Creep Rupture Behavior Research Articles

Effect of Salt-Deposit Hot Corrosion on Creep Rupture Behaviors of DZ466 Superalloy

Since gas turbine blades in engines suffer centrifugal stress and gas corrosion during service, a good creep resistance in hot corrosion environment is one of the important considerations to evaluate service performance of the blade materials. In this work, the creep rupture behaviors of the directionally solidified superalloy DZ466 with and without salt deposition at 760°C/765MPa, 850°C/500MPa and 950°C/220MPa are preliminarily investigated based on the creep properties measurement and microstructure observations. The effects of hot corrosion on the creep properties and fracture mode are examined. The results show that the creep-rupture life in salt-deposit environment is lower than that in air-exposure environment at different temperatures. The creep-rupture life reduction caused by hot corrosion is increased with increase of the creep temperature. The fracture mode is exhibited by transgranular fracture in all crepted specimens. The propagation directions of all the secondary cracks are almost perpendicular to the crept specimen surface or the stress axis. The surface cracks are mainly produced in air-exposure environment at low temperature or in salt-deposited environment at high temperature. Induced by the stress concentration, the internal cracks are initiated surrounding the carbides in both air-exposure and salt-deposit environments. The creep-rupture life is dependent on the crack initiation at low temperature but on the crack propagation at high temperature. The reduction of the active load bearing area in transversal direction is the main reason why the creep-rupture life is decreased at 950°C.

Creep Rupture Behavior and Microscopic Analysis of 304HCu Austenitic Stainless Steel Under Multiaxial State of Stresses at 973 K

As a part of materials development for the application of 304HCu SS in advanced ultra super critical power plants, an experimental study has been carried out to understand the response of material under various states of stresses at 973 K. In order to introduce the multiaxial state of stress, notches of different root radii ranging from 0.25 to 2.5 mm were incorporated in smooth specimens. The comparison of rupture life of the smooth and notched specimen revealed the ability of the steel to ‘notch-strengthen’. In order to understand and substantiate the above observations, detailed FE analysis has been carried out. The decrease in von-Mises stress below the net applied stress resulted in notch strengthening in the steel in presence of a notch. The rupture life of the steel was found to increase with notch sharpness. Detailed scanning electron microscopy investigation carried out on fracture surfaces of smooth and notched specimens exhibited ductile fracture for smooth and shallow notches; whereas mixed mode fracture for relatively sharper notches. Further, the effect of different stresses on fracture morphology has also been discussed.

Influence of Microstructural Inhomogeneity on the Creep Rupture Behaviour of 316LN SS Weld Joints

The fusion zone is usually the weakest link in austenitic steel weld joints and it is usually in this region where damage initiates during creep deformation. The heterogeneity in microstructure of the fusion zone enhances the complexity in understanding the damage evolution during creep exposure. The major source of heterogeneity in the microstructure is the weld thermal cycle, which the weld joint is inevitably subjected to during multi-pass welding. Subsequent thermal cycles are likely to alter the microstructure of the previously deposited fusion zone thereby influencing the mechanical properties of the joints. In this study, the effect of thermal cycle on creep rupture behaviour of 316LN SS weld joint has been studied. Weld joints were fabricated by two welding processes—activated tungsten inert gas (ATIG) and shielded metal arc (SMA) welding. In case of ATIG weld joints, the creep properties of single and dual-pass weld joints were compared and for the SMAW joints, the comparison was made between weld joints made with two different electrode sizes. In ATIG weld joint, the subsequent thermal cycle in the dual pass weld joint improved its creep rupture life. In the case of the SMAW joint, the weld joint made with larger electrode, which was subjected to less number of thermal cycles exhibited better creep properties. The weld thermal cycle induces a ‘thermo-mechanical treatment’ and changes the morphology of delta ferrite in the previously deposited fusion zone which has a synergistic role in dictating the creep rupture life of the 316LN SS weld joints.

Creep deformation and rupture behaviour of thermal aged P92 steel

Creep deformation and rupture behaviour of thermal aged P92 steel have been investigated over a stress range of 110–130 MPa at 923 K. Laves phase was observed in the thermal aged steel. Laves phase predominantly coarsens towards the intra-lath region assisted by substructure within the lath region. The stress dependence of minimum creep rate followed Norton's power law of creep with comparable apparent stress exponent value in the thermal aged, and normalised and tempered steels. The creep rate and creep rupture lives of both the steels were relatively comparable under lower stress level. The steel obeyed Monkman–Grant and modified Monkman–Grant ductility relationships. Enhanced precipitation of Laves phase has been observed at the necked region in both the conditions of the steel, which experienced higher plastic flow. Thermal aging of P92 steel has not significantly influenced the creep strength under longer creep duration.

9CrMAG鋼溶接継手のクリープ強度評価

Mod.9Cr steels are widely used as boiler materials in ultra-super critical power plants. In this study, creep tests were conducted using miniature specimens with 1mm diameter machined from a base metal, weld metal and a heat affected zone (HAZ) in a Mod.9Cr-1Mo welded joint. Creep deformation and rupture behaviors of the miniature base metal specimens were almost identical with those obtained from standard size specimens. Material constants of the Norton law were determined from obtained datum by the miniature creep specimens. Creep rupture times of the miniature welded joint specimens were much lower than those of the standard size specimens. The finite element creep analysis results indicated that the triaxiality factor in the HAZ of the miniature welded joint is smaller than that of the standard size welded joint resulting in faster creep strain rate in the miniature specimen under the same applied stress.

Research on representative stress and fracture ductility of P92 steel under multiaxial creep

Creep experiments on both plain and notched specimens were conducted at 650°C over a stress range of 120–185MPa. The notch strengthening effect was found to exhibit in notched specimens. By using stress components at the skeletal point, several expressions of representative stress were compared to validate their effectiveness in predicting creep rupture lives of P92 steel under multiaxial stress states. The results showed that Hayhurst representative stress was more suitable for life predictions of P92 steel. In the mean time, the relationship between the fracture ductility and multiaxiality was presented to investigate the influence of the multiaxial stress states on creep rupture behavior of P92 steel. A more reasonable prediction model was proposed, and the validity of the model was verified by experimental data.

Creep-Rupture Behavior of SUS 304H – IN 617 Dissimilar Metal Welds for AUSC Boiler Applications

Advanced Ultra Super-Critical (AUSC) power plants are envisaged for achieving higher thermodynamic efficiency by operating at temperatures and pressures of 710/720 °C and 310 ata, respectively which are significantly higher than sub-critical (conventional) and supercritical power plants. This has led to tremendous research in selection of new and advanced materials to meet high temperature requirements. Ni-base superalloys having known to have excellent creep-rupture behavior were selected for high temperature sections of boiler while austenitic stainless steels were selected for moderately high temperature sections considering the economical feasibility. Since both these materials have to be fabricated for application in boiler tubes, dissimilar metal welding is inevitable. This work discusses creep-rupture evaluation of one such case of dissimilar welds, i.e., between Inconel 617 (IN 617) and SUS 304H. IN 617 is a Ni-base superalloy while SUS 304H is a Cu-precipitated austenitic stainless steel. Welding was carried out using IN 617 filler material for producing defect free welds. Creep-rupture samples were prepared incorporating the complete cross-weldment for understanding mechanical behavior as a result of prolonged exposure to elevated temperature and stresses. Creep-rupture testing was carried out at 600, 650 and 700°C at suitable stresses to obtain rupture times in the excess of 3000 hours in a few test conditions. Detailed characterization studies when carried out on ruptured samples revealed the weakest zone to be heat affected zone towards SUS 304H which led to failure. This work also provides insight into the possible creep-mechanisms that operate in various zones of weldments. Phases formed as a result of exposure to stress and temperature for a prolonged duration are also discussed. Comparison of the data obtained with data available in literature was carried out and performance of weldments was analyzed and reported.

Creep deformation and rupture behavior of Alloy 617

A series of creep data was obtained from creep tests at different applied stresses at the temperatures of 850°C, 900°C, and 950°C for Alloy 617, which is a leading candidate material for high-temperature components in Gen-IV nuclear reactor systems. The creep deformation and rupture behavior were investigated in terms of Norton's power law, Monkman–Grant relation (MGR), modified Monkman–Grant relation (MMGR), creep damage tolerance factor (λ), Zener–Hollomon Parameter (Z), and fracture behavior. Alloy 617 did not exhibit textbook creep behavior and revealed somewhat differences from typical heat resistant steels. The MMGR provided improved correlation between creep rate and rupture life in Alloy 617. The Z parameter obeyed a good agreement for a function of Z=2.30×1033 (σ/E)5.87, and the same creep mechanism was operative within the ranges tested in the present study. The value of λ for Alloy 617 was found to be 2.40, and this was in agreement with materials exhibiting typical cavitation damage. The creep failure analysis revealed a dominant intergranular fracture mode, which proceeds via initiation, linking, and incorporation of the cavities.

A creep–rupture model of synthetic fiber ropes for deepwater moorings based on thermodynamics

The synthetic fiber ropes such as polyester, aramid and high modulus polyethylene (HMPE) are increasing applied to deepwater mooring systems for oil and gas exploitation. Due to that mooring ropes generally bear tensions for a long period, synthetic fiber ropes that are composed of the viscoelastic material would present creep behaviors and even the creep rupture, which is the failure mode of greatest concern especially for HMPE ropes and on which still less study can be found. A creep damage analysis of synthetic fiber ropes is of necessity to ensure the safe and economic operation of mooring systems. Therefore further investigation on the creep–rupture behavior is beneficial to fully establishing confidence in the viability of synthetic fiber ropes for deepwater moorings. In the present study, a creep–rupture model is proposed within the framework of thermodynamics to investigate the creep and damage behaviors of synthetic fiber ropes. Methods for identifying the model parameters are also proposed in detail, which apply to any component of fiber ropes such as the fiber, yarn, strand and rope. Experimental data of aramid yarns available from the literature are utilized to validate the constitutive model. Creep and creep–rupture tests of HMPE strands at different loading levels are specially performed to further examine the present model. The present work demonstrates that the proposed model can effectively describe the viscoelastic property and damage evolution of synthetic fiber ropes at different loading levels.

Small Punch Creep deformation and rupture behavior of 316L(N) stainless steel

The Small Punch Creep (SPC) behavior of 316L(N) stainless steel has been evaluated under various loads in the range 300–1000N over a temperature range 898–973K. The SPC curves clearly showed the transient, secondary and tertiary stages of creep. The transient creep deflection data have been analyzed on the basis of Garofalo’s equation. The trend of variations of rate of exhaustion of transient creep deflection and steady state deflection rate with applied load revealed that the SPC deformation obeyed first-order reaction rate theory throughout the transient region. Using Chakrabarty’s membrane stretch model, the creep strain–time curves were derived from deflection–time data measured in SPC tests. The steady state creep rate values calculated from such derived strain curves were closer to corresponding values obtained from conventional creep tests. From the load exponent of steady state deflection rate and estimated activation energy for creep, the mechanism that governs the SPC deformation of 316L(N) stainless steel under investigated conditions was found to be dislocation creep.

An assessment of creep deformation and rupture behaviour of 9Cr–1.8W–0.5Mo–VNb (ASME grade 92) steel

Creep deformation and rupture behaviour of 9Cr–1.8W–0.5Mo–VNb steel have been investigated at 873K, 923K and 973K over a stress range of 80–220MPa. The absence of clear primary creep regime and prolonged secondary stage of creep deformation have been noticed under lower stress level at 973K. The variation of minimum creep rate with applied stress obeyed Norton's power law of creep. The apparent stress exponents of 15.2, 12.3 and 5.8, and apparent activation energy of 619kJ/mole have been estimated for creep deformation of the steel. The apparent stress exponents and activation energy have been rationalised on the basis of threshold stress. The threshold stress values of 137.5MPa, 83.3MPa and 29.7MPa were obtained at 873K, 923K and 973K respectively. The threshold stress compensated true stress exponent of 4 and true activation energy of 244kJ/mole, and threshold stress normalised by Orowan stress confirms that the lattice diffusion assisted localised climb of dislocation is the rate controlling of creep deformation in the steel. The steel obeyed Monkman and modified Monkman–Grant relationships. Damage tolerance factor of 6 in the steel demonstrates that the microstructural degradation such as coarsening of precipitates and subgrain structure is the dominant creep damaging mechanism in the steel.

Review of current status of the LICON methodology

Since the LICON methodology was first developed in the late 1990s to predict the long term creep rupture behaviour of new generation martensitic 9% Cr steels from the results of relatively short duration multi-axial specimen tests, there have been a number of necessary refinements to this originally simple concept. These are reviewed.

Microstructural study in a Fe–Ni-base superalloy during creep–rupture at intermediate temperature

Creep–rupture behavior of a Fe–Ni-base alloy crept under various conditions has been studied using Electron Backscatter Diffraction (EBSD) and Transmission electron microscopy (TEM). The results indicate that grain orientation did not change after the alloy crept at 700°C/300MPa and changed greatly crept at 700°C/200MPa. The recrystallization texture near rupture–fracture surface was observed after the alloy crept at 750°C/150MPa. A better creep performance was found in the samples with the occurrence of grain rotation dependence of the microstructure and the grain store energy.

Microstructure of 10% Cr martensitic heat-resistant steel welded joints and type IV cracking behavior during creep rupture at 650 °C

The creep rupture behavior and microstructure changes of a new type of 10% Cr martensitic heat-resistant steel (MHRS) welded joints during short-term creep at 130–250MPa and 650°C were investigated. The results showed that type IV cracking took place in the fine-grained heat affected zone (FGHAZ). Creep cavities mainly formed at the intersection points of prior austenite grain (PAG) boundaries. M23C6 carbides coarsened significantly, coarse Laves phases formed next to M23C6 along the PAG boundaries and acted as the nucleation sites for creep cavities. It was considered that the high-density of PAG intersection points of FGHAZ in per unit area, combined with the coarsening of M23C6 precipitates and the formation of Laves phases on PAG boundaries caused the formation of type IV cracking.

Analysis of Creep Rupture Behavior of Cr-Mo Ferritic Steels in the Presence of Notch

Effect of notch on creep rupture behavior of 2.25Cr-1Mo, 9Cr-1Mo, and modified 9Cr-1Mo ferritic steels has been assessed. Creep tests were carried out on smooth and notched specimens of the steels in the stress ranging 90 to 300 MPa at 873 K (600 °C). Creep rupture lives of the steels increased in the presence of notch over those of smooth specimens, thus exhibiting notch strengthening. The strengthening was comparable for the 9Cr-1Mo and 2.25Cr-1Mo steels and appreciably more in modified 9Cr-1Mo steel. The strengthening effect was found to decrease with the decrease in applied stress and increase in rupture life for all the steels. The presence of notch decreased the creep rupture ductility of the steels significantly and the 2.25Cr-1Mo steel suffered more reduction than in the other two 9Cr-steels. Finite element analysis of stress distribution across the notch was carried out to understand the notch strengthening and its variation in the steels. The variation in fracture appearance has also been corroborated based on finite element analysis. Reduction in von-Mises stress across the notch throat plane resulted in strengthening in the steels. Higher reduction in von-Mises stress in modified 9Cr-1Mo steel than that in 2.25Cr-1Mo and 9Cr-1Mo steels induced more strengthening in modified 9Cr-1Mo steel under multiaxial state of stress.

Creep deformation and rupture behavior of CLAM steel at 823 K and 873 K

China Low Activation Martensitic (CLAM) steel is selected as the candidate structural material in Fusion Design Study (FDS) series fusion reactor conceptual designs. The creep property of CLAM steel has been studied in this paper. Creep tests have been carried out at 823K and 873K over a stress range of 150–230MPa. The creep curves showed three creep regimes, primary creep, steady-state creep and tertiary creep. The relationship between minimum creep rate (ε̇min) and the applied stress (σ) could be described by Norton power law, and the stress exponent n was decreased with the increase of the creep temperature. The creep mechanism was analyzed with the fractographes of the rupture specimens which were examined by scanning electron microscopy (SEM). The coarsening of precipitates observed with transmission electron microscope (TEM) indicated the microstructural degradation after creep test.

Creep life prediction of 9Cr–1Mo steel under multiaxial state of stress

Creep rupture life of 9Cr–1Mo steel under multiaxial state of stress has been assessed. Stress multiaxiality in cylindrical specimens during creep tests has been introduced by incorporating circumferential U-notches of different notch root radii. Creep tests were carried out on both smooth and notched specimens of the steel at 873K over the net applied stresses in the range of 110–210MPa. The creep rupture life of the steel was found to be higher in the presence of notch than that of smooth specimen indicating ‘notch strengthening’ behavior under multiaxial state of stress. The extent of strengthening tends to saturate for relatively sharper notches. Finite element analysis of stress and strain distributions across the notch was carried out to assess the notch strengthening behavior observed in the steel. The reduction in the von-Mises stress, extent of which increased and tends towards saturation with increase in notch root radii, resulted in increase in rupture life under multiaxial state of stress. Estimation of the creep rupture life under multiaxial state of stress has been assessed based on the different models, invoking the concept of skeletal point for calculating the representative stress. It has been observed that the creep rupture behavior of the steel under multiaxial state of stress is predominantly governed by the von-Mises stress. The creep rupture life has been predicted using finite element analysis coupled with continuum damage mechanics.

Prediction of creep rupture life of a V-notched bar in DD6 Ni-based single crystal superalloy

The circumferential V-type notched round bar has been designed to investigate the effect of multi-axial stress state on creep rupture behavior of DD6 Ni-based single crystal superalloys at 1100°C. Creep test on both smooth and notched specimens were carried out under 160MPa and 200MPa on the minimum net-section. Time to creep fracture was found to be higher in notched specimens than in smooth specimens. Finite element (FE) analysis coupled with continuum damage mechanics (CDM) was carried out to understand the stress distribution and the creep damage evolution under uniaxial and multi-axial stress states. The creep rupture life of the smooth specimen was successfully predicted by the classical Kachanov–Rabotnov damage law. The creep rupture life of the notched specimen predicted by finite element calculation incorporating a multi-axial creep CDM model was in good agreement with the experimental results. Both the fracture morphology and FE damage analysis indicate that rupture initiates first at the notch root in the notched specimen and finally towards to the center location.

Life assessment of die-cast Mg–5Al–1.7Ca alloys under creep service conditions

The creep rupture behavior of a Mg–5Al–1.7Ca alloy produced via die casting was investigated at temperatures between 423 and 498K, and the creep rupture life of the alloy was evaluated using the Larson–Miller parameter (LMP). It was demonstrated that the minimum creep rate and creep rupture life followed the phenomenological Monkman–Grant relationship. The value of the LMP was uniquely described by the logarithm of the applied stress when the Larson–Miller constant was set at 20. In addition, the applied stress and the LMP value followed a linear relationship below the yield stress. These results can be utilized to predict the long-term creep rupture life of the alloy under a given temperature condition and applied stress.

Mechanistic approach for prediction of creep deformation, damage and rupture life of different Cr–Mo ferritic steels

A mechanistic approach based on finite element analysis of continuum damage as proposed by Kachanov has been used to assess and compare creep deformation, damage and rupture behaviour of 2·25Cr–1Mo, 9Cr–1Mo and modified 9Cr–1Mo ferritic steels. Creep tests were carried out on the steels at 873 K over a stress range of 90–230 MPa. Modified 9Cr–1Mo steel was found to have highest creep deformation and rupture strength whereas 2·25Cr–1Mo steel showed the lowest among the three ferritic steels. Creep damage in the steels has been manifested as the microstructural degradation. 2·25Cr–1Mo steel was more prone to creep damage than 9Cr–steels. Finite element estimation of creep deformation and rupture lives were found to be in good agreement with the experimental results.