Formation Mechanism of AlN Inclusion in High-Nitrogen Stainless Bearing Steels

The existence of angular and hard AlN inclusions would seriously deteriorate the service life of high-nitrogen stainless bearing steels (HNSBSs). In this work, the formation mechanism of AlN inclusion in HNSBSs under as-cast, annealing and austenitizing states was systematically investigated by microstructure observation and thermodynamic, kinetic analyses. The results showed that the concentration product of Al and N could exceed the critical solubility of AlN inclusion at liquidus temperature with the Al content higher than 0.050 wt pct, which led to the formation of AlN inclusions about 1 to 5 mu m (equivalent diameter) in liquid steel. Based on the 'Clyne-Kurz' model, AlN inclusion could form at the solidifying front due to the enrichment of N in the residual liquid steel with the Al content higher than 0.030 wt pct. Besides, the precipitation of Cr2N and the extremely low diffusion coefficient of Al in alpha phase restrained the precipitation of AlN during annealing at 1023 K. However, AlN and AlN-MnS composite inclusions less than 0.6 mu m could precipitate during austenitizing at 1323 K with the Al content higher than 0.006 wt pct, which was the critical Al content to avoid AlN formation in HNSBSs after melting, solidification, and heat treatment processes.

Automated summary

This study systematically investigated the formation mechanism of AlN inclusions in high-nitrogen stainless bearing steels, finding that the concentration product of Al and N exceeding the critical solubility can lead to their formation. The precipitation of Cr2N and the low diffusion coefficient of Al in the alpha phase restrict the precipitation of AlN during annealing, while AlN and AlN-MnS composite inclusions can precipitate during austenitizing.