Microstructures and properties of carbidic austempered ductile Iron containing Fe3C particles and superfine ausferrite

Abstract

A new type heat treatment process comprising super-high temperature pretreatment and austempering treatment (S&A treatment) was used to process carbidic austempered ductile iron (CADI). The results showed that the netlike eutectic carbides were significantly reduced after super-high temperature pretreatment. Due to the increase in supercooling during super-high temperature pretreatment cooling, the inter-laminar spacing of pearlite was reduced from 300 nm to 100 nm. When the ductile iron with superfine pearlite was reheated, a large amount of Fe3C particles were retained inside the prior austenite grains. After austempering, a superfine ausferrite matrix with a length of 1–3 μm and a thickness of 50 nm was obtained. The main reason for this refinement is the increased number of nucleation-sites for the austenite grains and the retardation-effect of Cr on their growth. TEM results indicated that a possible orientation relationship between bainitic ferrite with a cubic lattice and precipitated Fe3C particles with a orthorhombic lattice is (011¯)α-Fe//(210)Fe3C, and their boundaries are coherent boundaries. The CADI obtained by S&A treatment has an impact toughness 120% higher than the traditional CADI without sacrificing hardness, and has excellent wear resistance under high wear load. Keywords: CADI, Super-high temperature pretreatment, Superfine ausferrite, Precipitates, Impact toughness