縱向靜磁場對定向凝固GCr15軸承鋼柱狀晶向等軸晶轉(zhuǎn)變的影響

縱向靜磁場對定向凝固GCr15軸承鋼柱狀晶向等軸晶轉(zhuǎn)變的影響Effect of Longitudinal Static Magnetic Field on the columnar to equiaxed transition in Directionally Solidified GCr15 Bearing Steel

進(jìn)行了外加縱向靜磁場下GCr15軸承鋼的定向凝固實(shí)驗(yàn)，考察了縱向靜磁場對試樣凝固過程中柱狀枝晶向等軸枝晶轉(zhuǎn)變 (columnar to equiaxed transition, CET) 的影響。實(shí)驗(yàn)結(jié)果表明，在溫度梯度(104 K/cm)和抽拉速率(20 μm/s)一定時，隨著磁場強(qiáng)度的增加(0 T至5 T)，試樣棒邊緣柱狀枝晶的生長逐漸地遭到破壞，從而發(fā)生不同程度的CET轉(zhuǎn)變；當(dāng)磁場強(qiáng)度和溫度梯度分別為4 T和104 K/cm時，在較低抽拉速率(5 μm/s)下，試樣的凝固組織發(fā)生了完全CET轉(zhuǎn)變；在試樣發(fā)生完全CET轉(zhuǎn)變后，其合金元素分布趨于均勻。結(jié)合數(shù)值模擬，可將這些現(xiàn)象歸結(jié)為縱向靜磁場與熱電流相互作用產(chǎn)生的熱電磁力對枝晶和熔體的作用所致。

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 μm/s 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.

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