The present work investigates how axial static magnetic field affects the solidification structure and the solute distribution in directionally solidified GCr18Mo steel. Experimental results show that grain refinement and the columnar to equiaxed transition is enhanced with the increases in the magnetic field intensity (B) and temperature gradient (G) and the decrease in the growth speed. This phenomenon is simultaneously accompanied by more uniformly distributed alloying elements. The corresponding numerical simulations verify a thermoelectric (TE) magnetic convection pattern in the mushy zone due to the interaction between the magnetic field and TE current. The TE magnetic convection in the liquid should be responsible for the motion of dendrite fragments. The TE magnetic force acting on the dendrite is one of the driving forces trigging fragmentation.Keywords GCr18Mo steel Á Grain refinement Á Axial static magnetic field Á Columnar to equiaxed transition Á Thermoelectric magnetic force Acta Metallurgica Sinica (English Letters) (2018) 31:681-691 https://doi.org/10.1007/s40195-017-0691-3( 0123456789().,-volV) (0123456789().,-volV)

The effects of nitrogen content on the microstructure and the mechanical properties of a cast nickel-base superalloy (CNS) have been investigated experimentally. Experimental results demonstrated that the grain structure of CNS samples was refined by increasing the nitrogen content, but the area percentage of microporosity has been augmented with increased nitrogen content. Increasing the nitrogen content resulted in the morphology evolution of carbide from an acicular or 'Chinese hieroglyphs' type to blocky one, while negligible change of the morphology of γ ′ precipitates was observed. Finally, it was found that the tensile strength has no obvious variation as the nitrogen content increases from 5 to 26 ppm, but it reduces sharply when the nitrogen content is raised to 34 ppm. The elongation decreases gradually with increasing nitrogen content.

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The effects of an axial static magnetic field (ASMF) on the columnar to equiaxed transition (CET) during directionally solidifying GCr18Mo steel were investigated by experiment and numerical simulation. Experimental results show that the CET has been promoted by the increases of the magnetic field intensity and temperature gradient and the decrease of the growth speed. The corresponding numerical simulations verify that a thermoelectric magnetic convection in the melts and a thermoelectric magnetic force acting on the secondary dendrite neck are produced by the interaction between ASMF and a thermoelectric current. Compared the experimental results with the numerical simulations, the mechanism for the CET with ASMF demonstrates that the application of ASMF contributes to the transport of the fragments in the melts and detachment of dendritic side arms. Based on these results, we propose a process window for the CET of GCr18Mo steel with ASMF.

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