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. Steel is an alloy of Iron and carbon Fe-C Phase diagram
Iron C < 0.02%
Steel  C  0.21%
Cast iron : 0.21% < 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 in phase 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

25. YR (Yield Ratio)

26. Steels for Building construction SN series High ratio decreases
the compliance of
structures such as
building .

27. in the thickness direction Lamellar tear
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 SM490, SM520, SM570…..
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 kJ/mm
to avoid HAZ softening, Low HAZ toughness

30. 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)

31. 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)

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)
1%Cr-9%Cr
JIS SCMQ series (Cr-Mo-V-(W))
9-12%Cr

34. Weldability of steels

35. 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
 : Arc thermal efficiency 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 (

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 (

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 Solidification cracking Liquation cracking
Hot cracking (>1200oC)
Solidification cracking
Liquation cracking
Cold cracking (<100oC)
(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 Under-bead crack (HAZ) Transverse crack (Weld metal)
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) C>0.11%
Hydrogen control method (Pcm) C<0.11%
BS5135 [EN A] (CEIIW)
CET method [EN B] (CET)
CEN method (CEN)
JWES IT -center (
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 > 15%, RAz > 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 Weld metal 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

66. HAZ toughness

67. Normalizing
Heat treatment
& HT490

68. Toughness of coarse grained zone
vTrs
Lower bainite
Upper bainite

69. HAZ toughness Smaller heat input (HI)welding
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)
Cellulosic
D__11
Organic substance
High titanium oxide (Rutile)
D__13
Low hydrogen
(Basic type)
D__16
Lime
Iron powder
D__26
Lime +
Flat
Horizontal
Non low hydrogen
HD > 30ml/100g
Low hydrogen
HD<7ml/100g

74. Gravity welding equipment

75. Flux type of covered electrode
Low hydrogen
Basic type CaCO 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. Low basicity Higher oxygen content Lower toughness
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

78. Moisture absorption of electrode
Baking condition for low hydrogen electrodes: oC x 30-60min
Drying condition for non-low hydrogen electores:70-100oC 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,
CO 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 sufficient strength

86. 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)

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

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 ferrite in 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)
1%Cr-9%Cr
JIS SCMQ series (Cr-Mo-V-(W))
9-12%Cr

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: Cr -1Mo – 350oC
9Cr – 1Mo – 350oC
PWHT (stress relief annealing) is required
to obtain tempered martensite in HAZ