pengenalan welding dasar.pdf

8/10/2019 Pengenalan Welding Dasar.pdf

1/38

Introduction to Welding

PATRIA DEVELOPMENT CENTER

PATRiA Development CenterPATRiA Development Center -- PDCPDCPT. UNITED TRACTORS PANDU ENGINEERINGPT. UNITED TRACTORS PANDU ENGINEERING

20122012


2/38

TUJUAN :

Memahami proses pengelasan dan efekyang terjadi kepada material

Memahami perubahan mikro strukturpengelasan dan kekerasan yang terjadi pada

daerah HAZ


Memahami cacat las, penyebab serta carapencegahannya


3/38


4/38

Apakah yang bisa dijual dari

pengelasan..??

Pengelasn merupakan sambungan paten yang kuat dan tahanlama

Bisa digunakan untuk hampir seluruh material logam


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


5/38

Keterbatasan Pengelasan

Upah man power/ welder cenderung mahal

Kebanyakan proses pengelasan melibatkan

penggunaan energi tinggi, yang berbahaya


sulit untuk melakukan pembongkarannya

Sambungan las bisa terjadi cacat yang sulit di deteksi,

dan hal ini bisa merusak struktur sambungan


6/38


7/38


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


8/38


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


9/38


Submerged Arc Welding

Arc welding process that uses a continuous, consumable bare

wire electrode, arc shielding is provided by a cover of granular

flux


10/38


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


11/38


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


12/38

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


13/38

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


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


14/38

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


15/38

Welding Deffect :

1. CracksDetection

Surface: Visual examination, magnetic particle, dye or

fluorescent penetrant inspection

Internal: Ultrasonic flaw detection, radiography



16/38

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



17/38

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


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


18/38

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


Prior buttering of surface beneficial

for susceptible plate


19/38

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


20/38

2. CavitiesDetection

Surface: Visual inspectionInternal: Ultrasonic flaw detection, radiography



21/38

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


22/38

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


atmosphere

If the weld wire used contains

insufficient deoxidant it is also

possible for carbon monoxide to

cause porosity


23/38

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


this instability


24/38

Surface Porosity

Causes: Excessive contamination from

grease, dampness, or atmosphere

entrainment

Occasionally caused by excessive

sulphur in consumables or parent


metal


25/38

Crater Pipes

Resulting from shrinkage at the endcrater of a weld run

Causes:

Incorrect manipulative technique or

current decay to allow for crater

shrinkage



26/38

3 Solid Inclusions

Detection

- normally revealed by radiography

Linear Slag Inclusions

Cause:

Incomplete removal of slag

in multi- ass welds often


associated with the

presence of undercut or

irregular surfaces in

underlying passes


27/38

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


multi-pass welds


28/38

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


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


29/38


ac o s e-wa us on ac o roo us on ac o n er-run us on

Lack of penetration


30/38

5 Imperfect Shape

Detection

- all shape defects can be determined by visual inspections

Linear Misalignment (HI-LO)

Cause:

Incorrect assembly or


distortion during fabrication


31/38

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


in question


32/38

Overlap

Causes: Poor manipulative technique

Too cold a welding conditions

(current and voltage too low)



33/38

Undercut

Results from the washing away ofedge preparation when molten

Causes:

Poor welding technique

Imbalance in welding conditions



34/38

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


can alleviate this problem in

difficult circumstances


35/38

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



36/38

5 Miscellaneous Faults

Arc StrikesCause:

Accidental contact of an

electrode or welding torch

with a plate surface remote

from the weld


Usually result in small hard

spots just beneath the

surface which may contain

cracks, and are thus to be

avoided


37/38

Spatter

Causes: Incorrect welding conditions

and/or contaminated

consumables or preparations,

giving rise to explosions within

the arc and weld pool


Globules of molten metal arethrown out, and adhere to the

parent metal remote from the

weld


38/38

pengenalan welding dasar.pdf

Documents