Relaxation spectra were determined for a high chromium-high nickel vacuum cast stainless steel. The specimens were tested in the following three conditions: (1) as-received and solution heat treated, (2) nitrided 20 hours and solution heat treated, and (3) nitrided 40 hours and solution heat treated. No internal friction peaks were found in any of the experimental runs. There was no precipitation of nitrides during testing in the torsional pendulum. This was verified by an electron microscopic examination of the specimens after testing. Since there were no nitrides precipitated, no internal friction relaxation peaks were evident. / M.S.

M.Ing. / During all manufacturing processes it is crucial to use certain design criteria and guidelines. Special care should be exercised when the final product of a manufacturing process is used in the automotive industry, because the failure of such a component may have tragic consequences. The failure of a bus chassis in the public transport sector is an example of a case where the failure of a product can have serious consequences. In recent years it has become common practice to use corrosion-resisting steel in the manufacture of these vehicles. The reason for this is the corrosion caused by a prolonged service life and adverse conditions such as salted road surfaces (The salt is used to melt the ice that forms on roads, particularly in European countries). These bus structures consist of tubes of varying size and geometry, and the manufacturing process of these tubes is considered in the present investigation. In a tube manufacturing process the design criteria may consist of such properties as the tube size and geometry, the thickness of the sheet that is used and the radius of the corners of the tube. Design criterion is also dependent upon the material that is used. The change in mechanical properties of the material during a manufacturing process is an important consideration during the establishment of design guidelines. The purpose of this investigation is to study the effects of particularly the cold forming manufacturing process on the mechanical properties of the material. The material used is 3CR12 corrosion resisting steel, a proprietary alloy also known as Type 1.4003, that was developed by Columbus joint venture as a cheaper alternative to stainless steels. 3CR12 is not a substitute for stainless steel but it is an alternative to treated mild steel, providing a cost-effective solution to corrosion. An experimental investigation is conducted into the forming of 40mm 3CR12 square tubes and normal plate bending of 3CR12. Various different wall thicknesses and bend radiuses are considered. A numerical investigation consisted of simulating the above-mentioned manufacturing processes using non-linear finite element analysis and then comparing its results to the experimental investigation. The experimental investigation showed that substantial work hardening occurred in the corner regions of the tube during forming. A loss of up to 70% of 3CR12's ductility may result in the corner regions. The work hardening at the inside of the tube was found to be higher than at the outside. A region of very little work hardening near the middle of the tube wall thickness was also identified (neutral axis). This neutral axis also seems to shift slightly more to the inside of the tube with thicker wall sections. The numerical analysis confirmed the experimental observations. An excellent correlation between the experimental and numerical results was achieved.

The mechanical behavior of structural materials used in nuclear applications is significantly degraded as a result of irradiation, typically characterized by an increase in yield stress, localization of inelastic deformation along narrow dislocation channels, and considerably reduced strains to failure. Further, creep rates are accelerated under irradiation. These changes in mechanical properties can be traced back to the irradiated microstructure which shows the formation of a large number of material defects, e.g., point defect clusters, dislocation loops, and complex dislocation networks. Interaction of dislocations with the irradiation-induced defects governs the mechanical behavior of irradiated metals. However, the mechanical properties are seldom systematically correlated to the underlying irradiated microstructure. Further, the current state of modeling of deformation behavior is mostly phenomenological and typically does not incorporate the effects of microstructure or defect densities. The present research develops a continuum constitutive crystal plasticity framework to model the mechanical behavior and deformed microstructure of bcc ferritic/martensitic steels exposed to irradiation. Physically-based constitutive models for various plasticity-induced dislocation migration processes such as climb and cross-slip are developed. We have also developed models for the interaction of dislocations with the irradiation-induced defects. A rate theory based approach is used to model the evolution of point defects generated due to irradiation, and coupled to the mechanical behavior. A void nucleation and growth based damage framework is also developed to model failure initiation in these irradiated materials. The framework is used to simulate the following major features of inelastic deformation in bcc ferritic/martensitic steels: irradiation hardening, flow localization due to dislocation channel formation, failure initiation at the interfaces of these dislocation channels and grain boundaries, irradiation creep deformation, and temperature-dependent non-Schmid yield behavior. Model results are compared to available experimental data. This framework represents the state-of-the-art in constitutive modeling of the deformation behavior of irradiated materials.