Carbide-free bainitic steels for rail wheel applications

In South Africa, forged wheels for rail cars are imported but cast wheels are manufactured locally, although recent developments indicate that forged wheels will in future be manufactured locally. The forged wheels are generally manufactured from AAR Class C steel, which is a high carbon, pearlitic steel used for rail application. Railway wheels are a costly component of the railway wagon as they experience wear and damage during application. Improvements in the mechanical properties are thus desired. Against this background, a project is reported on the development of durable rail steel alloys for railway wheel applications. Carbide-free bainite is a novel microstructure comprising bainitic ferrite and retained austenite/martensite but without coarse carbides in the interlath positions. The absence of carbides is achieved through the addition of a high silicon (similar to 2wt%) content to the steel. This carbide-free bainite can achieve high tensile strength (>1000MPa) and toughness (40J, 20 degrees C) as well as good wear resistance. These alloys have found application in areas where high strength, toughness and wear resistance are required, such as in rail steels, and have been deemed the "next generation" of rail steels. Carbide-free experimental alloys were produced in the laboratory and tested for mechanical properties such as hardness, tensile strength and impact toughness. The properties of the laboratory alloys were compared to those of standard Class C alloys used in South Africa to determine their potential use as railway wheel alloys.