Effect of Rare Earth Master Alloys on 4130

Abstract

This paper documents a series of experimental 4130 heats that had additions of rare earth master alloys with different rare earth oxide inclusions. Once melted, the heats were poured into keel block castings for mechanical testing and metallographic examination. Additionally, a single-thermocouple thermal analysis system was employed to examine the solidification reactions of each heat. Several interesting observations were made. First, the mechanical properties and solidification structure of the heats were similar, contrary to earlier work in 4130. Thermal analysis noted that the liquidus and peritectic did not shift with the master alloy additions. Inclusion characterization found that the inclusions in the final castings were significantly different from the ones in the master alloys. These inclusions contained low or no sulfur in them. Based on this, the author concludes that sulfur-containing rare earth inclusions are the likely nuclei in steels. This work identifies several important factors in the grain refinement of 4130. Not all rare earth inclusions cause refinement, supporting the conclusion by many that specific rare earth inclusions refine a steel’s structure by acting as heterogeneous nuclei. Sulfur-containing inclusions may be the compounds that are acting in this role. Additionally, shifts in the peritectic temperature appear related to the occurrence of refinement. No refinement in this work occurred, and no peritectic shift happened either. Other work in the literature observed an increase in the peritectic temperature and the onset of refinement. The peritectic reaction appears to play a role in steel solidification refinement.