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Metallurgy of High Strength Steel N. Yurioka

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Presentation on theme: "Metallurgy of High Strength Steel N. Yurioka"— Presentation transcript:

1 Metallurgy of High Strength Steel N. Yurioka
Visiting Professor at Osaka University

2 Crystalline lattice structure

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

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