19214680 Welding Properties of Fusion Welding

7/29/2019 19214680 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 weldsq 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 HAZBase

metal

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 gradientq 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 & corrosionresistance across welds


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Welding procedure

Selection of process conditions &consumables for a weld

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

Needs to consider service conditions of theweld


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Definition of weldability

The capacity of a to be weldedunder the imposed

into a specific, suitably designed &to satisfactorily in intended service.

(ANSI / AWS A3.0)


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Factors affecting weldability

Weldability is often considered to be amaterial 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

q Conductivity, expansion coefficient, thermalcapacity, latent heat

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

problems

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


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Service environment

Extreme environmentsq Corrosive

q Low temperature (brittle failure)

q High temperature (oxidation, creep, embrittlement)

q Others (wear, fatigue, nuclear)

The more extreme the environmentq The more difficult it is to find suitable materials

q The more restricted the welding procedure becomes toavoid service failure


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Welding Defects

Cracksq In weld metal or HAZ

Porosityq Gas bubbles entrapped in weld metal

Incomplete fusion (interrun or at preparation edge)

Inclusionsq Slag or other matter entrapped in weld

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

undercut


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Causes of weld defects

Poor design of weldmentq Lack of access to make weld

Mistakes by welderq Lack of skill

q Poor placement of weld metal

q Inadequate cleaning

Poor welding procedure


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Weld defects

Wormhole

porosity

Incompletefusion

Slaginclusion

Overlap


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Solidification cracking

Contraction strainscause rupture of theweld at the point where

the last materialsolidifies.q Solidification range

q Weld pool size & shape


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Residual stress & distortion

Uneven heating leads to constrained thermalexpansion

At completion of the weld thermal cycle theweldment either distorts or if restrained willcontain residual stress

Residual stress fields are complex, butstresses as high as yield often occur

Stresses may need to be removed by a stressrelief 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 beingjoinedq

Butt, tee, lap, corner, flareWelds - Geometry of weld detail selected tomake the jointq Butt, fillet, plug & slot


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Joint types

Butt Tee

Lap Corner

Edge


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Weld types

Butt weldq Between mating members

q Best quality

q High weld preparation cost

Fillet weldq Easy preparation

q Asymmetric loads, lower design loads

Plug & slot weldsq 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 oflimited use

Large fillets use a lot of weld metal &therefore are uneconomic


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Fillet weld terms

Root

Toe

Weld face

ToeThroatthickness

Apparent leg length

Gap


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Butt welds

Types:q Double welded butt

q

Permanent or temporary backingq 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 terms

Fusion face

Root face

Rootgap

Included angle

Bevel angle

Root run Toe

Toe

Reinforcement


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Weld preparation dimensions

Compromiseq Weld cross sectional area

Cost Ease of welding (risk of defects)

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

Fabricators risk


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Single welded butt joints

Access to one side onlyq Pipe & hollow section butt joints

Root run forms inside surface

High fit-up tolerances required

High skill requiredq Special welder qualification

Root may be contaminatedInside 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 weldlocation

OTHER SIDE

ARROW SIDE

Weld type symbol

Reference line


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Typical AS1101.2 symbols

6mm

6CJP

Documents

19214680 Welding Properties of Fusion Welding