Evaluation of surface and volume damages under HCF and VHCF loading for quenched and tempered steels

Achieving climate neutrality requires new advancements and adaptions in various engineering sectors, especially in the field of the steel industry. While recycling steels is a promising approach to reduce CO 2 emissions and production costs, the quality of recycled steels may deteriorate over time because of impurity enrichment. Apart from that, most steel components are prematurely recycled before the end of their structural integrity is reached. Re-use, before recycling, provides an attractive approach, particularly for the quenched and tempered SAE 4140 steel considered within this research, as recycling in electric arc furnaces consumes substantial energy. Considering the High-Cycle-Fatigue loading, fatigue damage develops predominantly at surface irregularities, whereas damage in the Very-High-Cycle-Fatigue regime is mainly shifted into the volume of the material. To evaluate the re-use potential, specimens were primarily loaded at defined stress amplitudes in the High-Cycle-Fatigue regime. In order to determine appropriate load levels, the lifetime prediction method StressLife was used. It enables the generation of a trend S-N curve with only three fatigue tests and is based on different empirical relations and a combination of non-destructive testing methods with conventional destructive fatigue testing. These methods provide information regarding the material response during fatigue experiments. In addition to that, the extent of fatigue damage induced is determined analytically in terms of optical- and scanning electron microscopy. The primarily loaded specimens were reconditioned to remove surface damages and then subjected to secondary loading with stress amplitudes lower than the primary loading amplitudes. Based on these insights the re-use potential can be estimated.