TWO INOCULATION METHODS FOR REFINING AS-CAST GRAIN STRUCTURE IN AUSTENITIC 316L STEEL

摘要

Grain refinement of the as-cast structure results in a casting that has higher strength, more isotropic properties, less segregation and porosity, better feeding, and a higher resistance to hot tearing. Inoculation techniques to refine the solidification structure are crucial for austenitic steel castings because significant cast structure modification cannot be achieved by heat treatment or mechanical working of near net shaped components. Inoculation introduces or promotes the formation of "foreign" heterogeneous nuclei prior to the beginning of solidification. These heterogeneous nuclei must: (i) be stable at steelmaking temperatures, (ii) be well dispersed throughout the melt, (iii) be active during austenite solidification (i.e. have lattice similarity, minimum interfaces energy), and (iv) have an appropriate size and shape that promotes nucleation. TiN precipitates are effective nuclei for 5-Fe grain refinement [1]; however, they are less potent at nucleating y-Fe [2], A larger lattice disregistry implies an increased difficulty to nucleate austenite phase in comparison to ferrite phase [3].In this article, two methods grain for refinement of cast austenitic 316L stainless steel are investigated. The first method is based on the in-situ formation of TiN by co-precipitation onto preexisting spinel inclusions in the melt [4]. The second method is based on a newly developed master alloy addition that contains preformed TiN nuclei.