Metallurgy of
High Strength Steel
N. Yurioka
Visiting Professor at Osaka University
Crystalline lattice structure
BCC
BCC
FCC
HCP
Crystalline lattice structure
a.
Face centered cubic (FCC)
Steel (at high temp.), Austenitic stainless steel,
Al, Cu,...
b.
Body centered cubic (BCC)
Steel (at low temp.), Ferritic stainless steel,
Ti (at high temp.)
c.
Hexagonally closed packed (HCP)
Ti (at low temp.)
Fe-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 - I
Pearlite (Composite of ferrite and cementite)
a
Fe3C
Phase transformation in cooling - II
Line expansion (Dilatation)
Dilatometry-I
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
Diffusion of carbon plays an important role in
phase transformation
Microstructure of steels -I
Martensite
Lower bainite
Martensite and lower bainite
Microstructure of steels -II
Rolling direction
Upper bainite
Ferrite and pearlite
Formation of upper bainite in cooling -I
Nucleation of ferrite
Growth of ferrite
Formation of upper bainite in cooling -II
Heat treatment of steels
Normalizing treatment of ferrite-pearlite steel
Grain refining
Hot rolling processes
Microstructure of hot rolled steel
As rolled
TMCP-II
Normalized
Quenched & tempered
Features of steels
• As rolled steel
Ferrite –pearlite
• Normalized steel
Grain-refined ferrite-pearlite
Higher strength and toughness
Low strength, Low YR
• 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)
• General structure
• Welded structure
• Building construction
SS series (SS400, SS490, etc…)
SM series
SN series ( Tensile strength )

Steels for
• Welded structures
SM series
YR (Yield Ratio)

Steels for
Building construction
SN series
Yield ratio  Yield /  Tensile
High ratio decreases
the compliance of
structures such as
building .
Lamellar tear
Reduction of P & S in steel
Increase of RAz
Reduction of area, RAZ
in the thickness direction

Steels for
• Building construction
SN series
High 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 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 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 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 steels
Welding 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) = h EI
h : Arc thermal efficiency 1.0 for SAW
0.8 for SMAW, GMAW
0.6 for autogenus TIG
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
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)
 45mm
Microstructure of HAZ
Normalizing heat treatment
CCT (Continuous Cooling Transformation) diagram
Cooling curve (log-scale)
CCT (Low-hardenability)
CCT (high hardenability)
HAZ maximum hardness
Hardness change against t8/5
Change in HAZ maximum hardness
Martensite
hardness
= f(C)
Hardenability
Carbon equivalent
CEIIW
CEWES
Prediction of HAZ hardness
• Welding conditions
t8/5
HAZ hardness
Heat input
Plate thickness
Preheat temperature
• Chemical composition of steel
C
Carbon Equivalent
JWES IT-Center
(http://www-it.jwes.or.jp/index_e.jsp)
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
Weld cracking
 Hot
cracking (>1200oC)
Solidification cracking
Liquation cracking
 Cold
cracking (<100oC)
(Hydrogen assisted cracking)
Hot cracking
Solidification crack
Liquation crack
Stainless steel, Al
Weld metal cracking
Segregation of impurities during solidification
Phase diagram
Residual liquid phase
Direction of solidification growth
H/W
Welding velocity
Cold cracks
Root crack (HAZ)
Toe crack (HAZ)
Under-bead crack
(HAZ)
Transverse crack
(Weld metal)
Generation and diffusion of hydrogen
Generation of hydrogen
Mineral water in flux, Moisture in flux
Moisture in atmosphere,
Rust, oil, grease in groove
Hydrogen diffusion in weld
Arc
H (hydrogen)
Effect of preheat on HAZ hydrogen
Cause of hydrogen-assisted cold cracking
Diffusible hydrogen
Weld metal hydrogen
Preheat temperature
Cold cracking
Hardness (HAZ, Weld metal)
Steel chemical composition
t8/5
HI, thickness
Tensile residual stress
Yield strength of weld metal
Notch concentration factor
Cold cracking
• Hydrogen assisted cracking, Delayed cracking
Determination 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 equivalents
Pc method
Necessary preheat temperature
Tph(oC) = 1440 Pc - 392
Cracking other than hot cracking and cold cracking
Lamellar tear
Reheat crack
Prevention 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 tear
Buttering pass
Reduction of
Deposited metal
Reheat crack
Weld metal
Coarse grained
HAZ
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
HAZ toughness
Normalizing
Heat treatment
& HT490
vTrs
Toughness of coarse grained zone
Lower bainite
Upper bainite
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
 Matrix
High HI
with high toughness
Low N, High Ni
High C
Impeding of austenite grain growth
Austenite grain boundary migration is stopped by
the pinning effect of particles.
Ti deoxidized steel
Island-like martensite
(MA, Martensite-Austenite constituent)
MA of very hard phase
Initiation site of brittle crack
Low carbon steel
Decrease of MA
Welding consumables
Typical covered electrodes
Low hydrogen
HD<7ml/100g
Non low hydrogen
HD > 30ml/100g
Hydrogen level
Type of covered
flux
Main ingredient
JIS
designation
Welding
position
Ilminite
D__01
Ilmenite
All
Lime-Titania
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
D__16
Lime
All
Iron powder
Low hydrogen
D__26
Lime +
Iron powder
Flat
Horizontal
(Rutile)
(Basic type)
(Impure rutile)
Gravity welding equipment
D4326
Flux type of covered electrode
Basic type
CaCO3
lime
CaO + CO2
High basicity
Low hydrogen
Decrease of partial pressure of H
Low oxygen in weld metal
Functions of the coating of covered
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 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 welding
Solid wire for building structure welding
Effect of Ti in solid wire
Deoxidization reaction in MAG welding
In welding arc,
CO2
CO + O
In molten weld metal and slag,
In the case of sufficient Si & Mn
Into slag
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
Blow hole
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
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
Flux cored wire
YFW – C 50 2 X
Flux type
( R:Rutile,M:Metalic,B:Basic, G:Other )
Charpy absorbed energy and temperature
Tensile strength
Shielding Gas (C:CO2, A:Ar+CO2)
Features 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 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 welding



Fused flux
Sintered flux
Bonded flux
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
Macro-structure of weld metal
As-solidified
(as cast)
Reheated
Low heat
input
welding
for lowtemperature
steel
kJ/mm
Microstructure of as-solidified weld metal
UpperUp
bainite
Ferrite + pearlite
t8/5  30s
Acicular ferrite
Intragranular nucleation of acicular ferrite
in as-solidified weld metal during cooling transformation
Welding of high temperature service steel
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)
1%Cr-9%Cr

JIS 4110 SCMQ series (Cr-Mo-V-(W))
9-12%Cr
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
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
9Cr – 1Mo
150 – 350oC
200 – 350oC
• PWHT (stress relief annealing) is required
to obtain tempered martensite in HAZ