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In this study, the microstructure and hardness of the fusion zone of 316L-316LN stainless steel multi-pass gas tungsten arc welding (GTAW) weld joint were systematically investigated. The results showed that there were two typical inhomogeneous intermediate mixed zones distributed in the weld metal along the 316L (filler metal)-316LN (base metal) stainless steel multi-pass GTAW weld joint fusion boundary, one was the dendritic structure region, the other was the cellular structure region. The dendritic structure region was duplex structure with ferrite in vermicular morphology in austenite. By contrast, the cellular structure region exhibited cellular austenite structure with a small amount of ferrite particles at the cellular boundary triple points. It was found that the cellular boundary was high-density tangled dislocation structure with Cr and Mo segregation. Obvious residual strain was found to be concentrated adjacent to cellular boundaries due to the thermal expansion/shrinkage during the rapid solidification process. Because of the high-density dislocations, grain refinement and solute micro-segregation at the cellular boundaries, the microhardness of the cellular structure region was 239 HV, higher than that of the dendritic structure region (227 HV), and the nano-hardness exhibited a same trend, which was 3.03 GPa and 2.74 GPa, respectively.

The grain boundary character distributions of TP347H stainless steel after different thermo-mechanical processes in terms of grain boundary engineering (GBE) were investigated by EBSD. DL-EPR and e...

The different fretting wear behaviors of alloy 690TT in air and in a flowing nitrogen environment were revealed. Moreover, grain refinement to the nanoscale caused by fretting wear in nitrogen was ...

The surface oxidation and subsurface microstructure evolution of Alloy 690TT can occur during partial slip fretting corrosion in high-temperature pure water. Detailed characterization methods such as laser scanning confocal microscopy, scanning electron microscopy, electron probe micro-analyzer, and transmission electron microscopy were used to reveal the related mechanism. The results showed that Cr2O3 oxides together with a small number of spinel oxides were formed in sticking region since a small quantity of high-temperature water could pass through the gaps between the asperities to oxidize the materials. Widespread distribution of oxides in microslip region consisted of (Ni, Fe)Cr2O4, because Ni2+ and Fe2+ ions could react with Cr2O3 to generate a small amount of non-stoichiometric spinel oxides. The oxides around micropitting in microslip region consisted of double-layer structure. The outermost layer contained (Fe, Cr)-rich oxides due to the effect of fretting leading to mechanical mixing between Cr2O3 and (Ni, Fe)(Fe, Cr)2O4. The inner layer consisted of (Fe, Ni)-rich oxides owing to the consumption of Cr2O3 by the reaction with Ni2+ and Fe2+ ions. The reciprocating motion of oxide particles in microslip region resulted in the stress–strain supporting the recrystallization for the formation and development of a tribologically transformed structure in subsurface and plowing effect by fretting in surface.

The fireside corrosion of a martensitic P92 steel with mixed sulfate deposit (Na2SO4/K2SO4 = 8:5, by weight) in the air at 650 °C was studied via X-ray diffraction, scanning electron microscope and electron probe microanalyzer and X-ray photoelectron spectroscopy. The results showed that P92 was highly susceptiblr to fireside corrosion attack. There were three corrosion layers: the Fe2O3 oxides in the outermost layer, the spinel oxides in the middle layer, and the mixed bands with oxide-sulfide between the inner oxide and the substrate. Sulfide particles were also distributed at the interface of martensitic lath in the matrix. There were different corrosion mechanisms in different corrosion layers. On the surface, basic fluxing of the oxide film destroyed the protective performance of the oxide film. At the forefront of the corrosion layer, the existence of sulfide and oxide mixed band and sulfide particles accelerated the mass transfer process and promoted the rapid oxidation of sulfide, making the oxidation rapidly penetrate the matrix. In nodular oxide, the formation of low melting point complex sulfate accelerated the mass transfer and oxidation process.

The dynamic recrystallization (DRX) behavior and microstructure evolution of 33Cr23Ni8Mn3N heat-resistant steel were studied by isothermal hot compression experiment within the temperature range of 1000 °C–1180 °C and strain rate of 0.01–10 s−1. Models for the DRX volume fraction and grain size of 33Cr23Ni8Mn3N heat-resistant steel were established, and comparison results showed high consistency for the predicted data from the models and those from scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The model was imported into Deform-3D, and hot compression was simulated by the finite element method (FEM). The simulation and experimental results were highly consistent. Thus, the established DRX model is an important reference for the microstructure evolution in 33Cr23Ni8Mn3N hot deformation behavior.

In this paper, a sliding mode variable structure control (SMC) strategy in αβ coordinate system is proposed for three-phase converter on grid side. The SMC strategy for grid-side converter in dq coordinate system not only occupies a lot of compute cycles to take coordinate transformation, but also causes nonnegligible errors and significant decline in calculation accuracy. In order to solve this problem, an αβ-based SMC strategy is proposed in this paper, which greatly reduces the calculation complexity in static coordinate system and achieves almost the same control effects as dq-based SMC strategy. At last, the proposed strategy is compared with dq-based SMC strategy and verified in the simulation based on Matlab/Simulink.

This paper proposed a single-loop Model Predictive Control (MPC) strategy for the Grid-side converter (GSC), which reduces the difficulty of controller design and improves the system transient performance. In field oriented control, double-loop cascade control structure is widely adopted by GSC system, but this control structure has some disadvantages, such as design and matching difficulty of inner and outer loop parameters, and controller delay. Therefore, there aspects are improved in this paper. First, a single-loop strategy is proposed for GSC to simplify the traditional cascade control structure and the system dynamic response capability is enhanced; Second, the integral discretization method is adopted to solve the predictive problem of second-order system, which reduces the predictive complexity; Third, to prevent over current cases, the current limit function is taken as a nonlinear additional term of cost function. Finally, the feasibility and effectiveness of the proposed control strategy are verified by simulation experiments.

At present, with the rapid development of power electronics, three-phase converter systems are more and more increasingly applied in various occasions. The traditional PI control strategy could not obtain satisfied steady and transient performance during the whole process. Therefore, this article presents a research on dq-based model predictive control strategy for three-phase converter. This article introduces from 3 aspects. First, a dq-based predictive control strategy is designed for three-phase converter in the dq coordinate system. Second, the principle of dq-based switching state of the IGBT is expressed in detail to make actual current track reference current to achieve the desired value. Finally, simulations are carried out to verify the proposed strategy feasibility and performance comparation with PI strategy.

The microstructure, surface potential value and corrosion properties of fusion boundary in 316L stainless steel/Inconel 182 dissimilar welds were investigated. The results indicated that there were two different types of fusion boundaries alternately distributed in the dissimilar welds, and there was a large potential value difference between two sides of the fusion line in different types of fusion boundaries. The oxides and oxidation behavior of different types of fusion boundaries were strongly dependent on their surface potential value change on the fusion boundaries.

Stress corrosion cracking in light water reactor is one of the most important factors threatening the safe operation of nuclear power plants. Due to the severity, generality and various safety and economic problems caused by this phenomenon, it is necessary to establish a model for predicting the stress corrosion cracking growth rates. This paper provides an overview of three main methods for predicting stress corrosion cracking growth rates in recent decades, i.e., empirical, deterministic and calculation methods, which are introduced in detail. Empirical models describe classical statistical analysis and emerging artificial neural network method, both of which are based on a large number of experimental test data mining. They are convenient and relatively accurate in predicting, but require extensive, time-consuming and expensive tests for different service environments. Deterministic models aim to establish a theoretical relationship between crack growth rate and various influencing parameters by studying the stress corrosion cracking mechanism. Many scholars have proposed different mechanisms to scientifically explain the stress corrosion cracking phenomenon and propose corresponding crack growth rate models. Calculation models reveal the mechanism of crack initiation and propagation in different layers of materials by means of finite element method based on fracture mechanics and multiscale method based on quantum mechanics. They provide new idea for future research on stress corrosion cracking and bridge the quantitative mechanism or model, but no specific stress corrosion cracking growth rate model is formed. The article concludes with the prospect, aim and direction for stress corrosion cracking mechanism and prediction model.

Fretting corrosion of Alloy 690 TT against Type 304 stainless steel (SS) can occur in the steam generators of a nuclear power plant under partial slip conditions. In this study, galvanic corrosion occurred before fretting, leading to faster corrosion of 304SS with larger polyhedral and flaky spinel oxides and slower corrosion of Alloy 690 TT with smaller polyhedral spinel oxides, which resulted in polygonal and flaky micropitting of Alloy 690 TT during fretting due to the spinel oxides sticking into the substrate of Alloy 690 TT. Additionally, the micropitting was widened and deepened by the motion of the displaced oxide particles. The oxide particles on the surface and inside the pits were fractured to produce wear debris.

Fretting wear of Alloy 690TT can occur in the steam generator of a nuclear power plant, in which the interfacial conditions are changed as the temperature varies. In this study, the gradual transfo...

The general corrosion behavior of Alloy ENiCrFe-7 in deoxygenated high-temperature and high-pressure water was investigated. The results showed that the precipitates of Alloy ENiCrFe-7 included nio...

The investigations on fretting wear behaviors of nickel based alloy in NaCl solution and atmosphere indicated that wear mechanism and regional transformation from partial slip regime (PSR) to the m...

The influence of cold rolling on the fretting wear behavior and mechanism of Inconel 690 alloy at 320 °C in air was studied. The wear volume and worn surface were obtained and analyzed using laser ...

Fretting wear behaviors of Alloy 600MA in dry air and deionized water conditions were investigated. Results indicated that partial slip regime (PSR) and gross slip regime (GSR) were separated in running condition fretting map (RCFM). In deionized water, the boundary of PSR/GSR shifted into lower amplitude. In material response fretting map (MRFM), the cracking and particle detachment were characterized. In dry air, white etching layer (WEL) transformation resulted in the crack propagation in PSR and the formation of wear debris layer (WDL) in GSR. In deionized water, WEL was more difficult to be formed, resulting in shorter crack and smaller thickness of WDL. In nanostructured WDL, there were non-stoichiometric oxides of NiO, (Ni,Fe)(Fe,Cr)2O4 and Cr2O3 for dry air. The unoxidized phase of (Ni, Cr, Fe) coupled with non-stoichiometric oxides existed in WDL for deionized water.

Control rod drive mechanism Canopy-seal weld mockup of PWR was taken as the object of study. The microstructure of oxidation film and the oxidation mechanism on the surface of 304L base metal near the weld, under the gas tungsten arc welding without inert gas shielding on the base metal, were investigated. The morphology, composition and structure of the oxide film were characterized by optical microscopy, scanning electron microscopy, energy disperse spectroscopy, laser Raman spectroscopy, Auger electron spectroscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. With decrease of distance to fusion line, oxygen content on the surface of the base metal increased, the size of oxide particles grew up, and cracks appeared on the oxide film, gradually. In the region at 2 mm distance from the fusion line, surface oxide film was mainly Cr2O3 and Cr-O amorphous. In the region at 7 mm distance from the fusion line, surface oxide film was mainly comprised of Fe3O4 with a few FeCr2O4 and Cr2O3. The temperature on the base metal during welding affected the growth rate, particle size, oxide species of oxide film on the surface of base metal. When the oxygen potential of the metal atom and the partial pressure of ambient oxygen are determined, the influence of temperature on diffusion is the main factor determining the composition of the oxide film.

A new thermomechanical processing of warm rolling by a low deformation followed by annealing, which was specially developed for grain boundary (GB) engineering of large-size materials, was performed on a large-size 316 stainless steel. To examine usefulness of the thermomechanical process, the GB-networks before and after this process were quantitatively compared, and intergranular attack susceptibility was measured. Results clearly demonstrate that the GB-engineered 316 has higher proportions of coincidence site lattice boundaries, larger grain-clusters, longer twin boundary chains, and lower susceptibility to intergranular attack, indicating a successful process for GB-engineering. Additionally, a new parameter called twin boundary chain was proposed to evaluate the GB-network optimization.

The oxide film formed during welding on the inner surface of 308L weld was investigated. The morphology, composition and structure of the oxide film were characterized by optical microscopy, scanning electron microscopy, laser Raman spectroscopy, X-ray diffraction, Auger electron spectroscopy and transmission electron microscopy. The results indicated that the oxidation degree of the weld metal was generally uniform from the weld center to the fusion line. Micro-scale cracks were found on the outer surface of the oxide film and nano-scale gaps scattered on the inner oxide film-weld interface. The oxide film exhibited a duplex structure including outer and inner layers. The outer layer, made of tightly bound particles, was mainly composed of Fe2O3 oxide particles with a diameter of 50–150 nm. And the inner layer, made of relative loosely bound particles, was mainly composed of Fe3O4 and FeCr2O4 oxide particles with a diameter of 10–50 nm. The corresponding oxidation behavior and mechanism of the oxide film were also discussed.

In order to clarify the reason for the differences in the fretting wear resistance of Inconel 690 and 600 plate against 304 stainless steel ball, which is conducive to deepening the understanding of degradation mechanism, the comparative study on wear scars of two materials have been conducted in the atmosphere. The results indicated that Inconel 690 exhibited more excellent wear resistance than Inconel 600, which could be attributed to the thicker and harder tribological transformed structure (TTS) formed in worn subsurface of Inconel 690. The further structural characterizations of subsurface showed that the transformation mechanism from the matrix (coarse grains) to TTS (nano-sized grains) was dynamic recrystallization. Compared with Inconel 600, Inconel 690 possessed lower stacking fault energy (SFE) for its high chromium content, which prompted the occurrence of dynamic recrystallization and eventually led to a harder and thicker TTS in Inconel 690 worn subsurface.

The microstructural evolution and mechanical behavior of cast duplex stainless steels (CDSSs) at 400 °C for different thermal aging times were investigated by transmission electron microscope (TEM) and small punch test (SPT). The results showed that the spinodal decomposition in ferrite was the main reason for the decrease in toughness, and G-phase did not play an important role in the embrittlement process. The change of membrane stretching zone (Wm) played an important role in the SPT load-displacement curve before and after thermal aging. During the deformation process of Wm in the SPT, for thermal aging for 10,000 h, some completely curved slip bands were generated inside the ferrite phase, which had no contact with the δ/γ phase interface and belonged to the slip bands produced by the independent deformation of ferrite. The combined effect of the curved slip bands and stress concentration led to the initiation of obvious micro-cracks at the δ/γ phase interface. The micro-cracks propagated along the ferrite phase curved slip bands, and eventually penetrated the entire hardened ferrite phase.

The deformation behavior and fracture mechanism of cast duplex stainless steel tubular specimens under different tensile stages were investigated through experimental and numerical evaluation. The results showed that the axial stress was redistributed due to the necking of the tubular specimen, the axial stress near the internal wall was larger than those near the external wall, and its maximum axial stress was distributed between the internal wall and the center of the wall thickness. Microcracks and voids were initiated under the maximum shear stress along the δ/γ phase interface and propagated to the ferrite interior. The voids were connected and merged into the main crack through the propagation of the microcracks. Moreover, the main crack first propagated to the internal wall and then rapidly propagated to the external wall. The fracture morphology can be divided into three types: shear lip zones that can be found on both the internal and external walls, and shear lip zones that can be found on either only the internal wall or the external wall.

The evolution of fretting wear behavior and damage mechanism in Alloy 690TT with cycle number was investigated via laser scanning confocal microscopy (LSCM), scanning electron microscopy (SEM), focus ion beam (FIB), and transmission electron microscopy (TEM). The results showed that the fretting running status underwent a transition from partial slip and mixed stick-slip to final gross slip with the transformation of Ft&ndash, D curves from the ellipse to the parallelogram. The coefficient of friction (COF) experienced three drops throughout the fretting process, which indicated the transformation from high-friction wear to low-friction wear. The first drop was due to the transition from two-body to three-body contact. The second and third drops were mainly related to the evolution of the glaze layer from a localized distribution to completely covering the whole contact surface. The competition between fretting induced fatigue cracking (FIF) and fretting induced wear (FIW) ran through the entire fretting wear process. Before the 1.2 &times, 104th cycle, the fatigue crack growth was faster than wear, and FIF won the competition. As the fretting cycle continued to increase, the wear velocity was obviously faster than that of FIF, which indicated that FIW defeated FIF. The tribologically transformed structure (TTS) participated in the competition between FIF and FIW. The gain boundaries and dislocations in the TTS were a suitable pathway for crack initiation and propagation and oxygen permeation.

Based on an 33Cr23Ni8Mn3N thermal simulation experiment, the application of an artificial neural network (ANN) in thermomechanical processing was studied. Based on the experimental data, a microstructure evolution model and constitutive equation of 33Cr23Ni8Mn3N heat-resistant steel were established. Stress, dynamic recrystallization (DRX) fraction, and DRX grain size were predicted. These models were evaluated by a variety of statistical indicators to determine that these models would work well if applied in predicting microstructure evolution and that they have high precision. Then, based on the weight of the ANN model, the sensitivity of the input parameters was analyzed to achieve an optimized ANN model. Based on the most widely used sensitivity analysis (SA) method (the Garson method), the input parameters were analyzed. The results show that the most important factor for the microstructure of 33Cr23Ni8Mn3N is the strain rate ( &epsilon, ˙ ). For the control of the microstructure, the control of the &epsilon, ˙ is preferred. ANN was applied to the development of processing map. The feasibility of the ANN processing map on austenitic heat-resistant steel was verified by experiments. The results show that the ANN processing map is basically consistent with processing map based on experimental data. The trained ANN model was implanted into finite element simulation software and tested. The test results show that the ANN model can accurately expand the data volume to achieve high precision simulation results.

The effect of scratch depth on high cycle fatigue behavior of Alloy 690TT stream generator (SG) tubes was investigated by experimental characterization and numerical simulation. Results indicated that fatigue cracks initiated at the scratch root of the SG tube external surface. The effect of scratch depth on fatigue life mainly depended on the loading stress levels, which could be considered as an exponential relationship between the loading stress and scratch depth for a given fatigue life. The fatigue limit decreased with the increase of scratch depth and changed in a small range of yield strength.

Effect of post-weld heat treatment(as-weld, 615℃/16h and 615℃/48h) on microstructure and corrosion behavior in ENiCrFe-7 weld overlay cladding materials had been investigated, the results showed that the heat treatment had an obvious influence on microstructure and corrosion properties. As-weld Alloy ENiCrFe-7 exhibited to be compressive residual stress, and it had a lot of homogeneously distributed microstructure defects(e.g. composition segregation), the pits were evenly distributed on the surface of the sample after polarization, its passive current density was the least due to formation of the densest passive film. With increasing heat treatment time to 16h, the compressive residual stress was rapidly decreased, the degree of Cr-depletion in the dendrites was obviously increased, and a large number of pits were concentrated in the Cr-depletion area. With increasing heat treatment time to 48h, the compressive residual stress was slightly increased compared with that at heat treatment time of 16h, both the degree of Cr-depletion in the dendrites and microstructure defects were improved, the looser passive film was formed, general corrosion was mainly performed during polarization.

The effect of heat treatment temperature on the microstructure and tensile properties of 316L stainless steel fabricated by wire and arc additive manufacturing was studied in this paper. Heat treatment was used to uniformize the microstructure, eliminate the anisotropic tensile properties in as-built 316L part, and improve the poor elongation caused by the multilayered structure of alternating re-melting area (RA) and overlapping area (OA). The results showed that the heat treatments below 1000 °C had no significant effect on the multilayered structure and columnar austenite grains, but only changed the morphology and content of ferrite and σ phase within the grains. With the increase of the heat treatment temperature, ferrite transformed into σ phase and the fraction of second phase gradually decreased, leading to the slight increase of elongation but decrease of material strength and its anisotropic degree. After solution annealing above 1000 °C, the ferrite and σ phase was nearly dissolved in the austenite matrix. The austenite recrystallized and the multilayered structure was destroyed, which played a major role in the tensile properties. With the increase of heat treatment temperature, the fraction of recrystallization increased, resulting in decrease of material strength. The elongation was greatly improved and reached that of the annealed state, and its anisotropic was almost eliminated.

In this paper, the in-situ fracture tests were carried out to study the fracture mechanism of different regions (base region, weld region and interface region) in a dissimilar metal welded joint of nuclear power plant. The local microstructural effect on short crack initiation and propagation behavior was investigated. The results indicated that the mechanism of crack initiation and propagation was strongly depended on the microstructures of the difference zone in the dissimilar metal welded joint. There were large numbers of twins in the base region, and the cracks tended to initiate along the twin boundary. The base region exhibited the characteristics of ductile fracture. There was no twin existed in the weld metal. The crack tended to initiate at the slip band and propagate along the columnar grain boundary due to existence of the second phase particles along the grain boundary. The fracture mode of the weld metal was intergranular. In the interface region, the crack tended to propagate from the weld zone to the epitaxial zone, and then along the grain boundary of the epitaxial zone. The fracture mode was the combination of the brittle and ductile fracture. Meanwhile, the weld region has a lower resistance of crack initiation and propagation compared with the base region and interface region.

An investigation on mechanical property and electrochemical behavior of the cladding materials aged at 400 °C for different times was carried out by small punch test (SPT) and double loop electrochemical potentiokinetic reactivation (DL-EPR) test. The results showed that thermal aging contributed to spinodal decomposition and G-phase formation in the aged ferrite phase, as well as increase of nano-hardness of ferrite phase. The SP energy by the small punch tests decreased with increase of thermal aging time, which was related to the lattice distortions in G-phase and aged ferrite phase and their interactions with dislocations during deformation process. Thermal aging promoted corrosion and intergranular corrosion susceptibility of the cladding materials. Combined effects of G-phase and spinodal decomposition lowered intergranular corrosion resistance of the cladding material.