Cause Analysis on Surface Pits of Imported E52100 Bearing Steel after Salt Bath Heat Treatment and Grinding

Little black surface pits were found on the surface of minor precision bearing parts which made of E52100 bearing steel disk with diameter of 1.6 mm and imported from USA after neutral salt bath heat treatment and finishing mill, and there was no regularity for number and position of the surface pits. Formation reason for the pits was studied by means of optical microscope, scanning electron microscope and X-ray energy dispersive spectrometer. The results show that there were many abnormal granular particles with different sizes, shapes and locations in material of the bearing parts. These particles were oxide inclusions which had high oxygen content and easy to fall off from the substrate and form the surface pits because of its high hardness and brittlement during finishing mill process. These nonmetallic oxide inclusions were innate in the imported E52100 bearing steel and it had nothing to do with the salt bath heat treatment.