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Thesis(M.S.)--Case Western Reserve University, 2009 / Title from PDF (viewed on 2010-01-28) Department of Materials Science and Engineering Includes abstract Includes bibliographical references and appendices Available online via the OhioLINK ETD Center

Thesis (Ph.D.)(Metallurgical Engineering)--University of Pretoria, 2006. / Includes summary. Includes bibliographical references. Available on the Internet via the World Wide Web.

Thesis (M. Sc.)(Applied Science)--University of Pretoria, 2006. / Includes summary. Includes bibliographical references. Available on the Internet via the World Wide Web.

The sensitisation of a 12% chromium ferritic stainless steel, conforming to EN 1.4003 and available commercially in South Africa under the trade name of 3CR12, was investigated during the course of this project. 3CR12 was designed to pass through the (<font face="symbol">a</font>+<font face="symbol">g</font>) phase field on cooling, with the austenite transforming to martensite on subsequent cooling to room temperature. The aim of this investigation was to verify that 3CR12 can sensitise during continuous cooling after welding, provided that low heat input levels are used. Two grades of 3CR12 with slightly different chemistries, designated 41220 (A) and 41311 (B), were evaluated. Grade 41220 has a higher austenite potential than grade 41311. 3CR12 plate was joined autogenously to AISI 316L by means of a series of square butt welds. Gas tungsten arc welding with argon shielding was used, and the heat input was varied from approximately 30 J/mm to 450 J/mm, in conjunction with welding speeds ranging from 2.36 mm/s to 33.3 mm/s. Rosenthal’s heat flow equations were used to calculate the cooling rate from 1500ºC to 800ºC for each experimental weld, and to illustrate the influence of the effective heat input and welding speed on the martensite content of the high temperature heat-affected zone. An increase in welding speed reduces the heat input and increases the cooling rate after welding. At lower heat input levels (less than approximately 100 J/mm), austenite nucleation was found to be suppressed by the rapid cooling rates, and a continuous network of ferrite-ferrite grain boundaries formed in the high temperature heat-affected zone. Higher heat inputs resulted in slower cooling with more martensite in the high temperature heat-affected zone after cooling. At heat input levels above approximately 250 J/mm, enough martensite formed during cooling to eliminate a continuous network of ferrite-ferrite grain boundaries in the high temperature heat-affected zone. Sensitisation was evaluated using an electrolytic oxalic acid etch (ASTM 763-99, Practice W), and a potentiostatic etch in 0.5M H2SO4. During the potentiostatic etch test, the potential was maintained at 0 VSCE to reveal the presence of any chromium depleted zones. Both grades of 3CR12 were found to be sensitised when a continuous network of ferrite-ferrite grain boundaries was present in the high temperature heat-affected zone (i.e. after welding at low heat input levels). When the heat input during welding was high enough to ensure the presence of martensite on the majority of the heat-affected zone grain boundaries, thereby effectively eliminating continuous ferrite-ferrite grain boundary networks, the welds were not in the sensitised condition. The austenite that forms during cooling acts as a carbon sink, absorbing any excess carbon. This prevents supersaturation of the ferrite and subsequent carbide precipitation that can lead to chromium depletion and sensitisation. Due to its higher austenite potential, grade 41311 can be welded at lower heat input levels and with faster cooling rates than grade 41220 without inducing continuous carbide precipitation and sensitisation. In order to prevent sensitisation, a fusion-line cooling rate of 80ºC/s should not be exceeded in 3 mm 3CR12 plate during welding. / Dissertation (MSc(Applied Science))--University of Pretoria, 2007. / Materials Science and Metallurgical Engineering / unrestricted

A study has been made of creep damge accumulation in two casts of l%.Cr-1/2%i.Mo low alloy steel. Creep tests and creep crack growth tests have been carried out at 823K to determine the nature of the damage accumulation and to attempt to relate microscopic damage mechanisms to the macroscopic fracture parameters. Four types of specimen were tested and failure of all occurred by the continuous nucleation. growth and coalescence of grain boundary cavities. A mechanism for the growth of cavities is suggested. based on grain boundary diffusion coupled with geometric constraint. The influence of continuous cavity nucleation has also been considered and it is suggested that this phenomenon initially increases the rate of diffusive cavity growth. However continuous nucleation decreases the growth rate once the latter becomes constrained. The effect of stress-state is also considered and increasing triaxiality is shown to have little effect on the unconstrained diffusive growth but it decreases the constrained growth rate by increasing the overall constraint in the specimen. Predicted growth rates give good agreement to those observed experimentally for both notched and un-notched creep specimens. Reasonable agreement is also observed to the predicted rupture lives although the predictions suggest notch strengthening whilst experimentally notch weakening is observed. This is thought to be due to non-uniform damage formation on loading. Based on the above concepts of cavity growth, constitutive equations are presented to predict the time dependence of creep strain. These are found to give good agreement to the experimentally determined strain rates, lending further support for the development of continuum damage mechanics as a means of assessing creep crack growth behaviour.

Thesis (Ph. D.)--Lehigh University, 2000. / Includes vita. Includes bibliographical references (leaves 306-314).

This thesis reports a study of the microstructural evolution of chromium containing duplex ferrite-martensite steels and examines the effects of the microstructure on the mechanical properties. Emphasis has been placed on determining the microstructural factors responsible for the persistent occurrence of anisotropy in a modified 12 wt% Cr steel designated 3CR12. in addition an investigation has been carried out in order to refine the grain structure of a ferritic steel containing 16-17 wt % Cr by inducing a duplex ferrite-martensite phase structure. The microstructural evolution of 3CR12 was studied during cooling from a solution heat treatment at 1380°C and the natures of the phase transformations evident were investigated. Energy dispersive X-ray spectroscopy (EDS), in association with a scanning electron microscope (SEM), was used to determine the composition of the phases arising from the solid state δ-ferrite to austenite transformation. It is shown that the high temperature δ-ferrite phase partially decomposes to austenite via a Widmanstatten growth mechanism and consequently a banded two phase structure is produced after hot rolling. The element partitioning which arises during the solid state δ-ferrite decomposition ieads to compositional banding with an indelible nature. A model is proposed for the events leading to the generation of the banded phase structure and the formation of an elongated ferritic microstructure in 3CR12 after sub-critical annealing. The type and distribution of non-metallic inclusions occurring in 3CR12 has also been assessed. Characteristic fracture modes developed during impact testing have been related to the grain morphology and the occurrence of non-metallic inclusions. It is shown that splits form parallel to the rolling plane when Charpy specimens are subjected to impact testing and that both impact energy and mode of fracture are dependent on the directional properties of the 3CR12 microstructure. Splitting is predominantly caused by the low energy crack path provided by long, undulating grain boundaries parallel to the rolling plane, and inclusions, particularly manganese sulphides (MnS), facilitate low energy modes of fracture associated with the splitting phenomenon. MnS inclusions are also found to affect the corrosion resistance of 3CR12 and careful control of the chemistry of the steel permits these inclusions to be restricted to levels at which acceptable impact and corrosion properties are maintained. Refinement of the grain structure of ferritic steels containing 16-17 wt % Cr was carried out by modifying the ratio of ferritising elements to austenitising elements in the steel chemistry. Suitable ruckel additions have been determined which provide alloys with sufficient austenitising ability to refine the high temperature δ-ferrite phase and consequently a duplex ferrite-martensite microstructure is produced. Tempering of these alloys at 700°C results in a lamellar ferrite-martensite structure which gives rise to an attractive combination of impact and tensile properties which may provide a stainless steel with superior cost effectiveness to austenitic grades.

In applications where resistance to localised corrosion is required, stainless steel alloys containing molybdenum are generally used thanks to their superior resistance to localised attack in aggressive environments. For ferritic stainless steels, vanadium additions have been found to also have a beneficial effect on the resistance to localised corrosion. In this study vanadium and molybdenum were compared directly as alloying elements in 18% chromium ferritic stainless steel as far as their effect on increasing the resistance to localised corrosion is concerned. Pitting potentials in a neutral chloride solution were used as the criterion for qualifying resistance to localised corrosion and it was shown that vanadium gave similar or slightly higher pitting potentials at addition levels of up to 4% (weight percent). It was subsequently found that the mechanism by which the molybdenum and the vanadium increase the resistance to localised corrosion, are not the same. The experimental data for the molybdenum containing alloys corresponded well with other work done in this field. The positive effect of molybdenum additions on the pitting resistance of these alloys could be explained through its effect in lowering the dissolution rate in the active dissolution region by enriching on the dissolving surface. The vanadium additions to these alloys were shown not to have an effect on the active dissolution kinetics. The effect of these two alloying elements on the initiation of metastable pits were examined, but no meaningful advantage for the vanadium containing alloys over the rest could be found. It is suggested that vanadium play a role in changing the dissolution kinetics of the salt film, which forms during the growth of a metastable pit. A delayed dissolution of salt film remnants would lead to a loss of the enriched pit solution, which would cause the metastable pit to repassivate. / Dissertation (M Eng (Metallurgical Engineering))--University of Pretoria, 2007. / Materials Science and Metallurgical Engineering / unrestricted

Ferritic chromium steels are important structural materials for future nuclear fission and fusion reactors due to their advantages over traditional austenitic steels, including low swelling rates, better thermal fatigue resistance, and lower thermal expansion coefficients. Radiation-induced segregation or depletion (RIS/RID) of solute atoms at grain boundaries is considered to be a potentially significant phenomenon for structural materials because of its potentially detrimental role in affecting microstructure and furthermore mechanical properties. However, the behaviour of Cr at grain boundaries in ferritic steels is not well understood. Both segregation and depletion of Cr at grain boundary under irradiation have been previously observed and no clear dependency on irradiation condition or alloy type has been presented. Furthermore, ferritic alloys are known to undergo hardening and embrittlement after thermal aging in the temperature range of 300-550DC and this phenomenon is related with a and a' phase separation occurring in the solid solution. However the low temperature a-a' miscibility gap in the currently used phase diagram is extrapolated from high temperature results and conflicts with many experimental observations. To understand the Cr behaviour at gram boundaries in ferritic steels under irradiation, a systematic approach combining SEM/EBSD, FIB specimen preparation and APT analysis has been developed and successfully applied to a Fe- 15.2at%Cr to investigate the effect of pre-irradiation chemistry, grain boundary misorientation, impurities, irradiation damage, irradiation depth, and other possible factors to get a better understanding of RIS/RID phenomena. Both low sigma boundaries and randomly selected high angle boundaries have been investigated in detail. Systematic differences between the behaviour of different classes of boundaries had been observed, and the operating mechanisms are also discussed in this thesis. The maximum separation method has been applied on APT data to study the C- enriched clusters and Cr-enriched clusters, which were not directly visible on the atom maps. The composition of the Cr-enriched clusters was consistent with a' phase and the irradiation was found to accelerate the nucleation rather than the growth of these clusters. Such results provided important information in re- determining the a-a' phase boundary.