Comparison of Rare Earth Refinement in 4130 and HY100

Solidification based grain refinement has gained wide interest by both researchers and industry. This method provides a route for refinement in processes where thermomechanical approaches are ineffective. Prior research into 4130 and HY100 found very different responses when rare earth additions were made. The 4130 was effectively refined while HY100 showed no response. The cause of this difference was not determined. The research presented in this paper examined heats of 4130 and HY100 with rare earth silicide or EGR additions. Characterization included macrostructure examination, mechanical testing, thermal analysis, and electron microscopy. Refinement was observed only in the treated 4130 heats and corresponded to an increase in the peritectic temperature. The HY100 heats had no changes in macrostructure or solidification reactions. Rare earth containing inclusions of similar compositions were observed in the treated 4130 and HY100 heats. These inclusions appear to be a good fit for austenite based on the 4130 data. It was proposed that the unresponsiveness of HY100 was due to the strong segregation of nickel before the peritectic in that alloy. Nickel promotes austenite, and its segregation may provide a stronger driving force for its formation than the energy barrier reduction caused by the presence of rare earth inclusions.