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Showing 11 to 18 of 18 (0.025 seconds)| 11 |
Two methods for processing an ultrafine ferritic grain size in steels and the thermal stability of the structurePan, L. (Longxiu) 19 October 2004 (has links)
Abstract
In this thesis, methods to process ultrafine ferritic (UFF) structures in steels, i.e. grain sizes below about 3 μm have been investigated. It is shown here, in accordance with the results in the literature, that a steel with a UFF grain size can be obtained by two methods, more or less convenient to mass production: deformation-induced ferrite transformation from fine-grained austenite (the DIF route) and the static recrystallization of various heavily cold-worked initial microstructures (the SRF/SRM route).
In the present work, the influencing factors in the processing of UFF structure in the DIF route have been systematically studied in four low-carbon steels: one C-Mn steel and Nb, Nb-Ti and Nb-high Ti microalloyed steels. A high strain, a low deformation temperature close to Ar3 and a fine prior austenite grain size are beneficial to promote the formation of UFF grains. Especially by using complex pretreatments to refine the prior austenite grain size, cold rolling, repeating the low-temperature reheating cycle or using martensitic initial microstructure, a UFF grain size can be obtained in these steels at the strain of 1.2 (70% reduction) at 780 °C. By controlling the cooling rate, the type of the second phase can be adjusted.
When using the static recrystallization route, it was found that UFF is difficult to obtain from a single-phase ferrite, but it is relatively readily obtained from deformed pearlite, bainite or martensite, especially in high-carbon steels with 0.3–0.8%C. In deformed pearlite, the cementite lamellae fragmented and spheroidised in the course of heavy deformation can provide numerous nucleation sites by the particle stimulated nucleation mechanism and retard the subgrain and recrystallized grain growth. Nucleation and retardation of grain growth are effective also in deformed bainite, martensite or high-carbon tempered martensite, as discussed in detail in the work.
The thermal stability of UFF grained steels was tested and found to be generally excellent, but it varies depending on the processing method. The UFF structure obtained by the SRM route has a thermal stability somewhat weaker than that of the DIF route. For a given steel, UFF grains may show different grain growth modes, related to the dispersion of second phase particles. In the DIF structure, abnormal grain growth occurs at 700 °C after about 2.5 h, while in the SRM structure, normal grain growth takes place slowly at 600 °C. Carbides on the grain boundaries seem to play an important role in inhibiting grain coarsening. No coarse-grained zone was formed at the HAZ of electron beam or laser welded seams, as performed at low heat inputs (up to 1.5 kJ/cm) on thin strips. The hardness even increased from the base metal towards the HAZ and the weld metal in all seams as an indication that they were hardened during the rapid cooling.
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Thermomechanical processing of eutectoid steels: strategies to improve the microstructure of the hot rolled stripsCaruso, Matteo 30 October 2013 (has links)
Eutectoid steel strips are designed for the production of parts for intensive use such as clutches, seat slides, and springs as they exhibit<p>excellent strength levels and wear resistance. These properties arise from the unique morphology of lamellar pearlite which can be considered<p>as a self-laminated nanoscale composite. However, a spheroidization annealing step is nowadays necessary to improve the cold forming properties before further cold rolling steps.<p>This thesis is aimed at improving the tensile ductility of the hot rolled products of eutectoid composition in order to eliminate the intermediate<p>annealing step. Two strategies are proposed.<p>The first is to transpose the concept of controlled rolling developed for HSLA to<p>eutectoid steels. Through a strict adjustment of the austenite processing and of the cooling strategy, it is possible to improve the ductility<p>of the final lamellar microstructure. The way the processing parameters influence the hot deformation of austenite, the eutectoid transformation and of the subsequent spheroidization annealing is deeply<p>investigated. It is found that refinement and pancaking of austenite<p>is beneficial as it reduces the pearlite block size improving the total<p>tensile elongation. Accelerated cooling is of paramount importance to<p>achieve fine Interlamellar spacing (ILS), which lead to high strength<p>levels and accelerate spheroidization during subsequent annealing.<p>The second approach involves intercritical or warm deformation. Warm processing of eutectoid steels is first explored by torsion testing<p>and then up-scaled to a pilot rolling-line. The interactions between thermomechanical parameters, rolling forces generated and microstructural<p>evolution are carefully scrutinized. During concurrent hot deformation, spheroidization of cementite takes place almost instantaneously<p>in both torsion and rolling. The restoration processes occurring in the ferrite matrix depends on the strain path and the strain rates. Low strain rates (0,1 s−1) and simple shear promotes the formation of a recrystallized-like HABs network of about 3μm in size.<p>Plane strain compression and high strain rates (10 s−1) leads to the formation of a typical recovered dislocation substructure (LABs) of 1μm in size. During annealing, no recrystallization occurs and the LABs substructure remains stable. This substructure influences drammatically the mechanical properties: the strength is very high and the work-hardening behavior is poor due to high recovery rate in the region close to the LABs. However, due to the presence of spheroidized<p>cementite particles the ductility of warm rolled eutectoid steels is higher than that of ultra fine grained low carbon steels. / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished
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Optimisation of local material parameters : Optimising local material parameters in ductile cast iron cylinder head castingMäkinen, Katri January 2021 (has links)
The constantly tightening emission regulations demand the engines to be moreefficient, to get more power out of smaller engines. Higher engine loads andcomponent temperatures are causing more stresses to engine components. Therefore,a company that produces engines wanted to study if it would be possible to increasethe capabilities of the components by optimising the used material. In this final project work, a cylinder head will be studied. The cylinder heads for theengines are made of ductile cast iron. The limits of that material are near safety limits,and therefore a better material is needed. In this work are some previous studiesanalysed and tried to find how to optimise the used material. The optimised materialshould have better thermal conductivity properties combined with sufficient strengthproperties. Previous studies were analysed to gather knowledge of the elements that affect thematerial parameters. Those studies showed that copper, silicon, pearlite fraction, andthe use of chills are the elements to be optimised. Silicon and pearlite fraction waschosen as optimisation parameters because of their effect on the thermal conductivityand strength properties. Copper was chosen as an optimisation variable due to its effecton the pearlite formation. Chills were used to affect the cooling rate and thereby thepearlite formation. The work was made using MAGMASOFT™ simulation software to simulate cylinderhead casting. The simulated cylinder head was divided into 4 parts for the simulations.For those sections were then set targets for pearlite fraction according to previousstudies. The silicon content was kept constant in the simulation, based on the studiespresented in this work. Copper content was simulated with variations from 0 to 0.7weight-%, and chill heights were simulated from 20 to 60 mm and without chills. After simulating the different variables, the results were analysed. Then the selectedcasting simulation result was mapped to finite element simulation mesh to include thelocal material parameters to finite element simulation. With the finite elementsimulation, the estimated lifetime of the component was simulated. By analysing the casting simulation results, an optimal combination was found. Theoptimal material parameters for a cylinder head casting would be copper 0.5weight-%, silicon 1.9 weight-% and chills thicker than 40 mm on the flame plate. Theoptimised material gives more possibilities to develop engines even further when thecomponent demands are growing.
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Nuclear Waste Canister : Evaluating the mechanical properties of cassette steel after castingFager, Fredrick, Chanouian, Serg January 2017 (has links)
Företaget Svensk Kärnbränslehantering AB (SKB) håller på att utveckla en slutförvaringskapsel som kommer innehålla avfall från den svenska kärnkraften. Det är dock fortfarande en process under utveckling och därför undersöks olika typer av metoder och kapselmaterial för att kunna tillverka en hållbar och säker kapsel. Kapseln består av ett hölje av kopparrör med svetsad botten och lock och en insats med stållock. Insatsen är en cylindrisk konstruktion av segjärn och innehåller en svetsad stålkassett för att skapa utrymmen till det använda kärnbränslet. Insatsen innehåller bland annat stålrör som under tillverkning får utstå en gjutprocess med segjärn och erhåller efter det icke homogena egenskaper. Målet med undersökningen är hur stor påverkan gjutningen har på stålets kemiska sammansättning samt mikrostrukturer. Det som orsakar de inhomogena egenskaperna är främst värmebehandlingen som driver diffusionen av kol från gjutjärnet till stålet, som då ger ett hårdare men sprödare material. Med hjälp av experiment och simuleringar upptäcks hur mycket kol som diffunderar in i stålet samt ändringar i den kemiska sammansättningen i de påverkade zonerna. Identifiering av fasomvandlingar, diffusion och ändringar i mikrostrukturer är stora faktorer som i sin tur ändrar de mekaniska egenskaperna i stålet. / The Swedish Nuclear Fuel and Waste Management Company (SKB) have developed a final storage canister that will contain waste from the Swedish nuclear power plants. However, it is still in a development phase and therefore different types of methods and canister materials are investigated to produce the most durable and safe canister. The canister is made of a copper tube with a welded bottom and lid with an insert. The insert is a cylindrical construction of nodular cast iron that contains a welded steel cassette, to make space for the spent fuel, and a steel lid. The steel tubes showed inhomogeneous properties after being exposed to a casting around them. The aim of this investigation is to clarify the impact of casting on the chemical composition of the steel as well as the microstructure. The cause to the inhomogeneous properties were the diffusion of carbon from the cast iron to the steel, which then produced a harder and more brittle material. Experiments and simulations were used to see the carbon diffusion into the steel as well as what happens with the chemical composition in the affected zones. Identification of phase changes, diffusion and microstructures contributed to changes of mechanical properties in the steel.
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Élaboration et genèse des microstructures dans les "aciers" fer-azote / Preparation and genesis of microstructures in iron-nitrogen "steels"Xiong, Xiao Chuan 13 November 2008 (has links)
L’industrie automobile cherche constamment à augmenter la part des pièces fabriquées à partir de tôles minces en aciers plus résistants et à plus bas coût. Le parallèle entre les diagrammes de phases Fe-N et Fe-C montre qu’il est possible de développer des aciers similaires dans le système Fe-N. Les objectifs de cette étude étaient l’élaboration des aciers binaires Fe-N et le développement des structures équivalentes à celles dans les aciers au carbone. Les approches envisagées s’articulent autour de : Elaboration : la nitruration gazeuse en phase austénitique suivie de traitements d’homogénéisation ont permis de charger des tôles minces de fer pur en concentrations importantes d’azote. Une simulation de la diffusion de l’azote a été proposée. Genèse des microstructures : Le refroidissement lent de l’austénite Fe-N conduit à des structures perlitiques lamellaires et globulaires, constituées de ferrite et du nitrure non-stoechiométrique Fe4N. Une structure aciculaire particulière a été identifiée. Il s’agit de la ferrite se développant dans le nitrure Fe4N. Le refroidissement lent de la ferrite Fe-N sursaturée conduit à la précipitation des nitrures stables Fe4N et métastable Fe16N2. Des microstructures multiphasées [alpha+alpha'+gamma] ont été obtenues par des maintiens dans le domaine intercritique suivis de trempe. Le domaine intercritique a été réexaminé en utilisant le modèle des sous-réseaux. Des essais in-situ en MET ont relevé l’évolution des précipités de Fe16N2 dans la ferrite au cours du vieillissement à 85 °C. Des proportions importantes de l’austénite résiduelle ont été relevées, ce qui serait à la base du développement des aciers TRIP à l’azote / Car designers are seeking ways to increase the proportion of parts made of sheet steels of higher strength and lower cost. The parallel between the Fe-N and Fe-C phase diagrams shows that it is possible to develop similar steels in the Fe-N system. The objective of this study was to prepare binary Fe-N steels and to develop structures equivalent to those in carbon steels. Approaches to meet the objectives are considered: Preparations of Fe-N steels: gas nitriding in austenite domain followed by homogenization treatments allowed to introduce high amount of nitrogen in pure iron sheet. A simulation of the nitrogen diffusion was proposed to describe the weight increase during nitriding. Genesis of microstructures: The slow cooling of the Fe-N austenite led to lamellar and globular pearlitic structures composed of ferrite and nitrideFe4N. An acicular microstructure, which is the consequence of the precipitation of the ferrite in the nitride Fe4N, was also identified. The slow cooling of the supersaturated Fe-N ferrite led to the precipitation of the stable nitride Fe4N and the metastable nitride Fe16N2, which witnessed a rapid diffusion of nitrogen in ferrite at low temperature, comparable to that of carbon. The multiphase microstructures [alpha+alpha'+gamma] were obtained by intercritical treatments followed by quenching. The intercritical domain was reviewed using the sub-lattice model. In-situ TEM investigations have identified the precipitation of Fe16N2 nitride in the ferrite during the aging at 85 °C. High amount of the residual austenite have been identified, which would be the basis for development of TRIP nitrogen steels
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Verformungsverfestigung bei zyklisch inkrementeller Torsion von Reineisen und dem Stahl 42CrMo4N / Strain hardening of pure iron and steel 42CrMo4N under cyclic incremental torsionKuprin, Corinna 12 February 2013 (has links) (PDF)
Die Arbeit widmet sich dem Fließverhalten von Stählen bei zyklisch inkrementeller Torsion. Dazu werden vergleichend Reineisen und der ferritisch-perlitische Stahl 42CrMo4N bei unterschiedlichen Verformungspfaden betrachtet. Vor allem in zyklischen Torsionsversuchen mit bleibendem Verformungsinkrement je Zyklus werden Fließkurvenverläufe und Verformungsverfestigung analysiert.
Die Ergebnisse belegen den Einfluss des lamellaren Zementit auf das Fließverhalten des Stahls 42CrMo4N, während die Eigenschaften des Reineisens von der entstehenden Versetzungszellstruktur bestimmt werden. Die Richtungsabhängigkeit der Fließspannung und die Verläufe der Fließkurven unterscheiden sich für die betrachteten Werkstoffe deutlich.
Fließortkurven dienen der quantitativen Beschreibung der Verfestigung. Die Vorgehensweise zu ihrer Ermittlung und ihre Abhängigkeit von den Versuchsbedingungen und den Verformungszuständen werden gezeigt. Bei Reineisen dominieren isotrope, bei dem Stahl 42CrMo4N kinematische Verfestigungsanteile das Fließverhalten. / Ferritic-pearlitic steel 42CrMo4N and pure iron under different strain paths are compared regarding their flow behaviour. Mainly in cyclic torsion tests with resulting strain increment per cycle shear stresses and strain hardening are analysed.
The results show, that the cementite lamellae determine the flow behaviour of the steel 42CrMo4N, whereas the properties of pure iron are governed by the evolving dislocation cell structure. The dependency of flow stress on the strain direction is different for the two materials.
Yield surfaces describe strain hardening quantitatively. The procedure for yield point detection and the dependency of subsequent yield surfaces on experimental conditions and strain states is shown. For pure iron isotropic hardening, for steel 42CrMo4N kinematic strain hardening dominates the flow behaviour.
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Verformungsverfestigung bei zyklisch inkrementeller Torsion von Reineisen und dem Stahl 42CrMo4NKuprin, Corinna 04 October 2012 (has links)
Die Arbeit widmet sich dem Fließverhalten von Stählen bei zyklisch inkrementeller Torsion. Dazu werden vergleichend Reineisen und der ferritisch-perlitische Stahl 42CrMo4N bei unterschiedlichen Verformungspfaden betrachtet. Vor allem in zyklischen Torsionsversuchen mit bleibendem Verformungsinkrement je Zyklus werden Fließkurvenverläufe und Verformungsverfestigung analysiert.
Die Ergebnisse belegen den Einfluss des lamellaren Zementit auf das Fließverhalten des Stahls 42CrMo4N, während die Eigenschaften des Reineisens von der entstehenden Versetzungszellstruktur bestimmt werden. Die Richtungsabhängigkeit der Fließspannung und die Verläufe der Fließkurven unterscheiden sich für die betrachteten Werkstoffe deutlich.
Fließortkurven dienen der quantitativen Beschreibung der Verfestigung. Die Vorgehensweise zu ihrer Ermittlung und ihre Abhängigkeit von den Versuchsbedingungen und den Verformungszuständen werden gezeigt. Bei Reineisen dominieren isotrope, bei dem Stahl 42CrMo4N kinematische Verfestigungsanteile das Fließverhalten. / Ferritic-pearlitic steel 42CrMo4N and pure iron under different strain paths are compared regarding their flow behaviour. Mainly in cyclic torsion tests with resulting strain increment per cycle shear stresses and strain hardening are analysed.
The results show, that the cementite lamellae determine the flow behaviour of the steel 42CrMo4N, whereas the properties of pure iron are governed by the evolving dislocation cell structure. The dependency of flow stress on the strain direction is different for the two materials.
Yield surfaces describe strain hardening quantitatively. The procedure for yield point detection and the dependency of subsequent yield surfaces on experimental conditions and strain states is shown. For pure iron isotropic hardening, for steel 42CrMo4N kinematic strain hardening dominates the flow behaviour.
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To Make Iron of Iron : A Comprehensive Analytical Study of Spade Shaped Iron BarsPappas Adlreburg, Nickolas January 2017 (has links)
This thesis aims to provide adequate analytical information on the spade shaped iron bars of Norrland and central Sweden. While their significance has been thoroughly debated for decades, analytical research on them has been confined to cases of single artefacts or theoretical interpretations of their value, meaning and origin. In this study a comprehensive approach is taken into consideration. Based on X-Ray fluorescence (XRF), scanning electron microscopy (SEM) and metallographical analysis this thesis seeks to facilitate new interpretations on quality, production centres and usage based on analytical results. Aiming to settle some of the long lasting questions regarding the artefacts while producing results which can further the discussion by raising new questions, previously unasked.
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