Industrial Machinery Repair Episode 2 Part 10 pot

The higher currents used with automatic welding result in a highweld metal deposition.. Gas-Shielded Metal Arc Welding GMAW The GMAW process, sometimes called metal inert gas MIG welding

● Check sheaves for wear Use a sheave gauge to ensure the sheave is notworn If worn, write a corrective maintenance work order to change thesheave at a later date.

Risk if the procedure is not followed: HIGH Belts will slip (even though

you may not hear the slippage), thus resulting in equipment not operating

to specifications

An important use of arc welding is the repair of plant machinery and ment In this respect, welding is an indispensable tool without which pro-duction operations would soon shut down Fortunately, welding machinesand electrodes have been developed to the point where reliable welding can

equip-be accomplished under the most adverse circumstances Frequently, ing must be done under something less than ideal conditions, and therefore,equipment and operators for maintenance welding should be the best.Besides making quick, on-the-spot repairs of broken machinery parts, weld-ing offers the maintenance department a means of making many itemsneeded to meet a particular demand promptly Broken castings, whennew ones are no longer available, can be replaced with steel weldmentsfashioned out of standard shapes and plates (see Figure 24.1) Specialmachine tools required by production for specific operations often can

weld-be designed and made for a fraction of the cost of purchasing a standardmachine and adapting it to the job Material-handling devices can be made

to fit the plant’s physical dimensions Individual jib cranes can be installed.Conveyors, either rolldown or pallet-type, can be tailor-made for specificapplications Tubs and containers can be made to fit products Grabs, hooks,and other handling equipment can be made for shipping and receiving Jigsand fixtures, as well as other simple tooling, can be fabricated in the main-tenance department as either permanent tooling or as temporary toolingfor a trial lot

The almost infinite variety of this type of welding makes it impossible to domore than suggest what can be done Figures 24.2 through 24.5 providejust a few examples of the imaginative applications of welding technologyachieved by some maintenance technicians The welding involved shouldpresent no particular problems if the operators have the necessary trainingand background to provide them with a knowledge of the many weldingtechniques that can be used

A maintenance crew proficient in welding can fabricate and erect many ofthe structures required by a plant, even to the extent of making structural

Original casting

Properlydesignedweldment

Not neededFigure 24.1

sub-Figure 24.3

Figure 24.4

to dynamic loads of several tons where a crane is involved Materials andjoint designs must be selected with a knowledge of what each can do Thenthe design must be executed by properly trained and qualified welders.Structural welding involves out-of-position work, so a welder must be able

to make good welds under all conditions Typical joints that are used inwelded structures are shown in Figures 24.6 through 24.9

Figure 24.5

Standard structural shapes, including pipe, which makes an excellent tural shape, can be used Electrodes such as the E6010-11 types are oftenthe welder’s first choice for this kind of fabrication welding because of theirall-position characteristics These electrodes, which are not low-hydrogentypes, may be used providing the weldability of the steel is such that neitherweld cracks nor severe porosity is likely to occur

struc-Scrap materials often can be put to good use When using scrap, however,

it is best to weld with a low-hydrogen E7016-18 type of electrode, sincethe analysis of the steel is unlikely to be known, and some high-carbonsteels may be encountered Low-hydrogen electrodes minimize crackingtendencies Structural scrap frequently comes from dismantled structuressuch as elevated railroads, which used rivet-quality steel that takes little or

no account of the carbon content

Shielded Metal Arc Welding (SMAW)

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5

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Within the shielded metal arc welding process, electrodes are readily able in tensile strength ranges of 60,000 to 120,000 psi (see Table 24.1) Inaddition, if specific alloys are required to match the base metal, these, too,are readily available (see Table 24.2)

avail-Field weld

A

1/4" for 8",10",12",14",8/16" beams 5/16" forbeams larger than 16"

Standard connection for simple

Figure 24.7

I-beam seat

Seat made from

Flux-Cored Arc Welding (FCAW)

Flux-cored arc welding is generally applied as a semiautomatic process

It may be used with or without external shielding gas depending on theelectrode selected Either method utilizes a fabricated flux-cored elec-trode containing elements within the core that perform a scavenging anddeoxidizing action on the weld metal to improve the properties of the weld

FCAW with Gas

If gas is required with a flux-cored electrode, it is usually CO2or a mixture

of CO2and another gas These electrodes are best suited to welding tively thick plate (not sheet metal) and for fabricating and repairing heavyweldments

rela-FCAW Self-Shielded

Self-shielded flux-cored electrodes, better known as Innershield, are alsoavailable In effect, these are stick electrodes turned inside out andmade into a continuous coil of tubular wire All shielding, slagging, and

Molten pool

Extruded covering Gaseous shield

Arc stream Base metal Slag

A typical application of semiautomatic and fully automatic equipment forFCAW is shown in Figure 24.11 For a given cross section of electrode wire,much higher welding amperage can be applied with semiautomatic and fullyautomatic processes This is because the current travels only a very shortdistance along the bare metal electrode, since contact between the current-carrying gun and the bare metal electrode occurs close to the arc In manualwelding, the welding current must travel the entire length of the electrode,

Note: Single values shown are maximum percentages, except where otherwise specified.

aThe suffixes A1, B3, C2, etc designate the chemical composition of the electrode classification.

bFor determining the chemical composition, DECN (electrode negative) may be used where

DC, both polarities, is specified.

cThese classifications are intended to conform to classifications covered by the military fications for similar compositions.

speci-dCopper shall be 0.10% max and aluminum shall be 0.05% max for E8018-NM electrodes.

eThe letters “XX” used in the classification designations in this table stand for the various strength levels (70, 80, 90, 100, 110, and 120) of electrodes.

fIn order to meet the alloy requirements of the G group, the weld deposit need have the minimum, as specified in the table, of only one of the elements listed Additional chemical requirements may be agreed between supplier and purchaser.

g

Figure 24.11

and the amount of current is limited to the current-carrying capacity ofthe wire The higher currents used with automatic welding result in a highweld metal deposition This increases travel speed and reduces weldingtime, thereby lowering costs

Gas-Shielded Metal Arc Welding (GMAW)

The GMAW process, sometimes called metal inert gas (MIG) welding, porates the automatic feeding of a continuous consumable electrode that

incor-is shielded by an externally supplied gas Since the equipment providesfor automatic control of the arc, only the travel speed, gun positioning, andguidance are controlled manually Process control and function are achievedthrough the basic elements of equipment shown in Figure 24.12 The gunguides the consumable electrode and conducts the electric current andshielding gas to the workpiece The electrode feed unit and power sourceare used in a system that provides automatic regulation of the arc length Thebasic combination used to produce this regulation consists of a constant-voltage power source (characteristically providing an essentially flat volt-ampere curve) in conjunction with a constant-speed electrode feed unit

Welding current, amperes

Square butt joint

GMAW for Maintenance Welding

In terms of maintenance welding applications, GMAW has the followingadvantages over SMAW:

1 Can be used in all positions with the low-energy modes

2 Produces virtually no slag to remove or be trapped in weld

3 Requires less operator training time than SMAW

4 Adaptable to semiautomatic or machine welding

5 Low-hydrogen process

6 Faster welding speeds than SMAW

7 Suitable for welding carbon steels, alloy steels, stainless steels,aluminum, and other nonferrous metals Table 24.3 lists recommen-ded filler metals for GMAW

Gas Selection for GMAW

There are many different gases and combinations of gases that can be usedwith the GMAW process These choices vary with the base metal, whether

a spray arc or short-circuiting arc is desired, or sometimes just according

to operator preference Recommended gas choices are given in Tables 24.4and 24.5

Table 24.3

filler metal

Aluminum 1100 ER1100 or ER4043 0.030 0.8 50–175 and 3003, 3004 ER 1100 or ER5356 3/64 1.2 90–250 aluminum 5052, 5454 ER5554, ER5356 A5.10 1/16 1.6 160–350

AZ80A ERAZ61A, ERAZ92A 3/64 1.2 160–320 2

ZE10A ERAZ61A, ERAZ92A 1/16 1.6 210–400 2

ZK21A ERAZ61A, ERAZ92A 3/32 2.4 320–510 2

Copper and Silicon Bronze ERCuSi-A

copper Deoxidized copper ERCu 0.035 0.9 150–300 alloys Cu-Ni alloys ERCuNi A5.7 0.045 1.2 200–400

Aluminum bronze ERCuA1-A1, A2 or A3 1/16 1.6 250–450 Phosphor bronze ERCuSn-A 3/32 2.4 350–550

nickel Monel 3 Alloy 400 ERNiCu-7 0.030 0.8 – alloys Inconel 3 Alloy 600 ERNiCrFe-5 A5.14 0.035 0.9 100–160

0.045 1.2 150–260 1/16 1.6 100–400 Titanium Commercially Use a filler metal one 0.030 0.8 –

titanium Ti-0.15 Pd ERTi-0.2 Pd A5.16 0.045 1.2 – alloys Ti-5A1–2.5Sn ERTi-5A1–2.5Sn or

Table 24.3 continued

filler metal

Steel Hot rolled or ER70S-3 or ER70S-1 0.020 0.5 – cold-drawn plain ER70S-2, ER70S-4 0.025 0.6 – carbon steels ER70S-5 ER70S-6 0.030 0.8 40-220

0.035 0.9 60-280 A5.18 0.045 1.2 125-380

0.052 1.3 260-460 1/16 1.6 275-450

2 Spray transfer mode.

3 Trademark-International Nickel Co.

Table 24.4

Metal Shielding gas Advantages

Aluminum Argon 0 to 1 in (0 to 25 mm) thick: best metal transfer

and arc stability; least spatter.

35% argon 1 to 3 in (25 to 76 mm) thick: higher heat input + 65% helium than straight argon; improved fusion characteristics

with 5XXX series Al-Mg alloys.

25% argon Over 3 in (76 mm) thick: highest heat input; + 75% helium minimizes porosity.

Carbon steel Argon Improves arc stability; produces a more fluid and

+ 1–5% oxygen controllable weld puddle; good coalescence and

bead contour, minimizes undercutting; permits higher speeds than pure argon.

Argon Good bead shape; minimizes spatter; reduces chance + 3–10% CO 2 of cold lapping; cut not weld out of position Low-alloy steel Argon Minimizes undercutting; provides good toughness.

+ 2% oxygen

Continued

Table 24.4 continued

Metal Shielding gas Advantages

Stainless steel Argon Improves arc stability; produces a more fluid and

+ 1% oxygen controllable weld puddle, good coalescence and

bead contour, minimizes undercutting on heavier stainless steels.

Argon Provides better arc stability, coalescence, and + 2% oxygen welding speed than 1 percent oxygen mixture for

thinner stainless steel materials.

Copper, nickel, Argon Provides good wetting; decreases fluidity of weld

Argon Higher heat inputs of 50 & 75 percent helium + helium tures offset high heat dissipation of heavier gages.

inert gas backing is required to prevent air nation on back of weld area.

contami-Table 24.5

Carbon steel 75% argon Less than 1/8 in (3.2 mm) thick: high

+ 25% CO2 welding speeds without burn-thru;

minimum distortion and spatter.

75% argon More than 1/8 in (3.2 mm) thick: minimum + 25% CO2 spatter; clean weld appearance; good puddle

control in vertical and overhead positions CO2 Deeper penetration; faster welding speeds.

Stainless steel 90% helium + 7.5% No effect on corrosion resistance; small

argon + 2.5% CO2 heat-affected zone; no undercutting;

minimum distortion.

Low alloy steel 60-70% helium Minimum reactivity; excellent toughness;

+ 25–35% argon excellent arc stability, wetting characteristics, + 4–5% CO2 and bead contour, little spatter.

75% argon Fair toughness; excellent arc stability, + 25% CO2 wetting characteristics, and bead contour;

Gas Tungsten Arc Welding (GTAW)

The gas tungsten arc welding (GTAW) process, also referred to as the sten inert gas (TIG) process, derives the heat for welding from an electricarc established between a tungsten electrode and the part to be welded(Figure 24.13) The arc zone must be filled with an inert gas to protectthe tungsten electrode and molten metal from oxidation and to provide aconducting path for the arc current The process was developed in 1941 pri-marily to provide a suitable means for welding magnesium and aluminum,where it was necessary to have a process superior to the shielded metal arc(stick electrode) process Since that time, GTAW has been refined and hasbeen used to weld almost all metals and alloys

tung-The GTAW process requires a gas- or water-cooled torch to hold the tungstenelectrode; the torch is connected to the weld power supply by a power cable

In the lower-current gas-cooled torches (Figure 24.14), the power cable isinside the gas hose, which also provides insulation for the conductor Water-cooled torches (Figure 24.15) require three hoses: one for the water supply,one for the water return, and one for the gas supply The power cable is

Copperbacking barArc

Shieldinggas in

Currentconductor

Figure 24.13

Optionalgas valve Gas

inlet

Figure 24.14

usually located in the water-return hose Water cooling of the power cableallows use of a smaller conductor than that used in a gas-cooled torch ofthe same current rating

Applicability of GTAW

The GTAW process is capable of producing very high-quality welds in almostall metals and alloys However, it produces the lowest metal deposition rate

of all the arc welding processes Therefore, it normally would not be used

on steel, where a high deposition rate is required and very high qualityusually is not necessary The GTAW process can be used for making rootpasses on carbon and low-alloy steel piping with consumable insert rings orwith added filler metal The remainder of the groove would be filled usingthe coated-electrode process or one of the semiautomatic processes such

as GMAW (with solid wire) or FCAW (with flux-cored wire)

A constant-current or drooping-characteristic power supply is required forGTAW, either DC or AC and with or without pulsing capabilities For water-cooled torches, a water cooler circulator is preferred over the use of tapwater

Handle

Gas in Water out

Water in

Gas orifice Power cable Cup

Electrode

Figure 24.15

For automatic or machine welding, additional equipment is required toprovide a means of moving the part in relation to the torch and feeding thewire into the weld pool A fully automatic system may require a programmerconsisting of a microprocessor to control weld current, travel speed, andfiller wire feed rate An inert-gas supply (argon, helium, or a mixture ofthese), including pressure regulators, flowmeters, and hoses, is required forthis process The gases may be supplied from cylinders or liquid containers

A schematic diagram of a complete gas tungsten arc welding arrangement

is shown in Figure 24.16

GTAW would be used for those alloys for which high-quality welds and dom from atmospheric contamination are critical Examples of these are thereactive and refractory metals such as titanium, zirconium, and columbium,where very small amounts of oxygen, nitrogen, and hydrogen can cause loss

free-of ductility and corrosion resistance It can be used on stainless steels andnickel-base superalloys, where welds exhibiting high quality with respect

to porosity and fissuring are required The GTAW process is well suited forwelding thin sheet and foil of all weldable metals because it can be controlled

at the very low amperages (2 to 5 amperes) required for these thicknesses.GTAW would not be used for welding the very low-melting metals, such astin-lead solders and zinc-base alloys, because the high temperature of thearc would be difficult to control