The effect of sodium microalloying on the rolling contact fatigue and mechanical properties of hot-deformed powder steels

Rolling bearing rings offer great opportunities for expanding the powder metallurgy production. At present, these opportunities are not fully realized. Hot forging of porous preforms makes it possible to obtain high-density materials for the manufacture of heavy-duty products, in particular rolling bearing rings. The problem of hot-forged bearing ring manufacturing is associated with a large amount of impurities in initial powders, as well as residual one-side open pores and microcracks in the surface layer of parts caused by cooling down of heated porous blanks in such process operations as hot repressing. The paper considers a potential improvement of mechanical properties and  rolling contact endurance of hot-deformed steels with eutectoid composition obtained on the basis of chrome-molybdenum  iron  powder, as well  as unalloyed  iron  powders with  various  impurity  contents due to  microalloying  by sodium. Sodium was  doped as bicarbonate. The  method proposed  previously for producing high-density iron-based chromium-bearing powder material was used in order to reduce the probability of heated porous preform oxidation during hot forging (HF). The method involves obtaining cold-pressed blanks with a porosity of 10–12 % with their sintering in a vacuum furnace and  subsequent HF. 10×10×55 mm prismatic samples were obtained for mechanical test and  structural analysis. Rolling contact endurance was studied using ∅ 26×6 mm cylindrical samples. The tests were carried out by running the flat surfaces of cylindrical samples with balls. Doping Na microadditives can  significantly increase the rolling contact endurance of powder steels compared to unalloyed samples, as well as with respect to check test pieces made of ShKh15 heat-treated steel due to a decrease in austenite grain size, an increase in the quality of interparticle jointing and  a decrease in surface porosity. Carbonaceous powder steels containing the optimum amount of sodium microadditive (0.2  wt.%)  can  be used to  manufacture structural products operating under contact loads.

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