Creep Exposure Research Articles

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.

Microstructural Studies in Understanding the Role of Metallurgical Parameters on Creep Behaviour of Austenitic Alloys

Studies on creep behaviour of austenitic steels as a function of metallurgical variables like: (a) extent of cold work (b) trace alloying elements (c) choice of joining method indicate the need to understand their role through microstructural studies. As the materials evolve during creep exposure in terms of defect structure and precipitation behaviour, extensive electron microscopy studies were carried out to understand the influence of the above variables. It was observed that cold work shows an increase in creep life as it is varied from 10 to 24 % due to the enhanced prior defect structure providing a complex tangled dislocation structure when the material undergoes creep deformation. The addition of Ti results in enhanced nanoscale precipitation of TiC which was found to influence the creep strength up to 0.23 % of Ti. Employing modified TIG joining over the conventional TIG process it has been found less creep cavitation in the former joint which led to the improvement in creep rupture strength. The paper would discuss in detail of the microchemical variations, microstructural evolution in terms of defect structure and precipitation behaviour on the creep life.

Brittle rupture of austenitic stainless steels due to creep cavitation

Basic creep cavitation models have been used to predict brittle rupture of austenitic stainless steels. It involves the grain boundary sliding models, which is the basis of the creep cavitation models, the recently developed cavity formation models and the constrained cavity growth models. The individual creep cavitation models are verified with experimental observations. Brittle rupture due to creep cavitation that appears as intergranular failure is found to be dominant at high temperatures and long creep exposure times.

Gamma Prime Precipitation, Dislocation Densities, and TiN in Creep-Exposed Inconel 617 Alloy

Inconel 617 is a solid-solution-strengthened Ni-based superalloy with a small amount of gamma prime (γ′) present. Here, samples are examined in the as-received condition and after creep exposure at 923 K (650 °C) for 574 hours and 45,000 hours and at 973 K (700 °C) for 4000 hours. The stress levels are intermediate (estimated, respectively, as of the order of 350, 275, and 200 MPa) and at levels of interest for the future operation of power plant. The hardness of the specimens has been measured in the gage length and the head. TEM thin foils have been obtained to quantify dislocation densities (3.5 × 1013 for the as-received, 5.0 × 1014, 5.9 × 1014, and 3.5 × 1014 lines/m2 for the creep-exposed specimens, respectively). There are no previous data in the literature for dislocation densities in this alloy after creep exposure. There is some evidence from the dislocation densities that for the creep-exposed samples, the higher hardness in the gage length in comparison with the creep test specimen head is due to work hardening rather than any other effect. Carbon replicas have been used to extract gamma prime precipitates. The morphology of γ′ precipitates in the ‘as-received’ condition was spheroidal with an average diameter of 18 nm. The morphology of these particles does not change with creep exposure but the size increases to 30 nm after 574 hours at 923 K (650 °C) but with little coarsening in 45,000 hours. At 973 K (700 °C) 4000 hours, the average gamma prime size is 32 nm. In the TEM images of the replicas, the particles overlap, and therefore, a methodology has been developed to estimate the volume fraction of gamma prime in the alloy given the carbon replica film thickness. The results are 5.8 vol pct in the as-received and then 2.9, 3.2, and 3.4 vol pct, respectively, for the creep-exposed specimens. The results are compared with predictions from thermodynamic analysis given the alloy compositions. Thermodynamic prediction shows that nitrogen content is important in determining the gamma prime volume fraction. This has not previously been identified in the literature. The higher the nitrogen content, the lower the gamma prime volume fraction. This may explain inconsistencies between previous experimental estimates of gamma prime volume fraction in the literature and the results here. The observed decrease in the γ′ volume fraction with creep exposure would correspond to an increase in TiN. At present, there are insufficient experimental data to prove that this predicted relationship occurs in practice. However, it is observed that there is a higher volume fraction of TiN precipitates in the gage length of a creep sample than in the head. This suggests that secondary TiN particles are precipitating at the expense of existing γ′ due to the ingress of N from the atmosphere, possibly via creep cracks penetrating in from the surface of the gage length. This effect is not expected to be observed in real components which are much larger and operate in different atmospheres. However, this highlights the need to be conscious of this possibility when carrying out creep testing.

Assessment of Creep Strain Distribution Across Base Metal of 316LN Austenitic Stainless Steel Weld Joint by an EBSD-Based Parameter

Electron backscatter diffraction (EBSD) analysis has been used to estimate the accumulated strain in base metal region of 316LN austenitic stainless steel weld joints, creep tested at 923 K (650 °C), and at stresses of 175 and 225 MPa. The variation in strength of weld metal, heat-affected zone (HAZ), and base metal-induced stress and strain gradients across the weld joint under creep exposure. Finite element analysis (FEA) of von-Mises stress distribution across the joint has been carried out on incorporating strength of different constituents of the joint, derived by miniature specimen testing techniques. The FEA simulations revealed preferential accumulation of von-Mises stress in the base metal region near to HAZ. The variation in accumulated plastic strain across the base metal has been estimated using a ‘crystal deformation’ (Cd) parameter which quantifies the orientation spread within a grain. This parameter was obtained by EBSD analysis carried out using a scanning electron microscope. The trend in variation of accumulated plastic strain across the base metal accounted well with the von-Mises stress variation, which causes plastic deformation. The plastic strain in the base metal in both the stress levels was found to accumulate preferentially near to the HAZ and reduced steadily toward the ridge at the end of specimen. Transmission electron microscopic study has been carried out to substantiate the findings of the EBSD investigation.

High-temperature creep resistance and effects on the austenite reversion and precipitation of 18 Ni (300) maraging steel

In this paper, the high-temperature creep resistance and effects on the austenite reversion and the dynamic evolution of precipitates of maraging 300 steel were investigated. The main strengthening mechanism in a solution treated and aged material is the fine needle shaped Ni3(Ti,Mo) precipitates densely dispersed in a single martensitic phase. The specimens were submitted to creep tests at temperatures of 550, 600 and 650°C and stress conditions of 200, 300 and 500MPa. Stress exponent (n) varied from 6.0 to 7.2 and activation energy for creep (Qc) from 364 to 448kJ/mol, associated to the tangled and cells arrangements of the dislocations, show that the dominant creep mechanism is controlled by dislocations climb and slip. The experimentally determined threshold stresses are about 25MPa at 550°C and close to 4MPa at 600 and 650°C. Due to high-temperature creep exposure, part of martensite was reverted to austenite in a range of 17.2% to 48.5%, depending upon the time, temperature and applied stress. At the same time, the Ni3(Ti,Mo) precipitates were coarsened and Fe2Mo precipitated, leading to undesirable alloy's strength reduction. Volume fraction of reverted austenite showed strong negative correlation with hardness. Fracture surfaces of specimens presented ductile failure consisting of equiaxed and bi-modal dimples in the fibrous zone surrounded by 45° shear lip.

Assessment of Tungsten Content on Tertiary Creep Deformation Behavior of Reduced Activation Ferritic–Martensitic Steel

Tertiary creep deformation behavior of reduced activation ferritic–martensitic (RAFM) steels having different tungsten contents has been assessed. Creep tests were carried out at 823 K (550 °C) over a stress range of 180 to 260 MPa on three heats of the RAFM steel (9Cr-W-0.06Ta-0.22V) with tungsten content of 1, 1.4, and 2.0 wt pct. With creep exposure, the steels exhibited minimum in creep rate followed by progressive increase in creep rate until fracture. The minimum creep rate decreased, rupture life increased, and the onset of tertiary stage of creep deformation delayed with the increase in tungsten content. The tertiary creep behavior has been assessed based on the relationship, $$ \varepsilon = \varepsilon_{\text{o}} + \dot{\varepsilon }_{\text{m}} t + \varepsilon_3{\exp }\left[ {p\left( {t - t_{\text{t}} } \right)} \right] $$ , considering minimum creep rate ( $$ \dot{\varepsilon }_{\text{m}} $$ ) instead of steady-state creep rate. The increase in tungsten content was found to decrease the rate of acceleration of tertiary parameter ‘p.’ The relationships between (1) tertiary parameter ‘p’ with minimum creep rate and time spent in tertiary creep deformation and (2) the final creep rate $$ \dot{\varepsilon }_{\text{f}} $$ with minimum creep rate revealed that the same first-order reaction rate theory prevailed in the minimum creep rate as well as throughout the tertiary creep deformation behavior of the steel. A master tertiary creep curve of the steels has been developed. Scanning electron microscopic investigation revealed enhanced coarsening resistance of carbides in the steel on creep exposure with increase in tungsten content. The decrease in tertiary parameter ‘p’ with tungsten content with the consequent decrease in minimum creep rate and increase in rupture life has been attributed to the enhanced microstructural stability of the steel.

Influence of Electron Beam Welding Parameters on Microstructure and Mechanical Properties of Boron-Added Modified 9CR-LMO Steel Weld

Modified 9Cr-lMo steel is used in high temperature applications due to its good thermo-physical, mechanical, weldability and formability. This steel is susceptible to Type IV cracking, in which failure of the weld joint occurs on creep exposure in the intercritical region of the heat- affected zone (ICHAZ). Improved resistance to Type IV cracking in modified 9Cr-1Mo steel weld joints can be achieve by addition of 100 ppm boron along with controlled nitrogen. However, in the absence of boron-added welding consumables, weld joints of this steel are presently being made using boron-free welding consumables. Consequently, during creep, the microstructure of boron-free weld metal is relatively less resistant to coarsening compared to the boron-added base metal and HAZ. The objective of the present investigation is to understand the effect of electron beam welding parameters on the microstructure and mechanical properties of the welds. Weld joints of the boron-free (P91) and boron-added (P91B) modified 9Cr-lMo steels were prepared by varying the welding parameters during electron beam welding. The soundness of the weld joints was confirmed by radiographic examination and bend testing. The tempering behavior of weld metal and base metal was studied and compared. The test results show that the ductile-to-brittle transition temperature of the P91B steel is comparable to that of the P91 steel but lower than that of boron-free weld metal. This paper discusses the influence of electron beam welding parameters on themicrostructure and mechanical properties of boron-added modified 9Cr-lMo steel welds.

Creep behavior of P91B steel in the presence of a weld joint

The paper presents creep test data on standard P91B steel specimens made from two distinct regions of a welded plate over a range of stresses (50–190MPa) and temperatures (600–650°C). The analysis of test data revealed that the samples having a weld zone within the gage length (cross-weld samples) have lower long term rupture strength than the samples made of the base metal. Estimated weld strength factors (WSF) of this steel were found to be higher than those reported for P91 steel. The study also showed that the effect of welding on loss of rupture ductility is much more prominent than its effect on the reduction in rupture strength. In presence of welded zone the extent of local deformation in ruptured samples was not as prominent as in the samples without weld. Creep damage tolerance factors (λ) were estimated from the creep strain versus time plots. This also showed that the magnitude of λ is significantly reduced in the presence of welding. Examination of microstructure and measurement of density revealed that this difference is primarily due to the formation of cavities in the heat affected zones of welded specimens. In the lower stress regime a few test specimens without any welded region did not fail even after very long creep exposure. Diameters of these specimens were found to have increased in spite of measureable increase in length due to creep. This unusual effect has been attributed to oxide scale growth. It shows up when the increase in diameter due to the growth of oxide scale becomes greater than the decrease in diameter due to the accumulation of creep strain.

Effect of Nb addition on Z-phase formation and creep strength in high-Cr martensitic heat-resistant steels

The effect of Nb addition on the creep strength of high-Cr martensitic heat-resistant steel was investigated at temperatures ranging from 839 to 894K. The short-term creep properties were improved by Nb addition due to the enhanced MX precipitation. However, Nb addition deteriorated the steel's long-term stability resulting in an even shorter creep-rupture life than the original alloy. Onset of the accelerated degradation in Nb-containing steel was attributed to the gradual formation of Z-phase at the expense of M2N precipitates during creep exposure.

Carbide evolution and service life of simulated post weld heat treated 2.25Cr–1Mo steel

Abstract To estimate the service life of the reactor internals made of 2.25Cr–1Mo steel in the high temperature gas-cooled reactor pebble-bed module demonstration plant, creep rupture test was performed on the simulated post weld heat treated steel at 773 K over a range of stress levels 300–200 MPa. It has been shown that the rupture time is 126–5532 h, correspondingly. Then, the service life under stress of 161.2 MPa at 648 K was calculated to be so long as 210,374,853 h (≈24,015 years). Four types of carbides M 3 C, M 2 C, M 7 C 3 and M 23 C 6 existed in both the steel plate and the post-rupture specimens. As the rupture time increases, the weight percentages of M 23 C 6 and M 2 C that are stable in thermodynamics in the post-rupture specimens increase while those of metastable M 3 C and M 7 C 3 decrease on the whole. During the long term creep exposure, the deformation promotes the precipitation of M 3 C, M 2 C and M 23 C 6 , according to the experimental results with the deformed and undeformed parts of the post-rupture specimens, but its influence on the weight percentage of M 7 C 3 seems rather complicated.

Laves-phase in the China Low Activation Martensitic steel after long-term creep exposure

Creep test at 600°C under 130MPa for the China Low Activation Martensitic (CLAM) steel was performed up to 7913h in this study. According to the stress level, the crept specimen was divided into three regions in order to investigate the influence of stress on Laves-phase formation. In addition to the expected M23C6 carbide and MX carbonitride, the amount and the size of Laves phase in these three regions on the crept specimen were characterized by transmission electron microscopy. Laves phase could be found in all the regions and the creep stress could promote the formation of Laves phase.

Advanced Microstructures for Increased Creep Rupture Strength of MARBN Steels

Over the past three decades a lot of effort was made to optimize the chemical compositionof 9% Cr martensitic steels, aiming to increase the operating temperature up to 923K and thus im-proving the efficiency of thermal power plants. Under these service conditions (high temperature andstress exposure), the creep strength of such steels is closely related to the long term stability of theirmicrostructure. The time to rupture can also be understood as an equivalent to the time of microstruc-ture deterioration. Optimization of the initial microstructure and understanding of the microstructureevolution during creep exposure are therefore decisive to improve the creep behavior of 9% Cr steels.Selected chemical compositions of MarBN steels (Martensitic 9% Cr steels strengthened by Car-bides, Nitrides and Boron) were subjected to different heat treatments to produce an optimized mi-crostructure to improve the creep rupture time. The initial microstructure before creep exposure wasinvestigated using optical microscopy, SEM and EBSD. Short term creep rupture tests at 923K and150MPa were performed, followed by systematic microstructure investigations.Comparative EBSD investigations confirm an optimized microstructure for creep exposure, pro-duced by an appropriate heat treatment. From comparative creep test results, it can be concluded thatadvanced microstructures increase the time to rupture of the selected MarBN steels by more than 10percent, without reduction of the ductility.

Microstructural evolution during creep of 316LN stainless steel multi-pass weld joints

Creep rupture behaviour of 316LN austenitic stainless steel weld joint fabricated by multi-pass shielded metal arc welding process has been studied at 923K over a stress range of 120–225MPa. The weld joint exhibited inferior creep rupture strength than the base metal and extensive creep cavitation in weld metal led to the premature failure. Weld metal microstructure was found highly inhomogeneous having different morphologies of delta (δ)-ferrite. The δ-ferrite transformed into intermetallic phases on creep exposure and the creep cavitation was associated with the intermetallic phases. Extensive creep cavitation in weld metal was confined to regions containing δ-ferrite with vermicular morphology. The region near the weld pass interface having globular δ-ferrite was less susceptible to creep cavitation. The globular δ-ferrite region possessed higher hardness than the vermicular δ-ferrite region. High dislocation density was observed in the globular δ-ferrite region which is a consequence of the microstructural modification by the heat input from the successive weld passes. The strength inhomogeneity between the globular and vermicular δ-ferrite regions together with the transformation of δ-ferrite into intermetallic phases produced pronounced creep cavitation in the vermicular δ-ferrite region which led to premature failure of the stainless steel weld joint.

Trunk Tissue Creep Can Increase Spine Forces During a Subsequent Lifting Task

OCCUPATIONAL APPLICATIONS Results from the current study show evidence of an adverse effect of prolonged trunk flexion on spine loads during consecutive lifting tasks. The time-dependent methodology introduced here can enhance task assessment based on the duration of flexion exposures. More generally, results demonstrate the importance of considering prior trunk exposures when assessing risk factors for lifting tasks. The proposed solution incorporates “time” as an independent variable, in addition to lifting weight and posture, to better assess spinal loads and maximum lifting capacity based on prior loadings. The current results also suggest that existing ergonomic guidelines or biomechanical models that do not incorporate the viscoelasticity of soft tissues or time-dependent neuromuscular alterations may underestimate spine forces and potential injury risk in some circumstances. To account for this, especially when assessing spine forces during lifting after exposure to prolonged flexion, additional safety margins should be considered.TECHNICAL ABSTRACT Background: Prolonged trunk flexion decreases soft tissue stiffness due to viscoelastic deformations and can also lead to altered kinematics when performing a subsequent lifting task. Yet, it remains to be determined if or how these changes and alterations might increase spine forces. Purpose: A previously developed viscoelastic model was used, along with experimental data, to predict changes in peak spine forces during a lifting task performed following a prolonged flexion exposure (creep). Methods: Model inputs were obtained from an experiment using ten participants, within which lifting kinematics and muscle activity were measured both before and after creep exposure. Two sets of simulations were performed; one in which kinematics were assumed to be unchanged by creep exposure and the other incorporating measured changes in kinematics following exposure. Results: Post-exposure changes in lifting kinematics involved a reduction in the peak relative sagittal-plane flexion of superior lumbar motion segments and an increase in these flexion among inferior lumbar motion segments. Creep exposure caused increases in predicted peak spine forces during lifting at all levels of the lumbar spine (65–241 N). A substantial portion (∼51%) of this increase was estimated to be the result of muscular compensations for reduced passive tissue stiffness. Conclusions: The current study demonstrates that both changes in lifting kinematics and viscoelastic deformations resulting from creep exposures can lead to increased trunk muscle forces and spine forces during a lifting task. This evidence suggests a potential mechanical basis for previous epidemiological evidence that indicates an increased risk of low back disorders for jobs involving both trunk flexion and lifting.

Microstructural Stability of P23/P91 Dissimilar Welds during Creep at 500°C

This paper deals with microstructural stability of P23/P91 dissimilar welds during long-term creep exposure at 500°C. Detailed TEM investigations on minor phase evolution were performed on specimens after long-term creep exposure. Microstructural changes in the decarburized layer of the P23 steel were studied using EBSD and TEM. Experimental data on minor phase evolution were compared to the results of thermodynamic (Thermocalc) and kinetic (Dictra) simulations. Evolution of minor phases in the P23/P91 welds during creep at 500°C was in a good agreement with results of microstructural modelling. Partial decarburization of the P23 occurred at the expense of dissolving M7C3 phase. Microstructure of the partly decarburized zone in the P23 steel remained bainitic in all specimens investigated. In spite of the partial decarburization in the P23 steel during longterm creep exposure at 500°C, creep failures occurred in the ICHAZ of the P23 steel.

Advanced intermetallic γ-TiAl based alloys with improved microstructural stability during creep

ABSTRACTEnsuring microstructural stability under technical relevant conditions is a determining criterion for the development of innovative high-temperature materials. In this work, the influ-ence of C and Si on the microstructural stability during creep exposure was investigated for a β-solidifying γ-TiAl based alloy with a nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (in at.%), named TNM. With a two-step heat treatment a microstructure consisting of fine lamellar α2/γ-colonies, surrounded by βo-phase and areas of discontinuous precipitation, starting from the boundaries of the lamellar colonies, was adjusted. Creep tests were carried out to examine the potential of C and Si to prevent microstructural instability during creep and hence improving the creep properties. At 815 °C the discontinuous precipitation process of the TNM alloy continues during ensuing creep testing leading to a reduced creep resistance. In comparison, the minimum creep rate of the TNM-0.3C-0.3Si alloy was significantly decreased caused by the lower βo-phase content and average lamellar spacing within the α2/γ-colonies, the precipitation of p-Ti3AlC carbides and the retarded kinetics of discontinuous precipitation.

Effect of Creep Exposure on Microstructure and Mechanical Properties of Modified 9Cr-1Mo Steel

The prolonged exposure at elevated temperatures under stress significantly alters the microstructure of steels resulting in degradation of the mechanical properties. Therefore, the assessment of structural integrity of high temperature components needs the evaluation of microstructure and mechanical properties. In this investigation, modified 9Cr-1Mo steel has been subjected to creep tests at 923K and at stress levels of 100, 120 and 140MPa The creep tests have been interrupted at different periods to assess the change in the microstructure and mechanical properties. While extensive TEM studies were carried out to determine the variation on defect structure and precipitation behaviour of the steel, hardness and ball-indentation testing were employed to evaluate the mechanical properties of creep exposed samples. Microstructural analysis using electron microscopy indicates that distinct morphological modifications in the martensitic lath structure, area fraction of carbides and formation of embrittling phases, which associate well with the decrease in the strength parameters as determined by small specimen test results. While the sudden drop in the strength of the steel has been correlated to the formation of z-phase, the continuous decrease in the strength as a function of time fraction of creep has been attributed to dislocation-free subgrain formation and extensive coarsening of carbides. Present paper would discuss the correlation of these structural variations with the mechanical properties.

Microstructure evolution and life assessment of T92 steel during long-term creep

Standard tests for creep and rupture strength were carried out on T92 martensitic heat-resistant steel at the temperatures of 600, 650, and 700°C and stress levels of 46–200MPa. The creep life of T92 steel was estimated through the Larson–Miller parameter, which was calculated from the aging temperature and aging time. Crystal structures and morphologies of precipitate phases after creep were investigated by scanning electron microscopy and transmission electron microscopy, and the composition was analyzed by energy dispersive X-ray spectrometry. Degeneration processes after creep exposure of the martensitic microstructure, namely, martensitic substructure recovery, as well as coarsening and precipitation of second phases, were also observed.

Change of precipitate free zone during long-term creep in 2.25Cr–1Mo steel

This work presents a residual life assessment method based on the quantitative analysis of changes in the fraction and width of a precipitate free zone (PFZ) around grain boundaries during long-term creep. Such changes resulted from precipitation and coarsening of carbides on grain boundaries and within the ferrite matrix during long-term creep exposure. Testing was conducted at creep temperatures ranging from 450 to 650°C, for a maximum time of 142415.1h. A relationship between the fraction of grain boundary with PFZ, PFZ width, and the Larson-Miller Parameter (LMP) was established, and the mechanism of PFZ formation was discussed. The increase in width, governed by the diffusion of alloy elements to grain boundaries, was mathematically modeled. The relationship between the width and creep exposure time was fitted to a square root law, and the PFZ broadening rate constant and activation energy for width growth were calculated. The proposed model will be applicable to determine the operating parameters of 2.25Cr–1Mo steel components in high-temperature equipment.