Creep Rupture Tests Research Articles

Effect of HIP on Creep Properties in the Single Crystal Superalloy

The Rene N5 is a second-generation single crystal superalloy containing 3wt% Re, commonly used in gas turbine blades for power generation at temperatures up to 1600°C due to its excellent creep life. Generally, single crystal (SX) and directionally solidified (DS) blades are not applied to hot isostatic pressing (HIP) because their porosities are lower than conventionally cast (CC) blades, and it causes recrystallization issue. However, in this study, HIP was chosen as a candidate to improve the mechanical properties of the second-generation single crystal superalloy Rene N5. HIP followed by ultra-rapid cooling rate of 660K/minute was applied in an attempt to address this issue. This approach was expected to enhance creep rupture properties through pore reduction and microstructural homogenization. Furthermore, by setting the HIP temperature and time identical to the solution treatment condition, the possibility of replacing solution treatment with HIP was discussed. As results of creep rupture tests, the rupture life was excellent in the order of specimens treated with HIP + solution + aging, solution + aging, and HIP + aging treatments. It is interesting that this is found to be primarily influenced by the size and shape of carbides and γ’ phases, with the effects of crystallographic orientation, shrinkage porosity, and eutectic phase being negligible.

Investigation of Precipitation Behavior of a Novel Ni-Fe-Based Superalloy during High-Temperature Aging Treatment.

The precipitation behavior of a novel Ni-Fe-based superalloy developed for advanced ultra-supercritical (A-USC) coal-fired power plant applications during high-temperature aging treatment was investigated. The results showed that the major precipitates in the novel alloy were randomly distributed MC carbides, M23C6 carbides at grain boundaries, and the γ'-Ni3 (Al, Ti) phase in grain interiors after aging. MC remained relatively stable during both short-term and long-term aging. M23C6 quickly precipitated and exhibited a discrete distribution at grain boundaries during short-term aging, and partly developed into continuous films during long-term aging. After uniform precipitation, the shape of γ' remained spherical, and the size kept increasing with aging time according to the Lifshitz-Slyozov-Wagner (LSW) model. The hardness of the novel alloy was mainly associated with the precipitation behavior of γ'; as γ' gradually precipitated, the hardness steadily increased; after complete precipitation, as the size of γ' increased, the hardness first increased and then decreased, reaching the peak hardness when the average radius of γ' achieved the critical size. In addition, the novel alloy exhibited abnormal coarsening behavior at grain boundaries during both short-term and long-term aging. The coarsened grain boundaries were actually precipitate-free zones (PFZs) and the coarsened and elongated rod-like particles inside were identified as γ' precipitates. The mechanism of strain-induced grain boundary migration and the discontinuous coarsening reaction is proposed for the formation of PFZs. Furthermore, PFZs were considered to be potential crack sources during the creep rupture test, leading to earlier failure of the material.

Microstructure, hardness and creep properties for P91 steel after long-term service in a ultra-supercritical power plant

3 P91 pipes enduring 5∼6 × 104 h service had been found in an ultra-supercritical power plant. Hardness tests, microstructure observation(OM, SEM/EBSD, TEM/EDS) and creep rupture tests were performed, and the relationship between hardness, microstructure and creep rupture strength were discussed. Results showed that 1 of the 3 pipes still featured lath martensite, and its mechanical properties complied with relevant criteria. Other 2 pipes were characteristic of a mixture of ferrite and highly recovered martensite. Their hardness and creep rupture strength experienced severe degradation. Laves phase was observed in all 3 pipes. The most distinctive feature of the 2 low hardness pipes was their coarse and intragranularly distributed M23C6. This might be caused by abnormal heat treatments which could lead to the mixed microstructure of martensite and ferrite, and this kind of microstructure would degrade fast during service. Thus the low hardness had nothing to do with the precipitation of Laves phase.

Effect of plasma treatment on hygrothermal creep behaviour of flax fibre composite

Atmospheric plasma activation and plasma coating with 3-aminopropyltriethoxysilane (APTES) were used to investigate the effect of a modified interface on the creep behaviour of flax fibre reinforced polyoxymethylene composite. The wetting parameters and the interfacial shear strength indicate that using air-based plasma activation is superior to plasma coating with APTES when improvement of interface and mechanical properties are concerned. Hygrothermal creep tests within the linear viscoelastic region reveal that the creep resistance of air plasma treated composites is significantly enhanced, evidenced by reduced instantaneous strain and slower viscoelastic flow when compared to the untreated composites. However, creep rupture (run to failure) tests demonstrate that the air plasma treatment extends the creep lifespan only under standard conditions (50 % RH and 23 °C). Under severe conditions (85 % RH and 23 °C), plasma-treated composites exhibit a somewhat shorter lifespan, possibly because some damage induced by the plasma treatment is exacerbated at high humidity.

Studying the formation of nodules in a Ni-Fe-Cr based superalloy used in fireside corrosion under the impact of creep stress

Creep rupture tests on a Ni-Fe-Cr based superalloy were conducted in a custom-built rig at 750 °C under simulated fireside corrosion environment. The impact of creep stress on corrosion was investigated by controlling load level (without loading, loads of 120–195 MPa). The phase composition, microstructure, and morphology of surface corrosion products and corrosion-affected area were studied. The mechanism responsible for the production of surface nodules and the observed uniform oxide scale during fireside corrosion are discussed. Results show that the presence of creep stress promotes the development of surface nodules and increases the corrosion-affected area.

Creep Performance Study of Inconel 740H Weldment Based on Microstructural Deformation Mechanisms

Abstract The creep behavior of Inconel 740H weldment at the temperature of 760 °C is investigated experimentally and analytically using the deformation-mechanism-based true stress (DMTS) model. The Inconel 740H weldment specimens are prepared with the gas tungsten arc welding technique. Creep testing is performed on the Inconel 740H weldment specimens under a range of stress levels from 190 MPa to 447 MPa at the temperature of 760 °C. The DMTS model is employed to analyze the creep curves and creep rates. The model parameters for Inconel 740H weldment are determined from the analyses of the creep testing data in combination with that from the previous studies of similar materials based on the same creep mechanisms that involve grain boundary sliding and intragranular dislocation climb-plus-glide with dislocation multiplication. The creep life predictions of the DMTS model for Inconel 740H weldment agree very well with creep rupture test data within a temperature range of 700–800 °C. The fractured surfaces and longitudinal sections of creep-tested Inconel 740H weldment specimens are examined using scanning electron microscopy, which corroborates the DMTS model inference that the creep failure of Inconel 740H weldment is in a mode of predominantly intergranular fracture. The present study suggests that grain boundary sliding is the most significant controlling factor for the creep failure of Inconel 740H weldment.

Evaluation of the effects of the addition of Al, Ti, and Zr on microstructure, carburization and creep resistance of the HPNb alloy

The process of thermally cracking hydrocarbons and steam in pyrolysis furnaces to obtain ethylene is essential in the production chain of products relevant to our daily lives, such as fertilizers, polymers, pharmaceuticals, etc. The operating temperature of these furnaces is limited by the occurrence of carburization and/or creep in the metallic alloy used. The objective of this work was to investigate the effect of adding Aluminum (Al) at approximately 3 wt%, Titanium (Ti) at around 0.2 wt%, and Zirconium (Zr) at about 0.2 wt% on the carburization and creep resistance of the HPNb alloy. This material is a high-performance nickel-chromium-iron (Ni–Cr–Fe) system alloy enriched with niobium (Nb). Carburization in the samples was investigated by exposure to 1150 °C for 480 h in a carburizing atmosphere. Creep was evaluated through creep-rupture tests with a constant load, performed under a tension of 17 MPa and a temperature of 1150 °C, in four metallic alloys produced by static pouring with different chemical compositions. The data obtained were analyzed using optical microscopy, SEM, EDS, and EBSD. This study’s main findings indicate that while Al enhances carburization resistance, its impact on creep resistance varies depending on exposure to carburizing conditions. The addition of Ti or Zr did not significantly alter carburization resistance but contributed to improved creep resistance under specific conditions. This study provides insights into optimizing alloy compositions for enhanced performance in high-temperature industrial applications.

Influence of heat treatment on the microstructure and creep response of additively manufactured cobalt superalloy Mar M509

ABSTRACT This work explores the influence of various post-build heat treatment procedures on the microstructural characteristics and creep response of laser powder bed fusion (L-PBF) Mar M509 superalloy. Besides MC-type carbides formed primarily during the L-PBF process, the obtained results indicate the influence of heat treatment on the nucleation of M6C and M7C3 carbides due to elemental segregation. Short-term creep-rupture tests have been conducted at 900°C/110MPa on eight specimens, each subjected to a specifically individual heat treatment procedure. Rupture times are found to be sensitive to the heat treatment applied and vary from ~178 to ~641 hours. Detailed microscopic characterisation of ruptured specimens highlights the synergistic influence of oxidation and M7C3/M6C precipitate-free zone formation on macroscopic intergranular failure facilitated by grain boundary sliding along favourably oriented small grains. Besides notable grain growth, the presence of a serrated grain boundary structure with relatively less MC-type carbide coarsening is found to offer the best creep resistance among the eight L-PBF specimens. Further improvement of the elevated temperature performance of the alloy is seen to require a focus on the L-PBF process conditions to optimise the morphology of MC carbides, as they form.

Microstructural evolution of 800H alloy after long-time creep rupture test at 675 °C

This study investigated the microstructural evolution of 800H alloy tubes and their welding through creep rupture test at 675 °C with different applied stresses. Vickers hardness testing was used to determine microhardness and optical microscopy, scanning electron microscopy, and transmission electron microscopy were used to examine the microstructure of the tubes. The microhardness of the base metal, heat-affected zones, and welding seams of the tubes after testing were all significantly higher than the as-received 800H tubes due to the combination of Cr23C6 precipitates, dislocation multiplication, and work-hardening effect. The base metal of all tubes had a coarse and equiaxed austenitic structure with large Ti-N-rich precipitates, parallel needle-like Cr23C6 precipitates within twins, and discontinuously dispersed chain-like Cr23C6 precipitates along grain boundaries. As testing progressed, spherical Cr23C6 were precipitated within grains then coarsened, and the intervals between parallel needle-like Cr23C6 within twins were increased due to their low thermodynamic stability and synergistic effect of time and stress. The heat-affected zones exhibited a low correlation with test time, with all samples displaying dense spherical and needle-like Cr23C6, whereas the welding seams showed insignificant microstructural evolution. γ΄ (Ni3(Al, Ti)) precipitates were observed along the sides of the Cr23C6, further coarsening of γ΄ was observed as the creep rupture test progressed.

Estimation the Remaining Life for Reformer Unit Tubes Furnace Operated Beyond the Design Life

The aim of this work is to study and model creep damage accumulation for remaining safe working life estimation of tube material used in petroleum industry plants operated at a high temperature for long time beyond the design life. Material constants and other data required for life estimation were extracted from the output results of real accelerated creep rupture tests conducted at three test temperatures (700, 725, and 750(oC and at three constant stress applications (120, 130, and 140 MPa). Also, a time-temperature parameter method (Larson-Miller Method) was modified to be applied based on the results of the accelerated creep rupture tests to build life predicted master curve for austenitic stainless steel type 321H. It is found that creep rupture time for stainless steel alloy decreases as temperature or stress increase. Also, it distinguished from the creep curves for the serviced stainless steel alloy, that the onset of tertiary creep stage starts early (about 50% of creep curve), this reflects the reduction in creep strength of the alloy as a result of the degradation in alloy properties that take place during the service life. It is also found that the estimated remaining working life for the serviced tube samples made from austenitic stainless steel type 321H is about (54703 hr) based on the calculated stress level (40 MPa) and service temperature 570 oC. The difference between the estimated remaining life and the experimental creep test life is due to the differences between the applied stresses, where higher level of stresses in the experimental creep testes are used to accelerate failure of specimens. The application for the result of this work can be the petroleum industries that where heaters tubes serviced at such industry need to be estimated for the remaining life after the design life expire.

Synergic effects of Si and V micro-additions on microstructural and mechanical properties of a dilute Al–Sc–Zr alloy containing L12 Al3(Sc, Zr, V) nanoprecipitates

This study investigates the impact of incorporating 0, 0.05, 0.10 at% V into a bare Al-0.06Sc-0.06Zr-0.18Si at% alloy on the precipitation behavior of Al3(Sc, Zr, V) (L12-structure) nanoprecipitates and the consequent creep resistance of the alloy. Various techniques were used to obtain results, including Vickers microhardness, Electrical conductivity, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and tensile creep rupture testing. The findings show that the amount of V added affected the nanoprecipitate structures, precipitation kinetics, coarsening resistance, and creep resistance of the alloys. After isochronal aging to 425 °C (25 °C/1 h step), alloy V0.10 had a peak microhardness value of 603 ± 14 MPa. TEM revealed a microstructure with precipitate diameters ranging from 3 to 20 nm. The nanoprecipitates had a core-shell structure with a Sc-enriched core, a Zr-enriched shell, and, in some cases, a V outer shell. The phenomenon of vanadium partitioning at the precipitate shells occurred due to the incremental influence of silicon, which enhanced the diffusion rate of Zr and V. While the alloy V0.10 had a lower steady-state strain rate than the Al-0.06Sc-0.06Zr at% alloy at stresses below 16 MPa, it showed better creep resistance at stresses above 16 MPa. The threshold stress of the alloy V0.10 (10 MPa) was almost equal to that of the previously studied Al-0.06Sc-0.06Zr at% alloy (12 MPa). These results can be explained by the fact that a variation in the lattice parameter of Al3(Sc, Zr, V) (L12) affects the ability of matrix dislocations to traverse the nanoprecipitates, even in the absence of measurable lanthanides.

Research on thermal creep behavior of Chinese low-activation ferritic/martensitic steel

A series of thermal creep experiments are conducted on Chinese low-activation ferritic/martensitic steel (CLF-1 steel) at a temperature of 550, 575, and 600 °C with various normal stress (from 150 to 300 MPa), respectively. The results are analyzed by creep curves, microstructure, and creep properties. Finished samples display adequate fracture surface in short creep rupture tests (within 3000 h). The comparative creep curve shows no difference related to the thickness of CLF-1 steel plate (10 and 30 mm). Microstructure images before and after creep reflect a tempered lath martensite structure and carbides decorating both in prior austenitic grain boundaries and lath boundaries. Comparison is discussed between 30 mm plate and 10 mm plate. All current results reflect that all CLF-1 steel samples shows good creep behavior and proves that a series excellent progress of CLF-1 steel R&D work has been successfully achieved. Besides, collected data will contribute a lot to the design and R&D work of the Chinese Helium Cooled Ceramic Breeding Test Blanket Module (CNHCCB-TBM).

Creep-damage modelling for micro gas turbine combustion chambers lifetime prediction

Micro Gas Turbines (MGTs) are nowadays largely used for electrical and thermal energy production in small buildings and households. Their reliability and compactness allow them to operate for thousands of hours with minimal maintenance. However, the long exposure at high temperatures in combustion chambers can promote creep, which can induce thermal fatigue and potential failure of these components. Creep-induced damage in MGTs has not yet been thoroughly investigated, due to the lack of numerical tools able to model these strongly coupled phenomena. This study presents the development of a Fortran-based subroutine integrated into ANSYS APDL. The code allows for a life assessment based on the Lemaitre-Chaboche creep damage model. Secondary creep and stress relaxation are modeled for the high-temperature resistant alloy Inconel718. A new set of temperature-dependent parameters for the Norton equation is provided, and the method to obtain these parameters from creep rupture tests is outlined. The model is validated and shows good agreement with experimental data. The subroutine correctly reproduces visco-plasticity, stress relaxation and damage under typical MGTs operating temperatures. This model constitutes the foundation of a life-assessment analysis for combustion chambers. The results highlight the impact of temperature and creep on the component’s life and the importance of integrating life assessment analysis into the preliminary design of combustion chambers.

Design and application of double stove wires high-temperature furnace in high-temperature creep rupture test

This paper presented a high-temperature furnace with double electric stove wires for the test of high-temperature creep rupture, adopted the 5 mm electric stove wire in diameter, and the wires are made up of the working stove wire and the alternate stove wire. When the tester system detects the working stove wire is fractured, it will automatically and quickly switch the power supply circuit from the working stove wire to the alternate stove wire. Then the alternate stove wire starts to heat to keep the high-temperature furnace working, reducing the loss caused by the interruption of the test on account of the fractured stove wire. Experimental result indicates that the high-temperature furnace with double electric stove wires can solve the problem that when the stove wire is fractured, the test is difficult to proceed for a long time and high-temperature creep rupture test with the classical high-temperature furnace. Besides, the 5 mm in diameter electric stove wire can also meet the temperature control on the ultra-high-temperature (such as 1200 °C) Creep test.

Creep Analysis and Microstructural Evaluation of a Novel Additively Manufactured Nickel-Base Superalloy (ABD®-900AM)

Abstract Nickel-base superalloys containing 30 to 50% gamma prime (γ') volume fraction are typically used in hot section components (e.g., guide vanes or blades) for power generating gas turbines, and suitable time-dependent properties are required for long-term elevated temperature operation. Additive manufacturing (AM) has recently been used to develop complex hot-section parts utilizing innovative designs with enhanced cooling features which improve efficiencies by reducing cooling air consumption. To further explore the opportunity to improve time-dependent AM superalloys, this paper focuses on a fundamental creep study and characterization of a novel nickel-base superalloy (ABD-900AM) that was manufactured using a laser-based powder bed fusion (LBPBF) AM process. The material was subjected to a subsolvus solution anneal and multistep aging heat treatment (HT) to produce a bi-modal distribution with ∼35% volume fraction of gamma prime without postprocessing hot isostatic pressing (HIP). Microstructural characterization was carried out for the as-built and fully heat-treated structures, and a creep-rupture test program was conducted to study the resultant creep properties. Activation energies and stress exponents in addition to rupture strength and deformation resistance were compared to traditionally cast IN939 and IN738 materials. After testing, specimens were evaluated using a variety of microscopy tools to determine location and features associated with creep damage. The optimized chemistry for ABD-900AM was printed crack free and fully dense in contrast to studies on similar alloys where significant process development and postbuild heat treatments were required. High-temperature mechanical properties in the heat-treated material showed some decrease in creep strength when compared to traditional casting. This strength and rupture life debit was dependent on build orientation, but a considerable increase in creep ductility was observed due to differences in the microstructure when compared with similar AM alloys. Analysis of creep data showed differences in creep mechanisms compared to traditional cast alloys. The relationship between microstructure and creep mechanisms is discussed, and ongoing work to further improve rupture strength through heat-treatment optimization will be highlighted.

Relationship between Grain Size and Sample Thickness on the Creep-Rupture Performance of Thin Metallic Sheets of INCONEL Alloy 740H

A study was conducted on INCONEL® alloy 740H® to examine the role of grain size and sheet thickness on the alloy’s creep-rupture behavior. Three different starting sheet thicknesses were utilized and multiple heat-treatment conditions anticipated for compact heat exchanger (CHX) manufacturing were applied to produce a range of grain sizes. Creep-rupture testing was conducted at 750 °C for times up to about 6000 h and the results were compared to wrought databases. The data show that both creep strength and ductility were important factors in the overall creep performance of the sheets. Reductions in performance were observed due either to accelerated creep when grain size was fine or loss of rupture ductility when grain sizes approached the sheet thickness. Some combinations of heat-treatment and thickness were able to produce typical expected wrought creep properties. Historically a ‘rule-of-thumb’ requirement for creep testing suggests 3-5 grains per sample minimum dimension to ensure homogeneous behavior. This research shows that to ensure representative wrought creep performance (i.e. no effect of sample size), the sample minimum cross-section should be 10 times the average ASTM grain size. Statistical analysis of the microstructures suggests the population of larger grains as a controlling feature in creep failure.

The effect of filler metal on microstructure and mechanical performance of GTAW inconel 625 at room and elevated temperature

The aim of this study was evaluating the influence of ER321 and ERNiCr-3 filler metals on the mechanical performance of the Inconel 625 gas tungsten arc welded joints at room and elevated temperature and correlating with the microstructure. Optical and scanning electron microscopy was conducted to characterize microstructure features at the fusion and the interfaces. Microhardness measurements, tensile test, Charpy impact test, and creep rupture test were applied to the weldments to investigate the effect of filler metal on mechanical properties of the joints. Creep rupture test was performed at 600 °C under load stresses of 150, 200 and 300 MPa. At high stress levels (300 MPa), nucleation and coalescence of voids accelerates and a ductile fracture with dimple morphology for both filler metals was seen. At lower stress level of 150 MPa, the creep deformation becomes dominant for both weldments and the voids which are mainly micro-creep voids get smaller in size and are located near the precipitates. A 100% increase in stress level (from 150 MPa to 300 MPa), reduces the creep rupture life of the ERNiCr-3 and ER321 welds by about 11 and 30 times, respectively. The results show that stringers of δ-Ferrite in inter-dendritic region of austenite appear in ER321 weldments along with some precipitates which are highly enriched in Ti and C. The formation of the unmixed zone with a different concentration gradient of the elements is observed at the Inconel 625/ER321 stainless steel interface. XRD pattern reveals the formation of (Fe2Mo) Laves phase in ER321 weld metal where the amount of Fe in the weld increases and Mo solubility decreases. ERNiCr-3 weldments show fully austenitic structure and some NbC precipitates. EDS analysis of ERNiCr-3 weld metal confirms high amount of Mo and Nb at the inter-dendritic region, whereas the dendritic core is depleted from Mo and Nb. By applying different filler metals, different fracture morphology was revealed under different loads. A non-uniform hardness profile and the minimum microhardness value of 120 HV were seen in the wide unmixed zone of ER321 weld metal due to its heterogeneous structure and also due to lack of weld metal dilution. It was shown that ER321filler metal does not give an appreciable tensile and impact properties comparing to ERNiCr-3 weldments. The tensile elongation was greater for ERNiCr-3 weldments (13.1%) than ER321 joints (10.2%) andthe average impact toughness of ERNiCr-3 weldments was about two times higher than ER321.

Evolution of creep rupture mechanism in advanced powder metallurgy superalloys with tantalum addition

To further understand the effect of Ta additive on the creep damage for polycrystalline Ni-base superalloys, creep rupture tests on powder metallurgy (PM) superalloys with various Ta were carried out under different conditions in this study. The results indicated that the multiple crack propagation and damage mechanisms could occur at the identical creep condition. These different fracture mode areas had distinct boundaries on the fracture surface, its size could be altered by Ta addition. However, the effects of Ta were radically changed by the creep conditions. The creep rupture mechanism transformed significantly with temperatures and applied stresses, and even subversive laws appeared in the creep life for high-Ta alloys. The substructure observation after creep rupture demonstrated that the annealing coherent twin boundaries gradually lose coherence with the matrix, which could become sites for cavities nucleation or crack propagation that followed. Furthermore, the creep-induced microtwins were critical for the creep damage and rupture. The electron back-scattered diffraction (EBSD), high-resolution transmission electron microscope (TEM) imaging, and quantitative elemental analysis assisted in comprehending these alterable creep rupture mechanisms.

Microstructures and high-temperature failure analyses of 25Cr-35Ni-Nb + MA/32Cr-20Ni-Nb dissimilar welded joint with different filler wires

To address the issue of lacking scientific guidance for welding 25Cr-35Ni-Nb + MA/32Cr-20Ni-Nb joint in petrochemical plant applications, the joint microstructures were studied by adopting four types of filler wires. The high-temperature tensile tests and creep rupture tests were performed to investigate joint failure mechanisms. Cellular/columnar dendritic grains were formed at the fusion line region with similar orientations to adjacent un-melted grains. The fine-sized dendritic grains generated at the central weld seam presented a larger misorientation owing to a lowered cooling rate. The lesser alloying element in filler wire led to a larger area with cellular/columnar dendritic grains. The carbide precipitates mostly located at grain boundaries either in the form of particles or necklace structures depending upon the filler wire compositions. The heterogeneous microstructure was thought to be the main reason for the failure at weld seam and fusion line region with large hardness fluctuations when joints welded by the filler wire with a more diverse alloying element. The filler wire with the least introduction of alloying elements, i.e., ERNiCrCoMo-1, gave rise to a more uniform microstructure and hence higher joint property without failing at the weld seam, which was therefore considered as the most suitable filler wire candidate.

Effect of manufacturing route on thermal creep behaviour of Zr-2.5Nb pressure tube alloy used in Indian PHWR

Indian Pressurized Heavy Water Reactors (IPHWRs) uses Zr-2.5Nb alloy pressure tube (PT) in cold work and stress relieved condition. These PTs are subjected to extreme conditions of stress, temperature and irradiation. Since past three decades of PHWR operation in India, fabrication routes of PTs for IPHWR has undergone several modifications to enhance its in-reactor performance. The deformation of PT in PHWR occurs due to irradiation creep, irradiation growth and thermal creep. In the present study, the different generations of PTs used in IPHWR are tested for its thermal creep behaviour. The accelerated creep rupture tests were carried out using flat tensile specimens with its axis parallel to the longitudinal direction of the PT in the temperature range of 350 to 450°C under stresses in the range of 0.7 to 0.9 times of yield strength at the respective temperatures. The minimum creep rates were evaluated along with the stress exponents and activation energies. The values of stress exponent and activation energy revealed the underlying creep mechanisms responsible for the creep deformation. Microstructural examination was carried out to understand the need and efficacy of the new fabrication route. The observed thermal creep behaviour of all the generations of pressure tubes were rationalised in terms of their respective texture and microstructures.