Effect of niobium and molybdenum addition on the wear resistance and the rolling contact fatigue of railway wheels

Abstract

Brazilian heavy-haul railway companies have drawn extensive research in the last years for developing solutions that can make the operation in this area more efficient and safer, such as friction management, the use of new mechanical assemblies, and the use of new materials for wheels and rails. Another solution implemented is increasing the weight transported by train wagons; however, the more weight added, the more deterioration caused to wheels and rails, for example, promoting higher maintenance costs and risks to safety. Thus, the key to extending the life cycle of railroad wheels is to reduce the wear and rolling contact fatigue (RCF) in developing new materials and in manufacturing them. Niobium (Nb) and molybdenum (Mo) are usually added to increase the mechanical strength of pearlitic steels by decreasing the interlayer spacing. This way, comparative twin-disc test for wheel material under dry conditions was performed to verify the wear resistance of a commercial railway wheel (7C) and a newly developed class D railway wheel steel (7 M) with Nb and Mo addition. Following the specifications of the Association of American Railroads (AAR) standard, a twin-disc tribometer with automatic control of load and speed was used. The slip ratio was obtained from the difference between the axis rotation. 7 M steel was observed to present lower mass loss compared with 7C steel. The Magnetic Barkhausen noise analysis showed higher residual stress close to the surface for 7 M steel, which correlated with the work-hardened depth. Such results, therefore, indicate that 7 M steel presented better performance than 7C steel regarding the specific characteristics of the tests. To confirm the feasibility of the wheel material for use in service, further twin-disc tests are proposed for both wheel materials (7 M & 7C) against the same rail material.