2d1r Flash Butt Welding

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<ul><li><p>Flash/Butt Welding</p></li><li><p>Flash Butt WeldingLesson ObjectivesWhen you finish this lesson you will understand: The flash and butt welding process for plain carbon steel The weld parameters which must be controlled to get good welds Typical flash/butt weld defectsLearning ActivitiesView Slides; Read Notes, Listen to lectureDo on-line workbookKeywordsFlash Weld (AC), Butt Weld (DC), Flashing Current, Upset Current, Upset Force, Upset Velocity, Upset Distance, Forging Temperature, Linear Platen Motion, Parabolic Platen Motion, Continuous Acceleration Platen Motion, Flat Spots, Penetrators</p></li><li><p>Introduction to Flash Welding[Reference: Welding Process Slides, The Welding Institute]</p></li><li><p>Basic Steps in Flash Welding(a)(c)</p><p>(b)(d)Electrodes[Reference: Welding Handbook, Volume 2, p.583, AWS]Position and Clamp the PartsApply Flashing Voltageand Start Platen MotionFlashUpset and Terminate Current</p></li><li><p>Equipment Example of Flash Welding[Reference: Welding Process Slides, The Welding Institute]Typical applications:(1) Butt welding of matching sections.(2) Chain links.(3) Railway lines.(4) Window frames.(5) Aero-engine rings.(6) Car wheel rims.(7) Metal strip in rolling mills.</p></li><li><p>Advantages of Flash WeldingFlexible cross sectioned shapesFlexible positioning for similar cross section partsImpurities can be removed during upset actsFaying surface preparation is not critical except for large partsCan weld rings of various cross sectionsNarrower heat-affected zones than those of upset welds</p></li><li><p>Limitations of Flash WeldingProduce unbalance on three-phase primary power linesThe ejected molten metal particles present a fire hazardRequire special equipment for removal of flash metalDifficult alignment for workpieces with small cross sectionsRequire almost identical cross section parts</p></li><li><p>Common Types of Flash Welds Cross Section After WeldingTransformerFixed PlatenMovable PlatenDiesAxially Aligned Weld[Reference: Welding Handbook, Volume 2, p.589, AWS]</p></li><li><p>Common Types of Flash Welds (CONT.)Cross Section After WeldingFixed PlatenMovable PlatenTransformerMiter Weld[Reference: Welding Handbook, Volume 2, p.589, AWS]</p></li><li><p>Common Types of Flash Welds (CONT.)Cross Section After WeldingFixed PlatenMovable PlatenTransformerRing Weld[Reference: Welding Handbook, Volume 2, p.589, AWS]ShuntCurrent</p></li><li><p>Typical Mill Forms and Products of Upset Welding[Reference: Welding Handbook, Volume 2, p.600, AWS]</p></li><li><p>Savage, Flash Welding, Welding Journal March 1962Systems Electrical Force Application</p></li><li><p>ApplicationsWheel Truck RimsBall Bearing RacewaysBar WeldingStrip Welding During Continuous ProcessingPipelines</p></li><li><p>Schematic of Typical Flash Weld CycleSavage, Flash Welding, Welding Journal March 1962</p></li><li><p> FlashingPartial Burn-offStage 1 - Heat SoakingIncreased Burn-offStage 2 - Steady StateExcessive Burn-offStage 3 - Heat out</p></li><li><p>Best Region For UpsetNippes, Temp Dist During Flash Welding, Welding Journal, Dec 1951</p></li><li><p>In Steady State, the Heat into the HAZ Equals the Heat OutStage 3 Occurs When More Heat Flows Out than is Flowing In</p></li><li><p>At UpsetShort Time AfterLong Time AfterForge TempUpset in the Steady State - Stage 2 Region</p></li><li><p>Nippes, Cooling Rates in Flash Welding,Welding Journal, July 1959</p></li><li><p>Temperature vs Time As a Function Of Distance From Interface At Moment of UpsetAt Moment Of Upset &amp; Short Time Thereafter</p></li><li><p>Nippes, Cooling Rates in Flash Welding,Welding Journal, July 1959</p></li><li><p>Factors Which Effect Extent of Stable Stage 2 Material Electrical &amp; Thermal Conductivity Platen Motion During Flashing Initial Clamping Distance Preheat Material Geometry</p></li><li><p>Electrical &amp; Thermal ConductivityHigh Resistance = More I2R HeatingLow Thermal Conductivity = Less Heat Out More Rapid Heating Longer Stage 2 Higher Temperature Wider HAZHAZ</p></li><li><p>Wide HAZNarrow HAZOxides TrappedAt InterfaceOxides Forced To Flashing</p></li><li><p>Platen MotionLinearParabolicContinuous AccelerationContinuous Acceleration lead to Stub Out</p></li><li><p>Nippes, Temp Dist During Flash Welding, Welding Journal, Dec 1951</p></li><li><p>Linear Flashing - Effect of Increased VelocityHigher Velocity</p></li><li><p>Parabolic FlashingNippes, Temp Dist During Flash Welding, Welding Journal, Dec 1951</p></li><li><p>Temperature Comparison of Linear and Parabolic FlashingNippes, Temp Dist During Flash Welding, Welding Journal, Dec 1951</p></li><li><p>Initial Clamping DistanceCloser Initial Clamping Shorter Stage 2 More Burnoff to Establish Steady State Steeper Temperature Gradient</p></li><li><p>Effect of PreheatBeneficialLarger HAZ</p></li><li><p>Thicker MaterialThicker Material is more of a Heat Sink</p></li><li><p>Turn to the person sitting next to you and discuss (1 min.): OK, we went back to the faster platen motion and told the night shift guy to keep his hands off, but the weld still seems to be too cold. What would you suggest?</p></li><li><p>DC Butt Welding</p></li><li><p>Introduction to Upset WeldingFinished Upset WeldHeated ZoneTo Welding TransformerClamping DieUpsettingForceMovable PartClamping DieStationary Part[Reference: Welding Handbook, Volume 2, p.598, AWS]</p></li><li><p>Schematic of Typical Butt Weld CycleMedar Technical Literature</p></li><li><p>Turn to the person sitting next to you and discuss (1 min.): Because the part are first touching as DC current is applied in butt welding, large current levels occur immediately. How would welding steels containing large manganese sulfide inclusions be effected by this?</p></li><li><p>FLASH/BUTT WELD DISCONTINUITIESMECHNICAL Misalignment Poor Scarfing Die BurnsHEAT AFFECTED ZONE Turned Up Fibers (Hook Cracks) HAZ SofteningCENTERLINE Cold Weld Flat Spots / Penetrators Pinholes Porosity Cracking</p></li><li><p>MisalignmentNotch: Stress Riser</p></li><li><p>NotchThin SectionPoor Scarfing</p></li><li><p>ArcingDie BurnsMartensiteCrack</p></li><li><p>Turned Up Fibers - Hook Cracks</p></li><li><p>Hook Cracks</p></li><li><p>Hardness Loss</p></li><li><p> Cold WeldCold Weld</p></li><li><p>Flat Spots &amp; Penetrators in Flash Welds</p></li><li><p>Factors During Upset Which Reduce Defects Upset Velocity Upset Current Upset Force Upset Distance Material Hot Strength/Chemistry</p></li><li><p>Upset VelocityHigher Velocity Helps extrude Centerline Oxides Out</p><p>1. Oxides Are Present Because MeltingPoints are high2. Oxides Tend to Solidify or Harden andGet entrapped at the Interface3. Rapid Velocity Helps Get Them Moving</p></li><li><p>Upset CurrentAdvantages Keeps Heat at Center Line During Upset Keeps Oxides Fluid Aids In Forcing Oxides Out</p><p>Disadvantages Excess Heating Can Produce Excess Upset More HAZ Fiber Turn Up</p></li><li><p>Upset ForceGenerally Use Maximum Available(Too Light a Force May Entrap Oxides)Upset DistanceNeed Enough Upset to Squeeze all Oxides Out(Rule of Thumb: 1/2 to 1.25 times the thickness)</p></li><li><p>Material Hot Strength/Chemistry Materials with higher hot strength require higher force during upset Materials producing refractory oxides or nitrides require higher upset distance to squeeze them out</p></li><li><p>Feedback Control on Platen Motion During FlashingFlashing Current Also Monitored; In Case of Short Circuit Motion is ReversedAcceptable Pre-Programmed RangeTorstensson, Electro-hydraulic Control of Flash Welding..Svetsaren, Feb 1975Monitor pre-programmed motion</p></li><li><p>CurrentVoltageFeedback Control on Platen MotionDuring FlashingMedar Technical Literature, Medar Flashweld Control with Programmable Adaptive CamMeasure Voltage and Current</p></li><li><p>Monitored DuringFlashingUpset Current UntilProportional Amount of Power AttainedDickinson Adapting HSLA Steel to Welded Wheel Rims,Welding Design &amp; Fab, May 1979</p></li><li><p>Flash Welding</p><p>Flash welding (FW) is a resistance welding process that produces a weld at the faying surface of a butt joint by a flashing action and by the application of pressure after heating is substantially completed. The flashing action, caused by the very high current densities at small contact points between the workpieces, forcibly expels material from the joint as the workpieces are slowly moved together. The weld is completed by a rapid upsetting of the workpieces.Two parts to be joined are clamped in dies (electrodes) connected to the secondary of a resistance welding transformer. Voltage is applied as one part is advanced slowly toward the other. When contact occurs at surface irregularities, resistance heating occurs at these locations. Initially, one or several current pulses may be applied to preheat the parts. Ther after, the force is instantaneoulsy removed before entering a flashing cycle. The preheat may not be used in all cases. Thereafter, the parts are brought back together. High amperage causes rapid melting and vaporization of the metal at the points of contact, and minute arcs form. This action is called flashing. As the parts are moved together at a suitable rate, flashing continues until the faying surfaces are covered with molten metal and a short length of each part reaches forging temperature. A weld is then created by the application of an upset force to bring the molten faying surfaces in full contact and forge the parts together. Flashing voltage may be terminated at the start of upset. Or, an upset voltage and current may continue during upset. If so, this is called upset voltage. All current flow is then ceased and the parts allowed to cool. The solidified metal expelled from the interface is called flash. This is often removed by a scarfing machine.</p><p>The basic steps in a flash welding sequence are as follows:(1) Position the parts in the machine.(2) Clamp the parts in the dies (electrodes).(3) Apply the flashing voltage.(4) Start platen motion to cause flashing.(5) Flash the normal voltage.(6) Terminate flashing.(7) Upset the weld zone.(8) Unclamp the weldment.(9) Return the platen and unload.The above slide illustrates these basic steps. Additional steps such as preheat, dual voltage flashing, postheat, and trimming of the flash may be added as the application dictates.The large industrial unit shown in the above slide has been purpose-built for welding chain links for ships. Note the molten metal expelled and section size of the workpiece in relation to the operator. The cycle time for welding this section would be about 1 min.Typical applications:(1) Butt welding of matching sections.(2) Chain links.(3) Railway lines.(4) Window frames.(5) Aero-engine rings.(6) Car wheel rims.(7) Metal strip in rolling mills.(8) Others.Butt joints between parts with similar cross section can be made by friction welding and upset welding, as well as by flash welding. The major difference between friction welding and upset and flash welding is that the heat for friction welding is developed by rubbing friction between the faying surfaces, rather than from electrical resistance. Upset welding is similar to flash welding except that no flashing action occurs.Listed in the above slide are some advantages of flash welding.Cross sectioned shapes other than circular (such as angle, H sections, and rectangles) can be flash welded. Rotation of parts is not required.Within limits parts of similar cross section can be welded with their axes aligned or at an angle to each other.The molten metal film on the faying surfaces and its ejection during upset acts to remove impurities from the interface.Preparation of the faying surfaces is not critical except for large parts that may require a bevel to initiate flashing.Rings of various cross sections can be welded.The heat-affected zones of flash welds are much narrower than those of upset welds.Some important limitations of flash welding are listed in the above slide.The high single-phase power demand produces unbalance on three-phase primary power lines.The molten metal particles ejected during flashing may cause a fire, an injure of the operator, or shafts and bearings damage. The operator should wear face and eye protection, and a barrier or shield should be used to block flying sparks.Removal of flash metal is generally necessary and may require special equipment.Alignment of workpieces with small cross sections is sometimes difficult.The parts to be joined must have near-identical cross sections.Three common types of welds made by flash welding are shown in this and the following two slides.The axially aligned weld is shown in the above slide.The miter weld is shown in the above slide.The ring weld is illustrated in the above slide. Because the distance across the flash welded surface is shorter than the distance around the hoop, current tends to flow across the interface making the flash weld. However, a sizable amount of shunt current follows the path around the hoop, thus higher currents are generally required when shunt paths like this are present. Often, the preheat cycle is eliminated as this would tend to preheat the entire part and additional heat at the interface would raise the resistance there and force more shut current into effect.Upset welding is used in wire mills and in the manufacture of products made from wire. In wire mill applications, the process is used to join wire coil to each other to facilitate continuous processing. The process also is used to fabricate a wide variety of products from bar, strip, and tubing. Typical examples of mill forms and products that have been upset welded are shown in the above slide. Wire and rod from 0.05 to 1.25-in. (1.27 to 31.75 mm) diameter can be upset welded.Lets now take a look at the systems needed in a flash wieldier to make the weld. First of all, there is the electrical system composed of a primary and secondary of a transformer. Usually tap switches are provided to adjust the overall current level. In addition, a welding contactor system for initiating the current flow is provided. In most cases, this contactor system is a phase shift heat control system such that current passage can be finely control and the level adjusted as well (see other areas where phase shift heat control is discussed).</p><p>The force application system consist of a fixed platen and a movable platen between which the part to be welded is clamped. A hydraulic or pneumatic cylinder is attached to the movable platen and the force applied on this piston causes motion of the movable part. The motion of the platen is controlled by a control mechanism which can be programmed in a number of motion patterns for a linear motion to an accelerating motion.Typical flash welding applications can range from as small as mico ball bearing raceways welded in clean room conditions to six foot diameter line pipe butt welds performed in a mater of seconds in the field environment.In the flash welding application itself, the current is turned on before the parts are physically brought into contact. (In some applications, a lower level preheat current is applied by bringing the parts together and passing current to preheat, but the parts are then separated before actual flashing). With current applied, the parts are moved until the aspirates begin to touch. When this happens, the current flows through these asperities, they rapidly melt and arcing occurs until the small volume of liquid is flash out of the interface. The interface layer, probably has an oxide layer, but when the steel melts and burns during the flashing operation it combines with oxygen from between the interface and the oxide layer making an atmosphere low in oxygen within the interface. As the asperities flash, they both heat t...</p></li></ul>