Creep Voids Research Articles

Investigation on creep behavior of welded joint of advanced 9%Cr steels

Abstract

Effect of hold time on the mechanical fatigue failure behavior of lead-free solder joint under high temperature

In this paper, effect of hold time on the low cycle mechanical fatigue failure behavior of Sn3.0Ag0.5Cu solder joint was investigated. Fatigue tests were conducted utilizing trapezoid waveform with four hold times (0, 2, 5, 10 s) under 348 K. The results show that samples tested under waveform with hold time has the faster maximum stress drop rate comparing to that without hold time. Accordingly, the fatigue life of solder joint is declined with increasing hold time. However, there exists a threshold value, and when the hold time exceeds this value, the fatigue life changes little. Stress relaxation occurs during the hold time, which is completed in a short time under high temperature. Deformation of the solder joint is not uniform, which is concentrated in the solder matrix near the substrate. Micro crack initiates at the corner of the solder joint and propagates in the strain concentrated zone with a direction parallel to the applied load. In addition, Creep voids are observed from the fracture morphology of solder joints tested with hold time, which nucleate at the slip bands and the dendrite boundary tensile ledges. Moreover, the fracture morphology appears an inter-granular rupture mode. However, for solder joints tested without hold time, the rupture mode is trans-granular with some dimples dispersed on the morphology.

Experimental investigation of specimen size effect on creep crack growth behavior in P92 steel welded joint

In order to clarify the effect of constraint induced by specimen size on creep crack growth behavior of P92 steel welded joint, creep crack tests were carried out on the compact tension specimens with thick thickness and thin thickness, the crack tip of which were located at different distinct zones of welded joint. Tested results revealed that even in thin thickness specimens, fine grained heat affected zone specimens exhibited a fast creep crack growth rate compared with other micro-zones specimens due to a low creep crack resistance and a high multistress state. The fractographies of these specimens exhibited an accelerated number of spherical particles that were caused by the coalescence of creep voids. Furthermore, the correlation of C* with creep crack growth rate was dependent on specimen thickness. As the specimen thickness increased from 10 to 30mm, the creep crack growth rate increased. This was due to the increase in constraint level ahead of crack tip during creep crack propagation.

Micro- and macro-creep damage formation for P92 under multiaxial stress related to circular notched specimen

In this study, to clarify the behaviour of micro- and macro-creep damage progression for P92 under multiaxial stress field, interrupted creep tests, analysis of multiaxial stress and detailed the cross-sectional observations were conducted on a circular notched round bar specimen which produces the multiaxial stress field due to the plastic constraint. As a result, creep voids were initiated at the early stage and they were formed up to the final fracture. These phenomena were found to be detected using direct current potential drop (DCPD) method. These results concern the development of the measurement of creep crack initiation. The distribution of high void area fraction was in good agreement with that of high hydrostatic stress and high multiaxial stress. This result indicates that multiaxial stress affects the void formation. Furthermore, the micro-creep damage of each interrupted specimen was evaluated by using the electron backscatter diffraction (EBSD) method which can analyse crystallographic misorientation caused by creep strain. The results of EBSD analyses indicated that the value of grain reference orientation deviation (GROD) closely concerns the void initiation.

Biaxial thermal creep of Inconel 617 and Haynes 230 at 850 and 950 °C

The biaxial thermal creep behavior of Inconel 617 and Haynes 230 at 850 and 950°C was investigated. Biaxial stresses were generated using the pressurized tube technique. The detailed creep deformation and fracture mechanism have been studied. Creep curves for both alloys showed that tertiary creep accounts for a greater portion of the materials’ life, while secondary creep only accounts for a small portion. Fractographic examinations of the two alloys indicated that nucleation, growth, and coalescence of creep voids are the dominant micro-mechanisms for creep fracture. At 850°C, alloy 230 has better creep resistance than alloy 617. When subjected to the biaxial stress state, the creep rupture life of the two alloys was considerably reduced when compared to the results obtained by uniaxial tensile creep tests. The Monkman–Grant relation proves to be a promising method for estimating the long-term creep life for alloy 617, whereas alloy 230 does not follow the relation. This might be associated with the significant changes in the microstructure of alloy 230 at high temperatures.

Creep fracture mechanism in welded joints of P91 steel

The creep rupture strength of the welded joint of P91 steel (9Cr–1Mo–V–Nb) is limited by that of the fine grained region of its heat affected zone (HAZ). The fine grained HAZ has the lowest creep rupture strength, and the creep voids are mostly concentrated in this region in comparison with the coarse grained HAZ, the weld metal and the base metal of the welded joint. This is mainly due to the higher rate of recovery of the subgrain structure (coarsening of subgrains and recovery of dislocations within the subgrains) in the fine grained HAZ. Type IV fracture occurred in the fine grained HAZ of the cross-weld specimens after the long term creep tests at low stresses at 600 and 650°C. It occurred by the formation of creep voids. There are two types of voids. The first is the voids that form wedge type cracks at the triple points between the prior austenite grain boundaries at intermediate stresses at 600 and 650°C. The second is the voids that form round type cracks on the prior austenite grain boundaries and the subgrain boundaries at low stresses in long term creep tests at 600 and 650°C. These voids form at the interfaces between the matrix and the coarse intergranular particles of Laves, Z and Cr rich carbide phases.

Study of Creep Damage in a 10.86% Cr Heat Resistant Steel using Synchrotron X-Ray Microtomography

Synchrotron X-ray microtomography(SR-μCT) scans have been carried out on sample coupons extracted from the fracture specimens of a 10.86% Cr heat resistant steel exposed to crep deformation at 873K over stresses of 120, 150, and 180 MPa. The 3D cavitation characteristics in terms of void volume fraction, numbwer density and size distribution as a function of the applied stress has been determined by quantitative analysis of the reconstructed tomograohy slice datasets. The relationship between heterogenous spatial distribution of creep voids and variation in rupture life has been exploited in terms of microstructural sites during the onset of creep embrittlement.

Fracture Analysis on Drainage Pipe Socket of Boiler

Rupture reason of boiler drainage pipe socket was analyzed by means of chemical composition analysis, macro-observation, metallurgical and SEM microstructure examination. Results showed that the rupture of the drainage pipe socket was a result of creep cracking initiated in intercritical zone of HAZ due to long period operation under high temperature. During creep process, carbides precipitated on fine grain boundaries of intercritical zone and grew up gradually into new coarse carbides as favorite nucleation site for creep void. With the growth, coalesce and connection of creep voids carries on, micro-cracks formed on original austenite boundaries and finally led to fracture of weld joint.

Creep-Induced Microstructural Changes in Large Welded Joints of High Cr Heat Resistant Steel

The creep damage process of high-Cr steel welded joints is characterized by the formation and growth of creep voids prior to the initiation of cracking, and this formation and growth accounts for a large proportion of creep life. Therefore, there has been much investigation into the detection and quantitative evaluation of creep voids and microcracks in welded joints for use in remaining life assessment. However, the microstructure around creep voids and microcracks is still not well known. In this study, a creep test on a large welded joint in Mod.9Cr-1Mo steel was conducted under 60 MPa at 650 °C. The observation of the microstructure in the heat-affected zone (HAZ) was made for the specimen interrupted the creep tests at 25% of a rupture life. The microstructure around the creep void was characterized using an electron backscatter diffraction pattern (EBSD) method. It was founded that creep voids formed and developed along random high angle grain boundaries that were not subject to K-S orientation relationship in the martensitic transformation. In addition, the initially formed void promoted preferential dynamic recovery and dynamic recrystallization in its surrounding microstructure, followed by sub-boundary formation.

SUS304HTB鋼におけるクリープ損傷過程のEBSD解析

Creep damage evaluation by EBSD (Electron BackScatter Diffraction pattern) method was conducted on the austenitic stainless steel SUS304HTB widely used as the boiler tube material. Creep damage was imposed under the condition of 650℃ /130MPa and 700℃ /75MPa for preparing various interruption time samples. The longitudinal mid-section of specimen was observed by EBSD equipment and then IPF (Inverse Pole Figure) maps, KAM (Kernel Average Misorientation) maps, GROD (Grain Reference Orientation Deviation) maps were made for damaged samples. IPF maps showed the increasing inhomogeneity incrystal orientation within each grain as the time fraction increased. Area averaged parameters of KAM and GROD, i.e., KAMave and GRODave showed monotonically increasing trend with creep time fraction. Compared with KAMave, GRODave showed better performance for insensitivity of EBSD pixel step size. As GRODave showed excellent proportionality with creep strain except for rupture stage, the linear relationship was used as the conversion function into creep strain. Using this relationship, it was suggested that creep void diameter was proportional to the creep strain derived from local GRODave measurement method proposed here. Therefore, GRODave could be a good measure for estimating the strains around creep voids and the void growth mechanisms of SUS304HTB.

高Cr耐熱鋼の溶接部におけるクリープ中の組織変化とType-IV損傷挙動

Creep strength of high Cr ferritic steel welds decreases than base metals due to Type-IV creep damages formed in the heat affected zone (HAZ) during long-term use at high temperatures. In the present study, creep tests of ASME Gr.91 and Gr.122 steel welded joints were interrupted at several time steps at 600°C. Microstructural changes and damage behaviors in the fine-grained HAZ of welds during creep were investigated by using SEM, TEM, and EBSD. KAM, grain boundary length and hardness of fine-grained HAZ decreased till 0.2 of life and saturated in the Gr.91 steel weld, whereas they decreased after 0.5 of life in the Gr.122 steel weld. It was found that the recovery of dislocation structures in the fine-grained HAZ was completed at early stage of life for the Gr.91 steel weld, whereas it occurred in the later stage of life for the Gr.122 steel weld. These differences of microstructural changes was considered to relate to the differences of Type-IV creep damage behavior; early initiation of creep voids at 0.2 of life in the Gr.91 steel weld and later damage evolution after 0.5 of life in the Gr.122 steel weld.

GS36 改良SPクリープS試験法による12%Cr耐熱鋼溶接継手のクリープ特性評価

Modified small punch (SP) creep test with a rectangular-type specimen (10 X 6 X 0.5 mm) was applied to the interrupted uniaxial creep specimen of welded joint ofKA-SUS410J3 (t/t_r=93%). The specimens consisting of weld metal, heat affected zone and base metal were removed at the different distances from the surface. All of the specimens were subjected to the SP creep test at 600°C and 250 N. Experimental results revealed that the SP creep properties deteriorated by the uniaxial creep and the deterioration was pronounced at around the surface of the interrupted creep specimen. The distribution of the decrease in SP creep strength throughout the thickness was qualitatively in good agreement with that of the area fraction of creep void. It was also found by the life fraction rule that the region just below the surface already consumed almost all the life, whereas the life fraction was determined to be about 70% at the central region of the thickness.

Study of creep cavitation behavior in tempered martensitic steel using synchrotron micro-tomography and serial sectioning techniques

Two three-dimensional (3-D) techniques, namely X-ray microtomography and serial sectioning, have been applied for characterization of creep cavitation behavior in tempered martensitic steel. For this purpose samples have been extracted from a series of specimens that were subjected to creep tests over the stress range of 120–180MPa at 600°C. The presence of creep voids in the series of samples was un-ambiguously detected in a non-destructive manner using synchrotron X-ray microtomography with a resolution of 1μm. The 3-D visualization of the datasets provided an assessment of the spatial distribution and morphology of the creep voids as a function of creep stress and high temperature exposure time. The quantitative analyses of the image datasets enabled the development of functional relationships between the macroscopic creep parameters (such as rupture ductility, applied stress, creep life) and cavitation characteristics (such as volume fraction, and number density). The quantitative analyses also provided an evaluation of manifestations of growth and coalescence processes in the respective datasets. A transition of cavitation behavior of the steel has been found to occur in the stress range of 120–150MPa at 600°C. The evolution in the pattern of cavitation and its relation to the prior-austenite boundary was explored by combining micro-tomography and serial sectioning techniques, which revealed a new possibility in the progress of cavitation in the long term creep exposed specimens of 9–12% Cr heat resistant steels.

Effect of groove configuration on Type-IV creep damages in Gr.122 steel welds

Creep strength of high Cr ferritic heat-resistant steel welds decreases more rapidly than base metals due to Type-IV creep damage formed in the heat-affected zone (HAZ) at high temperatures. This paper aims to elucidate the effect of groove configuration of welds on the formation and evolution of Type-IV creep damage. Creep tests using base metal, simulated HAZ and two-types of weld joint; gas tungsten arc and electron beam welds, of the ASME Gr.122 steel were conducted. Decreasing the groove angle and width of HAZ was effective in prolonging the creep life; however, it could not prevent Type-IV failure. Type-IV creep voids were mostly observed in the area about 1.0–3.0 mm below the specimen surfaces, whereas their detailed distributions were dependent on the groove configurations. Both the distribution of the multiaxial stress state and creep strain in HAZ can be considered to influence the formation of Type-IV creep damage. Damage mechanics analysis taking the multiaxial stress state into account can predict fairly well the distribution of Type-IV creep voids and failure times of the weld joints without regard to the groove configurations.

Geometrical influence for creep damage evaluation of IN738LC using electron backscatter diffraction

The Ni-based superalloy IN738LC is an outstanding heat-resistant material, but it does have the disadvantage of being brittle. Therefore creep degradation such as deformation of components or initiation of microscopic creep voids only appear at the end of its available creep life. To date, electron backscatter diffraction (EBSD) in conjunction with scanning electron microscopy has been used for damage evaluation. Some results showed that the misorientation was correlated with the magnitude of macroscopic plastic or creep strains. In this study, by using the EBSD method, the characteristics of the change and distribution tendencies of crystallographic misorientation on creep life have been investigated. In addition, the geometrical influence of stress concentrated areas for misorientation analysis has been investigated, which is important to evaluate damage in practical components like blades. As a result, the grain reference orientation deviation parameter which corresponds to the spread of creep strain has been found to be useful to estimate creep life among the misorientation parameters. Also, misorientation analysis allows the evaluation of creep damage before the appearance of obvious defects without being subjected to the geometrical influence of the stress concentrated areas.

Analysis of unified continuum damage mechanics model of gas turbine rotor steel: Life assessment

In this research, the unified creep continuum damage mechanics model of Lemaitre is used to model the stress–strain behavior of a gas turbine rotor. The unified damage law can model the creep and low cycle fatigue damage. It is based on the increment of the accumulated inelastic strain, which may be due to either creep damage or low cycle fatigue damage. The parameters of the unified continuum damage mechanics model, including Q, Norton constants, and constants of the nonlinear kinematic hardening model are determined and optimized by cyclic tension and relaxation tests. A careful three-dimensional thermal analysis is carried out and the temperature gradient is obtained. This analysis is used to determine stresses of the gas turbine rotor. Creep and low cycle damage are also modeled and stress–strain evolution in the gas turbine rotor is obtained. The replica tests are performed on four points of the rotor surface and formation of the creep voids is verified under a scanning electron microscope. Based on the continuum damage mechanics analysis, the creep–fatigue interaction is considered in modeling and the remaining life of the rotor is estimated.

Nucleation and Development of Creep Voids under Multiaxial Stress Using Circumferentially Notched Bar Specimens of Pure Aluminum

The aim of this work was to examine the nucleation and development of creep voids under multiaxial stress states. Type-IV creep damage has been identified as a troublesome in-service problem in welded sections and defined as the formation of creep damage in the fine-grained/intercritical heat affected zone (HAZ). It results in the degradation of the creep strength of structural components. The stress in the fine-grained HAZ is usually under a multiaxial stress state because the HAZ is wedged between the base and the weld metals. In general it is difficult to achieve various types of multiaxial stress states in experiments. Circumferentially notched bar specimens could used to develop the multiaxial stress state around the notch root. In this study, some interrupted creep tests were performed employing the circumferentially notched bar specimen of pure aluminum to examine the relation between nucleation/development of creep voids and the multiaxial stress state. After testing, grid lines divided the metallograph into equal size areas. Subsequently the number of creep voids in each segmented metallograph was counted using a technique similar to the A-parameter method and thus the distribution of creep voids area in each area was mapped. Experimental results showed that creep voids were mainly nucleated at the notch root and subsequently developed in the direction inclined from the loading axis by about 30 to 60 degrees. Elastic-plastic creep FE analyses were also performed to calculate the triaxial factor λ inside the notch root. It was shown that distributions of the triaxial factor λ were almost the same as those of creep voids.

PS01 配管溶接熱影響部におけるクリープボイドの三次元幾何形状の計測

PS01 配管溶接熱影響部におけるクリープボイドの三次元幾何形状の計測

The Microstructure Transformation of 10CrMo910 Steel after Running 147000h in High Temperature

Two specimens from original 10CrMo910 steel and the steel after running 147000h in high temperature were analyzed in this research by SEM, TEM, XRD and electron probe. The results show that after running 147000h in high temperature the 10CrMo910 steel has changed a lot: 1) The dislocation and subgrain boundary in bainite decreased largely while that in massive ferrite almost disappeared. 2) The strip-shape M3C carbides were nearly dissolved, the granular austenite was gradually transformed into martensite, the fine needle-shape Mo2C carbides coarsened, and granular carbides separated out in massive ferrite. 3) There were large particles of M23C6 carbides in crystal boundary between bainite and ferrite, especially in the boundary of any three phases. And creep voids appeared.

OS0804 SUS304HTB鋼のクリープ損傷過程におけるEBSD観察とひずみ評価

Creep damage was imposed on the austenitic stainless steel SUS304HTB for boiler tube use under the conditions of 650℃-130MPa and 700℃-75MPa. EBSD(Electron BackScatter Diffraction pattern) observation was conducted on the creep damaged samples using KAM(Kernel Average Misorientation) maps and GROD(Grain Reference Orientation Deviation) maps. Area averaged KAM and GROD values; KAM_<ave> and GROD_<ave> were used as the comparative parameter to time fraction and creep strain. While KAM_<ave> showed apparent dependence on EBSD pixel step size (i.e. observation magnification), GROD_<ave> did not show such dependence. KAM strain pa rameter, M_<εKAM> proposed here showed more unified and monotonic correlation with creep strain than KAM_<ave> GROD_<ave> showed much more unified and linear correlation with strain except for final rupture stage. Using GROD_<ave> creep void growth process can be quantitatively evaluated.