TIGBook_Chpt6.pdf

Cylinders should not be stored or used in a horizontal position.

This is because some cylinders contain a liquid which would

leak out or be forced out if the cylinder was laid in a flat position.

Handwheel

Welding torches and other cables should not be hung on or

near cylinders. A torch near a cylinder could cause an arc

against the cylinder wall or valve assembly, possibly resulting

in a weakened cylinder or even a rupture.

Double

Seating

Valve

It is very important to be absolutely sure of yourself before

attempting to use any welding equipment. Always think about

what you are doing, and if you are not sure of the next step

to take in any procedure, be sure to talk it over first with

your welding supervisor. Remember, safety is an impor-

tant factor not only for you, but for everyone around you!

Safety Cap

And Disc

It can be said that common sense is the most important tool a

welder can bring to the welding area. Common sense tells us

we must respect the basic safety steps which must be taken

to avoid both personal injury and injury to a fellow worker.

Horseplay or practical jokes have no place in the working area!

Outlet

Connection

Figure 5.8 Cross section of cylinder valve.

VI. Preparation for Welding

Certain basic preparations should be made prior to establishing

an arc. Preparations include base metal preparation, set up of

the machine and its controls. (Basic preparation of commonly

welded base metals will be covered later in this section.)

SMAW/GTAW Mode Switch

This switch should be set for the particular process being

used. It will disable various functions that are not required

when running one process or the other. For example, the gas

solenoid valves will not be active in the SMAW mode as they

are not required for this process.

Figure 6.1 illustrates the front panel of a typical AC/DC

machine designed for GTAW welding. Keep in mind that not

all power sources will have all the features or controls of this

machine. And the controls and switches mentioned in the

following paragraphs may be found in locations on the power

source other than the front panel. The various controls each

have a specific function and the operator changes or varies

them as the application changes. Power sources have

symbols that represent these various controls; table 10 in

Section XI covers these symbols.

Amperage Control Panel/Remote Switch

When a remote control device is being used, the switch must

be in the “remote” position. When amperage control is to be

at the front panel of the machine, the switch must be in the

“panel” position.

Output Control Panel/Remote Switch

When a remote output control device is being used, the

switch must be in the “remote” position. When using SMAW

and not using a remote output control device, the switch

must be in the “on” position. The “on” position means the

output terminal of the machine will have voltage applied as

soon as the power switch is turned on.

Preparing the Power Source

Power Switch

This switch controls the primary line power to the transformer.

When the switch is in the "on" position, voltage is applied to

the control circuit. Operation of the fan with the power switch

is dependent upon if the power source is equipped with Fan-

On-Demand ™ or not. In some cases, a pilot light will indicate

the power source is in the “on” mode. In other cases the LED

meters will indicate that the power is on. Before activatingthe

“On” switch make certain the electrode is not in contact

with the work lead or any portion of the work circuit!

Arc Force/Balance Control

On this particular power source, when the high-frequency

switch is enabled for GTAW welding, the arc force (Dig)

circuitry drops out, and this control becomes the balance arc

control. This will set the amount of time spent in the electrode

negative (maximum penetration equals more DCEN) and

electrode positive (maximum cleaning equals more DCEP)

portions of the AC cycle. For additional information, refer to

section II on GTAW fundamentals on the balance control. In

the SMAW or Stick electrode welding mode, the arc force

control will affect the arc action from a soft mushy type arc

(minimum arc control) to a driving-digging, forceful type arc

(maximum arc control). The arc force control is also referred

to as “dig”, and when used with the SMAW process it

increases the short circuit amperage. When set at 0, short

circuit amperage is the same as normal weld amperage.

Preflow and Postflow

Preflow control is not always a standard feature on all GTAW

power sources. It is made up of a timer control and an on/off

function switch. When “on”, the arc will not start until the pre-

flow timer has timed out assuring the arc will start in an inert

atmosphere. This reduces the possibility of air contaminating

the start of the weld. When “off” the preflow timer is out of

the circuit and the arc is able to start as soon as the remote

output control is activated. Postflow is a standard feature on

all GTAW power sources and consists of only a timer control.

It is used to allow the electrode, weld pool and filler rod to be

protected from the air while they cool down from welding

temperatures. Once they have cooled, they will no longer

oxidize. The postflow timer should time out and conserve

shielding gas. It is usually set to allow one second of postflow

time for each 10 amperes of welding current being used.

The tungsten should cool bright and shiny. Any bluing or

blackening indicates a lack of postflow.

Figure 6.1 Front panel of a typical AC/DC machine designed for

GTAW welding.

Weld Current Control or Amperage Control

This control sets the output current of the machine when no

remote current device is being used. With a remote device

attached, the control provides a percentage of total output.

For example, if the control is set at 50%, the remote device at

full output will deliver 50% of the machines available current.

Remote Amperage Control Receptacle

This receptacle is provided for connecting a remote hand

control or a remote foot control. This allows the operator to

have amperage control while welding at a work station which

may be a considerable distance from the power source. With

the foot control, the operator can vary the amperage as he

progresses along a joint. This is particularly helpful when

starting on a cold workpiece. Amperage may be increased to

establish a weld pool quickly, and as the material heats up,

the operator can decrease the amperage. When coming to the

end of a joint, the amperage can be further decreased to taper

off and "crater out".

Start Mode

The high-frequency switch has four (4) selections: start, off,

lift, and continuous.

When it is desired to use high frequency only to start the arc,

the “start” position is selected. The high frequency remains

on only until an arc is established and then it is automatically

removed from the circuit. This allows for starting the arc

without touching the electrode to the work. This setting

should be used when welding materials that might contaminate

the electrode.

AC/DC Selector and Polarity Switch

This three-position switch permits the operator to select

direct current electrode positive, direct current electrode

negative, or alternating current.

The “off” position is used when high frequency is not desired,

such as when scratch starts are suitable, or when the

machine is used for Stick electrode welding.

High-Frequency Intensity Control

This control allows the operator to choose the proper intensity

for the high-frequency output. As this control is increased,

the current in the high-frequency circuit is increased. It

should be set for the required intensity to start the arc. It is

recommended that this control be kept at a minimum setting

that will provide satisfactory weld starts. The higher the

setting the greater the amount of radiation which will cause

interference with communication equipment.

“Lift” arc is selected when high frequency is undesirable and

yet tungsten inclusions must be eliminated.

The “continuous” position is selected when welding with

alternating current on aluminum or magnesium. This provides

continuous high frequency for arc stabilization and starting.

Primary Overload Circuit Breaker

The circuit breaker provides protection against overloading

the main components of the welding machine. The circuit

breaker must be “on” before the primary contactor of the

machine can be energized.

Spark Gap Assembly

The spark gap points are normally set at the factory for

optimum performance. A feeler gauge can be used to check

the spacing or make adjustments on some machines.

Preparing the Weld Joint

Many GTAW problems, or supposed problems, are a direct

result of using improper methods to prepare the joint. Chief

among these is the improper use of grinding wheels to

prepare joints. Soft materials such as aluminum become

impregnated with microsized abrasive particles which, unless

subsequently removed, will result in excessive porosity.

Grinding wheels should be cleaned and dedicated exclusively

to the material being welded. The ideal joint preparation is

obtained with cutting tools such as a lathe for round or cylin-

drical joints, or a milling machine for longitudinal preparations.

When the electrode is positive and the work is negative

(reverse polarity or during one half of the AC cycle), the

positively charged gas ions are attracted to the negative

workpiece. These ions strike the surface with sufficient force

to chip away at the brittle oxide much like a miniature sand-

blasting operation. The electron flow from the work to the

electrode lifts the loosened oxide leaving clean base metal to

be welded.

Cleaning

Cleanliness of both the weld joint area and the filler metal is an

important consideration when welding with the Gas Tungsten

Arc Welding process. Oil, grease, shop dirt, paint, marking

crayon, and rust or corrosion deposits all must be removed

from the joint edges and metal surfaces to a distance beyond

the heat affected zone. Their presence during welding may

lead to arc instability and contaminated welds. If a weld is

made with any of these contaminants present, the result

could be a weld bead with pores, cracks, or inclusions.

Cleaning may be accomplished by mechanical means, by the

use of vapor or liquid cleaners, or by a combination of these.

Figure 6.2 Aluminum TIG Weld. Note bright area where oxides have been

removed through cleaning action of the arc.

This cleaning action should not be relied upon to do all the

cleaning. Mechanical or chemical cleaning methods should

be employed to remove heavy oxide, paint, grease, and oil, or

any other materials that will hinder proper fusion. Mechanical

cleaning may be done with abrasive wheels, wire brushes, or

other methods. Special abrasive wheels are available for

aluminum, and stainless steel wire brushes are recommended.

The important point is that the abrasive wheels and wire

brushes should be used only on the material being cleaned.

If a wire brush for example were used on rusty steel, and then

on aluminum, the brush could carry contaminants from one

piece to another. The vigorous brushing can impregnate the

contaminants carried in the brush into the aluminum. The

same is true of the abrasive wheel and equipment used to cut

and form aluminum.

Fixturing

Fixturing may be required if the parts to be welded cannot be

self supported during welding or if any distortion cannot be

tolerated or corrected by straightening. Fixturing should be

massive enough to support the weight of the weldment and

to withstand stresses caused by thermal expansion and

contraction. The decision to use fixturing for the fabrication of a

weldment is governed by economics and quality requirements.

There’s another problem that sometimes occurs when only

the side of the joint being welded is cleaned. Contamination

from the backside or between butting edges can be drawn

into the arc area. It is recommended that both sides of the

joint be cleaned if it contains foreign material.

Preheating

Preheating is sometimes required, the necessity being dictated

for the most part by the thickness of the material to be welded.

Preheating is most often achieved with the use of an oxy-

acetylene torch. However care must be taken when using this

method that localized overheating doesn’t occur, and the base

metal is not contaminated with combustion by-products of

the oxy-fuel process. Other methods of preheating include

induction coils, heating blankets and heating furnaces.

Another frequent source of contamination is the filler metal.

Aluminum filler wire and rod oxidizes just like the base metal.

If it is severe enough the rod must be cleaned prior to use.

The operator sometimes transfers contaminants from dirty

welding gloves onto the filler rod and consequently into the

weld area. Stainless steel wool is a good material to use for

cleaning filler wire and rod.

Preparing Aluminum for Welding

The preparation of aluminum deserves more consideration

than it is often times given. Aluminum is very susceptible to

contaminants which can cause considerable problems when

welding. First of all, aluminum has a surface oxide which

must be removed. This oxide removal is mentioned in detail

in the text devoted to Squarewave current. There have been

various theories as to how the arc action actually provides the

cleaning action. High speed photographs and films of the arc

let us observe the oxide removal.

Welding Aluminum

The information contained in Figures 6.3a and 6.3b will serve

as a guide to aluminum welding parameters.

Aluminum is a very good conductor of heat. The heat is rapidly

conducted away from the arc area and spread over the work-

piece. On small weldments, the entire part may heat up to a

Aluminum…Manual Welding – Alternating Current – High Frequency Stabilized

Gas

Flow-CFH

Tungsten Electrode

Diameter

Filler Rod Diameter

(If Required)

Metal Thickness

Joint Type

Amperage

Type

Butt

Lap

Corner

Fillet

1/16"

60 – 85

70 – 90

60 – 85

75 – 100

Argon

1/16"

Butt

Lap

Corner

Fillet

Butt

Lap

Corner

Fillet

3/32" – 1/8"

3/32"

125 – 150

130 – 160

120 – 140

130 – 160

180 – 225

190 – 240

180 – 225

190 – 240

Argon

1/8"

1/8" – 5/32"

1/8"

3/16"

Butt

Lap

Corner

Fillet

5/32" – 3/16"

3/16"

240 – 280

250 – 320

240 – 280

250 – 320

Argon

1/4"

Figure 6.3a Aluminum weld parameters.

Aluminum with Advanced Squarewave…Manual Welding – Alternating Current – High Frequency Stabilized

Filler

Material

Diameter

Electrode

Positive

Amperage

Electrode

Negative

Amperage

Frequency

Setting (Hz)

Metal

Thickness

Joint

Type

Tungsten

Size

Balance

Setting

Shielding

Gas

75% EN

70% EN

Butt

Lap

Corner

Fillet

1/16"

110

100% Argon

1/16"

Butt

Lap

Corner

Fillet

3/32"

120

125

140

175

70% EN

73% EN

68% EN

78% EN

100% Argon

1/8"

Lap

Corner

Fillet

3/32"

110

230

220

240

250

140

250

73% EN

70% EN

75% EN

100% Argon

1/4"

Figure 6.3b Aluminum with Advanced Squarewave weld parameters.

point that requires reduction of amperage from the original

setting. Remote foot amperage controls are advantageous in

these situations. When welding out-of-position, the amperages

shown in Figures 6.3a and 6.3b may be decreased by about

15%. A water-cooled torch is recommended for amperages

over 150. The electrode stickout beyond the cup may vary

from approximately 1/16" on butt joints to possibly 1/2" in

joints where it is difficult to position the torch. The normal

recommended arc length is approximately the same as the

electrode diameter.

magnetic and non-magnetic types of stainless steel. There are

a large number of alloy types and each type possesses some

specific properties as to corrosion resistance and strength. A

check with the manufacturer is recommended when in doubt

about the specific properties of an alloy.

When welding stainless steel, it should be thoroughly

cleaned. Protective paper or plastic coatings are applied to

many stainless sheets. Foreign material may cause porosity

in welds and carburetion of the surface which will lessen the

corrosion resisting properties. Any wire brushing should be

done with stainless steel wire brushes to prevent iron pick up

on the stainless surfaces. As with other welding procedures,

clean and dry filler metal should be used and proper

precautions taken to prevent contamination during welding.

Preparing Stainless Steel

for Welding

“Stainless steel” is a common term used when referring to

chromium alloyed and chromium-nickel alloyed steel. There are

Welding Stainless Steel

Figure 6.4 contains parameters which will serve as a guide for

welding stainless steel.

Rapid cooling through this range will help keep precipitation

to a minimum. On some alloys of stainless steel, columbium

or titanium are added to prevent carbide precipitation. It is

important that the filler metal used is of the same general

analysis as the material being welded.

Chromium-nickel stainless steels are considered readily

weldable. Normally the welding does not adversely affect the

strength or ductility of the deposit, parent metal, or fusion

zone. The filler metal used should be compatible, of similar

composition, to the base metal. The heat conductivity of

chrome-nickel stainless steels are about 50% less than mild

steel with a high rate of thermal expansion. This increases the

tendency for distortion on thin sections.

Preparing Titanium for Welding

Titanium’s light weight, excellent corrosion resistance, and high

strength-to-weight ratio make this a desirable metal for applica-

tions in the chemical, aerospace, marine and medical fields. Its

use in the petrochemical industry and in the manufacture of

sports equipment is some more recent application areas. Many

consider titanium as very hard to weld. Titanium alloys can be

embrittled by not following proper welding techniques, but

titanium is much more readily welded than typically believed.

Values shown in Figure 6.4 are for single pass welds on the

thinner sections, multiple pass welds on heavier material, and

for welding out-of-position. Job conditions will affect the

actual amperage, flow rate, filler rod, and tungsten used.

Some examples are:

Before welding titanium, it is essential that the weld area and

the filler metal be cleaned. All mill scale, oil, grease, dirt,

grinding dust and any other contamination must be removed.

If the titanium is scale free, degreasing is all that is required.

If oxide scale is present, it should be degreased prior to

descaling. An area at least 1 inch (25mm) from where the

weld is to be made should be cleaned. The joint edges should

be brushed with a stainless steel wire brush and degreased

with acetone just prior to welding. Any titanium part handled

after cleaning should be done so in a so-called “white glove”

procedure to eliminate recontamination of the weld area. The

cleaned parts should be welded within a few hours or properly

stored by wrapping in lint-free and oil-free materials.

Joint design and fit-up

■

Job specifications

■

Use of backing (gas, rings, bars)

■

Specific alloy

■

Operator

■

Heat input can be critical. In many applications, it is desirable

to keep the heat input as low as possible. In the weld and heat

affected zone, a metallurgical change takes place known as

carbide precipitation. If corrosion resistance is a big factor in the

completed weld, it should be noted that some of the corrosion

resistance properties are lost in the weld and adjacent areas

that are heated above the temperature where carbide precip-

itation occurs (800 – 1400˚ F). Keeping heat input to a

minimum is necessary in this situation. The longer the work is

at the 800 –1400˚ F temperature, the greater the precipitation.

If grinding or sanding is used to clean titanium or prepare a

joint, be very cautious of the fine titanium dust particles.

Titanium is flammable and the smaller the dust particles are,

the more flammable it becomes.

Stainless Steel…Manual Welding…Direct Current – Electrode Negative

Gas

Flow-CFH

Tungsten Electrode

Diameter

Filler Rod Diameter

(If Required)

Metal Thickness

Joint Type

Amperage

Type

Butt

Lap

Corner

Fillet

1/16"

40 – 60

50 – 70

40 – 60

50 – 70

Argon

1/16"

Butt

Lap

Corner

Fillet

Butt

Lap

Corner

Fillet

3/32"

65 – 85

90 – 110

65 – 85

90 – 110

100 – 125

125 – 150

100 – 125

125 – 150

Argon

1/8"

3/32"

1/8"

3/16"

Butt

Lap

Corner

Fillet

1/8"

5/32"

135 – 160

160 – 180

135 – 160

160 – 180

Argon

1/4"

Figure 6.4 Stainless steel weld parameters.

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: