Optimization of Melt Treatment for Austenitic Steel Grain Refinement
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HE size and morphology of primary grains are of particular importance for physico-chemical and mechanical properties of austenitic grades stainless steels. A typical cast macrostructure of austenitic grade stainless steels consists of a columnar zone formed by elongated dendrite crystals growing from externally cooled casting surfaces and an inner zone with equiaxed grains. Grain refinement of the cast structure is an important tool for: (i) reducing compositional microsegregation within grains, (ii) decreasing the large-scale macrosegregation of alloying elements within the entire casting, and (iii) control of structure and composition of the grain boundaries. Practical advantages of grainrefined cast structure in ferritic and austenitic steels have been studied.[1,2] In general, a fine-equiaxed grain structure can lead to a more uniform response in heat treatment, reduced anisotropy, and better properties compared to large columnar grains. Refining structure improves both alloy strength and ductility. In highly alloyed steels, regions with fine-equiaxed grain structures have superior homogeneity to the columnar regions with elongated dendrites. Castings with a refined cast grain structure can also exhibit reduced clustering of undesirable features, such as microporosity and
SIMON N. LEKAKH, Research Professor, is with the Missouri University of Science and Technology, Rolla, MO 65409. Contact e-mail: [email protected] JUN GE, Ph.D. Student, is with the University of Alabama at Birmingham, Birmingham, AL 35294. VON RICHARDS and RON O MALLEY, Professors, and JESSICA R. TERBUSH, Ph.D., Sr. Research Specialist, are with the Missouri University of Science and Technology, Rolla, MO 65409. Manuscript submitted August 17, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
nonmetallic inclusions. A small equiaxed grain structure is also preferred because it promotes resistance to hot tearing. Effective grain refinement requires the presence of a nucleating species with a high nucleation activity and nuclei that are sufficient in number and uniformly distributed in the melt. There are a variety of technical approaches that have been developed for cast structure grain refinement. These approaches are based on different principles which can be classified into two groups. The first group of methods can be referred to as physical methods, which mainly employ ‘‘self-stimulated’’ nucleation mechanisms induced by fragmented matrix crystals in magnetic field,[3] laser irradiation,[4] or mechanical mixing of the semisolid alloys.[5] The second group comprises the so called ‘‘chemical’’ methods, which employ special additives (dispersoids) to develop active heterogeneous nuclei for enhancing nucleation.[6] Both ‘‘physical’’ and ‘‘chemical’’ methods facilitate heterogeneous nucleation at low levels of thermal and constitutional melt undercooling. The classical analysis of the heterogeneous nucleation activity of a ‘‘foreign’’ solid in the melt is based on purely geometrical assumptions and predicts favorable conditions for nucleation when the interface
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