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

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Presentation on theme: "Metallurgy of High Strength Steel N. Yurioka Metallurgy of High Strength Steel N. Yurioka Visiting Professor at Osaka University Visiting Professor at."— Presentation transcript:

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

2 Crystalline lattice structure BCC FCCHCP BCC

3 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, Steel (at low temp.), Ferritic stainless steel, Ti (at high temp.) Ti (at high temp.) c. Hexagonally closed packed (HCP) Ti (at low temp.)

4 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

5 Phase transformation in cooling - I

6 Pearlite (Composite of ferrite and cementite) FeC Pearlite (Composite of ferrite and cementite) Fe 3 C

7 Phase transformation in cooling - II

8 Line expansion (Dilatation)

9 Dilatometry-I

10 Dilatometry-IITransformation In heating A: to start A c1 : to start A: to finish A c3 : to finish In cooling A: to start A r3 : to start A: to finish A r1 : to finish In rapid cooling (quenching) (quenching) M s : M start M s : M start M f : M finish M f : M finish

11 Diffusion of carbon plays an important role in phase transformation

12 Microstructure of steels -I MartensiteLower bainite

13 Martensite and lower bainite

14 Microstructure of steels -II Upper bainiteFerrite and pearlite Rolling direction

15 Formation of upper bainite in cooling -I Nucleation of ferriteGrowth 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 rolledNormalized TMCP-IIQuenched & 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 structureSS series (SS400, SS490, etc … )General structureSS series (SS400, SS490, etc … ) Welded structureSM seriesWelded structureSM series Building construction SN series ( Tensile strength )Building construction SN series ( Tensile strength )

23 Steels for Steels for Welded structuresSM seriesWelded structuresSM series

24

25 YR (Yield Ratio)

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

27 Lamellar tear Reduction of area, RA Z in the thickness direction in the thickness direction Reduction of P & S in steel Increase of RAz

28 Steels for Steels for Building construction SN seriesBuilding construction SN series

29 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

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 -160 o C

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 4110 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/ (J/mm), 1.7(kJ/mm) Heat Input HI(J/mm) = 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(C/s) at 540 o C CR( o C/s) at 540 o C t 8/5 (s): Cooling time between Cooling time between 800 o C and 500 o C 800 o C and 500 o C 1.7kJ/mm on 20mm thick 1.7kJ/mm on 20mm thick 7s in t 8/5 7s in t 8/5

37 Cooling rate, Cooling time Heat input Plate thickness Joint shape (Butt-joint, fillet-joint) Preheat temperature Prediction of cooling time, t 8/5 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 t 8/5

46 Change in HAZ maximum hardness Martensite hardness hardness = f(C) = f(C)Hardenability Carbon equivalent Carbon equivalent CE IIW CE IIW CE WES CE WES

47 Prediction of HAZ hardness HAZ hardness Welding conditions Heat input Plate thickness Preheat temperature t 8/5 Chemical composition of steel C Carbon Equivalent JWES IT-Center (http://www-it.jwes.or.jp/index_e.jsp)

48 Carbon equivalent CE IIW = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5 CE WES = C + Si/24 + Mn/6 + Ni/40 + Cr/5 + Mo/4 + V/14

49 Weld cracking Hot cracking (>1200 o C) Solidification cracking Liquation cracking Cold cracking (<100 o C) (Hydrogen assisted cracking)

50 Hot cracking Solidification crack Liquation crack Stainless steel, Al

51 Weld metal cracking

52 Segregation of impurities during solidification Phase diagram Residual liquid phase

53 Direction of solidification growth H/W Welding velocity

54 Cold cracks Root crack (HAZ)Toe crack (HAZ) Transverse crack (Weld metal) Under-bead 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 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

58 Cold cracking Hydrogen assisted cracking, Delayed crackingHydrogen assisted cracking, Delayed cracking

59 Determination of necessary preheat temperature AWS D1.1 Annex I Hardness control method (CE IIW ) C>0.11% Hydrogen control method (Pcm) C<0.11% BS5135 [EN A] (CE IIW ) CE T method [EN B] (CE T ) CE N method (CE N ) JWES IT -center (http://www-it.jwes.or.jp/index_e.jsp)http://www-it.jwes.or.jp/index_e.jsp Pc method (Pcm)

60 Carbon equivalents

61 Pc method Necessary preheat temperature Tph( o C) = 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 Reheat cracks are initiated at the weld toe during stress relief annealing 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 Weld metal Coarse grained HAZ Intergranular crack

66 HAZ toughness

67 & HT490 Normalizing Heat treatment

68 Toughness of coarse grained zone Lower bainiteUpper bainite vTrs

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 designatio n Main ingredient Welding position IlminiteD__01Ilmenite (Impure rutile) All Lime-Titania (Rutile) D__03Lime + Titanium oxide (Rutile) All CellulosicD__11Organic substance All High titanium oxide (Rutile) D__13Titanium oxide (Rutile) All Low hydrogen (Basic type) D__16LimeAll Iron powder Low hydrogen D__26Lime + Iron powder Flat Horizontal Non low hydrogen H D > 30ml/100g Low hydrogen H D <7ml/100g

74 Gravity welding equipment D4326

75 Flux type of covered electrode Basic type CaCO 3 CaO + CO 2 lime Decrease of partial pressure of H High basicity Low oxygen in weld metal Low hydrogen

76 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.

77 Non-low hydrogen electrode (H D > 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 (H D < 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: o C x 30-60min Drying condition for non-low hydrogen electores: o C 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 2 CO + O In molten weld metal and slag, In the case of sufficient Si & Mn Fe + O FeO Si + FeO SiO 2 + 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 CO 2 wire x Ar-CO 2 shielding gas Excessive Si & Mn in weld metal Excessive strength Ar-CO 2 wire x CO 2 shielding gas Insufficient Si & Mn in weld metal In sufficient strength

85

86 YFW – C 50 2 X Flux type ( R:Rutile,M:Metalic,B:Basic, G:Other ) ( R:Rutile,M:Metalic,B:Basic, G:Other ) Charpy absorbed energy and temperature Tensile strength Shielding Gas (C:CO 2, A:Ar+CO 2 ) Flux cored wire

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 Fused flux Sintered flux Sintered flux Bonded flux Bonded flux

90 PropertyFused typeBonded type Addition of alloying element Not possiblePossible Resistance to moisture absorption GoodPoor Diffusible hydrogen content Slightly highLow High speed weldingApplicableNot applicable Very high heat input welding Not applicableApplicable Comparison of SAW flux

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 Up Upper bainiteFerrite + pearlite Acicular ferrite t 8/5 30s

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 4110 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: 2.25Cr -1Mo 150 – 350 o C 9Cr – 1Mo 200 – 350 o C PWHT (stress relief annealing) is required to obtain tempered martensite in HAZ


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