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.

Heavy haul transportation (load over 30 tons/axle), as well as the axle load, has been more and more used in Brazil and worldwide. The stress generated in the wheel-rail contact with loads up to 30 tons/axle is around 760 MPa, which causes premature wear and cracks of conventional wheels (AAR (Association of American Railroads) class C). Microalloyed wheels are fundamental on heavy haul transport, whose main function is to combine high hardness, ductility, and yield strength of the material in order to prevent shelling. The main purpose of this research is to develop a new microalloyed wheel steel with niobium addition that meets all the requirements of the AAR class D material with mixed microstructure composed of pearlite and bainite. The 0.71C/0.020Nb steel developed in this study (Nb material) for railroad achieved the standards required for AAR class D in all mechanical properties, with fracture toughness higher than the usual vanadium microalloyed steel used in comparison. The Continuous Cooling Transformation (CCT) diagram showed the presence of bainite even at very low cooling rate, in the range between 0.3 - 2 °C/s. These cooling rates to form bainite are much lower than those observed in other steels with similar composition.