| 1. |
- Gyhlesten Back, Jessica
(författare)
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Modelling and Characterisation of the Martensite Formation in Low Alloyed Carbon Steels
- 2017
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The current work contains experimental and theoretical work about the formation of martensite from the austenitic state of the steel Hardox 450. Simulation of rolling and subsequent quenching of martensitic steel plates requires a model that can account for previous deformation, current stresses and the temperature history, therefore dilatometry experiments were performed, with and without deformation. Two austenitization schedules were used and in the standard dilatometry the cooling rates varied between 5-100 °C/s, in order to find the minimum cooling rate that gives a fully martensitic microstructure. Cooling rates larger than 40°C/s gave a fully martensitic microstructure. The cooling rate of 100 °C/s was used in the deformation dilatometry tests where the uniaxial deformation varied from 5-50 %. The theoretical work involved modelling of the martensite formation and the thermal/transformation strains they cause in the steel. Characterizations were done using light optical microscopy, hardness tests and electron backscatter diffraction technique. The parent austenite grains of the martensitic structure were reconstructed using the orientation relationship between the parent austenite and the martensite. Kurdjumov-Sachs orientation relationships have previously been proven to work well for low-carbon steels and was therefore selected.The standard implementation of the Koistinen-Marburger equation for martensite formation and a more convenient approach were compared. The latter approach does not require the storage of initial austenite fraction at start of martensite formation. The comparison shows that the latter model works equally well for the martensite formation. The results showed that the use of martensite start and finish temperatures calibrated versus experiments for Hardox 450 works better when computing thermal expansion than use of general relations based on the chemistry of the steel.The results from deformation dilatometry showed that deformation by compressive uniaxial stresses impedes the martensite transformation. The simplified incremental model works well for deformation with 5 % and 10 %. However, the waviness in the experimental curve for deformation 50 % does not fit the model due vi to large barrelling effect and the large relative expansion for the material that the sample holders are made of.Crystallographic reconstruction of parent austenite grains were performed on a hot-rolled as-received reference sample and dilatometry samples cooled with 60 °C/s and 100 °C/s. The misorientation results showed that the samples match with the Kurdjumov-Sachs orientation relationship in both hot rolled product and dilatometry samples. When misorientation between adjacent pixels are between 15° and 48°, then the boundary between them was considered as a parent austenite grain. The austenitic grain boundaries of the sample cooled at 100 °C/s is in general identical with the hot rolled sample when considering high angle boundaries (15°-48°). The results from the hardness tests showed that the rolled product exhibits higher hardness as compared to samples cooled by 100 °C/s and 60 °C/s. This can be attributed to the formation of transition-iron-carbides in the hot rolled product due to longer exposure of coiling temperature.
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| 2. |
- Yin, Jiaqing, 1989-
(författare)
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Formation of Bainite in Steels
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A systematic survey of morphology of bainite and proeutectoid ferrite was carried out in order to validate some old thoughts of bainite transformation mechanism. It is confirmed that there is no morphological evidence supporting a sharp change neither between Widmanstätten ferrite and the ferritic component of upper bainite, nor between upper and lower bainite. Both Widmanstätten ferrite and upper bainite start with precipitation of ferrite plates at a grain boundary while lower bainite starts with intragranular nucleation. In case of grain boundary nucleation, a group of parallel plates with same crystallographic orientation to the parent austenite grain forms. This process is followed by a second stage of decomposition of the austenitic interspace, which remained in between the primary ferrite plates. At high temperature, the austenitic interspace would either retain as thin slabs or transform into pearlite through a nodule originated from a grain boundary. At lower temperature, cementite precipitation starts to be possible and initiates simultaneous growth of ferrite. Generally, there are two modes of such eutectoid reactions operating in the second stage, i.e. a degenerate and a cooperative mode, which would lead to typical upper and lower bainite, respectively, in definition of carbides morphology. Both upper and lower bainite according to this definition are observed in a wide temperature range. A sharp temperature between the upper and lower bainite structures thus exists only when the definition is based on their nucleation sites, i.e. grain boundary nucleation for upper bainite and intragranular nucleation for lower bainite. Supposing that the first stage is a diffusionless process it should have a high growth rate to prevent carbon diffusion. This is not supported by lengthening rate obtained in current study as well as data from literature for Fe-C alloys. Finally, it is shown that the “subunits” play no role in the lengthening process of bainite.
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| 3. |
- Zeng, Zhipeng, 1977-
(författare)
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Deformation Behaviour, Microstructure and Texture Evolution of CP Ti Deformed at Elevated Temperatures
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the present work, deformation behavior, texture and microstructure evolution of commercially pure titanium (CP Ti) are investigated by electron backscattered diffraction (EBSD) after compression tests at elevated temperatures. By analysing work hardening rate vs. flow stress, the deformation behaviour can be divided into three groups, viz. three-stage work hardening, two-stage work hardening and flow softening. A new deformation condition map is presented, dividing the deformation behavior of CP Ti into three distinct zones which can be separated by two distinct values of the Zener-Hollomon parameter. The deformed microstructures reveal that dynamic recovery is the dominant deformation mechanism for CP Ti during hot working. It is the first time that the Schmid factor and pole figures are used to analyse how the individual slip systems activate and how their activities evolve under various deformation conditions. Two constitutive equations are proposed in this work, one is for single peak dynamic recrystallization (DRX), the other is specially for CP Ti deformed during hot working. After the hot compression tests, some stress-strain curves show a single peak, leading to the motivation of setting up a DRX model. However, the examinations of EBSD maps and metallography evidently show that the deformation mechanism is dynamic recovery rather than DRX. Then, the second model is set up. The influence of the deformation conditions on grain size, texture and deformation twinning is systematically investigated. The results show that {10-12} twinning only occurs at the early stage of deformation. As the strain increases, the {10-12} twinning is suppressed while {10- 11} twinning appears. Three peaks are found in the misorientation frequency-distribution corresponding to basal fiber texture, {10-11} and {10-12} twinning, respectively. A logZ-value of 13 is found to be critical for both the onset of {10-11} compressive twinning and the break point for the subgrain size. The presence of {10-11} twinning is the key factor for effectively reducing the deformed grain size. The percentage of low angle grain boundaries decreases with increasing Z-parameter, falling into a region separated by two parallel lines with a common slope and 10% displacement. After deformation, three texture components can be found, one close to the compression direction, CD, one 10~30° to CD and another 45° to CD.
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| 4. |
- Wedberg, Dan
(författare)
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Dislocation density based material model applied in FE-simulation of metal cutting
- 2010
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Simulation based design enables rapid development of products with increased customer value in terms of accessibility, quality, productivity and profitability. However simulation of metal cutting is complex both in terms of numeric and physics. The work piece material undergoes severe deformations. The material model must therefore be able to accurately predict the deformation behavior for a large range of strain, strain rates (>50000 s-1) and temperatures. There exist a large number of different material models. They can be divided into empirical and physically based models. The far most common model used in simulation of metal cutting is the empirical Johnson-Cook plasticity model, JC model. Physically based models are based on the knowledge of the underlying physical phenomena and are expected to have larger domain of validity. Experimental measurements have been carried out in order to calibrate and validate a physical based material model utilizing dislocation density (DD) as internal variable. Split-Hopkinson tests have been performed in order to characterize the material behavior of SANMAC 316L at high strain rates. The DD model has been calibrated in earlier work by Lindgren et al. based on strain rate up to 10 s-1 and temperatures up to 1300 °C with good agreement over the range of calibration. Same good correspondence was not obtained when the model was extrapolated to high strain rate response curves from the dynamic Split-Hopkinson tests. These results indicate that new deformation mechanisms are entering. Repeating the calibration procedure for the empirical JC model shows that it can only describe the material behavior over a much more limited range. A recalibrated DD model, using varying obstacle strength at different temperatures, was used in simulation of machining. It was implemented in an implicit and an explicit finite element code.Simulation of orthogonal cutting has been performed with JC model and DD model using an updated Lagrangian formulation and an implicit time stepping logic. An isotropic hardening formulation was used in this case. The results showed that the cutting forces were slightly better predicted by the DD model. Largest error was 16 % compared to 20 % by the JC model. The predicted chip morphology was also better with the DD model but far from acceptable. Orthogonal cutting was simulated using an updated Lagrangian formulation with an explicit time integration scheme. In this case were two hardening rules tested, isotropic hardening and a mixed isotropic-kinematic hardening. The later showed an improvement regarding the feed force prediction. A deviation of less than 8% could be noticed except for the feed force at a cutting speed of 100 m/min. The time stepping procedure in combination with the mesh refinement seems to be able to capture the chip segmentation quite well without including damage evolution in the material model.Further works will mainly focus on improving the DD-model by introducing relevant physics for high strain rates.
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