bab 29 welding design process selection

8/8/2019 Bab 29 Welding Design Process Selection

1/88

Metalurgi Pengelasan:Metalurgi Pengelasan:

Rancangan PengelasanRancangan Pengelasandan Pemilihan Prosesdan Pemilihan ProsesIr. Tri Prakosa, M. Eng.

Proses Manufaktur II, Januari 2010

1


2/88

IntroductionIntroduction

H eating the workpieces to a temperaturesufficiently high to produce a weld involvesimportant metallurgical and physical changes inthe materials being welded.The strength, toughness, and ductility of awelded joint depend on many factors.For example, the rate of heat application and

the thermal properties of metals are important inthat they control the magnitude and distributionof temperature in a joint during welding.

Institut Teknologi Bandung2


3/88

Introduction,Introduction, contcont

The microstructure and grain size of the welded joint depends on the magnitude of heat appliedand temperature rise , the degree of prior coldwork of the metals, and the rate of cooling after the weld is made.Weld quality depends on factors such as thegeometry of the weld bead and the presence of cracks , residual stresses , inclusions , and oxidefilms.Their control is essential to reliable welds thathave acceptable mechanical properties.



4/88

THE WELDED JO INTTHE WELDED JO INT



5/88

The welded jointThe welded joint

Three distinct zones can be identified:1. The base metal , that is, the metal to be joined .2. The heat-affected zone (HAZ).3. The weld metal , that is, the region that has melted

during welding.

The metallurgy and properties of the secondand third zones depend strongly on the metals

joined, the welding process, filler metals used, if any, and process variables.



6/88

The welded joint,The welded joint, contcont

A joint produced without a filler metal is calledautogenous , and the weld zone is composed of the resolidified base metal.A joint made with a filler metal has a centralzone called the weld metal and is composed of a mixture of the base and filler metals.



7/88

Typ ical fusion weld jointTyp ical fusion weld joint


1. The base metal ,that is, the metalto be joined.

2. The heat-affected zone (HAZ).

3. The weld metal ,that is, the regionthat has meltedduring welding.

Contoh karakteristik daerah fusi dari lasan padapengelasan busur gas oxyfuel.


8/88

S olidification of the weld metalS olidification of the weld metal

A fter applying heat and introducing filler metal, if any, into the weld area, the molten weld joint isallowed to cool to ambient temperature.The solidification process is similar to that incasting and begins with the formation of columnar ( dendritic ) grains .These grains are relatively long and form

parallel to the heat flow



9/88

S olidification of the weld metalS olidification of the weld metalBecause metals are much better heat conductors thanthe surrounding air, the grains lie to the plane of the twoplates or sheets being welded (Figure a).The grains in a shallow weld are shown in Figure b.


Struktur butir pada (a) lasan dalam (b) lasan dangkal. Perhatikan bahwa butir padalasan yang mengalami pendinginan orientasinya tegak lurus permukaan logamdasar. Pada lasan yang baik, garis pendinginan yang diperlihatkan sebagai garis

pada bagian tengah lasan dalam yang diperlihatkan pada (a) mempunyai migrasi butir yang menghasilkan kekuatan seragam pada manik/kampuh lasan.


10/88


G rain structure and size depend on the specificalloy , the welding process , and the filler metalused.The weld metal is basically a cast structure and,because it has cooled slowly, it generally hascoarse grains .Consequently, this structure has generally low

strength, toughness, and ductility .



11/88


H owever, the proper selection of filler-metalcomposition or heat treatments followingwelding can improve the joint's mechanicalproperties.The results depend on the particular alloy, itscomposition, and the thermal cycling to whichthe joint is subjected.

Cooling rates may, for example, be controlledand reduced by preheating the general weldarea prior to welding.



12/88


Preheating is particularly important for metalswith high thermal conductivity, such asaluminum and copper; otherwise, the heatduring welding rapidly dissipates.



13/88

K am p uhK am p uh LasanLasan


(a) Kampuh Lasan (pada cold-rolled nickel strip ) yang dihasilkan oleh sinar laser.(b) Profil kekerasan mikro penampang manik lasan. Perhatikan bahwa manik lasan mempunyai kekerasan relatif rendah dibandingkan dengan kekerasan logaminduk. Sumber: IIT Research Institute.


14/88

HeatHeat--affected zoneaffected zone

The heat-affected zone (HAZ ) is within the basemetal itself.It has a microstructure different from that of thebase metal before welding, because it has beensubjected to elevated temperatures for a periodof time during welding.The portions of the base metal that are far

enough away from the heat source do notundergo any changes during welding.



15/88


The properties and microstructure of the HAZ depend on:a. the rate of heat input and cooling; andb. the temperature to which this zone was raised.

The HAZ and the corresponding phase diagramfor 0.3 percent carbon steel are shown in thefollowing Figure



16/88

Daerah p ada Fusi di Zona LasanDaerah p ada Fusi di Zona Lasan


I lustrasi skematik berbagai daerah di dalam sona fusi lasan (dandiagram fasa yang sesuai) untuk 0.30% baja karbon. Sumber: Am erican Welding Society.


17/88


In addition to metallurgical factors (such asoriginal grain size , grain orientation , and degreeof prior cold work ), the specific heat and thermalconductivity of the metals influence the HAZ 's

size and characteristics .The strength and hardness of the heat-affectedzone depend partly on how the original strengthand hardness of the particular alloy wasdeveloped prior to welding .



18/88


They may have been developed by coldworking, solid-solution strengthening,precipitation hardening, or by various heattreatments.Of these strengthening methods, the simplest toanalyze is base metal that has been coldworked, say, by cold rolling or forging.

The heat applied during welding recrystallizesthe elongated grains (preferred orientation) of the cold-worked base metal.



19/88


20/88


The grain structureof such a weld-exposed tocorrosion by

chemical reaction- isshown in the Figure.The center verticalline is where the two

workpieces meet.


Korosi intergranular pada310

-stainless-steel welded tube setelah diekspos dengan larutan caustic. Garislasan terletak pada bagian tengah foto. Hasil scanm ikroskop elektron dengan pe m besaran 2 0 X. Sum ber:Courtesy of B. R. Jack, A llegheny Ludlu m Steel Corp.


21/88


The effects of heat during welding on the HAZ for joints made with dissimilar metals , and for alloys strengthened by other methods, arecomplex and beyond the scope of this lecture.



22/88

WELD QUA LITYWELD QUA LITY



23/88

Weld Qualit yWeld Qualit y

Because of a history of thermal cycling andattendant microstructural changes , a welded

joint may develop certain discontinuities .Welding discontinuities can also be caused byinadequate or careless application of established welding technologies or substandard operator training .

The major discontinuities that affect weld qualityare described as follow.



24/88

Porosit yPorosit y

Porosity in welds is caused by trapped gasesreleased during melting of the weld area andtrapped during solidification, chemical reactionsduring welding, or contaminants.Most welded joints contain some porosity , whichis generally spherical in shape or in the form of elongated pockets .

The distribution of porosity in the weld zone maybe random , or it may be concentrated in acertain region .



25/88

Porosit y,Porosit y, contcont

Porosity in welds can be reduced by thefollowing methods:{ Proper selection of electrodes and filler metals.{ Improving welding techniques, such as preheating

the weld area or increasing the rate of heat input .{ Proper cleaning and preventing contaminants from

entering the weld zone.{ Slowing the welding speed to allow time for gas to

escape.



26/88

S lag inclusionsS lag inclusions

Slag inclusions are compounds such as oxides ,fluxes , and electrode-coating materials that aretrapped in the weld zone.If shielding gases are not effective duringwelding, contamination from the environmentmay also contribute to such inclusions.Welding conditions are important, and with

proper techniques the molten slag will float tothe surface of the molten weld metal and not beentrapped.



27/88

S lag inclusions,S lag inclusions, contcont

Slag inclusions may be prevented by:{ Cleaning the weld-bead surface before the next layer

is deposited by using a hand or power wire brush.{ Providing adequate shielding gas .

{ Redesigning the joint to permit sufficient space for proper manipulation of the puddle of molten weldmetal.



28/88

Incom p lete fusion and p enetrationIncom p lete fusion and p enetration

Incomplete fusion (or lack of fusion) producespoor weld beads, such as those shown here.


Kualitas kampuh rendah sebagai akibat fusi yang tidak lengkap/penuh.Sumber: Am erican Welding Society.


29/88

Incom p lete fusion and p enetrationIncom p lete fusion and p enetration

A better weld can be obtained by:{ Raising the temperature of the base metal.{ Cleaning the weld area prior to welding.{ Changing the joint design and type of electrode.

{ Providing adequate shielding gas.Incomplete penetration occurs when the depth of the welded joint is insufficient. Penetration can beimproved by:

{ Increasing the heat input.{ Lowering travel speed during welding.{ Changing the joint design.{ Ensuring that the surfaces to be joined fit properly.



30/88

Weld p rofileWeld p rofileWeld profile is important not only because of itseffects on the strength and appearance of theweld, but also because it can indicateincomplete fusion or the presence of slag

inclusions in multiple-layer welds.Underfilling results when the joint is not filledwith the proper amount of weld metal (Figure a).



31/88

Weld p rofileWeld p rofile

Undercutting (Figure b) results from meltingaway the base metal and subsequentlygenerating a groove in the shape of a sharprecess or notch.



32/88

Weld p rofileWeld p rofileUnless it is not deep or sharp, an undercut canact as a stress raiser and reduce the fatiguestrength of the joint-and may lead to prematurefailure.

Overlap (Figure b) is a surface discontinuitygenerally caused by poor welding practice andselection of the wrong materials.


A proper weldis shown inFigure c.


33/88

C racksC racks

Cracks may occur in various locations anddirections in the weld area.The types of cracks are typically longitudinal,transverse, crater, underbead, and toe cracks(see next Figure).



34/88

JenisJenis- -jenis jenis retakanretakan p adap ada lasanlasan


J enis-jenis retakan (pada sambungan lasan) disebabkan oleh teganganthermal yang terjadi saat pendinginan dan kontraksi kampuh lasanserta struktur sekitar. (a) Retakan kawah. (b) Berbagai jenis retakan

pada sambungan tumpul ( butt) dan T.


35/88


36/88

C racksC racks

{ Inability of the weld metalto contract during cooling(Right Figure)-a situationsimilar to hot tears thatdevelop in castings andrelated to excessiverestraint of the workpiece


Retakan pada kampuh lasan,

berdasarkan kenyataan bahwa duakomponen tidak dimungkinkanmengalami kontraksi setelah lasanselesai. Sumber: S. L. Meiley, Packer

Engineering A ssociates, Inc.


37/88

C racksC racksCracks are classified as hot or cold cracks.H ot cracks occur while the joint is still at elevatedtemperatures.Cold cracks develop after the weld metal has

solidified.Some crack-prevention measures are:a. Change the joint design to minimize stresses from

shrinkage during cooling.

b. Change welding-process parameters, procedures, andsequence.

c. Preheat components being welded.d. A void rapid cooling of the components after welding.



38/88

Lamellar tearsLamellar tears

In describing the anisotropy of plasticallydeformed metals, we stated that because of thealignment of nonmetallic impurities andinclusions (stringers), the workpiece is weaker when tested in its thickness direction.This condition is particularly evident in rolledplates and structural shapes.



39/88

Lamellar tearsLamellar tears

In welding such components, lamellar tears maydevelop because of shrinkage of the restrainedmembers in the structure during cooling.Such tears can be avoided by providing for shrinkage of the members or by changing the

joint design to make the weld bead penetratethe weaker member more deeply.



40/88

S urface damageS urface damage

D uring welding, some of the metal may spatter and be deposited as small droplets on adjacentsurfaces.In arc welding processes, the electrode mayinadvertently contact the parts being welded atplaces not in the weld zone (arcstrikes).Such surface discontinuities may beobjectionable for reasons of appearance or subsequent use of the welded part.



41/88

S urface damageS urface damage

If severe, these discontinuities may adverselyaffect the properties of the welded structure,particularly for notch-sensitive metals.Using proper welding techniques andprocedures is important in avoiding surfacedamage.



42/88

Residual stressesResidual stresses

Because of localized heating and cooling duringwelding, expansion and contraction of the weldarea causes residual stresses in the workpiece.Residual stresses can cause:{ D istortion, warping, and buckling of the welded parts{ Stress-corrosion cracking.{ Further distortion if a portion of the welded structure

is subsequently removed, say, by machining or sawing.

{ Reduced fatigue life.



43/88

DistorsiDistorsi S etelahS etelah PengelasanPengelasan


D istorsi komponen setelah pengelasan: (a) sambungan tumpul; (b) Lasanfillet. D istorsi disebabkan oleh perbedaan ekspansi thermal dankontraksi dari komponen yang berbeda dari rakitan yang dilas.


44/88


The type and distribution of residual stresses inwelds is best described by reference to Figure a.



45/88


When two plates are being welded, a longnarrow region is subjected to elevatedtemperatures, whereas the plates as a wholeare essentially at ambient temperature.A s the weld is completed and time elapses, theheat from the weld area dissipates laterally tothe plates as the weld area cools.The plates thus begin to expand longitudinallywhile the welded length begins to contract.



46/88


These two opposing effects cause residualstresses that are typically distributed as shownin Figure b.



47/88


The magnitude of compressive residualstresses in the plates diminishes to zero at apoint away from the weld area.Because no external forces are acting on thewelded plates, the tensile and compressiveforces represented by these residual stressesmust balance each other.In complex welded structures, residual stressdistributions are three dimensional and difficultto analyze.



48/88


The preceding example involves two plates thatare not restrained from movement.In other words, the plates are not an integralpart of a larger structure.If they are restrained, reaction stresses will begenerated because the plates are not free toexpand or contract.

This situation arises particularly in structureswith high stiffness.



49/88

S tress relieving of weldsS tress relieving of welds

The problems caused by residual stresses, suchas distortion, buckling, or cracking, can bereduced by preheating the base metal or theparts to be welded.

Preheating reduces distortion by reducing thecooling rate and the level of thermal stresses(by reducing the elastic modulus).This technique also reduces shrinkage andpossible cracking of the joint.



50/88


The workpieces may be heated in a furnace or electrically or inductively, and for thin sections,by radiant lamps or hot-air blast.For optimum results, preheating temperaturesand cooling rates must be controlled carefully inorder to maintain acceptable strength andtoughness in the welded structure.Residual stresses can be reduced by stressrelieving the welded structure.



51/88


The temperature and time required for stressrelieving depend on the type of material andmagnitude of the residual stresses developed.Other methods of stress relieving includepeening, hammering, or surface rolling the weldbead area.These processes induce compressive residualstresses, thus reducing or eliminating tensileresidual stresses in the weld.



52/88


For multilayer welds, the first and last layersshould not be peened in order to protect themagainst possible peening-damage.Residual stresses can also be relieved, or reduced, by plastically deforming the structureby a small amount.This technique can be used in some weldedstructures, such as pressure vessels, bypressurizing the vessels internally (proof-stressing).



53/88


In order to reduce the possibility of suddenfracture under high internal pressure, the weldmust be made properly and be free fromnotches and discontinuities, which could act as

points of stress concentration.In addition to stress relieving, welds may alsobe heat treated by various techniques in order to modify their properties.



54/88

S tress relieving of weldsS tress relieving of weldsThese techniques include annealing,normalizing, or quenching and tempering of steels and solution treatment and aging of various alloys.



55/88

WELD AB ILITYWELD AB ILITY



56/88

Weldabilit yWeldabilit yWeldability of a metal as its capacity to bewelded into a specific structure that has certainproperties and characteristics and that willsatisfactorily meet its service requirements .

Weldability involves a large number of variables,making generalizations difficult.A s you have seen, the material characteristics-such as alloying elements, impurities,

inclusions, grain structure, and processinghistory-of the base metal and filler metal areimportant.



57/88

Weldabilit yWeldabilit y

Because of the melting, solidification, andmicrostructural changes involved, a thoroughknowledge of the phase diagram and theresponse of the metal or alloy to elevated

temperatures over a period of time is essential.A lso influencing weldability are the mechanicaland physical properties of strength, toughness,ductility, notch sensitivity, elastic modulus,specific heat, melting point, thermal expansion,surface tension characteristics of the moltenmetal, and corrosion.



58/88


Preparation of surfaces for welding is important,as are the nature and properties of surfaceoxide films and adsorbed gases.The welding process employed significantlyaffects the temperatures developed and their distribution in the weld zone.Other factors are shielding gases, fluxes,moisture content of the coatings on electrodes,welding speed, welding position, cooling rate,preheating, and post-welding techniques (suchas stress relieving and heat treating).



59/88


The following list states the general weldabilityof specific metals, which can vary if specialwelding techniques are used.a. Plain-carbon steels : Excellent for low-carbon steels;

fair to good for medium-carbon steels; poor for high-carbon steels.

b. Low-alloy steels : Similar to that for medium-carbonsteels.

c. H igh-alloy steels : G enerally good under well-controlled conditions.d. Stainless steels : Weldable by various processes.



60/88

Weldabilit yWeldabilit ye. A luminum alloys : Weldable at a high rate of heat input.

A lloys containing zinc or copper generally areconsidered unweldable.

f. Copper alloys : Similar to that of aluminum alloys.g. Magnesium alloys : Weldable with the use of protective

shielding gas and fluxes.h. Nickel alloys : Similar to that of stainless steels.i. Titanium alloys : Weldable with the proper use of

shielding gases.

j. Tantalum : Similar to that of titanium.k. Tungsten : Weldable under well-controlled conditions.l. Molybdenum : Similar to that of tungsten.m. Niobium (columbium) : G ood.



61/88

TE S TING WELDED JO INTSTE S TING WELDED JO INTS



62/88

Testing welded jointsTesting welded joints

A s in all manufacturing processes, the q uality of a welded joint is established by testing.Several standardized tests and test procedureshave been established and are available fromorganizations such as the A merican Society for Testing and Materials ( A STM), A mericanWelding Society ( A WS), A merican Society of Mechanical Engineers ( A SME), A mericanSociety of Civil Engineers ( A SCE), and federalagencies.



63/88

Testing welded jointsTesting welded joints

Welded joints may be tested either destructivelyor nondestructively.Each technique has certain capabilities,sensitivity, limitations, reliability, and need for special equipment and operator skill.Five methods of destructively testing welded

joints are commonly used.A

brief review of each method is provided asfollow.



64/88

Destructive techniquesDestructive techniques [Tension test][Tension test]

Longitudinal and transverse tension tests areperformed on specimens removed from actualwelded joints and from the weld metal area.Stress-strain curves are obtained.These curves indicate the yield strength (Y),ultimate tensile strength (UTS), and ductility of the welded joint in different locations anddirections.D uctility is measured in terms of percentageelongation and percentage reduction of area.



65/88

Destructive techniquesDestructive techniques [Tension[Tension- -shear test]shear test]

The specimens in the tension-shear test (Figurea and b) are specially prepared to simulateactual welded joints and procedures.



66/88

Destructive techniquesDestructive techniques [Tension[Tension- -shear test]shear test]

The specimens are subjected to tension, andthe shear strength of the weld metal and thelocation of fracture are determined.



67/88

Destructive techniquesDestructive techniques [Bend test][Bend test]Several bend tests have been developed todetermine the ductility and strength of welded

joints.


In one test,the weldedspecimen isbent arounda fixture(wrap-aroundbend test;Figure a).


68/88

Destructive techniquesDestructive techniques [Bend test][Bend test]In another test, thespecimens are tested inthree point transversebending (Figure b).

These tests help establishthe relative ductility andstrength of welded joints.


Destructive techniquesDestructive techniques


69/88


[Fracture Toughness test][Fracture Toughness test]

Fracture toughness tests commonly utilize theimpact testing techniques.Charpy V-notch specimens are prepared andtested for toughness.Other toughness tests include the drop-weighttest in which the energy is supplied by a fallingweight.




70/88


[C orrosion and cree p test][C orrosion and cree p test]

In addition to mechanical tests, welded jointsmay be tested for corrosion and creepresistance.Because of the difference in the compositionand microstructure of the materials in the weldzone, preferential corrosion may take place in it.Creep tests are important in determining thebehavior of welded joints at elevatedtemperatures. Testing weld hardness may alsobe used to indicate weld strength andmicrostructural changes in the weld zone.




71/88


[Testing of s p ot weld][Testing of s p ot weld]

Spot welded joints may be tested for weld-nugget strength using the(a) tension-shear

(b) cross-tension,




72/88



(c) twist,




73/88



and (d) peel test.




74/88

qq


Because they are easy to perform andinexpensive, tension-shear tests are commonlyused in fabricating facilities.The cross-tension and twist tests are capable of revealing flaws, cracks, and porosity in the weldarea.The peel test is commonly used for thin sheets.A

fter bending and peeling the joint, the shapeand size of the torn-out weld nugget isobserved.



75/88

Nondestructive techniquesNondestructive techniques

Welded structures often have to be testednondestructively, particularly for criticalapplications where weld failure can becatastrophic, such as in pressure vessels, load

bearing structural members, and power plants.Nondestructive testing techniques for welded

joints usually consist of visual, radiographic,magnetic particle, liquid penetrant, andultrasonic testing methods.



76/88

WELD DE S IGN AND PRO C ESS WELD DE S IGN AND PRO C ESS S ELE C TIONS ELE C TION



77/88

Weld Design and Process S electionWeld Design and Process S election

In addition to the material characteristicsdescribed thus far, selection of a joint and awelding process involves the followingconsiderations:

{ Configuration of the parts or structure to be weldedand their thickness and size.

{ The methods used to manufacture component parts.{ Service requirements, such as the type of loading

and stresses generated.{ Location, accessibility, and ease of welding.{ Effects of distortion and discoloration.



78/88


{A

ppearance.{ Costs involved in edge preparation, welding, and

post-processing of the weld, including machining andfinishing operations.

As in all manufacturing processes, the optimumchoice is the one that meets all design and

service requirements at minimum cost.



79/88

Weld Design and Process S electionWeld Design and Process S electionSome examples of weld characteristics areshown in the Figure, emphasizing the need for careful consideration of some of the factors justidentified.


Panduanrancangan untuk

pengelasan.Sumber: J. G.

Bralla (ed.), Handbook of Product Design for Manufacturing.


80/88


G eneral design guidelines may be summarizedas follows:a. Product design should minimize the number of

welds, as welding can be costly unless automated.

The weld location should be selected to avoidexcessive stresses or stress concentrations in thewelded structure and for appearance.

b. Parts should fit properly before welding. Themethod used to produce edges (sawing, machining,shearing, and flame cutting) can affect weld quality.

c. Some designs can avoid the need for edgepreparation.



81/88


d. Weld-bead size should be kept to a minimum toconserve weld metal. Weld location should beselected so as not to interfere with further processing of the part or with its intended use andappearance.

Standardized symbols used in engineeringdrawings to describe the type of weld and itscharacteristics are shown in the followingFigure.



82/88

S tandard Identification and S ymbols for WeldsS tandard Identification and S ymbols for Welds



83/88


These symbols identify the type of weld, groovedesign, weld size and length, welding process,sequence of operations, and various other information.



84/88

Ex am p le: Weld design selectionEx am p le: Weld design selection

In Figure a, the two vertical joints can be weldedexternally or internally.



85/88

Ex am p le: Weld design selectionEx am p le: Weld design selection

Full-length external welding takes considerabletime and requires more weld material than thealternative design, which consists of intermittentinternal welds.

Moreover, in the alternative method, theappearance of the structure is improved anddistortion is reduced.



86/88


87/88

Ex am p le: Weld design selectionEx am p le: Weld design selectionIn (c), the weld on the left requires about twice

the amount of weld material than the design onthe right.Note also that because more material must bemachined, the design on the left will requiremore time for edge preparation and more basemetal will be wasted.



88/88

bab 29 welding design process selection

Documents