Search

The electric resistance welding of thin-thickness duplex stainless steel (DSS) has an outstanding attraction in the plate-fin heat exchanger which is widely used in power machinery, diesel locomotive and other fields, but its structural integrity is threatened seriously by fatigue failure. Therefore, in this research, the formation of the heterogeneous microstructure of DSS electric resistance welding joints (ERWJ) was analyzed by the optical microscope (OM) and electron back-scattered diffraction (EBSD) observations, and the uniaxial tensile and fatigue tests were subsequently executed to disclose their mechanical properties. The intrinsic relevancy between phase-related characteristics and failure behavior was extensively analyzed. The results show that the ferrite was dominating at whole ERWJ, particularly at the energized area, accounting for up to 72%. More than 45% of high-angle boundaries (HABS) and the highest kernel average misorientation (KAM, 2.46° for austenite and 1.52° for ferrite), which indicates greater plastic deformation and residual stress, were observed at the weld edge. In addition, the failure occurred as a tear along the weld edge regardless of the monotonic and cyclic loadings under the deterioration of residual stress and island-like austenite. The distribution of fatigue life with cyclic amplitude performed an obvious turning point of 19.5 MPa due to the different failure mechanisms, providing a permissible range of the stress amplitudes and reinforcement strategies for the plate-shell heat exchanger ERWJ in the engineering. [ABSTRACT FROM AUTHOR]

• The correlation among heterogeneous microstructure, cyclic hardening/softening and fatigue failure behavior was analyzed extensively. • The fracture location was changed with the fatigue amplitude due to the amplitude-dependent and heterogeneous cyclic hardening/softening of BM and WM. • A modified cyclic constitutive model was proposed to precisely predict the heterogeneous cyclic mechanical response and failure location for both stress-controlled and strain-controlled conditions. This work performed experimental and modeling investigations on the fatigue failure mechanism of duplex stainless steel welded joints, and the correlation among heterogeneous microstructure, cyclic hardening/softening and fatigue failure behavior was analyzed extensively. A series of experiments including the characterization of microstructure by the electron back-scattered diffraction observation and fatigue mechanical tests assisted by digital image correlation technique were performed. The results show that at the lower stress amplitudes (approximately 520 MPa), the fatigue failure occurred in the weld metal, while it shifted to the base metal at higher amplitudes. Due to the grain refinement, the strain amplitude of weld metal was lower than that of base metal, particularly at the higher amplitudes, leading to the final failure at the base metal. However, at the lower amplitudes, due to the effect of Kurdjumov-Sachs orientation relationship at the weld metal, its cyclic softening behavior was much drastic than that of base metal, resulting in the overshoot of strain amplitude during fatigue process and finial rupture at the weld metal. A cyclic constitutive model was developed by considering the amplitude-dependent and heterogeneous cyclic hardening/softening, to accurately predict the cyclic mechanical response and fatigue failure location for both stress-controlled and strain-controlled conditions. This work can be used as guidance of structure integrity assessment and life prediction for the engineering components of duplex stainless steel. [ABSTRACT FROM AUTHOR]

Abstract In this study, the cyclic mechanical characters of 316L stainless steel at elevated temperature are extensively investigated by the experimental and cyclic constitutive models. The experiments include the monotonic tensile tests with different loading rates and the low cycle fatigue tests considering the effect of strain amplitudes, strain rates and loading sequences. The evolution of cyclic stress amplitudes, hysteresis loops and elastic modulus under various loading conditions are comprehensively analyzed. The experimental results show that the 316L steel at elevated temperature performs a typical three-stage cyclic mechanical response, i.e., initial hardening, subsequent saturation and final accelerated softening. The cyclic softening in both stiffness and flow stress is mainly caused by the nucleation of micro-voids or micro-cracks, and the subsequent coalesce and propagation. Furthermore, although the nearly rate-independent mechanical behavior is observed at monotonic tensile and first several fatigue cycles due to the DSA effect, the cyclic hardening/softening behavior shows a significant strain-rate and loading history dependence. Finally, inspired by the experimental observations and analyses, a damage-coupled cyclic elastic-viscoplastic constitutive model involving strain-range, strain-rate and loading history dependence is proposed to predict the complex cyclic behaviors of the material at elevated temperature. A hardening factor is incorporated into the Chaboche kinematic hardening equations to model the kinematic-induced hardening behavior. And the plastic strain memory surface and the maximum plastic strain rate are introduced to model the strain-range, strain-rate and loading history dependence of cyclic behavior. The proposed model is proved to effectively describe the complex evolution of not only cyclic stress amplitude but also hysteresis loops for the 316L steel at elevated temperature. Highlights • The cyclic response and fatigue mechanism for the 316L steel at room and elevated temperature were compared. • The various tensile and LCF tests considering the effect of strain range, strain rate and loading sequence were carried out. • The fatigue damage was determined experimentally, and incorporated into the cyclic elastic-viscoplastic constitutive model. • The relationship between strain rate, DSA effect and cyclic behavior were investigated and realized by the proposed model. • The proposed model can predict the evolution of not only cyclic stress amplitude but also hysteresis loops accurately. [ABSTRACT FROM AUTHOR]

• >Synthetical effect of material inhomogeneity and welding defect on fatigue behavior. • >New structure strain method including heterogeneous plasticity and stress singularity. • >Evolution of fatigue mode with geometric configurations and cyclic amplitudes. • >A damage tolerance-based fatigue design concept to guide manufacture of welded joints. This paper performed experimental and analytical investigations on the synthetical effect of material inhomogeneity and welding defects on the fatigue behavior of 2205 duplex stainless steel cruciform welded joints. A series of experiments were performed, including microstructure characterization by the electron back-scattered diffraction, fatigue mechanical tests assisted by digital image correlation technique and fracture morphology observations by scanning electron microscope. Results show that fatigue life and failure location of cruciform welded joints depended heavily on their geometric configurations for lower cyclic amplitudes. However, with the elevation of loading, material inhomogeneity played a greater roles, leading to the change of fatigue failure mode with both geometric configuration and cyclic stress amplitude. To quantize the effects of welding defects and material inhomogeneity, a modified structure strain method was proposed by considering the heterogeneous plastic properties, to accurately predict both fatigue lifetime and failure location of cruciform welded joints at both low and high cycles fatigue. Lastly, a damage tolerance-based fatigue design method was discussed to guide the highly-reliable manufacture and damage detection of duplex stainless steel cruciform welded joints in the engineering. [ABSTRACT FROM AUTHOR]

In this paper, the relaxation of weld residual stress in a 316L stainless steel weld joint under cyclic loading was researched by experimental and finite element method (FEM). Initially, the as-weld residual stresses were calculated by a sequential coupling thermo-mechanical FEM. Subsequently, a cyclic plasticity constitutive model was proposed to study the redistribution of the residual stress by the cyclic load. Significant residual stresses are released during the first few cycles. Especially, about 45–60% of the maximum residual stresses are released during the first cycle because of the plastic deformation caused by the superposition between the as-weld residual stress and the applied load. More residual stresses are released with the increase of the stress amplitude and cyclic number. An analytical model, which considers the effects of the initial residual stress, yield stress, stress amplitude, and number of cycles, was proposed to predict the relaxation of the weld residual stress by the cyclic load. In addition, experimental measurements were also performed to validate this model. Experimental results prove that the proposed model can be used as a valid tool to predict the relaxation of residual stress by the fatigue load. [ABSTRACT FROM AUTHOR]

• A multiscale model was proposed to accurately predict the heterogeneous and multiscale elasto-plastic properties of 2205 DSS welded joints. • The contribution of Peierls–Nabarro stress, solid solution strengthening, grain/phase boundary, and dislocations density on the macro-mechanical properties (YS and UTS) was quantified. • The evolution of heterogeneous micro-mechanical responses with two constitutive phases with the elevation of loads was predicted accurately. • The banded GBAs and its adjacent ferrite were more prone to plastic-deformed. This work aims to reveal the inherent dependency between the heterogeneous microstructure induced by the metallurgical transformation during welding of 2205 duplex stainless steel (DSS) and its macroscopic and microscopic elasto-plastic properties by experiments and models. A multiscale model was developed by combining a macro-constitutive model incorporating the physically-based yield strength and a crystal plasticity model considering both the micro-mechanical properties of two phases and their realistic morphology. The results show that the heterogeneous macro-mechanical properties of base metal (BM), heat affected zone (HAZ), and weld material (WM), as well as their micro-mechanical responses related to phase morphology, ratio, and grain size, can be accurately predicted. The contribution of Peierls–Nabarro stress, solid solution strengthening, grain/phase boundary, and dislocations density on macro-yield and tensile strengths was quantified, and WM was proven to have greater yield and tensile strengths due to the greater contribution of grain refinement and solid solution strengthening. In addition, a transition mechanism of the deformation-bearing phase from austenite to ferrite with the increase of load was proved, and the banded grain boundary austenite (GBA) and its adjacent ferritic grains were prone to plastic-deformation. This work can be used as guidance of structure integrity assessment and welding process optimization for the engineering components of DSS. [ABSTRACT FROM AUTHOR]

In this study, the microstructure characteristics and formation mechanism of DSS (duplex stainless steel) multi-pass weld joints were firstly investigated in terms of the OM (optical microscope) and EBSD (electron back-scattered diffraction) observation, and a series mechanical tests, including the monotonic tensile tests assisted by the DIC (digital image correlation) observation and stress-controlled fatigue tests, were subsequently performed to clarify the intrinsic correlation between mechanical behavior and microstructure. The fatigue fracture morphologies and cracks propagation behavior were analyzed by the OM and SEM (scanning electron microscope) observation. The results show that the significant ferritiszation and determined precipitations were found at the HAZ (heat-affected zone), whereas the austenite was dominating at weld filler zone due to the more formation of the IGA (intragranular austenite). A nearly balance phase ratio was obtained at weld back and cap zones, but the strip GBA (grain boundary austenites) was connected into piece along thickness direction. The monotonic tensile strength of weld joints was higher than that of the based materials due to the refined-grains structures, and the final rupture occurred at the BM (based material) zone. However, the fatigue lifetimes of weld joints were much shorter than that of based materials for the same cyclic amplitude. The fatigue cracks preferred to nucleate at weld back or cap passes although where phase ratio was approximately balance. The austenitic grains which were connected into pieces accelerated the initiation of fatigue cracks. Apart from maintaining phases balance and avoiding detrimental precipitation, the alternative distribution of austenite and ferrite should also be obtained in the engineering. [ABSTRACT FROM AUTHOR]

• Failure analysis of bolt-gasket-flange joints in ultra-high pressure pipelines was carried out by the combination of experimental and simulated methods. • The differences of microstructure and mechanical properties between serviced gaskets and unused gaskets were investigated by a series of tests. • An improved layer-variable parameter numerical model which considers the heterogeneous mechanical properties of the gasket along the thickness direction is to accurately predict its stress and deformation. • Gasket material degradation caused by frequent pressure fluctuations at the working environment of high temperature and high pressure is the main cause of seal failure. This study was focused on the conical gaskets in ultra-high pressure pipeline sealing devices. Failure analysis of the conical gaskets was carried out by combining gasket experiments and numerical simulations. Macroscopic dimension inspection, miniature specimen tensile tests, micro-hardness tests, metallographic tests, and scanning electron microscope (SEM) experiments were carried out to study the differences between used gaskets and unused gaskets. Results show that the mechanical properties of the used gaskets degenerate and microstructural changes occur, which is caused by frequent pressure fluctuations in the working environment of high temperature and high pressure. Besides, the stress distribution and deformation of the gasket, as well as the effect of pressure fluctuation on the gasket stress, were analyzed by numerical simulation. Meanwhile, an improved layer-variable parameter numerical model is proposed for the numerical simulation of the used conical gaskets. The model was used to compare the elastic resilience of the gaskets before and after service under different preload, respectively. Results show that the simulated results by the model is closer to the experimental results. The elastic resilience of the used gaskets is significantly less than that of the unused gaskets. Therefore, it can be concluded that the mechanical properties degradation and accumulation of plastic deformation of the gaskets caused the lack of the elastic resilience of the gasket, which may be the direct cause of the seal failure. [ABSTRACT FROM AUTHOR]