Effect of heat treatment on the microstructure and fatigue behaviour of AISI 4130 steel

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.