Learn Welding Processes Explained – Oxyfuel, Arc Welding (TIG, MIG, SMAW, SAW, FCAW), Beam Welding and Resistance Welding – Principles, Applications, Equipment and Selection Criteria

Introduction:

Welding involves the partial melting and fusion between two members to be joined. Fusion welding is defined as melting together and coalescing materials by means of heat. Filler metals which are metals added to the weld area during welding may also be used. Fusion welds made without the use of filler metals are known as autogenous welds. The major classes of fusion welding processes include oxyfuel gas welding, arc welding and high energy beam welding processes such as laser beam and electron beam welding. Each of these process families has important and unique applications in modern manufacturing. The selection of an appropriate welding process depends on numerous factors including the types of materials being joined, the thickness of the workpieces, the required joint strength, production volume and economic considerations.

Beyond the fundamental principles of each welding process, a comprehensive understanding of welding technology requires knowledge of weld-zone features and the variety of discontinuities and defects that can exist in welded joints. Additionally, the weldability of various ferrous and nonferrous metals and alloys varies considerably. Design guidelines for welding play a crucial role in ensuring successful joints. As in all manufacturing processes, the economics of welding is a significant aspect of the overall operation, encompassing welding processes, equipment and labor costs.

Oxyfuel Gas Welding:

Oxyfuel gas welding (OFW) is a general term used to describe any welding process that uses a fuel gas combined with oxygen to produce a flame. The flame is the source of heat used to melt the metals at the joint. The most common gas welding process uses acetylene and is known as oxyacetylene gas welding (OAW) typically used for structural metal fabrication and repair work. Developed in the early 1900s, OAW utilizes the heat generated by the combustion of acetylene gas in a mixture with oxygen. The heat generation occurs in accordance with a pair of chemical reactions. The primary combustion process which occurs in the inner core of the flame involves the dissociation of acetylene into carbon monoxide and hydrogen and produces about one third of the total heat generated in the flame. The secondary combustion process consists of the further burning of both the hydrogen and the carbon monoxide and produces about two thirds of the total heat. This reaction also produces water vapor. The temperatures developed in the flame can reach 3300°C.

Flame Types:

The proportion of acetylene and oxygen in the gas mixture is an important factor. At a ratio of 1:1, the flame is considered neutral. With a greater oxygen supply, the flame is an oxidizing flame which can be harmful for steels because it oxidizes the metal. Only in the welding of copper and copper based alloys is an oxidizing flame desirable because in those cases a thin protective layer of slag forms over the molten metal. If the oxygen is insufficient for full combustion, the flame is known as a reducing or carburizing flame with excess acetylene. The temperature of a reducing flame is lower making it suitable for applications requiring low heat such as brazing, soldering and flame hardening operations. Other fuel gases such as hydrogen and methylacetylene propadiene can also be used, but the temperatures developed are lower than those produced by acetylene. They are used for welding metals with low melting points and parts that are thin and small. The flame with pure hydrogen gas is colorless making it difficult to adjust by eyesight.

Filler Metals and Fluxes:

Filler metals supply additional metal to the weld zone and are available as filler rods or wire either bare or coated with flux. The purpose of the flux is to retard oxidation by generating a gaseous shield around the weld zone. The flux also helps to dissolve and remove oxides and other substances, contributing to the formation of a stronger joint. The slag developed protects the molten puddle of metal against oxidation as it cools.

Welding Practice, Equipment and Safety:

Oxyfuel gas welding can be used with most ferrous and nonferrous metals for almost any workpiece thickness but the relatively low heat input limits the process to thicknesses of less than 6 mm. Small joints may consist of a single weld bead while deep V-groove joints are made in multiple passes. Cleaning the surface of each weld bead prior to depositing a second layer is important for joint strength and in avoiding defects. Wire brushes may be used for this purpose. The equipment consists of a welding torch connected by hoses to high pressure gas cylinders and equipped with pressure gages and regulators. Safety equipment such as goggles with shaded lenses, face shields, gloves and protective clothing is essential. Proper connection of the hoses to the cylinders is important. Oxygen and acetylene cylinders have different threads so the hoses cannot be connected to the wrong cylinders. The low equipment cost is an attractive feature. Although it can be mechanized, this operation is essentially manual and slow. However it has the advantages of being portable, versatile and economical for simple and low quantity work.

Pressure Gas Welding:

In pressure gas welding, the welding of two components starts with the heating of the interface by means of a torch using typically an oxyacetylene gas mixture. After the interface begins to melt, the torch is withdrawn. A force is applied to press the two components together and is maintained until the interface solidifies. This results in the formation of a flash due to the upsetting of the joined ends.

Arc Welding Processes with Nonconsumable Electrodes:

In arc welding developed in the mid-1800s, the heat required is obtained from electrical energy. The process involves either a consumable or a nonconsumable electrode. An AC or DC power supply produces an arc between the tip of the electrode and the workpiece. The arc generates temperatures of about 30,000°C, much higher than those developed in oxyfuel-gas welding. In nonconsumable electrode welding processes, the electrode is typically tungsten. Because of the high temperatures, an externally supplied shielding gas is necessary to prevent oxidation. Direct current is typically used and its polarity is important. The selection of current levels depends on factors such as the type of electrode, metals to be welded and depth and width of the weld zone. In straight polarity (DCEN), the workpiece is positive and the electrode is negative, generally producing narrow and deep welds. In reverse polarity (DCEP), the workpiece is negative and the electrode is positive, resulting in less penetration and a shallower, wider weld zone preferred for sheet metals and joints with wide gaps. In the AC current method, the arc pulsates rapidly suitable for welding thick sections and using large diameter electrodes at maximum currents.

Gas Tungsten Arc Welding:

In gas tungsten arc welding (GTAW) formerly known as TIG welding, the filler metal is supplied from a filler wire. Because the tungsten electrode is not consumed, a constant and stable arc gap is maintained at a constant current level. The filler metals are similar to the metals to be welded, and flux is not used. The shielding gas is usually argon or helium. Welding with GTAW may be done without filler metals. Depending on the metals, the power supply is either DC at 200 A or AC at 500 A. AC is preferred for aluminum and magnesium because the cleaning action removes oxides and improves weld quality. Thorium or zirconium may be used in the tungsten electrodes to improve electron emission characteristics. The power supply ranges from 8 to 20 kW. Contamination of the tungsten electrode by the molten metal can be a significant problem because it can cause discontinuities, so contact of the electrode with the molten metal pool should be avoided. The GTAW process is used for a wide variety of metals, particularly aluminum, magnesium, titanium, and refractory metals and is especially suitable for thin metals. The cost of the inert gas makes this process more expensive than SMAW but provides welds of very high quality and surface finish. GTAW is used in critical applications with a wide range of thicknesses and shapes and the equipment is portable.

Plasma Arc Welding:

In plasma arc welding (PAW) developed in the 1960s, a concentrated plasma arc is produced and directed towards the weld area. The arc is stable and reaches temperatures as high as 33,000°C. A plasma is an ionized hot gas composed of nearly equal numbers of electrons and ions. The plasma is initiated between the tungsten electrode and the orifice by a low current pilot arc. What makes PAW unlike other processes is that the plasma arc is concentrated because it is forced through a relatively small orifice. Operating currents usually are below 100 A. When a filler metal is used, it is fed into the arc as in GTAW. Arc and weld zone shielding is supplied by an outer shielding ring and gases such as argon, helium or mixtures. There are two methods: in the transferred arc method, the workpiece is part of the electrical circuit and the arc transfers from the electrode to the workpiece, in the nontransferred method, the arc occurs between the electrode and the nozzle and the heat is carried to the workpiece by the plasma gas. Compared with other arc welding processes, PAW has better arc stability, less thermal distortion and higher energy concentration, permitting deeper and narrower welds. Higher welding speeds from 120 to 1000 mm/min can be achieved. A variety of metals can be welded with part thicknesses generally less than 6 mm. The high heat concentration can penetrate completely through the joint using the keyhole technique with thicknesses as much as 20 mm for some titanium and aluminum alloys. PAW is often used for butt and lap joints because of its higher energy concentration, better arc stability and higher welding speeds. Proper training and skill are essential, and safety considerations include protection against glare, spatter and noise.

Atomic Hydrogen Welding:

In atomic hydrogen welding (AHW), an arc is generated between two tungsten electrodes in a shielding atmosphere of hydrogen gas. The arc is maintained independently of the workpiece. The hydrogen gas normally is diatomic but where temperatures exceed 6,000°C near the arc, the hydrogen breaks down into its atomic form, simultaneously absorbing a large amount of heat. When the hydrogen strikes the cold surface of the workpieces, it recombines into its diatomic form and rapidly releases the stored heat. The energy in AHW can be varied by changing the distance between the arc stream and the workpiece surface. This process is being replaced by shielded metal arc welding mainly because of the availability of inexpensive inert gases.

Arc Welding Processes with Consumable Electrodes:

Shielded Metal Arc Welding:

Shielded metal arc welding (SMAW) is one of the oldest, simplest and most versatile joining processes. About 50% of all industrial and maintenance welding is performed by this process. The electric arc is generated by touching the tip of a coated electrode against the workpiece and withdrawing it quickly. The electrodes are thin, long rods held manually. The heat melts a portion of the electrode tip, its coating and the base metal. The molten metal consists of a mixture of the base metal, the electrode metal, and substances from the coating, forming the weld when it solidifies. The electrode coating deoxidizes the weld area and provides a shielding gas. A bare section at the end of the electrode is clamped to one terminal of the power source, while the other terminal is connected to the workpiece. The current which may be DC or AC usually ranges from 50 to 300 A. For sheet metal welding, DC is preferred because of the steady arc. Power requirements are less than 10 kW. SMAW has the advantages of being relatively simple, versatile and requiring a smaller variety of electrodes. The equipment consists of a power supply, cables and an electrode holder. SMAW is commonly used in general construction, shipbuilding, pipelines and maintenance work, and is especially useful in remote areas where a portable fuel powered generator can be used. SMAW is best suited for thicknesses of 3 to 19 mm, although this range can be extended using multiple-pass techniques. The multiple pass approach requires that the slag be removed after each weld bead. Unless removed completely, the solidified slag can cause severe corrosion and prevent fusion of weld layers. Before another weld is applied, the slag should be removed completely by wire brushing or weld chipping. Consequently both labor costs and material costs are high.

Submerged Arc Welding:

In submerged arc welding (SAW), the weld arc is shielded by a granular flux consisting of lime, silica, manganese oxide, calcium fluoride and other compounds. The flux is fed into the weld zone from a hopper by gravity flow through a nozzle. The thick layer of flux completely covers the molten metal preventing spatter and sparks and suppressing the intense ultraviolet radiation and fumes. The flux also acts as a thermal insulator promoting deep penetration. The unused flux can be recovered, treated and reused. The consumable electrode is a coil of bare round wire 1.5 to 10 mm in diameter, fed automatically through a tube. Electric currents range from 300 to 2000 A. The power supplies are connected to standard single or three phase power lines with a primary rating up to 440 V. Because the flux is gravity fed, SAW is limited largely to welds in a flat or horizontal position having a backup piece. Circular welds can be made on pipes and cylinders provided they are rotated during welding. SAW is automated and used to weld carbon and alloy steel and stainless steel sheets or plates at speeds as high as 5 m/min. The quality of the weld is very high, with good toughness, ductility and uniformity. SAW provides very high productivity, depositing 4 to 10 times the amount of weld metal per hour as SMAW. Typical applications include thick-plate welding for shipbuilding and pressure vessels.

Gas Metal Arc Welding:

In gas metal arc welding (GMAW), developed in the 1950s and formerly called MIG welding, the weld area is shielded by an inert atmosphere of argon, helium, carbon dioxide or various gas mixtures. The consumable bare wire is fed automatically through a nozzle into the arc by a wire feed drive motor. In addition to shielding gases, deoxidizers are present in the electrode metal to prevent oxidation. Multiple weld layers can be deposited. Metal can be transferred by three methods. In spray transfer, small molten metal droplets are transferred at several hundred droplets per second, spatter free and stable. High DC currents and voltages and large diameter electrodes are used with argon or argon rich gas. The average current can be reduced with a pulsed arc and the process can be used in all welding positions. In globular transfer, carbon dioxide rich gases are used and globules are propelled by arc forces, resulting in considerable spatter. High welding currents are used allowing greater penetration and higher speed than spray transfer, and heavier sections are joined by this method. In short circuiting, metal is transferred in individual droplets as the electrode tip touches the molten weld metal. Low currents and voltages are used with carbon dioxide rich gases and small diameter wire with power of about 2 kW. Temperatures are relatively low making this suitable only for thin sheets and sections less than 6 mm. GMAW is suitable for most ferrous and nonferrous metals and is used extensively in metal fabrication. Training is easy, and the process is versatile, rapid and economical with productivity double that of SMAW. GMAW can be automated easily and lends itself to robotics and flexible manufacturing systems.

Flux Cored Arc Welding:

The flux cored arc welding (FCAW) process is similar to GMAW except that the electrode is tubular and filled with flux. Cored electrodes produce a more stable arc, improve weld contour and produce better mechanical properties. The flux is more flexible than the brittle coating on SMAW electrodes so the tubular electrode can be provided in long coiled lengths. Electrodes are usually 0.5 to 4 mm in diameter, and power required is about 20 kW. Self shielded cored electrodes are also available and do not require external shielding gas. Small diameter electrodes make welding thinner materials possible and preferable, and they make it easy to weld parts in different positions. FCAW combines the versatility of SMAW with the continuous electrode feeding feature of GMAW. The process is economical and versatile, used for welding steels, stainless steels and nickel alloys. The higher deposition rate compared with GMAW has led to its use in joining sections of all thicknesses. A major advantage is the ease with which specific weld metal chemistries can be developed by adding alloying elements to the flux core. The process is easy to automate and is readily adaptable to flexible manufacturing systems and robotics.

Electrogas Welding:

Electrogas welding (EGW) is used primarily for welding edges of sections vertically in one pass with pieces placed edge to edge. It is classified as a machine welding process. The weld metal is deposited into a weld cavity between the two pieces enclosed by two water cooled copper dams to prevent molten slag from running off, with mechanical drives moving the shoes upward. Circumferential welds are also possible with the workpiece rotating. Single or multiple electrodes are fed through a conduit and a continuous arc is maintained by flux cored electrodes at up to 750 A or solid electrodes at 400 A with power requirements about 20 kW. Shielding is done by inert gas such as carbon dioxide, argon, or helium, provided from an external source from a flux-cored electrode or both. Equipment is reliable and training is simple. Weld thickness ranges from 12 to 75 mm on steels, titanium and aluminum alloys. Applications include bridges, pressure vessels, thick walled pipes, storage tanks and ships.

Electroslag Welding:

Electroslag welding (ESW) is similar to electrogas welding. The main difference is that the arc is started between the electrode tip and the bottom of the part. Flux is added and melts by the heat of the arc. After the molten slag reaches the tip of the electrode, the arc is extinguished. Heat is produced continuously by the electrical resistance of the molten slag. Because the arc is extinguished, ESW is not strictly an arc welding process. Single or multiple solid as well as flux cored electrodes may be used. The guide may be nonconsumable or consumable. ESW can weld plates with thicknesses from 50 mm to more than 900 mm in one pass. Current required is about 600 A at 40 to 50 V although higher currents are used for thick plates. Travel speed ranges from 12 to 36 mm/min. Weld quality is good. This process is used for large structural steel sections such as heavy machinery, bridges, oil rigs, ships and nuclear-reactor vessels.

Electron Beam Welding:

In electron beam welding (EBW) developed in the 1960s, heat is generated by high velocity narrow beam electrons. The kinetic energy of the electrons is converted into heat as they strike the workpiece. The process requires special equipment to focus the beam on the workpiece typically in a vacuum. The higher the vacuum, the more the beam penetrates, and the greater the depth-to-width ratio; methods are called EBW-HV, EBW-MV and EBW-NV. Almost any metal can be welded and workpiece thicknesses can range from foil to plate. Capacities of electron guns range up to 100 kW. The intense energy is capable of producing holes using the keyhole technique. Generally no shielding gas, flux or filler metal is required. EBW makes high quality welds that are almost parallel sided, deep and narrow with small heat affected zones. Depth to width ratios range between 10 and 30. The sizes of EBW welds are much smaller than those made by conventional processes. With automation and servo controls, parameters can be controlled accurately at speeds as high as 12 m/min. Almost any metal can be butt or lap welded at thicknesses up to 150 mm. Distortion and shrinkage are minimal. Weld quality is good and of very high purity. Applications include aircraft, missile, nuclear and electronic components as well as gears and shafts for automotive industry. EBW equipment generates X-rays so proper monitoring and periodic maintenance are essential.

Laser Beam Welding:

Laser beam welding (LBW) utilizes a high power laser beam as the source of heat. Because the beam can be focused onto a very small area, it has high energy density and deep penetrating capability. The beam can be directed, shaped and focused precisely on the workpiece. This process is particularly suitable for deep and narrow joints with depth to width ratios ranging from 4 to 10. LBW has become extremely popular and is used in most industries. In the automotive industry, welding transmission components are the most widespread application. Among numerous other applications is welding thin parts for electronic components. The laser beam may be pulsed with power levels up to 100 kW for spot welding of thin materials while continuous multi kW laser systems are used for deep welds on thick sections. LBW produces welds of good quality with minimum shrinkage or distortion. Laser welds have good strength and are generally ductile and free of porosity. The process can be automated and used on materials with thicknesses up to 25 mm particularly effective on thin workpieces. Tailor welded sheet metal blanks are joined principally by LBW using robotics. Typical metals welded include aluminum, titanium, ferrous metals, copper, superalloys and refractory metals. Welding speeds range from 2.5 m/min to as high as 80 m/min for thin metals. Welding can be done in inaccessible locations and operator skill required is minimal. Safety is important due to hazards to the eye and skin, solid state lasers are also dangerous. Major advantages of LBW over EBW are: a vacuum is not required and the beam can be transmitted through air; laser beams can be shaped, manipulated and focused optically by means of fiber optics, the beams do not generate X-rays and the quality of the weld is better with less tendency toward incomplete fusion, spatter and porosity and less distortion.

Arc Welding:

Arc welding is a fusion-welding process in which coalescence is achieved by the heat of an electric arc between an electrode and the work. The same basic process is used in arc cutting. An electric arc is a discharge of electric current across a gap, sustained by a thermally ionized column of gas called a plasma. To initiate the arc, the electrode is brought into contact with the work and then quickly separated. The electric energy produces temperatures of 5500°C or higher. A pool of molten metal, consisting of base metal and filler metal if used is formed near the tip of the electrode. As the electrode is moved along the joint, the molten weld pool solidifies. Movement is accomplished by a human welder or by mechanical means. The quality of manual arc welding depends on the skill of the welder. Productivity is measured as arc time, the proportion of hours worked that arc welding is being accomplished. For manual welding, arc time is around 20%. Frequent rest periods are needed due to fatigue. Arc time increases to about 50% for machine, automatic and robotic welding.

Electrodes and Arc Shielding:

Electrodes are classified as consumable or nonconsumable. Consumable electrodes provide filler metal and are available as rods and wire. Welding rods must be changed periodically, reducing arc time. Weld wire can be continuously fed avoiding interruptions. In both forms, the electrode is consumed and added as filler metal. Nonconsumable electrodes are made of tungsten or carbon, which resist melting. Despite the name they are gradually depleted through vaporization. For nonconsumable electrodes, filler metal is supplied by a separate wire. At high temperatures, metals react with oxygen, nitrogen and hydrogen in the air, degrading mechanical properties. Shielding is provided by a blanket of gas or flux. Common shielding gases include argon and helium. In welding ferrous metals, oxygen and carbon dioxide are used with argon and helium to control weld shape. Flux prevents formation of oxides and dissolves contaminants. During welding, flux melts and becomes a liquid slag, covering the operation and protecting the molten metal. The slag hardens and must be removed later. Flux serves additional functions: providing a protective atmosphere, stabilizing the arc and reducing spattering.

Power Source Considerations:

Both DC and AC are used. AC machines are less expensive but are restricted to ferrous metals. DC equipment can be used on all metals and provides better arc control. Power is the product of current and voltage. This power is converted into heat, but losses occur through convection, conduction, radiation and spatter. The effect of losses is expressed by the heat transfer factor. Heat transfer factors are greater for consumable electrode processes.

Resistance Welding:

Resistance welding is a group of fusion welding processes that uses heat and pressure, with heat generated by electrical resistance at the junction. Components include work parts, two opposing electrodes a means of applying pressure and an AC power supply. The operation results in a fused zone called a weld nugget. RW uses no shielding gases, flux or filler metal and electrodes are nonconsumable. RW is classified as fusion welding because the applied heat almost always causes melting.

Power Source and Heat Generation:

The heat energy depends on current flow, resistance and time. Current is very high typically 5000 to 20,000 A, while voltage is below 10 V. Duration is short, 0.1 to 0.4 seconds. High current is used because the squared term amplifies the effect of current and resistance is very low around 0.0001 ohms. Resistance is the sum of resistance of electrodes, work parts, contact resistances and contact resistance of faying surfaces. Heat is generated in all regions with the ideal being for the faying surfaces to have the largest resistance. Electrode resistance is minimized using copper and electrodes are water cooled. Work part resistance depends on resistivities and thicknesses. Contact resistances depend on contact areas and surface condition. Success depends on pressure as well as heat. Pressure forces contact and presses surfaces together for coalescence. Advantages include no filler metal, high production rates, ease of mechanization, lower operator skill and good repeatability. Drawbacks include high equipment cost and joints limited to lap joints.

Resistance Spot Welding:

Resistance spot welding is the predominant process widely used in mass production of automobiles, appliances and metal furniture. A typical car body has approximately 10,000 spot welds. RSW joins sheet-metal parts of thickness 3 mm or less using a series of spot welds. The size and shape of the weld spot is determined by the electrode tip, most commonly round. The weld nugget is typically 5 to 10 mm in diameter. If made properly, strength is comparable to the surrounding metal. Electrode materials are copper-based alloys and refractory metal compositions such as copper and tungsten combinations with the second group noted for superior wear resistance. Electrodes are designed with internal passageways for water cooling. Machines include rocker arm and press type spot welders and portable guns. Rocker arm welders have a stationary lower electrode and movable upper electrode mounted on a rocker arm controlled by a foot pedal. Press type welders have a straight line motion provided by a pneumatic or hydraulic press permitting larger forces. Portable guns consist of two opposing electrodes in a pincer mechanism connected to power and control sources. They are widely used in automobile assembly with robots becoming preferred.

Resistance Seam Welding:

In resistance seam welding, stick shaped electrodes are replaced by rotating wheels and overlapping spot welds are made along the lap joint. RSEW produces air tight joints for gasoline tanks, mufflers and containers. It is the same as spot welding except for wheel electrodes. Seams should be along straight or uniformly curved lines. Warping is a factor and fixtures are required. Spacing between nuggets depends on wheel motion relative to current application. In continuous motion welding, the wheel rotates at constant velocity and current is pulsed. Frequency is normally set for overlapping spots. If frequency is reduced, spaces appear, termed roll spot welding. In another variation, current remains constant for a continuous seam. In intermittent motion welding, the wheel is stopped periodically to make the spot. Seam welding machines are similar to press type spot welders with electrode wheels. Cooling is often necessary accomplished by directing water at the surfaces.

Resistance Projection Welding:

Resistance projection welding occurs at one or more small contact points determined by part design including projections, embossments or localized intersections. In a typical case, one part has embossed points to contact the other. Embossing increases cost but may be offset by savings in welding cost. Fasteners with projections can be joined to sheet or plate. Cross wire welding fabricates wire products such as fence, shopping carts and stove grills, with contacting round wires serving as projections.

Other Resistance Welding Operations:

In flash welding used for butt joints, surfaces are brought into contact or near contact and electric current is applied to heat surfaces to melting, then forced together. Arcing occurs, called flashing. Current is stopped during upsetting. Some metal is squeezed out and must be machined. Applications include butt welding of steel strips joining ends of wire and welding tubular parts. Equipment is expensive. Upset welding is similar except surfaces are pressed together during heating and upsetting with no arcing. Heating is by electrical resistance. When heated below melting, force is increased for upsetting and coalescence. Applications are similar to flash welding. Percussion welding is similar but duration is extremely short, 1 to 10 ms. Fast heating is by rapid discharge of electrical energy, followed immediately by percussion. Heating is localized, attractive for electronic applications. High frequency resistance welding uses high frequency AC for heating followed by upsetting. Frequencies are 10 to 500 kHz. In high frequency induction welding, heating current is induced by a high frequency induction coil without contact. Applications are continuous butt welding of longitudinal seams of metal pipes and tubes.

Oxyfuel Gas Welding:

Oxyfuel gas welding is the term for operations that burn fuels mixed with oxygen. Several gases are used. Oxyacetylene is the most important. Acetylene is the most popular because it is capable of higher temperatures than others up to 3480°C. The flame is produced in two stages. The first stage produces carbon monoxide and hydrogen. The second stage produces carbon dioxide and water vapor. The two stages are visible: the inner cone is bright white and the outer envelope is nearly colorless with blue to orange tinges. Maximum temperature is reached at the tip of the inner cone. The outer envelope spreads out and shields the work surfaces. Total heat liberated is 55 million joules per cubic meter. Power densities and heat transfer factors are relatively low from 0.10 to 0.30. Acetylene is highly flammable. Pure acetylene is colorless and odorless, commercial acetylene has a garlic odor. Acetylene is unstable above 1 atmosphere so cylinders are packed with porous filler saturated with acetone. Acetone dissolves about 25 times its own volume of acetylene. The welder wears eye and skin protection. Different screw threads are standard on cylinders and hoses. Equipment is inexpensive and portable, suited for low quantity production and repair. It is rarely used for stock thicker than 6.4 mm because of advantages of arc welding. Although mechanized, it is usually manual. Alternative gases include methylacetylene propadiene developed by Dow Chemical under the trade name MAPP. MAPP has heating characteristics similar to acetylene and can be stored under pressure as a liquid avoiding special storage problems.