The Size and Density of Intergranular Carbide Plates in 1020 HRS and Their Role during Brittle Fracture Initiation: A Preliminary Report

摘要

The purpose of this investigation is to determine the effect of microstructure on the dynamic and quasi-static fracture initiation in a plain carbon steel. For this purpose experiments were conducted on an AISI 1020 steel with either of two microstructures, the first a normalized microstructure, and the second an isothermally transformed (at 550 C) ferritic-pearlitic microstructure. An important distinction between these microstructures is the resulting density and size of grain boundary carbides that are found to initiate cleavage cracks. The high rate fracture toughness testing method employed was developed only recently. Tests were conducted over the temperature range -130C to 65C. The fracture toughness tests were complemented by high rate and quasi-static plasticity tests to provide necessary data on the mechanical behavior of both microstructure over the same temperature range and over approximately the same range of strain rates. Results show that the isothermally transformed microstructure is both stronger and tougher than the normalized microstructure at temperatures below about 18C where cleavage occurs. This appears to be due to the reduction in density of grain boundary carbides. At temperatures above 18C the fracture mode is fibrous in both microstructures and the toughness of the two microstructures are comparable. The quasi-static strength of the isothermally transformed microstructure is higher but the strength at higher strain rates is a little lower at temperatures above 18C. (Author)