Presentation on theme: "Chapter 31: Gas Flame and Arc Processes"— Presentation transcript:
1 Chapter 31: Gas Flame and Arc Processes DeGarmo’s Materials and Processes in Manufacturing
2 31.1 Oxyfuel-Gas WeldingUse the flame produced by the combustion of a fuel gas and oxygen as the source of heatCombustion of oxygen and acetylene (C2H2) produces a temperature of about 3250℃ (5850oF) in a two-stage reaction.First stage:C2H2 + O2 2CO + H2 + heatSecond stage:2CO + O2 2CO2 + heatH2 + 1/2O2 H2O + heat
3 Oxyacetylene Welding Torch FIGURE 31-1 Typicaloxyacetylene welding torch andcross-sectional schematic.(Courtesy of Victor EquipmentCompany, Denton, TX)
4 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 Typicaloxyacetylene flame and theassociated temperaturedistribution.
5 Oxyfuel-gas Welding FIGURE 31-3 Oxyfuel-gas welding with a consumable welding rod.
7 Oxyfuel ApplicationMonel: Alloy of Nickel, Copper, Iron and Manganese, with antacidity
8 31.2 Oxygen Torch CuttingOxyfuel-gas cutting (OFC), commonly called flame cutting, is the most common thermal cutting processWhen ferrous metal is cut, the process is according to one or more reactions at high temperature above 815℃(1500oF)Fe + O FeO + heat3Fe + 2O2 Fe2O3 + heat4Fe + 3O2 2Fe2O3 + heat
9 Flame CuttingFIGURE 31-4 Flame cutting ofa metal plate.
10 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 EquipmentCompany, Denton, TX)
11 Underwater Cutting Torch FIGURE 31-6 Underwatercutting torch. Note the extra setof gas openings in the nozzle topermit the flow of compressedair and the extra control valve.(Courtesy of Bastian-BlessingCompany, 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
12 31.3 Flame StraighteningUse controlled, localized upsetting as a means of straightening warped or buckled materials
13 Flame Straightening FIGURE 31-7 Schematic illustrating the theory of flame straightening.
14 31.4 Arc WeldingAn 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 AVoltage 20 to 50 V (low)
15 Arc Welding Schematic FIGURE 31-8 The basic electrical circuit for arc welding.
16 Arc Welding Polarity DCEN: direct-current electrode-negative, or SPDC -- direct current and straight polarityFast melt of electrode, i.e., high metal deposition rateShallow molten pool on the workpiece (weld penetration)DCEP: direct-current electrode-positive, orRPDC -- direct current and reverse polaritylow melt of electrode, i.e., low metal deposition rateBreak up any oxide films and give deeper penetration
17 Arc Welding ElectrodeManual 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.
18 Metal Transfer ModesFIGURE 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.
20 Shielded Metal Arc Welding Low cost equipmentFinite-length electrode (1.5 to 6.5 mm in diameter and 20 to 45 cm in length)Characteristics of shielding materialsVaporize to provide a protective atmosphereProvide ionizing elements to help stabilize the arc, reduce weld metal spatter, and increase efficiency of deposition
21 Shielded Metal Arc Welding Act as a flux to deoxidize and remove impurities from the molten metalProvide a protective slag coating to accumulate impurities, prevent oxidation, and slow the cooling of the weld metalAdd alloying elementsAdd additional filler metalAffect arc penetrationInfluence the shape of the weld bead
22 Welding Electrode Designation FIGURE Designation system for arc-welding electrodes.
23 Shielded Metal Arc Welding FIGURE A shieldedmetal arc welding (SMAW)system.
24 Schematic of SMAWFIGURE Schematic diagram of shielded metal arc welding (SMAW). (Courtesy of American Iron and Steel Institute, Washington, DC.)
26 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.
27 Flux-Cored Arc Welding FIGURE The flux-coredarc welding (FCAW) process.(Courtesy of The AmericanWelding Society, New York.)
29 Gas Metal Arc WeldingUse 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).
30 Schematic of Gas Metal Arc Welding FIGURE Schematicdiagram of gas metal arcwelding (GMAW). (Courtesy ofAmerican Iron and Steel Institute,Washington, DC.)
32 Submerged Arc WeldingNo 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.
33 Schematic of Submerged Arc Welding FIGURE (Top) Basicfeatures of submerged arcwelding (SAW). (Courtesy ofLinde Division, Union CarbideCorporation, Houston, TX)(Bottom) Cutaway schematic ofsubmerged arc welding.(Courtesy of American Iron andSteel Institute, Washington, DC.)
35 Stud WeldingUsed 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.
36 Stud Welding Gun FIGURE 31-16 Diagram of a stud welding gun. (Courtesy ofAmerican Machinist.)
37 Stud Welding ExamplesFIGURE (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)
39 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.
40 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.
41 Gas Tungsten Arc Welding Torch FIGURE 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)
42 Schematic of GTAW FIGURE 31-19 Diagram of gas tungsten arc welding (GTAW).(Courtesy of American Iron andSteel Institute, Washington, DC.)
43 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 Comparison of themetal deposition rates in GTAW withcold, hot, and oscillating-hot filler wire.(Courtesy of Welding Journal.)
45 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.
46 Schematic of Inert-Gas-Shielded Tungsten Arc Welding FIGURE Processschematic of spot welding bythe inert-gas-shielded tungstenarc process.
47 Example of GTAW FIGURE 31-22 Making a spot weld by the inert-gas-shieldedtungsten arc process. (Courtesyof Air Reduction Company Inc.,New York, NY)
48 Plasma Arc WeldingArc 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.
49 Types of Plasma Arc Torches FIGURE Two types ofplasma arc torches. (Left)Transferred arc; (right)nontransferred arc.
50 GTAW versus Plasma Arc Process FIGURE Comparison of the nonconstricted arc of gas tungsten arc welding and the constricted arcof the plasma arc process. Note the level and distribution of temperature. (Courtesy ASM International, Materials Park, OH.)
52 31.7 Welding Equipment Power source DC or AC Generally employ the “drooping voltage” characteristicsJigs or fixtures (also called positioners) are frequently used to hold the work in production welding.
53 Voltage Characteristics FIGURE Drooping-voltage characteristics of typical arc-welding power supplies. (Left) Directcurrent; (right) alternating current.
54 Welding EquipmentMost welding uses solid-state transformer-rectifier machinesFIGURE Rectifier-typeAC and DC welding powersupply. (Courtesy of LincolnElectric Company, Cleveland, OH)
55 31.8 Arc Cutting Carbon arc and shielded metal arc cutting Use arc from a carbon or shielded metal arc electrode to melt the metalRemove the molten metal from the cut by gravity or the force of arc itselfAir carbon arc cuttingArc maintain between a carbon electrode and the workpieceBlow the molten metal from the cut by the air
56 Arc Cutting Oxygen arc cutting Gas metal arc cutting Electric arc and a stream of oxygen are combined to make the cutElectrode is a coated ferrous-metal tubeExpel the molten metal from the cut by a high-velocity of jet of gasGas metal arc cuttingElectrode penetrate completely through the workpiece, cutting rather than welding occur.
57 Arc Cutting Gas tungsten arc cutting Plasma arc cutting Employ the same basic circuit and shielding gas as used in gas tungsten arc weldingExpel the molten metal from the cut by a high-velocity of jet of gasPlasma arc cuttingWith the nontransferred-type of torch, temperature can be about 16500℃With the transferred-type of torch, temperature can be as high as 33000℃
58 Example of Plasma Torch FIGURE Cutting sheetmetal with a plasma torch.(Courtesy of GTE Sylvania,Danvers, MA)
59 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.