AWEmaterials

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Metallurgy of High Strength Steel
N. Yurioka Visiting Professor
at Osaka University
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Crystalline lattice structure
BCC
BCC
BCC
FCC
HCP
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Crystalline lattice structure

• Face centered cubic (FCC)
• Steel (at high temp.), Austenitic stainless
  steel, Al, Cu,...
• Body centered cubic (BCC)
• Steel (at low temp.), Ferritic
  stainless steel,
• Ti (at high temp.)
• Hexagonally closed packed (HCP)
• Ti (at low temp.)

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Fe-C Phase diagram
Steel is an alloy of Iron and carbon
Iron C lt 0.02 Steel 0.02? C ? 0.21 Cast
iron 0.21 lt C
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Phase transformation in cooling - I
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Pearlite (Composite of ferrite and cementite)
a
Fe3C
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Phase transformation in cooling - II
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Line expansion (Dilatation)
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Dilatometry-I
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Dilatometry-II

• Transformation
• In heating
• Ac1 a to g start
• Ac3 a to g finish
• In cooling
• Ar3 g to a start
• Ar1 g to a finish
• In rapid cooling
• (quenching)
• Ms M start
• Mf M finish

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Diffusion of carbon plays an important role
inphase transformation
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Microstructure of steels -I
Martensite
Lower bainite
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Martensite and lower bainite
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Microstructure of steels -II
Rolling direction
Upper bainite
Ferrite and pearlite
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Formation of upper bainite in cooling -I
Nucleation of ferrite
Growth of ferrite
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Formation of upper bainite in cooling -II
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Heat treatment of steels
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Normalizing treatment of ferrite-pearlite steel
Grain refining
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Hot rolling processes
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Microstructure of hot rolled steel
As rolled
Normalized
TMCP-II
Quenched tempered
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Features 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)
• General structure SS series (SS400, SS490, etc)
• Welded structure SM series
• Building construction SN series ( Tensile
  strength )

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• Steels for
• Welded structures SM series

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YR (Yield Ratio)
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• Steels for
• Building construction SN series
• High ratio decreases
• the compliance of
• structures such as
• building .

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• Reduction of area, RAZ
• in the thickness direction

• Lamellar tear

Reduction of P S in steel
Increase of RAz
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• Steels for
• Building construction SN series

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High strength steel

• TS gt 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 purposes

• Lamellar tear resistant steel
• Ex. Z25 grade (RA gt 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 steels

• JIS SLA grade
• Al-killed steel (N or QT or TMCP)
• JIS SL grade
• 3.5Ni (NT, TMCP)
• 5Ni (NNT, TMCP)
• 9Ni (QQT, QLT, DQT)
• Austenitic stainless steel
• SUS304, SUS316
• Inver (34Ni-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 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)
• 1Cr-9Cr
• JIS 4110 SCMQ series (Cr-Mo-V-(W))
• 9-12Cr

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Weldability of steels
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Welding heat input

• Energy Input (AWS D1.1), Arc Energy(EN standard)
• EI(J/mm) 60 (EI/v)
• E(V), I(A), v(mm/min)
• 6025170/150 ? 1700 (J/mm),
  1.7(kJ/mm)
• Heat Input
• HI(J/mm) h? 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 time

• CR(oC/s) at 540oC
• t8/5(s)
• Cooling time between
• 800oC and 500oC
• 1.7kJ/mm on 20mm thick
• 7s in t8/5

?
?
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Cooling 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 HAZ
Normalizing heat treatment
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• CCT (Continuous Cooling Transformation) diagram

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Cooling curve (log-scale)
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CCT (Low-hardenability)
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CCT (high hardenability)
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HAZ maximum hardness
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Hardness change against t8/5
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Change in HAZ maximum hardness

• Martensite
• hardness
• f(C)
• Hardenability
• Carbon equivalent
• CEIIW
• CEWES

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Prediction of HAZ hardness

• Welding conditions
• Heat input
• Plate thickness
• Preheat temperature

t8/5
HAZ hardness

• Chemical composition of steel
• C
• Carbon Equivalent

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

• Hot cracking (gt1200oC)
• Solidification cracking
• Liquation cracking
• Cold cracking (lt100oC)
• (Hydrogen assisted cracking)

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Hot cracking

• Solidification crack

• Liquation crack

Stainless steel, Al
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Weld metal cracking
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Segregation of impurities during solidification

• Residual liquid phase

• Phase diagram

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Direction of solidification growth

• H/W

• Welding velocity

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Cold cracks
Transverse crack (Weld metal)
Under-bead crack (HAZ)
Root crack (HAZ)
Toe crack (HAZ)
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Generation and diffusion of hydrogen

• Generation of hydrogen

• Hydrogen diffusion in weld

Mineral water in flux, Moisture in flux Moisture
in atmosphere, Rust, oil, grease in groove
Arc
H (hydrogen)
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Effect of preheat on HAZ hydrogen
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Cause of hydrogen-assisted 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
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• Cold cracking
• Hydrogen assisted cracking, Delayed cracking

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Determination of necessary preheat temperature
AWS D1.1 Annex I Hardness control
method (CEIIW) Cgt0.11 Hydrogen
control method (Pcm) Clt0.11 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 equivalents
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Pc method
Necessary preheat temperature Tph(oC) 1440
Pc - 392
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Cracking other than hot cracking and cold
cracking

• Lamellar tear
• Reheat crack

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Prevention of lamellar tear

• Use steel with higher RA
• in the thickness direction
• RAz gt 15, RAz gt 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 tear
Buttering pass
Reduction of Deposited metal
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Reheat crack
Coarse grained HAZ
Weld metal
Reheat cracks are initiated at the weld toe
during stress relief annealing
Intergranular crack

• 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

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HAZ toughness
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Normalizing Heat treatment
HT490
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Toughness of coarse grained zone
vTrs
Lower bainite
Upper bainite
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HAZ 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 growth
Austenite grain boundary migration is stopped by
the pinning effect of particles.
Ti deoxidized steel
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Island-like martensite (MA, Martensite-Austenite
constituent)
MA of very hard phase Initiation site of
brittle crack Low carbon steel Decrease of
MA
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Welding consumables
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Typical covered electrodes
Hydrogen level Type of covered flux JIS designation Main ingredient Welding position
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 powder Low hydrogen D__26 Lime Iron powder Flat Horizontal
Non low hydrogen HD gt 30ml/100g
Low hydrogen HDlt7ml/100g
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Gravity welding equipment
D4326
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Flux type of covered electrode
Low hydrogen
Basic type CaCO3 CaO CO2
Decrease of partial pressure of H
lime
High basicity Low oxygen in weld metal
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Functions of the coating of covered electrode for
SMAW.

• 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 gt 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 lt 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 electrode
Baking condition for low hydrogen electrodes
300-400oC x 30-60min Drying condition for non-low
hydrogen electores70-100oC x 1hr
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Specification of solid wire for MAG welding
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Solid wire for building structure welding
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Effect of Ti in solid wire
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Deoxidization 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 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 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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Flux cored wire
YFW C 50 2 X Flux type (
RRutile,MMetalic,BBasic, GOther ) Charpy
absorbed energy and temperature Tensile
strength Shielding Gas (CCO2, AArCO2)
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Features 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 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 welding
• Fused flux
• Sintered flux
• Bonded flux

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Comparison of SAW flux
Property Fused type Bonded type
Addition of alloying element Not possible Possible
Resistance to moisture absorption Good Poor
Diffusible hydrogen content Slightly high Low
High speed welding Applicable Not applicable
Very high heat input welding Not applicable Applicable
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Macro-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 metal
Upper bainite
Ferrite pearlite
Up
t8/5 ? 30s
Acicular ferrite
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Intragranular nucleation of acicular ferritein
as-solidified weld metal during cooling
transformation
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Welding of high temperature service steel
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High 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)
• 1Cr-9Cr
• JIS 4110 SCMQ series (Cr-Mo-V-(W))
• 9-12Cr

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High 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 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