Two Inoculation Methods for Refining As-Cast Grain Structure in Austenitic 316L Steel

Two inoculation methods were utilized to introduce titanium nitride (TiN) particles into an AISI 316L steel melt to refine the as-cast grain structure during solidification. The design of the experimental melt treatments and grain refining additions was performed using thermodynamic simulations. The first inoculation method is based on in situ formation of heterogeneous nuclei by TiN co-precipitation on preexisting Mg–Al spinel inclusions. This method included a two-stage melt treatment using spinel forming additions followed by an addition of titanium in the ladle just prior to pouring. The second inoculation method used a newly developed master alloy that contains TiN precipitates which was added in the ladle during furnace tapping. In this method, protective conditions to prevent full dissolution of the TiN nuclei before the onset of solidification were determined by thermodynamic simulations. Grain refinement of the cast macrostructure was observed with both methods. The in situ method provided finer equiaxed grains than the master alloy method, while a thicker zone with columnar grains next to the chill was observed. A scanning electron microscope (SEM) with automated feature analysis was used to quantify the resulting inclusions. The master alloy method eliminated the need for spinel, gave better control of the amount and size of heterogeneous nuclei, and reduced clustering tendency in comparison with the in situ method. However, the in situ formed nuclei method is more effective to refine grain size. The effects of contact angle and nuclei surface geometry on the activity of heterogeneous nucleation were discussed. It is proposed that clustering TiN particles provides numerous sharp, concave corners which favors the heterogeneous nucleation of austenite grains. This is illustrated by SEM images of extracted TiN particles and electron backscatter diffraction analysis of grain orientation.