Influence of Sodium Microalloying on Rolling Contact Endurance and Mechanical Properties of Hot-Deformed Powder Steels

Rolling bearing rings comprise a large reserve for expanding powder metallurgy production. This reserve is being incompletely implemented. The hot forging of porous preforms makes it possible to produce high-density materials for the fabrication heavy loaded wares, in particular, rolling bearing rings. The problem of fabricating hot-forged bearing rings is associated with the presence of a large amount of impurities in the initial powders, as well as with residual one-side open pores and microcracks in the surface layer of parts caused by cooling a heated porous preform when performing operations of a hot repressing process. The possibility of improving the mechanical properties and rolling contact endurance of hot-forged steels of a eutectoid composition fabricated based on the chromium–molybdenum iron powder, as well as unalloyed iron powders with different impurity contents due to sodium microalloying, is considered. Sodium is introduced in the form of bicarbonate. To decrease the oxidation probability of a heated porous preform during hot forging (HF), the previously proposed method of fabricating the high-density iron-based chromium-containing powder material is used. This method foresees the formation of cold-pressed preforms with porosity of 10–12%, their sintering in a vacuum furnace, and subsequent HF. To determine the mechanical properties and perform the structural analysis, prismatic samples 10 × 10 × 55 mm in size are fabricated. The rolling contact endurance is investigated using cylindrical samples ∅26 × 6 mm. The tests are performed by rolling flat surfaces of cylindrical samples by balls. The introduction of Na microadditives makes it possible to substantially increase the rolling contact endurance of powder steels when compared with unalloyed samples, as well as with respect to test samples made of thermally treated steel ShKh15, which is caused by a decrease in the austenite grain size, increase in quality of interparticle intergrowth, and decrease in surface porosity. Carbon powder steels containing the optimal amount of sodium microadditive (0.2 wt %) can be used when fabricating structural products operating under contact loads.