The microvoid based Gurson-Tvergaard-Needleman (GTN) model is a powerful tool for predicting ductile fracture behavior, and application of such model to steels and welds needs the identification of ...microvoid related damage parameters. Currently, there is no standard damage parameter identification method available. In this study the previously proposed complete Gurson model (CGM) where a physical void coalescence mechanism is incorporated into the GTN model is revisited. According to the CGM, the void nucleation process dictates ductile fracture. By adopting the cluster nucleation model with an effective initial void volume fraction as the only controlling parameter, a method is proposed to explicitly determine the effective initial void volume fraction from the strain at maximum load and strain at fracture of a specially designed notched tensile specimen. The proposed equation has been experimentally verified by applying to three high strength materials, including a X80 pipeline steel and associated weld metal, and a 15CrMo steel. A general procedure for damage parameter identification is also suggested. It is argued that the obtained effective initial void volume fraction can be treated as a type of material ductility indicator.
Using the electrochemical noise method, this paper studies the effect of microstructure of weld seam on the stress corrosion cracking behavior of Ti–6Al–4V alloy weldment. Local rapid induction ...heating can significantly change the microstructure, and the acicular α′ martensite phase in the weld seam is decomposed into lathy α phase and transformed β phase. The higher heat temperature leads to more transformed β phase and less lathy α phase. The mechanical properties and stress corrosion cracking sensitivity during the slow strain rate tensile test are significantly affected by the microstructure of weld seam, which are improved with the decomposition of acicular α′ martensite and decrease of lathy α phase content combined with increase of transformed β phase content. Compared with the non-treated sample, the induction heating enhances the corrosion resistance of the sample and delays the occurrence time of localized corrosion during the slow strain rate tensile test. The higher corrosion resistance and the later occurrence time of localized corrosion are caused by the higher induction heating temperature. The microstructural evolution of the weldment significantly inhibits the stress corrosion cracking process.
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In this study, the residual stresses distribution induced by multi-pass arc welding of the steel S355J2+N are investigated experimentally and numerically. An extended approach is used for the ...simulations, which considers the change of the local microstructure properties due to multiple reheating. Experimental material data obtained from physical welding simulations with Gleeble® are used for the model calibration. The experimental stress study is performed using a neutron diffraction method on a fourier stress diffractometer. Numerical analysis of the welding stresses formation in the weldment is performed and compared to the experimental study. The results explain the influence of the welding thermal history on the resulting local thermo-mechanical properties in the heat-affected zone and, thus, on the residual stress distribution. The consideration of the local microstructure properties in the welding simulation leads to a significant increase in accuracy of the numerical results. The major influence factor on the residual stress formation is the change in the interpass microstructure yield strength. When a root pass with short cooling times is subjected to re-austenitisation in the fine-grained zone, the yield strength increases in this area and affects consequently the residual stress distribution. The influence of the reheating is detectable in the depth of the weldment, but it is less significant for the residual stress formation near the surface of the welded joint.
•Thermo-mechanical properties of S355 steel grade indicated during multiple thermal cycles•FE welding simulation considering microstructure properties as a function of the local multiple thermal cycles•Comparison of simulation results with and without consideration of the multiple heating effects•Validation of the numerical results through neutron diffraction measurements in the depth of the specimen
Ships and offshore structures may be operated in areas with seasonal freezing temperatures and extreme environmental conditions. While current standards state that attention should be given to the ...validity of fatigue design curves at subzero temperatures, studies on fatigue strength of structural steel at subzero temperatures are scarce. This study addresses the issue by analysing the fatigue strength of welded steel joints under subzero temperatures. Although critical weld details in large welded structures are mostly fillet‐welded joints, most published data are based on fatigue crack growth rate specimens cut out of butt‐welded joints. This study analyses fillet‐welded specimens at −20°C and −50°C against controls at room temperature. Significantly higher fatigue strength was measured in comparison to estimates based on international standards and data from design codes even at temperatures far below the allowed service temperature based on fracture toughness results.
The local creep properties of a CrMoV weldment were acquired by using miniature specimens of in-situ creep test. The creep performances of heat-affected zone (HAZ), weld pass metal in the welding ...direction (WP) and multiple weld passes through the thickness direction (MWP) were compared with each other. It is shown that the magnitudes of creep strength order satisfy WP > MWP > HAZ, while the creep ductility of these three samples exhibit an inverse order: WP < MWP < HAZ. In addition, the typical three-stage creep deformation curves are observed in both WP and MWP samples. However, no significant difference between the primary and the secondary stages is detected for HAZ sample.
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Effects of laser shock processing (LSP) on the cavitation erosion resistance of laser weldments were investigated by optical microscope (OM), scanning electron microscope (SEM) observations, ...roughness tester, micro hardness tester, and X-ray diffraction (XRD) technology. The morphological microstructures were characterized. Cumulative mass loss, incubation period, erosion rate, and damaged surface areas were monitored during cavitation erosion. Surface roughness, micro-hardness, and residual stress were measured in different zones. Results showed that LSP could improve the damage of morphological microstructures and mechanical properties after cavitation erosion. The compressive residual stresses were generated during the process of LSP, which was an effective guarantee for the improvement of the above mentioned properties.
Within stress‐based fatigue assessment concepts, causes that do not influence the fatigue stress parameters, such as temperature, can only be accounted for by means of modification factors. The ...strain energy density (SED) method allows to account for changing material support effects and Young's modulus with temperature directly. Thus, in this study, a concept is presented to extend the SED method for fatigue assessment of welded joints at sub‐zero temperatures. For this purpose, fatigue test results of welded joints made from normal and high‐strength structural steel are assessed in the range of 20°C down to −50°C. The results are evaluated based on the formula that is used to derive the SED control radii of welded joints and compared with results of studies on SED‐based assessment of notched components at high temperatures. From the estimates of the control radii, a temperature modification function for SED is derived for design purposes.
•The performance of alloy 80A weldment in a high-temperature aggressive environment was studied.•The effect of hot corrosion on different welding techniques (GTAW and PCGTAW) and filler wires ...(ERNiCrMo-3 and ERNiCr-3) were studied.•Influence of PCGTAW technique in protecting the surface of the welded substrate from an aggressive environment.IV) The mechanism of the formation of corrosive and protective oxides and its effect on the corrosion behaviour of welded substrates were studied.
One of the significant issues that shorten the life of components used in high-temperature applications is hot corrosion. The current study compares the performance of welded aerospace-grade 80A fabricated through continuous and pulsed gas tungsten arc welding techniques. Weld coupons are produced using two different filler wires (ERNiCrMo-3 (Mo-3) and ERNiCr-3 (Cr-3)). Welded substrates are subjected to 50 cycles of air oxidation and Na2SO4 + 60%V2O5 molten salt environmental conditions at 900 °C. Corrosion products were analysed through scanning electron microscope/energy dispersive spectroscopy and X-ray diffraction analyses. Thermogravimetric analysis revealed that all welded substrates trailed the parabolic rate of law kinetics. In the molten salt (MS) environment, gas tungsten arc welded (GTAW) Mo-3 substrate showed more weight gain. In contrast, a minor weight gain was observed in the pulsed current gas tungsten arc welded (PCGTAW) Cr-3 substrate. It indicates that accelerated corrosion kinetics was observed in the molten salt environmental condition that showed more weight gain than air oxidation. amongst the weldments, pulsed current gas tungsten witnessed superior corrosion-resistant behaviour. This phenomenon is observed due to grain refinement suppression of heat-affected zones and secondary phases in the weld fusion zone. In addition, the appearance of protective oxides such as Cr2O3, NbO and NiCr2O4 helps arrest the oxidation at the surface and sub-surface layer by providing good resistance against hot corrosion to the welded substrate. Thus, utilizing the PCGTAW technique during the fabrication/refurbishing process helps promote the material's corrosion resistance against a hot corrosion environment.
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