Abstract
The Heavy-Haul railroad wheels started to use higher wear resistance steels microalloyed with niobium, vanadium, and molybdenum [1]. During continuous cooling, these elements depress the temperature of the pearlite formation, producing smaller interlamellar spacing that increases the hardness of the steel, besides to favor the precipitation hardening through the formation of carbides [2, 3]. Also, they delay the formation of difusional components like pearlite and bainite during isothermal transformation. The effects of these alloy elements on microstructure during isothermal transformation were studied in this work using a Bähr 805A/D dilatometer. Three different compositions of class C railway wheels steels (two microalloyed and one, non microalloyed) were analyzed in temperatures between 200 and 700 °C. The microstructure and hardness for each isothermal treatment were obtained after the experiments. Comparing with non microalloyed steel (7C), the vanadium addition (7V steel) did not affect the beginning of diffusion-controlled reactions (pearlite and bainite), but delayed the end of these reactions, and showed separated bays for pearlite and bainite. The Nb + Mo addition delayed the beginning and the ending of pearlite and bainite formation and also showed distinct bays for them. The delays in diffusion-controlled reactions were more intense in the 7NbMo steel than in 7V steel. The V or Nb + Mo additions decreased the start temperature for martensite formation and increased the start temperature for austenite formation.
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