Creep Voids Research Articles

Creep Rupture Strength and Phase Stability of the Austenite Phase for KA-SUS304J1HTB

Metallurgical factor causing the heat-to-heat variation in creep rupture strength have been investigated for KA-SUS304J1HTB. In the long-term, there was a maximum difference of 3.5 times in creep rupture time between the heat with low creep strength and the heat with high creep strength. In the heat with low creep rupture strength, most of the creep voids occurred at the matrix/σ phase interface. Moreover, in the heat with low creep rupture strength, the area fraction of σ phase was larger than in the heat with high creep rupture strength. Considering that the difference in the area fraction of σ phase in each heat is related to the difference in phase stability of the austenite phase, the Md value in each heat was evaluated. The Md value is the parameter correlated with phase stability. The creep rupture time of each heat was correlated with the Md value. The smaller the Md value, the longer the creep rupture time. Therefore, the heat-to-heat variation in creep rupture strength is caused by the difference in the phase stability of each heat. In other words, in the heats with low phase stability, creep rupture strength is low because a large amount of σ phase precipitates during the creep test.

Tensile creep behavior of a heat treated β-solidified γ-TiAl alloy: The development of dynamic recrystallization and the role of B2 phase

The tensile creep behavior of a heat treated β-solidified γ-TiAl alloy has been investigated at 800 °C under 250–300 MPa. Results indicate that the heat treatment procedure proposed in this study can improve creep resistance of the alloy. Combined with kinetic analysis (stress exponent n = 3.36) and TEM observation, it can be inferred that creep deformation is dominated by viscous dislocation glide, and assisted by mechanical twinning. It is found that both twin boundary and TiB can hinder dislocation movement and strengthen creep resistance. During creep, there are obvious dynamic recrystallization (DRX) and B2 phase precipitation. On the one hand, DRX volume fraction increases with the increase of creep stress or the extension of creep time. Meanwhile, TiB can act as nucleation sites of recrystallized γ grains. On the other hand, there are two kinds of B2 phase precipitation behaviors: (ⅰ) thin B2 laths are formed in α2 lamellae; (ⅱ) blocky B2 phases are formed at colony boundary. Creep deformation is mainly completed by γ phase, B2 phase can involve in the deformation only under specific creep conditions. Due to poor deformation ability of B2 phase, creep voids prefer to nucleate at the γ-B2 interface and B2-colony interface. When voids are connected, cracks are formed and begin to propagate, eventually leading to creep failure in brittle-ductile mixed fracture.

Influence of microstructural heterogeneities on small-scale mechanical properties of an additively manufactured Al-Ce-Ni-Mn alloy

• Revealed the influence of local microstructural heterogeneity at melt-pool-boundaries in additively manufactured Al-rare earth alloy on nanomechanical properties • In situ nanoindentation and micropillar compression identified melt pool boundaries as weak spots • Identified the reasoning for creep void coalescence and failure initiation from melt pool boundaries Laser powder bed fusion-based additive manufacturing (AM) is a promising method to fabricate creep-resistant Al-rare earth alloys (Al-Ce-Ni-Mn) with stable microstructures at up to 400°C. However, creep testing of these alloys at high temperatures shows that void coalescence and failure initiation occurs along the melt pool boundaries in the microstructure. Hence it is crucial to understand how the local mechanical behavior of the melt pool boundaries would influence the global properties of the AM produced alloy. In this study, in situ nanoindentation conducted at room temperature and 300°C revealed a reduced hardness at the melt pool boundaries. Similarly, micro-pillar compression showed a slight decline in yield strength at these boundaries, indicating that they are the weak spots in the microstructure. Such multimodal local mechanical property studies are necessary for understanding the influence of melt pool boundaries on the bulk response of fusion-based AM alloys.

Mechanisms for creep rupture of 12Cr/alloy 4715 dissimilar weldments

The mechanism for creep rupture of 12%Cr /alloy 4715 welded joints was investigated by employing traditional uniaxial tensile creep in this study. The creep tests were maintained at 580 °C with creep loading stresses gradually decreasing from 300 to 240 MPa, and the time to failure was tested. The rupture location shifts from base metal (BM) to BM + fusion line (FL) then to FL + fine grain zone + coarse grain zone (CGZ), and finally to FL + CGZ as holding time increases. Meanwhile, a softening phenomenon is also observed in the heat-affected zone (HAZ) in crept specimens for a more extended holding time, where voids, carbides, and dislocation density are further discussed. The carbides are identified as Cr and Mo-rich M23C6 type, which forms mainly at grain boundaries during creep testing. In HAZ, M23C6 gradually increases and coarsens, providing nucleation sites for creep voids. Finally, the Larson–Miller parameter method is employed to provide a theoretical reference for predicting use cases that satisfy the industry service condition.

Creep Behavior of Fe–25Ni–15Cr‐Based Alumina‐Forming Austenitic Steel

The creep behavior of Fe–25Ni–15Cr‐based aluminum‐forming austenitic stainless steel is investigated at 700 °C, under the selected stress of 100, 140, 170, and 180 MPa, respectively. Creep properties are analyzed in terms of stress exponent (n) and creep damage tolerance factor (λ). Within the tested stress range, there is an order of magnitude difference between the minimum creep rate and steady creep rate. The evolution of the microstructures after creep test with the stresses of 140, 170, and 180 MPa is characterized by scanning electron microscopy and electron backscatter diffraction detectors, respectively. With the decreased creep stress, the creep void size shows an increasing trend. Higher stress would accelerate the process of recrystallization during the creep process at 700 °C. The proportion of high‐angle grain boundaries increases to a certain extent under high‐stress conditions. A large amount of Laves and NiAl phases are observed after creep test at 140, 170, and 180 MPa, respectively.

Effect of nitrogen concentration on creep strength and microstructure of 9Cr-ODS ferritic/martensitic steel

9Cr oxide dispersion strengthened steels with slightly different nitrogen concentrations (0.0034 − 0.029 wt%) were prepared and their creep property at 973 K was investigated with microstructural characterization before and after the creep test. The creep strength decreased significantly as the nitrogen concentration increased. Microstructural observation revealed that, in the higher nitrogen concentration specimen, coarse Y-rich inclusions were found along the boundary between transformed ferrite region and residual ferrite region. The solubility difference of nitrogen in α and γ phase would induce the localized increment of nitrogen concentration in the boundary region during the austenitizing process, resulting in the thermodynamic destabilization and subsequent coarsening of the dispersed oxide particles. The rows of creep voids were found near the rupture part of the crept specimen, and their fraction increased with increasing nitrogen concentration. This suggests that the coarse inclusions were one of the main starting points of creep void formation, leading to premature fracture in the higher nitrogen specimen.

Combined tensile and bending fatigue behavior and failure mechanism of a blade-like specimen at elevated temperature

Combined tensile and bending fatigue (CTBF) behavior and failure mechanism of a blade-like specimen, made of a Ni-based alloy DZ125, are experimentally investigated. In order to reveal the geometrical feature and the stress state of the fillet between turbine blade bottom and platform, a blade-like specimen was designed. A novel high temperature experimental methodology was developed to apply the cyclic tensions and high frequency transverse vibration, to the blade-like specimen. CTBF, dwell fatigue (DF) and high stress ratio bending fatigue (BF) tests were performed at 850 °C. Experiment results show that the introduction of high frequency vibration significantly reduced the cyclic life in comparison with DF, while the cyclic tension related dynamic strain aging (DSA) has a positive effect on high cycle fatigue (HCF) life. LCF-related and HCF-related multi-cracks are initiated at the surface due to the effect of bending stress gradient. During CTBF at elevated temperature, LCF, HCF, creep, oxidation, and their interactions contribute to the crack initiation, and linked creep voids, oxidation, and carbide precipitates accelerate the crack growth.

A Review on Failure Analysis of Turbine Blades of Aero Gas Turbine Engine

Almost all commercial electrical power on earth is generated with a turbine, driven either by wind, water, steam or burning gas. Turbine blades are subjected to very strenuous environments inside a gas turbine. They face high temperatures, high stresses, and a potentially high vibration environment. All these factors can lead to blade failure, resulting in catastrophic failure of turbine. In the event of turbine blade failure, turbine does not work and this leads to shutdown of power plant from one to four weeks or longer, depending on the extent of the damage and the procedure used to make the machine operational again, which results in economic loss and service to mankind also stops. For each day of a forced outage, a utility could lose hundreds of thousands of rupees in electrical power. The external and internal surface damages include corrosion, oxidation, crack formation, erosion, foreign object damage and fretting. The internal damage of microstructure include γ’ phase, CoNi3 [(Al, Ti)] phase aging (rafting), grain growth, brittle phases formation, carbides precipitation, creep and grain boundary void formation. These damages produce dimensional change which results in increase in operational stress that leads to deterioration in turbine efficiency. The deterioration of blade material is related to the high gas temperature, high steady state load levels (centrifugal load) and high thermal transient load (trips, start-ups, start downs). In this research, a review of common failures due to metallurgical defects found in gas turbine discussed is presented.

Creep behavior and microstructure evolution of Ni-based alloy IN617 at 1000 °C

The microstructure evolution of IN617 during creep under 30 MPa, 23 MPa, and 16 MPa at 1000 °C was characterized after creep ruptured tests and interrupted tests. As the applied stress was decreased, three different microstructure characteristics were found (i.e., dynamic recrystallization, classic creep voids, and precipitates coarsening), which were related to microstructural factors such as dislocation configuration, grain boundary (GB) weakening, and nitrogen diffusion and brittle-phase precipitation. The results indicated that the high stress of 30 MPa promoted the significant multiplication of dislocations that rearranged into high-density and low-energy substructures, leading to recrystallization and ductile rupture. When stress was decreased to 23 MPa, dislocations only formed loose tangles causing intragranular strengthening, and GB weakening was involved because most intergranular precipitates that could have exerted the pinning effect dissolved. Therefore, intergranular rupture occurred under the combined effect of intragranular strengthening and GB weakening. As stress was as low as 16 MPa, precipitates that existed in the as-received condition completely dissolved and abundant nitrogen diffused into the specimen from the furnace atmosphere through creep cracks, promoting the formation of brittle nitrides (π phase and AlN). The brittle π phase forming continuous networks along GBs was vulnerable to internal cracking, and the needle AlN precipitating inside grains caused the AlN/matrix interfacial cracking, which exerted a synergetic effect on brittle fracture.

Type IV creep crack initiation and propagation in mod.9Cr-1Mo steel welds

ABSTRACT The ratio of crack initiation and crack propagation life is important from the viewpoint of residual life prediction of mod. 9Cr-1Mo steel welds under Type IV creep damage. Both the creep void density distribution and the failure process with crack initiation/propagation/final failure are predicted for three types of welds in pipes and nozzles, which are compared with experimental results at 650°C. The ratio of crack initiation and final failure life depends on weld type and loading conditions; the ratio is about 0.95 in the cases where redistribution of stress occurs only in the thickness; the ratio is 0.59 ~ 0.65 in the cases where redistribution of stress occurs in both the thickness and circumferential directions of the pipe. Influence of stress ratio (axial vs. circumferential stress) is also predicted for circumferential welds subject to internal pressure/axial load and is compared with experimental results at 650°C.

Subsurface microstructural degeneration of Super304H boiler tube under the synergy of fireside corrosion and creep stress

The deterioration mechanism of creep rupture life of Super304H in a lab simulated fireside corrosion environment at 650 °C is investigated. This includes in-depth study of microstructural evolution in grain structure, elemental distribution and precipitates. Results reveal a two-thirds reduction in creep life by corrosive media. The major reason for this degradation under the synergy of high-temperature corrosion and creep stress is considered to be more serious dynamic recrystallization and martensitic transformation occurring in the surface layer. The fractography of the rupture specimens indicates creep voids coarsening is responsible for fracture in both conditions.

Failure cause assessment of steam reformer radiant tube after long‐term service

AbstractThis study investigates the damage, microstructure, and mechanical evolution of service‐exposed HP40NbTi radiant tubes of a steam reformer furnace. Tube failure was reported after 6‐year of working at 525°C–830°C in the form of visible longitudinal and transverse cracks. The microstructure was evaluated using optical and scanning electron microscopy equipped with energy‐dispersive X‐ray spectrum analysis. The carbide phase transformation was determined by the method based on the matrix dissolution technique and X‐ray diffraction. The results of this study showed that the primary cause of the longitudinal crack formation is the local overheating and the oxidation of the dendritic grain boundaries. The M7C3 carbide transforms completely into the metastable M23C6 phase during high‐temperature service. Metallographic examinations revealed the shrinkage cavities and creep voids that are nucleated on both Cr‐ and Nb‐carbides. The continuous oxide layers of Cr2O3 and SiO2 formed near the inner and outer tube surfaces and the matrix's chemical composition near the outer tube surface was found to be depleted of Cr and C. Mechanical properties are irregular across the samples. A little margin of safety is noted, while the complete ductility dip is detected at room temperature. The root cause analysis of the failure under investigation showed that the tube failed due to the creep.

Insight into the creep-fatigue interaction and remaining creep damage mechanisms in different micro-regions of 9%Cr steel welded joints

The deformation and damage mechanisms of welded joints have gained high interest for their widespread use in power plants. The inter-critical heat-affected zone (ICHAZ) is generally identified as the weakest region in the whole welded joint. However, the micro-regions' nanoindentation and creep results in the present work reveal that the weakest position moves to base material after fatigue loading. It is found from electron backscatter diffraction (EBSD) analysis that the early saturated grain growth in the ICHAZ contribute to the movement of the weakest region. It is important to note that compared with the creep strength in the as-received condition, prior fatigue loading enhances the creep deformation. It is also observed from three-dimensional X-ray tests that large fatigue cycles and long hold time of fatigue loading accelerate the nucleation of creep voids. Finally, a fatigue damage parameter based on the nanoindentation microhardness is proposed, which is validated to be capable of capturing the effect of various loadings on the subsequent creep life and creep fracture location. These results benefit the understanding of the creep behaviour of welded joints after fatigue damage and provide a guide for the design and damage evaluation of welded components.

Investigation of creep degradation in a Super304H austenitic steel and its nonlinear ultrasonic assessment method

ABSTRACT This paper reports the degradation behaviour of Super304H during the creep process. The failure mechanism of this Super304H is the typical intergranular damage, the creep voids and micro-crack are mainly concentrated on the GBs and triple point GBs. Meanwhile, there is a lot of precipitates inside void chain, those particles are the M23C6 phase, which indicate that the coarsening and growth of M23C6 phase are detrimental to the microstructural property of Super304H. Besides, the GND density of Super304H increased remarkably at the creep acceleration period, and the dislocation structure also changed from the pattern of straight-line to the zigzag. In addition, the nonlinear ultrasonic measurement results showed that there is a general upward trend of nonlinear parameter β’ of Super304H during creep. The increase of the nonlinearity effect in Super304H could be due to several factors such as the precipitates precipitation, dislocation density/structure change, and the microdefects formation.

Understanding the relation between creep-fatigue fracture mechanisms and intergranular dislocation accommodation of a high chromium steel using nanoindentation characterization

Relying on nanoindentation technology, the effect of intergranular dislocation accommodation on fracture behavior for the P92 steel that had experienced creep-fatigue (CF) loading was studied. The results indicate that the fracture mechanism of P92 steel gradually changes from fatigue-dominating to creep-fatigue interactive damage with increasing dwell time. According to the variations of hardness (H), elastic modulus (E), creep resistance and strain rate sensitivity (m), a normalized damage indicator ln(H/E)/m is proposed to characterize the fracture mechanism. Thermally activated process of intergranular dislocation accommodation results in the formation of creep voids at GBs and a remarkable stress relaxation, leading to a creep-fatigue fracture mechanism for the CF specimens with long dwells. However, mechanically driven of dislocation accommodation contributes to enhanced grain size and lower hardness, resulting in a fatigue-dominated fracture mechanism.

Effect of nickel-based filler metal types on creep properties of dissimilar metal welds between Inconel 617B and 10% Cr martensitic steel

Creep rupture behaviors of dissimilar metal welds (DMWs) between Inconel 617B and 10%Cr martensitic steel with two different filler metals Inconel 617 and 82 were comparatively investigated at 600 °C to study the effect of nickel-based filler metal types on creep properties. Both the crept DMWs ruptured on the steel side prematurely compared to steel base metal (BM). The failure mode of DMW shifts from interfacial failure at weld metal (WM)/steel interface to Type Ⅳ crack in the intercritical heat-affected zone (ICHAZ) of steel with longer rupture life at the same creep condition by replacing Inconel 617 filler metal with Inconel 82. Type Ⅳ crack was related to matrix softening and fine prior austenite grain boundaries (PAGBs) scarcely decorated by precipitates in the ICHAZ. Interfacial failure was controlled by creep voids nucleation, growth and coalescence at the interface, while oxide notch only triggered crack initiation. The failure mode shift is because Inconel 82 filler metal compared to Inconel 617 can achieve slighter creep deformation concentration in the HAZ closely adjacent to interface and avoid interfacial failure of the DMW due to the lower creep strength gradient across the interface it induced. Based on this study, creep strength compatibility should be considered when selecting nickel-based filler metals for similar DMWs to obtain prolonged creep life.

Microstructural changes and creep-strength degradation in 18Cr-9Ni-3Cu-Nb-N steel

The mechanism of long-term creep-strength degradation was investigated for 18Cr-9Ni-3Cu-Nb-N (KA-SUS304J1HTB, ASME Code Case 2328) steel, with a focus on changes in fracture mode and microstructure. At 650 to 800 °C, the long-term creep strength deviated from the trend of short-term data plots. The stress dependence of the minimum creep rate in the low-stress regime was different from that in the high-stress regime at 650 to 750 °C. The creep ductility initially increased with increasing Larson–Miller parameter and subsequently decreased at high values of the Larson–Miller parameter. M 23 C 6 , NbX, and Cu phase particles were confirmed to exist after short-term creep exposure, whereas a modified Z -phase and σ-phase were observed after long-term creep exposure. The fracture mode changed from creep void formation on grain boundaries to cracking at the interface between the σ-phase and the matrix in the long-term. The dislocations were pinned by precipitates in the grain interior, and the dislocation density in the grain interior remained high, even in the long term, at 600 to 700 °C. A precipitate-free zone formed around the σ-phase on grain boundaries after long-term creep exposure. The dislocation density was also low in the precipitate-free zone. The hardness of the precipitate-free zone was lower than that of the grain interior. We believe that the change in the fracture mode and the formation of a precipitate-free zone are responsible for the observed long-term degradation in the creep strength. • Creep strength degradation was observed in the long-term in 18Cr-9Ni-3Cu-Nb-N steel. • The dislocation density in grain interior was kept high even after creep rupture in the long-term. • Precipitation free zone (PFZ) was formed around σ phases on grain boundaries after long-term creep. • The hardness was lower in PFZ than in grain interior.

Effect of nano-metal additives on the creep behavior of AgSnO2 contact materials

The creep behavior of the AgSnO2 contact materials with different nano-metal additives (Fe, Ni, Cu) was investigated at room temperature. The fracture surface was characterized by scanning electron microscope (SEM). The creep behavior and mechanism of the AgSnO2 materials without addition and with Cu, Fe and Ni were simulated and analyzed. It was found that the creep strain and creep rate of the AgSnO2 materials were obviously decreased due to the improvement of interface wettability between Ag and SnO2 by the incorporation of metal additive. The Cu additive exhibited the most remarkable effect on improving the creep resistance, following by Fe, Ni. The creep strain of the AgSnO2(Cu) materials was significantly decreased, about 60% lower than that of the material without additives. The experimental creep curves of the AgSnO2 materials with different nano additives agreed well with the Norton-Bailey power law. Moreover, the nano metal additives played a role in preventing or slowing creep voids growth which could be attributed to the interface strength enhancement and the improved load-transfer capability, evidenced by ABAQUS software. This work can provide a useful guideline for the optimization and design of Ag-matrix contact materials.

Thermomechanical fatigue behaviour and damage mechanisms in a 9% Cr steel: Effect of strain rate

Thermomechanical fatigue (TMF) loading is inevitable during the operation of ultra-supercritical power plants. In the present work, TMF tests are carried out at strain rates of 5 × 10−5/s to 4 × 10−4/s and in-phase (IP) and out-of-phase (OP) to investigate the cyclic deformation and damage mechanisms of P92 steel. The results reveal that the increase in strain rate leads to a significant improvement in fatigue life under both IP-TMF and OP-TMF, in which OP-TMF induces more severe damage to fatigue life than IP-TMF. It was found that the fatigue life reaches a maximum after a strain rate of 1 × 10−4 s−1. It is important to note that the action of dynamic strain ageing (DSA) is strain rate-, fatigue cycle- and phase angle-dependent. It was also demonstrated that the effect of the strain rate on the fatigue life is a combined effect of DSA, fatigue cracks, creep voids and oxidation. Additionally, the predictive abilities of various traditional life prediction methods under various loading conditions are comparatively assessed.

Creep rupture mechanisms and life prediction of IN617 for VHTR applications

The creep rupture mechanisms of IN617 were systematically investigated by establishing the relationship between microstructural characteristics and creep performance, including rupture time, percentage elongation, microhardness, and minimum creep rate. The Wilshire equation was employed to extrapolate long-term creep life by normalizing the stress, σ , using the ultimate tensile strength, σ TS . The results indicated that at temperatures ≤950 °C, when σ / σ YS values were >0.2 ( σ YS = yield strength), the creep rupture mechanism involved the sliding and migration of recrystallized grain boundaries, leading to matrix softening and necking fractures. When applied stresses ranged from 0.12 σ TS to 0.2 σ YS , creep voids along the grain boundaries became a pronounced feature, implying the rupture mechanism of the weakening of the grain boundaries without sufficient secondary-phase pinning. At a low stress level ( σ / σ TS < 0.12), creep ruptures were governed by a composite mechanism of microstructural degradation and internal oxidization, which was reflected in the brittle fractures caused by the connection of cracks along carbide networks and internal oxidized voids. A temperature–stress–rupture mechanism map was constructed by combining a comparison and summary from the literature. The Wilshire extrapolation curve described the creep rupture times well with temperatures and stresses, and the turning point of the curve suggested that microstructural degradation and internal oxidization increased the minimum creep rates and accelerated creep rupture.