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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]

The creep damage in high temperature resistant titanium alloys Ti60 is measured using the nonlinear effect of an ultrasonic Lamb wave. The results show that the normalised acoustic nonlinearity of a Lamb wave exhibits a variation of the “increase-decrease” tendency as a function of the creep damage. The influence of microstructure evolution on the nonlinear Lamb wave propagation has been analyzed based on metallographic studies, which reveal that the normalised acoustic nonlinearity increases due to a rising of the precipitation volume fraction and the dislocation density in the early stage, and it decreases as a combined result of dislocation change and micro-void initiation in the material. The nonlinear Lamb wave exhibits the potential for the assessment of the remaining creep life in metals. [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]

Near-threshold fatigue crack growth behavior in 25Cr2NiMo1V steel with different microstructures was investigated by utilizing the load-shedding technique at ambient temperature. Crack surface morphology was observed by SEM with special emphases on the incidence of intergranular fracture and the influence on crack growth rates. Results show that the maximum intergranularity occurs at the ΔK corresponding to the cyclic plastic zone size being equivalent to the prior austenitic grain size. Two types of crack growth mode were observed in the near-threshold regime, i.e., the crystallographic mode of crack growth and the striation mode of crack advance. The incidence of faceted fracture was mainly rationalized by comparing the cyclic plastic zone size with the grain size. It is concluded that, in the crystallographic mode, lower crack growth rates in samples with higher heat treatment temperatures are caused by a greater degree of roughness-induced crack closure (RICC), faceted fracture induced crack closure (FFICC), and oxide-induced crack closure (OICC). The faceted fracture shows negligible influence on crack growth rates when cracks grow in a striation controlled mode. [ABSTRACT FROM AUTHOR]

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: 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]

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]

Transitional behavior of fatigue crack growth is observed for initial notches located at base metal, weld metal and heat-affected zone of 25Cr2Ni2MoV steel welded joint. Results indicate that both microstructures and load ratio R contribute to the transition. Greater roughness at both weld metal and base metal indicates a more pronounced roughness-induced crack closure in the near-threshold regime than that at heat-affected zone, especially at lower R . This transition is dominated by both Δ K and K max which depends on R . An estimation of Δ K at transition point is proposed, which agrees fairly well with the experimental data. [ABSTRACT FROM AUTHOR]

Abstract: The cumulative second-harmonic analysis of ultrasonic Lamb wave has been performed to study the precipitation kinetics and microvoid initiation of ferritic Cr–Ni alloy steel during the ageing process. Ageing of ferritic Cr–Ni alloy materials have been done at 1223K and 1173K for different degradation time intervals and air cooled. The results show that the normalized acoustic nonlinearity of Lamb wave increases with the formation of fine precipitates at the early stage of ageing till about 1000h and keeps as a plateau with the precipitates dynamic balance for a long-term ageing, and then decreases gradually at the final holding time with the coarsening of precipitates and initiation of microvoids. The results also show that the variation of nonlinear Lamb wave follows the same trend as that of hardness in materials. Therefore, the cumulative second-harmonic of ultrasonic Lamb waves has been found to be strongly sensitive to the precipitates behavior and microstructure evolution during the thermal ageing of ferritic Cr–Ni alloy steel. [Copyright &y& Elsevier]

Abstract: In the present paper the crack behavior of multi-component material systems is investigated under extensive creep condition. The validation of the creep fracture parameters C * and C(t) is firstly examined at the microscale level. It is found that the C * value is no longer path-independent when mismatch inclusions are embedded into the matrix. To characterize the crack fields in inhomogeneous material the integral value defined at the crack tip as is introduced to reflect the influence of the inclusion. The interaction effects between microcrack and inclusion are systematically calculated with respect to different mismatch factors, various inclusion locations and inclusion numbers. The analysis results show that the value is not only influenced by the inclusion properties but also depends on the microstructure near the crack tip. [Copyright &y& Elsevier]

Fatigue crack growth behavior of Inconel 740H and Haynes 282 nickel-based alloys at 20 °C, 700 °C and 750 °C was studied. Results showed increased fatigue crack growth rate for both the two alloys, while increased fatigue threshold in Inconel 740H at high temperatures. The size and distribution of MC carbides, serrated grain boundaries and twins deflected crack growth path. The strengthening γ' phase was interacted with dislocations by Orowan bypass and shear mechanisms at room temperature, while γ' was sheared in Haynes 282 and Orowan bypassed in Inconel 740H alloy at elevated temperatures. The cross slip, dislocation climbing and entanglement reduced the resistance of dislocation movement and accelerated the accumulation of damage. The higher content of Cr in Haynes 282 alloy promoted formation of M 23 C 6 carbide while the lower content in Inconel 740H alloy promoted thicker oxide layer. The Cr content needed careful tuning as it was superior to microstructural factors for higher fatigue threshold at elevated temperatures. [ABSTRACT FROM AUTHOR]

A through transmission nonlinear ultrasonic measurement has been proposed to characterize the creep degradation of titanium alloy that was conducted by creep tests at temperature of 600 °C. The experimental results show a change of “N”-like shape of the acoustic nonlinearity versus the creep loading time, which reveal based on metallographic studies that the variation of acoustic nonlinearity is closely related to the microstructure evolutions. An analytical model calculation has revealed a good agreement with the measured result, which indicates that the precipitate–dislocation interaction is likely the dominant mechanism responsible for the change of acoustic nonlinearity in the crept materials. [ABSTRACT FROM AUTHOR]