Flash/Butt Welding Plain Carbon Steel. Flash Welding of Plain Carbon Steel Lesson Objectives When you finish this lesson you will understand: The flash.

  • Published on
    12-Jan-2016

  • View
    217

  • Download
    0

Transcript

Flash/Butt WeldingPlain Carbon SteelFlash Welding of Plain Carbon SteelLesson 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 ActivitiesLook up KeywordsView Slides; Read Notes, Listen to lectureDo on-line workbookDo homeworkKeywordsFlash 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, PenetratorsSavage, Flash Welding, Welding Journal March 1962ApplicationsWheel Truck RimsBall Bearing RacewaysBar WeldingStrip Welding During Continuous Processing In Steel MillsPipelinesSchematic of Typical Flash Weld CycleSavage, Flash Welding, Welding Journal March 19620.05.10.15Initial FlashingPartial Burn-offStage 1 - Heat SoakingIncreased Burn-offStage 2 - Steady StateExcessive Burn-offStage 3 - Heat outBest Region For UpsetNippes, Temp Dist During Flash Welding, Welding Journal, Dec 1951In Steady State, the Heat into the HAZ Equals the Heat OutStage 3 Occurs When More Heat Flows Out than is Flowing InAt UpsetShort Time AfterLong Time AfterForge TempUpset in the Steady State - Stage 2 RegionNippes, Cooling Rates in Flash Welding,Welding Journal, July 1959Temperature vs. Time As a Function Of Distance From Interface At Moment of UpsetAt Moment Of Upset & Short Time ThereafterNippes, Cooling Rates in Flash Welding,Welding Journal, July 1959Turn to the person sitting next to you and discuss (1 min.): The night shift flash weld operator said that he felt the platen velocity was too fast so he slowed it down. What do you think will result by this change?Factors Which Effect Extent of Stable Stage 2 Material Electrical & Thermal Conductivity Platen Motion During Flashing Initial Clamping Distance Preheat Material GeometryElectrical & Thermal ConductivityHigh Resistance = More I2R HeatingLow Thermal Conductivity = Less Heat Out More Rapid Heating Longer Stage 2 Higher Temperature Wider HAZHAZWide HAZNarrow HAZOxides TrappedAt InterfaceOxides Forced To FlashingPlaten MotionLinearParabolicContinuous AccelerationContinuous Acceleration lead to Stub OutNippes, Temp Dist During Flash Welding, Welding Journal, Dec 1951Linear Flashing - Effect of Increased VelocityHigher VelocityParabolic FlashingNippes, Temp Dist During Flash Welding, Welding Journal, Dec 1951Temperature Comparison of Linear and Parabolic FlashingNippes, Temp Dist During Flash Welding, Welding Journal, Dec 1951Initial Clamping DistanceCloser Initial Clamping Shorter Stage 2 More Burnoff to Establish Steady State Steeper Temperature GradientEffect of PreheatBeneficialLarger HAZThicker MaterialThicker Material is more of a Heat SinkTurn 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?DC Butt WeldingSchematic of Typical Butt Weld CycleMedar Technical LiteratureTurn 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?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 CrackingMisalignmentNotch: Stress RiserNotchThin SectionPoor ScarfingArcingDie BurnsMartensiteCrackTurned Up Fibers - Hook CracksHook CracksHardness Loss Cold WeldCold WeldFlat Spots & Penetrators in Flash WeldsFactors During Upset Which Reduce Defects Upset Velocity Upset Current Upset Force Upset Distance Material Hot Strength/ChemistryUpset VelocityHigher Velocity Helps extrude Centerline Oxides Out1. Oxides Are Present Because MeltingPoints are high2. Oxides Tend to Solidify or Harden andGet entrapped at the Interface3. Rapid Velocity Helps Get Them MovingUpset CurrentAdvantages Keeps Heat at Center Line During Upset Keeps Oxides Fluid Aids In Forcing Oxides OutDisadvantages Excess Heating Can Produce Excess Upset More HAZ Fiber Turn UpUpset 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)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 outIn this module we will finish our discussion about resistance welding of plain carbon uncoated steels by looking at the flash welding and the butt welding processes.In flash butt welding, the parts are connected to both a fixed and a movable platen and a current is passed though the interface as the parts are slowly moved toward each other. In this case a ring is being welded but the opposite faces of the ring hoop are approaching each other just as in the case of conventional butt welded parts. The current is controlled through a welding transformer and an electronic contactor unit. AC current is used. Platen motion is controlled by a hydraulic platen controller which supplies fluid to the piston and cylinder mechanism at a controlled rate. Flashing occurs as the asperities on the opposite surfaces touch.Some of the plain carbon steel applications which use flash or DC butt welding for manufacture are listed here.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 the surrounding base metal heat affected zone and they cause some molten region. Any surface oxide remaining, will be carried out of the interface as flashing continues and as the final upset is performed.This is a schematic of the process. The platen travel is continuous starting at the time of flashing and progressing until upset. This may be a linear motion, a constantly accelerating motion or a parabolic motion as illustrated here. At upset the platens are rapidly squeezed together for upset. The flashing current starts low as only a few asperities touch, but as the platen moves, more and more asperities touch and the current increases. At upset, the current may either be immediately terminated or it may continue for a short time (called upset current) as shown here.The top figure shows the same platen travel motion, but the lower figure show the instantaneous temperature of the flash weld at a point just below the interface (the points which ultimately will end up in contact with each other when upset occurs. This temperature increases with time (and thus with the amount of material burned off during flashing) until it reaching the forging temperature at which time the upset is initiated. Thus the weld region actually occurs at temperatures below the melting point and this process is really a solid state process. If there is some upset current applied, the temperature of the interface may raise slight above the forging temperature. After the current is cut off the temperature cools and the weld is made.Lets look a little closer at that flashing operation because there are several different stages that occur. In the top figure, the initial flashing stage is represented. With each flash, heat soaks back into the heat affected zone. We can record the temperature at various distances (here represented by the triangles) from the instantaneous interface as new fresh material is presented into this interface and flashed off. During partial burn-off or the Stage 1 time, heat soaks into the HAZ and the temperature rises. With increased burn-off at stage 2 a steady state temperature profile is obtained. In this region, there is just as much heat coming in to this region from the flashing as there is heat exiting back into further depths of the metal part. When burn off is excessive, more heat is sucked out into the holding dies than is acquired from flashing and a heat loss stage 3 is entered.When we look at the temperature as a function of burn-off at each of the distances curve represent here are obtained. The point closest to the interface heats in stage 1, reaches steady state stage 2 for a considerable amount of burn-off and then slowly enters stage 3. Points at further removed distances follow the same general pattern but do not attain as high a maximum temperature. Good welds are produced where the temperature for some measurable distance behind the instantaneous interface reach a steady stage 2 region and whose temperature is at or above the forging temperature of the material.Once again, in the steady state stage 2 region, heat from flashing soaks into the region at the same rate that heat is removed back into the part and through the grips. In the stage 3 region, so much material has burned-off that more heat now exits the part than is added by flashing. In such a condition, the region is cooling and the time for upset has pasted and unsuccessful welds will be made.Upset should occur when the heating stage 2 is reached. Looking at the instantaneous temperature profile while in steady state stage 2, a profile like the top profile is attained. A significant amount of material behind the instantaneous interface is above the forging temperature so upset will cause metal flow and a weld to occur. A short time after upset, general cooling occurs at the interface as heat soaks back and the temperature raises at greater depth. Thereafter the entire profile begins to drop as no new heat is added.Here is actual test data confirming the schematic profiles show previously.At the moment of upset and a short time after, we can examine the temperature vs.. time curves at the various locations back from the instantaneous interface. The temperate pulses are presented her. Once again, here is actual data on plain carbon steel.As mentioned, the best place for upset to occur is in that steady state stage 2 region. There are a few factors which effect the size and temperature of this region as listed here.Materials with higher electrical resistance will have more heating and lower thermal conductivity will retain the heat at the interface with less heat sucked out into the cool regions, so the heating is more rapid and the stage 2 is longer and the temperature are higher in stage 2 region and this results in a wider heat affected zone. These are all shown here. This will make the squeezing out of metal which might contain interface oxide easier with the production of better flash welds.However, if the heat affected zone gets too wide, the metal flow pattern may be adjusted so that the metal from the centerline interface does not squeeze out as well, but rather the deformation occurs behind this interface. So it is possible to have an overheating condition occur.Platen motion also has an effect. There are three types of platen motion that people have used, linear, Parabolic, and continuous Acceleration. With the continuous acceleration mode, the rate of the platen motion is continuously accelerating and thus at the point where upset is normally applied, the ram is move so fast that no new upset increase is needed. This type of motion is often so fast that stub out can occur during flashing and is often therefore not used. Therefore linear or parabolic motion are most often preferred. This slide just shows that platen displacement vs.. time for the linear and parabolic motion.With linear flashing, as speed is increased, the flashing rate increase, and heating occurs faster and the stage 2 region gets wider. Usually there is little if any increases in stage 2 temperature, however. The temperature distribution from the interface tends to drop off just a little faster as the faster flashing does not allow as much time for thermal diffusion. So we have just slightly narrower heat affected zones with higher speed linear flashing.With parabolic flashing rates we find dramatic increase in the size of the stage 2 region as seen here. So many people are beginning to use that parabolic flashing rate.Comparing the temperature distributions for linear and parabolic the temperature reaching some temperature as illustrated here for 800F. With parabolic flashing it reaches that temperature at a much closer distance from the interface than with linear flashing.Initial clamping distance also has some effect. It the initial clamping distance gets too close, there is a shorter stage 2 region because we are sucking heat out through those clamps. It takes more burn off to establish that steady state. You can see that the burnoff in stage 1 takes much longer to get up to steady state. And the instantaneous temperatures tends to be a little bit lower at the interface also because se are sucking out heat. And we have steeper temperature gradients because the heat is being removed. Thus, close initial clamping distance is not usually good.The effect of preheat, either a gas preheat or a preheat given by current passage, tends to be beneficial by raising the instantaneous temperature profile making the stage 2 region much larger. But it also promotes larger heat affected zones so some inclusion problems may be encountered.Thicker materials tend to be more of a heat sink so it takes longer burn-off to get the heat to steady state and the stable stage 2 region is shorter and the temperature is reduced, making a steeper temperature gradient.Lets take just a quick look at the butt welding process. The first thing is that the current used for this process is DC rather than the AC current used in flash welding.IN the butt welding cycle, the parts are brought together first, a current is applied and most of the heating occurs just by the I2R heating. There should be no flashing in the upset butt process. And because when we start to heat the parts begin to heat and expand, it tends to push the platen back a little until it reaches a temperature where the parts get hot enough for general yielding and the force of the platen begins to push the parts back again. When the part finally gets hot enough, full upset is applied where a rapid closure occurs.Lets take a look at the types of discontinuities that can occur in these types of processes. WE can divide them up into mechanical discontinuities, those occurring in the heat affected zone and those occurring at the centerline as listed here.The first mechanical type is misalignment. If we bring the parts together in a misaligned fashion after flashing and upset, we wind up with a weld looking like that illustrated here. A large amount of flashing occurs out of the top and bottom areas. And a notch stress riser results. This can be adjusted just by some mechanical adjustment of the clamps.With poor alignment of the scarfing tool, one can end up with too deep and cut on one side or too narrow a cut on the other resulting in a notch and a thinned section.Die burns are another problem when the clamping dies get contaminated with flash spatter and making uniform contact impossible. When this happens, some arcing may occur as current is passed thus melting and burning small areas of the part right under the dies. The rapid cooling from the die can cause martensite formation and cracks emanating from this hard strained martensite die burns. This can be improved by periodic cleaning of the spatter off the dies by the operator.The first heat affected zone problem we will look at is the turned up fibers or hook cracks. In steels wherein have not been properly process, the occurrence of manganese sulfide stringers can occur. The are roll out in the rolling direction during processing and during flash welding they tend to turn up during flashing and upset. Under the action of cooling the stress distribution set up can cause these planes of weakness to open up into cracks. Under cut in scarfing as mentioned earlier can accentuate this problem.This is a micrograph of some hook cracks. Note the manganese sulfide stringers which as they approach the centerline on the far left of this figure are tuned up and form the hook cracks.Another problem in the heat affected zone of concern are losses in hardness. For materials that are cold rolled or processed through martensitic hardening, and even HSLA steels which get some strengthening by precipitation, we can find softening in the heat affected zone as illustrated here. The cold rolled steels get annealed, in the HSLA we get overaging of the precipitates and strength loss, in the martensitic steels temper softening is observed with rehardening near the centerline. Subsequent processing can also have an effect. A illustrated here, HSLA steels which experienced softening in the manufacture of wheel rims in the HAZ were subsequently roll formed which selectively cold worked the softer material causing hardening in these regions until the strength in the soft region reached that of its neighboring material. Thereafter uniform hardening would occur for the remainder of the forming. This works well as long as tearing does not occur in the softer material during the initial yielding before it hardened sufficiently.As we discussed earlier, we need to get the flashing current, flashing time and platen motion just right so that we get the temperature just behind the interface to the forging temperature. If these conditions are not set correctly then a cold weld occurs as illustrated here. When a cold weld occurs, we never get the material to forge out during upset and thus inclusions are not squeezed out and centerline defects occur.A specific type of centerline defect called a flat spot or a penetrator can sometimes occur. When control of the flash welding current is controlled by phase shift heat control, and low percentage of heat is applied, it becomes possible for the asperities to touch and deform during the times when the current is shut off. This occurs because the platen continues to move closer to each other without current flow. When the current again comes on, a large spike in current can result. This causes a large shorting at this asperity and a large pit is expelled. If this happens late in the flashing cycle just before upset, the upset may not be enough to fully mate the adjacent surfaces leaving a hole, flat spot or penetrator behind. A phase shift heat control of near 100% reduces this effect. In this case the current needs to be controlled by transformer tap settings rather than large phase sift heat control.Here is an example of one of those pinholes. As illustrated, the hole was cut out and opened up so a scanning electron micrograph of the face of the hole could be viewed as in the next slide. The micrograph showed a lot of oxides including aluminum oxide and manganese oxides sitting in the bottom of the hole. They even make the surface look kind of flat looking thus the term flat spot.This is another view of a similar flat spot. Note the flat looking characteristics of region A. Note that in the holes there are oxides when the upset operation was just not sufficient to eliminate and squeeze out these oxides. So, some factors during upset which will reduce these types of defects are listed here. Higher upset velocities, the application of upset current to keep heat on during upset, increased upset force to keep material flowing, increase upset distance to be sure to squeeze out all the interfacial material, and chemistry leading to lower hot strength all assist in the reduction of these defects.Upset velocity helps as indicted.Her are the advantages and disadvantages of upset current.Upset force and upset distance also helps.The effect of material hot strength is listed here.

Recommended

View more >