pengenalan welding dasar.pdf
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Introduction to Welding
PATRIA DEVELOPMENT CENTER
PATRiA Development CenterPATRiA Development Center -- PDCPDCPT. UNITED TRACTORS PANDU ENGINEERINGPT. UNITED TRACTORS PANDU ENGINEERING
20122012
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TUJUAN :
Memahami proses pengelasan dan efekyang terjadi kepada material
Memahami perubahan mikro strukturpengelasan dan kekerasan yang terjadi pada
daerah HAZ
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Memahami cacat las, penyebab serta carapencegahannya
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Apakah yang bisa dijual dari
pengelasan..??
Pengelasn merupakan sambungan paten yang kuat dan tahanlama
Bisa digunakan untuk hampir seluruh material logam
PATRIA DEVELOPMENT CENTER
Jika elektroda yang digunakan memiliki kekuatan tarik yang melebihikekuatan tarik dari material induk
Teknik menyambungkan logam yang paling mudah dan efisienhingga saat ini
Dapat dilakukan dimana saja, di lapangan maupun di dalam
pabrik
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Keterbatasan Pengelasan
Upah man power/ welder cenderung mahal
Kebanyakan proses pengelasan melibatkan
penggunaan energi tinggi, yang berbahaya
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sulit untuk melakukan pembongkarannya
Sambungan las bisa terjadi cacat yang sulit di deteksi,
dan hal ini bisa merusak struktur sambungan
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PATRIA DEVELOPMENT CENTER
Shielded Metal Arc Welding (SMAW)
An arc welding process that uses a consumable electrodeconsisting of a filler metal rod coated with chemicals that provide
flux and shielding
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PATRIA DEVELOPMENT CENTER
Gas Metal Arc Welding (GMAW) Arc welding process in which the electrode is a consumable bare
metal wire and shielding is accomplished by flooding the area
with gas
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PATRIA DEVELOPMENT CENTER
Submerged Arc Welding
Arc welding process that uses a continuous, consumable bare
wire electrode, arc shielding is provided by a cover of granular
flux
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PATRIA DEVELOPMENT CENTER
Resistance Welding
A fusion welding process that utilizes a combination of heat and
pressure to accomplish coalescence, the heat being generatedby electrical resistance to current flow at the junction to be
welded
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PATRIA DEVELOPMENT CENTER
HEAT25.12
HEAT2
2222
2222
OHCOOHCO
HCOOHC
Oxyacetylene Welding
A fusion welding process performed by a high-temperature flame
from a combustion of acetylene and oxygen
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Fusion Weld JointPATRIA DEVELOPMENT CENTER
Fusion Zone
A mixture of filler metal and base metal that has completely
melted
High degree of homogeneity among the component metals thathave been melted during welding
The mixing of these components is motivated largely by
convection in the molten weld pool
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Weld Interface
The narrow boundary that separates the fusion zone and theheat affected zone
This interface consists of a thin band of base metal that wasmelted or partially melted (localized melting within the grains)during the welding process, but immediately solidified before anymixing could take place
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The metal in this region has experienced temperature below itsmelting point, but high enough to change the microstructure
This metal consists of the base metal which has undergone aheat treatment due to the welding temperatures, so that itsproperties have been altered.
The amount of metallurgical damage in the HAZ depends on theamount of heat input, peak temp reached, distance from fusionzone, time at elevated temp, cooling rate, and the metalsthermal properties
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Heat Affected Zone (HAZ) contd The effect on the mechanical properties is usually negative, and
it is most often the region of the weld joint where failure occurs
Unaffected Base Metal Zone Where no metallurgical change has occurred
The base metal surrounding the HAZ is likely to be in a state ofhigh residual stress, due to the shrinkage in the fusion zone
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Welding Deffect :
1. CracksDetection
Surface: Visual examination, magnetic particle, dye or
fluorescent penetrant inspection
Internal: Ultrasonic flaw detection, radiography
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Solidification Cracking
Causes: Large depth/width ratio of weld
bead
High arc energy and/or preheat
Sulphur, phosphorus or niobium
pick-up from parent metal
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Hydrogen Induced HAZ Cracking
Causes: Hardened HAZ coupled with the
presence of hydrogen diffused from
weld metal
Susceptibility increases with the
increasing thickness of section
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especially in steels with high carbonequivalent composition
Can also occur in weld metal
Increase welding heat beneficial
Preheating sometimes necessary
Control of moisture in consumablesand cleanliness of weld prep
desirable
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Lamellar Tearing
Causes: Poor ductility in through-thickness
direction in rolled plate due to non-
metallic inclusions
Occurs mainly in joints having weld
metal deposited on plate surfaces
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Prior buttering of surface beneficial
for susceptible plate
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Retak Panas
Occurs in creep resisting and some
thick section structural low alloysteels during post weld heat
treatment
Causes:
Poor creep ductility in HAZ coupled
with thermal stress
Accentuated by severe notches
such as preexisting cracks, or
tears at weld toes, or unfused root
of partial penetration weld
Heat treatment may need to
include low temperature soaking Grinding or peening weld toes after
welding can be beneficial
X 200
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2. CavitiesDetection
Surface: Visual inspectionInternal: Ultrasonic flaw detection, radiography
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Worm Holes
Resulting from the entrapment of gasbetween the solidifying dendrites ofweld metal, often showingherringbone array ( B )
Causes: The gas may arise from
contamination of surfaces to bewelded, or be prevented fromescaping from beneath the weld by
joint crevices
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Porousity
Resulting from the entrapment of gas
in solidified weld metal
Causes:
Gas may originate from dampness
or grease on consumables or
workpiece, or by nitrogen
contamination from the
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atmosphere
If the weld wire used contains
insufficient deoxidant it is also
possible for carbon monoxide to
cause porosity
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Restart Porosity
Causes: Unstable arc conditions at weld
start, where weld pool protection
may be incomplete and temperature
gradients have not had time to
equilibrate, coupled with inadequate
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this instability
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Surface Porosity
Causes: Excessive contamination from
grease, dampness, or atmosphere
entrainment
Occasionally caused by excessive
sulphur in consumables or parent
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metal
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Crater Pipes
Resulting from shrinkage at the endcrater of a weld run
Causes:
Incorrect manipulative technique or
current decay to allow for crater
shrinkage
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3 Solid Inclusions
Detection
- normally revealed by radiography
Linear Slag Inclusions
Cause:
Incomplete removal of slag
in multi- ass welds often
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associated with the
presence of undercut or
irregular surfaces in
underlying passes
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Isolated Slag Inclusions
Causes:
Normally by the presence of mill
scale and/or rust on prepared
surfaces, or electrodes with
cracked or damaged coverings
Can also arise from isolated
undercut in underlying passes of
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multi-pass welds
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4. Lack of Fusion and Penetration
Detection This type of defect tends to be sub surface and is therefore
detectable only by ultrasonics or X-ray methods
Lack of side wall fusion which penetrates the surface may be
detected using magnetic particle, dye or fluorescent
penetrant inspection
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Cause
Incorrect weld conditions (eg. low current) and/or incorrect
weld preparation (eg. root face too large)
Both cause the weld pool to freeze too rapidly
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PATRIA DEVELOPMENT CENTER
ac o s e-wa us on ac o roo us on ac o n er-run us on
Lack of penetration
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5 Imperfect Shape
Detection
- all shape defects can be determined by visual inspections
Linear Misalignment (HI-LO)
Cause:
Incorrect assembly or
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distortion during fabrication
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Excessive Reinforcement
Causes: Deposition of too much weld metal,
often associated with in adequate
weld preparation
Incorrect welding parameters
Too large of an electrode for the joint
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in question
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Overlap
Causes: Poor manipulative technique
Too cold a welding conditions
(current and voltage too low)
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Undercut
Results from the washing away ofedge preparation when molten
Causes:
Poor welding technique
Imbalance in welding conditions
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Excessive Penetration
Causes:
Incorrect edge preparation
providing insufficient support at
the weld root
Incorrect welding conditions
(too high of current)
The rovision of a backin bar
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can alleviate this problem in
difficult circumstances
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Root Concavity
Causes:
Shrinkage of molten pool at
weld root, due to incorrect root
preparation or too cold of
conditions
May also be caused by
incorrect welding technique
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5 Miscellaneous Faults
Arc StrikesCause:
Accidental contact of an
electrode or welding torch
with a plate surface remote
from the weld
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Usually result in small hard
spots just beneath the
surface which may contain
cracks, and are thus to be
avoided
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Spatter
Causes: Incorrect welding conditions
and/or contaminated
consumables or preparations,
giving rise to explosions within
the arc and weld pool
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Globules of molten metal arethrown out, and adhere to the
parent metal remote from the
weld
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