Research On Fatigue Life Of Helical Springs Of Railway Freight Wagons

Helical compression springs are critical components in railway freight wagons as they support and damp whole wagons. In recent years as freight wagons are required to run at increased speed and loading capacity, loading conditions on freight wagons have taken great changes, which therefore put forward higher requirements on bogie helical compression springs. The premature break of springs not only poses a great threat to the safe operation of railway freight wagons, but also causes enormous direct and indirect economic losses.Statistics have shown that fatigue break accounts for 80% to 90% of break failure for springs. Therefore it is very important to do research on fatigue life of springs. Factors affecting fatigue life of springs are very complicated, including design, raw materials quality, heat treatment, spring surface quality and physical dimensions.The paper presents extensive experimental and theoretical research and analysis on significant effect of 60Si2CrVAT raw materials on fatigue life of springs in chemical composition, segregation, non-metallic inclusions, and local residual spheroidal carbide of larger grain and quantity in structure. Research in hot rolled temperature of spring steel, heat treatment curve and other factors in heat treatment process has demonstrated that in different current common spring production processes, residual heat quenching and reheat quenching could cause change in fatigue life.By mathematic stress model and extensive fatigue tests on materials, effect of rolled physical dimensions on spring fatigue life is demonstrated. The physical dimensions of contact between spring supporting coil and working coil are especially important, as it is the weak point in spring fatigue life. In load application process, if the contact between supporting coil and working coil is longer linear, the contact stress will be reduced. In the paper, effect of shot blast treatment on spring fatigue life is also shown. After the shot blast, a cold hardening layer (case penning layer) is formed, where a large residual compressive stress exist. Therefore when the spring is loaded variably, such residual compressive stress can offset a certain amount of tensile stress in variable load, favorably affecting fatigue life of springs.Based on achievements made in the above researches, springs quality has been improved distinctively by reasonable selection of raw materials, change of traditional production dimensions of springs, adoption of complete quenching heat treatment process and shot blast treatment. All springs produced in such way have met standard requirements in fatigue life. All springs fitted to 7200 freight wagons can completely meet operation requirements of freight wagons at higher speed, and no quality issue has appeared. Therefore it can be concluded that all the above achievements can be applied to springs to freight wagons with increased speed.