Remaining Creep Life Research Articles

Creep deformation behavior and residual life prediction of low hardness P91 steel welded joints

The remaining creep life of the reheat section pipeline of low hardness P91 steel replaced in a certain power plant was studied. The rupture behavior of the joint was studied using metallographic microscope (OM), scanning electron microscope (SEM), and Electron Back Scatter Diffraction (EBSD), and the creep life was predicted. The results show that the rupture behavior of P91 steel and its joint during service is related to external stress. When the stress is high, the failure position of the joint is concentrated at the lowest hardness position, dominated by low strength martensitic deformation cracking; When the stress is low, the joint fails from FGHAZ in the form of Type IV cracking, resulting in a combination of martensitic structure degradation and pore nucleation. Type IV cracking has a significant impact on the life of joints, and the polygonal line OSD method based on fracture mode for life prediction can effectively improve the accuracy of material life prediction.

Evolution and criteria for early creep damage

ABSTRACT The life-limiting fitness for service of high-temperature components is of interest in design, fabrication and later assessments of remaining creep life. Of the associated indicators, strain reflects creep in design and in service, while the discontinuities like creep cavities are targeted in the in-service inspections. Microstructure and hardness can provide supporting information on the material condition. Here, we assess such indicators for the creep-associated damage, particularly at the early stages. Improvements appear possible, e.g. in microscopy to support metallographic inspections and in utilising the widening inspection experience on newer materials. The present work successfully integrated the Wilshire/LCSP creep strain and rupture models with FE analysis for predicting creep strain evolution. Since the model applies for the whole creep curve, it can be used for early stages of creep down to the limit of negligible creep and to the lower limit of the window where creep cavitation damage can be observed.

Microstructure Analysis and Finite Element Modelling for Creep Failure Prediction and Fitness-For-Service Assessment of Superheater Tubes

Abstract This study aimed to perform a fitness-for-service assessment and investigate the root cause of failure of Grade 14CrMo3 steel seamless tubes typically used in superheaters in power generation plants. For this purpose, samples were taken from in-service superheater tubes in a 320 MW power plant. Thickness and hardness measurements were taken from the samples, and microstructural analyses were performed using scanning electron microscopy equipped with energy dispersive X-ray spectroscopy and X-ray diffraction. The results showed the presence of vanadium (V) and sulfur (S) elements on the tubes' external surface (flue gas facing-side), which is indicative of fuel ash corrosion. The formation of low melting point salts, such as Na2SO4, NaVO3, Na2O, and V2O5 (particularly between 10 and 2 o'clock positions) and degradation of the protective oxide layer led to loss of tube wall thickness. On the steam side of the tubes, the formation of an iron oxide layer (particularly between 12 and 2 o'clock positions) and the presence of water in the steam due to the improper function of the steam drum created an insulated zone leading to the formation of localized hot spots, creep microvoids, and spheroidization of carbides. In addition, a thickness reduction of 18% resulted in a considerable increase in hoop stresses having a detrimental effect on the remaining creep life. To explain the creep damage mechanism and determine the remaining creep life, the Larson–Miller criteria and API 579-1/ASME fitness-for-service-1 guidelines were utilized. The effects of the reduction in wall thickness were considered by performing a three-dimensional finite element analysis. The results showed that a temperature increase of only 50 °C (from 480 °C) could decrease the life of the tubes from 30 years to less than a year.

Creep Life Assessment Method for Super304H Boiler Tubes Based on HTS-SQUID Gradiometer

This paper presents a novel method for the creep life assessment of Super304H (KA-SUS304J1HTB, ASME code: 2328) boiler tubes based on a discrete-type eddy-current-testing (ECT) HTS-SQUID nondestructive evaluation (NDE) system. Six specimens were subjected to internal pressure creep tests at a stress level of 110 MPa and a temperature of 700°C. The specimens were inspected at various creep test times using our NDE system, which included an ECT probe and a HTS-SQUID gradiometer. By studying the physical characteristics and microstructures of the specimens at different test times, we found that the electrical conductivity and permeability of the specimens varied with an increasing test time. The detected signals may have been generated from creep voids and/or surface microcracks. Based on the NDE inspection results, creep life assessment curves that can be used for assessing the remaining creep life of the Super304H boiler tubes were plotted. The results confirmed the possibility of assessing the remaining creep life using our HTS-SQUID-based system in a nondestructive manner.

Endurance Strength and Residual Life Assessment of T23 Steel after 96000 Hours of High Temperature and High Pressure Service

The macroscopic morphology, oxide layer characteristics, rupture strength and residual life of T23 steel after 96000 hours of service at high temperature and high pressure were analyzed. The results show that T23 steel pipe has good macroscopic appearance, without obvious abnormal features such as cracks, folds, rolling breaks, double skin, scabs, delaminations and sharp scratches. The chemical composition of different pipe sections conforms to the standard, and the structure is mainly bainite, with uniform distribution and no obvious coarse phase. There is an obvious interface between the metal substrate and the internal oxide layer, and there is an obvious wavy line at the interface. The oxide layer is relatively dense and continuous. The tensile properties at room temperature and high temperature are good. At room temperature, the yield strength exceeds 420MPa, the tensile strength exceeds 540MPa, and the elongation exceeds 19%. After 96000 hours of use, the remaining creep life of T23 split platen superheater submitted for inspection is 84000 hours, and the remaining creep life of T23 rear platen superheater submitted for inspection is 40000 hours.

Remaining Creep Life Prediction Method for In-Service Boiler Pipe Weldment Using Small Scale Specimen

Abstract The author proposes a remaining creep life prediction method for weldment of Grade 91 steel using a small specimen taken not from the weldment but from the base metal. Analysis of creep data obtained from long-term used steels indicates that the shape of the master rupture curve given by the Larson-Miller parameter remains almost the same regardless of the heat-to-heat variation of the creep life of the steels, and the life of each steel can be expressed by the parallel displacement of the master rupture curves along the abscissa direction. Therefore, we can estimate the life of used steels with only one datum even if it is obtained with a short-time creep test. This approach of extrapolation is effective for the proposed method because the number of test specimens taken from the pipe is at most two or three in power plants in Japan. The results of investigation of the relationships among creep strength, deformation, and ductility suggest that the creep deformation property primarily determines the creep life of the materials used in this study. Finally, the proposed method is applied to the in-service boiler pipe weldment of Grade 91 steel, and its features are described in comparison with the conventional life assessment method.

Degradation of microstructural and mechanical properties with serviced turbine blades

To study the degradation of directionally solidified turbine blades with different service times, four parts of four cross sections are selected to examine their microstructure through scanning electron microscopy. The width of the γ-phase channel, γ’ volume fraction, types of carbides, and elemental composition are chosen to characterize the damage. Creep tests are carried out at four heights on the upper and lower parts of the turbine blades, and the residual creep life is determined. The degrees of degradation of the blades are examined based on the microstructures and creep test results of the samples with respect to various service times. The results indicate that the quantitative description of the microstructure is consistent with the experimentally obtained results for the remaining creep life. The relationships between the residual lives of the small samples and the width of the γ-channel and γ'-phase volume fraction are presented in a three-dimensional graph. Based on the creep damage model, a method for predicting the remaining life of the turbine blades in service is established. The predicted results of the remaining life fit well with the model results. • The study of serviced turbine blades can obtain first-hand information. • The volume fraction and size of the γ' phase are closely related to the mechanical properties. • The degradation degree of the same part of the blades increases with the large increase of service time. • There is a good correlation between the mechanical properties and the icrostructural features of the blades. • The quantitative description of the microstructure is consistent with the test results of the remaining creep life . • Based on the creep damage model, a method for predicting the remaining lives of turbine blades in service was established.

Characterization of the cyclic softening and remaining creep behaviour of P92 steel weldment

Different micro-regions of weldment are expected to present various cyclic softening behaviours during creep–fatigue interaction (CFI), which has a significant influence on the remaining mechanical properties. The present work aims to investigate the cyclic softening and remaining creep behaviour of different micro-regions of P92 steel weldment using electron backscattered diffraction (EBSD) and nanoindentation technology. CFI tests were performed and interrupted at various pre-set fatigue cycles at elevated temperature of 650 °C and hold time of 180 s, and thereafter EBSD analyses, nanoindentation tests and high-temperature creep tests were conducted on these interrupted specimens. Experimental results reveal that the microstructure of the parent metal and fine austenite grain heat-affected zone, which is adjacent to the intercritical heat affected zone, are sensitive to CFI loading. Notably, the microstructural evolution in intercritical heat affected zone almost reaches saturated after fatigue cycles of 20%Nf. Nanoindentation results uncover that the intercritical heat affected zone exhibits the lowest elastic modulus and microhardness, and with increasing the number of fatigue cycles, the weakest region changes. Accordingly, creep fracture location transits and creep life was degraded. Finally, a fatigue damage parameter based on the microhardness was developed to capture the effects of CFI on remaining creep life.

Effects of microstructure and lattice misfit on creep life of Ni-based single crystal superalloy during long-term thermal exposure

Abstract According to the microstructural evolution during longterm thermal exposure at 1100 °C, the creep rupture life of Ni-based single crystal superalloys at 980 °C/270 MPa was evaluated. The microstructure was characterized by means of scanning electron microscopy, X-ray diffraction and related image processing methods. The size of γ’ precipitates and the precipitation amount of topologically close-packed increased with the increase in thermal exposure time, and coarsening of the γ’ precipitates led to the simultaneous increase of the matrix channel width. The relationship between the creep rupture life and the lattice misfit of γ/γ’, the coarsening of γ’ precipitate and the precipitation of TCP phase are systematically discussed. In addition, according to the correlation between γ’ phase evolution and creep characteristics during thermal exposure, a physical model is established to predict the remaining creep life.

The correlation between prior fatigue damage and remaining creep properties in a high chromium steel welded joint

Strain cycling prior to a constant load exposure is expected to influence the creep properties. The present work aimed to evaluate the effect of prior low cycle fatigue (LCF) damage on the remaining creep properties of P92 steel welded joints. Strain amplitudes of 0.25%, 0.4% and 0.6% and fatigue cycles ranging from 10% to 80% of the number of cycles to failure (Nf) were introduced in creep specimens to represent fatigue damage. A critical analysis of the creep test responses revealed that a high strain amplitude and high fatigue cycles induced a clear reduction in the remaining creep life; conversely, prior LCF also led to a significant increase in the creep rupture strain. It was found that prior LCF loading produced its saturated effect after fatigue cycles at 70% Nf or a strain amplitude of 0.4%. It is important to note that the degradation of the remaining creep life of a welded joint was more severe than that found in homogeneous base metal. It was also observed that the failure location and development of creep voids were dependent on both the number of fatigue cycles and strain amplitude. Additionally, the prior fatigue damage was evaluated using a defined damage parameter that revealed two nonlinear stages of fatigue damage.

Remaining creep properties and fracture behaviour of P92 steel welded joint under prior low cycle fatigue loading

The alternation of creep properties is unavoidable in welded structures after suffering from cyclic damage, which may lead to the premature failure of power plant units and hard to predict. This work is devoted to clarify the influence of prior low cycle fatigue (LCF) loading on subsequent creep properties and fracture behaviour of P92 steel welded joint. Prior LCF tests ranging from 0%Nf to 70%Nf and subsequent creep tests were performed at 650 °C. Results indicate that the remaining creep life of P92 steel welded joint is significantly reduced by prior LCF loading, whereas the creep ductility has a pronounced increase. Importantly, the reduction of remaining creep life of welded joint is greater than that of homogeneous base metal, while the creep rupture strains of welded joint and base metal show opposite evolution. Fractography of failed specimens reveals that the creep fracture location depends on prior LCF damage. Additionally, the remaining creep properties were evaluated with a proposed fatigue damage parameter, and the sensitive regions of fatigue damage on remaining creep properties were determined.

ICME Framework for Damage Assessment and Remaining Creep Life Prediction of In-Service Turbine Blades Manufactured with Ni-Based Superalloys

Accurate creep life prediction is necessary for the evaluation of the remaining creep lives of in-service turbine blades and for the design of new turbine blades in aircraft engines. In this study, an integrated computational material engineering methodology for predicting the remaining creep life of in-service turbine blades was developed by taking a microstructural criterion and creep strain criterion into consideration, and combining artificial neural networks with a modified θ projection model to assess the service temperature, stress, degradation time, and existing creep strain. To explore the application of the method and verify its accuracy, the microstructural degradations at different locations of two directionally solidified superalloy DZ125 turbine blades, which were in-service for 300 h and 980 h in different engines, were characterized and quantified. Using these results, the remaining creep life of the microstructures at different locations of the blade was predicted. Finally, these creep life prediction results were experimentally verified using miniature creep test specimens. The development of this new method provides a reference for the design and service evaluation of turbine blades made of directional solidified and single-crystal Ni-based superalloys.

Accelerated Creep Life Assessment of In-Service Power Plant Components

Structural metals used in plant components are subject to aging from a combination of fatigue, creep, and corrosion. Exposure to elevated temperatures promotes creep. Aged metals lose toughness, or the ability to absorb energy at stress above the yield point and cannot endure an occasional high load without fracturing. Creep is one of the most critical factors for determining the structural integrity of components. The main objective of the current study is to assess the remaining creep life of various 20-year old power plant engineering components such as the high temperature fasteners. Due to time constraints, the approach followed in this study was to utilize the accelerated high temperature creep testing in addition to Scanning Electron Microscopy (SEM) analysis to assess the remaining life of 4 different samples. The accelerated high temperature creep tests were conducted at a stress level of 61 MPa and at a temperature of 1000°C for samples Sample 1 (original), Sample 2, Sample 3 and Sample 4; these samples were collected from different parts of the power plant. SEM analysis was carried out for all the samples. The results of the accelerated high temperature tests were compared to similar materials’ theoretical creep data using Larson Miller curve. The Larson Miller actual creep lives of the tested samples were much higher than the experimental ones, which suggest that the samples are critically aged. SEM analysis on the other hand, showed that all samples have high percentage of creep voids

Response Surface Methodology for the Remaining Creep Life Estimation by the Small Punch Creep Test

Abstract In this article, a method for the estimation of the remaining creep life of steel alloy was studied using the response surface methodology (RSM) based on the central composite design (CCD). Small punch creep tests (SPCT) were performed on the specimens from the aged materials of SA-304L alloy, with different consumed creep life ratios at elevated temperatures. Then, two equations were developed to predict the fracture time and the minimum displacement rate with respect to the consumed creep life ratio of samples, the temperature, and the load of SPCT. The contour plots of responses show that there is a linear relationship between the fracture time and the minimum displacement rate in the logarithmic scale, in accordance with the Monkman-Grant relation.

Small ring testing of high temperature materials

In service components such as steam pipes, pipe branches, gas and steam turbine blades, etc. which operate in engineering applications such as power plant, aero-engines, chemical plant etc., can operate at temperatures which are high enough for creep to occur. Often, only nominal operating conditions (i.e. pressure, temperatures, system load, etc.) are known and hence precise life predictions for these components, which may be complex in terms of geometry or weld characteristics, are not possible. Within complex components it can also be the case that the proportion of the material creep life consumed may vary from position to position within the component. It is therefore important that non-destructive techniques are available for assisting in the making of decisions on whether to repair, continue operating or replace certain components. Small specimen creep testing is a technique which can allow such analyses to be performed. Small samples of material are removed from the component to make small creep test specimens. These specimens can then be tested to give information on the remaining creep life of the component. This paper presents the results of small ring specimens tested under creep conditions and shows the comparison to standard (full size) creep testing for materials used under high temperature in industry.

Phase field analysis and simulation for remaining creep life assessment of H13 steel

Abstract A traditional phase field model was used to develop and improve the remaining creep life assessment of H13 steel. In this modified phase field model, the microstructure was characterized by order parameter ϕ. The changes of ϕ were considered to be a function of subgrain formation or growth during creep. Due to the linear relationship between yield strength and hardness in steel, hardness was used in equation of motion for studying material flow at steady state creep. Creep tests consist of fracture and 1% creep ductility, were conducted according to ASTM E139 at 500, 550 and 600 °C and stress levels of 872–926.5 MPa. The mechanical properties and microstructural characteristics of H13 steel were examined and used through simulation process.

Thermal load non-uniformity estimation for superheater tube bundle damage evaluation

Industrial boiler damage is a common phenomenon encountered in boiler operation which usually lasts several decades. Since boiler shutdown may be required because of localized failures, it is crucial to predict the most vulnerable parts. If damage occurs, it is necessary to perform root cause analysis and devise corrective measures (repairs, design modifications, etc.). Boiler tube bundles, such as those in superheaters, preheaters and reheaters, are the most exposed and often the most damaged boiler parts. Both short-term and long-term overheating are common causes of tube failures. In these cases, the design temperatures are exceeded, which often results in decrease of remaining creep life. Advanced models for damage evaluation require temperature history, which is available only in rare cases when it has been measured and recorded for the whole service life. However, in most cases it is necessary to estimate the temperature history from available operation history data (inlet and outlet pressures and temperatures etc.). The task may be very challenging because of the combination of complex flow behaviour in the flue gas domain and heat transfer phenomena. This paper focuses on estimating thermal load non-uniformity on superheater tubes via Computational Fluid Dynamics (CFD) simulation of flue gas flow including heat transfer within the domain consisting of a furnace and a part of the first stage of the boiler.

Experimental study of remaining creep life of SA-304L stainless steel using small punch creep test

ABSTRACTIn this paper, the remaining creep life of SA-304L stainless steel at elevated temperatures is studied. The small punch creep tests (SPCT) were performed on SA-304L virgin and aged materials at constant loads. Times to fracture and the minimum deflection rate in SPCTs were recorded, and the time-temperature parametric analysis was performed, based on experimental results. The constants of the Larson–Miller parameter and the Monkman–Grant relationship were obtained for different consumed creep life ratios, and eventually the equations were established to explain the variation of constants of the Larson–Miller parameter and the Monkman-Grant relationship with respect to the consumed creep life ratio.

Small Ring Testing of High Temperature Materials

In service components such as steam pipes, pipe branches, gas and steam turbine blades, etc. which operate in engineering applications such as power plant, aero-engines, chemical plant etc., can operate at temperatures which are high enough for creep to occur. Often, only nominal operating conditions (i.e. pressure, temperatures, system load, etc.) are known and hence precise life predictions for these components, which may be complex in terms of geometry or weld characteristics, are not possible. Within complex components it can also be the case that the proportion of the material creep life consumed may vary from position to position within the component. It is therefore important that non-destructive techniques are available for assisting in the making of decisions on whether to repair, continue operating or replace certain components. Small specimen creep testing is a technique which can allow such analyses to be performed. Small samples of material are removed from the component to make small creep test specimens. These specimens can then be tested to give information on the remaining creep life of the component. This paper presents the results of small ring specimens tested under creep conditions and shows the comparison to standard (full size) creep testing for materials used under high temperature in industry.

High-temperature Tensile Properties and Creep Life Assessment of 25Cr35NiNb Micro-alloyed Steel

Reformer tubes in petrochemical industries are exposed to high temperatures and gas pressure for prolonged period. Exposure of these tubes at severe operating conditions results in change in the microstructure and degradation of mechanical properties which may lead to premature failure. The present work highlights the high-temperature tensile properties and remaining creep life prediction using Larson-Miller parametric technique of service exposed 25Cr35NiNb micro-alloyed reformer tube. Young’s modulus, yield strength, and ultimate tensile strength of the steel are lower than the virgin material and decreases with the increase in temperature. Ductility continuously increases with the increase in temperature up to 1000 °C. Strain hardening exponent increases up to 600 °C, beyond which it starts decreasing. The tensile properties are discussed with reference to microstructure and fractographs. Based on Larson-Miller technique, a creep life of at least 8.3 years is predicted for the service exposed material at 800 °C and 5 MPa.