Effects of Nitriding and Nitro-carburizing on the Fatigue Properties of Ductile Cast Iron

Abstract

The effects of nitro-carburizing or nitriding treatments on rotating–bending fatigue properties were investigated on four kinds of pearlitic ductile cast iron samples. In this study, we produced alloyed ductile cast iron samples containing V (0.1 %), Al (0.1 %) + Cr (0.1 %) and Al (0.1 %) + V (0.1 %). Tensile and hardness characteristics of the nitro-carburized samples were compared to those of the nitrided ones and as-cast ductile cast irons without alloying element. The iron nitride (γ′-Fe4N) formed on the surfaces of the nitride samples, and the iron complex nitrides (e-Fe2–3N) formed on the surfaces of the nitro-carburized ones. As a result, comparing with the same alloying content, the tensile strength and elongation of the nitro-carburized samples show the higher value than that of nitride ones. The micro-Vickers hardness tends to decrease with increase in distance from the sample surfaces. From the viewpoint of the nitride method, the hardness of nitride samples shows the higher value than that of the nitro-carburizing samples. Also, the maximum micro-Vickers hardness of nitrided layer at distance of 0.03 mm from sample surfaces increased with the increasing practical depth of nitrided layer. Therefore, the fatigue lives existing in the higher stress ranging from 500 to 650 MPa were found to be longer in the order of as-cast, nitro-carburized and nitrided samples. However, the fatigue limit of the lower stress zone existing at 410–450 MPa became no longer significant on endure limit by differences of nitiriding method. This means that the larger nitride layer depth and/or the higher hardness in the vicinity of surface acts to delay the fatigue crack generation on the fatigue characteristics of ductile cast iron samples.