Metalurgi Pengelasan:
Metalurgi Pengelasan:
Rancangan Pengelasan
Rancangan Pengelasan
dan Pemilihan Proses
dan Pemilihan Proses
Ir. Tri Prakosa, M. Eng.
Proses Manufaktur II, Januari 2010
1
Introduction
Introduction

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.
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Introduction, con’t
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
THE WELDED JOINT
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The welded joint
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
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
Typical fusion weld joint
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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.
Solidification of the weld metal
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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Solidification of the weld metal

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

mechanical properties.

The results depend on the particular alloy, its composition,
and the thermal cycling to which the joint is subjected.

Cooling rates may, for example, be controlled and reduced by
preheating the general weld area prior to welding.
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Solidification of the weld metal
Solidification of the weld metal

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
Kampuh Lasan
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(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.
Heat-affected zone
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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Heat-affected zone
Heat-affected zone

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
Daerah pada Fusi di Zona Lasan
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Ilustrasi skematik berbagai daerah di dalam sona fusi lasan (dan
diagram fasa yang sesuai) untuk 0.30% baja karbon. Sumber:
American Welding Society.
Heat-affected zone
Heat-affected zone

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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Heat-affected zone
Heat-affected zone

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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Heat-affected zone
Heat-affected zone

Grains that are away from the weld metal will recrystallize
into 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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Heat-affected zone
Heat-affected zone

The grain structure of
such a weld-exposed to
corrosion by chemical
reaction- is shown in the
Figure.

The center vertical line is
where the two workpieces
meet.
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Korosi intergranular pada 310-stainless-steel welded
tube setelah diekspos dengan larutan caustic. Garis
lasan terletak pada bagian tengah foto. Hasil scan
mikroskop elektron dengan pembesaran 20 X. Sumber:
Courtesy of B. R. Jack, Allegheny Ludlum Steel Corp.
Heat-affected zone
Heat-affected zone

The effects of heat during welding on the HAZ for joints made
with dissimilar metals, and for alloys strengthened by other
methods, are complex and beyond the scope of this lecture.
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WELD QUALITY
WELD QUALITY
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Weld Quality

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
Porosity

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, 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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