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AWS New Welding Technologies The Key to Higher Productivity Controlled Short Circuit GMAW – Root Pass Pipe Weldling. Jim Cuhel Welding Engineer Miller Electric Mfg. Co. Typical STD MIG Waveform. 150 IPM .035" S-6. 42. 350. Voltage. 36. Current. 300. 30. 250. 24. 200. - PowerPoint PPT Presentation

### Transcript of Jim Cuhel Welding Engineer Miller Electric Mfg. Co

• Jim CuhelWelding EngineerMiller Electric Mfg. Co.

• Short Circuit Transfer

• Short ArcRMDTaking Control

• What Is RMD?Current (amps)Time (Ms)300301.5303304.5306307.5309310.5050100150200250300

• dlWirePuddleNecking Region Power Density in the Necking RegionMolten Wire

• TimePower in Necking RegionNecking BeginsShort Circuit Clears (standard MIG)Detect Clearing EventReduce Current (hence, power)Short Circuit Clears at Much Lower Power Level (RMD)

• Heat InputArc Heating TermResistive Heating

• How Much Energy is Needed to Burn off the Incoming Wire? First we need to bring the wire temperature from something near room temp up to the melting point of the wire: Temperature Change: Heat Input Required to Effect Temperature Change:(where C=specific heat of material and Mwire = Mass of the wire being heated)Then, we need to supply sufficient energy to cause a phase transformation from solid to liquid (we need to melt it):(where Hm=latent heat of fusion of material)So, the total* energy required to burn off the incoming wire is:

• Putting This Knowledge to UseHeat In =+ I2(t)*I(t)*

• ARC ONLittle lLittle lI(t)I(t)*I2(t)*1/2-JsetArc HeatingResistive HeatingJset = (Heat in @ Stickout)Error TermARC OFFJset

• TballWidth of ball pulse is adjustedin response to the heat input in the wireDuration of Ball Phase is Modified Based Upon Heat Content of Wire

• Stick out variation video

• Constant Voltage GMAW ComparisonRMD: 0.035 ER70S-6 on 8 Sch. 80Conventional GMAW

• Establishing Good TechniqueAs with any welding process, success with RMD process requires establishing and maintaining good preparation and welding techniques. The following guidelines lead to proven success and increased productivity for welding pipe

• Joint ConfigurationStandard 75 degree included angleLand: 0 3/32Root Opening: 1/8

• Five Critical Items For Stainless SteelsThe techniques for welding carbon are the same for stainless alloysTo qualify procedures for welding 300 series stainless steel piping Without backing gas, fabricators should do the following:1.)Ensure a minimum 1/8 gap around the entire circumference of the joint. This gap allows the shielding gas to flow through to protect the backside of the joint from oxidation

• Five Critical Items Cont.2.)Clean the pipe both inside and out to remove any contaminates or unwanted substances. Use a wire brush to clean at least 1 in. back from the edge of the joint3.)Use only a stainless steel wire with a high silicon content, such as 316LSi or 308LSi. Higher silicon contents helps the puddle wet out and acts as a deoxidizer

• Five Critical Items Cont.4.)For optimum performance, use a Tri-H gas thats 90 He/ 7 Ar/ 2 CO2 Alternatively, use 98 Ar/ 2 CO25.)For best results, use a tapered nozzle for the root pass because it localizes the shielding gas coverage. Tapered nozzles with built-in gas diffusers provide exceptional coverage

• Thank YouAny Questions

*Voltage redCurrent blueCV processMay wonder where the constant part come fromThe time scale is in msecCV control works to maintain a voltage setpointThe difference between the voltage setpoint and the actual instantaneous voltage is called the voltage errorThe current level is adjusted to attempt to bring the voltage error back to zeroThe short circuit process alternates between the low voltage short condition and the high voltage arc conditionWhen the short occurs, the current rises in order to restore the voltage to the set point level.The rate at which the current rises is determined by the inductance setting of the power supply.A low inductance allows the current to rise quickly, a high inductance allows the current to rise more slowlyYoure all familiar with the trade-offs of adding inductance.The current during the short will continue to rise until the short clears. Sometimes this requires a very high current levelThe higher the current level at short clearing the more spatter is generated and the more the puddle is agitatedOnce the short clears and the arc is reestablished, the voltage level at the short clearing current is typically much higher than the set pointThe voltage error in then in the opposite direction and the control loop reduces the current to bring the voltage downIn this regard, CV control for short arc is a lot like trying to maintain an average speed of 40 MPH by alternately mashing on the accelerator and brake Despite the short comings, short circuit MIG with its relatively low heat input has served the welding industry well for the last 50 years and continues to do so today.However, the spatter levels and tendency to puddle agitation and cold lap limit its usefulness in some areas.

Spray transfer requires very little finesse in the dynamics of the welding power supply (except of course, the start transient) The droop and inductance of are of little import in a true open arc spray transfer.However, it took some time (about 10 years) before the importance of these concepts and how they related to an acceptable short circuit transfer was understood. From a heat input standpoint, short circuit wire welding was just the ticket. By effectively turning the arc on and off about 100 times/sec so that there was no arc at all 25% of the time, it was apparent that arc power could be reduced significantly with short circuit welding. There are reasons it took 10 years to refine this seemingly logical transition from spray to short arc. The balancing act of clearing that short circuit every time without blowing away the weld puddle as you did, took some time to resolve. Too much current, too fast and the spatter generation becomes unacceptable. Too little, too late and the short does not clear and the wire stubs and you have a relaxation oscillator that continues in a stub/flare cycle indefinitely. Short arc no longer required current above the spray threshold for a given wire. Actually, the average current was not sufficient to burn-off the incoming wire so that short circuits were inevitable.

*Cold lapping from excessive puddle agitation. Produces poor weld tie in at toes. Arc cant reach base material in order to melt it.*Wet: Current is slightly reduced to allow the surface tension between the weld puddle and the ball on the electrode to break and allow the two members to join as one.Pinch: Current is rapidly increased to induce electromagnetic fields, which begins to pinch the ball at the end of the electrode.Clear: Current is gradually increased at a linear rate, while the prediction circuitry looks for the short to clear.Blink Predict: Approximately 50 microseconds before the short clears, the inverter is disabled allowing the secondary welding current to drop.Blink Arc: A low current setting allows the arc to be reinitiated, thought of as a quelling time, as to not agitate the puddle. Largest difference between RMD and GMAW. Ball: Current is increased and held constant to allow the electrode to form a ball for the next short circuit transfer.Background: Current is dropped down to allow the overall heat input to be minimized while also allowing the electrode to begin approaching the puddle.Pre-Short: Current is again allowed to drop down to a relatively low value, allowing the electrode to touch the weld puddle and form the next short circuit.

Each phase has at least three adjustable variablesTarget Current ValueRamp Rate to get to that Target Current ValueTime, includes Ramp Rate time

Plus another approx 20 variables per taught point =50 parameters that need to be set per taught point. About 6 or 7 TP per program = 300-350 parameters per program.*Two main points1.) Power is equal to the current squared times the resistanceWhere as the resistance is equal to the small segment of wire times the resistivity (function of temperature) divided by the area (pi R^2).As the area approaches zero, the overall resistance in the circuit dramatically increases.2.) Looking at the overall power equation, the resistance does increase from the area decreasing, but its the value of the current squared that has the largest appreciable affect.Consider the difference between 300 amps squared vs. 100 or 150 amps squared

*This diagram graphically shows what would happen if the preceding equations were cared.

Pinching Down the electrode, goes off, then clears the shortIf we can catch that, predict when the short is about to clear, then we can begin to optimize the process.This is vital to the process

The curve is relatively flat in the beginning because the current and area has not began to change.

**When looking at the heat input of GMAW, we use the above formulaThe heat input is a function of the process being in an arc and the constant contributions of the resistance heating.Arc heating term: is the current, voltage , work function of the material (amount of energy required to remove electrons), and thermal energy. The 3 comes from the three directions of movement by the electrons. Resistive Heating: is the resistive term (B, heat energy), current squared, and this applies to the entire stick out of the electrode while welding.

*There are other phase transitions with iron before it melts: alpha (BCC) to gamma (FCC) @ 912 degC- gamma to delta (BCC) @ 1394 degC

Hmelt: is the amount of energy required for an amount of material to change forms (solid to liquid), and not change temperature. In other words a phase transformation.

Q = mL where:Q is the amount of energy released or absorbed during the change of phase of the substance (in kJ or in BTU), m is the mass of the substance (in kg or in lb), and L is the specific latent heat for a particular substance (kJ-kgm1 or in BTU-lbm1); substituted as Lf to represen