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Microstructure Design of Low Alloy Transformation-Induced Plasticity Assisted SteelsZhu, Ruixian 03 October 2013 (has links)
The microstructure of low alloy Transformation Induced Plasticity (TRIP) assisted steels has been systematically varied through the combination of computational and experimental methodologies in order to enhance the mechanical performance and to fulfill the requirement of the next generation Advanced High Strength Steels (AHSS). The roles of microstructural parameters, such as phase constitutions, phase stability, and volume fractions on the strength-ductility combination have been revealed.
Two model alloy compositions (i.e. Fe-1.5Mn-1.5Si-0.3C, and Fe-3Mn-1Si-0.3C in wt%, nominal composition) were studied. Multiphase microstructures including ferrite, bainite, retained austenite and martensite were obtained through conventional two step heat treatment (i.e. intercritical annealing-IA, and bainitic isothermal transformation-BIT). The effect of phase constitution on the mechanical properties was first characterized experimentally via systematically varying the volume fractions of these phases through computational thermodynamics. It was found that martensite was the main phase to deteriorate ductility, meanwhile the C/VA ratio (i.e. carbon content over the volume fraction of austenite) could be another indicator for the ductility of the multiphase microstructure.
Following the microstructural characterization of the multiphase alloys, two microstructural design criteria (i.e. maximizing ferrite and austenite, suppressing athermal martensite) were proposed in order to optimize the corresponding mechanical performance. The volume fraction of ferrite was maximized during the IA with the help of computational thermodyanmics. On the other hand, it turned out theoretically that the martensite suppression could not be avoided on the low Mn contained alloy (i.e. Fe-1.5Mn-1.5Si-0.3C). Nevertheless, the achieved combination of strength (~1300MPa true strength) and ductility (~23% uniform elongation) on the low Mn alloy following the proposed design criteria fulfilled the requirement of the next generation AHSS.
To further optimize the microstructure such that the designed criteria can be fully satisfied, further efforts have been made on two aspects: heat treatment and alloy addition. A multi-step BIT treatment was designed and successfully reduced the martensite content on the Fe-1.5Mn-1.5Si-0.3C alloy. Microstructure analysis showed a significant reduction on the volume fraction of martensite after the multi-step BIT as compared to the single BIT step. It was also found that, a slow cooling rate between the two BIT treatments resulted in a better combination of strength and ductility than rapid cooling or conventional one step BIT. Moreover, the athermal martensite formation can be fully suppressed by increasing the Mn content (Fe-3Mn-1Si-0.3C) and through carefully designed heat treatments. The athermal martensite-free alloy provided consistently better ductility than the martensite containing alloy.
Finally, a microstructure based semi-empirical constitutive model has been developed to predict the monotonic tensile behavior of the multiphase TRIP assisted steels. The stress rule of mixture and isowork assumption for individual phases was presumed. Mecking-Kocks model was utilized to simulate the flow behavior of ferrite, bainitic ferrite and untransformed retained austenite. The kinetics of strain induced martensitic transformation was modeled following the Olson-Cohen method. The developed model has results in good agreements with the experimental results for both TRIP steels studied with same model parameters.
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Precipitation, recrystallization and solute strengthening in microalloyed steelsAkben, Melek G. January 1980 (has links)
Constant strain rate compression and torsion tests were carried out isothermally at temperatures of 875 to 1075(DEGREES)C on a series of six steels. The base steel had a composition of 0.06% C and 1.43% Mn and the others contained one of the following sets of additions: (i) 0.035% Nb; (ii) 0.035% Nb + 0.115% V; (iii) 0.035% Nb + 0.30% Mo; (iv) 0.035% Nb + 1.90% Mn; (v) 0.115% V. The tests were conducted to determine the effects of Mn, Mo, Nb and V, singly and in combination, on the high temperature flow and recrystallization behavior of the materials. The dynamic precipitation kinetics for Nb(CN) and VN were determined by the Weiss method. The two PTT curves were similar, with the nose of the VN curve being situated at a slightly lower temperature (885 vs. 900(DEGREES)C) and at a somewhat longer time (26 vs. 16 s), in agreement with the lower equilibrium solution temperature of VN. The dynamic precipitation kinetics of Nb(CN) were retarded by the addition of Mn, V or Mo. This retardation is attributed to the increased carbonitride solubility that follows the addition of these elements because of the way in which they decrease the C and N activity coefficients. / RTT curves were constructed for dynamic recrystallization in the six steels investigated. These were derived from the peak strains of the compression flow curves, as determined at a strain rate of 3.7 x 10('-2)s('-1). Recrystallization occurred earliest in the plain C steel followed fairly quickly by the 0.115% V steel. All of the Nb bearing steels recrystallized considerably later, with the greatest retardation being noted in the 0.3% Mo steel, where it was nearly twice that due to Nb addition alone. This very large effect, and the retardation due to each of the transition elements, is explained in terms of the electronic differences between iron and the particular element. The effect of the atomic size differences with respect to iron is also considered. / The strengthening due to the presence of Mn, Mo, Nb and V in solution was determined from the yield strengths of these steels. The increment in yield strength over that of the plain C steel was determined as 70% and 7% per 0.1 at.% of Nb and V when each is added singly. The strengthening increased to 80% and 8% respectively for these elements when present jointly in austenite. The strength increments were 9% for Mo and 1.3% for Mn per 0.1 at.% when added to a 0.035% Nb steel. The rank order of these effects is also explained in terms of the electronic and atomic size differences, and a possible reason for the synergistic effect (e.g. in the case of Nb and V in a Nb-V steel) is proposed.
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Plate yield slenderness criteria for structural members fabricated from high strength steelsTang, Louis (Ruo Biao) January 2008 (has links)
Increasing demand from flourishing construction markets led to the successful development of high strength steels (HSS). The new structural steel has exceptional high strength, high fracture toughness, long fatigue life, high corrosion resistance, and better weldability making the material attractive for structural design applications in the modern steel buildings and bridges. With their high strength, typically in the range of 500~700 MPa, and reduced weight/dimensions, it frees imaginations of modern designers and opens up new possibilities. Although HSS cost more, this is more than offset by reduced fabrication and erection costs. The advantage of the intrinsic properties of the HSS makes it possible to achieve successful applications in a cost-effective manner. At present, the Australian steel design standard, AS 4100 (SA, 1998), is limited to conventional low strength steels (LSS) with yield stress less than 450 MPa, (i.e. fy . 450 MPa). As a result steel structural members fabricated from HSS in Australia are usually designed according to overseas specifications, such as AISC-LRFD (AISC, 2003) which allows the design for structures fabricated from HSS materials. However, the design provisions of AISC-LRFD were mainly based on experimental and analytical studies on standard LSS. HSS exhibits mechanical properties that are quite different from conventional LSS. On the other hand, the design procedure and approach of the American specifications (AISC, 2003) are unfamiliar with Australian design engineers, which explains why practising engineers in Australia are reluctant to use AISC-LRFD specification in the design of HSS members. Therefore research into the behaviour of HSS members is essential to address this shortcoming. However, since the use of HSS often leads to smaller sections, hence thinner plates, the elastic and inelastic instability of these thin-walled and HSS members become highly critical. Conservatively, the local instabilities of the constituent plate element interactions in the cross-section have been ignored in the current steel practices. Increasing the slenderness of either plate elements within a cross-section leads to a significant reduction in the section capacity of the structural member. Therefore, the interactive effects between flange and web plate elements have to be considered in the strength, stability and deformation studies of HSS members. Furthermore, the current definitions and values of the plate slenderness limits also vary among major steel design codes (AS4100, 1998; AISC, 2003; EN1993, 2003; BS5950, 2000). The main aim of this research project is to investigate the structural behaviour of Ishaped HSS members subjected to local buckling effects in the elastic and inelastic ranges. For this purpose, it will use advanced numerical analyses and laboratory experiments to study the structural behaviour of these HSS members in compression and bending, respectively. The critical review has found that various inconsistencies among the major steel design specifications (AS4100, 1998; AISC, 2003; EN1993, 2003, BS5950, 2000) in the current practice produce conflicting design predictions of section capacities. The experimental measurements of residual stress distributions have confirmed that the ECCS recommendation (1984) is inappropriate for crosssections fabricated from typical HSS materials (i.e. BISPLATE80). The experimental measurements and numerical studies carried out in this project have produced a better understanding of the structural behaviour of HSS members subjected to local instabilities. The study has enabled to provide a series of proposals for proper assessment of plate slenderness limits for structural members made of HSS materials. It may also enable the inclusion of future version of the AS4100 code for HSS materials to be used in the design of steel building and bridge constructions. It is believed that the use of HSS in building and bridge constructions will increase significantly in the very near future, and to fully-facilitate this, the future versions of national and international steel design specifications must include rational and reliable design rules for members made of all steel grades by including the effects of HSS special characteristics and true interactive local buckling behaviour of HSS members. This research project has contributed towards this.
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Wear and microstructure of eutectoid steels /Danks, Daniel, January 1989 (has links)
Thesis (Ph. D.)--Oregon Graduate Center, 1989.
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Seismic retrofitting of rectangular reinforced concrete columns with partial interaction plating /Wu, Y. F. January 2002 (has links) (PDF)
Thesis (Ph.D.)--University of Adelaide, Dept. of Civil and Environmental Engineering, 2002. / "June 2002" Includes bibliographical references (leaves 349-374).
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Development of a self-lubricating plasma sprayed coating for rolling/sliding contact wear /Niebuhr, David V., January 1997 (has links)
Thesis, (Ph. D.)--Oregon Graduate Institute, 1997.
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Development of high performance concrete for use on Tennessee bridge decks and overlaysHamblin, Charles Herbert, January 2004 (has links) (PDF)
Thesis (M.S.)--University of Tennessee, Knoxville, 2004. / Title from title page screen (viewed Sept. 22, 2004). Thesis advisor: Edwin G. Burdette. Document formatted into pages (ix, 82 p. : ill. (some col.)). Vita. Includes bibliographical references (p. 62-68).
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An investigation into possible means of increasing the strength of lightweight high strength concrete /Edwards, Derek Oswald. January 1993 (has links)
Thesis (M. Phil.)--University of Hong Kong, 1993.
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Production and quality control of high performance concrete in Texas bridge structures /Myers, John Joseph, January 1998 (has links)
Thesis (Ph. D.)--University of Texas at Austin, 1998. / Vita. Includes bibliographical references (leaves 529-539). Available also in a digital version from Dissertation Abstracts.
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Friction stir welding of high-strength automotive steel /Olsen, Eric, January 2007 (has links) (PDF)
Thesis (M.S.)--Brigham Young University. School of Technology, 2007. / Includes bibliographical references (p. 73-75).
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