Yudong Zhang 1,2, Xiang Zhao 1, Changshu He 1, Weiping Tong 1, Liang. Zuo 1, Jicheng He 1 and Claud. Esling 2 Effects of a High Magnetic Field on the Microstructure Formation in 42CrMo Steel during Solid Phase Transformations Sino-German Workshop on EPM, Oct.11-12, 2004, Shanghai Univ. Shanghai, China 12

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

Introduction Theoretical simulation of the effect of magnetic field on ferrite/austenite and austenite/ferrite phase equilibrium; Morphological features appearing during ferrite to austenite and austenite to ferrite transformation ; Kinetic characteristics of proeutectoid ferrite transformation under magnetic field. 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: Now, the research on these issues is on its initial stage!

Part I －  under high magnetic field (1) Materials: Hot Rolled 42CrMo Steel Chemical composition(wt.%)

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

Part I －  under high magnetic field (3) Heat treatment Temp. 880°C 33min Time 10°C/min Slow cooling Fast cooling Temp. 880°C 33min 46°C/min Temp. Time B 0 =6; 10; 14T 880°C 33min 10°C/min Temp. Time B 0 =14T 880°C 33min 46°C/min Time

Microstructure Part I －  under high magnetic field (4) Heated at 880  C for 33min and cooled at 10  C/min 0T RD 50  m 6T 50  m 10T 50  m 14T RD//MFD

Magnetic field increases the amount of product ferrite Area percentage of ferrite, % Intensity of magnetic field, Tesla Image analysis Image analysis (for the slow cooling group) c Steel b' b a    B 0 =10 T B 0 =0 T Temperature, °C carbon content, wt% Part I －  under high magnetic field (5)

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

Part I －  under high magnetic field (7) t ---- transformation time A, B, C, & E ---- constants T----absolute temperature R----gas constant Q ---- activation energy for diffusion T ---- absolute temperature  ---- interfacial energy  G V ---- 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 When a high magnetic field is applied The magnetic- field induced extra energy difference between the ferrite and the austenite (1) (2) According to Johnson-Mehl equation, The kinetic equation of proeutectoid ferrite transformation from austenite can be expressed as Eq.(1) As a consequence, t for transformation is reduced As a consequence, t for  transformation is reduced

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

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

Hot-working …… AnnealingMachiningQuenching+Tempering  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 Complicated processes Not satisfactory ! Rapid annealing under high magnetic field(1) 50  m Banded structure obtained by conventional annealing 50  m Original austenite grain structure by special etching  RD Conventional annealing 860°C 30min Temp. Time Furnace cooling 1°C/min General processing procedures

  hardness Conventionally: Rapid: Optimum hardness for machining : HV192.75~211 (HB 197~210) HV164.8~174 (HB 170~179) HB160~230   cooling rate 24.4% 23.1% 50  m  RD//MFD   Advantages of rapid annealing Rapid annealing under high magnetic field B 0 =14T 880°C 33min 46°C/min Effectively avoiding the formation of banded microstructure; improving microstructure (refining and homogenizing) simplifying processes by   shortening treatment time   leaving out subsequent additional treatments Rapid annealing under high magnetic field(2)   ferrite% 1°C/min 46 °C /min A potential alternative Y.D. Zhang et al. Adv.Eng. Mater., 2004,6(5):310  RD//MFD

Part II － Tempering Behaviors in High magnetic field Quenching   High Temperature Tempering 650°C 60min Temp. Time Temp. Time 860°C 20min Water cooling B 0 =14T 650°C 60min Temp. Time 200°C 60min Temp. Time B 0 =14T 200°C 60min Temp. Time  Carbide Precipitation  Matrix Recovery   Low Temperature Tempering  Carbide Precipitation

Part II － Tempering Behaviors in High magnetic field   High Temperature Tempering (650°C×60min) 1m1m 1m1m 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.  Carbide Precipitation

Spherical cementite has the lowest magnetostrictive strain 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 Part II － Tempering Behaviors in High magnetic field   High Temperature Tempering(650°C×60min) Schematic illustration of cementite/ferrite interfacial energy Why magnetic field can influence the morphology and distribution of carbide

5m5m 5m5m 0T 14T The magnetic field obviously retards the recovery of the matrix Part II － Tempering Behaviors in High magnetic field 014 Percentage, % Induction of the magnetic field, Tesla Recovered Tempered at 650°C 7.24% 5.42% EBSD maps(blue area are recovered regions)  Matrix Recovery   High Temperature Tempering(650°C×60min) Y.D. Zhang et al. Acta. Mater., 52 (2004), p

Transformation Martensite Precipitation of transition carbides °C°C  -Fe 2 C or  -Fe 2 C  -Fe 5 C 2 Fe 3 C Precipitation sequence For a given time duration 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.

Part II － Tempering Behaviors in High magnetic field   Low Temperature Tempering (200°C×60min)  Carbide Precipitation 1m1m  -Fe 5 C 2 monoclinic 1m1m  -Fe 2 C orthorhombic 0T 14T (a)  -Fe 2 C formed during non-magnetic tempering (b)  -Fe 5 C 2 formed during magnetic tempering Diffraction patterns and their indexing Magnetic field has an obvious effect on changing the precipitation sequence by skipping the precipitation of  -Fe 2 C.

Part II － Tempering Behaviors in High magnetic field   Low Temperature Tempering (200°C×60min)  Carbide Precipitation Temperature variations of magnetization of  -Fe 2 C,  -Fe 5 C 2 and  -Fe at 14 T Gibbs free energy vs. carbon concentration for  ’ martensite,  -Fe 5 C 2 and  - Fe 2 C at 200  C 14T Carbon content 0T Gibbs free energy FeC Gibbs free energy  -Fe 2 C  - 5 C 2 ’’

  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 matrix when 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 Summary A high magnetic field was applied to the austenite to ferrite transformation and tempering processes in a 42CrMo steel:

This work was supported by the National Science Fund for Distinguished Young Scholars (Grant No ), the National Natural Science Foundation of China (Grant No ) 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. Acknowledgement

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