welding
DESCRIPTION
Pemilihan Bahan & ProsesTRANSCRIPT
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The Metallurgy of welding :
Welding Design and Process selection
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Topics to Discuss
Introduction
Welded joint
Weldability
Testing welding joint
Weld design and process selection
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Introduction
welded joint Kekuatan, keuletan, ketangguhan
Struktur mikro dan ukuran butir tergantung pada temperatur
Kualitas las tergantung pada geometri, adanya retak, tegangan sisa, inklusi dan lapisan oksida…
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Welded Joint
3 daerah yang berbeda Base metal Heat affected zone (HAZ) Weld metal (logam las)
Metalurgi dan sifat pada HAZ dan logam las sangat bergantung pada jenis logam yang disambung, proses pengelasan, filler metal yang digunakan dan variable proses
Daerah logam las merupakan resolidified base
metal
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Fusion Weld Zone
Fig : Characteristics of a typical fusion weld zone in oxyfuel gas and arc welding.
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Grain Structure
Fig : Grain structure in (a) a deep weld (b) a shallow weld. Note that the grains in the solidified weld metal are perpendicular to the surface of the base metal. In a good weld, the solidification line at the center in the deep weld shown in (a) has grain migration, which develops uniform strength in the weld bead.
(a) (b)
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Solidification of Weld metal
Proses pembekuan dimulai dengan pembentukan butir kolumnar yang mirip dengan proses pengecoran
Butir relatif panjang dan membentuk paralel terhadap heat flow
Struktur, ukuran butir dan sifat daerah las tergantung pada jenis paduan, komposisi dan siklus termal
Logam las memiliki struktur pengecoran karena proses pendinginan yang lambat
Pre-heating sangat penting untuk logam logam yang memiliki konduktivitas termal tinggi
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Weld Beads
Fig : (a) Weld bead (on a cold-rolled nickel strip) produced by a laser beam.
(b) Microhardness profile across the weld bead. Note the lower hardness of the weld bead compared to the base metal.
(a) (b)
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Heat affected Zone
Heat effected zone is within the metal itself
Sifatnya tergantung pada rate of heat input dan cooling dan temperatur yang dialami pada HAZ, ukuran butir asli, arah butir, degree of prior cold work
Kekuatan dan kekerasan tergantung pada kekuatan dan kekerasan original logam induk
Panas yang diaplikasikan selama welding akan menghasilkan proses rekristalisasi (apabila base metal telah mengalami pengerjaan dingin, maka elongated grains akan berubah)
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Corrosion
Fig : Intergrannular corrosion of a 310-stainless-steel welded tube after exposure to a caustic solution. The weld line is at the center of the photograph. Scanning electron micrographs at 20X.
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Weld Quality
Welding discontinuities can be caused by inadequate or careless application
The major discontinuities that affect weld quality are
Porosity
Slag Inclusions
Incomplete fusion and penetration
Weld profile
Cracks
Lamellar tears
Surface damage
Residual stresses
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Disebabkan oleh gas yang dilepaskan selama proses pencairan di daerah las tetapi terperangkap pada saat proses solidifikasi, reaksi kimia dan komtaminan
Bentuknya bulat atau terelongasi
Porositas dapat dikurangi dengan cara:
Proper selection of electrodes
Improved welding techniques
Proper cleaning and prevention of contaminants
Reduced welding speeds
Porosity
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Slag Inclusions
• Yaitu : senyawa seperti oksida, fluxes, and electrode-coating materials yang terperangkan pada daerah las
• Pencegahan dapat dilakukan dengan cara: • Membersihkan permukaan yang akan dilas untuk setiap layer
yang akan dilakukan
• Menggunakan gas pelindung
• Redesigning the joint
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Incomplete Fusion and Penetration
Produces lack of weld beads
Practices for better weld :
Raising the temperature of the base metal
Cleaning the weld area, prior to the welding
Changing the joint design and type of electrode
Providing enough shielding gas
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Penetration:
Incomplete penetration occurs when the depth of the welded joint is insufficient
Penetration can be improved by the following practices :
Increasing the heat Input
Reducing the travel speed during the welding
Changing the joint design
Ensuring the surfaces to be joined fit properly
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Weld Profile:
Under filling results when the joint is not filed with the proper amount of weld metal.
Undercutting results from the melting away of the base metal and consequent generation of a groove in the shape of a sharp recess or notch.
Overlap is a surface discontinuity usually caused by poor welding practice and by the selection of improper material.
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Discontinuities in Fusion Welds
Fig : Schematic illustration of various discontinuities in fusion welds.
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Cracks
Cracks occur in various directions and various locations
Factors causing cracks:
Temperature gradients that cause thermal stresses in the weld zone
Variations in the composition of the weld zone.
Embrittlement of grain boundaries
Inability if the weld metal to contract during cooling
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Cracks
Fig : Types of cracks (in welded joints) caused by thermal stresses that develop during solidification 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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Cracks
Cracks are classified as Hot or Cold.
Hot cracks – Occur at elevated temperatures
Cold cracks – Occur after solidification
Basic crack prevention measures :
Change the joint design ,to minimize stresses from the shrinkage during cooling
Change the parameters, procedures, the sequence of welding process
Preheat the components to be welded
Avoid rapid cooling of the welded components
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Cracks in Weld Beads
Fig : Crack in a weld bead, due to the fact that the two components were not allowed to contract after the weld was completed.
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Lamellar tears :
Occurred due to the shrinkage of the restrained components in the structure during cooling.
Can be avoided by providing for shrinkage of the members
Changing the joint design
Surface Damage : These discontinuities may adversely affect the properties of welded structure, particularly for notch sensitive metals.
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Residual Stresses:
Caused because of localized heating and cooling during welding, expansion and contraction of the weld area causes residual stresses in the work piece.
Distortion,Warping and buckling of welded parts
Stress corrosion cracking
Further distortion if a portion of the welded structure is subsequently removed
Reduced fatigue life
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Distortion after Welding
Fig : Distortion of parts after welding : (a) butt joints; (b) fillet welds. Distortion is caused by differential thermal expansion and contraction of different parts of the welded assembly.
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Residual Stresses developed during welding
Fig : Residual stresses developed during welding of a butt joint.
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Stress relieving of welds :
Preheating reduces reduces problems caused by preheating the base metal or the parts to be welded
Heating can be done electrically,in furnace,for thin surfaces radiant lamp or hot air blast
Some other methods of stress relieving : Peening, hammering or surface rolling
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Weldability:
Capacity to be welded into a specific structure that has certain properties and characteristics and will satisfactorily meet service requirements
Thorough knowledge of the phase diagram is essential
Factors such as strength, toughness, ductility, notch sensitivity, elastic modulus, specific heat, melting point, thermal expansion, surface tension characteristics of the molten metal, corrosion resistance.
Testing Welded Joints
• Quality of the welding joint is established by welded joint
• Each technique has capabilities ,limitations and sensitivity reliability
and requirement for special equipment and operator skill.
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Destructive Techniques
Tension Test : Longitudinal and transverse tension tests are performed
Stress strain curves are obtained
Tension-Shear Test Specifically prepared to simulate actual welded joints and procedures.
Specimen subjected to tension and shear strength of the weld metal
Bend test : Determines ductility and strength of welded joints.
The welded specimen is bend around a fixture
The specimens are tested in three-point transverse bending
These tests help to determine the relative ductility and strength of the welded joints
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Destructive Techniques
Fig : Two types of specimens for tension-shear testing of welded joints.
Fig : (a) Wrap-around bend test method. (b) Three-point bending of welded specimens.
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Other destructive testing
Fracture Toughness Test:
Corrosion and creep tests
Testing of spot welds
Tension-Hear
Cross-tension
Twist
Peel
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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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Testing of Spot Welds
Fig : (a) Tension-shear test for spot welds. (b) Cross-Tension test. (c) Twist test. (d) Peel test
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Weld design & Process Selection
Considerations:
Configuration of the components or structure to be welded, and their thickness and size
Methods used to manufacture the components
Service requirements, Type of loading and stresses generated
Location, accessibility and ease of welding
Effects of distortion and discoloration
Appearance
Costs involved
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Welding Design Guidelines
Fig : Design guidelines for welding
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General Design Guidelines
Fig : Standard identification and symbols for welds
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Weld Design Selection
Fig : Weld Design Selection
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