Abstract
AISI 4130 steel is widely used in the aircraft industry, mainly for frame structures. It is a medium carbon low-alloyed (25CrMo4) steel predetermined for precipitation of carbides in the microstructure during heat treatment. Six different heat treatments were applied to study the effect of the resulting microstructure (ferritic-pearlitic in a normalised state or martensitic with carbides in a hardened state) on the fatigue life of AISI 4130. Tension-tension cycling under force control was used to determine the S-N curve of flat specimens with a central circular notch. The best fatigue properties were achieved for the material state HT125, i.e., hardened & tempered to 125 ksi (863 MPa), exhibiting the maximum stress at a fatigue limit of 225 MPa, which is approximately 5-15 % better compared to the rest of the applied heat treatments. Microstructural investigations using scanning electron microscopy explained the acquired fatigue life results of AISI 4130 concerning different heat-treated conditions.

Key words
medium carbon steel, heat treatment, fatigue life, microstructure, scanning electron microscopy

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