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Abstract The evolution of grain size and structure morphology across the CrMoV welded joint were investigated. The tensile properties of subzones in the weldment were measured by miniature specimens. And the measured results were compared to the results acquired from the standard round bar specimen. It is found that the strength characterized by miniature specimen is lower than that obtained by standard specimen. The deviation of strength is acceptable. However, the measured Young's modulus is not consistent with the elastic modulus of standard specimen, due to the jig deformation and clamped effects of the miniature specimen. Although HAZ is the weakest location due to the deteriorated microstructure and the reduced strength, the failure pattern is different comparing the measured results of the miniature specimen and that of the standard round bar cross-weld specimen. For the miniature specimen, fracture occurred at the HAZ region close to the base metal (BM). However, the fracture of the standard specimen occurred in the BM instead of the HAZ region. The difference is ascribed to the different constraints of specimens with various geometric dimension. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Effect of inhomogeneous microstructure of a CrMoV multi-pass weld metal on creep behavior was investigated experimentally. Four subzones in the multi-pass weld metal were identified in terms of the grain size and morphology. Compared to the micro-hardness and tensile strength, a remarkable different creep behavior was achieved for different subzones. Especially, the heterogeneous creep deformation was observed during the tertiary creep stage. Such a heterogeneous creep behavior of multi-pass weld metal was ascribed to the coupling effect of inhomogeneous microstructure and loading direction. [ABSTRACT FROM AUTHOR]

Crack closure and opening behavior of physically short fatigue crack tip was investigated in microstructures with different grain sizes by using the in-situ SEM fatigue test. A gradually decreased crack closure zone near the crack tip was observed which depended on the microstructures, crack size and stress levels. A new parameter involving the load level, grain size, specimen dimensions and crack length was proposed to interpret the physically short crack closure development. The contribution of the gradient crack closure zone on the physically short fatigue crack growth behavior was evaluated by using the modified partial crack closure model. A generalized fatigue crack growth driving force was finally determined and correlated well with the physically short crack growth rate. [ABSTRACT FROM AUTHOR]

Abstract: Very high cycle fatigue (VHCF) tests at 370°C were carried out for a Cr–Mo–V steel to investigate the influence of microstructure and microdefects on fatigue behavior. Duplex S–N curves were obtained with the crack initiation switched from specimen surface to interior. A new elliptical type of fish-eye pattern originated from microstructure heterogeneities was observed. In the high cycle fatigue regime, the influence of microstructures on fatigue performance was greater than that of microdefects. In the VHCF regime, by contrast, the fatigue performance was dominated by inclusions and could be interpreted by using the proposed life controlling parameter Z. [Copyright &y& Elsevier]

Abstract: In order to reveal the physical mechanisms of ratchetting process under creep–fatigue loadings, following ratchetting tests in advanced 9–12% chromium ferrite steel, a study of associated fracture modes and dislocation patterns explored by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations is presented in this paper. Two main domains were observed depending both on the peak hold time and on the stress ratio, in which the ratchetting deformation and failure mechanisms were different. These two damage domains correspond to two distinct creep–ratchetting interaction mechanisms. Particular attention was paid to the dependence of ratchetting damage behavior on the stability of dislocation substructure. In addition, an attempt is made to correlate the results of the microstructural investigations with the variations of internal stress. [Copyright &y& Elsevier]

Abstract: The mechanochemical effects between dissolved hydrogen and deformation on active dissolution of 304 stainless steel in acid chloride solution were investigated by use of constant loading test method. A theoretical model was proposed in terms of the stored strain energy and electron work function. Results indicated that active dissolution was affected mainly by Gibbs free energy produced by the part of plastic deformation and surface dislocation structure. A good agreement between the prediction and test results revealed that the synergistic effect of stress and dissolved hydrogen only promoted the process of active dissolution. [Copyright &y& Elsevier]

Abstract: Correlation between microstructure, hardness and strength was investigated in heat affected zone (HAZ) of dissimilar welding joints of newly developed rotor steels in terms of both the traditional micro-hardness testing and the nanoindentation technique. Relationships between micro-hardness and nano-hardness were obtained for HAZ of welds, where the mechanical properties were microstructure dependent. Lath width of the tempered martensites was selected as the characteristic microstructure size for correlating the micro-hardness. The detailed strength distribution in HAZ and the correlation with the characteristic microstructure size were discussed. A good agreement was observed for the correlated strength and the experimental results in both weld metal (WM) and base metal (BM), and therefore gave us confidence for the application of the proposed relationship between the hardness and the mechanical properties in HAZ of the dissimilar weld. [Copyright &y& Elsevier]

Abstract: Fatigue crack growth behavior was investigated in a newly developed steel 25Cr2NiMo1V with different heat treatments to meet different property requirements of high-pressure (HP) and low-pressure (LP) parts in the combined steam turbine rotor. The load-shedding method and constant load amplitude techniques were adopted. Fractography including crack surface observation and fatigue crack growth path analyses was carried out. Results show that in the threshold regime, fatigue crack growth resistance of HP is clearly superior to that of LP. The distributed bainitic microstructures and larger prior austenite grain size in HP result in more tortuous crack propagation path than that in LP. Compared with ferritic blocks in HP, the tempered martensitic laths in LP do not play a dominant role in stopping the fatigue crack advance. Nevertheless, no clear difference in fatigue crack growth in HP and LP is observed in the Paris regime. [Copyright &y& Elsevier]

Abstract: In this work, a survey on the closed form solutions for the prediction of interfacial shear and peeling stresses of bonded structures is first given. And then, comparisons between the closed form solutions, elastic finite element analysis (FEA) and elastic-creep FE results are performed for the bonded structures subjected to thermal loading. To elucidate the creep effects on interfacial stresses, at last, creep damage FE analysis is carried out referring to the K–R damage creep law. Results indicated that both creep and creep-damage analysis will lead to interfacial stress redistribution and thus reduce the shear stress and peeling stress at interface. Creep damage analysis yields the lowest stresses among the linear elastic and elastic creep analysis results. The maximum damage is produced at the edge of the interface where the failure often occurs and is proportional to the modulus ratio of substrate to film. On the other hand, the damage distribution along the interface is very similar to the trend of peeling stress distribution. Compared to the FE results, the existing closed solutions from and can only give an accurate prediction of the interfacial shear stress but give a rough estimation to the interfacial peeling stress. As far as effects of creep are concerned, the existing closed form solutions based on elastic analysis are inapplicable anymore. [Copyright &y& Elsevier]

Abstract: Microscopic tensile test was conducted on 316LSS vacuum diffusion-bonded joints to investigate the microstructure evolution and effect of micro-voids on interfacial failure mechanism. In situ observation of the interfacial crack initiation and propagation was carried out during the whole testing. The results showed that the most likely sites for cracks initiation are grain boundaries. The favorable grain boundaries containing cracks are oriented at 0–20° to the loading axis. Intergranular cracks play a dominant role in interfacial failure. Micro-voids do not link up each other until the load is increased to 352MPa (63% σ b). For 316LSS diffusion-bonded joints, interfacial failure depends mainly on microstructure of joints. [Copyright &y& Elsevier]

Abstract: Vacuum diffusion bonding was carried out on 316L stainless steel. Metallographic inspections and micro-hardness testing were conducted near the interface of diffusion-bonded joints. Fatigue tests were performed to investigate the mechanical performance of diffusion-bonded joints under cyclic loading. Results indicate that, although the static strength of joints closes to that of base metal, fatigue life of the diffusion-bonded joint is markedly lower than that of the base metal. Increments of electrical resistance of specimens were monitored and recorded at different cycles. By taking the electrical resistance as a damage parameter, the evolution of fatigue damage of diffusion-bonded joints was interpreted and the relationship between fatigue life and increments of electrical resistance was established. Overall good agreement between the experimental results and predicted life was obtained which provides confidence in the use of the developed approach for life prediction. [Copyright &y& Elsevier]

The metallurgical notch effect of weldment is critical for the structural integrity assessment and to avoid the premature failure especially at elevated temperature. The local nonhomogeneous microstructure located in heat affected zone (HAZ) and multi-pass weld metal area (WM) is one of the largest contributions for the material weakening of the entire weldment. Although the type IV failure related to the HAZ has been intensively discussed, few researchers focus on the creep damage on the weld metal. In fact, the limited long-term creep tests of welded joints conducted by Tabuchi et al. [4]. has emphasized the necessity of monitoring creep damage not only in the HAZ but also in the weld metal. In this article, the nonhomogeneous microstructure related creep damage of the CrMoV multi-pass weld metal is discussed in detail. Different fracture morphology subjected to different weld properties are compared with each other, including the distribution of creep cavities, the precipitates and the substructures of crept specimens sampled from weld pass metal in the welding direction (WP), the overlap of different weld passes (WPO) and multiple weld passes through the thickness direction (MWP). Further, the microscopic analysis on the heterogeneous creep damage behavior are conducted. It is found that the grain length difference perpendicular to the loading direction is the dominant reason for anisotropic creep damage of multi-pass weld metal. In addition, the mismatched creep properties of subzones in the multi-pass weld metal also affect the damage field distribution. [ABSTRACT FROM AUTHOR]

Relationship between the nano-mechanical behavior and microstructure in the wavy interface zone of explosive welded 304L/533B steel plates was explored. Nanoindentation tests were carried out on the interfacial wavy region across the wave peak and wave valley. The microstructure and chemical components of the interfacial region were obtained. Both across the wave peak and wave valley of interfacial 533B steel, the hardness gradually decreased and then went back to a plateau value of ~ 3.5 GPa. There was no distinct difference in hardness with the distance from interface between the wave peak and wave valley. The reduction of cementite content and the decarburization of interfacial 533B steel were responsible for the decreasing of hardness in the interfacial regions. Moreover, this variation in hardness with the distance from interface was basically inverse to that in ferritic grain size. On the side of 304L austenite steel, the nanohardness gradually increased from the outer region to the interface both across wave peak and wave valley. The relationship between the hardness and the grain size in the graded interfacial zone of 304Lsteel followed the classical Hall-Petch strengthening equation. The graded variation in hardness could be due to the combined strengthening mechanisms of gradual refined grain size, as well as martensitic transformation strengthening, carbon precipitation strengthening and dislocation strengthening. Also, the hardness in the wave peak zone was larger than that in the wave valley zone of 304L steel, which could be due to the severer plastic deformation occurring in the peak zone. [ABSTRACT FROM AUTHOR]

In this paper, in-situ SEM images with both low and high resolution near fusion line of a Cr-Ni-Mo-V steel welded joint were taken during interrupted monotonic tensile tests, with which macro- and micro-strain were measured based on digital image correlation technique and correlated with microstructures. An error analysis of strain measurement indicated sufficient resolution for image correlation with intrinsic microstructures as speckles. Results from digital image correlation showed that base metal, heat-affected zone and weld metal of the welds exhibited different responses to external tensile loading with mismatched strain distribution. A transfer of strain localisation from tempered martensites at base metal to lathy bainites at weld metal was observed around the fusion zone, due to varied hardening capabilities among non-homogeneous microstructures. Micro-strain could be several times higher than macro-strain at the same strained area, implying severe non-uniform deformation at microstructural scale. The most straining zone around the fusion line was found to be localized at blocks of soft δ-ferrites. [ABSTRACT FROM AUTHOR]

Axially loaded cyclic tests of a nickel-based alloy were conducted at 550 °C, 630 °C and 650 °C and stress ratios of −1, −0.3 and 0.5 up to high cycle fatigue regime. Multiscale characterizations and quantitative analyses of microstructural damage were carried out to illustrate the physics and mechanics of interior microstructure induced fatigue strength degradation and cracking. Results showed that the fatigue strength increased with the decreases of twin spacing, the size and aspect ratio of carbide, and γ′ phase size. Twin collapse due to slip band-twin interaction was found for the first time and confirmed as a form of crack initiation process, followed by short crack growth forming facets with significant contribution from shear stress. A new mechanistic fatigue life model of interior microstructure induced cracking was established with sound agreement with experimental data. All these combined into a design-manufacturing integrated approach which are capable of ensuring the safe operation and maintenance of service structures in long-life regime. • A shorter twin spacing, a reduced size and aspect ratio of carbide, and a smaller γ′ phase size were found beneficial for higher fatigue strength. • Slip bands-twin interaction induced twin collapse was found for the first time, which was the first step to interior facet induced cracking. • Dislocation slip, the size and location of facet was combined into a fatigue life model to evaluate interior facet induced fatigue failure. • A design-manufacturing integrated approach to anti-fatigue for long-life service structures was established. [ABSTRACT FROM AUTHOR]

The effect of tensile plastic deformation on the nonlinear ultrasonic response of austenitic stainless steel was examined, using nonlinear longitudinal wave. The plastic deformation induced the formation of twins and martensite needles in grains. The density of martensite needles increased with increasing plastic strain. The nonlinear ultrasonic parameter increased with increasing the plastic strain experienced by plastically deformed austenitic stainless steel. A simple power–law relation was proposed between the nonlinear ultrasonic parameter and the plastic strain, taking account of the contribution of local microstructures and microplastic deformation. Using this relationship, the stress exponent index was found to be 1.55. Considering the contributions of local microstructures, the deformation-induced phase change and the deformation associated with the formation of new phase, it is suggested that the acoustic nonlinearity of heterogeneous materials depends on local microstructures, phases, misfit, and high-order elastic constants of individual phases. [ABSTRACT FROM AUTHOR]

Abstract: The aim of this paper was to address the effect of WC content on the tensile properties of WC/Ni composite coatings deposited by laser cladding on 45# steel substrates. The tensile tests were performed in the chamber of a scanning electron microscope. Experimental results showed that the mechanical properties and fracture behavior of WC/Ni coatings were strongly dependent on the volume fraction of WC reinforcing particles in the coatings. The tensile properties of the coating decreased with increasing content of WC particle. When the WC content was relatively high, multiple through-width cracks (TWC) in the coating can be found. The number of through-width cracks in the coating as a function of applied strain was measured. From the in-situ observation of coating surfaces, most of cracks were initiated from the WC particle, and then propagated throughout the coatings. This phenomenon was attributed to the stress concentration in the WC particle or near the WC particle, which was confirmed by using finite element analysis. The present research may provide some insights on the development of designing methodology of composite coatings. [Copyright &y& Elsevier]

Abstract: A probabilistic model for prediction of bonding time in diffusion bonding is presented based on the stochastic characteristics of surface finish of bonded area. The probabilistic distribution of surface roughness is introduced into a deterministic model of diffusion bonding by taking the parameters of void radius and height as random variables. Monte Carlo simulation technique in conjunction with numerical integration is used to calculate the bonding time and its probabilistic distribution. The sensitivity analysis of parameters is then performed to evaluate the effects of creep exponent and surface roughness on the bonding time. Comparison is also made between the theoretical predictions and available experimental results from previous work on Ti–6Al–4V alloy in order to validate the probabilistic model. The proposed model showed a good agreement with the experimental results. [Copyright &y& Elsevier]

Cyclic deformation response of an austenitic Ni-based alloy was investigated by symmetrical/asymmetrical strain cycling tests at room temperature (RT) and 350°C. The effects of strain amplitude/ratio and ambient temperature on the cyclic deformation behavior were discussed in terms of microstructural features and internal stresses. Results indicate that the cyclic hardening/softening at RT is mainly attributed to the increase/decrease in back stress. Whilst at 350°C, the contribution of effective stress to cyclic deformation response becomes important due to the planar slip promoted by the occurrence of dynamic strain aging (DSA) throughout the fatigue process. Accordingly, the continuous cyclic hardening at 350°C comes from the combined action of back stress and effective stress. • DSA is present throughout the fatigue life at 350°C. • Physical origin of discrepancy in cyclic deformation response between RT and 350°C is clarified. • Continuous cyclic hardening at 350°C results from the increase of both back and effective stress caused by DSA effect. [ABSTRACT FROM AUTHOR]

Creep damage is governed by different microstructural features (e.g. precipitate coarsening, substructure growth, cavity/crack nucleation and growth) during long term high temperature exposure in 9–12%Cr steels. The dependences of stress states (e.g. equivalent stress and stress tri-axiality) on these damage mechanisms are clarified in this work. Various stress states are produced in FB2 steel notched components with different root radii for creep tests at 605 °C. Results indicate that equivalent stress and stress tri-axiality present different influences on creep damage behavior of notched components at elevated temperatures. The equivalent stress and stress tri-axiality both enhance the precipitation of the carbide, indicating larger mean diameter and higher area of the precipitates. Meanwhile, the equivalent stress plays a more significant role on the coarsening of the substructure of the material than the stress tri-axiality. Creep cavities are mainly found at several hundred micrometers away from the notch root, which seems to be induced by the combining action of equivalent stress and stress tri-axiality. These microstructural degradation can be proved by the distribution of the hardness in the notch region of component accompanied by the variation of stress states. • Dependences of stress states on microstructural evolution and creep damage behavior are clarified. • Equivalent stress and stress tri-axiality present different influences on creep damage behavior of notched components. • Equivalent stress and stress tri-axiality both enhance the precipitation of the carbide. • Equivalent stress plays a more significant role on the coarsening of the substructure than stress tri-axiality. [ABSTRACT FROM AUTHOR]