metalurgi welding

8/10/2019 Metalurgi WELDING

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The Metallurgy of welding :Welding Design and Process selection


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PendahuluanWelded jointWeldabilityPengujian welding jointWeld design dan process selection


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Kekuatan, keuletan, ketangguhansambungan lasan ( welded joint )

Struktur mikro dan ukuran butir

tergantung pada temperaturKualitas lasan tergantung pada: geometry Retak ( cracks )

Tegangan sisa ( residual stresses ) Inklusi ( inclusions ) Lapisan oksida ( oxide films )


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3 daerah yang berbeda: Base metal Heat affected zone Weld metal

Metallurgy and properties of second and third zonedepend stronglyOn metals joined , welding process, filler metalsused & on process variables.Autogenously is joint produced without the filler

metalWeld zone is composed of resolidified base metal


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Fig : Characteristics ofa typical fusionweld zone inoxyfuel gas andarc welding.


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Fig : Grain structure in (a) a deep weld (b) a shallow weld. Note that the grains in the solidified weld metalare perpendicular to the surface of the base metal. In a good weld, the solidification line at the center inthe deep weld shown in (a) has grain migration, which develops uniform strength in the weld bead.

(a) (b)


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Solidification begins with formation of columnargrains which is similar to castingGrains relatively long and form parallel to the heatflowGrain structure and size depend on the specific

alloyWeld metal has a cast structure because it hascooled slowly, it has grain structureResults depends on alloys ,composition andthermal cycling to which the joint is subjected.Pre-heating is important for metals having highthermal conductivity


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Fig : (a) Weld bead (on a cold-rolled nickel strip) produced by a laser beam. (b) Microhardness profileacross the weld bead. Note the lower hardness of the weld bead compared to the base metal.

(a) (b)


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Heat effected zone is within the metal itselfProperties depend onRate of heat input and coolingTemperature to which the zone was raised

Original grain size ,Grain orientation , Degree ofprior cold workThe strength and hardness depend partly on howoriginal strength and hardness of the base metalwas developed prior to the weldingHeat applied during welding Recrystalliseselongated grains of cold worked base metal


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Fig : Intergrannularcorrosion of a 310-stainless-steel weldedtube after exposure to acaustic solution. Theweld line is at thecenter of thephotograph. Scanningelectron micrographs at20X.


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Welding discontinuities can be caused byinadequate or careless applicationThe major discontinuities that affect weldquality arePorositySlag InclusionsIncomplete fusion and penetrationWeld profileCracksLamellar tearsSurface damageResidual stresses


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PorosityCaused by gases released during melting of the weldarea but trapped during solidification, chemicalreactions, Contaminants

They are in form of spheres or elongated pocketsPorosity can be reduced by : Proper selection of electrodes Improved welding techniques Proper cleaning and prevention of contaminants Reduced welding speeds

Slag nclusionsCompounds such as oxides ,fluxes, and electrode-coating materials that are trapped in the weld ZonePrevention can be done by following practices :Cleaning the weld bed surface before the next layer isdepositedProviding enough shielding gasRedesigning the joint


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Incomplete penetration occurs when the depth of thewelded joint is insufficient

Penetration can be improved by the following practices :Increasing the heat InputReducing the travel speed during the weldingChanging the joint designEnsuring the surfaces to be joined fit properly


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Under filling results when the joint is notfiled with the proper amount of weld metal.

Undercutting results from the melting away

of the base metal and consequentgeneration of a groove in the shape of asharp recess or notch.

Overlap is a surface discontinuity usuallycaused by poor welding practice and by theselection of improper material.


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Fig : Schematic illustration of various discontinuities in fusion welds.


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Cracks occur in various directions and variouslocations

Factors causing cracks:

Temperature gradients that cause thermal

stresses in the weld zoneVariations in the composition of the weld zone.

Embrittlement of grain boundaries

Inability if the weld metal to contract duringcooling


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Fig : Types of cracks (in welded joints) caused by thermal stresses that develop duringsolidification and contraction of the weld bead and the surrounding structure. (a) Crater cracks(b) Various types of cracks in butt and T joints.


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Fig : Crack in a weld bead, due to thefact that the two components were

not allowed to contract after the weldwas completed.


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Occurred due to the shrinkage of therestrained components in the structureduring cooling.

Can be avoided by providing for shrinkageof the members

Changing the joint design

Surface Damage : These discontinuities mayadversely affect the properties of weldedstructure, particularly for notch sensitivemetals.


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Caused because of localized heating andcooling during welding, expansion andcontraction of the weld area causes residualstresses in the work piece.

Distortion,Warping and buckling of weldedparts

Stress corrosion cracking

Further distortion if a portion of the weldedstructure is subsequently removed

Reduced fatigue life


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Fig : Distortion of parts after welding : (a) butt joints; (b) fillet welds. Distortion is caused by differentialthermal expansion and contraction of different parts of the welded assembly.


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Fig : Residual stresses developed during welding of a butt joint.


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Preheating reduces reduces problemscaused by preheating the base metal or theparts to be welded

Heating can be done electrically,infurnace,for thin surfaces radiant lamp or hotair blast

Some other methods of stress relieving :Peening, hammering or surface rolling


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Capacity to be welded into a specific structure that hascertain properties and characteristics and will

satisfactorily meet service requirementsThorough knowledge of the phase diagram is essential

Factors such as strength, toughness, ductility, notchsensitivity, elastic modulus, specific heat, melting point,

thermal expansion, surface tension characteristics of themolten metal, corrosion resistance.

Testing Welded Joints Quality of the welding joint is established by welded joint

Each technique has capabilities ,limitations and sensitivity reliabilityand requirement for special equipment and operator skill.


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Tension Test :Longitudinal and transverse tension tests are performedStress strain curves are obtained

Tension-Shear TestSpecifically prepared to simulate actual welded joints andprocedures.

Specimen subjected to tension and shear strength of theweld metal

Bend test :Determines ductility and strength of welded joints.

The welded specimen is bend around a fixtureThe specimens are tested in three-point transversebendingThese tests help to determine the relative ductility andstrength of the welded joints


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Fig : Two types of specimens for tension-sheartesting of welded joints.

Fig : (a) Wrap-around bend test method. (b)Three-point bending of welded specimens.


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Fracture Toughness Test:Corrosion and creep testsTesting of spot weldsTension-HearCross-tensionTwistPeel

Non-Destructive testing : Often weld structures need to be tested Non-Destructively Non-Destructive testing are :

Visual Radiographic Magnetic-particle Liquid-penetrant Ultrasonic


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Fig : (a) Tension-shear test for spot welds. (b) Cross-Tension test. (c) Twist test. (d) Peel test


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Considerations:

Configuration of the components or structure to bewelded, and their thickness and size

Methods used to manufacture the components

Service requirements, Type of loading and stressesgenerated

Location, accessibility and ease of welding

Effects of distortion and discolorationAppearance

Costs involved


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Fig : Design guidelines for welding


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Fig : Standard identification and symbols for welds


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