Nodular Graphite Formation in P/M Products from Cast Iron Swarf Powder and Fe–Si–C Mixed Powders

Abstract

This paper confirms at first that the P/M products from 1.4%C cast iron swarf powder shows nodular graphite, though the products from 2.0%C powder reveals flake graphite. Secondly this paper investigates in detail the variation of graphite morphology and its effects on tensile strength using Fe–Si–C mixed powders. Fe–Si–C mixed powders were prepared from reduced iron, atomized ferrosillicon (17%Si), and graphite powders. Blended mixtures contained 2.5%Si and various C contents in the range of 0–2.7%. The preforms were compacted at 600 MPa. Some of them were sintered at 1423 K in dry H₂ atmosphere for various times up to 43 ks, followed by air cool (1 K/s), the others were powder-forged at about 1100 K, 1200 MPa during a sintering.
The product containing about 1%C, sintered for a long-time or powder-forged, showed high tensile strength (900–1200 MPa), because graphite was nodularized in them. It was not, however, the case of the products containing more than 2%C. In the former, sintering proceeds in the state of all C being dissolved in γ phase; consequently, pores between powder particles spheroidize in a long-time sintering or powder-forging process. During cooling, the supersaturating C atoms crystallize as graphite on the surface of spheroidal pores and fill them up. In the latter, graphite which cannot be dissolved in the γ phase remains along powder particle boundaries. During cooling, the supersaturating C atoms precipitate as graphite onto the residual graphite and/or unspheroidal pores.