Effect of inverse peritectic reaction on microstructural spheroidization in semi-solid state

The semi-solid metal processing involves reheating feedstock slugs that have a fine spherical microstructure obtained normally by continuous casting of round bars under the effect of electromagnetic stirring or SIMA (strain-induced melted activation). These methods require additional production processing to produce the feedstock billets previously. Through the research on the microstructural evaluation of the PR (peritectic reaction) stainless steels and the NPR (non-peritectic reaction) stainless steels a new approach of spheroidization is proposed in this paper. Type 310S stainless steel which melts in gamma --> L + gamma --> L transformation and type 430 stainless steel which melts in delta --> L + delta --> L transformation are employed as NPR steel in this investigation. The melting of the steels commences with the place of low melting point composition and develops towards the place of high melting point composition until the end of melting. on the other hand, the type 304, type 316 and AUS304J3-L stainless steel, which melt in gamma + delta --> L + gamma + delta --> L + delta --> L transformation, are also observed as PR steel in this investigation. The result shows that a perfect and fine spherical structure of ferrite particles surrounded by liquid can be obtained through an inverse peritectic reaction gamma <-> L + delta although it exhibited a strong banded L + delta + gamma structure in the prior half of partial remelting state. The new approach to microstructural spheroidization of semi-solid metal is so called inverse peritectic-induced spheroidization (IPIS). This method offers advantages over traditional semi-solid manufacturing techniques: the need for specialty, added-cost raw material is eliminated, capital cost required for material handling is reduced and the issue of oxide formation on billets during reheating is avoided. (C) 2008 Elsevier B.V. All rights reserved.