Heat Treatments to Develop High-Strength Ferritic Ductile Iron

Abstract

The influence of grain refinement on the mechanical properties of ferritic ductile iron was investigated. Alloys containing nominally 2%, 3%, and 4% silicon were produced for this study. Various heat treatments were employed to produce a fine-grained ferritic structure. Hardness, tensile, and Charpy impact properties were determined. The three alloys were austempered at 385 °C (725 °F). The austempered materials were then annealed between 540 and 732 °C (1000–1350 °F) to produce a fine-grained, 100% ferrite matrix. Intercritical heat treatments were also performed to produce a fine-grained acicular microstructure. Samples were both air-cooled and oil-quenched from the intercritical temperature and subsequently annealed to ferritize the material and enhance toughness. All three alloys were also tested in the fully annealed ferritic condition. All approaches to refine ferrite grain size were successful; the heat-treated alloys developed strengths comparable to the fully annealed SSSF 4%Si alloy. Based on Charpy impact testing, the grain-refined alloys displayed nil ductility temperatures below room temperature, whereas that of the 4% Si alloy was above room temperature.