Crystal-structure changes during tempering of hardened high-strength cast irons with nodular graphite

Abstract

1. The martensite of high-strength cast iron is decomposed by a two-phase mechanism in the first stage of tempering (up to 100°C); the second stage of martensite decomposition occurs at 100–300°C and has the same character as that of high-carbon steel. 2. The decomposition of residual austenite occurs in two stages: The austenite is decomposed with the segregation of α-phase crystals and the carbon enrichment of intact austenite segments in the first stage (100–400°C); the decomposition is completed in the second stage (400–550°C) with the segregation of carbide from the carbon-enriched residual austenite. 3. Decomposition of residual austenite during tempering begins at 100°C in silicon cast iron, and at 200°C in manganese cast iron.