Modification of non-metallic inclusions to improve the fatigue properties of nitriding steels
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameMaster of Engineering
Nitriding processes are widely used to improve the fatigue properties of steel. Fatigue failures in nitriding steels initiate from sub-surface non-metallic inclusions, which are usually oxide inclusions. It is known that duplex oxysulphide inclusions are less harmful to the fatigue properties of steel than oxide inclusions alone, since the sulphide outer phase acts as a buffer to the oxide in the core, and thus reduces the fatigue-initiating stress influence of oxide inclusions. Twelve nitriding steel melts were produced, and non-metallic inclusions, modified by adding CaSi cored wire, were investigated. Since calcium has a high vapour pressure and a low solubility in molten steel, it is difficult to effectively modify non-metallic inclusions by the addition of CaSi cored wire in small furnaces. However, it was found that the addition of CaSi cored wire at a lower temperature was favourable to higher calcium injection yield in a small furnace, and it was also observed that the reaction with liquid steel was less violent, resulting in the effective modification of non-metallic inclusions. Analysis by a scanning electron microscope with energy dispersive X-ray technique showed a proportion of duplex oxysulphide inclusions formed in ingots from 11A, with the remainder being predominantly globular calcium aluminate inclusions. Specimens for direct stress fatigue testing were produced from steels containing both modified and unmodified inclusions, and the fatigue properties were compared. It was found that calcium aluminate inclusions had a more deleterious effect on the fatigue properties of steel than alumina inclusions. In calcium modified steels, at lifetimes of more than 5x10⁵ cycles, fatigue cracks always initiated from calcium aluminate inclusions, which were partly or completely debonded from the matrix. The fatigue properties of the calcium modified steel was found to be inferior to those of the unmodified steel. No duplex oxysulphide inclusions were found among those inclusions which produced fatigue crack initiations, and this indicated that when they were present calcium aluminate inclusions governed the fatigue properties of steel whether or not duplex oxysulphide inclusions were present in the steel. It is inferred that the fatigue properties of steel can be improved if all large calcium aluminate inclusions are modified to duplex oxysulphide inclusions.