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Yudong Zhang 1,2 , Xiang Zhao 1 , Changshu He 1 ,

Effects of a High Magnetic Field on the Microstructure Formation in 42CrMo Steel during Solid Phase Transformations. Yudong Zhang 1,2 , Xiang Zhao 1 , Changshu He 1 , Weiping Tong 1 , Liang. Zuo 1 , Jicheng He 1 and Claud. Esling 2. 1. 2.

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Yudong Zhang 1,2 , Xiang Zhao 1 , Changshu He 1 ,

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  1. Effects of a High Magnetic Field on the Microstructure Formation in 42CrMo Steel during Solid Phase Transformations Yudong Zhang1,2, Xiang Zhao1, Changshu He1, Weiping Tong1, Liang. Zuo1, Jicheng He1 and Claud. Esling2 1 2 Sino-German Workshop on EPM, Oct.11-12, 2004, Shanghai Univ. Shanghai, China

  2. Outline Introduction Part I－Characteristics of Phase Transformation from Austenite to ferrite in High magnetic field Part II－Tempering Behaviors in High magnetic field Summary

  3. Introduction The introduction of magnetic field to solid phase transformations in steels has been a subject of much attention in materials science. If the parent and product phases are different in saturation magnetization and are allowed to transform under the magnetic field, the transformation temperature and transformed amount can be considerably affected, as the Gibbs free energy of a phase can be lowered by an amount according to its magnetization. Previous work has focused on the influence of a magnetic field on the martensitic phase transformation in some materials with lower martensitic transformation start temperatures. Quite recently, attention has been shifted to the high temperature diffusional transformations. Research on this topic is mainly on following aspects: • Theoretical simulation of the effect of magnetic field on ferrite/austenite and austenite/ferrite phase equilibrium; • Morphological features appearing during ferrite to austeniteand austenite to ferrite transformation ; • Kinetic characteristics of proeutectoid ferrite transformation under magnetic field. Now, the research on these issues is on its initial stage!

  4. S P Si Mn C Cr Mo Fe 0.04 0.50 -0.80 0.15 -0.25 0.38-0.45 balance 0.90 -1.20 0.20 -0.40 0.04 Part I－ gaunder high magnetic field (1) Materials: Hot Rolled 42CrMo Steel Chemical composition(wt.%)

  5. Part I－ gaunder high magnetic field (2) Heat treatment arrangement Furnace Ar Cooling water Magnetic field direction Hot-rolling direction Pt-Rh thermocouple Pt heater Magnets Zero force area Specimen Cooling-jacket Ar

  6. Temp. Temp. 880°C 880°C 33min 10°C/min B0=6; 10; 14T 33min 10°C/min Time Time Temp. 880°C 880°C B0=14T 46°C/min 33min 33min 46°C/min Time Time Part I－ gaunder high magnetic field (3) Heat treatment Slow cooling Fast cooling Temp.

  7. Microstructure RD RD//MFD m 50 m 0T m 50 m m 50 m 6T 10T 14T Part I－ gaunder high magnetic field (4) Heated at 880C for 33min and cooled at 10C/min

  8. 31 Magnetic field increases the amountof product ferrite 30 29 Area percentage of ferrite, % 927 28 B =10 T 0 27 B =0 T Steel 877 0 26 g 6 8 10 12 14 Intensity of magnetic field, Tesla Temperature, °C 827 a+g a 777 a b b' c 727 0.0 0.2 0.4 0.6 0.8 1.0 1.2 carbon content, wt% Part I－ gaunder high magnetic field (5) Image analysis (for the slow cooling group)

  9. RD RD//MFD 50m 50m Part I－ gaunder high magnetic field (6) Microstructure Heated at 880C for 33min and cooled at 46C/min 0T, bainite 14T, ferrite+pearlite Ferrite Fraction:2% Ferrite Fraction 23.1%

  10. As a consequence, t for gatransformation is reduced Part I－ gaunder high magnetic field (7) According toJohnson-Mehl equation, The kinetic equation of proeutectoid ferrite transformation from austenite can be expressed as Eq.(1) t ---- transformation time A, B, C, & E ---- constants T----absolute temperature R----gas constant Q ---- activation energy for diffusion T ---- absolute temperature s ---- interfacial energy GV ---- Gibbs volume free energy difference between the product and the parent phase x, x ---- solubility values of austenite and ferrite at T x ---- carbon content of the material (1) When a high magnetic field is applied (2) The magnetic- field induced extra energy difference between the ferrite and the austenite

  11. Part I－ gaunder high magnetic field (8) How and Why the band structure formed during slow-cooling under magnetic field Original austenite grains (3) the nucleation barrier: (4) RD C---- constants T----absolute temperature R----gas constant Q ---- activation energy for diffusion T ---- absolute temperature s ---- interfacial energy GV ---- Gibbs volume free energy difference between the product and the parent phase High temperature nucleation nucleation on Grain boundaries The magnetic- field induced extra energy difference between the ferrite and the austenite

  12. S pole N pole Magnetic field Ferrite Austenite Part I－ gaunder high magnetic field (9) Dipolar interaction between ferrite nuclei The schematic illustration of nucleation of ferrite at austenite grain boundary triple junctions along magnetic field direction

  13. Temp. RD 860°C Furnace cooling 1°C/min 30min 50m 50m Time Conventional annealing …… Hot-working Annealing Machining Quenching+Tempering Banded structure obtained by conventional annealing • Formation of banded structure in conventional full annealing Hot working history nucleation on grain boundaries due to slow cooling • Eliminating method Normalizing+high temperature tempering Not satisfactory ! Original austenite grain structure by special etching Complicated processes Rapid annealing under high magnetic field(1) General processing procedures

  14. 50m Rapid annealing under high magnetic field 880°C 33min 46°C/min B0=14T Rapid annealing under high magnetic field(2) A potential alternative Y.D. Zhang et al. Adv.Eng. Mater., 2004,6(5):310  RD//MFD  RD//MFD • cooling rate • hardness • ferrite% 1°C/min 24.4% Conventionally: HV164.8~174 (HB 170~179) 23.1% 46 °C /min Rapid: HV192.75~211 (HB 197~210) Optimum hardness for machining : HB160~230 • Advantages of rapid annealing • Effectively avoiding the formation of banded microstructure; • improving microstructure (refining and homogenizing) • simplifying processes by • shortening treatment time • leaving out subsequent additional treatments

  15. Temp. Temp. 200°C 650°C 60min 60min Temp. B0=14T B0=14T 860°C Time Time 20min Water cooling Time Part II－ Tempering Behaviors in High magnetic field • High Temperature Tempering Temp. 650°C • Carbide Precipitation 60min • Matrix Recovery Time Quenching • Low Temperature Tempering Temp. 200°C • Carbide Precipitation 60min Time

  16. 1m 1m Part II－ Tempering Behaviors in High magnetic field • High Temperature Tempering (650°C×60min) • Carbide Precipitation 0T 14T Cementite precipitated during tempering (650C for 60 min)---bright areas Magnetic field effectively prevents cementite from growing directionally along boundaries and shows spheroidization effect.

  17. Schematic illustration of cementite/ferrite interfacial energy • Magnetic field increases the cementite/ferrite interfacial energy Sphere and particle like cementite has minimum interface area, which is advantageous to minimize the final total interfacial energy Spherical cementite has the lowest magnetostrictive strain energy Part II－ Tempering Behaviors in High magnetic field • High Temperature Tempering(650°C×60min) Why magnetic field can influence the morphology and distribution of carbide

  18. 8 7.24% Recovered Tempered at 650°C 7 5.42% 6 5 Percentage, % 4 3 2 5m 5m 1 0 0 14 Induction of the magnetic field, Tesla Part II－ Tempering Behaviors in High magnetic field • High Temperature Tempering(650°C×60min) • Matrix Recovery 0T 14T EBSD maps(blue area are recovered regions) The magnetic field obviously retards the recovery of the matrix Y.D. Zhang et al. Acta. Mater., 52 (2004), p3467-3474

  19. Transformation For a given time duration Precipitation sequence Fe3C Precipitation of transition carbides Martensite -Fe5C2 °C -Fe2C or -Fe2C Part II－ Tempering Behaviors in High magnetic field • Low Temperature Tempering (200°C×60min) For low temperature tempering, the main change in microsturcture is the precipitation of transition carbides. They are metastable at different temperatures and change their form when tempering temperature rises.

  20. 1m 1m -Fe5C2 monoclinic Part II－ Tempering Behaviors in High magnetic field • Low Temperature Tempering (200°C×60min) Diffraction patterns and their indexing • Carbide Precipitation -Fe2C orthorhombic 0T (a) -Fe2C formed during non-magnetic tempering (b) -Fe5C2 formed during magnetic tempering 14T Magnetic field has an obvious effect on changing the precipitation sequence by skipping the precipitation of -Fe2C.

  21. a’ c - Fe C 5 2 h - Fe C a-Fe 0T 2 15 -Fe5C2 -Fe2C Gibbs free energy Gibbs free energy 14T 10 Magnetization, JT-1mol-1 5 0 0 - - 200 - 100 0 0 100 100 200 200 300 300 400 400 Temperature, °C Fe C Carbon content Part II－ Tempering Behaviors in High magnetic field • Low Temperature Tempering (200°C×60min) • Carbide Precipitation Temperature variations of magnetization of -Fe2C, -Fe5C2 and -Fe at 14 T Gibbs free energy vs. carbon concentration for ’ martensite, -Fe5C2 and - Fe2C at 200C

  22. Summary A high magnetic field was applied to the austenite to ferrite transformation and tempering processes in a 42CrMo steel: • The thermodynamic and kinetic effects of the high magnetic field on the austenite to ferrite transformation show that it can obviously increase the amount of the product ferrite and accelerate the transformation by enhancing the Gibbs free energy difference between the parent and product phases. • Magnetic field can effectively prevent the cementite from growing directionally along the plate and twin boundaries and retard the recovery process of the ferrite matrixwhen high temperature tempering is conducted. • In the case of low temperature tempering, magnetic field can change the precipitation sequence of transition carbides, distribution and size of carbides and improve the impact toughness

  23. Acknowledgement This work was supported by the National Science Fund for Distinguished Young Scholars (Grant No. 50325102), the National Natural Science Foundation of China (Grant No.50234020) and the National High Technology Research and Development Program of China (Grant No. 2002AA336010). We also gratefully acknowledge the support obtained in the frame of the Chinese-French Cooperative Research Project (PRA MX00-03) and the Key International Science and Technology Cooperation Program (Grant No. 2003DF000007). The authors would like to thank the High Magnetic Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University, for the access to the magnetic field experiments.

  24. Thank you for your attention!

  25. Part I－ gaunder high magnetic field (5) EBSD Analysis Inverse pole figures of aligned ferrite grains formed in 14T at a cooling rate of 10℃/min.

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