17. Weldability

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Copyright 2004, TWI Ltd World Centre for Materials Joining Technology

Types of fracture

Appear when yielding and deformationprecedes failure

Ductile fracture


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it is the result of overloading

evidence of gross yielding or plasticdeformation

the fracture surface is rough and torn

the surface shows 45 shear lips or havesurfaces inclined at 45 to the loaddirection (because maximum shearplane is at 45 to the load!)

Types of fracture

Ductile fracture distinguish features:


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Brittle fracture is a fast, unstable type offracture.

Types of fracture

Brittle fracture


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Types of fracture

Factors affecting brittle fracture: Temperature (transition curve, convergence of YS

and UTS as the temperature is reduced)

Crystalline structure (b.c.c. vs. f.c.c.)

Material toughness

Residual stress

Strain rate (YS increase but UTS remain constant)

Material thickness (restrain due to surroundingmaterial)

Stress concentrations/weld defects


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Types of fracture

Causes for brittle fracture:

Presence of weld defects (poor quality)

Poor toughness in parent material (wrongchoice)

Poor toughness in HAZ (to high heat input)

High level of residual stress (no PWHT,wrong design)



Types of fracture

If a material is subjected to a static load,final rupture is preceded by very largestrains.

If the same material is subjected torepeated loads, failure may occur:

At stress well below elastic limit

With little or no plastic deformation



Fatigue fracture occurs in structures subject torepeated application of tensile stress. Crack growth isslow (in same cases, crack may grow into an area oflow stress and stop without failure).

Types of fracture

Fatigue fracture



crack growth is slow

it initiate from stress concentration points

load is considerably below the design or yield stress level

the surface is smooth the surface is bounded by a curve

bands may sometimes be seen on the smooth surface -beachmarks. They show the progress of the crack frontfrom the point of origin

the surface is 90 to the load final fracture will usually take the form of gross yielding

(as the maximum stress in the remaining ligamentincrease!)

fatigue crack need initiation + propagation periods

Types of fracture

Fatigue fracture distinguish features:



Avoiding fatigue fracture

Use smooth shapes and transitions

if possible, position welds in low stress areas

Check weld joint classification Check effect of possible weld defects;define weld quality

Use improvement techniquesProvide for inspection in service for fatiguecracks


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Types of fracture

Creep is a time-temperature dependantphenomenon

Section under stress continue to deformeven if the load is maintained constant

Creep is most likely when operating nearthe recrystallization temperature of that

material Usually appear in case of process plant

equipment, due to heating and coolingcycles

Creep fracture distinguish features:



Welding Defects

Classified by Shape

Longitudinal

Transverse

Branched

Chevron

CracksClassified by PositionHAZCenter line

CraterFusion zoneParent metal



Welding Defects

4 Crack Types

Solidification cracks

Lamellar tearing

Hydrogen induced cracks

Reheat cracks

Cracks


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Welding Defects

Solidification Cracking



Welding Defects

Deeper and narrow weldbeads are prone tosolidification cracking(depth to width ratio

over 2:1)In order to avoid

solidification cracking,reduce penetration andincrease bead width(depth to width ratio0,5:1)



Welding Defects

Lamellar Tearing

Step like appearance

Occurs in parent material or HAZ Only in rolled direction of the parent material

Associated with restrained joints subjected tothrough thickness stresses on corners, tees and

fillets Requires high sulphur or non-metallic inclusions

Cracks


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Welding Defects

Lamellar Tearing


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Welding Defects

Re-heat cracking

Occurs mainly in HAZ of low alloy steels duringpost weld heat treatment or service at elevated

temperatures Occurs in areas of high stress and existing defects

Prevented by toe grinding, elimination of poorprofile material selection and controlled post weld

heat treatment

Cracks


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Welding Defects

Hydrogen Induced

Requires susceptible grain structure, stress andhydrogen

Hydrogen enters via welding arc

Hydrogen source - atmosphere or contamination ofpreparation or electrode

Moisture diffuses out into parent metal on cooling

Most likely in HAZ

Cracks


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Cold (Hydrogen) Cracking

Also known as Toe, Underbead, HICC, delayed andchevron cracking.

Occurs in carbon; C/manganese; Low, medium and

high alloy steels:-FERRITIC/MARTENSITICsteels.

Very rarely in austenitic or duplex stainless steels,

never in Ni or Cu alloys. i.e.Body centered cubic metals NOT face centered cubicmetals


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Typical sites for cold cracking


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Toe cracking in MMA fillet weld


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Occurs :-

At temperatures below 300oc

May be up to 72hrs after completion

In weld metal, HAZ, parent metal.

At weld toes, under weld beads, at stressraisers.


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Presence of hydrogen

From moisture in the consumables damp

electrodes, damp flux, water in shield gas.

Condensation on parent metal

Dirt/grease on consumables or weld preps


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Reduce Hydrogen Level

Select lower hydrogen potential process e.g.

BASIC vs. RUTILEMAG vs. MMA

Increase hydrogen diffusion with increased preheat

Maintain preheat after welding allowing diffusion from weld

Bake basic MMA electrodes/SAW fluxes - manufacturersrecommendations!

Cleanliness/dryness of consumables and weld preparations e.g.rust scale grease cutting fluids

Use austenitic or nickel fillers


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Susceptible microstructure

Hard brittle structure MARTENSITIC Promoted by:

A) High Carbon Content, Carbon Equiv (CE)

Heat input = Amps x Volts x arc time

Run out length x 10-3 (1000) Kj/mm

C + Mn + Cr+Mo+V + Ni+Cu

6 5 15B) high alloy content

C) fast cooling rate:-

Cold Material, Thick Material and Low Heat Input.


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COMBINED THICKNESS - used to calculatecombined chilling effect of joint type andthickness.

Combined Thickness t1+t2 + t375mm


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Preheat Temperature Control

TEMPILSTIKS - crayons, melt at set temps. Will notmeasure max temp.

Pyrometers - contact or remote, measure actualtemp.

Thermocouples - contact or attached, very accurate,measure actual temp.


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Application Of Preheat

Heat 75mm either side of joint

Heat opposite face

Measure temp 2mins after heat removal

Always best to heat complete component rather thanlocal if possible

If procedure requires preheat

So do tack welds and attachments

Even if procedure does not require preheat tack weldsand attachments may

Preheat always higher for fillet than butt


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Preheat application

Furnace - heats entire component - best

Electrical preheat elements -controllable; Portable; Siteuse; Clean; Component cannot be moved.

Gas burners - direct flame impingement; Possible localoverheating; Less controllable;Portable; Manual operationpossible; Component can be moved.

Radiant gas heaters - capable of automatic control; Noflame impingement; No contact with component; Portable.

Induction heating - controllable; Rapid heating (mins nothours); Large power supply; Expensive equipment


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Manual gas preheating



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Electrical preheat of large steel structure


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Prevention

Slow the cooling rate

Reduce hydrogen level

Reduce residual stress


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WELD DECAY

The risk of weld decay has reduced significantly in recent

years through the development of low carbon grades (e.g.304L, 316L) which contain 0.03%C, and 'stabilised' grades(e.g. 321, 347) containing either Ti or Nb, to form carbidespreferentially to Cr.

In both types of steel, the amount of free carbon in solutionis sufficiently low to ensure that Cr carbide formation isminimal and therefore that sensitisation is not usually ofpractical significance during welding.

Carbon levels in the standard austenitic grades have alsobeen reduced in recent years, usually to levels of 0.05%,reducing the tendency for sensitisation so that this is verymuch less of a practical problem than was the case in thepast.

C


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WELD DECAY

Sensitisation range where peak temperatures inthe HAZ reaches about 6000C to 8500C

WELD DECAY


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WELD DECAY

When heated in the range 6000C to 8500Ccarbides form at the grain boundaries

Chromium migrates tosite of growing carbide

WELD DECAY


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WELD DECAY

Grain boundaries become depleted of chromiumand lose their corrosion resistance

WELD DECAY


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WELD DECAY

WELD DECAY


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WELD DECAY

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17. Weldability