Industrial Machinery Repair Episode 2 Part 12 doc

GTAW uses a nonconsumable tungsten electrode, with aluminum alloyfiller material added separately, either from a handheld rod or from a reel.Alternating current AC is preferred by many us

Maintenance Welding 509

density, surface tension, and cooling rate, horizontal, vertical, and overheadwelds can be made with relative ease High deposition rates are practical,producing less distortion, greater weld strength, and lower welding coststhan can be attained with other fusion welding processes

GTAW uses a nonconsumable tungsten electrode, with aluminum alloyfiller material added separately, either from a handheld rod or from a reel.Alternating current (AC) is preferred by many users for both manual andautomatic gas tungsten arc welding of aluminum because AC GTAW achieves

an efficient balance between penetration and cleaning

Copper and Copper Alloys

Copper and its alloys can be welded with shielded metal arc, gas-shieldedcarbon arc, or gas tungsten arc welding Of all these, gas-shielded arc weld-ing with an inert gas is preferred Decrease in tensile strength as temperaturerises and a high coefficient of contraction may make welding of copper com-plicated Preheating usually is necessary on thicker sections because of thehigh heat conductivity of the metal Keeping the work hot and pointing theelectrode at an angle so that the flame is directed back over the work willaid in permitting gases to escape It is also advisable to put as much metaldown per bead as is practical

Control of Distortion

The heat of welding can distort the base metal; this sometimes becomes

a problem in welding sheet metal or unrestrained large sections Thefollowing suggestions will help in overcoming problems of distortion:

a Avoid overwelding Use as little weld metal as possible by takingadvantage of the penetrating effect of the arc force

b Use correct edge preparation and fit-up to obtain required fusion atthe root of the weld

c Use fewer passes

d Place welds near a neutral axis

e Use intermittent welds

f Use back-step welding method

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a Preset the parts so that when the weld shrinks, they will be in thecorrect position after cooling

b Space parts to allow for shrinkage

c Prebend parts so that contraction will pull the parts into alignment

parts is insufficient to resist contraction)

a Balance one force with another by correct welding sequence sothat contraction caused by the weld counteracts the forces of weldspreviously made

b Peen beads to stretch weld metal Care must be taken so that weldmetal is not damaged

c Use jigs and fixtures to hold the work in a rigid position with sufficientstrength to prevent parts from distorting Fixtures can actually causeweld metal to stretch, preventing distortion

Special Applications

Sheet Metal Welding

Plant maintenance frequently calls for sheet metal welding The principles

of good welding practice apply in welding sheet metal as elsewhere, butwelding thin-gauge metals poses the specific challenges of potential distor-tion and/or burn-through Special attention should therefore be given toall the factors involved in controlling distortion: the speed of welding, thechoice of proper joints, good fit-up, position, proper current selection, use

of clamping devices and fixtures, number of passes, and sequence of beads.Good welding practice normally calls for the highest arc speeds and thehighest currents within the limits of good weld appearance In sheet metalwork, however, there is always the limitation imposed by the threat of burn-through As the gap in the work increases in size, the current must bedecreased to prevent burn-through; this, of course, will reduce weldingspeeds A clamping fixture will improve the fit-up of joints, making higherspeeds possible If equipped with a copper backing strip, the clamping fix-ture will make welding easier by decreasing the tendency to burn-throughand also removing some of the heat that can cause warpage Where possible,

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sheet metal joints should be welded downhill at about a 45 degrees anglewith the same currents that are used in the flat position or slightly higher.Tables 24.16 and 24.17 offer guides to the selection of proper current, volt-age, and electrodes for the various types of joints used with 20- to 8-gaugesheet metal

Resisting Abrasive Wear

Abrasive wear is resisted by materials with a high scratch hardness Sandquickly wears metals with a low scratch hardness, but under the same con-ditions, it will wear a metal of high scratch hardness very slowly Scratchhardness, however, is not necessarily measured by standard hardness tests.Brinell and Rockwell hardness tests are not reliable measures for determin-ing the abrasive wear resistance of a metal A hard-surfacing material of thechromium carbide type may have a hardness of 50 Rockwell C Sand willwear this material at a slower rate than it will a steel hardened to 60 Rock-well C The sand will scratch all the way across the surface of the steel Onthe surfacing alloy, the scratch will progress through the matrix materialand then stop when the sand grain comes up against one of the micro-scopic crystals of chromium carbide, which has a higher scratch hardnessthan sand If two metals of the same type have the same kind of microscopicconstituents, however, the metal having the higher Rockwell hardness will

be more resistant to abrasive wear

∗F—fat position; V—vertical; O—overhead.

† Electrode negative, work positive.

∗F—fat position; V—vertical; O—overhead.

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Figure 24.31

Resisting Impact Wear

Whereas abrasive wear is resisted by the surface properties of a metal, impactwear is resisted by the properties of the metal beneath the surface To resistimpact, a tough material is used, one that does not readily bend, break,chip, or crack It yields so as to distribute or absorb the load created byimpact, and the ultimate strength of the metal is not exceeded Included

in impact wear is that caused by bending or compression at low velocitywithout impact, resulting in loss of metal by cracking, chipping, upsetting,flowing, or crushing

Types of Surfacing Electrodes

Many different kinds of surfacing electrodes are available The problem is

to find the best one to do a given job Yet because service conditions vary

so widely, no universal standard can be established for determining theability of the surfacing to resist impact or abrasion Furthermore, there is

no ideal surfacing material that resists impact and abrasion equally well

In manufacturing the surfacing electrodes, it is necessary to sacrifice onequality somewhat to gain the other High impact resistance is gained bysacrificing abrasion resistance, and vice versa

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Price is no index of electrode quality An expensive electrode ingredientdoes not necessarily impart wear resistance Therefore, the user of surfacingmaterials must rely on a combination of the manufacturer’s recommenda-tions and the user’s own tests to select the best surfacing material for aparticular purpose

Choosing Hard-Facing Material

The chart shown in Figure 24.32 lists the relative characteristics of manualhard-facing materials This chart is a guide to selecting the two items in thefollowing list:

material has not produced the desired results

Example 1

Application: Dragline bucket tooth, as shown in Fig 24.32

Service: Sandy gravel with some good-sized rocks

Maximum wear that can be economically obtained is the goal of most facing applications The material chosen should rate as highly as possible inthe resistance-to-abrasion column, unless some other characteristics shown

hard-in the other columns make it unsuited for this particular application.First, consider the tungsten carbide types Notice that they are composed ofvery hard particles in a softer and less abrasion-resistant matrix Althoughsuch material is the best for resisting sliding abrasion on hard material, insand the matrix is apt to scour out slightly, and then the brittle particles areexposed These particles are rated poor in impact resistance, and they maybreak and spall off when they encounter the rocks

Next best in terms of abrasion, as listed in the chart, is the high-chromiumcarbide type shown in the electrode size column to be a powder It can beapplied only in a thin layer and also is not rated high in impact resistance.This makes it of dubious use in this rocky soil

The rod-type high-chromium carbides also rate very high in abrasion tance but do not rate high in impact resistance However, the second doesshow sufficient impact rating to be considered if two or three different mate-rials are to be tested in a field test Given the possibility that it has enough

(Rockwell scale) 30

C 50C 70C Particles

Particles Matrix

Matrix

Work hardened

Work hardened Work hardened

Work hardened

Heat treated Heat treated

Heat treated

Heat treated

Heat treated

Heat treated

Figure 24.32

Thin deposits

No limit $ 10.00 per hr forCost figured at

labor and overhead

Cost per cu in.

`of deposit Cost per pound

of electrode

(Powder)

A B C

Grind

Figure 24.32 continued

Example 2

Application: Same dragline tooth used in Example 1

Service: Soil changed to clay and shale

The semiaustenitic type selected in the first example stands up well, but theteeth wear only half as long as the bucket lip With double the wear on theteeth, only half the downtime periods would be needed for resurfacing, andboth teeth and bucket could be done together Since impact wear is nownegligible with the new soil conditions, a material higher in the abrasioncolumn should be considered A good selection would be the first high-chromium carbide rod, which could give twice the wear by controlling thesize of bead applied while still staying within a reasonable cost range

Example 3

Application: Same dragline tooth used as in Examples 1 and 2

Service: Soil changed to obtain large rocks

If the earth contains many hard and large rocks, and the teeth arefailing because of spalling under impact, one should move down theabrasion-resisting column to a more impact-resistant material, such as thesemiaustenitic type

These examples demonstrate that where a dragline operates in all kinds

of soils, a material that is resistant to both impact and abrasion, such as asemiaustenitic type, is the best choice When the same type of reasoning isused to check the important characteristics, an appropriate material can bechosen for any application If, for any reason, the first choice does not provesatisfactory, it is usually easy to improve the next application by choosing

a material that is rated higher in the characteristic that was lacking Wherefailures occur because of cracking or spalling, it usually indicates that amaterial higher in impact or ductility rating should be used Where normal

518 Maintenance Welding

wear alone seems too rapid, a material with a higher abrasion rating isindicated

Check Welding Procedure

Often, hard-facing failures due to cracking or spalling may be caused byimproper welding procedures Before changing the hard-surfacing material,consider whether or not the material has been properly applied For almostany hard-facing application, very good results can be obtained by followingthese precautions:

Remove any defective areas down to sound base metal

crack-ing and spallcrack-ing This minimum temperature should be maintained untilwelding is completed The exception to the rule is 11 to 14% manganesesteel, which should be kept cool

insulating material such as lime or sand

When more than normal buildup is required, apply intermediate layers ofeither medium carbon or stainless steel This will provide a good bond tothe base metal and will eliminate excessively thick layers of hard-surfacingmaterial that might otherwise spall off Stainless steel is also an excellentchoice for intermediate layers on manganese steels or for hard-to-weld steelswhere preheating is not practical

Check Before the Part Is Completely Worn

Whenever possible, examine a surfaced part when it is only partly worn.Examination of a part after it is completely worn is unsatisfactory Did thesurface crumble off, or was it scratched off? Is a tougher surface needed, or isadditional abrasion resistance required? Should a heavier layer of surfacing

be used? Should surfacing be reduced? All these questions can be answered

by examination of a partially worn part and with a knowledge of the surfacingcosts and service requirements

When it is impossible to analyze the service conditions thoroughly inadvance, it is always on the safe side to choose a material tougher than

a part which is normally surfaced with a tough, semiaustenitic electrode,

it may be possible to get additional abrasion resistance without sacrificingresistance to cracking A little of the powdered chromium carbide materialcan be fused to critical areas where additional protection is needed.Many badly worn parts are first built up to almost finished size with a high-carbon electrode, then surfaced with an austenitic rod, and finally a fewbeads of chromium carbide deposit are placed in spots requiring maximumprotection against abrasion Regardless of the circumstances, a careful anal-ysis of the surfacing problem will be well worthwhile Examples of jobs areshown in Figures 24.33 to 24.35

Hard Surfacing with SAW

The submerged arc process offers several advantages for hard surfacing Thegreater uniformity of the surface makes for better wearing qualities Thespeed of SAW creates major economies in hard-surfacing areas that requirethe deposition of large amounts of metal These areas may be either flat orcurved surfaces Mixer bottom plates, scraper blades, fan blades, chutes,

522 Maintenance Welding

Figure 24.38

Figure 24.39

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to the deposit through the flux rather than through the wire Also availableare tubular wires that contain alloying material in the hollow portion of themild-steel tube All the methods have specific advantages With SAW, con-siderable variation in the hard-surfacing deposit can be made by changingthe welding procedure to control admixture and the heat-treatment effect

of the welding cycle Methods and procedures should be established withthe help of qualified engineers

Selection and Maintenance of Equipment

Machines

Satisfactory welding can be accomplished with either alternating or directwelding current Each type of current, however, has particular advantagesthat make it best suited for certain types of welding and welding conditions.The chief advantage of alternating current is its elimination of arc blow,which may be encountered when welding on heavy plate or into a corner.The magnetic fields set up in the plate deflect the path of the arc Alternatingcurrent tends to minimize this deflection and also will increase the speed of

type of electrodes

The chief advantages of direct current are the stability of the arc and the factthat the current output of the motor-generator type of welder will remainconstant in spite of variations in the input voltage that affect a transformer-type welder Direct current, therefore, is a more versatile welding current.Certain electrodes, such as stainless, require a very stable arc; these elec-trodes operate much better with direct current Direct current, because

of its stability, is also better for sheet metal welding, where the danger ofburn-through is present The DC arc also can be more readily varied to meetdifferent welding conditions A wider range of control over both voltage andcurrent permits closer adjustment of the arc for difficult welding conditions,such as might be encountered in vertical or overhead welding Because ofits versatility, direct current should be available for maintenance welding.Direct-current welders (Figs 24.40 and 24.41) are made either as motor-generator sets or as transformer-rectifier sets Motor-generator sets arepowered by AC or DC motors Generators are also powered by small air-cooled gasoline engines (Figure 24.42) The advantage of this type of set