Made available in DSpace on 2014-10-09T12:31:41Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:01:07Z (GMT). No. of bitstreams: 1 02594.pdf: 1955228 bytes, checksum: 34c0873d245b469a86c13624608fed2b (MD5) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares, IPEN-CNEN/SP

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.

The effect of surface finish on fatigue limit of two types of austenitic stainless steels (AISI 304L and AISI 316L) has been investigated. Fatigue specimens having two different surface conditions were obtained by changing the final cutting condition; annealing was performed to separate the residual stress effects from surface roughness. Electropolished samples were tested as a reference for each material. A generic mechanistic model for short fatigue crack propagation proposed by Navarroand Rios (N-R model) was implemented to assess its suitability for predicting the fatigue behaviour of specimens with various controlled surface conditions, obtained by machining. The surface/material properties required to implement this model were obtained by electron backscatter diffraction (EBSD), surface profilometry, hardness testing and X-ray diffraction residual stress measurement. The fatigue limits were determined using rotating-bending by means of the staircase method. The fatigue limits predicted by the N-R fatigue model were compared with the results of the fatigue tests. There was no agreement between the prediction and observations, indicating that the original form of the N-R model is not appropriate for austenitic stainless steels. In AISI 304L, the surface residual stresses are the dominant parameter, allowing prediction of the effects of machining on fatigue resistance while, the surface roughness developed by machining has no significant effect. In AISI 316L, the effect of surface roughness is found to be negligible, with a weaker effect of surface residual stress than has been observed for AISI 304L. Crack nuclei in run-out (>107 cycles) fatigue tests were observed to arrest at twins and martensite packets, developed by fatigue in AISI 316L and AISI 304L, respectively. Good agreement with experiments was achieved by using a modification to the fatigue model, which takes account of the observed effect of the plastic deformation on the microstructure.

The bending and spring back behaviors of DP780 and DP980 were investigated using experiments, analytical models, and PEA. An PEA study using 3D shells was first performed of the work by Queener and DeAngelis (1968) and demonstrated that the 3D shell element behavior in bending and springback, is similar to that from simple bending theory. Experimental and FE studies were then performed using DP780 and DP980 steels under simple and general bending conditions. Simple bending conditions were studied in V -die bending. General bending was studied for DP780 using a commercial bending machine. The PEA examined the effect of element formulation and material hardening assumptions on springback, bending stresses, and the residual stress distributions after springback. Corresponding simple and general bending analytical models were also compared. The simple bending model was from Queener and DeAngelis ( 1968), and the general bending model was from Tan et al. ( 1995), modified for Ludwig hardening. Overall, the PEA predicted the springback magnitude in the order; 2D continuum >3D continuum> 3D shells, and kinematic hardening> mixed hardening> isotropic hardening. In the V -die bending study the 3D shell PEA, using a calibrated mixed hardening assumption, produced the most accurate results. The PEA using pure isotropic hardening demonstrated that bending and springback behaviors for both steels were consistent with that described by simple bending theory. The behaviors demonstrated by the FEA using mixed or pure kinematic hardening were not. In the general bending study, the 3D continuum element FEA using pure kinematic hardening was the most accurate. The 3D continuum element FEA captured the bending stress interaction with the hardening assumption as well as thinning deformation, in agreement with the analytical model and thinning measured experimentally. 3D Shell elements could not capture these behaviors and significantly under-predicted springback under the pure isotropic hardening assumption. / Thesis / Master of Applied Science (MASc)

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Recovery, recrystallization, grain growth and precipitation constitute the fundamentals of thermomechanical controlled processing (TMCP) of microalloyed steels. In-depth understanding of these phenomena is indeed needed. In this work, the individual components and some of the potential mutual interactions have been investigated deliberately. The effect of alloying elements of Mn, Si, and Al on recovery and recrystallization has been systematically studied by conducting the stress relaxation tests on binary Fe-0.1%C and ternary Fe-0.1%C-X alloys. The effect of temperature on recovery kinetics was also investigated. The effects were considered by fitting the recovery model through the activation volume term. Higher temperature or lower solute content will accelerate the recovery process and then facilitate the onset of recrystallization. NbC precipitation behavior has been investigated using a nickel-based model alloy, having samples deformed at both room temperature and elevated temperature and subjected to annealing at 700℃ for different times, in order to elucidate the stages of nucleation, growth and coarsening for precipitation. The microstructures preserved by water quenching were examined using transmission electron microscopy (with both metal foil and carbon replica specimens). Results from mechanical response and microstructural evolution are linked and discussed. The precipitate number density and size evolution show good agreements with predictions from a classical strain-induced precipitation model. The in-situ laser-ultrasonics measurement of C-Mn steels provides a unique way to evaluate grain size evolution during TMCP, for different strains of 0.15, 0.25 and 0.35, at 950℃ and 1050℃. Effects of temperature and strain on recovery, recrystallization and grain growth have been covered and elucidated. Higher strains facilitate the onset of recrystallization and grain size refinement. However, higher temperatures only shorten the onset of recrystallization but lead to larger grain size. The effect of microalloying element of Nb on softening kinetics was also investigated by comparing C-Mn/C-Mn-Nb steels at the same conditions. The solute drag effect of Nb can be seen by the onset-delays of recrystallization and larger grain sizes. The laser-ultrasonics results can match well with stress relaxation measurements. The in-situ grain size evolution data has given the possibility to develop robust thermomechanical processing (TMP) models combining deformation, recovery, precipitation, recrystallization and grain growth. The application and validation of the TMP models have been attempted and remain ongoing. / Thesis / Doctor of Philosophy (PhD)

Yes / The effects of 0·5 wt-%Mo addition on the processing, microstructure, and strength of PM Fe–3·5Mn–0·7C steel are described. Water atomised and sponge irons, Astaloy 1·5Mo, milled ferromanganese, and graphite were the starting powders. During sintering in 75H2 /25N2 or pure hydrogen the dewpoint was controlled and monitored; in particular the effects of improving it from -35 to -60°C were investigated. Faster heating rates (20 K min-1), sufficient gas flowrates, milling the ferro alloy under nitrogen, a low dewpoint (<-60°C), and a getter powder can all contribute to the reduction or prevention of oxidation of the manganese, in particular formation of oxide networks in the sintered steels. For 600 MPa compaction pressure densities up to 7·1 g cm-3 were obtained; these were not significantly affected by sintering at temperatures up to 1180°C. The sintered microstructures were sensitively dependent on the cooling rate. Irrespective of the presence of Mo, slow furnace cooling at 4 K min-1 resulted in mainly pearlitic structures with some ferrite and coarse bainite, whereas fast cooling at 40 K min-1 produced martensite and some retained austenite, very fine pearlite, bainite, and some ferrite. Young's modulus, determined by tensile and ultrasonic tests, was in the range 110–155 GPa. Sintering with -60°C dewpoint resulted in tensile and transverse rupture strengths of 420 and 860 MPa for the Mn steel, rising to 530 and1130 MPa as a result of the Mo addition. This contrasts with strength decreases observed when processing included use of high oxygen containing ferromanganese and sintering with -35°C dewpoint.

The recent development of tool steels and high-speed steels has led to a significant increase in alloy additions, such as Co, Cr, Mo, N, V, and W. Knowledge about the phase relations in these multicomponent alloys, that is, the relative stability between different carbides or the solubility of different elements in the carbides and in the matrix phase, is essential for understanding the behaviour of these alloys in heat treatments. This information is also the basis for improving the properties or designing new alloys by controlling the amount of alloying elements. Thermodynamic calculations together with a thermodynamic database is a very powerful and important tool for alloy development of new tool steels and high-speed steels. By thermodynamic calculations one can easily predict how different amounts of alloying elements influence on the stability of different phases. Phase fractions of the individual phases and the solubility of different elements in the phases can be predicted quickly. Thermodynamic calculations can also be used to find optimised processing temperatures, e.g. for different heat treatments. Combining thermodynamic calculations with kinetic modelling one can also predict the microstructure evolution in different processes such as solidification, dissolution heat treatments, carbide coarsening, and the important tempering step producing secondary carbides. The quality of predictions based on thermodynamic calculations directly depends on the accuracy of the thermodynamic database used. In the present work new experimental phase equilibria information, both in model alloys containing few elements and in commercial alloys, has been determined and was used to evaluate and improve the thermodynamic description. This new experimental investigation was necessary because important information concerning the different carbide systems in tool steels and high-speed steels were lacking. A new thermodynamic database for tool steels and high-speed steels, TOOL05, has been developed within this thesis. With the new database it is possible to calculate thermodynamic properties and phase equilibria with high accuracy and good reliability. Compared with the previous thermodynamic description the improvements are significant. In addition the composition range of different alloying elements, where reliable results are obtained with the new thermodynamic database, have been widened significantly. As the available kinetic data did not always predict results in agreement with new experiments the database was modified in the present work. By coupling the new thermodynamic description with the new kinetic description accurate diffusion simulations can be performed for carbide coarsening, carbide dissolution and micro segregation during solidification. / QC 20100929