.
• Joint – The junction of the edges of members that are to be joined or have been joined.
• Lap Joint – A joint type in which the non-butting ends of one or more workpieces overlap approximately parallel to one another.
• Leg of Fillet Weld – The distance from the root of the joint to the toe of the fillet weld.
• Metal Cored Arc Welding – An arc welding process with a tubular electrode where the hollow electrode contains alloying materials.
• Metal Cored Electrode – A composite tubular electrode consisting of a metal sheath and a core of various powdered materials, producing no more than slag islands on the face of the weld bead. External shielding is required.
• Peening – The mechanical working of metals using impact blows.
• Plug Weld – A circular weld made through a hole in one member of a lap or T-joint.
• Porosity – A hole-like discontinuity formed by gas entrapment during solidification.
• Post Weld Heat Treatment (PWHT) – Any heat treatment subsequent to welding.
• Preheating – The application of heat to the base metal immediately before welding, brazing, soldering, thermal spraying, or cutting.
• Preheat Temperature – The temperature of the base metal immediately before welding is started.
• Procedure Qualification Record (PQR) – The demonstration that the use of prescribed joining processes, materials and techniques will result in a joint exhibiting specified soundness and mechanical properties.
• Reinforcement – Weld metal, at the face or root, in excess of the metal necessary to fill the joint.
• Root Opening – A separation at the joint root between the work pieces. Root Crack – A crack at the root of a weld.
• Self-Shielded Flux Cored Arc Welding (FCAW-S) – A flux-cored arc welding process variation in which shielding gas is obtained exclusively from the flux within the electrode.
• Shielded Metal Arc Welding (SMAW) – A process that welds by heat from an electric arc between a flux-covered metal electrode and the work. Shielding comes from the decomposition of the electrode covering.
• Shielding Gas – Protective gas used to prevent atmospheric contamination.
• Submerged Arc Welding (SAW) – A process that welds with the heat produced by an electric arc between a bare metal electrode and the work. A blanket of granular fusible flux shields the arc.
• Tack Weld – A temporary weld used to hold parts in place while more extensive, final welds are made.
• Tensile Strength – The maximum stress a material subjected to a stretching load can withstand without tearing.
• Weld Bead – The metal deposited in the joint by the process and filler wire used.
• Welding Leads – The work piece lead and electrode lead of an arc welding circuit.
• Weld Metal – The portion of a fusion weld that has been completely melted during welding.
• Weld Pass – A single progression of welding along a joint. The result of a pass is a weld bead or layer.
• Weld Pool – The localized volume of molten metal in a weld prior to its solidification as weld metal.
• Weld Puddle – A non-standard term for weld pool.
• Welding Sequence – The order in which weld beads are deposited in a weldment.

Reprinted from the Michigan Department of Transportation Field Manual for Structural Welding.

Controlling Hazardous Fume and Gases During Welding

From OSHA FactSheet courtesy of Hotfoil-EHS

Welding joins materials together by melting a metal work piece along with a filler metal to form a strong joint. The welding process produces visible smoke that contains harmful metal fume and gas by-products. This article discusses welding operations, applicable OSHA standards, and suggestions for protecting welders and coworkers from exposures to the many hazardous substances in welding fume.

Types of welding

Welding is classified into two groups: fusion (heat alone) or pressure (heat and pressure) welding. There are three types of fusion welding: electric arc, gas and thermit. Electric arc welding is the most widely used type of fusion welding. It employs an electric arc to melt the base and filler metals. Arc welding types in order of decreasing fume production include:

• Flux Core Arc Welding (FCAW) filler metal electrode; flux shield
• Shielded Metal Arc (SMAW) electrode provides both flux and filler material
• Gas Metal Arc (GMAW or MIG) widely used; consumable electrode for filler metal, external gas shield
• Tungsten Inert Gas (GTAW or TIG) superior finish; non-consumable electrode; externally-supplied inert gas shield

Gas or oxy-fuel welding uses a flame from burning a gas (usually acetylene) to melt metal at a joint to be welded, and is a common method for welding iron, steel, cast iron, and copper. Thermit welding uses a chemical reaction to produce intense heat instead of using gas fuel or electric current. Pressure welding uses heat along with impact-type pressure to join the pieces.

Oxy-fuel and plasma cutting, along with brazing, are related to welding as they all involve the melting of metal and the generation of airborne metal fume. Brazing is a metal-joining process where only the filler metal is melted.

What is in welding fume?

Metals: Aluminum, Antimony, Arsenic, Beryllium, Cadmium, Chromium, Cobalt, Copper, Iron, Lead, Manganese, Molybdenum, Nickel, Silver, Tin, Titanium, Vanadium, Zinc.

Gases:

• Shielding—Argon, Helium, Nitrogen, Carbon Dioxide.
• Process—Nitric Oxide, Nitrogen Dioxide, Carbon Monoxide, Ozone, Phosgene, Hydrogen Fluoride, Carbon Dioxide.

Factors that affect worker exposure to welding fume

• Type of welding process
• Base metal and filler metals used
• Welding rod composition
• Location (outside, enclosed space)
• Welder work practices
• Air movement
• Use of ventilation controls

Health effects of breathing welding fume

• Acute exposure to welding fume and gases can result in eye, nose and throat irritation, dizziness and nausea. Workers in the area who experience these symptoms should leave the area immediately, seek fresh air and obtain medical attention.
• Prolonged exposure to welding fume may cause lung damage and various types of cancer, including lung, larynx and urinary tract.
• Health effects from certain fumes may include metal fume fever, stomach ulcers, kidney damage and nervous system damage. Prolonged exposure to manganese fume can cause Parkinson’s–like symptoms.
• Gases such as helium, argon, and carbon dioxide displace oxygen in the air and can lead to suffocation, particularly when welding in confined or enclosed spaces. Carbon monoxide gas can form, posing a serious asphyxiation hazard.

Welding and Hexavalent Chromium

• Chromium is a component in stainless steel, nonferrous alloys, chromate coatings and some welding consumables.
• Chromium is converted to its hexavalent state, Cr(VI), during the welding process.
• Cr(VI) fume is highly toxic and can damage the eyes, skin, nose, throat, and lungs and cause cancer.
• OSHA regulates worker exposure to Cr(VI) under its Chromium (VI) standard, 29 CFR 1910.1026 and 1926.1126.
• OSHA’s Permissible Exposure Limit (PEL) for Cr(VI) is 5 μg/ m3 as an 8-hour time-weighted average.

Reducing exposure to welding fume

• Welders should understand the hazards of the materials they are working with. OSHA’s Hazard Communication standard requires employers to provide information and training for workers on hazardous materials in the workplace.
• Welding surfaces should be cleaned of any coating that could potentially create toxic exposure, such as solvent residue and paint.
• Workers should position themselves to avoid breathing welding fume and gases. For example, workers should stay upwind when welding in open or outdoor environments.
• General ventilation, the natural or forced movement of fresh air, can reduce fume and gas levels in the work area. Welding outdoors or in open work spaces does not guarantee adequate ventilation. In work areas without ventilation and exhaust systems, welders should use natural drafts along with proper positioning to keep fume and gases away from themselves and other workers.
• Local exhaust ventilation systems can be used to remove fume and gases from the welder’s breathing zone. Keep fume hoods, fume extractor guns and vacuum nozzles close to the plume source to remove the maximum amount of fume and gases. Portable or flexible exhaust systems can be positioned so that fume and gases are drawn away from the welder. Keep exhaust ports away from other workers.
• Consider substituting a lower fume-generating or less toxic welding type or consumable.
• Do not weld in confined spaces without ventilation. Refer to applicable OSHA regulations (see list below).
• Respiratory protection may be required if work practices and ventilation do not reduce exposures to safe levels.

Some OSHA standards applicable to welding:

• Welding, Cutting & Brazing—29 CFR 1910, Subpart Q
• Welding & Cutting—29 CFR 1926 Subpart J
• Welding, Cutting & Heating—29 CFR 1915, Subpart D
• Permit-required confined spaces—29 CFR 1910.146
• Confined & Enclosed Spaces & Other Dangerous Atmospheres in Shipyard Employment—29 CFR 1915 Subpart B
• Hazard Communication—29 CFR 1910.1200
• Respiratory Protection—29 CFR 1910.134
• Air Contaminants—29 CFR 1910.1000 (general industry), 29 CFR 1915.1000 (shipyards), 29 CFR 1926.55 (construction)

More Information:

For more information on hexavalent chromium exposure, visit OSHA’s website at www.osha.gov.

Article reprinted here courtesy of Hotfoil-EHS.

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Air-Arc Gouging

Air arc gouging uses a generated electric arc to melt metal between the tip of a carbon electrode and the workpiece. High velocity air is shot down the electrode to blow the molten metal away, leaving a clean gouge. Gouging works on any conductive metal, including stainless steel, mild steel, copper, and aluminum. Typical applications include removal of surface and internal defects, removal of excess metal around welds, and edge preparation before welding.

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Prevent Electrostatic Precipitator and Baghouse Hopper Blockage with Hopper Heaters

Coal-fired power plants in the U.S. require the use of electrostatic precipitators or bag houses to filter out very fine fly ash particles incorporated into flow gas. The ash is collected while the flue gas passes through filter bags or large electrodes and then falls into hoppers. As the hot fly ash cools, it may condense on the hopper walls. The mixture of dry, sulfur-rich fly ash and water is very problematic, so it is very important that there is no condensation in the collection hoppers.

The mixture of water and fly ash can cause the hopper to block up (or "pluggage "), and most importantly, residual sulfur in the flue gas will combine with condensate to form sulfuric acid. The sulfuric acid attacks the inside of the hopper walls, causing corrosion, weakening walls and generating significant (and costly) maintenance problems over time.

Efficient and continuous removal of fly ash is essential for all coal-fired power plants. Collection hoppers are an integral part of the removal process. Plugging or inoperable hoppers are a known problem for engineers and maintenance crews. Constant maintenance and excess downtime seriously prevent a plant's ability to manage the production rate of fly ash. Slower fly ash production means a reduction in energy production and efficiency. The power generation of a power station is directly proportional to its rate of combustion of coal, which in turn directly affects the production of fly ash. The maintenance personnel usually attempt to remedy ash system failures in real time by disabling the affected hopper, while continually generating electricity and ash. In some situations (to prevent shutdowns of boilers), ash will be dumped on the floor, requiring costly cleaning.

Evacuation and management of fly ash is much easier if the ash is kept warm. One of the most common ways of maintaining high fly ash and hopper temperatures is by connecting electric hopper heaters to the outside hopper walls. Hopper heaters play a very important role in removing the fly ash from precipitators and bag filter walls by keeping the hopper temperatures over the flue gas acid dew point. The only function of the hopper heater is to preheat the hopper and the internal environment to prevent the formation of moisture, fly ash clumping and the development of sulphuric acid.

Hopper heaters are designed for a dirty, high-vibration power plant environment. They provide the optimum watt density for proper thermal transfer through the hopper wall and uniform heating. They are available in square, rectangular and trapezoidal shapes for any hopper design. For poke tubes, man-ways and cylindrical throats, ancillary flexible heating cloths are available. The use of electric hopper heaters in electrostatic precipitator and bag house fly ash collection systems is an effective time-tested way to prevent condensation and the resulting clumping and corrosive acids in hopper products, thus providing better opportunities for continuous production of fly ash.