1MetalsMetals StructureStructureand Weldabilityand Weldability

Indian Institute of Welding ANB

Refresher Course Module 01

Contents

Structure of Metals

Structure of Steels

Weldability of Steels

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A Metallurgical Process

Welding is the joining of two or morepieces of metal by applying Heat or Pressure or both . to form a

Localized union through Fusion Re-crystallization across the interface

Why should Welding Technologistslearn metallurgy

Welding is mostly done for fabrication of metalsand alloys

The final properties of the welded assembly willdepend on the metallurgical structure of the parentmetal and the weld.

All welding processes involve heating and coolingof the components being welded

Thus to ensure a satisfactory welded component,it is necessary to understand metallurgicalstructures and how they and the weld thermalcycle, determine the properties of the weld joint.

3Welding a major fabrication process General Engineering

Construction - Earthmoving equipment, cranes

Infrastructure - Buildings , bridges , roads, flyovers, tunnels

Projects -, refineries, fertilizers, steel plants, chemical &petrochemical plants

Automotive sector - 2- wheelers, cars, trucks, buses

Railways - Coaches, locomotives, wagons

Shipbuilding and aircraft

Power plants & pressure vessels

Consumer durable - Refrigerators, ACs, Almirahs

Defence - Tanks, APCs, Aircraft, Rockets

Food processing - Dairy, brewery, cooking, freezing eqpt.

Materials of Construction

Mild steels, High strength low alloy steelsAll general engineering, Infrastructure, Automotive, Shipbuilding,Railways

High tensile steelsDefence, penstocks for hydel plants

Creep resisting steels.Boilers and piping in thermal power plants

Stainless steels - AusteniticChemical & petrochemical plant, refineries, cryogenic plant, foodprocessing, pharmaceuticals

AluminiumLight structurals, boats, dairy equipment, busbars

Copper, Nickel and alloys, TitaniumVessel, piping & heat exchangers in chemicals & food

4Structure of Metals

All metals and alloys are crystalline bodies with their atomsarranged in regular order, which is periodically repeated inthree directions

They distinguish them from amorphous bodies whose atomsare in random order

Metals obtained by conventional methods are polycrystallinebodies, consisting of great number of fine crystals differentlyoriented with respect to one another

All typical properties of metals can be explained by the factthat they contain highly mobile electrons.

Structure of metals

5Common Properties of Metals

Out of more than 106 elements known, 76 are metals All metals do exhibit some typical properties,

common to them: high thermal and electrical conductivity - due to presence of

free electron

positive temperature co-efficient of electrical resistivity thermo-ionic emission good reflectivity of light lend themselves to plastic deformation - due to ordered

arrangement of atom

Pure Metals & Alloys

In their ordinary structural state pure metals are oflow strength and do not possess requiredphysicochemical and structural properties forrequired service, in most cases. Consequently theyare seldom used in engineering applications.

Overwhelming majority of metals are thus used asalloys.

Example : Steel, Cast iron, Copper alloys, Aluminium alloys etc

6Structure of metals

The basic structure of a metal or alloy is a crystal consistingof the metal atoms located in a specific 3-dimensionalarrangement or lattice

For iron you have 2 crystal structures - polymorphism

Alpha iron upto 912 deg C Gamma iron 912 1394 deg CDelta iron 1394 1539 deg C

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HCP- Hexagonal Close packed

7Single Crystal

Unit Cell

Polycrystal

Grainboundary

8Crystal boundary orGrain boundary

In these regions there exists a film of metals, somethree atoms thick, in which atoms do not conform toany pattern

This crystal boundary is of amorphous nature Metallic bond acts within and across the crystal

boundary and therefore not necessarily an area ofweakness

Impurity atoms has got tendency to segregate atgrain boundary or crystal boundary.

Depending on the nature of impurity atom they maystrengthen or weaken the boundary

Grain Boundary

9Defects in Metals - Dislocations

Any real crystal always has defects in itsstructure and deviates from perfect periodicity

These defects are called Lattice defects / Latticeimperfections / Dislocations

Metals and alloys get deformed whendislocations are forced to move by theapplication of force

Any solute atom, phase or inter-metallic thatresists the flow of dislocations are thestrengthening agents in any alloy system

Structure of metals

Phases are distinct states of aggregation ofmatter Gases : Always single phase Liquids : Pure liquid or solution single phase,

immiscible liquids eg. Oil & water two phases Solids : Different crystal structures ( even having

the same composition ) form different phases.Can be single or multi-phase.

A phase is a homogeneous and physicallydistinct portion of the material

Microstructure, as seen under a microscopereveals the phases that exist in the materialtogether

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Structure of metals..

Grains During solidification from the liquid

phase or re-crystallization from onesolid phase to another, crystalsnucleate at different points withinthe parent phase and grow untilthey impinge on one another andform individual grains.

Structure Structure of a metal / alloy implies

the metallurgical phases present,their dispersion, shape, orientationand grain size. All of these go todetermine its physical andmechanical properties

Structure of Steels

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Carbon & Alloy steels

Steels are alloys of iron with a max.carbon content of 2%

Plain carbon steels contain less than 1.65Mn, 0.6 Si and 0.6 Cu

Alloys steels contain Mn, Si, Cu in greaterquantities or other alloying elements Alloying additions enhance their mechanical

properties Typical alloying elements are Ni, Cr, Mo, V

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Phases in steel

Ferrite: solid solution of carbon in -

iron; Maximum solubility of C:0.022% at 727C

Austenite: solid solution of carbon in -

iron; Maximum solubility of C:2.11% at 1146C

Delta() ferrite: solid solution of carbon in delta

iron; Maximum solubility of C:0.09% at 1495C Austenite orAustenite or ironiron

Ferrite orFerrite or ironiron

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Phases in steel

Graphite: crystalline form of carbon having a

hexagonal crystal structure. Onlyforms on very slow cooling

Pearlite: Lamellar structure consisting of

alternate bands of Ferrite andCementite

Cementite (Fe3C): an inter-metallic compound having a

complex orthorhombic structure; C -6.67% by wt. Even though this is ameta-stable phase, carbon is almostalways present in this form in steels.

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PearlitePearlite

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Fe-C phase diagram

The Fe-C (iron-carbon diagram)helps us to understand the phasesin steel

Important Concepts to understandare: This is an Equilibrium diagram Steels & Irons a clear distinction Phase fields & reactions Critical temperatures

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25

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Phase transformation reactionsin steel

Peritectic reaction(1495C) Liquid Fe + -Ferrite =

Austenite Eutectic reaction (1146C)

Liquid Fe = Austenite +Cementite (Eutecticmixture of austenite andcementite is calledLedeburite)

Eutectoid reaction (727C) Austenite = Ferrite +

Cementite (Eutecticmixture of Ferrite andCementite called Pearlite)

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Phase transformation reactionsin steel..

Micro-structures ofslowly cooled steels Eutectoid steel

( 0.77% C ) fullypearlitic

Hypo-eutectoid steel (0.77% C ) Pro-

eutectoid cementite+ Pearlite

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Effect of composition & cooling rate onmicrostructure

A large variety of microstructures canbe developed in ferritic steel bychanging composition & cooling rate

Austenite: fcc

Ferrite: bcc

Effect of cooling rate on Pearlite

Pearlitic structure is lamellarwith alternate bands of ferrite+ pearlite

Faster coolingV. Fine pearlite 35 40 Rc

Fast coolingFine pearlite 20 25 Rc( air cooled )

Slow coolingCoarse pearlite 5 10 Rc( furnace cooled )

Cooling rate

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Temperature Time TransformationT-T-T Diagrams

Bainite

Formed in alloyed steelswhen austenite is cooledrapidly passed the nose ofthe C-curve .

Extremely fine mixture offerrite + carbide but notlamellar like pearlite

Formed between 500 220C Upper Bainite or lowerBainite depending ontemp.

Has higher hardness andtoughness than pearlite

Bainite + accicular ferrite

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Martensite

Martensite :

Very hard and brittle phase. Formed on rapid cooling below

Ms temperature

Tempered Martensite : howeverhas a good combination ofstrength and toughness and isa useful structure and isdeveloped by re-heatingmartensite

Hardness depends on carboncontent of steel

Martensite

Carbon % 0.1 0.2

Hardness Rc 38 44

0.3 0.4 0.5 0.6 0.8

50 57 60 63 65

Martensite formation

For carbon steels very fast cooling

rates required to form Martensite

3 deg C / sec Fine pearlite 35 deg C / sec - Very fine pearlite +

martensite

140 deg C / sec martensite

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Effect of alloying additions

Alloying elements such as Ni, Cr, Mn, Si, Mo & V shift thenose of the C-C-T curve to the right. Exception Cobaltwhich shifts it to left

This is because they slow down growth of pearlite. Eg -0.5% Mo slows growth rate X 100

Martensite can thus be formed at much slower cooling rates In a Ni-Cr-Mo low alloy steel cooling rate of

8 deg C / sec Full martensite0.3 deg / sec Bainite + martensite0.02 deg / sec PearliteNote : Alloy elements do not affect the hardness of theMartensite they only affect the ease with which Martensiteforms

Structural features of ferritic steel

Ferrite +Pearlite

TS = f(P) ~ %C; grainsize & carbide spacingContinuous plate

Upper Bainite TS = f(plate width %carbide & its spacing)Broken platelets

Lower Bainite TS = f(plate width, %carbide & its spacing)Broken fine platelets

Martensite TS = f(%C)Brittle & unstableNo ppt.Needle (lens) shape

F P

M

B

B

Coolin

g ra

te / s

tren

gth

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Mild steel for structural purposes

Carbon 0.15 0.25 % ( covered by IS: 2062 ) Used in as-rolled and air-cooled condition in the

form of plates, channels & other structuralsections

Structure : Ferrite + 25% fine pearlite Properties : Y.S. 300 to 350 Mpa

UTS - 400 to 450 MpaEl - 26 30

Low carbon steels 0.1% C Structure : Mainly ferrite + small amount pearlite Properties : YS - 200300 mpa, UTS - 300370 mpa

elongation 2840% Very good ductility, used as cold rolled sheets in

automobile and white goods industry

High strength low alloy structural steels

Carbon in same range as mild steels 0.15 0.25% Low amounts of alloying elements Mo, Cr, Cu, Ni etc

added eg. weathering steels to IS: 11587 Structure accicular ferrite and bainite or ferrite and

tempered martensite Sronger and tougher than pearlitic steels with higher

strength Hardenability is increased which affects weldability

YS 400-700 MPaUTS 500-800 MPaElongation 18-25%

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Micro alloyed HSLA steels

Fine dispersion of alloy carbides results in strengthening byprecipitation hardening

Small amounts of carbide forming elements eg. Nb, V, Ti etc addedTotal amount 0.20% max as such called Micro-alloyed steels

Controlled rolling at low finish roll temperatures results in very finegrain size ASTM 12 14. Also improves strength.

Range of medium and high tensile steel developed to give improvedstrength and toughness without impairing weldability. Covered byIS:8500 - 1991

Gives comparitively lower elongation but better toughness than lowalloy HSLA steels

Properties : UTS 600 650 MPaYS 400 500 MPa

Elongation 20 22 %

Properties of typical Micro-alloyed steels

Grade / Tradename

% C % Mn % Si % MA YS

MPa

UTS

MPa

ASTM A633

Gr C

0.20 1.50 0.50 0.05 Nb 350 min 600 min

SAILMA 410 0.25 1.50 0.50 Nb+V+Ti=0.20

410 min 540 - 660

SAILMA 450 0.25 1.50 0.50 Nb+V+Ti=0.20

450 min 570 - 720

SAILMA 450HI 0.20 1.50 0.50 Nb+V+Ti=0.20

450 min 570 720

CVN = 19.6J

Min at 20C

TISTEN 60 0.20 1.80 0.50 0.20 440 min 590 min

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Weldability of Steels

Weldability

Weldability maybe defined as the capacity ofa metal to be welded under the fabricationconditions imposed, into a suitable designedstructure, and to perform satisfactorily in theintended service

Weldability is the ease with which a metalcan be welded to give the required service

Weldability is the amount and nature ofproblems you face to weld a material

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Weldability Problems

Cracking - In the weld - solidification cracks- micro-fissuring

- In the HAZ H2 induced cold cracks- liquation cracks- reheat cracks

Porosity Oxidation of reactive metals Reduced joint strength In the weld

- In the HAZ Reduced corrosion resistance

Examples of Weldability ProblemsOxidation of reactivemetals

Aluminium, Magnesium,Titanium

Inert gas shielding, activefluxes

Gas-metal reaction ordissolution

Porosity : N2 in steel , O2 in Cu &NI, H2 in Al & Ti

Use of de-oxidisers in fillermetal. Inert gas

Vaporisation of low B.P.metals

Porosity : Zinc in brasses Use of Sn-bronze filler andlow currents

Hot cracking in weld Due to low meltingconstituents , impurities eg.S, P, Pb

Use of 2-phase fillers egSS electrodes with 5%ferrite.

Hot cracking in HAZ Embrittlement, liquation Heattreatable alloys of aluminium

Use of lower M.P. alloys

Cold cracking in HAZ Hydrogen cracking of C-Mnand alloy steels

Use of pre-heat and low H2electrodes

Reduction in HAZstrength

Precipitation / Age hardenedalloys

Control heat inputSolution anneal and heat-treat after weld.

Reduction in corrosionresistance

HAZ of SS welds due toChrome carbide precipitation

Use of stabilised or ELCsteels

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Weldability is Process Related

Fusion Welding Processes High heat input of the welding arc / heat source

and influence of arc atmosphere Solidification of the molten filler metal and

fused portion of base metal into a separate weldzone

Parent metal on both sides of the weld affectedby the weld thermal cycle Heat affected zone( HAZ )

Metallurgical effects on both reheating andcooling

Weldability is Process Related

Solid / Plastic state welding processes- Diffusion welding, ultra-sonic welding, forge

welding, explosive welding, forge welding,friction welding, friction stir welding

Below melting point of metals No arc atmosphere / effect of gases No filler metal Bonding through diffusion / plastic state mixing

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Weldability is Process Related

Whereas the fusion welding processes have moreweldability problems, they are in general more versatile,economic and suitable over a wide range of shapes andsizes of fabricated products.

The solid state processes may have advantages in lessweldability constraints but limitations in practicalapplicability and economics.

Demands on materials of construction

Higher strength Improved toughness down to cryogenic

temperatures Resistance to corrosion by a wide variety of

chemicals and corrosive media. High temperature oxidation resistance Resistance to creep at high temperatures Higher strength : weight ratio Wear and erosion resistant Should be weldable

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Mechanisms used by metallurgists forimproving strength, toughness etc.

Can have adverse effect on weldability

Strength / hardness improved by : Solid solution hardening Dispersion of second phase Phase transformation eg martensitic transformation Precipitation hardening carbides / nitrides /

intermetallic compounds Ageing ( time dependent precipitation hardening ) Work hardening

Toughness improved by : Grain refinement / fine grain size Low impurity level Austenite phase promoted by Nickel, Manganese

etc.

Creep resistance improved by : Finely dispersed carbides of chromium, molybdenum,

vanadium etc, formed after tempering of martensitic/ bainitic steels. eg 1Cr-0.5Mo, 2Cr-1Mo steels uptoP92 steels

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Materials Grouping for Weldability

Materials have been gouped under ASME section IXand ISO/TR 15608 based on comparable base metalcharacteristics such asCompositionWeldabilityBrazeabilityMechanical Properties

The objective is to reduce the number of welding andbrazing procedure qualifications Under ASME these groups are assigned P-NumbersFerrous metals which have specified impact testrequirements have been assigned Group Numberswithin P-Numbers.

Slno

Material ASME Sec IXP nos

ISO/TR 15608Groups

1 Steels 1, 3 11 1 112 Aluminium and Al alloys 21 25 21 263 Copper and Cu alloys 31 35 31 384 Nickel and Ni alloys 41 47 41 485 Titanium and Ti alloys 51 53 51 546 Zirconium and Zr alloys 61 & 62 61 & 627 Cast Iron nil 71 76

ASME Sec IX and ISO/TR 15608material groups

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Weldability Problems in C - Mn steels

Hydrogen induced coldcracking ( HICC ) HAZ cracking Delayed cracking

Solidification cracking Hot cracking in the weld Centerline cracking

Lamellar tearing Occurs predominantly in plate

material Due to presence of non metallic

inclusions

Solidification cracking

Steels having unfavourable Mn-S

ratio are prone to such cracking.

Due to presence of S, Pand other impurityelements which form lowmelting films at grainboundariesReduced by higherManganese content

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Mechanism of HICC3 factors causing Hydrogen induced cold cracking A brittle martensitic micro-structure produced by rapid

cooling in HAZ area heated above A1 line Presence of Hydrogen from the welding process Presence of contractional and residual stressesMechanism Hydrogen absorbed by the weld pool diffuses to the fusion

zone and HAZ as the weld solidifies and cools Forms pockets of molecular hydrogen which exerts

additional stress on the susceptible microstructure In combination with existing stresses causes cracking

generally in HAZ but can also take place in multi-passwelds

Factors influencing HICC

Presence of Hydrogen Process

Presence of stress Weld design & procedure

Formation of hard microstructure

Chemical composition ( intrinsic to material )

Cooling rate - Combined thickness of joint- Heat input of process- Degree of preheat if any and inter-pass temp

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Hydrogen levels for differentprocesses and consumables

Scale A : Above 15 ml / 100 gm diffusible hydrogencontent in weld Rutile electrodes, LH electrodes whichhave been exposed to moisture

Scale B : 10 15 ml / 100 gm diffusible hydrogencontent - LH electrodes redried at 250 C

Scale C : 5 10 ml / 100 gm diffusible hydrogen content Gas Metal arc welding ( MIG ) process, LH electrodesredried at 350 C

Scale D : below 5 ml / 100 gm diffusible hydrogencontent Gas Tungsten Arc welding ( TIG ) process, LHelectrodes redried at 450 C

Carbon Equivalent

Chemical composition expressed in terms of carbonequivalent C.E. is the measure of the susceptibility of thematerial to form a hard microstructure ( martensite )

Thus Carbon Equivalent has become synonymous withWeldability of a steel

C.E. = %C + % Mn / 6 + % (Cr + Mo + V ) / 5 + % (NI + Cu) / 15

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Variation in cooling rate produces a variety ofmicro-structures and hence properties in steel

Combined thickness of joints

Butt welds & corner welds ofequal thickness - T1 + T2

Butt welds & corner welds ofunequal thickness

Av of T1 over 75 mm + T2

Fillet welds T1 + T2 + T3

Directly opposed simultaneousfillet welds T1 + T2 + T3 / 2

Two rods - D1 + D2 / 2

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Effect of process heat input ongrain size

Grain size significantly influences the propertiesof a steel.

Finer the grain size higher the strength andtoughness

The original or re-crystallized austenite grainsize determines the ferrite and pearlite grainsize.

Higher the process heat input and longer thetime above 1050 C in austenite range coarserthe grain size in the previous runs and HAZ

Heat input during welding

Is calculated from the Arc energy divided by thewelding speed

Arc voltage X Welding current----------------------------------------------- kJ / mm

Welding speed ( mm / sec ) X 1000

For other welding process divide by followingfactors

SAW ( single wire ) - 0.8GTAW - 1.2GMAW - 1.0

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Procedures to reduce weldabilityproblems in C Mn steels

Pre-heating To reduce cooling rates and producesofter micro-structures in the HAZ

Inter-passtemperaturecontrol

To control process heat input to theweld & HAZ to produce finer grainstructure for improved toughness

Post heating Eliminate H2 by diffusion from the weldby maintaining heating at around 300 Cwithout allowing the weld to cool down

Post WeldHeat-Treatment

Heating below the lower criticaltemperature to relieve internal stresses,reduce hardness & improve ductility

Practical requirements of Welding Engineer

Given a steel of known composition or C.E. Upto what combined thickness can be welded with

normal rutile electrodes, without danger of HAZcracking

Upto what thickness can be welded using LowHydrogen electrodes

Upto what thickness can be welded using LowHydrogen electrodes properly redried as permanufacturers recommendations

Above what thickness pre-heat is required anddegree of pre-heat.

Is it necessary to impose any restrictions on heatinput by the welding process and parameters used

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IS 9595 : 1996 recommendations for welding ofcarbon and carbon manganese steels

Annexure F gives detailed guidance on pre-heatrequirements and inter- pass temperatures foravoiding hydrogen induced cold crackingconsidering the following factors

- Carbon equivalent of steel- Combined thickness to be welded- Heat input of process in kJ / mm- Hydrogen level of process in Scales A to D

Simplified table for Fillet welds Detailed graphs for other conditionsCovers steels under IS : 2062 - 1992 and IS 8500

1991 of C.E. upto 0.53

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Acknowledgements

We gratefully acknowledge the contributions of

the following faculty members for developing

this module

Mr.Soumya Sarkar Mr.R.Banerjee Mr.A.A.Deshpande Dr.Shaju Albert

THANK YOU