Effect of Longitudinal Static Magnetic Field on the Columnar to Equiaxed Transition in Directionally Solidified GCr15 Bearing Steel
Yuan HOU, Zhongming REN(), Jiang WANG, Zhenqiang ZHANG, Xia LI
State Key Laboratory of Advanced Special Steel, Shanghai University, Shanghai 200072, China
Cite this article:
Yuan HOU, Zhongming REN, Jiang WANG, Zhenqiang ZHANG, Xia LI. Effect of Longitudinal Static Magnetic Field on the Columnar to Equiaxed Transition in Directionally Solidified GCr15 Bearing Steel. Acta Metall Sin, 2018, 54(5): 801-808.
Columnar to equiaxed transition (CET) generating a fine-grain structure of GCr15 bearing steel with the homogeneity of the solute contents and the rather small amount of internal defects is often desired in solidification processes. In recent years much attention has been paid to the effect of static magnetic fields on the CET of Al base alloys, Pb-Sn alloys and Ni base superalloys. However, there are few papers to investigate the effect of static magnetic fields on the CET of GCr15 bearing steel. The present work investigates how longitudinal static magnetic fields affect the CET in directionally solidified GCr15 bearing steel. Experimental results show that columnar dendrites degenerate and transform into equiaxed dendrites at the edge of the sample as the longitudinal static magnetic field increases at pulling rate of 20 μm/s and temperature gradient of 104 K/cm. The dendritic morphology without the longitudinal static magnetic field is regular and columnar at pulling rate of 5 and 50 μm/s and temperature gradient of 104 K/cm. When the 4 T longitudinal static magnetic field is applied, the dendritic morphology is still regular and columnar at pulling rate of 50 μm/s and temperature gradient of 104 K/cm. However, the CET occurs at low pulling rate of 5 μm/s and temperature gradient of 104 K/cm. This phenomenon is simultaneously accompanied by more uniformly distributed alloying elements. The corresponding numerical simulations verify that the thermoelectric (TE) magnetic force is induced by the interaction between the longitudinal static magnetic field and TE current. Owing to TE magnetic force localized into the root of the dendrite, the dendritic fragments detach from the primary dendrites. Then the TE magnetic convection induced by TE magnetic force acting on the melt transports the fragments from the interdendritic spacing to the region ahead of columnar dendrites. It can be deduced from above phenomena that the TE magnetic force leads to the CET under the longitudinal static magnetic field.
Fund: Supported by National Natural Science Foundation of China (Nos.U1560202, 51604171 and 51690162), Shanghai Municipal Science and Technology Commission (No.17JC1400602) and United Innovation Program of Shanghai Commercial Aircraft Engine (Nos.AR910 and AR911)
Fig.1 Schematic of the Bridgman solidification apparatus in a superconductor magnet
Fig.2 Longitudinal solidification microstructures near the solid/liquid interface of GCr15 bearing steel specimen at pulling rate of 20 μm/s and temperature gradient of 104 K/cm without (a) and with 1 T (b), 2 T (c) and 5 T (d) longitudinal static magnetic fields (The letters B and G with the arrow indicate the direction of the magnetic field and the temperature gradients, respectively)
Fig.3 Longitudinal solidification microstructures of GCr15 bearing steel specimen at pulling rates of 5 μm/s (a, c) and 50 μm/s (b, d) and temperature gradient of 104 K/cm without (a, b) and with 4 T (c, d) longitudinal static magnetic fields
Fig.4 Radial distribution of the Cr content at 15 mm from the solid/liquid interface in GCr15 bearing steel specimen at pulling rate of 5 μm/s and temperature gradient of 104 K/cm without and with 4 T longitudinal static magnetic field
Parameter
Unit
Value in solid
Value in liquid
Absolute thermoelectric power S
VK-1
-1×10-6
-4×10-6
Dynamic viscosity μ
Pas
-
5.5×10-3
Electrical conductivity σ
Ω-1m-1
8.5×105
7.2×105
Density ρ
kgm-3
7.4×103
7.02×103
Thermal conductivity λ
Wm-1K-1
32.5
31.2
Table 1 Physical properties and parameters in numerical simulation[32,33,34]
Fig.5 Numerical simulation for the thermoelectric (TE) magnetic force in the directionally solidified GCr15 bearing steel at pulling rate of 20 μm/s and temperature gradient of 104 K/cm with 5 T longitudinal static magnetic field (a) geometry of computation domain (b) computed TE current (c) distribution of the computed TE magnetic force acting on a columnar dendrite
Fig.6 Numerical simulation for the TE magnetic effects in the directionally solidified GCr15 bearing steel at the pulling rate of 50 μm/s and temperature gradient of 104 K/cm with 5 T longitudinal static magnetic field (VTEMC—magnitude of computed TE magnetic convection) (a) geometry of computation domain (b) computed TE current (c) computed TE magnetic convection (d) computed TE magnetic convection in the x-y plane at different positions in the mushy zone
Fig.7 Schematic of the columnar to equiaxed transition (CET) in the GCr15 bearing steel during directional solidification without and with the longitudinal static magnetic field (VP—pulling rate, FTE—thermoelectric magnetic force)
Parameter
Unit
Value
Heterogeneous nuclei density N0
m-3
9×108
Supercooling necessary for nucleation ΔTN
K
1.5
Diffusion coefficient D
m2s-1
4.79×10-9
Partition coefficient k
-
0.34
Liquidus slope m
K%-1
-78
Gibbs-Thomson parameter Γ
Km
1.9×10-7
Table 2 Related parameters used in the CET map[31,32,42]
Fig.8 CET map for the GCr15 bearing steel during directional solidification without and with the longitudinal static magnetic field (The solidification morphologies of GCr15 bearing steel specimen under 4 T longitudinal static magnetic field are shown in the the black dotted bordered rectangle)
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