Research Information System
Thesis Type: Postgraduate
Institution Of The Thesis: Ataturk University, Fen Bilimleri Enstitüsü, Makine Mühendisliği Anabilim Dalı, Turkey
Approval Date: 2022
Thesis Language: Turkish
Student: FAİK YILAN
Supervisor: Halim Kovacı
Abstract:
Purpose: The fatigue life of machine parts operating under cyclic loads is significantly reduced due to notches, cracks, and geometric irregularities on their surfaces. For this reason, it is necessary to improve the notch effect, which is effective on the initiation and propagation of fatigue cracks in machine parts. In this study, it is aimed to improve the fatigue strength of notched parts by plasma nitriding process.
Method: Four different radii (r=1, 2, 4 and 8mm) variations were created from AISI 4140 steel material with theoretical stress concentration factors of Kt= 1.63, 1.41, 1.27, and 1.19 respectively and then, plasma nitriding processes were applied at 480℃ using a gas mixture of %25N2-%75H2 for 2 and 9 hours. The structural and phase properties of untreated and plasma nitrided samples were investigated by XRD, SEM and optical microscope. The surface hardness values and thickness of the diffusion layer were determined by microhardness measurements. The fatigue strength of the untreated and plasma-nitrided samples was determined by a rotating bending fatigue test machine.
Findings: As the Kt values increased, the fatigue strength of untreated and plasma nitriding notched parts decreased. All plasma nitrided notched parts showed higher fatigue strength than untreated ones and the level of increase in fatigue strength decreased with the increasing Kt value. It was found that the fatigue properties of plasma nitrided notched parts improved in comparison to untreated parts, depending on the compressive residual stresses on the surface and the diffusion layer thickness.
Results: It was observed that in plasma nitrided notched parts that fatigue crack initiation occurred in the inner parts of the materials and as a result, the fatigue strength increased in comparison to untreated samples. Up to 72% improvement was achieved in the fatigue properties of the samples with the lowest stress concentration factor and nitrided for long periods.