Creep Exposure Research Articles

Effect of notch angle on creep deformation and rupture behavior of 304HCu SS

Abstract Multiaxial creep deformation behaviour of U-notched 304HCu austenitic stainless steel (304HCu SS) has been studied by altering the angle of the U-notch, by 30°, 60°, 90°, and 120°, for a fixed root radius of 1.25 mm. The associated stress concentration factor for the U-notch geometry is about 1.8–1.86 with decrease in notch angle. Creep rupture lives of all notched specimens are above the un-notched specimen. However, decrease in rupture life and increase in creep rupture ductility has been observed with an increase in notch angle. At all notch angles, nearly intergranular creep cavitation failure with isolated cavities was observed at the notch center. In contrast, at the notch root, with decrease in notch angle the fracture mode changed from mixed mode to intergranular fracture. Finite element analysis (FEA) of stationary state of stress distribution indicated that von-Mises stress throughout the notch throat plane and maximum principal stress near the notch root region were found to increase with increase in notch angle. However, away from the notch root the maximum principal stress was found to decrease with increase in notch angle. Hydrostatic stress distribution remained same near the notch root at all the notch angles, but tends to increase towards the specimen center with decrease in notch angle. Similarly, at the notch throat plane, triaxiality factor (TF) was found to increase with decrease in notch angle. The creep damage evaluated through FEA coupled with continuum damage mechanics (CDM) proposed by Kachanov-Rabotov (K-R) model was found to increase with an increase in notch angle, resulting in decrease in creep rupture life. Creep damage evaluated as a function of notch depth, during initial creep exposure, revealed significant damage for 1.675 mm notch depth in comparison to 2 and 2.5 mm notch depths which resulted in lower rupture life for 1.675 mm notch depth.

Creep behaviour of dissimilar weld joint between buttered Grade 91 steel and Alloy 800

Creep rupture behaviour of dissimilar weld joint (DWJ) between buttered Grade 91 steel (G91) and Alloy-800 fabricated by Shielded Metal Arc Welding (SMAW) process is investigated at 873 K over a stress range of 90–160 MPa. DWJ exhibited lower creep rupture strength than the G91 steel and the failure location of the DWJ shifted with applied stress. At high stresses (≥140 MPa) fracture occurred in the over-tempered region of base metal, whereas it shifted to HAZ at lower stresses. In contrast to interface failures being considered the life-limiting factor in DWJs, the ruptured specimens in the present study revealed a clear interface (between the buttering layer and G91) free from deleterious coarse carbides and insignificant cavitation even after creep exposure of ≈ 5800 h. It’s important to highlight that the buttered G91 DWJ exhibited higher rupture life and weld strength reduction factor compared to joints fabricated without buttering by the SMAW process.

Effect of PWHT of X6Cr-NiNbN 25-20 Steel on the Evolution of the Sigma Phase during Creep Exposure at 700 °C

Effect of PWHT of X6Cr-NiNbN 25-20 Steel on the Evolution of the Sigma Phase during Creep Exposure at 700 °C

Creep behaviour and microstructure evolution in Super 304H–P92 heterogeneous welds

ABSTRACT The paper deals with results of long-term stress rupture tests on “cross-weld“ specimens made of Super 304 H – P92 heterogeneous welds. Stress rupture tests were carried out at temperatures of 625°C and 650°C up to 20,000 hours to rupture. Creep rupture strength and WSF values of Super 304 H – P92 welds for 10,000 hours at both 625°C and 650°C were calculated. The preferential location of failure was the intercritical part of the heat affected zone in P92 steel. Hardness of both base materials did not show significant changes during creep exposure at both 625°C and 650°C. However, gradual hardening of the weld metal occurred. Metallographic studies were performed in individual parts of heterogeneous welds. A special attention was paid to precipitation reactions in both base materials and heat affected zones. A residual content of vanadium in Super 304 H steel affected precipitation processes. Vanadium partly substituted niobium in the modified Z-phase ((V,Nb)CrN).

Generation of allowable stress for indigenously developed 304HCu Stainless steel

The 304HCu stainless steel (SS) is the candidate material for reheater and superheater tubes in Advanced Ultra Supercritical (AUSC) coal-fired power plants of India. In the present study, the creep design data have been generated for indigenously developed 304HCu SS using the creep properties (creep strain and the rupture life) and tensile properties available at the temperatures 923 K, 973 K and 1023 K. The time dependent allowable stresses were obtained for the above steel according to the stress criteria specified in ASME Section III Division 5 Subsection HB. The allowable stresses were also derived in the creep as well as non-creep regimes as per the guidelines in ASME Section IID. In both the cases, the maximum allowable stress at each temperature was dictated by the stress to cause rupture. The allowable stress determined as per Section IID was the same as that estimated according to section III.5 HB for creep exposure of 100,000 h. Further, it must be mentioned that, at the temperature range 923–1023 K, the indigenously developed 304HCu SS possess allowable stresses that are in par with the corresponding values specified in ASME code case 2328–2.

Creep damage mechanism of 20Cr32Ni1Nb steel at high temperatures

In the petrochemical industry, the centrifugally cast austenitic 20Cr32Ni1Nb stainless steel used in steam reformer furnace's hot outlet manifold experiences creep failure and microstructural degradation, impacting its structural integrity. Understanding the causes of these issues is crucial for addressing the associated challenges. This study examines the creep deformation behaviour, damage mechanisms, and microstructural evolution of 20Cr32Ni1Nb steel under stresses of 22–50 MPa at temperatures of 1163 K (890 °C) and 1223 K (950 °C). Analysis of creep and creep strain rate curves revealed the presence of both primary and tertiary creep stages, with the power law relationship governing the stress-dependent minimum creep rate and rupture life. The variance of creep damage tolerance factor was also analysed, with results showing that in high-stress regimes, loss of section and necking along with cavity growth may initiate tertiary creep, while in low-stress regimes, microstructural degradation, specifically G-phase formation, may initiate tertiary creep. As the duration of creep exposure increases, microstructures were observed to degrade, with the growth and coarseness of NbC and M23C6 carbides. The structure of the space group for the Ni–Nb–Si enriched G-phase in 20Cr32Ni1Nb steel was found to be Fm3‾m, with a lattice parameter of about 1.14 nm.

Laves phase evolution in 10Cr-3W-3Co steel during thermal exposure and creep

The formation of Laves phase was studied in experimental 10Cr-3 W-3Co creep-resistant steel. The steel was exposed to the temperature of 650 °C, and just thermal and creep exposure was compared. Laves phase content, particle size, and morphology were quantitatively analysed by image analysis of micrographs. Significant differences were observed between the creep and thermal exposure. Thermal exposure resulted in fine Laves phase particle nucleation and their gradual coarsening. On the other hand, creep exposure induced the formation of coarse particles from the early stages of exposure. The differences seem to originate in a different state of the matrix, where martensite crystal boundaries facilitate the precipitation of fine Laves phase particles.

Creep Resistance and Microstructure Evolution in HR3C-P92 Heterogeneous Welds

The paper deals with results of long-term stress rupture tests on „cross-weld“ specimens made of HR3C–P92 heterogeneous welds. Stress rupture tests were carried out in air at temperatures of 625 and 650 °C up to ca 20 000 h to rupture. Creep rupture strength values of HR3C–P92 welds for 104 h at both 625 and 650 °C were calculated. The preferential location of failure was the intercritical part of the heat affected zone in P92 steel. Local changes of hardness during creep exposure were evaluated by hardness profiles across the welds. Metallographic studies were performed in individual parts of heterogeneous welds. A special attention was paid to precipitation reactions in both base materials and heat affected zones.

A simulation of softening during creep exposure of grade 91 steel in a time range over 100,000 h around 600°C

Hardness reduction is a useful indicator of microstructural degradation progressing in Gr. 91 steel during its creep exposure. Grade 91 steel is strengthened by its fine and elongated subgrain microstructure, and its hardness decreases with increase in its subgrain width. Subgrain width of the steel is stabilized by pinning force from M23C6 particles. Aggregation of the particles due to thermal aging (without creep) brings about coarsening of subgrain width (thermal recovery). Creep deformation assists the aggregation, accelerating the thermal recovery during creep. It also directly induces coarsening of subgrain width. These three mechanisms cause hardness reduction during creep exposure of the steel. On the basis of this model, hardness reduction during creep exposure of Gr. 91 steel in a time range over 100,000 h around 600 °C is simulated, and results of the simulation are compared with hardness of steam pipes measured after their service exposures. The simulation can well explain the hardness reduction of normalized and tempered Gr. 91 steel. The model is also verified with hardness measurement during thermal aging up to 233,000 h.

Effect of prior heat treatment on the microstructure evolution and creep resistance of ZK60 Mg alloy under tensile creep loading along normal direction

Tensile creep tests were performed along the normal direction on the as-solutioned (AS) and as-aged (AA) samples of a commercial ZK60 alloy in this work. The results showed that the AA sample obtained a stronger 0.2% proof stress but a poorer creep resistance in comparison to the AS sample. It was revealed that the creep deformation in the AS sample was attributed to basal slip and twinning while the creep behavior of the AA sample was controlled by basal slip, pyramidal <c+a> slip and twinning. Besides, the reasons for the poorer creep resistance of the AA sample were unveiled. Pyramidal dislocations towards various moving directions were found to accelerate the creep deformation, and basal dislocations kinking at twin boundaries were found to induce heavy stress concentration. Furthermore, massive dynamic precipitates including β1′ and β2′ types were formed in the AS sample during the creep exposure, contributing to the superior creep resistance. But in the AA sample, plentiful β1′ and β2′ phases generated during the prior peak-ageing treatment, dissolved into the matrix fast and only β1′ phases re-precipitated during the creep process. With the decreased area fractions of precipitates, the age-hardening effect slacked off and failed in enhancing the creep resistance.

A unified constitutive approach for creep response of AISI 316 steel produced by conventional technologies or additive manufacturing techniques

ABSTRACT The creep response of AISI 316 and AISI 316 L was analysed to provide a coherent picture of the material behaviour, valid for both conventional wrought and additively manufactured steels. Literature evidences were considered. The analysis demonstrated that the presence of a fine distribution of particles, precipitated during creep exposure in both AISI 316 and 316 L(N), should be taken into account. A recent model, expressly developed for particle-strengthened alloys, was successfully used to describe the minimum creep rate dependence on applied stress for AISI 316 and 316 L(N). The same set of constitutive equations, in combination with the composite model, was then used for describing creep data obtained by testing the AISI 316 L produced by additive manufacturing. The main difference with wrought materials was that the microstructure of the additively manufactured alloy was composed by cells (soft zones) and thick walls heavily decorated with ultrafine oxide particles (hard zones).

Numerical simulation of creep notched bar of P91 steel

Numerous components designed for use at elevated temperatures now exhibit multiaxial stress states as a result of geometric modification and material inhomogeneity. It is necessary to anticipate the creep rupture life of such components when subjected to multiaxial load. In this work finite element analysis has been performed to study the influence of different notches, namely blunt and medium notches on the stress distribution across the notch throat during the creep exposure. Within the FE model, a ductility exhaustion model based on the Cocks and Ashby model was utilized to forecast the creep rupture time of notched bar P91 material. The lower and upper bound of creep ductility are employed in the FE analysis. Different notch specimens have different stress and damage distribution. It is shown that for both types of notches, the von Mises stress is lower than the net stress, indicating the notch strengthening effect. The accumulation of creep damage in the minimum cross-section at each element across the notch throat increases over time. The point at which damage first occurs is closer to the notch root for the medium notch than for the blunt notch. The long-term rupture life predicted for blunt notch specimens appears to be comparable to that of uniaxial specimens. The upper bound creep ductility better predicts the rupture life for medium notches.

Precipitation Evolution in the Austenitic Heat-Resistant Steel HR3C upon Creep at 700 °C and 750 °C.

HR3C (25Cr-20Ni-Nb-N) is a key material used in heat exchangers in supercritical power plants. Its creep properties and microstructural evolution has been extensively studied at or below 650 °C. The precipitation evolution in HR3C steel after creep rupture at elevated temperatures of 700 °C and 750 °C with a stress range of 70~180 MPa is characterized in this paper. The threshold strength at 700 °C and 750 °C were determined by extrapolation method to be 57.1 MPa and 37.5 MPa, respectively. A corresponding microstructure investigation indicated that the main precipitates precipitated during creep exposure are Z-phase (NbCrN), M23C6, and σ phase. The dense Z-phase precipitated dispersively in the austenite matrix along dislocation lines, and remained stable (both size and fraction) during long-term creep exposure. M23C6 preferentially precipitated at grain boundaries, and coarsening was observed in all creep specimens with some continuous precipitation of granular M23C6 in the matrix. The brittle σ phase formed during a relatively long-term creep, whose size and fraction increased significantly at high temperature. Moreover, the σ phases, grown and connected to form a large “island” at triple junctions of grain boundaries, appear to serve as nucleation sites for high stress concentration and creep cavities, weakening the grain boundary strength and increasing the sensitivity to intergranular fracture.

Effect of Graphite on Residual Lifetime of Superheater Tubes Made of Steel 16Mo3 after Creep Exposure

Carbon and low-alloys steel components are used in various stages of steam power plants which are operating at elevated temperatures (up to 530°C). Long-term exposure of low-alloy steel components at elevated temperatures inevitably result in some kind of structural degradation; for example, creep cavitation, carbide coarsening and/or spheroidization, and, less commonly, also graphitization. Graphitization can be detected mainly in carbon steels, but also in 16Mo3 steel where the ferritic matrix is strengthened by Mo in the solid solution. The absence of strong carbide forming elements (typically Cr, V, Nb) is the principal reason why these steels are susceptible to cementite decomposition and graphitization much more than the other creep resistant steels. The paper describes microstructure and material properties of superheater 16Mo3 tube, in which graphite was found after 70,000 hours of exposure at 445 °C. The actual material properties were compared to another tube of the same material delivered in the as-received state. The comparison was based on evaluation and testing of mechanical strength, fracture properties and microstructure. Creep resistance of the steel was studied using small punch creep tests (SPCT).

An improved creep damage mechanics model and its application to evaluate creep strain design codes for hot outlet manifold in steam reformer furnace

Considering the effect of carbide coarsening and cavity nucleation and growth on the creep damage of Fe–Cr–Ni-based austenitic stainless steels, an improved creep damage constitutive model is established based on the hyperbolic sine function. The material parameter optimisation procedure and the associated ABAQUS user subroutine CREEP of the model are also developed. In addition, the improved model is calibrated and validated by comparing the extrapolated failure times for the three creep damage models, and applied to an idealized pipe bend to evaluate the inelastic strain limits in the ASME-NH code. The results show that the proposed model is capable to reflect the creep behaviour of a hot outlet manifold steel under different stress levels at 890 °C. During the long-term creep exposure of the manifold component, the limits of 1% creep membrane strain and 2% creep membrane strain + bending strain ruled in ASME-NH code may not be conservative. If only the cumulative inelastic strain is considered in the structural design, membrane strain is the primary rule for investigating the failure of the pipe bending.

Evolution of precipitation particles with respect to grain boundary energy in Grade 91 steel during creep exposure

ABSTRACT Precipitation particles evolution in ASME Grade 91 steel after 233,823 h of creep exposure at 600 °C was characterised after grain boundary energyclassification in terms of misorientation angle (MOA): Low energy boundary(LEB) with MOA up to 15°, high energy boundary (HEB) with MOA from 15° to 45°, andmedium energy boundary (MEB) with MOA from 45° to 63°. The number density of the Laves phase stronglydepends on the boundary energy classification by MOA: the largest in the HEB,followed in order by the MEB and the LEB. The Z-phase particles is preferentiallylocated in grain boundaries where at least one of adjacent boundaries is theHEB or the MEB. Even though the block boundaries belong tothe MEB in terms of MOA, they are not the preferential site for the Z-phase due to their exceptionally smallboundary energy.

Issues and solutions for integrity of pressure vessels and piping subjected to long-term creep exposure in supercritical thermal plants

ABSTRACT ASME BPVC Section VIII Division 2 (‘ASME VIII(2)’) and ASME B31.1 are being used not only for low-temperature design but also for high-temperature design in the creep range in supercritical thermal power plants. However, creep effects in ASME VIII(2) and B31.1 are taken into account implicitly, and there could be significant technical issues in terms of conservatism in the case of long-term operation in the creep range because their design evaluation results do not change across plant operation time. The conservatisms of ASME VIII(2) and B31.1 have been quantified based on comparisons with nuclear-grade design rules of ASME BPVC Section III Division 5 and RCC-MRx which take creep hold time into account explicitly. It was shown that ASME VIII(2) and B31.1 results could be non-conservative if operation time exceeds a certain time limit, which raises concerns around non-conservatism in the case of long-term operation within the creep range.

Transmission electron microscopy of precipitation in fine-grained heat-affected zone of Grade91 steel weld during creep exposure

Precipitation during creep exposure at 550 °C in the fine grain heat-affected zone of a Grade91 steel weld produced by gas tungsten arc welding with no filler and post-weld heat treatment at 760 °C for 30 min. has been investigated. The applied tension load during creep testing was 165 MPa, the creep rupture time was 1138 hrs. (100% creep aging), and the creep exposure times were 911, 569, 341, 223 and 114 hrs. for 80%, 50%, 30%, 20% and 10% creep aging, respectively. Light microscopy and scanning electron microscopy were used to examine the heat-affected zone in the steel weld. Grain size in the fine grain heat-affected zone was measured by a circular intercept method on secondary electron images from scanning electron microscopy. Precipitates were detached from the fine grain heat-affected zone by a carbon-film extraction replica technique and investigated by transmission electron microscopy. During creep, a continual increase in grain size was observed in the fine grain heat-affected zone, starting from about 1.70 µm after post-weld heat treatment to about 2.30 µm at 100% creep aging. Precipitates found during creep were M23C6, M7C3, M6C, MX, M2X and high-Si Laves phase. M23C6 was found in all creep conditions until rupture, mainly at prior austenite grain boundaries and sub-boundaries. M7C3, M6C and high-Si Laves phase were observed during creep aging, mainly nearby grain boundaries and sub-boundaries. Relatively fine MX and M2X were found after 20% creep aging until rupture, mainly within grain interiors. Detailed criteria for identification of these precipitate phases, based mainly on energy dispersive X-ray spectrometry and selected area electron diffraction using transmission electron microscopy, is given and a major cause of premature loss of creep strength in Grade91 steel weld is discussed. This is important in understanding creep behavior of steel parts used for boiler tubing, heat exchangers, and steam piping systems in fossil-fuel power plants.

Study on microstructure characterization of fracture frontier of post welds heat treatment and type IV cracking of P92 steel welded joint

In the research work presented in this study microstructure evolution at fracture frontier of crept P92 weld, creep rupture life and effect of creep exposure time on microstructure evolution in fine-grained heat affected zone were performed. Microstructure evolution and creep rupture behavior of metal arc welded joint of P92 steel plate in the as-welded have been studied. The different states of post weld heat treatment (PWHT). (i). post welded heat treatment at 760 o C for the 2h (ii). re-austenitizing at 1040 o C for 60 min and air cooled and tempering at 760 o C for 2h. In PWHT condition, most common type IV cracking was observed creep exposure 620 o C / 150 MPa. The martensitic matrix fracture is also observed in PWNT 1 condition. A move away from the fracture frontier, the cavities still remain in the microstructure while the martensitic matrix fracture is difficult to observe. The line mapping also confirmed the increase in weight percentage of Cr and Mo in M 23 C 6 . The elemental mapping of PWHT 2 condition is also carried out in FGHAZ which confirm the formation of Mo and Cr-rich M 23 C 6 precipitates.

Microstructure Evolution and Coarsening of θ′-Al2Cu Precipitates in a Creep Aged Al-Cu-Mn Alloy

This paper describes the microstructure evolution in a creep aged Al-Cu-Mn alloy using optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Results show the sub-grains formation and accelerated coarsening of θ′ precipitates in creep aged samples. It is observed that at an applied stress of 50MPa, as the creep temperature increases the average length of θ′ precipitates increases and their aspect ratio first increases then decreases. It is also found that after 100h of creep exposure at 200°C/50MPa, θ′ precipitates have grown to an average length of 279nm; at 225°C/50MPa to an average length of 425nm and at 250°C/50MPa they have grown to an average length of 844nm. Coarsening/growth rate of the precipitates is described by Lifshitz-Slyozov-Wagner (LSW) model.