Quantification of Structural Transformations during Heat Treatment in Ultra-high Carbon Steels

Carnegie Mellon University, Pittsburgh PA

Investigators

Abstract

Steel is a metallic material with iron and carbon as the primary components. The properties of steel can vary greatly depending on carbon level and processing. Steels with ultra-high carbon levels behave more like ceramics than metals; they are extremely hard and very brittle. These steels are primarily utilized in tool components to shape softer steels and other metals, but could see greater usage if their brittleness can be reduced. To date, brittleness has been reduced by cycles of heating and forming, methods that are challenging to implement on a large production scale. It is possible to decrease brittleness in these materials by simply heating alone; however existing heat treatments are energy intensive and far from optimized. This award supports fundamental research on the structural transformation behavior of ultra-high carbon steels during heating. The research utilizes novel experimental methods to observe changes to the steel structure in real time during heating and to develop new metrics that can be used to optimize processing and properties. These fundamental studies will lead to improved heat treatment strategies that will enable production of ultra-high carbon steels with improved combinations of properties while also reducing energy usage. Results from this research will also introduce materials science undergraduate students to fundamental topics of modern steelmaking and an outreach component will expose middle and high school students to digital image analysis and methods used to characterize materials. Ultra-high carbon steels can exhibit superior hardness and strength compared to other steels of lower carbon contents. However, the high fraction of carbides in these steels render them too brittle for many structural applications. This project seeks to investigate microstructural transformations at elevated temperatures in steels containing ultrahigh carbon contents (1.7-2.3 weight percent) and the impact of these transformations on mechanical properties: toughness, hardness, strength and ductility. This study will apply in-situ confocal scanning laser microscopy and ex-situ electron microscopy to study and quantify microstructural evolution, specifically examining the effects of time, temperature and alloy chemistry on carbide dissolution kinetics and their final distribution in heat treated steels. A comprehensive assessment of important mechanical properties will be correlated to the final microstructural state. These pursuits will result in an analytical model that can accurately describe carbide dissolution and precipitation behavior at temperature ranges employed for heat treatments of ultra-high carbon steels.