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AWEmaterials

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Metallurgy of High Strength Steel N. Yurioka Visiting Professor at Osaka University HAZ toughness & HT490 Normalizing Heat treatment Toughness of coarse grained zone ... – PowerPoint PPT presentation

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Title: AWEmaterials


1
Metallurgy of High Strength Steel
N. Yurioka Visiting Professor
at Osaka University
2
Crystalline lattice structure
BCC
BCC
BCC
FCC
HCP
3
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.)

4
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
5
Phase transformation in cooling - I
6
Pearlite (Composite of ferrite and cementite)
a
Fe3C
7
Phase transformation in cooling - II
8
Line expansion (Dilatation)
9
Dilatometry-I
10
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

11
Diffusion of carbon plays an important role
inphase transformation
12
Microstructure of steels -I
Martensite
Lower bainite
13
Martensite and lower bainite
14
Microstructure of steels -II
Rolling direction
Upper bainite
Ferrite and pearlite
15
Formation of upper bainite in cooling -I
Nucleation of ferrite
Growth of ferrite
16
Formation of upper bainite in cooling -II
17
Heat treatment of steels
18
Normalizing treatment of ferrite-pearlite steel
Grain refining
19
Hot rolling processes
20
Microstructure of hot rolled steel
As rolled
Normalized
TMCP-II
Quenched tempered
21
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

22
  • Mild steels (JIS standard)
  • General structure SS series (SS400, SS490, etc)
  • Welded structure SM series
  • Building construction SN series ( Tensile
    strength )

23
  • Steels for
  • Welded structures SM series

24
(No Transcript)
25
YR (Yield Ratio)
26
  • Steels for
  • Building construction SN series
  • High ratio decreases
  • the compliance of
  • structures such as
  • building .

27
  • Reduction of area, RAZ
  • in the thickness direction
  • Lamellar tear

Reduction of P S in steel
Increase of RAz
28
  • Steels for
  • Building construction SN series

29
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
30
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)

31
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)
32
? -160oC
33
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

34
Weldability of steels
35
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

36
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

?
?
37
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)

38
? 45mm
39
Microstructure of HAZ
Normalizing heat treatment
40
  • CCT (Continuous Cooling Transformation) diagram

41
Cooling curve (log-scale)
42
CCT (Low-hardenability)
43
CCT (high hardenability)
44
HAZ maximum hardness
45
Hardness change against t8/5
46
Change in HAZ maximum hardness
  • Martensite
  • hardness
  • f(C)
  • Hardenability
  • Carbon equivalent
  • CEIIW
  • CEWES

47
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)
48
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
49
Weld cracking
  • Hot cracking (gt1200oC)
  • Solidification cracking
  • Liquation cracking
  • Cold cracking (lt100oC)
  • (Hydrogen assisted cracking)

50
Hot cracking
  • Solidification crack
  • Liquation crack

Stainless steel, Al
51
Weld metal cracking
52
Segregation of impurities during solidification
  • Residual liquid phase
  • Phase diagram

53
Direction of solidification growth
  • H/W
  • Welding velocity

54
Cold cracks
Transverse crack (Weld metal)
Under-bead crack (HAZ)
Root crack (HAZ)
Toe crack (HAZ)
55
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)
56
Effect of preheat on HAZ hydrogen
57
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
58
  • Cold cracking
  • Hydrogen assisted cracking, Delayed cracking

59
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)
60
Carbon equivalents
61
Pc method
Necessary preheat temperature Tph(oC) 1440
Pc - 392
62
Cracking other than hot cracking and cold
cracking
  • Lamellar tear
  • Reheat crack

63
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

64
Prevention of lamellar tear
Buttering pass
Reduction of Deposited metal
65
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

66
HAZ toughness
67
Normalizing Heat treatment
HT490
68
Toughness of coarse grained zone
vTrs
Lower bainite
Upper bainite
69
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

70
Impeding of austenite grain growth
Austenite grain boundary migration is stopped by
the pinning effect of particles.
Ti deoxidized steel
71
Island-like martensite (MA, Martensite-Austenite
constituent)
MA of very hard phase Initiation site of
brittle crack Low carbon steel Decrease of
MA
72
Welding consumables
73
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
74
Gravity welding equipment
D4326
75
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
76
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.

77
  • 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

78
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
79
Specification of solid wire for MAG welding
80
Solid wire for building structure welding
81
Effect of Ti in solid wire
82
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
83
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

84
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
85
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86
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)
87
Features of MAG welding processes
Slag type of FCW All position welding with
high current Self shield arc welding No
supply of shielding gas
88
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)

89
  • Flux for submerged arc welding
  • Fused flux
  • Sintered flux
  • Bonded flux

90
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
91
Macro-structure of weld metal
As-solidified (as cast) Reheated
Low heat input welding for low- temperature
steel
kJ/mm
92
Microstructure of as-solidified weld metal
Upper bainite
Ferrite pearlite
Up
t8/5 ? 30s
Acicular ferrite
93
Intragranular nucleation of acicular ferritein
as-solidified weld metal during cooling
transformation
94
Welding of high temperature service steel
95
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

96
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
97
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
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