Influence of the grain size of rolled sparingly alloyed steel on the quality of manufacture of gears

A rational set of mechanical properties and high hardenability of the diffusion layer ensure the use of sparingly alloyed steels for large‑module gears. The technology currently used by metallurgical plants for the production of hot‑rolled carburized steel does not ensure a stable production of a fine‑grained structure in products. The results of studies of rolled samples of sparingly alloyed structural steel 21KhGNMBA, heat‑treated according to different technological schemes are given. It has been established that at the same austenitic grain of the metal, rolled products from two suppliers differ significantly in the size of the actual grain. Coarse‑grained microstructure structure is explained by peculiarities of heat treatment of rolled products with long delayed cooling at incomplete annealing without recrystallization. It was obtained that the initial coarse‑grained structure of rolled products increased the tendency to grain aggregation of products during subsequent high‑temperature treatment, which led to an increase in the hardenability of steel and increased kinematic error of gears made of this rolled product.