Transcript
Page 1: Welding- properties of fusion welding

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Properties of fusion welds

VIJAYAN M G

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Welding v casting

Weld must adhere to mould wall Heat is added continually to welds

High temperature gradient

Welds solidify much faster than castings In welds, the surface shape is constant There is strong mixing of weld pools

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Fusion weld structure

HAZWeld metal

HAZBasemetal

Fusion line

Weld preparation

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Thermal gradients in HAZ

Time

Temperature

Fusion lineFusion line + 2mmFusion line + 5 mm

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HAZ structure

High peak temperature High temperature gradient

Variable cooling rate

Superimposed HAZs in multipass welds Welding stresses affect transformation

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Multipass fusion weld

Last weld run

Previous weld run

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Weld properties

Weld metal has different composition & thermal history to base metal

Welding heat modifies adjacent base metal (HAZ)

Variation in strength, ductility & corrosion resistance across welds

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Welding procedure

Selection of process conditions & consumables for a weld

Designed to match base material properties as closely as possible & avoid weld defects

Needs to consider service conditions of the weld

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Definition of weldability

The capacity of a material to be welded under the imposed fabrication conditions into a specific, suitably designed structure & to perform satisfactorily in intended service.

• (ANSI / AWS A3.0)

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Factors affecting weldability

Weldability is often considered to be a material property, however the effect of other variables should not be ignored.

Design of weldment Its service conditions Choice of welding process

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Material properties

Melting & vaporisation temperatures Electrical & thermal properties

Conductivity, expansion coefficient, thermal capacity, latent heat

Affinity of weld metal for O, N & H Susceptibility to porosity, embrittlement or wetting

problems

Presence of a surface film on base metal Oxide, paint or metallic surface coating

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Service environment

Extreme environments Corrosive Low temperature (brittle failure) High temperature (oxidation, creep, embrittlement) Others (wear, fatigue, nuclear)

The more extreme the environment The more difficult it is to find suitable materials The more restricted the welding procedure becomes to

avoid service failure

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Welding Defects

Cracks In weld metal or HAZ

Porosity Gas bubbles entrapped in weld metal

Incomplete fusion (interrun or at preparation edge) Inclusions

Slag or other matter entrapped in weld

Defective profile Under-weld, over-weld, lack of penetration, overlap,

undercut

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Causes of weld defects

Poor design of weldment Lack of access to make weld

Mistakes by welder Lack of skill Poor placement of weld metal Inadequate cleaning

Poor welding procedure

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Weld defects

Wormholeporosity

Incompletefusion

Slag inclusion

Overlap

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Solidification cracking

Contraction strains cause rupture of the weld at the point where the last material solidifies.

Solidification range Weld pool size & shape

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Residual stress & distortion

Uneven heating leads to constrained thermal expansion

At completion of the weld thermal cycle the weldment either distorts or if restrained will contain residual stress

Residual stress fields are complex, but stresses as high as yield often occur

Stresses may need to be removed by a stress relief heat treatment

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XX XX

Residual stress in a butt weld

sx

sy

sx

0 TensionCompression

XX XX

y Tension

Compression

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Weld detailing

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Weld positions - plate

Flat(1G)

Horizontal(2G)

Vertical(3G)

Up or Down

Overhead(4G)

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Welding positions - pipe

Axis vertical2G

Axis horizontal5G

Axis inclined 45°6G

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Weld details

Joints - Arrangements of members being joined

Butt, tee, lap, corner, flare

Welds - Geometry of weld detail selected to make the joint

Butt, fillet, plug & slot

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Joint types

Butt Tee

Lap Corner

Edge

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Weld types

Butt weld Between mating members Best quality High weld preparation cost

Fillet weld Easy preparation Asymmetric loads, lower design loads

Plug & slot welds Modified fillet welds in lap joints, using

holes through one member

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Fillet welds

Simple & cheap to assemble & weld Stress concentrations at toes & root Notch at root (fatigue, toughness) Critical dimension is throat thickness Root gap affects throat thickness Radiography & ultrasonic testing is of

limited use Large fillets use a lot of weld metal &

therefore are uneconomic

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Fillet weld terms

Root

ToeWeld face

ToeThroatthickness

Apparent leg length

Gap

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Butt welds

Types: Double welded butt Permanent or temporary backing Single welded butt

Lower stress concentration Easier ultrasonic testing or radiography Expensive preparation

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Butt weld types

Single veecan be single

or double welded

Single bevel Double vee

Backed butt (permanent or temporary)

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Butt weld termsFusion face

Root face

Root gap

Included angle

Bevel angle

Root run Toe

Toe

Reinforcement

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Weld preparation dimensions

Compromise Weld cross sectional area

• Cost• Ease of welding (risk of defects)

Standard preparations AS1554, AWS D1.1, ANSI B31.3

Fabricator’s risk

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Single welded butt joints

Access to one side only Pipe & hollow section butt joints

Root run forms inside surface High fit-up tolerances required High skill required

Special welder qualification

Root may be contaminated Inside is difficult to inspect

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‘J’ & ‘U’ Preparations

Land

Root radius“U” preparation

Double “U” butt

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AS1101.2 drawing symbols

Tail

Arrow points to weld location

OTHER SIDE

ARROW SIDE

Weld type symbol

Reference line

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Typical AS1101.2 symbols

6mm6

CJP


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