Thesis (Ph. D.)--Oregon Graduate Center, 1980

A study of the constitution, transformation and precipitation effects in ternary Fe-Cr-Ni alloys and quaternary Fe-Cr-Ni-X alloys containing Mo, Nb, Ti, and Si was carried out. A systematic approach was adopted so that the microstructural effects observed as a result of ageing the selected iron base ternary alloys could be directly compared to the quaternary alloys. A series of ageing curves were plotted for the six ternary alloys over the temperature range 400°-900°C and for the ten quaternary alloys in the temperature range 650°-850°C. Optical and electron metallography were used to study the transformation and precipitation of intermetallic phases and carbides in the aged microstructures. The transformation of delta ferrite to sigma phase in a duplex (gamma+delta) ternary alloy was studied. The transformation was preceded by the precipitation of a cellular structure which formed on the delta/gamma grain boundaries and consisted of M23C6 and new austenite. A complex transformation product, which resembled a region of imiscibility, then formed at the delta/(gamma + M23C6) interface, this product consumed the delta ferrite grain, eventually transforming to sigma phase and new austenite. The alpha' phase, more commonly referred to as 475°C embrittlement, was also observed precipitating in the delta ferrite grains in two of the iron based ternary alloys in the temperature region 400°-500°C. Overageing of the alpha' precipitates after 1000 hours at 500°G was accompanied by the precipitation of a rod like austenite. The elements 2%Mo, 1%Nb, 1/4%Ti and 1% and 2%Si were added totwo Fe-Ni-Cr base alloys. One alloy was an austenitic (20Cr,23Ni) and the other was a transformable alloy (18Cr, 7Ni). The quaternary element additions had no effect on the constitution of the austenitic ternary alloy. However the addition of Mo to the 18Cr, 7Ni ternary alloy caused the transformation of delta ferrite to sigma phase over ageing temperature range 650°-850°C. The addition of Si moved the constitution of the ternary alloy further into the gamma+delta phase field, a small amount of sigma phase was observed in the 2%Si quaternary alloy on ageing at 650°C for 1000 hours. Irradiation damage studies were performed using High Voltage Electron Microscope and Variable Energy Cyclotron. The effect ofthe quaternary alloying additions Mo, Si, and Nb on the voidswelling behaviour of a 20Cr, 23Ni alloy were studied using 46MeV Ni6 ions in the Variable Energy Cyclotron, irradiating to a total dose of 10dpa at 550°C. All additions reduced void swelling, the largest reduction was observed in the Si containing allov. The Insert A Thermal ageing showed the presence of M23C6 the amount of which increased with increasing ageing temperature. No delta ferrite was observed in these alloys.

No description available.

Transformation Induced Plasticity (TRIP) steels have both high strength and high ductility. Retained austenite in the microstructure, upon straining, transforms to martensite and this absorbs energy and improves the work hardening of the steel, giving improved elongation. The transformation can be either stress-assisted or strain-induced and the initiation and the mechanism depend on the composition of, the size and shape of, and the phases surrounding, the austenite grains. It is important to understand the relationship between these variables and the properties of the TRIP steel. The aim of this work was to determine how the microstructure of the TRIP steel affects the transformation. Four experimental microstructures were developed, containing austenite grains with different sizes, shapes, and surrounding phases. The Fine microstructure had thin elongated austenite laths between fine bainitic ferrite laths, the Coarse microstructure had elongated austenite grains between coarser bainitic ferrite laths, the Equiaxed microstructure had equiaxed austenite grains in a matrix of equiaxed ferrite and the Acicular microstructure had elongated austenite grains surrounded by recovered ferrite laths. Tensile tests were performed and detailed characterization, using neutron diffraction, was done of samples with the four microstructures. The variation in the amount of austenite during deformation was measured. The tensile tests revealed that the microstructures had different mechanical properties and different transformation behaviours. Fine had the lowest elongation and the highest strength. Acicular and Equiaxed had good elongation but lower strength. Coarse had intermediate strength and Equiaxed had sustained work hardening. The transformation in Fine and Coarse was minimal. Coarse had some slow, steady transformation, but Fine may have had none. The transformation in Equiaxed was larger. It started quickly and then slowed at higher strains. The austenite in Acicular transformed steadily. The predominant mechanism of transformation was stress-assisted transformation, with strain-induced transformation occurring only in Equiaxed. The results of this work showed that the influence of the surrounding phases on the stability of the austenite is significant. The differences in the transformation behaviour of the four microstructures seemed to be due more to the surrounding phases than the grain size or the composition, although both these factors also played a role. / Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2007-12-14 13:35:07.248

Low carbon bainitic steels microalloyed with Nb, Ti and V are widely used for the pipeline, construction and automobile industries because of their excellent combination of strength, toughness and weldability. Boron as another major alloying element has been also frequently used in this type of steels since the 1970s. The purpose of adding boron is to improve the hardenability of the steel by promoting bainite formation. / It has been realized that Boron can only be effective as a strengthening element when it is prevented from forming BN and/or Fe23(C, B) 6 precipitates. Therefore, Boron is always added together with other alloying elements which are stronger Nitride or Carbide formers, such as Ti and Nb. However, the formation of complex bainitic structures and the interaction with precipitates at industrial coiling temperature are not adequately understood. / In this study, the effect of boron on the microstructure and mechanical properties of a low carbon Nb-B steel was studied by a hot compression test (50% reduction at 850°C) followed by quenching samples into a salt bath. The microstructures of the tested samples were examined through optical microscopy and SEM; and the mechanical properties of these samples were investigated by micro-hardness and shear punch tests. / The results indicate that during thermo-mechanical controlled rolling (TCR), the final properties of the products not only depend on the applied deformation but also depend on the coiling temperature where phase transformation takes place. According to the investigation, two strengthening mechanisms are responsible for the strength of the steel at the coiling temperature: phase transformation and precipitation. Under optical microscopy, the microstructures of all specimens appear to be bainite in a temperature range from 350°C to 600°C without distinct differences. However, the SEM micrographs revealed that the microstructures at 550°C are very different from the microstructures transformed at the other holding temperatures. / Two strength peaks were observed at 350°C and 550°C in the temperature range studied. It is believed that the NbC precipitates are the main contributor to the peak strength observed at 550°C because the kinetics of NbC is quite rapid at this temperature. The strength peak at 350°C is mainly due to the harder bainitic phase, which formed at relatively lower temperature.

Thesis (M.S. in Mechanical Engineering and Mechanical Engineer)--Naval Postgraduate School, December 1990. / Thesis Advisor(s): Fox, Alan G. "December 1990." Description based on title screen as viewed on April 2. 2010. DTIC Identifier(s): Steel, Microstructure, Mechanical Properties, Copper, Quenching, Tempering, Processing, Naval Vessels, HSLA-100 Steel, Theses, Age Hardening, Modulus of Elasticity, Charpy V Notch Tests. Author(s) subject terms: HSLA-100, Mechanical Properties, Copper Precipitation, Carbide. Includes bibliographical references (p. 66-68). Also available in print.

Thesis (Ph.D)--Mechanical Engineering, Georgia Institute of Technology, 2010. / Committee Chair: David L. McDowell; Committee Member: G. B. Olson; Committee Member: K. A. Gall; Committee Member: Min Zhou; Committee Member: R. W. Neu. Part of the SMARTech Electronic Thesis and Dissertation Collection.

No description available.

The rolling-sliding dry-wear behaviour of a series of bainitic steels and a standard pearlitic rail steel have been compared over a range of contact stress and creepage conditions applicable to the British Rail network. A rolling-sliding wear machine has been constructed - LEROS - which allows very high contact stresses to be combined with high creepages under well controlled conditions. Materials were tested on LEROS and on an Amsler machine. Limited vibration analyses were carried out on both machines and compared with the frequencies of disc surface periodic undulations. No direct linkage was determined. Despite better standard mechanical properties, the wear resistance of lower carbon bainitic steels was inferior to that of the pearlitic steel. A bainitic steel with the same carbon content as the pearlitic steel wore a little less, but at considerable expense to the pearlitic wheel steel counter-material in the wear couple. The wear resistance of bainitic steels depends upon the volume fraction of hard phase, such as carbide and martensite-austenite phase, for rolling-sliding as well as other types of dry wear loading. Pearlitic steel performs exceptionally well under certain rolling-sliding conditions, such as the majority seen in these tests, since the lamellar microstructure is modified so as to present a greater area fraction of carbide hard phase at the wear surface, a fraction in excess of bulk volume fraction. Recommendations are made for the dry wear applicability of the steels.

The aim of this work is to nondestructively characterize the industrial dual phase (ferritic-martensitic) steels (DPS) by the Magnetic Barkhausen Noise (MBN) method. By quenching of AISI 8620 steel specimens having two different starting microstructures, from various intercritical annealing temperatures (ICAT) in the ferrite-austenite region, the microstructures consisting of different volume fractions of martensite and morphology have been obtained. The microstructures, strength properties and hardness values were determined by conventional metallographic and mechanical tests. The measurements of the Magnetic Barkhausen Noise (MBN) were performed by using both Rollscan and &amp / #956 / SCAN sensor connectors. A good correlation between the martensite volume fraction, hardness and MBN signal amplitude has been obtained. MBN emission decreased as the ICAT, therefore the volume fraction of martensite increased. Moreover, MBN emission decreased as the martensite morphology become thinner. It has been concluded that MBN method can be used for nondestructive characterization of industrial dual phase steels.