Bab 29 welding design process selection

welding design process selection

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Metalurgi Pengelasan:

Rancangan Pengelasan

Proses Manufaktur II, Januari 2010

1

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 Heating the workpieces to a temperature sufficiently high to produce a weld involves important metallurgical and physical changes in the materials being welded

 The strength, toughness, and ductility of a welded joint

depend on many factors

 For example, the rate of heat application and the thermal

properties of metals are important in that they control the

magnitude and distribution of temperature in a joint during welding

Institut Teknologi Bandung 2

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Introduction, con’t

 The microstructure and grain size of the welded joint depends

on the magnitude of heat applied and temperature rise, the degree of prior cold work of the metals, and the rate of

cooling after the weld is made

 Weld quality depends on factors such as the geometry of the weld bead and the presence of cracks, residual stresses,

inclusions, and oxide films

 Their control is essential to reliable welds that have

acceptable mechanical properties

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THE WELDED JOINT

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The 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 second and third zones depend strongly on the metals joined, the welding process, filler metals used, if any, and process variables

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The welded joint, con’t

 A joint produced without a filler metal is called autogenous , and the weld zone is composed of the resolidified base metal

 A joint made with a filler metal has a central zone called the weld metal and is composed of a mixture of the base and

filler metals

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Typical fusion weld joint

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.

Contoh karakteristik daerah fusi dari lasan pada

pengelasan busur gas oxyfuel.

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Solidification of the weld metal

 After applying heat and introducing filler metal, if any, into the weld area, the molten weld joint is allowed to cool to ambient temperature

 The solidification process is similar to that in casting and

begins with the formation of columnar (dendritic) grains

 These grains are relatively long and form parallel to the heat flow

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 Because metals are much better heat conductors than the surrounding air, the grains lie to the plane of the two plates 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 pada lasan yang mengalami pendinginan orientasinya tegak lurus permukaan logam dasar 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.

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 Grain structure and size depend on the specific alloy, the welding process, and the filler metal used

 The weld metal is basically a cast structure and, because it has cooled slowly, it generally has coarse grains

 Consequently, this structure has generally low strength,

toughness, and ductility

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 However, the proper selection of filler-metal composition or heat treatments following welding can improve the joint's

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 Preheating is particularly important for metals with high thermal conductivity, such as aluminum and copper; otherwise, the heat during welding rapidly dissipates

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Kampuh Lasan

(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

logam induk Sumber: IIT Research Institute.

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Heat-affected zone

 The heat-affected zone (HAZ) is within the base metal itself

 It has a microstructure different from that of the base metal before welding, because it has been subjected to elevated temperatures for a period of time during welding

 The portions of the base metal that are far enough away from the heat source do not undergo any changes during welding

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 The properties and microstructure of the HAZ depend on:

a the rate of heat input and cooling; and

b the temperature to which this zone was raised.

 The HAZ and the corresponding phase diagram for 0.3 percent carbon steel are shown in the following Figure

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Daerah pada Fusi di Zona Lasan

Ilustrasi skematik berbagai daerah di dalam sona fusi lasan (dan

diagram fasa yang sesuai) untuk 0.30% baja karbon Sumber:

American Welding Society.

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 In addition to metallurgical factors (such as original grain size, grain orientation, and degree of prior cold work), the specific heat and thermal conductivity of the metals influence the

HAZ's size and characteristics

 The strength and hardness of the heat-affected zone depend partly on how the original strength and hardness of the

particular alloy was developed prior to welding

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 They may have been developed by cold working,

solid-solution strengthening, precipitation hardening, or by various heat treatments

 Of these strengthening methods, the simplest to analyze is base metal that has been cold worked, say, by cold rolling or forging

 The heat applied during welding recrystallizes the elongated

grains (preferred orientation) of the cold-worked base metal

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 Grains that are away from the weld metal will recrystallizeinto fine equiaxed grains

 However, grains close to the weld metal, having been

subjected to elevated temperatures for a longer period of time, will grow

 This growth will result in a region that is softer and has less strength

 Such a joint will be weakest in its heat-affected zone

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 The center vertical line is

where the two workpieces

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 The effects of heat during welding on the HAZ for joints madewith dissimilar metals, and for alloys strengthened by other methods, are complex and beyond the scope of this lecture

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WELD QUALITY

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Weld Quality

 Because of a history of thermal cycling and attendant

microstructural changes, a welded joint may develop certain discontinuities

 Welding discontinuities can also be caused by inadequate or careless application of established welding technologies or substandard operator training

 The major discontinuities that affect weld quality are

described as follow

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 Porosity in welds is caused by trapped gases released during melting of the weld area and trapped during solidification,

chemical reactions during welding, or contaminants

 Most welded joints contain some porosity, which is generally spherical in shape or in the form of elongated pockets

 The distribution of porosity in the weld zone may be random,

or it may be concentrated in a certain region

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Porosity, con’t

 Porosity in welds can be reduced by the following 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.

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Slag inclusions

 Slag inclusions are compounds such as oxides, fluxes, and electrode-coating materials that are trapped in the weld zone

 If shielding gases are not effective during welding,

contamination from the environment may also contribute to such inclusions

 Welding conditions are important, and with proper techniquesthe molten slag will float to the surface of the molten weld

metal and not be entrapped

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Slag inclusions, con’t

 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 weld

metal.

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Incomplete fusion and penetration

 Incomplete fusion (or lack of fusion) produces poor weld beads, such as those shown here

Kualitas kampuh rendah sebagai akibat fusi yang tidak lengkap/penuh

Sumber: American Welding Society.

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Incomplete fusion and penetration

 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 be improved by:

{ Increasing the heat input.

{ Lowering travel speed during welding.

{ Changing the joint design.

{ Ensuring that the surfaces to be joined fit properly.

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Weld profile

 Weld profile is important not only because of its effects on the strength and appearance of the weld, but also because it can indicate incomplete fusion or the presence of slag inclusions

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Weld profile

 Undercutting (Figure b) results from melting away the base metal and subsequently generating a groove in the shape of

a sharp recess or notch

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Weld profile

 Unless it is not deep or sharp, an undercut can act as a

stress raiser and reduce the fatigue strength of the joint-and may lead to premature failure

 Overlap (Figure b) is a surface discontinuity generally caused

by poor welding practice and selection of the wrong

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Jenis-jenis retakan pada lasan

Jenis-jenis retakan (pada sambungan lasan) disebabkan oleh tegangan thermal yang terjadi saat pendinginan dan kontraksi kampuh lasan serta struktur sekitar (a) Retakan kawah (b) Berbagai jenis retakan

pada sambungan tumpul (butt) dan T.

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{ Hydrogen embrittlement.

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{ Inability of the weld metal

to contract during cooling

(Right Figure)-a situation

similar to hot tears that

develop in castings and

related to excessive

restraint of the workpiece

Retakan pada kampuh lasan,

berdasarkan kenyataan bahwa dua

komponen tidak dimungkinkan

mengalami kontraksi setelah lasan

selesai Sumber: S L Meiley, Packer

Engineering Associates, Inc.

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 Cracks are classified as hot or cold cracks

 Hot cracks occur while the joint is still at elevated temperatures

 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, and

sequence.

c Preheat components being welded.

d Avoid rapid cooling of the components after welding.

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Lamellar tears

 In describing the anisotropy of plastically deformed metals,

we stated that because of the alignment of nonmetallic

impurities and inclusions (stringers), the workpiece is weaker when tested in its thickness direction

 This condition is particularly evident in rolled plates and

structural shapes

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Lamellar tears

 In welding such components, lamellar tears may develop

because of shrinkage of the restrained members 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 penetrate the weaker member more deeply

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Surface damage

 During welding, some of the metal may spatter and be

deposited as small droplets on adjacent surfaces

 In arc welding processes, the electrode may inadvertently

contact the parts being welded at places not in the weld zone (arcstrikes)

 Such surface discontinuities may be objectionable for

reasons of appearance or subsequent use of the welded part

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Surface damage

 If severe, these discontinuities may adversely affect the

properties of the welded structure, particularly for

notch-sensitive metals

 Using proper welding techniques and procedures is important

in avoiding surface damage

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Residual stresses

 Because of localized heating and cooling during welding, expansion and contraction of the weld area causes residual stresses in the workpiece

 Residual stresses can cause:

{ Distortion, 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.

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Distorsi Setelah Pengelasan

Distorsi komponen setelah pengelasan: (a) sambungan tumpul; (b) Lasan fillet Distorsi disebabkan oleh perbedaan ekspansi thermal dan

kontraksi dari komponen yang berbeda dari rakitan yang dilas.

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 When two plates are being welded, a long narrow region is subjected to elevated temperatures, whereas the plates as a whole are essentially at ambient temperature

 As the weld is completed and time elapses, the heat from the weld area dissipates laterally to the plates as the weld area cools

 The plates thus begin to expand longitudinally while the

welded length begins to contract

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residual stresses must balance each other.

 In complex welded structures, residual stress distributions are three dimensional and difficult to analyze

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 The preceding example involves two plates that are not

restrained from movement

 In other words, the plates are not an integral part of a larger structure

 If they are restrained, reaction stresses will be generated because the plates are not free to expand or contract

 This situation arises particularly in structures with high

stiffness

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Stress relieving of welds

 The problems caused by residual stresses, such as

distortion, buckling, or cracking, can be reduced by

preheating the base metal or the parts to be welded

 Preheating reduces distortion by reducing the cooling rate

and the level of thermal stresses (by reducing the elastic

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Stress relieving of welds

 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 temperatures and cooling rates must be controlled carefully in order to maintain

acceptable strength and toughness in the welded structure

 Residual stresses can be reduced by stress relieving the

welded structure

Tiêu đề	Bab 29 Welding Design Process Selection
Trường học	Institut Teknologi Bandung
Chuyên ngành	Manufacturing Engineering
Thể loại	thesis
Năm xuất bản	2010
Thành phố	Bandung

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