Creep Damage Assessment Research Articles

Mechanical properties and non-destructive evaluation of chromium–molybdenum ferritic steels for steam generator application

The paper presents high temperature mechanical properties and non-destructive evaluation (NDE) of chromium–molybdenum (Cr–Mo) ferritic steels widely used as structural materials for steam generator (SG) applications. Creep and low cycle fatigue deformation and damage (the important design considerations for SGs) of ferritic steels are presented. Recent trend and advances in ferritic steels for SG applications are discussed. The paper also highlights the recent results obtained on creep and fatigue properties of indigenously developed steels. Weld and weld joints of ferritic steels are known to be the weak links affecting the life of the components. The creep properties of weld joints are presented. Apart from mechanical behaviour of ferritic steels, the paper deals with some of the recent results on the characterisation of microstructural features such as grain size, nucleation and growth of secondary phases, degradation of microstructure due to thermal ageing and creep, and assessment of creep and fatigue damage of ferritic steels using advanced NDE techniques.

228 SiC 粒子分散強化 A6061 合金のクリープ損傷評価

Creep damage of SiC particle reinforced A6061 aluminum alloy, with various volume fraction of particle SiCp/A6061 composite, has been studied. Unloading modulus was measured from the unloading stress-strain curve at arbitrary creep testing time, during creep test for the qualitative assessment of creep damage. Densimetric change of the composite was also investigated and found to be associated with microstructural aspect of debonding at matrix/particle interfaces and cracks within SiC particles. The unloading modulus decreased with increasing creep time and with decreasing change of density of the composite. Damage parameter was found to have good relative correlations to unloading modulus and creep strain. The damage parameter derived from unloading modulus was formulated by a unique equation as a function of creep strain independent of the volume fraction of SiC particle. It has been concluded that unloading modulus was applicable to quantitative assessment of the creep damage of SiCp/A6061 composite.

Estimation of Creep Damage for the Components of Mod. 9Cr-1Mo Steel. 3rd Report, Creep Damage Assessment for Seam Weld in Internally Pressurized Pipe.

In order to establish the assessment method of creep damage for high temperature piping components made of modified 9Cr-1Mo steel with seam weld, subjected to internal pressure, the internal pressure creep tests and FEM stress analysis were done for cylinders with seam weld. In internal pressure creep tests, the creep rupture occurred at fine grained heat affected zone (F. G. HAZ), caused by cracking originated from the formation of voids. The correlation between void area fraction and creep damage was obtained. The morphology of voids were quite similar to that observed in welded joint specimens in uni-axial creep tests. Stress distribution of seam weld cylinder under internal pressure creep showed the higher circumferential stresses in F. G. HAZ where creep rupture occurred. The distribution of circumferential stress and creep damage calculated by using circumferential stress along specimen thickness were quite similar to that of void area fraction in internal pressure creep test. The creep damage in seam weld pipe under internal pressure creep could be estimated by the observation of void area fraction in F. G. HAZ.

Estimation of Creep Damage for the Components of Mod. 9Cr-1Mo Steel. 4th Report. Creep Damage Assessment for Circular Seam Welded Joint in Internally Pressurized Pipe.

In order to establish the assessment method of creep damage for high temperature piping components made of mod. 9Cr-1Mo steel with circular seam welded joint, the internal pressure creep tests and FEM stress analysis were done. Internal pressure creep tests showed the creep rupture occurred in the coarse grained heat affected zone (C. G. HAZ), caused by cracking originated from the formation of voids on grain boundaries. The void parameters such as void area fraction, density of void population and A parameter correlated well with time fraction in internal pressure creep tests. Vickers hardness decreased with creep times and correlated well also with time fraction of internal pressure creep tests. Stress distribution of cylinder with circular seam welded joint under internal pressure creep showed the maximum circumferential stress in C. G. HAZ where creep rupture occurred. The distribution of circumferential stress and creep damage calculated by using circumferential stress along specimen thickness were quite similar to that of void area fraction. The creep damage of circular seam welded joint under internal pressure creep could be estimated by the observation of void and the FEM stress analysis.

Statistical and probabilistic modelling of uniaxial creep life of a Ti-6246 alloy

Repeat data on a Ti-6246 alloy at 773 K and 580 MPa provided a unique opportunity to identify stochastic creep properties and to use this information to build a probabilistic creep damage assessment for this alloy. The stochastic nature of creep properties both under a single test condition (using a generalised gamma distribution) and under various test conditions (by combining this distribution with the Monkman–Grant relationship) were identified. Under the single test condition log failure times had a distribution skewed to the right and log minimum creep rates had a distribution skewed to the left. Over various test conditions a failure time distribution similar to the Weibull distribution was most supported by the data. The observed log time to failure at 480 MPa was well within the extrapolated 95% confidence limits obtained from the Monkman–Grant model. Uncertainty about the exact distributional form increased this 95% confidence limit.

Standardization of Surface Replication Procedures for Life Assessment of High Temperature Facilities

Surface replication is playing an important role in the assessment of creep damage and remaining life of high temperature components. As the replication procedures, however, have not been standardized in domestic industry, its standardization is proposed in this study. For this purpose, the 2.25Cr-IMo steel was heat treated(5 min at 1,300 0C and oil quenched) to produce a simulated HAZ microstructure, and crept in air at 575 0C and under 120 MPa to produce artificial cavities. Then, the effect of surface preparation procedures on the quality of replicas was investigated using this sample. As a result, it was demonstrated that the presence of cavities may be observed readily or missed depending on the surface preparation procedures followed. Therefore it is essential to repeat three polishing/etching cycles at least in order to reveal cavitation damage accurately, even though it may be tedious or time-consuming.

Estimation of Creep Damage for the Components of Mod. 9 Cr-1 Mo Steel. 1st Report. Creep Damage Assessment for Base Metal by Means of Hardness Measurement.

The correlation between creep damage and hardness was obtained for the modified 9 Cr-1 Mo steel specimens crept in laboratory. The hardness became softer as the creep damage increased. The theoretical formulation of softening was done based on the kinetics of the dislocation structure. The formula gave quantitative correlation among the creep conditions of stress, temperature, creep time and Vickers hasdness. The effect of the creep conditions and initial hardness of the material on the creep damage vs. hardness correlation was analyzed, and a practical creep damage curve was proposed which should be used for the creep damage assessment in actual and aged components made of the modified 9 Cr-1 Mo steel.

Estimation of Creep Damage for the Components of Mod. 9Cr-1Mo Steel. 2nd Report. Creep Damage Assessment for Welded Joint by Means of Void Observation.

The voids were observed in the fine grained heat affected zone of the creep damaged modified 9Cr-1 Mo steel welded joint. And, the correlation between creep damage and area fraction of void was obtained. The area fraction of void increased as the creep damage increased depending on the creep conditions. The theoretical formula of the correlation among the area fraction of void, creep damage and creep conditions was derived based on the creep deformation kinetics. The distribution of voids was not uniform along the radial axis of the round bar creep specimen. The stress state under creep was analyzed by FEM and the effect on the distribution of voids was investigated. In the fine grained heat sffected zone, the tri-axial stress state was realized under the uni-axial creep test and it was shown that the creep damage by axial stress played an important role on the distribution of voids.

A Metallographic Technique for High Temperature Creep Damage Assessment in Single Crystal Alloys

The use of single crystal (SX) nickel-base superalloys will increase in the future with the introduction of SX blades into large gas turbines for base-load electricity production. Prolonged periods of use at high temperatures may cause creep deformation and the assessment of damage can give large financial savings. A number of techniques can be applied for life assessment, e.g., calculations based on operational data, nondestructive testing or material interrogation, but because of the uncertainties involved the techniques are often used in combination. This paper describes a material interrogation (metallographic) technique for creep strain assessment in SX alloys. Creep tests have been performed at 950°C on the SX alloy CMSX-4 and quantitative microstructural studies performed on specimens interrupted at various levels of strain. It was found that the strengthening γ′-particles, initially cuboidal in shape, coalesced to form large plates or rafts normal to the applied stress. The γ-matrix phase also formed plates. CMSX-4 contains ∼70 vol % γ-particles and after creep deformation the microstructure turned itself inside out, i.e., the gamma “matrix” became the isolated phase surrounded by the γ′-“particles.” This can cause problems for computerized image analysis, which in this case, were overcome with the choice of a suitable measurement parameter. The rafts reached their maximum length before 2 percent strain, but continued to thicken with increasing strain. Although of different dimensions, the aspect ratios (length/thickness ratio) of the gamma-prime rafts and the gamma plates were similar at similar levels of strain, increasing from ∼1 at zero strain to a maximum of ∼ 3 at about 1–2 percent strain. Analysis of microstructural measurements from rafting studies on SX alloys presented in the literature showed that the aspect ratios of the γ and γ′-phases were similar and that at a temperature of 950–1000°C a maximum length/thickness ratio of about 2.5–3.5 is reached at 1 to 2 percent creep strain. Measurement of gamma-prime raft or (or gamma plate) dimensions on longitudinal sections of blades is thus a suitable method for high temperature creep damage assessment of SX alloys. This gives a considerable advantage over conventional Ni-base superalloys whose microstructures are usually very stable with respect to increasing creep strain.

Creep damage assessment and remaining life evaluation

The approach of Kachanov and Rabotnov to the assessment of damage has been related to the creep of components at elevated temperature. The relation between the damage function and void formation is discussed, together with recent developments in which remaining life can be estimated using available materials constants and strain rate measurements.

Cryo-Cracking: a technique for creep damage assessment in high temperature components

The characterisation of fracture surfaces of service exposed SA welds created under cryogenic conditions leads to a new fractographic analysis method called “Cryo-Cracking”. The “Cryo-Cracking” method takes advantage of an ability to separate the previous (service) creep damage from the “Cryo- Cracking” induced fracture features. This allows the evaluation of cavities and service damaged grain boundaries and permits a quantitative definition of damage level; aided by the SEM's high resolution and enhanced depth of field. In addition, directed EDS can provide chemical information regarding cavitation and the particulates which nucleate cavities. The “Cryo-Cracking” method does not depend on personal metallographic skills and is thus considered less sensitive to misinterpretation due to specimen preparation techniques.

Stress relaxation properties of alloy HP50Nb and their application to life assessment of reformer furnace tubing

Traditional life assessment techniques based on an inverse design approach do not explain the premature failures frequently encountered in reformer tube applications. This is because no account is taken of the through wall stresses generated by the thermal gradient. In order to account for the thermal stresses in reformer tubes, the stress relaxation properties of alloy HP50Nb in both as-cast and in-service aged conditions have been determined for input to a ductility exhaustion model for assessment of creep damage due to thermal cycling. The ductility exhaustion model proposed is that used in the Nuclear Electric R5 high temperature assessment procedures. The model predicts that thermal cycling contributes significant levels of damage. While variations in stress relaxation properties are significant, the overwhelming influence on predicted life is material ductility.

Assessment of creep damage of ferromagnetic material using magnetic inspection

Results of inspection creep damage by magnetic hysteresis measurements on Cr-Mo steel are presented. It is shown that structure sensitive parameters such as coercivity, remanence and hysteresis loss are sensitive to the creep damage. Previous metallographic studies have shown that creep changes the microstructure of the material by introducing voids, dislocations, and grain boundary cavities. As cavities develop, dislocations and voids move out to the grain boundaries; therefore the total pinning sources for domain wall motion are reduced. This, together with the introduction of demagnetization field due to the cavities, results in the decrease of both coercivity and remanence. Numerical computations with a modified Jiles-Atherton model are presented which are consistent with the proposed mechanisms.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Assessment of creep damage in austenitic steel X-8-CrNiMoNb-16-16 at elevated temperature

A phenomenological model of creep deformation is developed by introduction of a function that represents intrinsic creep resistance to damage. The Lévy-Mises relation in plasticity is modified in deriving the time dependent creep strain and/or strain rate expression. Results correlated well with the experimental creep data of the X-8-CrNiMoNb-16-16 austenitic steel.

Multiaxial Relaxation and Creep Damage Assessments—A Comparison of ASME N-47 and GLOSS Time-Scaling Methods

Two simplified methods for determining multiaxial relaxation and creep damage, the GLOSS time-scaling and the ASME Code Case N-47 method, are described in this paper. The theoretical basis and applicability of the methods to various pressure component configurations are discussed in detail. The G-factor in the ASME formula in effect speeds up or slows down the uniaxial relaxation process, similar to the GLOSS time-scaling method. The two methods are compared, and the differences are attributed to local follow-up not being accounted for in the ASME formula.

Observations of the ripple-loading effect: Horstmann, M. and Gregory, J.K. Scr. Metall. Mater.25 11 (Nov 1991) 2503–2506

This paper is devoted to parametric study on creep-fatigue endurance of the steel type 316N(L) weldments at 550 °C identified as type 3 according to R5 Vol. 2/3 procedure classification. The study is implemented using a direct method known as the Linear Matching Method (LMM) and based upon the creep-fatigue evaluation procedure considering time fraction rule for creep-damage assessment. Seven configurations of the weldment, characterised by particular values of a geometrical parameter ρ, are proposed. Parameter ρ, which represents different grades of TIG dressing, is a ratio between the radius of the fillet of the remelted metal on a weld toe and the thickness of welded plates. For each configuration, the total number of cycles to failure N☆ in creep-fatigue conditions is assessed numerically for different loading cases defined by normalised bending moment M∼ and dwell period Δt. The obtained set of N☆ is extrapolated by the analytic function dependent on M∼,Δt and parameter ρ. Proposed function for N☆ shows good agreement with numerical results obtained by the LMM. Therefore, it is used for the identification of Fatigue Strength Reduction Factors (FSRFs) effected by creep, which are intended for design purposes, and dependent on Δt and ρ.

Creep damage assessment of austenitic steel specimen subjected to time varying load

Developed in this work is a spring/dashpot model that characterizes the creep behavior of metal under time varying loads. The characteristics parameters are assumed to be time dependent as intrinsic to the material response process. One of them is defined as the average internal resistance of the material based upon which predictions on lifetime could be made. An application is made for the austenitic steel X-8-CrNiMoNb-16-16 at 700°C under cyclic loading. Lifetime predictions are made and compared with those found from the Robinson's linear rule. The results are in good agreement with experimental data.

Effects of metallographic preparation procedures on creep damage assessment

Specimens taken from a ruptured CrMo steel steam tube and a cast austenitic reformer tube exposed at high temperature for extended time have been examined for creep damage, using mechanical polishing procedures. It is demonstrated that the presence of damage may be observed readily or missed depending on the surface preparation procedure followed. The nature of the observed damage is discussed together with the ways in which the surface preparation procedures affect its disclosure. The implications of metallographic preparation procedures on the assessment of creep damage and remaining life estimation are reviewed, and a procedure to ensure reliable identification of such damage is recommended.

A Ductility Exhaustion Approach for Axial Fatigue—Creep Damage Assessment Using Type 316 Stainless Steel

A Ductility Exhaustion Approach for Axial Fatigue—Creep Damage Assessment Using Type 316 Stainless Steel

Assessment of Creep Damage by NDT

AbstractResidual lifetime analysis of power plant components requires information on the degree of degradation of the material. In case of hightemperature creep, material damage can be related to cavity formation or to the accumulated creep strain. At present, only metallographic replication technique is widely used for in-service inspection to detect creep cavitation. Other NDT-techniques like ultrasonic velocity and electrical resistivity, which have potential for detection of low pore concentrations, are being developed for practical application. The use of capacitive strain gages yields encouraging results for the long-time monitoring of creep deformation. Potential and limits of these NDT-techniques together with industrial needs for creep damage assessment are reviewed.