Heat Treatments to Develop High-Strength Ferritic Ductile Iron
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Copyright 2020 American Foundry Society https://doi.org/10.1007/s40962-020-00489-0
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 grainrefined alloys displayed nil ductility temperatures below room temperature, whereas that of the 4% Si alloy was above room temperature.
Introduction
It is well known2 that strength in wrought ferrous alloys can be increased through grain refinement. It is proposed that grain refinement might also enhance the properties of ferritic ductile iron without increasing the DBTT temperature at the same time. In this investigation, novel methods of heat treatment have been employed to produce grain refinement in ferritic ductile iron.
Ductile iron castings with a ferritic (BCC) matrix exhibit high-energy fracture in a ductile tearing mode. However, at lower temperatures and/or high loading rates, they are subject to low-energy, brittle fracture. For the traditional ferritic (pearlite-free) grades with low to intermediate silicon contents, the ductile-to-brittle transition in notched impact testing occurs below room temperature. However, when the application requires increased strength, and stronger ductile iron grades are needed, the transition temperature rises well above room temperature, toughness suffers, and brittle fracture can occur. This is true for both the pearlitic grades and the high-Si, SSSF ductile iron grades of EN1563.1 As a consequence, when catastrophic brittle failure is possible and can cause danger to life, the lower-Si ferritic grades of ductile iron are chosen due to their low ductile-to-brittle transition temperature (DBTT).
This paper is an invited submission to IJMC selected from presentations at the 6th Keith Millis on Ductile Iron held October 23–26, 2018 at the Sonesta Resort, Hilton Head Island, SC. It is published in the IJMC by permission of AFS and DIS (Ductile Iron Society).
International Journal of Metalcasting
Keywords: ferritic ductile iron, heat treatment, grain refinement, hardness, microstructure, tensile strength, Charpy impact toughness, nil ductility,
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