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This paper studied the creep rupture behavior of Hastelloy C276-BNi2 brazed joint by experimental. Based on a series of creep rupture tests at 600 °C under 110–250 MPa, a relation between the stress and rupture time was achieved to evaluate the creep life of brazed joint. The results show that all the brazed joints were fractured in brazed seam immediately after the second creep stage. The creep model, fracture behavior and failure mechanism are dependent on the level of applied stress. The creep behavior of the brazed joint should be depicted by two-regime Norton (2RN) creep model rather than single Norton model. The creep damage tolerance factor was used to identify the cause of creep damage. From low to high stress level, the failure mode is changed from intergranular brittle to transgranular ductile fracture due to the increases of damage tolerance factor with applied stress increases. [ABSTRACT FROM AUTHOR]

Creep crack growth behavior of the 9Cr-1Mo steel under multi-axial stress state at 600 °C was investigated by a modified creep damage model. Firstly, a modified creep damage model was proposed and incorporated into the finite element software ABAQUS by the CREEP subroutine. And then the creep and damage behaviors of the 9Cr-1Mo steel were simulated by the notched bar and CT specimen. The results indicate that the multi-axial creep ductility and the life of the high temperature components calculated by the modified model are corresponding well with the experimental data, and the life prediction precision has been greatly improved compared to that from the K-R model. This demonstrates that the proposed model can be used to accurately predict the creep crack growth behaviors of the high temperature materials. The creep crack growth rate a ̇ presents a linear relationship with the crack driving force parameter C * in log-log coordinate system. Based on the established relationship between the a ̇ and C *, the creep crack growth rate of the 9Cr-1Mo steel components working at high temperatures can be predicted. [ABSTRACT FROM AUTHOR]

Neutron diffraction measurement, contour method and finite element method have been combined together to study the distribution of weld residual stresses in a thick plate considering the effect of back chipping. The comparison among the three methods, the effect of back chipping and the formation mechanism of the residual stresses in the thick plate were analyzed. The results show that the residual stress distribution exhibits an “M” shape across the specimen width. Large tensile residual stresses were generated in the back weld and top weld layer. Back chipping has a great influence on the residual stress profile. It can change not only the distribution shape of residual stress but also the position of the peak value. The residual stresses increase with the increase in back chipping thickness. The residual stress distribution is mainly formed in the welding of back weld. As the layer number increases, the residual stress distribution in the main weld does not change obviously and only the stress values change. [ABSTRACT FROM AUTHOR]

We developed an analytical model to predict the equivalent creep strain rate of a lattice truss panel structure. The model, which takes into account the effects of the bonded node and the intersection node of the trusses, is well validated by finite element analysis. Compared with Hodge and Dunand model, this model obtains a more accurate prediction result. The creep deformation of the panel structure is controlled by the creep of vertical trusses parallel to the applied load. The equivalent creep strain rate is determined by five key parameters including punching angle, cutting angle, truss thickness, width and length. A slight change of truss dimension can lead to a big variation of the creep rate by orders of magnitude. With the increase of punching angle and cutting angle, the relative density decreases and the stresses in the trusses increase, leading to an increase of creep rate. With the increase of truss thickness and width, the creep rate decreases because the relative density increases and the stresses in the truss decrease. As the truss length increases, the creep rate increases due to the decrease of relative density and the increase of stresses in the truss. [ABSTRACT FROM AUTHOR]

Abstract: The lattice truss sandwich structures are considered as the most promising advanced lightweight materials used in modern industries and aircrafts. Most sandwich panel structures are fractured at brazed joints named node failure during static and dynamic testing, which is mainly influenced by brazing residual stresses. Finite element method (FEM) was used to study the brazing residual stresses in a stainless steel X-type lattice truss sandwich structure. And the nanoindentation experiment is used to verify the validity of FEM. The effects of braze processing parameters including applied load, face sheet thickness, truss thickness and truss length on residual stresses have been investigated. It is shown than the residual stresses are concentrated on the brazed joint, which has a significant effect on node failure. As the applied load increases, the residual stresses decrease first and then remain unchanged, and the optimal applied load is around 1MPa. As the face sheet thickness increasing, the residual stresses are increased. Too thin face sheet can cause large residual stresses on the top surface of face sheet. With truss thickness and truss length increase, the residual stresses are decreased first and then increased. The optimized face sheet, truss thickness and truss length are found to be 2mm, 1mm and 26mm. [Copyright &y& Elsevier]

Ultra-thick duplex stainless steel welding plates are widely used in various industries. However, one of the significant concerns is the inhomogeneous microstructure and mechanical properties along the thickness, which reduces welding efficiency and degrade service performance. In this study, a 55 mm thick duplex stainless steel welding joint with an X-type weld groove was prepared to study the variation of microstructure and mechanical properties along the through-thickness direction by both the miniature tensile specimen and cross-weld tensile specimen. The correlation between the microstructure and mechanical property was also studied. The results showed that the weld metal was characterized by abundant columnar grains with decreasing austenite content from surface to mid-thickness. Besides, the total fraction of low angle grain boundaries increased more than one-fold, while the total fraction of high angle grain boundaries decreased from 48.03% to 4.52% with increasing depth to the mid-thickness. Furthermore, substantial substructured grains and recrystallized grains were dominated in the weld metal and base metal. Along the thickness direction, the mechanical strength almost decreased and the elongation of welding joint increased with the increasing distance from the surface. Microhardness had a similar trend to that of strength, which all can be attributed to the variation of austenite content and grain boundary character distribution. Along the transverse direction, weld metal had the highest strength of 597.67 MPa, followed by the heat-affected zone of 549.42 MPa and base metal of 510.20 MPa, while the trend of elongation was opposite to that of strength. The microhardness distribution was not consistent with that of strength and had a maximum at the heat-affected zone. [ABSTRACT FROM AUTHOR]

The effects of borides dissolution during the homogenization treatment on the mechanical properties and fracture behavior of austenitic stainless steel brazed joints are investigated. The microstructure evolution of brazed joints is investigated by field emission scanning electron microscope (FE-SEM) and electron probe microanalyser (EPMA). The mechanical properties of brazed joints are investigated by microhardness test and tensile test of T-shape brazing specimen. In particular, electron back scatter diffraction (EBSD) measurement is conducted to study the fracture behavior of brazed joints. The results show that the diffusion affected zone of the brazed joints contains substantial Cr enriched boride precipitates with different morphologies. The homogenization treatment effectively eliminates the boride precipitates and significantly reduces the difference in chemical composition between different zones. The boride precipitates experience the fragmentation and dissolution during the homogenization treatment. In addition, the microhardness of the diffusion affected zone gradually decreases as the homogenization time increases. The maximum force of the tensile test increases first and then decreases as the homogenization treatment time increases. The fracture mechanism of the brazed joints is discussed. After the homogenization treatment, the fracture position of brazed joints changes from the diffusion affected zone to the base metal. It is attributed to that the dissolution of boride precipitates enhances the binding force of grain boundaries and improve the cracking resistance of the diffusion affected zone. [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]