The effects of a new mechanical surface treatment method, called ultrasonic nanocrystal surface modification (UNSM), on near-surface microstructures and residual stress states as well as on the fatigue behavior of an austenitic steel AISI 304 are investigated and discussed. The results are compared with consequences of other mechanical surface treatment methods such as deep rolling or shot peening.
Prior studies have noted that gas nitriding has a considerable effect for wear resistance. The aim of this paper is to study the influence of gas nitriding time (12, 24, 36 and 48 h) in the wear behaviour of 42CrMo4 steel. It has been assessed by micro hardness, pin-on-disc tribosystem, and SEM through the nitrided layer for each nitriding time. The study relates to the performance of the compound layer and the diffusion layer with respect to adhesive wear. The results were analyzed in terms of the weight lost during wear, for nitrided steel with and without the compound layer, and for untreated steel. It has been observed that wear rate varies as a function of the tests conditions due to the presence of different wear mechanisms. Thus, for short tests conditions wear rate depends on two mechanisms: plastic deformation and adhesive wear, whereas for large tests conditions the mechanisms controlling wear rate are abrasive and oxidative wear. Furthermore, this study contains an analysis of the wear mechanisms of a nitrided part, founded on scanning electron microscopy (SEM) observations of the wear traces at various stages of the evolution of wear. The SEM examination of worn surfaces revealed signatures for the adhesion, abrasion, delamination and tribochemical (oxidative) modes of wear. This is an important issue for future research.
Kurzfassung
Es werden die Auswirkungen kombinierter thermischer und mechanischer Festwalzbehandlungen am Beispiel des Werkstoffs Ck 45 in unterschiedlichen Wärmebehandlungszuständen studiert. Dabei handelt es sich um das Festwalzen bei erhöhten Temperaturen sowie um eine an die Festwalzbehandlung angeschlossene Auslagerungsbehandlung. In beiden Fällen werden im Vergleich mit konventionellen Festwalzbehandlungen Steigerungen der Schwingfestigkeit beobachtet. Ein besonderes Augenmerk wird auf die erzeugten randnahen Eigenspannungszustände sowie Mikrostrukturen und ihre Stabilität bei Ermüdungsbeanspruchung gelegt. Die versagensrelevanten oberflächennahen Werkstoffzustände werden durch die Tiefenverläufe von Eigenspannungen, Interferenzlinienhalbwertsbreiten und die Härte charakterisiert.
In comparison with traditional room temperature processes, deep rolling at elevated temperature leads to further increased fatigue strengths and lifetimes. This is demonstrated using notched bending fatigue samples made of quenched and tempered steel SAE 4140, which have been processed at room temperature and at 300 °C resp. near surface properties were analyzed before and after fatigue loading. It is shown that the positive effect of warm deep rolling is due to stabilized residual stress states which relax only slightly in the course of fatigue loading. The underlying microstructural processes are static as well as dynamic strain aging effects.
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