No description available.

No description available.

Made available in DSpace on 2014-10-09T12:36:19Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:59:16Z (GMT). No. of bitstreams: 1 12908.pdf: 2121499 bytes, checksum: c2d2abf4d18418855c9e93a43337b8ad (MD5) / Dissertacao (Mestrado) / IEA/D / Escola Politecnica, Universidade de Sao Paulo - POLI/USP

Made available in DSpace on 2014-10-09T12:36:19Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:59:16Z (GMT). No. of bitstreams: 1 12908.pdf: 2121499 bytes, checksum: c2d2abf4d18418855c9e93a43337b8ad (MD5) / Dissertacao (Mestrado) / IEA/D / Escola Politecnica, Universidade de Sao Paulo - POLI/USP

This thesis is concerned with the physical principles, methodology and applications of neutron diffraction in the measurement of residual stress. Work on three main areas is presented. 1) Carbon steels 2) Data and Peak Broadening analysis and 3) Single lap glue shear joints. The Carbon steels section shows the drastic effect of the content of carbon on the measured stress. This is an aspect which has been somewhat neglected in the past. The carbon is in the form of cementite, which is a hard compound and causes the carbon steel to act like a composite material, the ferrite acting as a soft matrix and the cementite as a reinforcement. The consequence of this is that the two components develop high microstresses with plastic deformation. This is clearly illustrated in the work of [Bon 97] where values of approx. 460 MPa in the residual stress in the ferrite are balanced by negative residual stresses of 2300 MPa in cementite yielding an overall macro residual stress of zero. In this work it has been shown that even knowledge of the cementite and ferrite residual stresses and fractions may not be sufficient to accurately calculate the macro stress since the ferrite unloading curve is non linear. The use of a single valued constant modulus to convert from strain to stress is hence not valid. Peak shape analysis enables dislocation density and cell size estimates to be made. The thesis examines several methods of data weighting and deconvolution in order to asses the best means of extracting this information from standard residual stress data. Care should be taken for the peaks with very low backgrounds when finding the Gaussian and Lorentzian components. A weighting that avoids the strong bias of zero and I counts in the detector channels should be used e.g. W = I / ( 10 + Y). Lorentzian and Gaussian components can be successfully extracted from asymmetrical peaks (of peaks that broaden symmetrically), using deconvolution method 1, although the data should be of good quality. Reproducibility has been shown in the Gaussian, Lorentzian and FWHM for different instruments at different institutes. This is extremely important for the use of these values for peak broadening analysis and for estimation of the plastic deformation within a sample. The neutron diffraction technique has been used to investigate the longitudinal stresses in the adherend produced as a result of cure and due to the application of a tensile load in a single lap shear joint. The results throw doubt on widely used finite element predictions.

The annealing behaviour, including studies of recrystallisation kinetics and development of crystallographic texture, of two low carbon steels after different cold rolling reductions have been investigated using Optical Microscopy (OM), Electron Back-Scatter Diffraction (EBSD) and Transmission Electron Microscopy (TEM). The primary recrystallisation behaviour of 20, 50, 70 and 90% cold rolled Interstitial Free (IF) and High Strength Low Alloy (HSLA) steels was studied. The HSLA was initially processed to give a volume fraction of about 0.2 of fine pearlite colonies, which acted as mechanically hard particles. The presence of such particles on the HSLA steel significantly reduced the temperature needed for recrystallisation by enhancing the recrystallisation and acting as nucleation sites by the Particle Stimulated Nucleation (PSN) mechanism. The inhomogeneous deformation and the local orientation changes introduced in the neighbourhood of the carbide particles (i.e. the particle deformation zone) were observed using TEM and selected area electron diffraction. The JMAK model was used to analyse the recrystallisation kinetics of the two steels. The experimental data plotted according to the JMAK model could be represented by straight lines with a JMAK exponent n falling in the range from 1.4 to 2.0. The development of crystallographic textures after cold rolling reductions and subsequent recrystallisation has been investigated. The texture development is shown to be largely dependent on the rolling reduction. With increasing rolling reduction, the annealing texture show gradual intensification of α- and γ-fibre components. However, the intensity of both fibres is weaker in the HSLA steel. Despite the dominance of PSN in HSLA steel, the recrystallisation textures were similar to that of the rolling textures with weaker intensity of both fibres. To help clarify the reasons for that, in-situ EBSD experiments of recrystallising HSLA steel deformed to 50% and 70% have been carried out. It shows that the formation of the nucleus seems to occur within the deformation zones in regions away from the particle surface leading to recrystallisation textures similar to that of the rolling textures (i.e. both α- and γ-fibre exist). The validity of this assumption has been confirmed by the use of Monte Carlomodelling. This model was used to simulate, in general way, and study the phenomenon of PSN during the recrystallisation process. The simulation shows the discontinuous evolution of the subgrains in the deformation zone to form recrystallisation nuclei around the particle. It shows also the subsequent growth of these nuclei to consume the matrix region around the particle. The simulation results are shown to match with the experimentally observed features of the recrystallisation phenomena in low carbon steel containing coarse cementite particles.

Martensitic steels have become very important engineering materials in modern society. Crucial parts of everyday products are made of martensitic steels, from surgical needles and razor blades to car components and large-scale excavators. Martensite, which results from a rapid diffusionless phase transformation, has a complex nature that is challenging to characterize and to classify. Moreover the possibilities for modeling of this phase transformation have been limited, since its thermodynamics and kinetics are only reasonably well understood. However, the recent development of characterization capabilities and computational techniques, such as CALPHAD, and its applicability to ferrous martensite has not been fully explored yet. In the present work, a thermodynamic method for predicting the martensite start temperature (Ms) of commercial steels is developed. It is based mainly on information on Ms from binary Fe-X systems obtained from experiments using very rapid cooling, and Ms values for lath and plate martensite are treated separately. Comparison with the experimental Ms of several sets of commercial steels indicates that the predictive ability is comparable to models based on experimental information of Ms from commercial steels. A major part of the present work is dedicated to the effect of carbon content on the morphological transition from lath- to plate martensite in steels. A range of metallographic techniques were employed: (1) Optical microscopy to study the apparent morphology; (2) Transmission electron microscopy to study high-carbon plate martensite; (3) Electron backscattered diffraction to study the variant pairing tendency of martensite. The results indicate that a good understanding of the martensitic microstructure can be achieved by combining qualitative metallography with quantitative analysis, such as variant pairing analysis. This type of characterization methodology could easily be extended to any alloying system and may thus facilitate martensite characterization in general. Finally, a minor part addresses inverse bainite, which may form in high-carbon alloys. Its coupling to regular bainite is discussed on the basis of symmetry in the Fe-C phase diagram. / <p>QC 20120824</p> / Hero-m

In this study a new method is introduced to produce bimodal grain structures in low carbon steels. This method is based on cold rolling of dual phase structures and appropriate annealing treatments. The difference in the recrystallization behaviour of ferrite and martensite yields a heterogeneous microstructure with a distribution of coarse and fine grains. These types of microstructures are of interest for optimizing the balance of strength and uniform elongation in ultra-fine grained low carbon steels.

Made available in DSpace on 2014-10-09T12:25:36Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:03:46Z (GMT). No. of bitstreams: 1 06790.pdf: 5752105 bytes, checksum: 9b66a692f462ce07b2dd97fe9220e337 (MD5) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Made available in DSpace on 2014-10-09T12:47:51Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:06:33Z (GMT). No. of bitstreams: 1 08319.pdf: 6875638 bytes, checksum: 6b5eeabf820dd27ca2fa6fcb250bcc1b (MD5) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP