Abstract
Abstract This research shows two new microalloyed steels with niobium (Nb) for railroad wheels (Nb1 and Nb2) applied on heavy haul transportation (load over 30 tons/axle). The steels have mixed microstructure composed of pearlite and bainite (finished wheel rim section), and another one composed of martensite with bainite (black wheel section removed on machining) obtained by continuous cooling quenching heat treatment followed by tempering. The bainitic and martensitic microstructure are sensitive to the temper embrittlement phenomenon; therefore, knowing the embrittlement temperature range of these steels is mandatory in order to avoid such an event in the tempering process. Charpy specimens were extracted from the rim of railroad wheels, austenitized (860 oC), quenched in water for four minutes, and tempered at different temperatures to obtain the embrittlement curve. Three materials were compared: AAR (Association of American Railroads) Class C steel with pearlitic microstructure (not sensitive to embrittlement), Nb1, and Nb2 microalloyed steels with martensitic, bainitic, and pearlitic microstructures. The microalloyed steels have niobium (Nb), molybdenum (Mo), and silicon (Si) addition. The effect of Si content and the microstructure (analyzed by Scanning Electronic Microscope (SEM)), regarding the embrittlement phenomenon, was also studied. The microstructure of Nb2 steel was analyzed by with Transmission Electronic Microscope (TEM) for improving its characterization.
Highlights
There has been a significant development in steel for railroad wheels over the last 10 years, especially with the introduction of microalloyed steel wheels, called class D wheels, by the AAR (Association of American Railroads) in 2011
The average Charpy impact test result increases continuously with tempering temperature, which indicates the presence of martensite and/or bainite
Difference of 20 °C refers to the Si content addition to the Nb1 material, which delays the embrittlement phenomenon by stabilizing Ɛ-carbide
Summary
There has been a significant development in steel for railroad wheels over the last 10 years, especially with the introduction of microalloyed steel wheels, called class D wheels, by the AAR (Association of American Railroads) in 2011. The pearlitic microstructure (standard for railroad wheels) provides its resistance limit. For this reason, other options in terms of wheel microstructure, such as bainite, have become necessary1;2. Bainite has higher mechanical resistance associated with higher toughness than those of pearlite, which are two important characteristics for wheel material[4]. Wear tests show that microstructures consist of ductile and brittle phases; structures with more than one phase, such as pearlitic and bainitic, are more efficient in abrasion resistance than the monophasic ones, like martensitic[5]. In twin-disk wear tests carried out with niobium microalloyed railway wheels with pearlitic and bainitic microstructures, bainite proved to be more
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