Cleanliness Improvement and Microstructure Refinement of As-Cast High-Nitrogen Stainless Bearing Steel by Magnesium Treatment

Abstract

The influence of magnesium treatment on cleanliness and microstructure characteristics of as-cast high-nitrogen stainless bearing steel (HNSBS) was systematically investigated. Results manifested that as the magnesium content increased from 0.0003 to 0.0054 wt pct, the oxygen and sulfur contents in steel, along with the number density and average size of inclusions significantly decreased due to the strong thermodynamic affinity and the removal of inclusions. Meanwhile, the inclusion evolution processes were Al₂O₃ → MgO·Al₂O₃ → MgO and MnS → MgS + Mg₃N₂, and the magnesium content in HNSBS should not exceed 0.0047 wt pct to prevent the formation of deleterious Mg₃N₂ inclusion. Additionally, the secondary dendrite spacing and the area fraction of precipitates (M₂₃(C, N)₆ and M₂(C, N)) at the 1/2 radius of ingots decreased form 83 ± 25 μm and 17 pct to 63 ± 16 μm and 12 pct, respectively. The dendrite structure was refined owing to the increase in effective nucleation sites for γ-Fe provided by MgO·Al₂O₃ and MgS inclusions, as well as the enrichment of magnesium in the liquid phase at solidifying front. The area fraction and size of precipitates were reduced due to the decrease of chromium activity. The finer and more dispersed precipitates was attributed to the reduction of growth space and increase in effective nucleation sites. This work provides theoretical guidance for preventing the formation of deleterious inclusions (especially for nitrides) in high-nitrogen alloy systems and refining the microstructure of alloy systems containing M₂₃(C, N)₆ and M₂(C, N) precipitates.