Abstract
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.
Highlights
Industrial needs create a continual drive for improving the strength and ductility of low alloy steels
The treated steels had peritectic reactions higher than the prediction and Baseline sample. This increase likely indicates that the Rare earth (RE) inclusions in the melt acted as nuclei for austenite and allowed the reaction to occur at a higher temperature, which was consistent with expected results for heterogeneous nucleation [36]
A higher solidus than predicted by thermodynamic software has been observed in other work and appears to be related to the original experimental data employed as the basis for a particular thermodynamic xxFFOORRPPEEEERRRREEVVIEIEWW
Summary
Industrial needs create a continual drive for improving the strength and ductility of low alloy steels. Considerable research effort focuses on new steel alloys, but existing ones must be improved. Obtaining improvements in strength and ductility at the same time limit the strengthening strategies available to reducing grain size. Several approaches can be employed limiting grain size: slight alloying element modifications for creating grain pinning precipitates, improved thermomechanical processing, and better nucleation during solidification [1]. Grain pinning and improved thermomechanical processing have been extensively examined but are of minimal use in net shape casting applications. Mechanical deformation of the steel in this application is not possible. Manipulating the nucleation potential of the melt during solidification remains the most logical route
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