Made available in DSpace on 2014-10-09T12:28:34Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:56:03Z (GMT). No. of bitstreams: 0 / Tese (Doutoramento) / IEA/T / Universidade Karlsruhe, Alemanha

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The effect of surface finish on fatigue limit of two types of austenitic stainless steels (AISI 304L and AISI 316L) has been investigated. Fatigue specimens having two different surface conditions were obtained by changing the final cutting condition; annealing was performed to separate the residual stress effects from surface roughness. Electropolished samples were tested as a reference for each material. A generic mechanistic model for short fatigue crack propagation proposed by Navarroand Rios (N-R model) was implemented to assess its suitability for predicting the fatigue behaviour of specimens with various controlled surface conditions, obtained by machining. The surface/material properties required to implement this model were obtained by electron backscatter diffraction (EBSD), surface profilometry, hardness testing and X-ray diffraction residual stress measurement. The fatigue limits were determined using rotating-bending by means of the staircase method. The fatigue limits predicted by the N-R fatigue model were compared with the results of the fatigue tests. There was no agreement between the prediction and observations, indicating that the original form of the N-R model is not appropriate for austenitic stainless steels. In AISI 304L, the surface residual stresses are the dominant parameter, allowing prediction of the effects of machining on fatigue resistance while, the surface roughness developed by machining has no significant effect. In AISI 316L, the effect of surface roughness is found to be negligible, with a weaker effect of surface residual stress than has been observed for AISI 304L. Crack nuclei in run-out (>107 cycles) fatigue tests were observed to arrest at twins and martensite packets, developed by fatigue in AISI 316L and AISI 304L, respectively. Good agreement with experiments was achieved by using a modification to the fatigue model, which takes account of the observed effect of the plastic deformation on the microstructure.

The austenitic stainless steel 316L has been compared to duplex stainless steels to be able to highlight a choice of material for manufacturing of spare tanks used for cooling water at nuclear power stations on the Swedish west coast. In this report 316L and the duplex stainless steels 2205, 2304 and LDX 2404 have been compared according to corrosion resistance, strength, manufacturing aspects and prices. The steels arranged by increasing corrosion resistance: 316L < 2304 < LDX 2404 < 2205. The steels arranged by increasing strength (considering the thickness of the plates needed for construction): 316L < 2304 < LDX 2404 and 2205. The steels arranged by increasing price/tank: 2304 < LDX 2404 < 2205 < 316L. One of the duplex stainless steels is recommended rather than the austenitic stainless steel 316L. In terms of price 2304 is preferable to 2205 and LDX 2404. When it comes to corrosion resistance 2205 is superior to 2304 and can sometimes be considered as unnecessary good and therefore not relevant for this application.

Stainless steels are high-alloyed, usually with multiple components and often also dual matrix phases, as for duplex stainless steels. This make predictions and calculations of alloying effects on equilibria and transformations complicated. Computational thermodynamics has emerged as an indispensable tool for calculations within these complex systems with predictions of equilibria and precipitation of phases. This thesis offers examples illustrating how computational methods can be applied both to thermodynamics, kinetics and coarsening of stainless steels in order to predict microstructure and, to some extent, also properties. The performance of a current state-of-the-art commercial thermodynamic database was also explored and strengths and weaknesses highlighted. / <p>QC 20130429</p>

Slurry coating technique, as one of the most popular deposition methods, is widely used to produce various material coatings. This method includes two processes: spraying, brushing or dipping of slurry, and sintering heat treatment of the coated specimen. Superalloys and stainless steels are the most common materials used as either coating materials or substrate materials because of their excellent corrosion, wear, high-temperature and mechanical properties. This research is aimed at investigating the influence of the sintering heat treatment in the slurry coating process developed at Kennametal Stellite Inc. on the microstructure, hardness and wear behavior of superalloys and stainless steels. Low-carbon Stellite 22, cobalt-based Tribaloy T-400C, martensitic AISI 420 and AISI 440C stainless steels are studied in this research. The microstructure, hardness and wear resistance of these alloys before and after the heat treatment are investigated, stressing the influence of the heat treatment on these material characteristics. The hardness and wear tested are conducted on these alloys at both room temperature and at elevated temperatures. The worn surfaces of each specimen are analyzed using a Scanning Electron Microscope (SEM) with backscatter electron imaging (BEI) and energy dispersive X ray (EDX) spectrum. It is demonstrated that the heat treatment alters the microstructures of these alloys differently; it increases the hardness but affects the wear resistance more complexly than hardness. At room temperature, the wear resistance of these alloys is governed by their microstructures. However, at high temperatures, oxidation, resulting in formation of oxide films on the specimen surface, influences the wear resistance significantly.

Des industriels du domaine aéronautique se sont regroupés autour du projet MEKINOX (Mécanique Inoxydable) visant à développer l'utilisation de différentes nuances d'aciers inoxydables en raison de leur haute résistance mécanique et de leur résistance à la corrosion. Cependant ces aciers sont réputés sensibles au frottement. Dans ce contexte, ce travail de thèse est dédié dans un premier temps à l'étude de l'effet des différents traitements thermiques et de surface sur la microstructure des aciers inoxydables visant à améliorer leur aptitude au frottement. Nous avons mis en évidence différents mécanismes de durcissement : par précipitation, par changement de phase et par solution solide. Dans un deuxième temps, nous avons mis en place une démarche expérimentale permettant de comparer la réponse des différents couples de matériaux sous contact roulant et glissant extrêmes. Ces essais nous ont permis de caractériser l'évolution du coefficient de traction et de classer les différents états métallurgiques selon leur résistance à l'usure. Dans un troisième temps, nous avons caractérisé les différents types d'endommagements se produisant en surface et en sous-couche. Enfin, l'utilisation de l'EBSD nous a permis caractériser la déformation plastique en sous couche et de tenter de corréler la valeur de la densité de HAGB (Angles à forte désorientation) avec la résistance à l'usure des différents états métallurgiques des aciers étudiés. / Aeronautics industrials gathered thought MEKINOX project ("Mécanique Inoxydable") in order to develop the use of different stainless steels grades, because both their high strength and corrosion resistance. However, these steels are deemed sensitive to friction. In this context, this thesis is dedicated firstly to study the effect of different thermal and surface treatments on the microstructure of stainless steels to improve their ability to friction. Different hardening mechanisms were observed: precipitation, phase change and solid solution. Secondly, we have implemented an experimental approach for comparing the response of the various materials pairs under extreme sliding and rolling contact. These tests have allowed us to characterize the friction coefficient evolution and classify the different metallurgical materials and treatments according to their wear resistance. Thirdly we have characterized the different types of damage occurring at surface and in the subsurface. Finally, the use of EBSD allowed us to characterize the plastic deformation in the subsurface and to correlate the value HAGB (High Angle Grains Boundaries) density with the wear resistance of the different metallurgical materials and treatments of the examined steels.

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