Metallurgy ofMetallurgy of High Strength Steel High Strength Steel

N. YuriokaN. Yurioka Visiting Professor at Osaka UniversityVisiting Professor at Osaka University

Crystalline lattice structureCrystalline lattice structure

BCC

BCC

FCC HCP

BCC

Crystalline lattice structureCrystalline lattice structure

a.a. Face centered cubic (FCC)Face centered cubic (FCC)Steel (at high temp.), Austenitic stainless steel,Steel (at high temp.), Austenitic stainless steel,

Al, Cu,...Al, Cu,...

b.b. Body centered cubic (BCC)Body centered cubic (BCC) Steel (at low temp.), Ferritic stainless steel,Steel (at low temp.), Ferritic stainless steel, Ti (at high temp.)Ti (at high temp.)

c.c. Hexagonally closed packed (HCP)Hexagonally closed packed (HCP)Ti (at low temp.) Ti (at low temp.)

Fe-C Phase diagramFe-C Phase diagram

Steel is an alloy of Iron and carbon

Iron C < 0.02%

Steel 0.02 C 0.21%

Cast iron : 0.21% < C

Phase transformation in cooling - IPhase transformation in cooling - I

Pearlite (Composite of ferrite and cementite)Pearlite (Composite of ferrite and cementite) FeFe3CC

Phase transformation in cooling - IIPhase transformation in cooling - II

Line expansion (Dilatation) Line expansion (Dilatation)

Dilatometry-IDilatometry-I

Dilatometry-IIDilatometry-II

TransformationTransformation

In heatingIn heating AAc1: : to to start start AAc3: : to to finish finish

In coolingIn cooling AAr3: : to to start start AAr1: : to to finish finish

In rapid cooling In rapid cooling (quenching)(quenching) MMss: M start: M start MMff: M finish: M finish

Diffusion of carbon plays an important role inDiffusion of carbon plays an important role inphase transformationphase transformation

Microstructure of steels -IMicrostructure of steels -I

Martensite Lower bainite

Martensite and lower bainiteMartensite and lower bainite

Microstructure of steels -IIMicrostructure of steels -II

Upper bainite Ferrite and pearlite

Rollin

g

dire

ctio

n

Formation of upper bainite in cooling -IFormation of upper bainite in cooling -I

Nucleation of ferrite Growth of ferrite

Formation of upper bainite in cooling -IIFormation of upper bainite in cooling -II

HeatHeat treatment of steels treatment of steels

Normalizing treatment of ferrite-pearlite steelNormalizing treatment of ferrite-pearlite steel

Grain refining

Hot rolling processesHot rolling processes

Microstructure of hot rolled steelMicrostructure of hot rolled steel

As rolled Normalized

TMCP-II Quenched & tempered

Features of steelsFeatures of steels

• As rolled steel Ferrite –pearlite Low strength, Low YR

• Normalized steel Grain-refined ferrite-pearlite Higher strength and toughness

• TMCP-II (controlled rolling and accelerated cooling) steel Grain-refined ferrite + low temperature transformation product High strength and toughness, low CE (better weldability)

• Quenched and tempered steel Tempered martensite, highest strength, high YR, high CE (preheating)

Cautions for TMCP and QT steels:

Heat input limitation ( 4.5kJ/mm), No hot forming

Mild steels (JIS standard)Mild steels (JIS standard)• GeneralGeneral structure structure SSSS series series (SS(SS400400, SS, SS490490, etc, etc……))

• Welded Welded structurestructure SMSM series series• BuildingBuilding construction construction SNSN series series ( Tensile ( Tensile

strength )strength )

Steels forSteels for• Welded Welded structuresstructures SMSM series series

YR (Yield Ratio)YR (Yield Ratio)

Steels forSteels forBuildingBuilding construction construction

SNSN series series

High ratio decreasesHigh ratio decreases

the compliance of the compliance of

structures such asstructures such as

building .building .

TensileYield / ratio Yield

Lamellar tearLamellar tearReduction of area, RAReduction of area, RAZZ

in the thickness directionin the thickness direction

Reduction of P & S in steel

Increase of RAz

Steels forSteels for• BuildingBuilding construction construction SNSN series series

High strength steelHigh strength steel

• TS >= 490MPa SM490, SM520, SM570…..

• Reduction of weight of structures Bridge, Storage tank, Pressure vessel Submarine,……

• Increase of production efficiency (Reduction of welding passes) Pipeline,…….

Welding of QT steel, TMCP steel

Max allowable heat input 4.5kJ/mm to avoid HAZ softening, Low HAZ toughness

Steels for specific purposesSteels for specific purposes

Lamellar tear resistant steel Ex. Z25 grade (RA >= 25%)

Steel for very high heat input welding

Fire resistant steel

Hot-dip galvanizing crack resistant steel

Atmospheric corrosion resistant steel (Weathering steel, SMA series)

Low temperature service steelsLow temperature service steels

JIS SLA grade Al-killed steel (N or QT or TMCP) JIS SL grade 3.5%Ni (NT, TMCP)

5%Ni (NNT, TMCP) 9%Ni (QQT, QLT, DQT) Austenitic stainless steel SUS304, SUS316 Inver (34%Ni-Fe)

Welding of low temperature steels (QT, TMCP) Low heat input welding ( 35kJ/mm desired)

-160oC

High temperature service steelsHigh temperature service steels

JIS G3103 SB series (C, Mo) Boilers JIS G3119 SBV series (Mn-Mo, Mn-Mo-Ni) JIS G3120 SQV series (Mn-Mo, Mn-Mo-Ni) Nuclear pressure vessels JIS G4109 SCMV series (Cr-Mo) 1%Cr-9%Cr JIS 4110 SCMQ series (Cr-Mo-V-(W)) 9-12%Cr

Weldability of steelsWeldability of steels

Welding heat inputWelding heat input

Energy Input (AWS D1.1), Arc Energy(EN standard)

EI(J/mm) = 60 · (E·I/v) E(V), I(A), v(mm/min)

60·25·170/150 1700 (J/mm), 1.7(kJ/mm)

Heat Input

HI(J/mm) = EI : Arc thermal efficiency 1.0 for SAW 0.8 for SMAW, GMAW 0.6 for autogenus TIG

Welding cooling rate, cooling timeWelding cooling rate, cooling time

CR(CR(oC/s) at 540C/s) at 540ooCC

tt8/58/5(s):(s):

Cooling time betweenCooling time between

800800ooC and 500C and 500ooCC

1.7kJ/mm on 20mm thick1.7kJ/mm on 20mm thick

7s in t7s in t8/58/5

Cooling rate, Cooling timeCooling rate, Cooling time

Heat input

Plate thickness

Joint shape (Butt-joint, fillet-joint)

Preheat temperature

Prediction of cooling time, t8/5

JWES IT-Center (http://www-it.jwes.or.jp/index_e.jsp)

45mm

Microstructure of HAZMicrostructure of HAZ

Normalizing heat treatment

CCT (Continuous Cooling Transformation) diagram CCT (Continuous Cooling Transformation) diagram

Cooling curve (log-scale)Cooling curve (log-scale)

CCT (Low-hardenability)CCT (Low-hardenability)

CCT (high hardenability)CCT (high hardenability)

HAZ maximum hardnessHAZ maximum hardness

Hardness change against tHardness change against t8/58/5

Change in HAZ maximum hardnessChange in HAZ maximum hardness

Martensite Martensite

hardnesshardness

= f(C)= f(C)

HardenabilityHardenability

Carbon equivalentCarbon equivalent

CECEIIWIIW

CECEWESWES

Prediction of HAZ hardnessPrediction of HAZ hardness

HAZ hardness

• Welding conditions Heat input Plate thickness Preheat temperature

t8/5

• Chemical composition of steel C Carbon Equivalent

JWES IT-Center (http://www-it.jwes.or.jp/index_e.jsp)

Carbon equivalentCarbon equivalent

CEIIW = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5

CEWES = C + Si/24 + Mn/6 + Ni/40 + Cr/5 + Mo/4 + V/14

Weld crackingWeld cracking

Hot cracking (>1200oC)

Solidification cracking Liquation cracking

Cold cracking (<100oC) (Hydrogen assisted cracking)

Hot crackingHot cracking

Solidification crackSolidification crack Liquation crackLiquation crack

Stainless steel, Al

Weld metal crackingWeld metal cracking

Segregation of impurities during solidificationSegregation of impurities during solidification

Phase diagramPhase diagram Residual liquid phaseResidual liquid phase

Direction of solidification growthDirection of solidification growthH/WH/W

Welding velocityWelding velocity

Cold cracksCold cracks

Root crack (HAZ) Toe crack (HAZ) Transverse crack (Weld metal)

Under-bead crack(HAZ)

Generation and diffusion of hydrogenGeneration and diffusion of hydrogen

Generation of hydrogenGeneration of hydrogen Hydrogen diffusion in weldHydrogen diffusion in weld

Mineral water in flux, Moisture in fluxMoisture in atmosphere, Rust, oil, grease in groove

ArcH (hydrogen)

Effect of preheat on HAZ hydrogenEffect of preheat on HAZ hydrogen

Cause of hydrogen-assisted cold crackingCause of hydrogen-assisted cold cracking

Cold cracking

Diffusible hydrogen

Weld metal hydrogen Preheat temperature

Hardness (HAZ, Weld metal)

Steel chemical composition t8/5 HI, thickness

Tensile residual stress Yield strength of weld metal Notch concentration factor

Cold cracking Cold cracking • Hydrogen assisted cracking, Delayed crackingHydrogen assisted cracking, Delayed cracking

Determination of necessary preheat temperatureDetermination of necessary preheat temperature

AWS D1.1 Annex I Hardness control method (CEIIW) C>0.11% Hydrogen control method (Pcm) C<0.11%

BS5135 [EN 1011-2 A] (CEIIW)

CET method [EN 10110-2 B] (CET)

CEN method (CEN)

JWES IT -center (http://www-it.jwes.or.jp/index_e.jsp)

Pc method (Pcm)

Carbon equivalentsCarbon equivalents

Pc methodPc method

Necessary preheat temperature

Tph(oC) = 1440 Pc - 392

Cracking other than hot cracking and cold cracking Cracking other than hot cracking and cold cracking

Lamellar tearLamellar tear

Reheat crackReheat crack

Prevention of lamellar tearPrevention of lamellar tear

Use steel with higher RA in the thickness direction RAz > 15%, RAz > 25%

Avoid excessive amount of deposited weld metal

Employ buttering pass sequence

Prevent cold crack which may initiate lamellar tear

Prevention of lamellar tearPrevention of lamellar tear

Buttering pass

Reduction ofDeposited metal

Reheat crackReheat crack

Reheat cracks are initiated at the weld toe during stress relief annealing

Prevention of reheat crack

Reduce stress concentration at the weld toe by grinding, etc.

Use appropriate steel

with reduced amount of precipitation element such as Cr, Mo, V, Nb

Low heat input welding

Weld metal Coarse grained HAZ

Intergranular crack

HAZ toughnessHAZ toughness

& HT490

NormalizingHeat treatment

Toughness of coarse grained zoneToughness of coarse grained zone

Lower bainite Upper bainite

HAZ toughnessHAZ toughness

Refined grain at the coarse grained zone of HAZ

Smaller heat input (HI)welding Steel with dispersed fine particles (TiN, oxide)

Microstructure with high toughness

Increase of lower bainite Decrease of upper bainite and MA(island-like martensite)

Low HI High HI High C

Matrix with high toughness

Low N, High Ni

Impeding of austenite grain growthImpeding of austenite grain growthAustenite grain boundary migration is stopped by the pinning effect of particles.

Ti deoxidized steel

Island-like martensiteIsland-like martensite (MA, Martensite-Austenite constituent) (MA, Martensite-Austenite constituent)

MA of very hard phase Initiation site of brittle crackLow carbon steel Decrease of MA

Welding consumablesWelding consumables

Typical covered electrodesTypical covered electrodes

Hydrogen level

Type of covered flux

JIS designation

Main ingredient Weldingposition

Ilminite D__01 Ilmenite(Impure rutile)

All

Lime-Titania (Rutile)

D__03 Lime +Titanium oxide (Rutile)

All

Cellulosic D__11 Organic substance

All

High titanium oxide (Rutile)

D__13 Titanium oxide (Rutile)

All

Low hydrogen(Basic type)

D__16 Lime All

Iron powderLow hydrogen

D__26 Lime +Iron powder

FlatHorizontal

Gravity welding equipmentGravity welding equipment

D4326

Flux type of covered electrodeFlux type of covered electrode

Basic type CaCO3 CaO + CO2

lime

Decrease of partial pressure of H

High basicity Low oxygen in weld metal

Low hydrogen

Functions of the coating of covered Functions of the coating of covered electrode for SMAW.electrode for SMAW.

(a) It enables easy arc ignition.

(b) It stabilizes the arc.

(c) It generates neutral gas for shielding weld from the air.

(d) It forms slag which covers and protects the weld metal from air.

(e) It makes de-oxidation and refines weld metal.

(f) It improves the properties of weld by adding effective alloying elements

(g) It increases deposition rate by adding iron powder.

Non-low hydrogen electrode (HD > 30ml/100g) High hydrogen Only for mild steel Low basicity Higher oxygen content Lower toughness Rutile (Ti-oxide) Good workability Less generation of spatter and blowholes

Low hydrogen electrode (HD < 7ml/100g)

Low hydrogen For mild steel and high strength steel Basic type of flux Lower oxygen content Higher toughness Poorer workability More generation of spatter and blowholes

Moisture absorption of electrodeMoisture absorption of electrode

Baking condition for low hydrogen electrodes: 300-400oC x 30-60min

Drying condition for non-low hydrogen electores:70-100oC x 1hr

Specification of solid wire for MAG weldingSpecification of solid wire for MAG welding

Solid wire for building structure weldingSolid wire for building structure welding

Effect of Ti in solid wireEffect of Ti in solid wire

Deoxidization reaction in MAG weldingDeoxidization reaction in MAG welding

In welding arc,

CO2 CO + O

In molten weld metal and slag, In the case of sufficient Si & Mn

Fe + O FeO

Si + FeO SiO2 + Fe

Mn + FeO MnO + Fe

In the case of insufficient Si & Mn

Fe + O FeO C + FeO CO + Fe

Into slag

Blow hole

Prevention of blowholePrevention of blowhole

Cause of blowhole

• Hydrogen Decrease of moisture, rust in welding materials

• CO gas Entry of air into shielding gas Stable flow of shielding gas (appropriate gas flow rate) Wind velocity 2 m/s (7km/hr) Avoidance of excessively long arc length

Yield of Si & Mn in MAG weldingYield of Si & Mn in MAG welding

CO2 wire x

Ar-CO2 shielding gas

Excessive Si & Mn in weld metal Excessive strength

Ar-CO2 wire x

CO2 shielding gas

Insufficient Si & Mn in weld metal In sufficient strength

YYFWFW –– C C 5050 2 2 XX

Flux type Flux type

( R:Rutile,M:Metalic,B:Basic, G:Other )( R:Rutile,M:Metalic,B:Basic, G:Other )

Charpy absorbed energy and temperatureCharpy absorbed energy and temperature

Tensile strengthTensile strength

Shielding Gas (C:COShielding Gas (C:CO22, A:Ar+CO, A:Ar+CO22))

Flux cored wireFlux cored wire

Features of MAG welding processesFeatures of MAG welding processes

Slag type of FCW : All position welding with high current

Self shield arc welding : No supply of shielding gas

Efficiency of welding consumablesEfficiency of welding consumables

Deposition efficiency(%) = Weight of deposited metal / weight of melted consumable

Melting rate (g/min) = Melting speed of consumable per unit time (wire diameter, welding current, wire extension)

Spatter loss (%) = Total weight of spatter / weight of melted consumable

Deposition rate (g/min) = Weight of deposited metal per unit time (melting rate, penetration)

Flux for submerged arc weldingFlux for submerged arc welding Fused fluxFused flux Sintered flux Sintered flux Bonded fluxBonded flux

Property Fused type Bonded type

Addition of alloyingelement

Not possible Possible

Resistance to moisture absorption

Good Poor

Diffusible hydrogencontent

Slightly high Low

High speed welding Applicable Not applicable

Very high heat inputwelding

Not applicable Applicable

Comparison of SAW fluxComparison of SAW flux

Macro-structure of weld metalMacro-structure of weld metal

As-solidified (as cast)

Reheated

Low heat input welding

for low-temperature steel

kJ/mm

Microstructure of as-solidified weld metalMicrostructure of as-solidified weld metal

UpUpper bainite Ferrite + pearlite

Acicular ferrite

t8/5 30s

Intragranular nucleation of acicular ferriteIntragranular nucleation of acicular ferritein as-solidified weld metal during cooling transformationin as-solidified weld metal during cooling transformation

Welding of high temperature service steelWelding of high temperature service steel

High temperature service steelsHigh temperature service steels

JIS G3103 SB series (C, Mo) Boilers JIS G3119 SBV series (Mn-Mo, Mn-Mo-Ni) JIS G3120 SQV series (Mn-Mo, Mn-Mo-Ni) Nuclear pressure vessels JIS G4109 SCMV series (Cr-Mo) 1%Cr-9%Cr JIS 4110 SCMQ series (Cr-Mo-V-(W)) 9-12%Cr

High temperature service steelHigh temperature service steel

Cr: Oxidation resistance at high temperatures by Cr oxide film

Mo and Cr(less than 1%): Creep resistance

Creep : Grain boundary slip Creep rupture

Creep rupture is likely in fine grained zone Highest creep resistance Single crystal

Welding of high temperature service steelWelding of high temperature service steel

• High Cr and Mo High CE (Highly hardenable)

100% martensite in HAZ

• Preheating is required to avoid cold cracking at HAZ

Ex: 2.25Cr -1Mo 150 – 350oC

9Cr – 1Mo 200 – 350oC

• PWHT (stress relief annealing) is required to obtain tempered martensite in HAZ