An Example of Incidence of Non-Metallic Inclusions in an Alloy Steel

There was a large incidence of surface defects on the crank pins and journals and other areas of crank shafts of a high power automotive engine. The steel used was a Cr-Mo type of nitriding steel. Metallographic observations conclusively proved that the defective areas were entrapment of foreign bodies, resulting from steel making/deoxidizing/teeming stages. The occasionally globular nature of the foreign particles suggested these were formed at the liquid condition of the steel. The ratio of Mn-Si as seen on electron probe microanalysis also suggested the globules high in Mn content might have resulted in deoxidizing stage. Particularly the absence of Fe in some areas in the inclusion was indicative of precipitation deoxidation by ferromanganese/ferrosilicon. The defects apparently did not have time to coalesce and rise up to the top.