Welding
• Economical way of joining metal
• Approximately over 50% of the GNP of the U.S.A. is welding
• History
– Bronze Age - 1000 BC - Pressure Welding
– Iron Age - 500 BC - Egyptians welded pieces of iron together
– Middle Ages, Forge Welding
– Welding today - 19th century
Welding
• Welding risks - Estimated 562,000 employees
– exposure to chemical and physical hazards
• Fatality rate - four in one thousand workers
• Deaths include:
– explosions, electrocutions, asphyxiation, falls and crushing injuries
• Construction industry
– welders flash accounts for 5.6% of all eye injuries
• Other risks
– fumes, gases and ionizing radiation, heavy metal poisoning, lung cancer, metal fume fever, flash burns, etc. Risks vary, depending upon the type of welding materials and welding surfaces
Process Terminology
• Common welding processes include:
– Gas Welding
– Arc Welding
– Resistance Welding
– Brazing
– Soldering
• Thermit and Laser Beam Welding are exotic processes
Gas Welding
• One of the most popular welding methods uses a gas flame as a source of heat. In the oxyfuel gas welding process, heat is produced by burning a combustible gas, such as MAPP (methylacetylene-propadiene) or acetylene, mixed with oxygen
• Acetylene and MAPP (methylacetylenepropadiene) are two types of gases used in the gas welding process. These gases require a special torch which mixes with pure oxygen to raise the temperature of two similar pieces of metal to their fusion point, allowing them to flow together. A filler rod is used to deposit additional metal
Arc Welding
• Arc welding is a process that uses an electric arc to join the metals being welded. A distinct advantage of arc welding over gas welding is the concentration of heat. In gas welding the flame spreads over a large area, sometimes causing heat distortion
• All arc-welding processes have three things in common: a heat source, filler metal, and shielding. The source of heat in arc welding is produced by the arcing of an electrical current between two contacts. The power source is called a welding machine or simply, a welder
Resistance Welding
• Resistance welding is a process used to join metallic parts with electric current. There are several forms of resistance welding, including spot welding, seam welding, projection welding, and butt welding.
• In all forms of resistance welding, the parts are locally heated until a molten pool forms. The parts are then allowed to cool, and the pool freezes to form a weld nugget
• To create heat, copper electrodes pass an electric current through the work pieces. The heat generated depends on the electrical resistance and thermal conductivity of the metal, and the time that the current is applied
Brazing
• Brazing is the process of joining metal by heating the base metal to a temperature above 800°F and adding a nonferrous filler metal that melts below the base metal
• Brazing is sometimes called hard soldering or silver soldering because the filler metals are either hard solders or silver-based alloys. Brazing requires distinct joint designs
• Brazing offers important advantages over other metal joining processes. It does not affect the heat treatment of the original metal as much as welding does, nor does it warp the metal as much. The primary advantage of brazing is that it allows you to join dissimilar metals
Soldering
• Soldering is a method of using a filler metal (commonly known as solder) for joining two metals without heating them to their melting points
• Soldering is not classified as a welding or brazing process, because the melting temperature of solder is below 800°F. Welding and brazing usually take place above 800°F. The one exception is lead welding that occurs at 621°F
Thermit Welding
• Thermit Welding normally uses the exothermic reaction of a mixture of iron oxide and aluminum powder or other similar mixtures to weld or repair large forgings and castings, and join pipes and railroad rails
• Thermit welding (TW) is a welding process, which produces bonding of metals by heating them with superheated liquid metal from a chemical reaction between a metal oxide and aluminum with or without the application of pressure
Laser Beam Welding
• Laser beam welding (LBW) is a welding process, which produces bonding of materials with the heat obtained from the application of a concentrated, coherent light beam focussed upon the surfaces to be joined
• The focused laser beam has the highest energy concentration of any known source of energy
• The laser beam is a source of electromagnetic energy or light that can be projected without diverging and can be concentrated to a precise spot
Personal Protective Equipment
• Eye hazards
– stray flashes, reflected glare, flying sparks, and globules of molten metal
• Helmets and welding goggles
– made from nonflammable insulating material
– fitted with a removable protective colored filter and a clear cover lens
• Filter lenses
– variety of shades designated by number
– low number lighter the shade; high number, darker the shade
Personal Protective Equipment
• Gloves (two and five finger)
– Both protect the hands from heat / metal spatter
– The two-finger gloves are better
• less weld spatter / sparks between fingers
• reduce finger chafing
Personal Protective Equipment
• Clothing
– most welding processes require special flameproof clothing Protect against
• radiated heat, splashes of hot metal, or sparks
– consists of aprons, sleeves, combination sleeves and bib, jackets, and overalls
Cylinders (not in book)
• Gas cylinders
– made of high-quality steel
– High-pressure gases stored in seamless cylinders
– low-pressure gases may be welded or brazed
– Cylinders are tested at pressures above maximum permissible charging pressure
• Cylinders are color coded
• Decals
– two decals should be applied on the shoulder of each cylinder
– indicate the name of gas, precautions for handling, and use
Cylinders
• Color Coding Examples
General Cylinder Safety (not in book)
• Transporting & Moving
– Valve protection caps should be in place / secured
– Cylinders should be secured in a cradle, slingboard, or pallet when hoisted
– Cylinders should be moved by tilting and rolling them on their bottom edges. They should not be intentionally dropped, struck, or permitted to strike each other violently
– When cylinders are transported by powered vehicles, they should be secured in a vertical position
General Cylinder Safety
• Transporting & Moving
– Valve protection caps should not be used for lifting cylinders
– Bars should not be used under valves or valve protection caps to pry cylinders
– Regulators should be removed and valve protection caps put in place before cylinders are moved
– A suitable cylinder truck, chain, or other steadying device should be used to keep cylinders from being knocked over while in use
– Cylinder valve should be closed when work is finished, cylinders are empty, or when cylinders are moved at any time
– Compressed gas cylinders should be secured in an upright position at all times
General Cylinder Safety
• Storage
– Oxygen cylinders should be separated from fuel-gas cylinders or combustible materials a minimum distance of 20 feet or by a noncombustible barrier at least 5 feet high having a fire-resistance rating of at least one-half hour
– For inside storage, cylinders should be stored in a well-protected, well-ventilated, dry location
– Cylinders should be stored away from elevators, stairs, or gangways
General Cylinder Safety
• Cylinder Placement
– Cylinders should be kept far enough away from the actual welding or cutting operation so that sparks, hot slag, or flame will not reach them. When this is impractical, fire resistant shields should be provided.
– Cylinders should be placed where they cannot become part of an electrical circuit. Electrodes should not be struck against a cylinder to strike an arc.
General Cylinder Safety
• Cylinder Placement
– Fuel gas cylinders should be placed with valve end up whenever in use
– They should not be placed in a location where they would not be subject to open flame, hot metal, or other sources of artificial heat.
– Cylinders containing oxygen or acetylene or other fuel gas should not be taken into confined spaces.
General Cylinder Safety
• Treatment of Cylinders
– Cylinders, whether full or empty, should not be used as rollers or supports.
– No person other than the gas supplier should attempt to mix gases in a cylinder
– No one except the owner of the cylinder or person authorized by him, should refill a cylinder
– No one should use a cylinder's contents for purposes than those intended by the supplier
– No damaged or defective cylinder should be used.
Gas Welding Hazards
• Acetylene Hazards
– Acetylene is a flammable fuel gas
– When burned with oxygen, acetylene produces a hot flame, (5700°F and 6300°F)
– Acetylene is a colorless gas, having a disagreeable odor that is readily detected
– When a portable welding outfit is used, acetylene is obtained directly from the cylinder
– In the case of stationary equipment, the acetylene can be piped to a number of individual cutting stations
Gas Welding Hazards
• Hazards
– Pure acetylene is self-explosive if stored in free state at 29.4 psi
– Acetylene becomes extremely volatile if used Ý 15 psi
– Gas cylinders are equipped with fusible plugs that relieve excess pressure
– The standard acetylene cylinder contains 225 cf of acetylene weighing about 250 lbs
– The acetylene cylinder is yellow, and all compressed-gas cylinders are color-coded for identification
Mapp Gas Hazards (not in book)
• MAPP (methylacetylene-propadiene) is an all-purpose industrial fuel having the high-flame temperature of acetylene but safer handling characteristics
– MAPP (a liquid) is sold by the pound, rather than by cubic foot, as with acetylene (a gas)
– One cylinder (70 lbs) of MAPP = 6 1/2 acetylene cylinders (225 cf)
• MAPP Gas Safety
– Mapp gas is more stable than Acetylene
– The explosive limits of MAPP gas are 3.4 percent to 10.8 percent in air VS 2.5 percent to 80 percent explosive limits of Acetylene in air. As shown on the slide, you can see these limits are narrow in comparison with that of acetylene
Mapp Gas Hazards (not in book)
Oxygen Hazards
• Oxygen
– colorless, tasteless, odorless gas
– nonflammable but supports combustion with other elements
– cylinders are high pressure (2000 psi), and can become rockets it the valve is broken off
– caps are also slotted for venting in case of rapid release of pressure.
– any substance can be explosively combustible in the presence of pure oxygen
Oxygen Hazards
• Oxygen Safety
– Oxygen cylinders and fittings should be kept away from oil or grease
– Cylinders, cylinder caps and valves, couplings, regulators, hose, and apparatus should be kept free from oil or grease
– Oxygen should not be directed at oily surfaces, greasy clothes, or within a fuel oil or other storage tank or vessel
Torches and Apparatus
– Torch Safety
• Clogged torch tip openings should be cleaned with suitable cleaning wires
• Torches should be inspected for leaking shutoff valves, hose couplings, and tip connections
• NEVER use matches to light the torch; their length requires bringing the hand too close to the tip. Accumulated gas may envelop the hand and, upon igniting, result in a severe burn
Torches and Apparatus
• Backfire
• caused by touching tip against the work
• by overheating the tip
• by operating the torch with incorrect gas pressures
• by a loose tip or head, or dirt on the seat
– If backfire occurs
• close torch valves, check connections, and review operational techniques before relighting torch
Torches and Apparatus
• Flashback
• occurs when flame burns back inside the torch, usually with a shrill hissing or squealing noise
– If flashback occurs
• close the torch oxygen valve that controls the flame to stop the flashback at once
– Flashbacks indicate that something is wrong, either with the torch or with the way it is being operated. Every flashback should be investigated to determine its cause before the torch is relighted
Manifolds
• Fuel gas and oxygen manifolds should be marked for their substance
• Manifolds have non-interchangeable connections
• Hose connections should be kept free of grease / oil
• When not in use, manifold and header hose connections should be capped
• Nothing should be placed on top of a manifold, which will damage the manifold or interfere with the quick closing of the valves
Regulators
• Most common problem is gas leakage between regulator seat and nozzle
• Caused by worn / cracked seats or foreign matter lodged between seat and nozzle
• Particularly dangerous with fuel-gas regulators and the explosive hazard. Ensure these regulators are removed from service for repair
Hoses
• Must be made to withstand internal pressures that can reach as high as 100 psig
• The rubber used in hose manufacture is specially treated to remove the sulfur that could cause spontaneous combustion
• Defective hose should be removed from service
Hoses
• If evidence of severe wear or damage, should be tested at 300 p.s.i.
• Boxes used for storage of gas hose should be ventilated
• Hoses, cables, and other equipment should be kept clear of passageways, ladders, and stairs
Ground Returns and Machine Grounding
• A ground return cable should have a safe current-carrying capacity equal to or exceeding specified maximum output capacity of the arc welding or cutting
• Pipelines containing gases or flammable liquids, or conduits containing electrical circuits, should not be used as a ground return
• When a structure or pipeline is continuously employed as a ground return circuit, all joints should be bonded, and inspected routinely
• The frames of all arc welding and cutting machines should be grounded
Fires and Explosions
Welding in Confined Spaces