Abstract: Asymmetric rolling is a promising forming technique offering numerous possibilities of material properties modification and the improvement of technological process parameters. This geometry of deformation is relatively easy to implement on existing industrial rolling mills. Moreover, it can provide large volume of a material with modified properties. The study of microstructure, crystallographic texture and residual stress in asymmetrically rolled titanium (grade 2) is presented in this work. The above characteristics were examined using EBSD technique and X-ray diffraction. The rolling asymmetry was realized using two identical rolls, driven by independent motors, rotating with different angular velocities ω1 and ω2. This ensured a wide range of rolling asymmetry: A=ω1/ω2. It was found that a strong shear stress induced in the asymmetrically rolled material allowed to obtain a microstructure refinement, texture homogenization and lowering of residual stress.
Abstract: Starting during the shut-down of the HZB research reactor BER-II in 2011/2012 the E3 residual stress and texture diffractometer in Berlin underwent a comprehensive upgrade. The investments were broken down into different criteria, such as enhancing the instrument performance and accuracy as well as extending the range of applications for the user community. Here, we report about the gains achieved after integrating and commissioning the individual hardware and software tools included in the upgrade project, namely a motorized primary slit to accurately adjust the gauge volume, a secondary optics radial collimator and a laser scanner to precisely and quickly align the sample. The integration of the presented devices is further supported by software developments to shorten the instrument alignment procedure and measurement time. The upgrade has improved the efficiency of the available neutron beamtime.
Abstract: Current status of a dedicated neutron diffractometer (TAKUMI) for investigations of stresses and crystallographic structures in engineering components, that has been built at Materials and Life Science Facility of J-PARC, was introduced as well as the engineering studies done there. Several topics recently performed at TAKUMI were selected to be introduced; internal strains in an ITER central solenoid conductor sample, internal strains in a rebar embedded in an air-cured concrete, deformation behavior of TRIP steels at RT and tensile behavior of LPSO MgZnY alloys. These topics and the relevant results showed that TAKUMI is a powerful neutron diffractometer for various engineering studies.
Abstract: There are two major types of solid state phase transformations in metallic materials; the formation of second phase particles during heat treatments, and the transformation of the matrix from one crystalline packing arrangement to another during either heating or cooling. These transformations change the spacing between adjacent atoms and can thus influence the residual stress levels formed. The heating and cooling cycles of materials processing operations using lasers such as cladding and melting/heating, can induce phase transformations depending on the character of the material being processed. This paper compares the effects of the different phase transformations and also the influence of the type of laser processing on the final residual stress formed. The comparisons are made between laser clad AA7075, laser clad Ti-6Al-4V and laser melted nickel-aluminium bronze using neutron diffraction and the contour method of measuring residual stress.
Abstract: An important issue to be considered in the life assessment of power plant components is the effects of prior creep damage on subsequent fatigue crack growth and fracture behavior. To examine these effects, creep damage has been introduced into 316H stainless steel material by interrupting creep crack growth (CCG) tests on compact tension, C(T), specimens at 550 °C. During the CCG tests, the specimen is loaded in tension, crept and unloaded after a small amount of crack extension. This process introduces compressive residual stress fields at the crack tip, which may subsequently affect the fatigue crack growth test results. In this work, neutron diffraction (ND) measurements have been conducted on interrupted CCG test specimens, which contain creep damage local to the crack tip, and the results are compared to predictions obtained from finite element (FE) simulations. Reasonable agreement has been found between the FE predictions and ND measurements.
Abstract: A mirror furnace for in situ residual stress measurements by neutron diffraction is designed. Bulk and stick sample can be in situ heated up to 1000 degrees centigrade. The stability of the temperature is better than ±1 degree centigrade. This furnace is designed mainly for residual stress neutron diffraction experiments. And also it can be applied to some other neutron instruments such as neutron texture diffractometer, neutron powder diffractometer and triple axis spectrometer.
Abstract: Carbon-carbon composites are deemed as candidate materials for application in very high temperature reactors. In a very high temperature reactor, carbon-carbon composite materials would experience severe environmental impacts from high temperatures. As a result, we applied non-destructive ex-situ diffraction experiments to investigate the microstructure changes of the carbon-carbon composite materials experiencing different temperatures. In this study, the samples were prepared in a format of a three-dimensional pitch-based carbon-carbon composite. The samples were heated to 500 (°C), 700 (°C), and 900 (°C) for 2 minutes, respectively. In order to understand the temperature effect on carbon-carbon composite, we facilitated the high penetration of the synchrotron X-ray diffraction at National Synchrotron Radiation Research Center to examine the evolution of microstructures subjected to heat treatment. The results show that the lattice parameters of a-axis and c-axis evolve upon heating. The molecular dynamics simulation results suggest that the early-stage rearrangement is originated from the release of the defects.
Abstract: Residual stress distributions in fillet welds in 8 mm 900 MPa steel have been mapped perpendicular and parallel to the weld line and also through the thickness in the vicinity of weld toe position. Measurements were carried out on four welds when two of them were performed with conventional and two with the so called LTT (low transformation temperature) filler materials. Both neutron and X-ray diffractions were used for determination of the residual stress distribution. Fatigue properties have also been evaluated for all test welds. Neutron diffraction measurements showed that the stress profiles perpendicular to the weld toe qualitatively did not depend on filler material type although the absolute stress levels differed. Trends were similar for positions 2, 4 and 6 millimetres below the surface for all three stress components; σx (direction perpendicular to the weld), σy (parallel to the weld) and σz (through the thickness). X-ray diffraction showed difference in residual stress level at the weld toe. Lower residual stress levels have been identified for LTT filler material when compared to the conventional consumable compositions. The effect of residual stress is discussed in relation to fatigue properties of all four welds. Remarkable higher fatigue strength has been measured for welds prepared by the LTT filler materials.
Abstract: Welding processes create a complex transient state of temperature that results in post-weld residual stresses. The current work presents a finite element (FE) analysis of the residual stress distribution in an eight-pass slot weld, conducted using a 316L austenitic stainless steel plate with 308L stainless steel filler metal. A thermal FE model is used to calibrate the transient thermal profile applied during the welding process. Time-resolved body heat flux data from this model is then used in a mechanical FE analysis to predict the resultant post-weld residual stress field. The mechanical analysis made use of the Lemaitre-Chaboche mixed isotropic-kinematic work-hardening model to accurately capture the constitutive response of the 316L weldment during the simulated multi-pass weld process, which results in an applied cyclic thermo-mechanical loading. The analysis is validated by contour method measurements performed on a representative weld specimen. Reasonable agreement between the predicted longitudinal residual stress field and contour measurement is observed, giving confidence in the results of measurements and FE weld model presented.