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Metallurgy ofMetallurgy of High Strength Steel High Strength Steel
N. YuriokaN. Yurioka Visiting Professor at Osaka UniversityVisiting Professor at Osaka University
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Crystalline lattice structureCrystalline lattice structure
BCC
BCC
FCC HCP
BCC
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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.)
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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
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Phase transformation in cooling - IPhase transformation in cooling - I
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Pearlite (Composite of ferrite and cementite)Pearlite (Composite of ferrite and cementite) FeFe3CC
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Phase transformation in cooling - IIPhase transformation in cooling - II
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Line expansion (Dilatation) Line expansion (Dilatation)
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Dilatometry-IDilatometry-I
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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
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Diffusion of carbon plays an important role inDiffusion of carbon plays an important role inphase transformationphase transformation
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Microstructure of steels -IMicrostructure of steels -I
Martensite Lower bainite
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Martensite and lower bainiteMartensite and lower bainite
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Microstructure of steels -IIMicrostructure of steels -II
Upper bainite Ferrite and pearlite
Rollin
g
dire
ctio
n
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Formation of upper bainite in cooling -IFormation of upper bainite in cooling -I
Nucleation of ferrite Growth of ferrite
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Formation of upper bainite in cooling -IIFormation of upper bainite in cooling -II
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HeatHeat treatment of steels treatment of steels
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Normalizing treatment of ferrite-pearlite steelNormalizing treatment of ferrite-pearlite steel
Grain refining
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Hot rolling processesHot rolling processes
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Microstructure of hot rolled steelMicrostructure of hot rolled steel
As rolled Normalized
TMCP-II Quenched & tempered
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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
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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 )
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Steels forSteels for• Welded Welded structuresstructures SMSM series series
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YR (Yield Ratio)YR (Yield Ratio)
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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
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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
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Steels forSteels for• BuildingBuilding construction construction SNSN series series
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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
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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)
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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)
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-160oC
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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
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Weldability of steelsWeldability of steels
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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
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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
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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)
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45mm
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Microstructure of HAZMicrostructure of HAZ
Normalizing heat treatment
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CCT (Continuous Cooling Transformation) diagram CCT (Continuous Cooling Transformation) diagram
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Cooling curve (log-scale)Cooling curve (log-scale)
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CCT (Low-hardenability)CCT (Low-hardenability)
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CCT (high hardenability)CCT (high hardenability)
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HAZ maximum hardnessHAZ maximum hardness
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Hardness change against tHardness change against t8/58/5
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Change in HAZ maximum hardnessChange in HAZ maximum hardness
Martensite Martensite
hardnesshardness
= f(C)= f(C)
HardenabilityHardenability
Carbon equivalentCarbon equivalent
CECEIIWIIW
CECEWESWES
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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)
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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
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Weld crackingWeld cracking
Hot cracking (>1200oC)
Solidification cracking Liquation cracking
Cold cracking (<100oC) (Hydrogen assisted cracking)
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Hot crackingHot cracking
Solidification crackSolidification crack Liquation crackLiquation crack
Stainless steel, Al
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Weld metal crackingWeld metal cracking
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Segregation of impurities during solidificationSegregation of impurities during solidification
Phase diagramPhase diagram Residual liquid phaseResidual liquid phase
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Direction of solidification growthDirection of solidification growthH/WH/W
Welding velocityWelding velocity
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Cold cracksCold cracks
Root crack (HAZ) Toe crack (HAZ) Transverse crack (Weld metal)
Under-bead crack(HAZ)
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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)
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Effect of preheat on HAZ hydrogenEffect of preheat on HAZ hydrogen
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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
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Cold cracking Cold cracking • Hydrogen assisted cracking, Delayed crackingHydrogen assisted cracking, Delayed cracking
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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)
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Carbon equivalentsCarbon equivalents
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Pc methodPc method
Necessary preheat temperature
Tph(oC) = 1440 Pc - 392
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Cracking other than hot cracking and cold cracking Cracking other than hot cracking and cold cracking
Lamellar tearLamellar tear
Reheat crackReheat crack
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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
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Prevention of lamellar tearPrevention of lamellar tear
Buttering pass
Reduction ofDeposited metal
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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
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HAZ toughnessHAZ toughness
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& HT490
NormalizingHeat treatment
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Toughness of coarse grained zoneToughness of coarse grained zone
Lower bainite Upper bainite
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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
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Impeding of austenite grain growthImpeding of austenite grain growthAustenite grain boundary migration is stopped by the pinning effect of particles.
Ti deoxidized steel
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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
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Welding consumablesWelding consumables
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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
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Gravity welding equipmentGravity welding equipment
D4326
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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
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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.
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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
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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
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Specification of solid wire for MAG weldingSpecification of solid wire for MAG welding
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Solid wire for building structure weldingSolid wire for building structure welding
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Effect of Ti in solid wireEffect of Ti in solid wire
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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
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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
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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
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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
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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
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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)
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Flux for submerged arc weldingFlux for submerged arc welding Fused fluxFused flux Sintered flux Sintered flux Bonded fluxBonded flux
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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
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Macro-structure of weld metalMacro-structure of weld metal
As-solidified (as cast)
Reheated
Low heat input welding
for low-temperature steel
kJ/mm
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Microstructure of as-solidified weld metalMicrostructure of as-solidified weld metal
UpUpper bainite Ferrite + pearlite
Acicular ferrite
t8/5 30s
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Intragranular nucleation of acicular ferriteIntragranular nucleation of acicular ferritein as-solidified weld metal during cooling transformationin as-solidified weld metal during cooling transformation
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Welding of high temperature service steelWelding of high temperature service steel
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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
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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
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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