Chapter 31: Gas Flame and Arc Processes DeGarmo’s Materials and Processes in Manufacturing

31.1 Oxyfuel-Gas Welding Use the flame produced by the combustion of a fuel gas and oxygen as the source of heat Combustion of oxygen and acetylene (C2H2) produces a temperature of about 3250℃ (5850oF) in a two-stage reaction. First stage: C2H2 + O2  2CO + H2 + heat Second stage: 2CO + O2  2CO2 + heat H2 + 1/2O2  H2O + heat

Oxyacetylene Welding Torch FIGURE 31-1 Typical oxyacetylene welding torch and cross-sectional schematic. (Courtesy of Victor Equipment Company, Denton, TX)

Oxyacetylene Flame Temperatures Inner cone: the first stage of combustion is complete. Most welding should be performed with the torch positioned this point just above the metal being welded. FIGURE 31-2 Typical oxyacetylene flame and the associated temperature distribution.

Oxyfuel-gas Welding FIGURE 31-3 Oxyfuel-gas welding with a consumable welding rod.

Oxyfuel-Gas Welding Process

Oxyfuel Application Monel: Alloy of Nickel, Copper, Iron and Manganese, with antacidity

31.2 Oxygen Torch Cutting Oxyfuel-gas cutting (OFC), commonly called flame cutting, is the most common thermal cutting process When ferrous metal is cut, the process is according to one or more reactions at high temperature above 815℃(1500oF) Fe + O  FeO + heat 3Fe + 2O2  Fe2O3 + heat 4Fe + 3O2  2Fe2O3 + heat

Flame Cutting FIGURE 31-4 Flame cutting of a metal plate.

Oxyacetylene Cutting Torch Acetylene is by far the most common fuel used in oxyfuel-gas cutting, the process is often referred to as oxyacelylene cutting (OFC-A) Fuel gases: acetylene (OFC-A), natural gas (OFC-N), propane (OFC-P) , and hydrogen (OFC-H) FIGURE 31-5 Oxyacetylene cutting torch and cross-sectional schematic. (Courtesy of Victor Equipment Company, Denton, TX)

Underwater Cutting Torch FIGURE 31-6 Underwater cutting torch. Note the extra set of gas openings in the nozzle to permit the flow of compressed air and the extra control valve. (Courtesy of Bastian-Blessing Company, Chicago, IL) An auxiliary skirt surrounds the main tip, and a additional set of gas passages conducts a flow of compressed air that provides secondary oxygen for the oxyacetylene flame and expels water from the zone where the burning of metal occurs

31.3 Flame Straightening Use controlled, localized upsetting as a means of straightening warped or buckled materials

Flame Straightening FIGURE 31-7 Schematic illustrating the theory of flame straightening.

31.4 Arc Welding An arc between two electrodes was a concentrated heat souse that could approach 4000°C. Current 1 to 4000 A (large) Typically range from 100 to 1000 A Voltage 20 to 50 V (low)

Arc Welding Schematic FIGURE 31-8 The basic electrical circuit for arc welding.

Arc Welding Polarity DCEN: direct-current electrode-negative, or SPDC -- direct current and straight polarity Fast melt of electrode, i.e., high metal deposition rate Shallow molten pool on the workpiece (weld penetration) DCEP: direct-current electrode-positive, or RPDC -- direct current and reverse polarity low melt of electrode, i.e., low metal deposition rate Break up any oxide films and give deeper penetration

Arc Welding Electrode Manual arc welding is almost always performed with shielded (covered) electrodes. Continuous bare-metal wire can be used as the electrode in automatic or semiautomatic arc welding, but this is always in conjunction with some form of shielding and are–stabilizing medium and automatic feed-control devices.

Metal Transfer Modes FIGURE 31-9 Three modes of metal transfer during arc welding. (Courtesy of Republic Steel Corporation, Youngstown, OH) As the electrode melts, the arc length and the electrical resistance of the arc length vary. To maintain a stable and satisfactory welding conditions, the electrode must be moved toward the work at a controlled rate.

31.5 Consumable-Electrode Arc Welding Shielded metal arc welding (SMAW) Flux-cored arc welding (FCAW) Gas metal arc welding (GMAW)  MIG Submerged arc welding (SAW)

Shielded Metal Arc Welding Low cost equipment Finite-length electrode (1.5 to 6.5 mm in diameter and 20 to 45 cm in length) Characteristics of shielding materials Vaporize to provide a protective atmosphere Provide ionizing elements to help stabilize the arc, reduce weld metal spatter, and increase efficiency of deposition

Shielded Metal Arc Welding Act as a flux to deoxidize and remove impurities from the molten metal Provide a protective slag coating to accumulate impurities, prevent oxidation, and slow the cooling of the weld metal Add alloying elements Add additional filler metal Affect arc penetration Influence the shape of the weld bead

Welding Electrode Designation FIGURE 31-10 Designation system for arc-welding electrodes.

Shielded Metal Arc Welding FIGURE 31-11 A shielded metal arc welding (SMAW) system.

Schematic of SMAW FIGURE 31-12 Schematic diagram of shielded metal arc welding (SMAW). (Courtesy of American Iron and Steel Institute, Washington, DC.)

Process Summary of SMAW

Flux-Cored Arc Welding Overcome some of the shielding metal arc limitations by moving the powdered flux to the interior of a continuous tubular electrode. Compared to the stick electrodes of the shielded metal arc process, the flux-cored electrode is both continuous and less bulky, since binders are no longer required to hold the flux in place.

Flux-Cored Arc Welding FIGURE 31-13 The flux-cored arc welding (FCAW) process. (Courtesy of The American Welding Society, New York.)

Process Summary of FCAW

Gas Metal Arc Welding Use of supplementary shielding gas, such as CO2 flowing through the torch to protect arc and molten metal. No longer a need for the volatilizing flux. continuous, solid, uncoated metal wire as an electrode (or a continuous hollow tube with powdered alloy additions in the center, known as a metal-cored electrode), or referred to as metal inert-gas welding (MIG).

Schematic of Gas Metal Arc Welding FIGURE 31-14 Schematic diagram of gas metal arc welding (GMAW). (Courtesy of American Iron and Steel Institute, Washington, DC.)

Process Summary of GMAW

Submerged Arc Welding No shielding gas is used. Instead, a thick layer of granular flux is deposited just ahead of a solid bare-wire consumable electrode. Portion of flux melts and acts to remove impurities from the rather large pool of molten metal, while the unmelted excess provides additional shielding.

Schematic of Submerged Arc Welding FIGURE 31-15 (Top) Basic features of submerged arc welding (SAW). (Courtesy of Linde Division, Union Carbide Corporation, Houston, TX) (Bottom) Cutaway schematic of submerged arc welding. (Courtesy of American Iron and Steel Institute, Washington, DC.)

Process Summary of SAW

Stud Welding Used to attach studs, screws, pins, or other fasteners to a metal surface. Inserted stud acts as an electrode and a dc arc is established between the end of the stud and the workpiece. After a small amount of metal is melted, the two pieces are brought together under light pressure and allowed to solidify.

Stud Welding Gun FIGURE 31-16 Diagram of a stud welding gun. (Courtesy of American Machinist.)

Stud Welding Examples FIGURE 31-17 (Left) Types of studs used for stud welding. (Center) Stud and ceramic ferrule. (Right) Stud after welding and a section through a welded stud. (Courtesy of Nelson Stud Welding Co, Elyria, OH)

31.6 Nonconsumable-Electrode Arc Welding Gas tungsten arc welding  TIG  nonconsumable electrode Gas tungsten arc spot welding Plasma arc welding

Gas Tungsten Arc Welding Torch Known as tungsten inert-gas (TIG) welding or Heliarc welding when helium was the shielding gas. Nonconsumable tungsten electrode provide the arc but not the filler metal. Tungsten electrode: alloyed with thorium oxide, zirconium oxide, cerium oxide, or lanthanum oxide to provide better current-carrying and electron-emission characteristics and longer electrode life.

Gas Tungsten Arc Welding Torch Operating in an inert environment using inert gas as shielding gas, such as argon, helium, or mixture of them, NOT CO2. Filler metal is optional. When filler metal is required, it is generally selected to match the chemistry and/or tensile strength of the metal being welded. Maximum penetration is obtained with direct current electrode negative (DCEN) conditions.

Gas Tungsten Arc Welding Torch FIGURE 31-18 Welding torch used in nonconsumable electrode, gas tungsten arc welding (GTAW), showing feed lines for power, cooling water, and inert-gas flow. (Courtesy of Linde Division, Union Carbide Corporation, Houston, TX)

Schematic of GTAW FIGURE 31-19 Diagram of gas tungsten arc welding (GTAW). (Courtesy of American Iron and Steel Institute, Washington, DC.)

Metal Deposition Rate Comparison The hot-wire process is not practical when welding copper or aluminum because it is difficult to preheat the low-resistivity filler wire. FIGURE 31-20 Comparison of the metal deposition rates in GTAW with cold, hot, and oscillating-hot filler wire. (Courtesy of Welding Journal.)

Process Summary of GTAW

Gas Tungsten Arc Spot Welding Access it limited to one side of the joint or where thin sheet is being attached to heavier material. Weld nugget: Arc spot welding – form at the surface where the gun makes contact. Resistance spot welding – form at the interface between the two members.

Schematic of Inert-Gas-Shielded Tungsten Arc Welding FIGURE 31-21 Process schematic of spot welding by the inert-gas-shielded tungsten arc process.

Example of GTAW FIGURE 31-22 Making a spot weld by the inert-gas-shielded tungsten arc process. (Courtesy of Air Reduction Company Inc., New York, NY)

Plasma Arc Welding Arc is maintained between a nonconsumable electrode and either the welding gun (nontransferred arc) or the workpiece (transferred arc). Characterize as a high rate of heat input and temperatures on the order of 16500℃ (30000oF). Offer fast welding speeds, narrow welds with deep penetration (a depth-to-width ratio of about 3), narrow heat-affected zone, less distortion.

Types of Plasma Arc Torches FIGURE 31-23 Two types of plasma arc torches. (Left) Transferred arc; (right) nontransferred arc.

GTAW versus Plasma Arc Process FIGURE 31-24 Comparison of the nonconstricted arc of gas tungsten arc welding and the constricted arc of the plasma arc process. Note the level and distribution of temperature. (Courtesy ASM International, Materials Park, OH.)

Process Summary of PAW

31.7 Welding Equipment Power source DC or AC Generally employ the “drooping voltage” characteristics Jigs or fixtures (also called positioners) are frequently used to hold the work in production welding.

Voltage Characteristics FIGURE 31-25 Drooping-voltage characteristics of typical arc-welding power supplies. (Left) Direct current; (right) alternating current.

Welding Equipment Most welding uses solid-state transformer-rectifier machines FIGURE 31-26 Rectifier-type AC and DC welding power supply. (Courtesy of Lincoln Electric Company, Cleveland, OH)

31.8 Arc Cutting Carbon arc and shielded metal arc cutting Use arc from a carbon or shielded metal arc electrode to melt the metal Remove the molten metal from the cut by gravity or the force of arc itself Air carbon arc cutting Arc maintain between a carbon electrode and the workpiece Blow the molten metal from the cut by the air

Arc Cutting Oxygen arc cutting Gas metal arc cutting Electric arc and a stream of oxygen are combined to make the cut Electrode is a coated ferrous-metal tube Expel the molten metal from the cut by a high-velocity of jet of gas Gas metal arc cutting Electrode penetrate completely through the workpiece, cutting rather than welding occur.

Arc Cutting Gas tungsten arc cutting Plasma arc cutting Employ the same basic circuit and shielding gas as used in gas tungsten arc welding Expel the molten metal from the cut by a high-velocity of jet of gas Plasma arc cutting With the nontransferred-type of torch, temperature can be about 16500℃ With the transferred-type of torch, temperature can be as high as 33000℃

Example of Plasma Torch FIGURE 31-27 Cutting sheet metal with a plasma torch. (Courtesy of GTE Sylvania, Danvers, MA)

31.9 Metallurgical and Heat Effects in Thermal Cutting Low rate of heat input, oxyacetylene cutting will produce a rather large HAZ Plasma arc cutting is so rapid, and the heat is so localized. Imbalanced residual stress will induce distortion.

Cutting Process Comparison

Reference Problems Review Questions 1 (3), 10, 18, 22, 35, 38, 43, 50, 64,