Remnant Life Assessment Research Articles

Comparative Study on Remaining Life Assessment of a Pressure Vessel, Working in Severe Conditions, Based on Results of Metallographic Replicas and Creep Tests

The paper presents and compares the results on the reliability and remaining life assessment of a reactor (coxing box) from a petrochemical plant. The reactor shell is made of 16Mo5 (W1.5423) steel, with a thickness of 25 mm, plated with 3 mm thick X6CrAl13 (W1.4002) stainless steel. The assessment was made in two steps. For preliminary remnant life assessment, specifications of section VII of the ASME code was used followed by iRiS‑Thermo expert system. Further, experimental creep and metallographic replica analysis were performed. Results comparison of the two methods applied revealed a reduction of the preliminary estimated remaining live obtained using metallographic replica analysis. Based on the results obtained, the possibility to extend the service duration of the coxing box in the safety condition, using current process parameters, with of 20.000 hours was highlighted.

Assessment of Power Plant Components With Flaws and Defects Operating in the Long-Term Creep Range

Abstract Originating from defects and flaws in high temperature components crack initiation and crack propagation under service conditions can occur. Fracture mechanics data and procedures are needed to study crack problems and to support an advanced remnant life evaluation. During subsequent research in the past 35 years, data were determined for different high temperature materials. Methodologies and concepts taking into account the specific material behavior were developed in order to be able to describe crack initiation and crack growth and have appropriate assessment methods available. For creep crack initiation, two criteria principles were used and for creep crack growth assessment based on the integral C* parameter were applied. Furthermore, a method for determination of critical crack length was developed allowing decisions whether modified stress analysis methods are sufficient or more complicated fracture mechanics methods are needed. To provide data and methodologies in a user-friendly way, a program system combining data and methods was implemented. The paper describes developed features and shows comparisons to other methods. The methods can be applied for design purposes as well as remnant life assessments.

An evaluation of high temperature hydrogen attack resistance and a possible new microstructure development based remnant life assessment method of a clean ASME Gr.91 thick section steel plate

A thick section plate specimen of a completely refined Gr.91 steel plate met the requirements of ASME code concerning the toughness and creep strengths due to exact control of the chemical composition and lowering of impurities. Low impurities also resulted in almost the same HTHA (High Temperature Hydrogen Attack) resistance as that of 2.25Cr-1Mo steel at 500ºC at 20 MPa hydrogen pressure. The long-term creep strength of Gr.91 steel is reduced by the presence of weld heat-affected-zones. The weakening phenomenon is due to the formation of ‘Type IV damage’. In this study, a crystal misorientation diagram transition analysis method using electron-back-scattering-diffraction analysis, EBSD, is applied to predict the remnant creep life of the HAZ. The trial prediction agrees with the actual rupture time.

Creep damage and life assessment of thick-walled spherical reactor using Larson–Miller parameter

Creep damage and remnant life assessment of a thick-walled spherical reactor made of 316LN austenitic stainless steel (316LN SS) have been investigated. The Robinson's linear damage accumulation rule has been used to obtain damage and remnant life assessment in which time to rupture is determined by Larson-Miller Parameter (LMP). Due to high temperature, creep is the most significant damage mechanism exhausting the lifetime of the reactor. The material properties, except Poisson's ratio, are assumed to depend on the temperature. An analytical solution employed to calculate the stress rates followed by an iterative method using initial thermoelastic stresses at zero time to obtain effective stress histories and then using LMP to calculate the damage and remnant life assessment. It is concluded that the temperature gradient has a significant effect on the effective stress histories so that effective stresses are decreasing with time in a uniform temperature field while they are increasing in the presence of a thermal gradient.

Small Punch Creep Testing Technique for Remnant Life Assessment

Small punch creep (SPC) testing technique is a material non-intensive testing technique for evaluating creep behavior of materials using miniature specimens. It can be used for remnant life assessment (RLA) studies on components in service, by scooping out limited material for testing without impairing the strength of component. In order to ensure the reliability of use of SPC technique for RLA, it is necessary to establish sound database on SPC properties of the material before putting into service. In this investigation, SPC technique was used to evaluate creep properties of 316LN stainless steel using specimens of size 10 x 10 x 0.5 mm. SPC tests were conducted in load controlled mode at 923 K and at various loads. SPC curves clearly exhibited primary, secondary and tertiary creep stages. The minimum deflection rate increased and rupture life decreased with an increase in applied load. Like in conventional creep test results, the minimum deflection rate obeyed Norton’s power law and Monkman-Grant relationship. SPC test was correlated with corresponding conventional creep test. Good correlation was established between creep rupture life values evaluated from SPC tests and conventional creep tests.

Uncertainty in Damage Assessment and Remaining Life Prediction of Engineering Materials Used In Petrochemical Industry

In this paper creep damage assessment of about 11 years’ service exposed HP-40 grade of steel used in hydrogen reformer of a petrochemical industry has been carried out in terms of a discontinuous Markov process. Experimentally determined conventional creep data under identical testing condition were used in the present investigation. Scatter and damage accumulation due to creep deformation were evaluated through microstructural assessment using light optical microscope and scanning electron microscope. Quantification of creep damage was made from replicated creep data in terms of two damage parameters A and A*. Statistical analysis of void area fraction has been carried out extensively for the both top and bottom portions of the reformer tube at 870 o C in the stress range of 52-68 MPa. In addition, the proposed probabilistic model has been compared with the Kachanav’s Continuum Damage Mechanics (CDM) model. Both the approaches displayed quantitative experimental support. A residual life of A¢ÂŸÂ© 10 years is estimated at 870 o C/ operating stress. For 55 years’ service exposed Catalytic Cold Cracking (CCU) reactor vessel and Feed Processing Unit (FPU) distillation column materials of a petrochemical industry remnant life assessment studies were estimated by incorporating the uncertainty involved in calculation of LMP (Larson Miller Parameter) values and from extrapolation of stress vs. LMP plot. Variability of normalized creep damage for reactor and column materials is well approximated with the aid of Weibull distribution. As expected, it is observed that the distributions shift towards the higher range of damage with increase in service exposure time.

Analysis Remnant Life of Power Transformer Based on Health Index

According to the transformer testing information, the factors affecting of transformer remnant life assessment were analyzed. The formula which reflects the transformer health condition and remnant life is deduced and established according to the current information. The model thus is used in the health condition assessment and calculation of the remnant life for transformers. The results show that the process is consistent with the practical situation.

Remnant Life Assessment and Microstructural Studies on Service Exposed Primary Reformer Tubes of a Catalytic Converter of an Ammonia Plant

Abstract13.5 year service exposed (SE) catalyst primary reformer tube material made of H39WM micro-paralloy grade used in feritilizer plant was assessed for remaining life. The investigation includes mechanical properties evaluation; microstructural analysis and accelerated stress rupture tests. Failed tube portions showed coarsening primary carbides of Chromium and Niobium along the grain boundaries. Degradation of Niobium carbide (NbC) into Ni-Nb-Si phase and partial conversion this phase back to NbC was observed. Secondary carbides within grains were almost absent. Degradation in tensile strength for a range of temperature from 1123 to 1223 K was also observed but they were within the specified limits. Premature failures within 3–5 years service exposure are more common in reformer tubes. The failure was attributed to localized overheating leading to creep damage. The cast tube material may undergo microstructural changes during service exposure which is the main cause of degradation in strength and hardness changes. Accelerated stress rupture tests were performed in the range of 1173–1248 K on samples machined from 13.5 years at 1191 K and 15.4 MPa exposed reformer steel tubing, did not reveal any degradation of rupture behavior compared to that of the virgin alloy. An additional life of at least 10.6 years is predicted at the operating stress-temperature conditions.

Determination of creep property of 1.25Cr0.5Mo pearlitic steels by small punch test

Small punch creep tests (SP-C) have been developed to determine creep data of a 1.25Cr0.5Mo pearlitic steel. The test results have been compared with those of conventional creep tests. The comparison shows that overall shapes of the SP-C tests’ creep curves are similar to those of conventional creep tests. For the same creep rupture life, ratio of load of SP-C tests to equivalent stress of conventional creep tests is approximately 2.7. Some empirical formulae such as Larson-Miller law, Dorn equation and Monkman–Grant correlation, which are widely used for conventional creep tests, have also been used to analyze the results of SP-C tests. The analytic results are in good consistency with those obtained from conventional creep tests. Therefore, it can be concluded that small punch creep tests can be potentially used as a non-destructive testing method to evaluate creep properties of in-service structural components for remnant life assessment.

Microstructural studies and remnant life assessment of eleven years service exposed reformer tube

Utilization of appropriate inspection during plant shut down, in combination with high temperature mechanical properties, microstructural analysis and judicious accelerated creep provided an effective strategy for short term assessment of 11 years service exposed reformer tube integrity. Barring development of corrosion cracks and decarburization of the inner and outer walls of the tube, the microstructure of the service-exposed alloy was free from any signs of deterioration like creep cavitation or voids, and the hardness was within the expected ranges; however, tensile testing revealed an apparent loss of strength from 25 °C to 900 °C. Accelerated 900–980 °C stress rupture testing of samples machined from 11 years at 950 °C and 19 MPa exposed KHR 35 CT reformer steel tubing, did not reveal any degradation of rupture behaviour compared to that of the virgin alloy. An additional life of at least 32,822 h is predicted at the operating stress–temperature conditions.

A detailed study of the relationship between fatigue crack growth rate and striation spacing in a range of low alloy ferritic steels

It was shown that the measured average fatigue striation spacings predicted the fatigue crack growth rates for low alloy ferritic steels to within a range of ±2% to 35% with an overall average error band value of ±10.1%. When we consider the fatigue stress range this average error was reduced to only some ±4%. This was good news to both failure analysts and other workers involved in the field of component remnant life and life extension since such predicted fatigue stress ranges use real fracture characteristics observed at some point on the actual component fracture surface. These findings were applied to a real cracking problem recently reported in a steam raising plant, viz., a cracked attemperator reducer weld. In this case an NDT assessment indicated that the maximum crack depth was 7 mm while the lower bound critical crack depth was estimated at 10 mm. As such, remnant life assessments can be estimated for a series of fatigue stress ranges through the use of a reported 450 °C fatigue crack growth law for C–Mn steels. Remnant life estimates of a 7 mm deep crack for a range of stress ranges varied from 3000 to 4000 starts where the chances of the real remnant life values being greater than the calculated values was only 1 in 2. However when a realistic failure probability which reflected the serious implications of a failure event of E−4 was taken the remnant life values were reduced to around 100 starts or some 6 months of normal service.

Hydrogen-Assisted Cracking in Boiler Steam Drums

A statutory inspection of a boiler steam drum that had seen 10 years of service revealed a few scattered cracks on the inner surface of the drum. The drum was used in the refinery and petrochemical industry. Remnant life assessment, including inspection of other drums, showed presence of several cracks in two out of seven boiler inspected. In situ microstructural analysis revealed a variation in the microstructure in the cracked regions compared to the microstructure in uncracked regions. Additionally, a wide variation in hardness was associated with the microstructural variation. Detailed study on a sample extracted from a cracked region demonstrated that the cracking occurred in a crescent zone similar in appearance to that of a heat affected zone (HAZ) generally associated with a spot weld or other sources of hot spots in the material. Subsequent examination of more samples confirmed that repair welding was carried out at several places on the inner surface of the drums before installation and commissioning and all the cracks were around the repair welds. The failure occurred by crack initiation in the HAZ of the repair welds and the cracks then propagated progressively across the inner surface of the drums. The failure mechanism was identified to be cold cracking and the failure analysis showed that all the factors required for cold cracking, namely, vulnerable microstructure, residual stresses and hydrogen atmosphere, during welding had been present in the drum material.

High Temperature Rotors: Failure Mechanisms and Remnant Life Assessment

This paper presents the common failure mechanisms of high temperature rotors and the engineering approaches to their remnant life prediction. In fatigue crack growth at the rotor bore, cracks from original forging defects or induced during long service life may grow under cyclic loading to its critical size causing fast fracture. In fatigue-creep interaction at the shaft surface, high tensile residual stress relaxation under high operating temperature causes creep crack initiation. The cracks may then grow under the combination of cyclic loading and high operating temperature. Remnant creep life at the center of the rotor is based on the time while accumulated creep strain reaches its threshold level. Creep rupture could occur at other locations such as the outside surface of the shaft at disks∕shaft radii or blade fixings. Finite element analyses were carried out to analyze stresses, temperature transients, creep strain, and reference stress for creep rupture. Fracture mechanics analyses with R5 and R6 approaches were used to estimate the crack initiation and growth rates, the critical crack sizes, and the type of the failure. Appropriate Paris law and Norton creep laws were used for fatigue and creep crack growth. Depending on the failure mechanism, a rotor’s remnant life is defined in terms of allowable starts and operating hours.

A failure analysis and remnant life assessment of boiler evaporator tubes in two 250 MW boilers

It has been established from (a) the tube thermal flux calculations, (b) the failure surface fractographic features, (c) the data sheets on evaporator tube temperatures from both Mitsubishi and Seimens, (d) the reported data from the literature and (e) to a certain extent, the tube strain gauge data that the recent waterwall tube failures in two 250 MW boilers were the result of high stresses being active at certain boiler locations. At start-up these stresses were estimated from fractographic features to be around 370–420 MPa in magnitude, which resided between the tube materials yield and ultimate tensile strengths. The defect extension process in the present waterwall tube failures was shown to be fatigue crack growth, which is the most common failure mode in waterwall boiler tubes. By adopting two separate approaches, viz., an “upper bound” ASME XI design line and an influence diagram model approach which was designed specifically for waterwall tubes, detailed remnant life values could be estimated for a number of different situations. In the case of new replacement tubes, both approaches estimated a remnant life of about 9 years. Note that the failure probability of the upper bound ASME XI predictions was demonstrated to be around 10 −5. Also the influence diagram model approach was extremely helpful in identifying the probable root cause of corrosion fatigue failures in waterwall boiler tubes. With respect to the remnant life estimates two separate cases were considered, viz., (1) tubes with defect populations as recorded during the recent NDT study and (2) new replacement evaporator tubes which contained zero defects.

Advances in Residual Stress Modeling and Measurement for the Structural Integrity Assessment of Welded Thermal Power Plant

The safe operation of both thermal and nuclear power plant is increasingly dependent upon structural integrity assessment of pressure vessels and piping. Furthermore, structural failures most commonly occur at welds so the accurate design and remnant life assessment of welded plant is critical. The residual stress distribution assumed in defect assessments often has a deciding influence on the analysis outcome, and in the absence of accurate and reliable knowledge of the weld residual stresses, the design codes and procedures use assumptions that yield very conservative assessments that can severely limit the economic life of some plant. However, recent advances in both the modeling and measurement of residual stresses in welded structures and components open up the possibility of characterising weld residual stresses in operating plant using state-of–the–art fully validated Finite Element simulations. This paper describes research undertaken to predict residual stresses in stainless steel welds in order to provide validated reliable, accurate Structural Integrity assessment of nuclear power plant components

The structural integrity management of a deaerator feed water vessel approaching the end-of-life condition

The present paper describes the integrity management of a deaerator feedwater vessel over a 20-month service period within which it approached the end-of-life condition. A series of four remnant life assessments were carried out using both Crackwise and the R6 failure assessment programmes which determined critical crack sizes for vessel failure and a leak-before-break event. Interestingly, actual crack growth data obtained from this particular feedwater vessel over a 12 year service period was used in the sub-critical crack growth rate assessments. Remnant life values were reduced from only 25% of the normal guideline period of two years to re-inspect repaired weld cracks to around 12% or only three months within a 30-month service period. Also at each inspection a large amount of cracks, which numbered over one hundred, were found. From this it was clear that the vessel was approaching an end of life condition and the final assessment deemed that the vessel should be removed from service and all the problematic weld runs be replaced with new welds.

Application of Thermal Barrier Coatings on Combustion Chamber Liners – A Review.

Thermal barrier coatings (TBC) help to reduce the temperature of combustion chamber liner by 473-573K during operation on being aided by swirls of film cooling air. Versatility and low production cost make air plasma spray (APS) TBCs more attractive on liners. However spinal formation at the top to bond coat interface induces ceramic sintering rate and forms thermally grown oxide (TGO) growth. Lifing of engines is attempted to utilize existing design and remnant life within design constrains by giving emphasis on coating philosophies. A proper and efficient substitute with multimetallic bond coat and ceramic topcoat yields longer hours of exposure. Parallel removal of harmful elements in the liner material, restriction on the use of poor quality fuels, and atmospheric effects increase life. Studies of remnant life assessment have been found to be based on control of parameters that check TGO growth, increase adhesion of thicker TGO and restrict ceramic top coat sintering. For example, ceria or lanthana stabilized zirconia transform at comparatively higher temperature than yttria stabilized zirconia. The current scenarios of protection have been changed to replacements by continuous fiber ceramic composite (CFCC), and ceramic matrix composites (CMC) component; e.g. Sylramic, Nicalon, Naxtal, and Ceracurb. Non-destructive examination of ceramic translucence based on Parker's optical property produces in-situ information about ceramic degradations.

Cross-weld Creep Behavior and Life Prediction of Low Alloy Ferritic Steels

The reliability of elevated temperature components depends upon understanding the creep behavior of welds, which often determine the components' lives. The most likely manner of failures at high temperatures for the serviced welds fabricated from low alloy ferritic steels should be Type IV cracking. In the present work using 1.25Cr-0.5Mo and 2.25Cr-1Mo steel welds, however, the location and the morphology of the ultimate failures were influenced by a couple of factors, namely, the magnitude of stress, temperature, time to rupture, specimen geometry and inherent creep properties owned by each alloy. Thus, the consistent relationship between the feature of creep damage and the life consumption to be utilized for the remnant life assessment of welds has not been derived. As the alternative, the authors have examined the cross-weld creep behavior at the tertiary creep stage. Despite the variation of failure types, which were Type I, Type III, Type IV and ductile rupture at the parent material, the omega method predicted the rupture life with the accuracy of a factor of 2. Furthermore, it was found that the linear relationship between e and e appearing at the tertiary creep stage was also available in the life prediction. The slope of strain rate versus strain derived from the above relationship was correlated with the rupture life independently of failure types, specimen geometry and materials and the same level of accuracy as that of the omega method was obtained. From the usefulness of this technique and observation of grain boundary damage at the Intercritical HAZ (ICZ), the authors has interpreted that grains around creep cavities are still subjected to a significant level of stress, suggesting that apparent feature of grain boundary damage dose not necessarily exhibit the remaining creep strength of welds.

Creep Behavior of 2.25Cr-1Mo Steel Shield Metal Arc Weldment

In order to guarantee the safety of high temperature components and prevent unscheduled outage, remnant life assessment for creep is quite important. The experiences in actual components reveal that almost all the problems relating to creep are generated at the weldment, which contains microstructures with different creep properties from those presumed in the parent material. Nowadays, it is generally accepted that a terminal failure mode of low alloy ferritic steels or tempered martensitic steels become Type IV, taking place at the Intercritical HAZ or Fine Grained HAZ. However, in the present work using 2.25Cr-1Mo steel welds fabricated with basic coated Shield Metal Arc Weld (SMAW) consumables, the most prevalent failure mode was Type I in the center of weld metal rather than Type IV. And this tendency was more pronounced at low stresses, suggesting the possibility of Type I cracking in actual components. It was concluded that poor creep strength of 2.25Cr-1Mo weld metal was ascribed to its fully bainitic microstructure in which carbides evolution was accelerated and M 2 C depletion took place earlier than base metal.

The Effect of Grain Boundary Cavities on the Tertiary Creep Behavior and Rupture Life of 1.25Cr-0.5Mo Steel Welds

In general, the ultimate creep failure mode of welds fabricated from low alloy ferritic steels can be considered to be Type IV cracking resultant from coalescence of grain boundary damage. Thus, observation of grain boundary feature using a replication technique has been one of the most common practices for the remnant life assessment of welds operated at high temperatures. The experimental results in the present work and those found in previous works, however, prove that the feature of grain boundary damage highly depends upon testing conditions. Therefore, the relationship between the extent of damage and remaining life would not necessarily be effective for the practical use if it were obtained under the extremely accelerated laboratory conditions. In contrast, strain rate measurement predicted the rupture life with reasonable accuracy independently of the susceptibility to grain boundary cavitaion. The increase in the strain rate due to the presence of creep cavities was not observed in the experiments using heat treated specimens, which had the microstructure expected at the Intercritical HAZ in whole gauge length. This fact suggests the effectiveness of strain rate measurement for the life assessment and the necessity of reconsidering the physical meaning of grain boundary damage.