Development of Cleaner Steels for Gear Applications

Abstract

Steel components such as bearings, wheel hubs, transmission gears, and shafts that are part of critical systems in transportation equipment are especially impacted by steel cleanliness. Under high contact loads, nonmetallic inclusions in steel can reduce the life of these components. The trend in the automotive industry of increasing power density and downsizing gears and bearings translates to higher loads and stresses on the parts in service. These trends, coupled with the electrification of vehicle powertrains, have brought increased demand for cleaner steels. Carburized gears for automotive transmissions are typically manufactured from low-carbon alloy steel grades, such as SAE 5117, SAE 5120, and SAE 4120. These steels are usually designed with minimum aluminum (Al) and sulfur (S) ranges to guarantee fine grain size and machinability, respectively. However, although aluminum and sulfur have beneficial effects, they also combine with other elements to form inclusions during the steelmaking process, a condition that negatively affects castability during continuous casting of the steel. Therefore, the challenge for steel producers is to develop a stable manufacturing process that results in steel with a low frequency of small inclusions that translates into better performance of the manufactured parts. This research focused on the impact on continuous casting and the resulting steel cleanliness of various levels of aluminum and sulfur in low-carbon alloy steels. Steel cleanliness was evaluated using several techniques, including optical microscopy, scanning electron microscopy, and ultrasonic testing using low and high frequencies. The proper design of steel chemistry in conjunction with optimized steelmaking and a range of complementary test methods has allowed for the serial production of high-quality steels for gearing applications.