Soldagem de topo com centelhamento de trilhos ferroviários: caracterização microestrutural, propriedades mecânicas e desgaste.

Abstract

In general, heavy haul railways in Brazil have been subjected to operating conditions that demand better performance of the materials, as the speed of operation and the loads increase to comply with the increasing demand for transportation. For joining the rails, the Flash Butt Welding (FBW) process stands out, due to the better quality obtained on the welded joints. However, the properties of the steels may be compromised, because to the undue parameterization of the process introducing weld joint defects and heterogeneities, particularly in the Heat-Affected Zone (HAZ).Experimental reports using dilatometry to simulate the HAZ region and its properties have been published recently, however, there is a lack of detailed studies of the effects of welding parameters in the microstructure of rails. In this work, an experimental matrix was proposed, which aimed to study four parameters of the FBW process (pulse current, number of pulses, upsetting force and carbon equivalent) in both Premium and Super Premium rail steels. The matrix generated eight welding conditions. The results made it possible to understand the relationship between the microstructure and the microhardness maps on the longitudinal section of the welded joint. Tensile tests showed that the welding condition that corresponds the highest heat input presented the lowest yield strength and ultimate tensile strength. Fracture analysis of Premium rails showed ductile fracture, with double-necking in the partial austenitization region, while Super Premium rails showed brittle fracture, except for the more extreme welding condition. Sliding wear tests showed that the partial austenitization region presented more larger loss of mass in relation to the other regions, being the region most affected.In summary, the welding conditions with the lowest heat input showed the best behavior for railroad applications.

Evaluation of the Effect of Heat Input and Cooling Rate of Rail Flash-Butt Welding using Finite Element Method Simulation

“…Thus, if there is a chance of a martensitic transformation (or any acicular microconstituents) these parameters had a higher chance. Also, this set of welding parameters has presented a better mechanical property for the welded joint [6,7,19]. As this weld represents a HAZ size similar to one from the simulations and had a higher cooling rate, these parameters were chosen in order to validate the microstructure (and the eventual presence of acicular microconstituents).…”

“…This pro-eutectoid ferrite is a characteristic of flash-butt welded rails with a carbon content close to the eutectoid composition that undergoes decarburization during the flashing step of the welding. This singularity is well known, mainly in research related to rail welding by FBW [6,7,19,20,26,30].…”

“…Although the welded rail shown in Figures 14 and 15 is more similar to the condition E2 h30 (see Tables 4 and 5), there are no traces of acicular microstructure, especially martensite, along the rail weld. As the martensite would be formed in all simulations with E1 energy, due to the critical cooling rate of the CCT diagram from Ma et al [19] being very low, simulations with E2 energy would only be possible with a heat transfer coefficient h of 30 W/(m 2 K). The microstructures of Figures 14 and 15 are in agreement even with the CCT diagram from Ma et al [19].…”

“…As the martensite would be formed in all simulations with E1 energy, due to the critical cooling rate of the CCT diagram from Ma et al [19] being very low, simulations with E2 energy would only be possible with a heat transfer coefficient h of 30 W/(m 2 K). The microstructures of Figures 14 and 15 are in agreement even with the CCT diagram from Ma et al [19]. At the time that the cooling rate is in agreement with that observed in FBW rail welding [9,20], the simulations should not provide fractions of acicular microconstituents.…”

Influência da microestrutura antes e depois da soldagem de topo por centelhamento de trilhos ferroviários: uma abordagem de fadiga de contato de rolamento, transformações de fases e simulação por elementos finitos.

“…Os resultados mostraram que houve estricção na região de esferoidização e a presença desta microestrutura diminuiu em média 17% do limite de escoamento e 13% do limite de resistência se (carbonetos) e γ) e a região intercrítica de transformação parcial da austeníta (γ) [33]. Nishikawa et al [33], Bauri [52] e Alves et al [53]. Além disso, nenhum destes resultados foi validado por algum método de metalurgia qualitativa (metalografia).…”