Effects of Heat Treatments and Compositional Modification on Carbide Network and Matrix Microstructure in Ultrahigh Carbon Steels

This dissertation investigates microstructure/property relations in ultrahigh carbon steel (UHCS) with the aim of improving toughness while retaining high hardness. Due to high C contents (ranging from 1 to 2 wt%), UHCS exhibit high strength, hardness, and wear resistance. Despite this, applications for UHCS are currently limited because they typically contain a continuous network of proeutectoid cementite that greatly reduces ductility and toughness. In previous research, thermomechanic processing had seen considerable success in breaking up the network. However, the processing is difficult and has thus far seen very limited industrial application. Chemical modification of the steel composition has also seen some success in network break-up, but is still not well understood. There have been relatively few fundamental studies of microstructure evolution in UHCS; studies in the literature typically focused on lower C steels (up to 1 wt% C) or on cast irons (>2.1 wt% C). Thus, this work was undertaken to gain a better understanding of microstructural changes that occur during heat treatment and/or chemical modification of UHCS with a focus on the distribution of proeutectoid cementite within the microstructure. This dissertation is composed of eight chapters. The first chapter presents an introduction to phases found in UHCS, descriptions of research materials used in each chapter, and the hypotheses and objectives guiding the work. The second chapter describes a study of the microstructure found in a 2C-4Cr UHCS before and after an industrial-scale austenitizating heat treatment that increased hardness and toughness and also produced discrete carbide particles in the matrix. The third chapter establishes and demonstrates a metric for measuring connectivity in carbide networks. The fourth chapter describes a series of heat treatments designed to investigate kinetics of spheroidization and coarsening of carbide particles and denuded zones near cementite network branches in 2C-4Cr UHCS. The fifth chapter describes an additional series of heat treatments comparing coarsening kinetics in 2C-1Cr and 2C-4Cr UHCS. Lowering the Cr content caused clustering of cementite particles near grain boundaries, in contrast to the denuded zones observed in the higher Cr UHCS. The fifth chapter details four in situ confocal laser scanning microscopy heat treatments of 2C-4Cr UHCS. The seventh chapter investigates the effects of a 2wt% Nb addition on 2C-4Cr UHCS. The eighth and final chapter summarizes the findings of all the experiments of the previous chapters and revisits the objectives and conclusions.