Oxide Accelerating Primary Ferrite Nucleation of Austenitic Stainless Steel Weldment

在本研究中，我们评价了Ti2O_3、(Ti，Al)_2O_3和MgAl_2O_4对TiN形成的促进作用，以促进初生铁素体的形核。Ti_2O_3、(Ti，Al)_2O_3和MgAl_2O_4均能促进TiN的形成，使δ铁素体形核，其中添加Al_2O_4的样品细化δ-铁素体的效果最好。Ti2O_3和(Ti，Al)_2O_3的晶格失配率较低，分别为0.6%和0.6~8.1%，取向关系为{0001}氧化物//{111}TiN和[10-10]氧化物//[110]TiN，可以作为TiN的非均相晶核。尖晶石型MgAl2O4与TiN具有完全平行的取向关系，并导致每个晶面与TiN的失配4.8%。与Ti2O-TiN和(Ti，Al)_2O_3-TiN形成75次/mm~2相比，MgAl_2O_4能显著促进TiN的形成，形成面密度在250次/mm~2以上。TiN和δ铁氧体之间存在Baker-Nutting(B-N)取向关系。研究了初生铁素体细化对奥氏体形核的影响。加入MgAl2O4-TiN后，初生铁素体明显细化，具有无规取向和多重取向，与温度梯度关系不大。残余铁素体形态呈蠕虫状，呈细小弥散分布。奥氏体形变温度高于固相线，奥氏体形核并独立长大为初生铁素体，奥氏体呈柱状组织。

In this study, we evaluate the accelerating effect of Ti2O3, (Ti,Al)2O3, and MgAl2O4 on TiN formation to promote primary ferrite nucleation. Ti2O3, (Ti,Al)2O3, and MgAl2O4 could all accelerate TiN formation to nucleate δ-ferrite, the specimen with MgAl2O4 showed the optimal effect of δ-ferrite refinement. Ti2O3 and (Ti,Al)2O3 could act as heterogeneous nuclei for TiN because low lattice misfits of 0.6% and 0.6~8.1% could be reached as for Ti2O3 and (Ti,Al)2O3 with orientation relationship of {0001}oxide//{111}TiN and [10-10] oxide//[110]TiN. Spinel type MgAl2O4 was identified to have a fully paralleled orientation relationship with TiN and lead to a 4.8% misfit with TiN in each crystal plane. MgAl2O4 could significantly accelerate TiN formation to form MgAl2O4-TiN with a higher area density of above 250 counts/mm2 compared to 75 counts/mm2 regarding Ti2O3-TiN and (Ti,Al)2O3-TiN. Baker-Nutting (B-N) orientation relationship was confirmed between TiN and δ-ferrite. The formed TiN could nucleate δ-ferrite.The effect of primary ferrite refinement on austenite formation was investigated. When MgAl2O4-TiN was introduced, the primary ferrite was greatly refined with random and multiple orientations, having little relationship with the temperature gradient. The morphology of residual ferrite showed vermicular morphology with fine-dispersed character. The austenite shows columnar morphology because the formation temperature of austenite is higher than the solidus and the austenite nucleated and grew independently to primary ferrite.