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The chemical cleaning of the vertical superheater needs to be considered more; the cleaning steps and agent selection are incorrect, and the cleaning flow rate is lower, resulting in a large amount of residual liquid and residue accumulation in pipes. An apparent leakage occurs in the weld zone inside the pipes during service. To study the failure in the steam superheating section piping of the hydrogen production from natural gas, macroscopic inspections, mechanical properties testing, chemical composition analysis, metallographic microstructure, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and energy backscattering diffraction (EBSD) and chemical cleaning, etc., technologies were used. The experiment's results confirm the reason for cracks occurrence in 12Cr1MoVG steel piping that the alkali stress corrosion cracking occurs. A large amount of sodium citrate in the residual liquid decomposes CO2 at high temperatures, and NaOH in the running liquid reacts with CO2 to produce Na2CO3 precipitation. The NaOH and Na2CO3 aqueous solutions were concentrated at high temperatures to produce strong alkalinity. At the same time, the hardness of welded joints exceeds the standard, the internal stress is higher, and alkali stress corrosion cracking occurs. On this basis, suggestions and countermeasures for the failure of steam pipes of hydrogen production from natural gas are put forward. [ABSTRACT FROM AUTHOR]

The creep-fatigue behaviors of P92 steel under strain range of 0.3 %–0.5 % and test temperature of 600–650 °C was studied carefully in this paper. With the increase of temperature, the creep-fatigue life is significantly reduced, and more vulnerable to temperature than strain amplitude. In addition, the dislocation density decreases with increasing creep fatigue, and the martensite laths become coarser. Furthermore, the increase of strain amplitude leads to more significant secondary cracks and fatigue striation. The higher temperature causes much deeper and larger dimples. During the test, the growth and accumulation of precipitates inevitably lead to stress concentration, resulting in material fracture and destruction. Finally, the linear cumulative damage (LCD), the modified ductility exhaustion (MDE) and the frequency separation life (FSL) model are used to predict the creep-fatigue life of P92 steel, and it is found that the frequency separation life model had the highest prediction accuracy among the threes. [ABSTRACT FROM AUTHOR]

In this paper, tensile test and creep‐fatigue test at 650°C were carried out for 316L steel with and without prior thermal aging. The effect of aging on creep‐fatigue interaction of 316L steel is analyzed in detail. Microscopic observations are performed on fatigue and creep‐fatigue specimens to study the microstructure characteristics affected by aging, creep, and fatigue. Actually, three strain amplitude loadings of ±0.3%, ±0.4%, and ±0.5% are adopted in creep‐fatigue tests. The creep‐fatigue behaviors of unaged and aged 316L steel are investigated in depth for varying strain amplitude. The creep‐fatigue constitutive model is constructed by using Chaboche mixed softening and hardening model combined with strain hardening creep. Life prediction models are compared and used to do prediction. The applicability of different models is recommended. The accuracy of the models is analyzed by comparing with the test data. The agreement is achieved between theoretical and experimental predictions. Highlights: Influences of thermal aging on creep‐fatigue behaviors of 316L steel are investigated.The creep‐fatigue behaviors are characterized with cyclic stress–strain response curve.Microstructural observations are performed to identify the failure mechanism.Life prediction is compared between theoretical and test data with various models. [ABSTRACT FROM AUTHOR]

The technology of pulse plasma modification of the working surface of the stamp of 4Kh5MF1S tool steel (analogues: in the EU - X40CrMoV5-1; in China - 4Cr5MoSiV1) is considered. The mentioned tool is used for stamping billets of titanium VT6 alloy (wt.%: Al – 3.0…6.8; V – 3.5...5.0; Ti – base), which is performed at temperatures up to 700 °С. The surface of the stamp is heated, which leads to its oxidation and diffusion redistribution of alloying elements. Pulse-plasma stamping leads to the formation of elastic-plastic deformations of the surface layer in tool steel, which in combination with pulsed thermal and electromagnetic effects provides refinement of the alloy structure and intensifies the diffusion mechanisms of alloying elements. Studies showed that the modified layer (over 80 μm thickness) in 4Kh5MF1S steel, formed in the process of pulse-plasma treatment, contains up to 2.5% carbon, up to 12% oxygen and up to 3% tungsten. In the mentioned layer the presence of nanocrystalline structures with a size of less than 100 nm was revealed. The hardness of the modified layer is more than 700 HV 0.025. The surface roughness after pulse-plasma treatment did not change. Experience of industrial use of this technology showed that modification of a surface of a stamp from 4Kh5MF1S steel provided its high efficiency at deep drawing of products from the heated (to 700 °C) sheet of VT-6 titanium of 3 mm thickness. 11 Ref., 1 Tabl., 6 Fig. [ABSTRACT FROM AUTHOR]

[Display omitted] • A Tube-to-Tube sliding wear test rig is developed. • Sliding wear volume of Inconel 690 tube increases with increasing cycle number. • Sliding wear volume of Inconel 690 tube is larger under larger contact force. • Wear volume is smaller under lubrication of water, especially at beginning. • Water reduces oxidative wear due to lower oxygen concentration. A Tube-to-Tube sliding wear test rig was developed in the current work. The reciprocating sliding wear behaviors on Inconel 690 heat transfer tubes were experimentally studied under various conditions. The sliding amplitude and frequency have little influence on wear, and the wear volume is in general positively proportional to the sliding cycle number and the contact force. Therefore, the Archard wear equation can be adopted to calculate the macroscale wear volume of the heat transfer tubes. Based on the results obtained by optical microscopy and scanning electron microscopy, the main wear mechanisms of the tube are the adhesive wear, abrasive wear and oxidative wear. Under the water environment, the wear rate is evidently lower than that of the dry condition and the wear mechanisms were not altered. [ABSTRACT FROM AUTHOR]

• GNS layer induced by SMRT enhanced the strength-ductility synergy of 316L steel. • Gradient microstructure effectively inhibits strain localization during deformation. • SMRT 316L displays phase transformation and cyclic softening under fatigue at 650 ℃. • Fatigue life is well predicted by CPFE model with plastic-slip-based indicator. • Multi-scale failure mechanism is studied for GNS 316L under strain fatigue at 650 ℃. Gradient nanostructured materials have excellent fatigue resistance. At present, there are few research on the fatigue properties of gradient nanostructured (GNS) materials at high temperature. Herein, the multi-scale failure mechanism of GNS 316L steel at 650 °C is studied, and the high temperature fatigue tests under different strain amplitudes are conducted. Then, post-test microstructure observations were carried out to reveal the damage mechanisms, and the fatigue behavior was numerically simulated by crystal plasticity model. The experimental results show that the strength of the GNS 316L steel is still higher than that of the coarse-grained (CG) 316L steel at 650 °C, and the fatigue life of the GNS 316L steel is also higher than that of the CG one under strain fatigue loadings. The fatigue fracture results show that the fatigue crack presents a mixed propagation mode, and the crack source of GNS 316L transfers from surface to subsurface under low strain amplitude. The numerical results directly reflect the influence of the GNS surface layer on the plastic slip behavior. The fatigue failure mechanism of GNS 316L steel under strain-controlled cyclic loading at 650 °C was explained from a multi-scale perspective. This study provides data and method support for the application of GNS 316L steel at high temperature. [ABSTRACT FROM AUTHOR]

• Fatigue failure mechanism was elucidated for multi-stage centrifugal pump shaft. • Specific technical measures to prevent pump shaft failure were proposed. • Fatigue resistance design is performed for pump shaft structure and coupling. • Feasibility of the new pump shaft system has been verified by experiment. In the operation of multistage centrifugal pump, the pump shaft of 304 stainless steel is subjected to a variety of alternating loads. It is one of the parts prone to failure. The fracture microscopic observation, chemical composition analysis and mechanical property measurement of the fractured pump shaft are carried out. Through the fracture analysis, it is found that the pump shaft failure is caused by fatigue mechanism with characteristic of multi-source crack initiation, and there are many cavities and inclusions at the fracture. From the numerical analysis, it is found that there is significant stress concentration at the fracture, which exceeds the fatigue limit of the material and eventually leads to the fatigue fracture. Besides, the appropriate fillet radius and elastic coupling are designed for the pump shaft, which can effectively reduce the stress concentration and improve the service life of the pump shaft. Finally, the fatigue life of the improved pump shaft was experimentally verified. [ABSTRACT FROM AUTHOR]

Al 2 O 3 nanoparticles incorporated NiCoW ternary alloy coatings were synthesized by direct current electrodeposition from sulphate-citrate electrolyte. The influence of applied current density and electrolyte composition on the evolution of surface morphology, element composition, microstructure, microhardness and sliding wear behavior of the as-deposited coatings was investigated. The as-deposited coatings exhibit colony surface morphology and the inter-colony boundaries lead a leaf vein-like cross-sectional structure. The composition of the as-deposited coatings is controlled by the induced co-deposition mechanism and the competition among the reductions of Ni, Co, Ni-W and Co-W. The as-deposited coatings change from crystalline to amorphous structure with the decrease of Ni content in coatings and the critical Ni content for the transformation is about 40 at.%. The incorporation of Al 2 O 3 nanoparticles leads to a little increase of microhardness, degrades adhesive wear and oxidation wear, then significantly improve wear resistance. Variations in electrodeposition conditions lead the as-deposited coatings with different wear behavior, including abrasive wear, adhesive wear and oxidation wear, accompanying with plastic deformation. An attempt is made to establish the relationship between the electrodeposition conditions and wear behavior. [Display omitted] • Al 2 O 3 reinforced NiCoW ternary alloy coatings were prepared by DC electrodeposition. • Electrodeposition mechanism of NiCoW/Al 2 O 3 composites is discussed. • Al 2 O 3 nanoparticles could degrade adhesive wear and oxidation wear of NiCoW coatings. • Relationship between electrodeposition conditions and wear behavior is established. [ABSTRACT FROM AUTHOR]

• The multiaxial creep tests of 16MND5 steel for reactor pressure vessel are carried out at 700℃, and the macroscopic properties are obtained. • The mechanism of material failure under uniaxial stress and multiaxial stress is explained by microscopic observation (OM, SEM). • A new model based on the continuous damage theory is established by considering the activation energy parameter Q and stress combination. • The simulation results are in good agreement with the tests results. The effect of stress triaxiality on creep damage is fully understood,. • It is found that the highly-concentrated damage occur at the center as well as the notch root with increasing the notch acuity ratio. In-vessel retention (IVR) is an effective strategy of ensuring the structural integrity of RPV under severe accidents. In the IVR condition, due to the geometric discontinuity of pressure vessel, multiaxial creep is the main form of failure of RPV. Moreover, 16MND5 steel undergoes a phase transition from bainite to austenite. In order to study the multiaxial creep failure mechanism, creep tests on different notched bar are carried out at 700℃. The influence of grain boundary precipitation on creep behaviors was studied by micro-structural observation. Furthermore, based on the Kachanov-Robotnov continuous damage model (CDM), the term of activation energy Q is adopted in the newly-developed model as the correction parameter that is used to study the damage evolution and multiaxial creep behaviors of 16NND5 steel. Finally, the good agreement is achieved between theoretical prediction and experimental results. [ABSTRACT FROM AUTHOR]

In the process of production and service of pressure pipeline, local thinning defects may occur due to dielectric corrosion, manufacturing defects and other reasons. These defects reduce the carrying capacity of the pipeline and lead to leakage and rupture of the pipeline. Therefore, the integrity assessment of components and structures is required. Traditionally, the inelastic finite element method (FEM) is highly dependent on the specification of adequate mesh density and an assurance of numerically stable solutions. However, in engineering design, independent validation methods, such as Elastic Modulus Adjustment Procedures (EMAP), provide fast and stable solutions at relatively low cost. On the basis of elastic finite element analysis (FEA), the upper and lower bounds of limit load can be obtained by specifying the variations of elastic modulus in EMAP method. In this paper, an elastic modulus adjustment parameter k , which can improve the convergence of EMAP, is used to achieve stable convergence in the application of elbow with double defects. The EMAP method is also proved to be more efficient than the inelastic finite element analysis (FEA). Furthermore, the effect of defect size and distribution on the limit load of the elbow is studied. • The limit load of the elbow with defects can be quickly determined. • Elastic modulus adjustment method can achieve good convergence. • Theoretical results obtained by EMAP are validated with test data. • The interaction of double defects on limit load is investigated in detail. • The EMAP method is comprehensively evaluated by using parameters. [ABSTRACT FROM AUTHOR]

• The fatigue life as well as stress relaxation are greatly affected by dwell creep period for long-term holding. • The creep fatigue test with type of short-term hold loading has notable characteristics of fatigue fracture. • Pre-fatigue damage will significantly reduce the duration of the stable stage in the subsequent creep fatigue cycles. • For the test data obtained currently, the life prediction accuracy of strain energy density exhaustion method is the highest. • Damage evolution mechanism of creep fatigue interaction was explained from the perspective of microstructure change. In this paper, the creep fatigue test at 550–650 °C of P92 steel is carried out, which not only reveals the damage evolution mechanism, but also analyzed the creep fatigue behaviors. The influence of temperature, load holding time and pre-fatigue damage on the creep fatigue behavior of P92 steel was studied in detail. The effects of the temperature and holding time on the hysteresis loops of stress–strain are evaluated quantitatively. Actually, the load holding time mainly affects the stress relaxation behavior. Secondly, pre-fatigue damage will replace the initial stage of cycle stress response and significantly reduce the duration of the stable stage. The linear cumulative damage model (LCD), strain range division model (SRD), strain energy density exhaustion model (SEDE) were used to predict the life of P92 steel. Actually, the SEDE model is found to be most suitable for the creep fatigue life prediction. Furthermore, the microstructure of the specimen and fracture at pre- and post- test was observed to explain the damage evolution mechanism of the creep fatigue interaction. The microstructure analysis shows that the main body of P92 steel is tempered martensite in lath shape, and a large number of M 23 C 6 carbides are dispersed at the grain boundary. The M 23 C 6 carbides have poor thermal stability, which will coarsen and connect after creep, reducing the material properties. The analysis of fracture morphology shows that the creep fatigue specimen with short-term load holding has notable characteristics of fatigue fracture, while the creep fatigue specimen with long-term load holding has notable characteristics of creep fracture. [ABSTRACT FROM AUTHOR]