The effect of heat strengthening on the strength characteristics of freight bogie side frames made of low-alloy cast steels

Freight bogie side frames operate under conditions of increased dynamic loads, which may exceed the specified loads. Side frames are made of 20GL, 20GTL, and 20GFL steels and must be heat-treated—either normalizing or normalizing with first-grade annealing. After normalization, the steel must have a grain size of at least number 8 according to GOST 5639-82 and a ferritic-pearlite microstructure. The potential of 20GL and 20GTL steels, determined by their chemical composition, has been exhausted and does not allow for increasing their strength. The inadequacy of the cast side frame’s load-bearing capacity for high operational loads can be addressed by heat-hardening 20GFL steel. In this steel, finely dispersed vanadium carbide, precipitated from austenite during cooling, promotes structural refinement. The greatest structural refinement of 20GFL steel occurs during martempering (quenching and high-temperature tempering). However, the developers limited themselves to normalization, fearing warpage after water quenching and tempering.A characteristic of cooling low-carbon, low-alloy steel is the need for a high quenching rate, which is achieved using a water-air mixture. It has been established that as the cooling rate increases in the martensitic range, the probability of fracture of the hardened parts first increases and then decreases to zero. Self-tempering is necessary to prevent quench cracks. The use of a new heat-strengthening technology (quenching and self-tempering) on cast side frames made of 20GFL steel, pre-heat treated (normalized), allows for a reduction in grain size (9-10 points) and the achievement of a sorbite-tempered structure, increasing strength without reducing cold resistance.This study was supported by the Ministry of Industry and Trade of the Russian Federation (GK No. 0000000002018Q8T0002).