3rd WIS5 Weldability n.ppt 1

Faisal YusofCopyright © 2003 TWI Ltd

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Welding InspectionWeldability

Course Reference WIS 5


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Plain Carbon Steels

Steels are classified into groups as follows

1. Low Carbon Steel 0.01 – 0.3% Carbon

2. Medium Carbon Steel 0.3 – 0.6% Carbon

3. High Carbon Steel 0.6 – 1.4% Carbon

Plain carbon steels contain only iron & carbon as main alloying elements, traces of Mn, Si, Al, S & P may also be present

Classification of SteelClassification of Steel


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IRON CARBON DIAGRAM


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TTT DIAGRAM


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Diagram showing the Relationship between Carbon Content, Mechanical Properties, Microstructure and Uses of Plain Carbon

Steels in the Normalised Condition


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G Y An Alloy steel is one that contains more than

Iron & Carbon as a main alloying elements

Alloy steels are divided into 2 groups

1. Low Alloy Steels < 7% extra alloying elements

2. High Alloy Steels > 7% extra alloying elements

Classification of SteelClassification of Steel


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(a) substitutional (b) interstitial

Solid solution


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Carbon: Major element in steels, influences strength,toughness and ductility

Manganese: Secondary only to carbon for strength toughness and ductility, secondary deoxidiser and also acts as a desulphuriser.

Silicon: Primary deoxidiser

Molybdenum: Effects hardenability, and has high creep strength at high temperatures. Steels containing

molybdenum are less susceptible to temper brittleness than other alloy steels.

Chromium: Widely used in stainless steels for corrosion resistance, increases hardness and strength but reduces ductility.

Nickel: Used in stainless steels, high resistance to corrosion from acids, increases strength and toughness

Steel Weld MetallurgySteel Weld Metallurgy


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Aluminium:Deoxidiser,grain refinement

Sulphur: Machineability

Tungsten: High temperature strength

Titanium: Elimination of carbide precipitation

Vanadium: Fine grain – Toughness

Copper: Corrosion resistance and strength

Steel Weld Metallurgy


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G Y Increased strength: C, Si, Cu, Mn, Mo (also Nb and V;

their exact effect depends on other factors also such as the rolling temperature and time, amount of carbon and nitrogen present, etc.)

Hardening capacity: C, Mn, Mo, Cr, Ni, Cu

Toughness: Ni, grain refinement (achieved via the presence of Nb, V, Al, Ti)

Elevated Temperature Properties: Cr, Mo, V

Atmospheric corrosion Resistance: Cu, Ni


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Steel Weld MetallurgySteel Weld Metallurgy

The type and number of elements present in the

material

The temperature reached during welding and or

PWHT.

The cooling rate after welding and or PWHT

The grain structure of steel will influence its weldability, mechanical properties and in-service performance. The grain structure present in a material is influenced by:


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Cooling RateCooling RateThe cooling rate of the weld zone depends on the following factors:

•Weld heat : Also call arc energy, is the amount of electrical

energy that is supplied to the welding arc

over a given weld length ( an inch or mm)

•Thickness of material

•Preheating


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Heat Affected ZoneHeat Affected Zone

The parent material undergoes microstructure changes due to the influence of the welding process. This area, which lies between the fusion boundary and the unaffected parent material, is called the heat affected zone (h.a.z.).


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Material composition

Cooling rate, fast cooling higher hardness

Arc energy, high arc energy wider HAZ

The HAZ can not be eliminated in a fusion weld

The extent of changes will be dependent upon the following :-


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Arc energyArc energy

Heat input = 1.6 kJ/mm

Amps = 200 Volts = 32

Travel speed = 240 mm/min

Arc energy= Amps x volts Travel speed mm/sec X 1000

Heat input = 200 X 32 X 60 240 X 1000


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G Y High heat input - slow cooling

Low toughness

Reduction in strength

Heat InputHeat Input

Low heat input - fast cooling

Increased hardness

Hydrogen entrapment

Lack of fusion


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Carbon EquivalentCarbon Equivalent

The CE of steel primarily relates to its hardenability.

Higher the CE, lower the weldability

Higher the CE, higher the susceptibility to brittleness

The CE of a given material depends on its alloying elements

The CE is calculated using the following formula

CE = C + Mn + Cr + Mo + V + Cu + Ni6 5 15

Hardenability:The relative ability of a ferrous alloy to form martensite when quenched from high temperatures.


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Weldability Weldability

Weldability can be defined as the ability of a material to

be welded by most of the common welding processes,

and retain the properties for which it has been designed.

A steel which can be welded without any real dangerous

consequences is said to possess Good Weldability.

A steel which can not be welded without any dangerous

consequences occurring is said to possess Poor

Weldability. Poor weldability normally generally results in

the occurrence of some sort of cracking problem


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Weldability is a function of many inter-related

factors but these may be summarised as:

Composition of parent material

Joint design and size

Process and technique

Access


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It is very difficult to asses weldability in absolute terms therefore it is normally assessed in relative terms


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Weldability Weldability There are many factors which affect weldabilty e.g. material type, welding parameters amps, volts travel speed, heat input.


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Other factors affecting weldabilty are welding position and welding techniques.


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Weldability Weldability Basically speaking weldabilty is the ease with which a material or materials can be welded to give an acceptable joint


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CracksCracks


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Process Cracks Process Cracks

Hydrogen induced cold cracking (HICC)

Solidification cracking (Hot Tearing)

Lamellar tearing

Weld Decay


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Cracks Cracks

When considering any type of crack mechanism, three elements must be present for it’s occurrence:

Stress: stress is always present in weldments,through local expansion and contraction.

Restraint: may be a local restriction, or through the plates being welded.

Susceptible microstructure: the structure is often made susceptible to cracking through welding, e.g high hardness


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Hydrogen Hydrogen CracksCracks


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Hydrogen Cracking Hydrogen Cracking

Hydrogen causes general embrittlment and in welds may lead directly to cracking,


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A combination of four factors is necessary to cause HAZ hydrogen cracking


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Hydrogen Cracking Characteristics Also known as hydrogen induced cold cracking ,

delay cracking , underbead cracking and chevron. Hydrogen is the major influence to this type of

cracking. Source of hydrogen may be from moisture or

hydrocarbon such as grease , paint on the parent material, damp welding fluxes or from condensation of parent material

Hydrogen is absorbed by the weld pool from the arc atmosphere.


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• During cooling, much of this hydrogen escapes from the solidified bead by the diffusion but some also diffuses into the HAZ of the parent metal.

• Type of cracking is intergranular along grain boundaries or transganular

• Requires susceptible grain structure, stress and hydrogen and low temperature is reached.

• Most likely in HAZ for Carbon Manganese steel and in weld metal for HSLA steel.


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Hydrogen induced weld metal cracking

Hydrogen induced HAZ cracking


Micro Alloyed Steel Carbon Manganese Steel


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Hydrogen Cracking

Factors responsible:

Hydrogen cracking occurs when the conditions outlined in 1 – 4 occur simultaneously :

1.Susceptible grain structure – hardness value > 350 V.P.N

That part of HAZ which experiences a high enough temperature for the parent steel to transform rapidly from ferrite to austenite and back again,produces microstructures which are usually harder and more susceptible to hydrogen embrittlement.

2.Hydrogen level - > 15 ml/100g

This is inevitably present, derived from moisture in the fluxes used

in welding and from other sources.


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Hydrogen Cracking

3.Temperature < 200oC for any steel and < 150oC for structural steel.

The greatest risk of cracking occurs when temperatures near ambient are reached and cracking may thus take place several hours after welding has been completed ( normally after 72 hours )

4.Stress > 50% yield strength of parent metal

These arise inevitably from thermal contractions during cooling and may be supplemented by other stresses developed as a result of rigidity in the parts to be joined.


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G Y Pre heat, removes moisture from the joint preparations, and slows down

the cooling rate

Ensure joint preparations are clean and free from contamination

The use of a low hydrogen welding process such as TIG or MIG/MAG

The use of Nickel and Austenitic filler metal

Ensure all welding is carried out under controlled environmental

conditions

Ensure good fit-up as to reduced stress

The use of a PWHT with maintaining the pre- heat temperature

Avoid poor weld profiles

Use low hydrogen electrodes and baked as per manufacturer instructions


Precautions for controlling hydrogen cracking


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Solidification Solidification CracksCracks


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Solidification Cracking Characteristics

Also known as hot cracking or center line cracking or crater cracking and liquation cracking

Solidification cracking is intergranular type of cracking that is along the grain boundaries of the weld metal.

It occurs during the terminal stages of solidification,when the stresses developed across the adjacent grains exceed the strength of the almost completely solidified weld metal.

Impurities such as sulphur and phosphorous and carbon pick - up from parent metal increase the risk of cracking

High joint restraint which produce high residual stress will

increase the susceptibility to this type of cracking.


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• Occurs during weld solidification process from liquidus to solidus and at the last area to solidified.

• Steels with high sulphur content (low ductility at elevated temperature ) whereby produce hot shortness to the weld metal

• FeS form films at the grain boundaries whereby reduce the strength of the weld metal.

• Addition of manganese will form MnS and forms globules instead of films( FeS)

• Occur longitudinally down center of weld• Welding process that most susceptible to this type

of cracking are SAW and MIG/MAG with spray transfer due to high dilution rate.

Solidification Cracking


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Intergranular liquid film along the grain

boundary

Solidification Cracking Solidification Cracking


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Weld Centerline



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Solidification Cracking Factors responsible :

Metallurgical factors

a) Freezing temperature range –higher freezing range more susceptible to solidification cracking due to presence of FeS

b) Primary solidification Phase – Less than 5% delta ferrite

c) Surface tension – concave more susceptible than convex weld shape

d) Grain structure of fusion zone – Coarse columnar grain more susceptible especially with high energy welding process.

Mechanical factors

a) Contraction stresses – Thicker material more susceptible.

b) Degree of restraint – poor fit - up


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Use low dilution welding process

The use of high manganese and low carbon content fillers

Maintain a low carbon content

Minimise the amount of stress / restraint acting on the joint

during welding

The use of high quality parent materials, low levelsof impurities

Use proper joint design, use Single J instead of single V

Clean joint preparations, free from oil, paints and any other

sulphur containing product.

Joint design selection depth to width ratios


Precautions for controlling solidification cracking


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Lamellar Lamellar TearingTearing


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Lamellar Tearing Characteristics Lamellar tearing has a step like appearance due to the solid inclusions linking up under the influences of welding stressesOccurs at beneath of HAZ or near HAZ It forms when the welding stresses act in the short transverse direction of the material (through

thickness direction)

Low ductile materials containing high levels of impurities are very susceptible


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G Y • Occur only in rolled direction of the parent material

• Associated with restrained joints subjected to through thickness stresses on corners and tees

• Presence of elongated stringers such of nonmetallic inclusion such as silicates and sulfides parallel to steels rolling plane will produce poor through thickness ductility of the plate.

• Tearing will triggered by this such non metallic inclusion near the weld or it just outside HAZ during weld contraction.

Lamellar Tearing


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Step like appearance

Cross section

Lamellar Tearing Lamellar Tearing


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Tee fillet weld Tee butt weld (double-bevel)

Corner butt weld(single-bevel)



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Critical area

Critical area

Critical area



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Lamellar TearingPrecautions for controlling lamellar tearing

The use of high quality parent materials, low levels of impurities

( Z type material )


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Friction Welded Caps

Short Tensile Specimen

Through Thickness Ductility

Sample of Parent Material

A test for a materials susceptibility to lamellar tearing

Short Tensile TestsShort Tensile Tests

The results are given as a STRA valueShort Transverse Reduction in Area


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( Z type material )

Change joint design


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Modifying a Tee joint to avoid lamellar tearing

Susceptible

Susceptible Improved

Non-susceptible

Non-susceptible

Gouge base metal and fill with weld metal before welding the joint

Susceptible Less susceptible

Prior buttering of the joint with a ductile layer of weld metal may avoid lamellar tearing


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Modifying a corner joint to avoid lamellar tearing

Susceptible Non-Susceptible

Prior welding both plates may be grooved to avoid lamellar tearing

An open corner joint may be selected to avoid lamellar tearing


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( Z type material )

Change joint design

Minimise the amount of stress / restraint acting on the joint during welding

The use of buttering runs with low strength weld metal

Hydrogen precautions e.g use low hydrogen electrodes

Shift welding process such as Electro slag welding

Use forging or casting joint.

Place soft filler wire between the joint e.g T joint to reduce stresses during expansion and contraction of weld metal.

Pre heating helps on removal of Hydrogen on the plate.


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Weld DecayWeld Decay


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Weld Decay Characteristics

Weld decay may occurs in unstabilized austenitic stainless steels with carbon content above 0.1%

Also known as knife line attack or crack

Chromium carbide precipitation takes place at the critical range of 450oC-850oC (sensitising temperature )

At this temperature range carbon is absorbed by the chromium, which causes a local reduction in chromium content by promoting chromium carbides.

Loss of chromium content results in lowering the materials resistance to corrosion attack allowing rusting to occur


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Precautions for Weld Decay Precautions for Weld Decay The use of a low carbon grade stainless steel e.g.

304L, 316L, 316ELC with carbon content < 0.03%

The use of a stabilized grade stainless steel e.g.

321, 347, 348 recommended for severe corrosive

conditions and high temperature operating conditions

Standard grades may require PWHT, this

involves heating the material to a temperature

over 1100oC and quench the material, this

restores the chromium content at the grain boundary,

a major disadvantage of this heat treatment is the

high amount of distortion


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Fatigue Fatigue CracksCracks


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Fatigue TestingFatigue Testing


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Fatigue Cracks Fatigue Cracks Fatigue cracks occur under cyclic stress

conditions

Fracture normally occurs at a change in section, notch and weld defects i.e stress concentration area

All welded materials are susceptible to fatigue cracking

Fatigue cracking starts at a specific point referred to as a initiation point

The fracture surface is smooth in appearance sometimes displaying beach markings

The final mode of failure may be brittle or ductile or a combination of both


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Initiation points / weld defects

Fatigue fracture surface smooth in appearance

Secondary mode of failure ductile fracture rough fibrous appearance

Fatigue Cracks Fatigue Cracks


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Fatigue Cracks Fatigue Cracks

A fatigue failure on a small bore pipe work


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Precautions against Fatigue Cracks Precautions against Fatigue Cracks

Toe grinding, profile grinding.

The elimination of poor profiles

The elimination of partial penetration welds and weld defects

Operating conditions under the materials endurance limits

The elimination of notch effects e.g. mechanical damage cap/root undercut

The selection of the correct material for the service conditions of the component


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Any Questions?Any Questions?


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G Y QU 1. Briefly discuss the four essential factors for hydrogen

cracking to occur

QuestionsQuestions

QU 2. State four precautions to reduce the chance of hydrogen cracking

QU 3. In which type of steel is weld decay is experienced and state how it can be prevented

QU 4. State the precautions to reduce the chances of solidification cracking

QU 5. State four the essential factors for lamellar tearing to occur

3rd WIS5 Weldability n.ppt 1

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