Bending Fatigue Performance in Low Pressure Carbonitriding SAE 8620 with and without Niobium Addition and Aluminum Reduction

Low pressure carbonitriding followed by high pressure gas quenching and tempering can be performed on mechanical components with the objective of improving mechanical performance, especially with respect to fatigue. An adequate chemical composition of the steel alloy, together with the heat treatment, also contributes to improve the mechanical performance. The presence of elements that contribute for grain refinement in mechanical components are relevant, especially during high temperature heat treatments. Aluminum is frequently added to the steel as a deoxidant, also contributing on grain refinement by forming aluminum nitrides. Another important element for austenite grain refinement is niobium, which is added to the steel exclusively to refine the average grain size, improving the strengthening and toughness. Despite of the contribution of aluminum to refine the average grain size, aluminum as a deoxidant also forms aluminum oxides, which is undesired because they are potential fatigue crack nucleation sites. Reducing the aluminum content and adding niobium in steel alloys (semi-killed alloys), is a strategy to reduce the addition of deoxidizing elements such as aluminum, consequently reducing the formation of aluminum oxides, and still have refined austenitic grains. For this work, two SAE 8620 alloys – as a base material and its modified version with addition of niobium and reduction of aluminum content, were low pressure carbonitrided and evaluated in terms of fatigue strength. Bending fatigue was performed in Brugger specimens to simulate the fillet of a gear tooth and were evaluated through the staircase method. Retained austenite and residual stress profiles, prior austenite grain measurement, and hardness profiles were obtained. The main objective of this work was evaluating if the reduction of aluminum content and addition of niobium has an impact in the fatigue performance of the low pressure carbonitrided steel alloys, considering lower aluminum oxide population, which was calculated through Thermo-Calc.