Metallurgy of High Strength Steel: Properties and Transformations

1. Metallurgy of High Strength SteelN. 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 < 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)aFe3C

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

25. YR (Yield Ratio)

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

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

28. Steels for • Building 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.  -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 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) • 60·25·170/150  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 (>1200oC) • Solidification cracking • Liquation cracking • Cold cracking (<100oC) • (Hydrogen assisted cracking)

50. Hot cracking Solidification crack Liquation crack Stainless steel, Al