Development of Niobium Microalloyed Steel for Railway Wheel with Pearlitic Bainitic Microstructure

Abstract

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.

Sliding Wear of Rail and Wheel Steels: Effect of Hardness Ratio, Normal Load and Lubrication

“…They are in agreement to those specified by the respective standards for these components, with hardness of 321 HB being at the lower limit of the specification for hardness on the wheel's rolling surface by the AAR standard, a value 10 % distant from the value of 350 HB considered usual/expected for wheels of this class. This lower hardness value was probably due to tempering of the bainite formed on the wheel surface during thermal processing that follows cooling, as well as evidenced in a sample of forged wheel that received thermal treatment with austenitizing temperature of 890 °C and tempering temperature of 590 °C [12]. The same value of 321 HB was measured on the wheel surface, a value 10 % distant from the value of 350 HB considered usual/expected for wheels of this class.…”

“…Cooling can be "online", while the steel is still austenitic immediately after hot rolling, or via "offline" processes by reheating laminated rails [11]. After wheel forging or casting steps, the wheel standard specifies that these must be heat treated by means of accelerated cooling with water spray cooling (normalization) on the wheel's rolling surface, and then receives a second heat treatment (tempering) for stress relief [12,13]. This heat treatment provides reduction in interlamellar spacing of the pearlite and increase in surface hardness.…”

Relação entre a microestrutura e a resistência ao desgaste na pista de rolamento de rodas ferroviárias em suas múltiplas vidas.

“…Para obter uma microestrutura bainítica nas rodas ferroviárias são adicionados ao aço elementos de liga ou é utilizada uma maior taxa de superresfriamento na etapa do tratamento térmico. Segundo Minicucci (2019) as rodas podem ser microligadas com a adição de fortes formadores de carbonetos como vanádio, nióbio ou molibdênio. A composição química deve ser rigorosamente controlada para que não ocorra a formação de martensita.…”