Materials Science Forum Vols. 490-491

Abstract: High strength and ductility of the TRIP steels is often attributed to the transformation induced plasticity effect resulting from the strain induced martensitic transformation of the retained austenite in the bainite microstructure. The present work reports results of in-situ neutron diffraction experiments focused on monitoring the phase evolution in two TRIP steel samples (two different thermomechanical treatments) subjected to tensile loading at room temperature. Comparison of the single lineprofile analysis of reactor data (TKSN-400 at NPI Rez) and multi lineprofile analysis of data obtained at spallation neutron source (diffractometer ENGIN-X at ISIS RAL Chilton) suggests that the former can be used in the first approximation for in-situ monitoring of the phase evolution in TRIP steels subjected to mechanical loads.

Abstract: Thin films deposited by physical vapor deposition (PVD) were studied in terms of residual stress by the authors. The final purpose of our study is to evaluate the stress state at the interface between a substrate and a thin film. In this study, JIS-SKH55 tool steel without thin-film deposition was used as the specimen. SKH55 is a dual-phase steel consisting of martensite a’Fe and alloyed carbide M6C2. The specimens were heated to 573K, 798K, 843K and 893K. Recently, the relationship between the misfit of plastic strain and stress obtained by X-ray stress measurement has been proposed by the authors using the Eshelby/Mori-Tanaka model (EMT model). The residual stress and the misfit of plastic strain were determined by X-ray stress measurement using the EMT model. Results showed that as annealing temperature increased, the compressive residual stress remained nearly constant up to about 800K, and decreased above 800K in both phases. The misfit of plastic strain also remained nearly constant up to about 800K, and reached zero above 800K.

Abstract: Synchrotron x-rays are used for in situ determination of oxide strain, during oxide formation on a Kanthal A1 FeCrAlZr substrate at 1160°C. The measurements rely on use of high-energy (~80keV) x-rays and transmission geometry, and the methodology of the strain measurements is presented. Oxide growth strains at elevated temperature, relative to pure alumina, were seen to be small, while temperature excursions induced significant strains. Furthermore, significant strain relaxation was observed during isothermal holds, suggesting oxide creep as a major relaxation mechanism. Upon cooling to room temperature, significant residual strains developed, with a corresponding in-plane residual stress of -3.7 GPa.

Abstract: The contour method, a newly-invented sectioning technique for residual stress measurement, has the potential to measure the cross-sectional residual stress profile of a weld in a simple and time-efficient manner. In this paper we demonstrate the capability of the contour method to measure cross-sectional residual stress profiles, which are compared with neutron diffraction measurements and show excellent agreement. The results provide useful information for safetycritical design of welded components and optimization of welding parameters, and also illustrate the potential of the contour technique as a powerful tool for residual stress evaluation.

Abstract: Drill pipe is made from 35CrMnMo. The used drill pipes were curved, when they were aligned and deformed, there is high macro residual stress in the aligned used drill pipe. In active service, the macro residual stress releases and thus causes considerable deformation of the aligned used drill pipe. The vibratory stress relief is introduced to the aligned used drill pipe. The vibratory stress relief of the aligned used drill pipe was carried out. The mechanical properties of the aligned used drill pipe were measured before and after vibration. Seven points vibratory stresses were measured. The macro residual stress was measured before and after vibration. The macro residual stress decreased notably. The mechanical properties changed slightly. The vibratory wave is standing wave. The vibratory stress relief is valid for macro residual stress relieving of the aligned used drill pipe. The aligned used drill pipes were steady in latter active service. The mechanism of the macro residual stress relieving by vibration on the aligned used drill pipe is discussed. The “double-dynamic mechanism” is introduced to explain the course of macro residual stress relieving in the test.

Abstract: Surface generation using conventional machining processes shows a residual stress distribution which mainly depends on the process parameters. This distribution affects the mechanical stability of the layers affected by the process. Particularly, when a specific level of the residual stresses is reached, a stress corrosion cracking (SCC) of the 316L austenitic stainless steel occurs in the MgCl2 (40%) solution at 142°C. Results obtained in this investigation have shown that the non-cracking residual stress threshold value depends not only on the couple workspace material / environment, but on the surface generation process also. Indeed while, a threshold SCC value of +380 MPa was measured in the case of surfaces generated using the turning process, a value of +280 MPa only was evaluated for the ground surfaces. This difference was discussed on the basis of the relation between the mechanism of crack initiation under SCC and the electrochemical reactivity of the generated surfaces using a specific process. Hence, one can conclude that the residual stress heterogeneity at the surface as well as in the under layer and the finished surface reactivity explain the morphology of the generated cracks by SCC and the sense of the non-cracking threshold must be revised.

Abstract: The change of magnetic flux as a function of applied tensive stress has been investigated under geomagnetic field for a cuboid specimen of 0.45% wt C steel. A hole was drilled in the center of the specimen to generate stress concentration and the stress concentration of the specimen was theoretic calculated. The magnetic flux near the surface was measured using a reluctance effect sensor with sensitivity of 5 2 10 Gs − × in directions parallel and perpendicular to tensile direction. To each specimen magnetic field distributions of 9 stress cycles, amplitude form 0kN to 120kN, were measured. The results showed a reversible dependence of this field on tensile after the first few stress cycles. Correlation coefficients of these data also show this trend. During the cycles, domain motion and rotation occurred in response to tensile stress. This change included reversible and irreversible process. After several tensile cycles at same stress amplitude, all irreversible components were overcome and the change of magnetic field became mostly reversible. Comparing with the previous work, the reversible magnetic field versus tensile stress shows clearer and more valuable relationship between stress and field.

Abstract: In this paper, The effects of laser-shock peening and high temperature deep rolling on nearsurface microstructures, residual stress states and fatigue behavior of various metallic materials are investigated and discussed. Similar to warm peening (shot peening at elevated temperatures), high temperature deep rolling may induce several favourable effects, especially in ferritic steels, where dynamic strain aging by carbon atoms can be exploited as a major strengthening mechanism. But also in materials without ‚classical‘ strain aging high temperature deep rolling is effective in improving the fatigue behaviour by inducing favourable, e.g. precipitation-hardened, nearsurface microstructures. As a consequence, these modified near-surface microstructures directly alter the thermal and mechanical relaxation behaviour of residual stresses. Laser-shock peening is already used in the aircraft industry (as a mechanical surface treatment for fan-blades) and owes its benefial effects to deep layers of compressive residual stress and work hardening and a relatively smooth surface roughness. Characteristic examples of microstructures and residual stress profiles as generated by laser-shock peening are presented. Moreover, the impact on the fatigue behavior of steels and a titanium alloy is outlined and discussed.