Industrial Machinery Repair Episode 2 Part 11 pps

Argon is used more extensively than helium for GTAW because:1 It produces a smoother, quieter arc action; 2 It operates at a lower arc voltage for any given current and arc length; 3 The

Argon is used more extensively than helium for GTAW because:

1 It produces a smoother, quieter arc action;

2 It operates at a lower arc voltage for any given current and arc length;

3 There is greater cleaning action in the welding of materials such asaluminum and magnesium in the AC mode;

4 Argon is more available and lower in cost than helium;

5 Good shielding can be obtained with lower flow rates;

6 Argon is more resistant to arc zone contamination by cross drafts;

7 The arc is easier to start in argon

The density of argon is approximately 1.3 times that of air and 10 timesthat of helium For this reason, argon will blanket a weld area and be moreresistant than helium to cross drafts Helium, being much lighter than air,tends to rise rapidly and cause turbulence, which will bring air into thearc atmosphere Since helium costs about three times as much as argon,and its required flow rate is two to three times that for argon, the cost ofhelium used as a shielding gas can be as much as nine times that of argon.Although either helium or argon can be used successfully for most GTAWapplications, argon is selected most frequently because of the smoother arcoperation and lower overall cost Argon is preferred for welding thin sheet

to prevent melt-through Helium is preferred for welding thick materialsand materials of high thermal conductivity such as copper and aluminum

Electrode Material for GTAW

In selecting electrodes for GTAW, five factors must be considered: material,size, tip shape, electrode holder, and nozzle Electrodes for GTAW areclassified as pure tungsten, tungsten containing 1 or 2% thoria, tungstencontaining 0.15 to 0.4% zirconia, and tungsten that contains an internallateral segment of thoriated tungsten The internal segment runs the fulllength of the electrode and contains 1 or 2% thoria Overall, these elec-trodes contain 0.35 to 0.55% thoria All tungsten electrodes are normallyavailable in diameters from 0.010 to 0.250" and lengths from 3" to 24".Chemical composition requirements for these electrodes are given in AWSA5.12, “Specification for Tungsten Arc Welding Electrodes.”

Pure tungsten electrodes, which are 99.5% pure, are the least expensivebut also have the lowest current-carrying capacity on AC power and a low

resistance to contamination Tungsten electrodes containing 1 or 2% thoriahave greater electron emissivity than pure tungsten and, therefore, greatercurrent-carrying capacity and longer life Arc starting is easier, and the arc ismore stable, which helps make the electrodes more resistant to contamina-tion from the base metal These electrodes maintain a well sharpened pointfor welding steel.

Tungsten electrodes containing zirconia have properties in between those ofpure tungsten and thoriated tungsten electrodes with regard to arc startingand current-carrying capacity These electrodes are recommended for ACwelding of aluminum over pure tungsten or thoriated tungsten electrodesbecause they retain a balled end during welding and have a high resistance

to contamination Another advantage of the tungsten-zirconia electrodes istheir freedom from the radioactive element thorium, which, although notharmful in the levels used in electrodes, is of concern to some welders

GTAW Electrode Size and Tip Shape

The electrode material, size, and tip shape (Figure 24.21) will depend onthe welding application, material, thickness, type of joint, and quantity.Electrodes used for AC or electrode-positive polarity will be of largerdiameter than those used for electrode-negative polarity

The total length of an electrode will be limited by the length that can

be accommodated by the GTAW torch Longer lengths allow for moreredressing of the tip than short lengths and are therefore more econom-ical The extension of the electrode from the collet or holder determinesthe heating and voltage drop in the electrode Since this heat is of no value

to the weld, the electrode extension should be kept as short as necessary toprovide access to the joint

It is recommended that electrodes to be used for DC negative-polarity ing be of the 2% thoria type and be ground to a truncated conical tip.Excessive current will cause the electrode to overheat and melt Too low

weld-a current will permit cweld-athode bombweld-ardment weld-and erosion cweld-aused by the lowoperating temperature and resulting arc instability Although a sharp point

on the tip promotes easy arc starting, it is not recommended because it willmelt and form a small ball on the end

For AC and DC electrode-positive welding, the desirable electrode tip shape

is a hemisphere of the same diameter as the electrode This tip shape onthe larger electrodes required for AC and DC electrode-positive weldingprovides a stable surface within the operating current range Zirconia-type

Electrode for use with

DC electrode negative

Electrode

tip angle

Tipradius

TipradiusElectrode

diameter

Electrode diameterElectrode

Figure 24.21

electrodes are preferred for AC and DC electrode-positive operation becausethey have a higher current-carrying capacity than the pure tungsten elec-trodes, yet they will readily form a molten ball under standard operatingconditions Thoriated electrodes do not ball readily and, therefore, are notrecommended for AC or DC electrode-positive welding

The degree of taper on the electrode tip affects weld penetration, wherethe smaller taper angles tend to reduce the width of the weld bead andthus increase penetration When preparing the tip angle on an electrode,grinding should be done parallel to the length of the electrode Specialmachines are available for grinding electrodes These can be set to accuratelygrind any angle required

GTAW Electrode Holders and Gas Nozzles

Electrode holders usually consist of a two-piece collet made to fit eachstandard-sized tungsten electrode These holders and the part of the GTAWtorch into which they fit must be capable of handling the required weld-ing current without overheating These holders are made of a hardenablecopper alloy

The function of the gas nozzle is to direct the flow of inert gas around theholder and electrode and then to the weld area The nozzles are made of

a hard, heat-resistant material such as ceramic and are available in various

sizes and shapes Large sizes give a more complete inert gas coverage ofthe weld area but may be too big to fit into restricted areas Small nozzlescan provide adequate gas coverage in restricted areas where features of thecomponent help keep the inert gas at the joint Most nozzles have internalthreads that screw over threads on the electrode holder Some nozzles arefitted with a washer-like device that consists of several layers of fine-wirescreen or porous powder metal These units provide a nonturbulent orlamellar gas flow from the torch, which results in improved inert gas cover-age at a greater distance from the nozzle In machine or automatic welding,more complete gas coverage may be provided by backup gas shielding fromthe fixture and a trailer shield attached to the torch.

Characteristics of GTAW Power Supplies

Power supplies for use with GTAW should be of the constant-current,drooping-voltage type (Figure 24.22) They may have other optional fea-tures such as up slope, down slope, pulsing, and current programmingcapabilities Constant-voltage power supplies should not be used forGTAW

Constant currentpower source

The power supply may be a single-phase transformer-rectifier, which alsocan supply AC for welding aluminum Engine generator-type power sup-plies are usually driven by a gasoline or diesel engine and will produce DCwith either a constant-current, drooping-voltage or constant-voltage char-acteristics Engine alternator power supplies will produce AC for GTAW Apower supply capable of operating on either constant current or constantvoltage should be set for the constant-current mode for GTAW.

Power supplies made specifically for GTAW normally will include a frequency source for arc starting and valves that control the flow of inertgas and cooling water for the torch Timers allow the valves to be opened

high-a short time before the high-arc is initihigh-ated high-and closed high-a short time high-after the high-arc

is extinguished The high frequency is necessary for arc starting instead oftorch starting, where tungsten contamination of the weld is likely It should

be possible to set the high frequency for arc starting only, or for continuousoperation in the AC mode

Power supplies should include a secondary contactor and a means of trolling arc current remotely For manual welding, a foot pedal wouldperform these functions of operating the contactor and controlling weldcurrent A power supply with a single current range is desirable because itallows the welder to vary the arc current between minimum and maximumwithout changing a range switch

con-The more advanced power supplies incorporate features that permit pulsingthe current in the DC mode with essentially square pulses Both backgroundand pulse peak current can be adjusted, as well as pulse duration and puls-ing frequency (Figure 24.23) In the AC mode, the basic 60-Hz sine wavecan be modified to produce a rectangular wave Other controls permit the

AC wave to be balanced or varied to favor the positive or negative half-cycles.This feature is particularly useful when welding aluminum and magnesium,where the control can be set to favor the positive half-cycle for maximumcleaning In the DC mode, the pulsing capability allows welds to be made inthin material, root passes, and overhead with less chance of melt-through

or droop

GTAW Torches

A torch for GTAW must perform the following functions:

1 Hold the tungsten electrode so that it can be manipulated along theweld path

PeakcurrentBackgroundcurrentPulse timeCycle time

2 Provide an electrical connection to the electrode

3 Provide inert-gas coverage of the electrode tip, arc, and hot weld zone

4 Insulate the electrode and electrical connections from the operator ormounting bracket

Typical GTAW torches consist of a metallic body, a collet holder, a collet,and a tightening cap to hold the tungsten electrode The electrical cable isconnected to the torch body, which is enclosed in a plastic insulating outersheath For manual torches, a handle is connected to the sheath Power, gas,and water connections pass through the handle or, in the case of automaticoperation, through the top of the torch In the smaller, low-current torches,the electrode, collet, and internal components are cooled by the inert-gasflow Larger, high-current torches are water-cooled and require connections

to tap water and a drain or to a water cooler circulator A cooler circulator

with distilled or deionized water is preferred to prevent buildup of mineraldeposits from tap water inside the torch.

Inert gas flows through the torch body and through holes in the collet holder

to the arc end of the torch A cup or nozzle is fitted over the arc end ofthe torch to direct inert gas over the electrode and the weld pool Thenozzles normally screw onto the torch and are made of a hard, heat-resistantceramic Some are made of high-temperature glass such as Vicor and arepressed on over a compressible plastic taper Some nozzles can be fitted with

an insert washer made up of several layers of fine-wire screen sometimescalled a gas lens This produces a lamellar rather than turbulent flow of inertgas to increase the efficiency of shielding

On most manual GTAW torches, the handle is fixed at an angle of mately 70 degrees to the torch body Some makes of torches have a flexibleneck between the handle and torch body which allows the angle betweenthe handle and the torch body to be adjusted over a range from about 50degrees to 90 degrees

approxi-Manual GTAW Techniques

To become proficient in manual gas tungsten arc welding, the welder mustdevelop skills in manipulating the torch with one hand, while controllingweld current with a foot pedal or thumb control and feeding filler metalwith the other hand (see Figure 24.24) Before welding is started on anyjob, a rough idea of the welding conditions, such as filler material, current,shielding gas, etc., is needed

Establishing Welding Parameters for GTAW

The material, thickness, joint design, and service requirements will mine the weld current, inert gas, voltage, and travel speed This informationmay be available in a “welding procedure specification” (WPS) or from hand-book data on the material and thickness If welding parameters are notprovided in a WPS, the information given in Tables 24.6, 24.7, and 24.8 can

deter-be used as starting-point parameters for carbon and low-alloy steels, less steels, and aluminum These should be considered starting values; finalvalues should be established by running a number of test parts

stain-Gas Tungsten Arc Starting Methods

The gas tungsten arc may be started by touching the work with the trode, by a superimposed high frequency pulse, or by a high-voltage pulse

elec-Figure 24.24

The touch method is not recommended for critical work because there is

a strong possibility of tungsten contamination with this technique Mostweld power supplies intended for GTAW contain a high-frequency genera-tor (usually a spark-gap oscillator), which superimposes the high-frequencypulse on the main weld power circuit When welding with DC electrode-negative or -positive, the high-frequency switch should be set in the HTstart position When the welder presses the foot pedal to start welding, atimer is activated, which starts the high-frequency pulse and stops it whenthe arc initiates Once started, the arc will continue after the high-frequencypulse stops as long as the power and proper arc gap are maintained Whenwelding with AC, the switch should be set in the HF continuous position

to ensure that the arc restarts after voltage reversal on each half-cycle.High-frequency generators on welders produce frequencies in the radiocommunications range Therefore, manufacturers of power supplies mustcertify that the radio frequency radiation from the power supply does notexceed limitations established by the Federal Communications Commission(FCC) The allowable radiation may be harmful to some computer andmicroprocessor systems and to communications systems These possibili-ties for interference should be investigated before high-frequency starting

is used Installation instructions provided with the power supply should bestudied and followed carefully

Table 24.6

Welder input Ampere 3-conductor cable, Grounding

Amp input Wire size (3 in conduit) Wire size (3 in free air)

With-Volts With out With out Ground With out Ground Welder input condsr condsr condsr condsr conduct condsr condsr conduct.

Table 24.8

Machine size,

Cable sizes for lengths (electrode plus ground) amp Up to 50 ft 50–100 ft 100–250 ft

∗Recommended longest length of 4/0 cable for 400-amp

welder, 150 ft; for 600-amp welder, 100 ft For greater distances, cable size should be increased; this may be a question of cost- consider ease of handling versus moving of welder closer to work.

Oxyacetylene Cutting

Steel can be cut with great accuracy using an oxyacetylene torch (seeFigure 24.25) However, not all metals cut as readily as steel Cast iron,stainless steel, manganese steels, and nonferrous materials cannot be cutand shaped satisfactorily with the oxyacetylene process because of theirreluctance to oxidize In these cases, plasma arc cutting is recommended.The cutting of steel is a chemical action The oxygen combines readily withthe iron to form iron oxide In cast iron, this action is hindered by thepresence of carbon in graphite form, so cast iron cannot be cut as read-ily as steel Higher temperatures are necessary, and cutting is slower Insteel, the action starts at bright-red heat, whereas in cast iron, the tem-perature must be nearer the melting point in order to obtain a sufficientreaction

Because of the very high temperature, the speed of cutting is usually fairlyhigh However, since the process is essentially one of melting without anygreat action, tending to force the molten metal out of the cut, some provisionmust be made for permitting the metal to flow readily away from the cut.This is usually done by starting at a point from which the molten metal can

(DC-)

Cutting gasShieldinggas

Shieldingcup

Air-Carbon Arc Cutting and Gouging

Air-carbon arc cutting (CAC-A) is a physical means of removing base metal

or weld metal using a carbon electrode, an electric arc, and compressedair (see Figure 24.26) In the air-carbon arc process, the intense heat ofthe arc between the carbon electrode and the workpiece melts a portion

of the base metal or weld Simultaneously, a jet of air is passed throughthe arc of sufficient volume and velocity to blow away the molten material.This sequence can be repeated until the required groove or cut has beenobtained Since CAC-A does not depend on oxidation to maintain the cut, it

is capable of cutting metals that oxyacetylene cutting will not cut It is used

to cut carbon steel, stainless steel, many copper alloys, and cast iron.Arc gouging can be used to remove material approximately five times asfast as chipping Depth of cut can be closely controlled, and welding slagdoes not deflect or hamper the cutting action, as it would with cuttingtools Gouging equipment generally costs less to operate than chippinghammers or gas cutting torches An arc-gouged surface is clean and smoothand usually can be welded without further preparation Drawbacks of theprocess include the fact that it requires large volumes of compressed air,and it is not as good as other processes for through-cutting

excel-Power Sources

While it is possible to arc air gouge with AC, this is not the preferred method

A DC power source of sufficient capacity and a minimum of 60 open circuit