Roman Wawszczak

Residual stress field in steel samples during plastic deformation and recovery processes

An X-ray diffraction method was applied to measure residual stresses and stored elastic energy in deformed and annealed polycrystalline ferritic and austenitic steel samples. The orientation distribution of plastic incompatibility second-order stresses created during elastoplastic deformation was determined and presented in Euler space. Using deformation models, these stresses were correlated with different types of intergranular interactions occurring in the studied materials. An important decrease of the first- and the second-order residual stresses was observed during recovery and recrystallisation processes. Diffraction peak widths, related to dislocation density, were studied and correlated with stress variation during annealing process. Differences in stress relaxation between ferritic and austenitic samples were explained by different values of the stacking fault energy, which influences dislocation climb and cross-slip.

Problem of elastic anisotropy and stacking faults in stress analysis using multireflection grazing-incidence X-ray diffraction

Multireflection grazing-incidence X-ray diffraction (MGIXD) was used to determine the stress- and strain-free lattice parameter in the surface layer of mechanically treated (polished and ground) tungsten and austenitic steel. It was shown that reliable diffraction stress analysis is possible only when an appropriate grain interaction model is applied to an anisotropic sample. Therefore, verification of the X-ray stress factors (XSFs) was accomplished by measuring relative lattice strains during an in situ tensile test. The results obtained using the MGIXD and standard methods ( and geometries) show that the Reuss and free-surface grain interaction models agree with the experimental data. Moreover, a new interpretation of the MGIXD results was proposed and applied for the first time to measure the probability of stacking faults as a function of penetration depth for a polished and ground austenitic sample. The XSF models verified in the tensile test were used in the analysis of residual stress components.

Incompatibility Stresses and Elastic Energy Stored in Polycrystalline Materials

ray diffraction method is used to determine the stress field in polycrystalline materials. The measurement of peak shifts enables the determination of the macrostresses and the plastic incompatibility stresses (intergranular stresses). In the interpretation of the experimental results self-consistent model of elatoplastic deformation is used. In the present work, the plastic incompatibility stresses and the elastic energy stored in cold rolled brass and ferritic steel were determinate. The results are discussed and presented in Euler space.

Evolution of Residual Stresses and Stored Elastic Energy in Ferritic Steel during Recovery Process

Diffraction method was applied to determine the residual stresses in deformed and annealed polycrystalline samples of ferritic steel. The specific stored elastic energy corresponding to the grain stresses was calculated and presented in Euler space. An important decrease of the first and second order residual stresses and consequently stored elastic energy was observed during recovery and recrystallization. The evolution of stresses was correlated with the variation of diffraction peak width (related to dislocations density) and crystallographical texture.

Residual Stress in α-Brass during Annealing

The evolution of residual stress and crystallographic texture during thermal treatment was studied using X-ray diffraction. Polycrystalline α-brass samples were examined after cold rolling and afterwards after annealing at different temperatures in the range of 50 0C - 450 0C. Additionally, the width of the diffraction peak was measured in order to estimate the variation of the dislocation density. The interpretation of experimental data was based on a fitting procedure for which the anisotropic diffraction elastic constants calculated by a self-consistent approach were used. As the result of analysis, the values of the first order and second order stresses were determined in each sample.

A multireflection and multiwavelength residual stress determination method using energy dispersive diffraction

Multireflection grazing-incidence X-ray diffraction was used to investigate the structure and residual stress gradients in the near-surface region of mechanically treated titanium samples. The development of this method by using a white synchrotron beam during an energy dispersive diffraction experiment is proposed.

Study of Stress Partitioning in a 0.68 wt%C Pearlitic Steel Using High Energy X-Ray Synchrotron Radiation

In the present work, the evolution of the phase stresses in a 0.68 wt%C pearlitic steel is analyzed by synchrotron diffraction during uniaxial tensile loading, at room temperature. The diffraction measurements were done at ESRF beamline ID15B (Grenoble, France). The microstructure of the studied material, obtained after an austenitizing at 1050°C for 7 minutes followed by cooling under blowing air, corresponds to fully pearlitic steel with a cementite volume fraction of about 12.5%. As expected, the results show a clear effect of elastic and plastic anisotropy in the both phases. For the interpretation of the diffraction data, different models are compared. In elastic range and for small plastic deformation, the self-consistent model presents the best agreement with the experimental data. For large plastic deformation, this model does not predict correctly the stress partitioning between the phases as well as the macro behavior of the studied steel. Therefore a mixture model “(1-x)*self-consistent model + x*Taylor” was used to take into account the interaction between the phases. Introduction Offering an excellent combination of ductility, strength and cost, the fully pearlitic steels are the most used plain carbon steels in manufacturing to produce wires for reinforcing tires, cables for suspension bridges, engineering springs for automotive and railroads. The role of the microstructure and specially the interlamellar space in the mechanical behavior and fatigue resistance of pearlitic steels has been studied in previous works [1, 2]. In-situ diffraction technique during mechanical loading is a powerful method to investigate the mechanical behavior of the phases and the stress partitioning between the cementite and the ferrite [2-5]. Only few results concerning the role of the Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 521-526 doi: http://dx.doi.org/10.21741/9781945291173-88 522 lamellar cementite in the hardening of the fully pearlitic steel have been reported [1, 2]. We propose in this study to apply to fully pearlitic steel an approach based on the analysis of the mechanical properties of the polycrystalline material at the grain and phase scale using synchrotron X-ray diffraction technique and elasto-plastic models. Material The pearlitic steel EN C70 (SAE 1070) investigated in this study was produced by ASCOMETAL France Company in the form of bars of 80 mm in diameter. Its chemical composition is given in Table 1. The bars were submitted to an austenitizing at 1050°C for 7 minutes followed by cooling under air blown. The resulting microstructure consists of entirely pearlite colonies having an average size of 78 μm with cementite plates lamellae spaced by 170 nm (Fig. 1). The cementite fraction was estimated at 12.5 vol % using Rietveld phase quantification from X-ray diffraction data. The mechanical properties of the obtained microstructure are reported in Table 2. Table 1Chemical composition of EN C70 pearlitic steel (wt%) C Si Mn S P Ni Cr Mo Cu Al Fe 0.68 0.192 0.846 0.010 0.010 0.114 0.160 0.027 0.205 0.042 balance Table 2Mechanical properties of the annealed EN C70 pearlitic steel Yield Stress [MPa] Ultimate Stress

The Role of Intergranular Stresses in Plastic Deformation Studied Using a Diffraction and Self-Consistent Model

Diffraction methods are commonly used for the determination of the elastic lattice deformation from the displacement and broadening of the diffraction peak. The measurements are performed selectively, only for crystallites contributing to the measured diffraction peak. When several phases are present in the sample, measurements of separate diffraction peaks allow the behaviour of each phase to be investigated independently [e.g. 1-4]. Comparison of experimental data with a multi-scale model allows us to understand the physical phenomena which occur during sample deformation at the level of polycrystalline grains. In the present work the methodology combining diffraction experiment and self-consistent calculation was used to study the mechanical behaviour of groups of grains within stainless duplex steel and Al/SiC composite. Special attention has been paid to the role of second order stresses on the yield stresses of the phases, as well as on the evolution of these stresses during the deformation process. The intergranular stresses were determined from lattice strains measured “in situ” during tensile tests. The diffraction measurements were done using synchrotron (ID15B, ESRF, Grenoble, France) and neutron (EPSILON, FLNP, JINR, Dubna, Russia) radiations. Experimental methodology To study the elastoplastic behavior of two phase materials the diffraction measurements were performed “in situ” during a tensile test. Two materials, i.e., an aged duplex steel UR45N (50% of ferrite and 50% of austenite; microstructure, texture and composition are given in [3,4]) and a particle reinforced Al/SiCp composite (Al2124 matrix with 17% of SiC particles with size of 0.7 μμ subjected to the T1 treatment ) were studied using diffraction methods of two types. The Al/SiCp Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 551-556 doi: http://dx.doi.org/10.21741/9781945291173-93 552 composite was obtained by powder metallurgy processing followed by the T1 thermal treatment (air cooled from elevated temperature forming process). For the steel specimen monochromatic synchrotron radiation with an energy of about 90keV (λ = 0.14256Å) and a beam size of 100 × 100 μμ2 was applied. The measurements were carried out at the European Synchrotron Radiation Facility in Grenoble on the ID15B beamline. The diffraction pattern in the range of 2θ = 1.7 − 7.5° was collected on two-dimensional detector PIXIUM4700 in the form of concentric rings corresponding to different hkl reflections for both phases: ferrite and austenite. The necessary conversion of two-dimensional images into 2θ diffractograms was performed with FIT2D software [5]. The peak positions were determined using the MULTIFIT software [6] and after that Braggs’ law was employed to determine the interplanar spacings dhkk. The second experiment was performed for the particle reinforced Al/SiCp composite specimen using the time-of-flight (TOF) neutron diffraction method enabling simultaneous measurement of different hkl reflections for both phases in the studied composite. For data acquisition two of nine detector banks covering a 2θ-range of 82°≤2θ≤98° were employed on the EPSILON-MDS diffractometer at the Joint Institute For Nuclear Research, Dubna, Russia [7]. A geometry of this kind allowed to determine stress tensor if the lattice strains are measured for two sample orientations with respect to the experimental setup. The measurement was performed for the initial material and after deformation of the sample during a tensile test. The incident beam of 10 mm width was pointed at the sample of 4.4 mm x 4.4 mm cross-section. Evolution of deviatoric stresses in duplex steel during plastic deformation Initial state of the specimen The analysis of the initial stresses and of the evolution of the lattice parameter for both phases was performed for the duplex steel measured using synchrotron radiation. Firstly the stress tensor was determined for the initial non-deformed sample. The principal stresses were decomposed into two parts: hydrostatic (p) and deviatoric (q, r, s), according to Eq. 1. The deviatoric stresses were determined directly from the measured lattice strains, while the additional assumption p = 0 (zero values of hydrostatic stresses) was introduced in order to calculate the initial values of the lattice parameter a0 for both phases. The results are presented in Table 1. � σ11 0 0 0 σ22 0 0 0 σ33 � = � p 0 0 0 p 0 0 0 p � + � q 0 0 0 r 0 0 0 s � (1) Table 1. Initial stresses and lattice parameters determined for both phases of the studied steel assuming zero value of hydrostatic stresses. The parameters used in the self-consistent model are also presented. Austenite Ferrite Initially measured values (q) σRR[MMa] 134 ± 15 -155 ± 19 (r) σTR[MMa] 84 ± 15 -44 ± 19 (s) σNR[MMa] -218 ± 15 199 ± 18 a0 [Å] 3.6102 ± 0.0001 2.8791 ± 0.0001 Single crystal elastic. constants c11, c12, c44[GMa] 198 , 125 , 122 231, 134, 116 Slip systems 〈111〉{110} and 〈111〉{211} 〈110〉{111} Parameters of Voce law τ0 [MMa] 170 370

Residual stress in ferrite and austenite after rolling and recovery processes

The relation between residual stresses occurring in plastically deformed material and after subsequent annealing is of practical and theoretical importance. In the present work the X-ray multi-reflection method was applied to determine residual stresses and their orientation distribution in rolled and annealed ferrite and austenite steel samples. An important decrease of the first- and the second-order residual stresses was observed during recovery and recrystallization processes. Diffraction peak width was also studied and correlated with stress variation during annealing. Different kinetics of stress relaxation in ferrite and austenite were explained by different levels of stacking fault energy and different types of intergranular interactions occurring in these materials.

Residual Stresses in Austenitic Steel during Plastic Deformation and Recovery Processes

X-ray diffraction method was applied to measure residual stresses in deformed and annealed polycrystalline austenitic steel. An elastoplastic deformation model was used in analysis of experimental data. As the result, the orientation distribution function of grain stresses, created during elastoplastic deformation was determined and presented in the Euler space. An important decrease of the first and the second order residual stresses was observed during recovery process. It was found that the magnitude of the stresses decreases, while their distribution between different grain orientations remains almost unchanged.