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A significant portion of failures in grade 91-92 steels have been attributed to poor fabrication that results from a high production variability and repair/management activities such as post weld heat treatment. This can result in microstructures which are detrimental to creep and fatigue properties of the structure. The objective of this article is to use Rayleigh wave nonlinear ultrasonics to differentiate and classify the different microstructures. Eight different microstructures were chosen for testing with as-received, gradual and gross degradation in the microstructure. The nonlinear 2nd and 3rd harmonics were measured for each microstructural condition and used for analysis. The destructive analysis included hardness measurement and microscopy to get the average grain sizes. The hardness was used to calculate the dislocation density, which was further correlated with the 2nd and 3rd harmonic nonlinear parameters. The results suggest that the 3rd harmonic is better at classifying the microstructures compared to the 2nd harmonic or the linear Rayleigh wave velocity. Conjectures were developed destructive and nondestructive results. • NLU was used to classify eight different microstructures found in P91 steel. • Beta and delta parameters were correlated with hardness and grain size. • Multi-phase and precipitates contributions to NLU response is not well understood. • Conjectures were developed based on DE and NDE observations. [ABSTRACT FROM AUTHOR]

The exposure of structural materials to the environment in nuclear power plants leads to aging through thermal and/or irradiation effects. One of the consequences of this aging is the shift of the ductile to brittle transition temperature ( D B T T ) towards higher temperatures. Characterising the aging of structural materials is therefore a safety relevant topic especially in the context of long term operation of nuclear plants. For testing in service materials and to reduce the health risks and costs associated with handling of irradiated materials miniature testing techniques are being developed. The small punch (SP) technique is one of these methods. In a tensile/fracture SP test, a punch with a hemispherical tip is pushed with a constant displacement rate through the center of a small cylindrical disk specimen. A series of tensile SP tests on the ferritic/martensitic steel Gr. 91 at two different displacement rates show no impact of the displacement rate on the D B T T . A new method using fracture energies normalized to the maximum force for deriving the D B T T from force-deflection data is presented. Post-test analysis by means of X-ray computed tomography is used for deriving the D B T T from fracture strains. Both methods give consistent values for the transition temperature. However, the D B T T determined in this study is higher than what can be found in the literature. Supplementary tests indicate that the punch diameter has an effect on the SP defined D B T T . [ABSTRACT FROM AUTHOR]

Cr-Mo creep strength enhanced ferritic (CSEF) steels are mainly used in nuclear reactors and ultra-supercritical (USC) power plants for superheater tubes and header. The present research deals with the analysis of fracture surface of the tensile and impact tested specimen of cast and forged (C&F) modified 9Cr-1Mo (P91) steels, which are subjected to different heat treatment regimes. The heat treatment temperatures were 350 °C, 650 °C, 760 °C and 1000 °C, respectively. The heat treatment was carried out a particular temperature for 2 h duration. The fracture surface of tensile and impact tested specimen were also studied for a varying time duration from 2 h to 8 h for a fixed tempering temperature of 760 °C. The heat treatment effect on tensile properties, toughness, Vickers hardness and particle size was also studied. Heat treatment has a noticeable effect on mechanical properties of C&F Grade 91 (X10CrMoVNNB9-1) steel. Fracture morphology is strongly affected by the microstructure and presence of secondary phase particles. The fracture surface was analyzed by using the field-emission scanning electron microscope (FE-SEM). The fractured tensile sample mainly indicates the presence of transgranular ductile dimples and transgranular cleavage facets for heat treatment temperatures of 350 °C, 1000 °C and 1040 °C. The percentage of cleavage facets on the tensile fracture surface was found to decrease for sample heat treated at 760 °C. Less amount of ductile dimples was noticed on the fracture surface for the samples heat treated at 650 °C and 760 °C (furnace-cooled). The so-called ‘splitting’ fracture was noticed for the sample heated at 760 °C. The ‘splitting’ fracture becomes more pronounced with the increase in tempering duration from 2 h to 8 h. The sample heat treated for 1000 °C, mainly indicates the cleavage facets on the fracture surface. The fracture mode of impact tested specimen is more complex and shows both ductile dimple tearing and quasi-cleavage facets for heat treatment temperature of 650 °C, 760 °C and as-received condition. The impact failure zone of sample heat treated at 350 °C and 1000 °C indicates the presence of so-called ‘river pattern’ on the fracture surface. [ABSTRACT FROM AUTHOR]

The Small Punch (SP) technique is a miniature test used for characterizing irradiated materials or when a testing material is available only in small quantities. In this work Finite Element (FE) models are developed to support the parametric analysis of SP fuel cladding tube specimens in comparison to standard flat ones. FE analysis shows that there are practically no differences between circular and rectangular flat specimens. The tube specimen results in only slightly higher maximal force (F m ). However, F m is attained at significantly lower displacements. This is attributed to the curvature of the specimen. The friction linearly increases the F m and to a lesser extent the displacement at F m . FE analysis also shows that the yield stress and different hardening, while keeping ultimate tensile strength constant, practically do not affect the F m . Significant specimen deformation can be expected at already small (0.1 mm) puncher deflection, which could limit the applicability of the small punch test to ductile materials. Varying degree of clamping in the experimental procedure can cause large scatter in the yield stress estimates. In all cases good agreement between the simulation and experimental results was obtained, best with a friction coefficient of 0.2. [ABSTRACT FROM AUTHOR]

The small sample impression creep test method has recently been of interest, as it can give a good indication of expected creep rates in uniaxial creep testing with minimal use of material. The compressively loaded test has also been shown to provide accurate results under multi-step loading conditions for a low alloy steel (½Cr½Mo¼V) to further extract value from a single test specimen. The Electric Power Research Institute (EPRI) has conducted step tests (step temperature and step loading changes) on another low alloy steel (Grade 22), as well as a tempered martensitic 9 Cr steel (Grade 91). Results have shown that there may be potential problematic areas when conducting step-up and step-down steps in these materials. Additional posttest evaluations have shown that material effects, such as strain hardening and strain softening, may add additional complexities when comparing strain rates of multi-stepped loaded stain rates. Hardness testing on posttest impression creep specimens have confirmed strain softening of tempered martensitic Grade 91 and no observed effect for an ex-service Grade 22 alloy. These findings have shown that careful considerations must be made before using creep rates obtained from multi-stepped loaded tests in situ of single loaded tests.

Cross-weld creep testing conducted at 650 °C under 69 MPa stress has reproduced either the Type I or Type IV failure mode in Grade 91 welds, depending on the post-weld heat treatment procedures. Welds post-weld heat treated below the A C 1 temperature have ruptured in the Type VI failure mode, while welds heat-treated above the A C 1 temperature of the alloy have ruptured in the Type I failure mode. Heat-treatments at lower temperatures and shorter durations have produced a reduced creep rate. The accelerated short term stress-relaxation test has been conducted to obtain the input creep rates for a finite element model for the cross-weld creep testing. The model predicted secondary creep rates are in good agreement with the results from the conventional cross-weld creep tests. From the finite element model, the creep damage by cavitation is believed to start at regions where the first principal stress and stress triaxiality concentrate. [ABSTRACT FROM AUTHOR]

Four different case studies of premature failures of Grade 91 materials in Heat Recovery Steam Generators (HRSG) and power piping at combined cycle power plants throughout the world are described. The impact of plant design, the metallurgical aspects of the failures and results from finite element simulations are incorporated. The failure modes discussed are brittle, creep failure, creep and creep-fatigue (CF) failures. All of the investigated cases had life calculations showing a creep life greater than the intended service life of the components. However, since all cases suffered pre-mature failures, it can be concluded that sufficient design life using design temperature and hoop stress is not a guarantee for safe design. The failures can be accredited to improper material properties associated with deficient fabrication and Post Weld Heat Treatment, increased service loading due to geometrical effects not included in design calculations and increases in local temperatures due to gas turbine (GT) load changes. Recommendations to mitigate the risk of component failures are presented. [ABSTRACT FROM AUTHOR]

Abstract: In Creep Strength Enhanced Ferritic steels control of both composition and heat treatment of the parent steel is necessary to avoid producing components which have properties below the minimum expected by applicable codes. The degree of tempering involved in manufacture will modify the material hardness. While under most conditions hardness is reduced by tempering, exceeding the AC1 temperature can lead to an increase in hardness. In this heat treatment the properties will be relatively poor even though the measured hardness may be apparently acceptable. Thus, care should be exercised in imposing an acceptance test of components based on simple hardness alone. Differences in parent material heat treatment and composition apparently have remarkably little influence on the creep life of the heat affect zone (HAZ). Thus, Type IV cracking in the fine grained or intercritically heat treated regions of the HAZ does not appear to directly depend on the strength of the base steel. This form of in-service damage is relatively difficult to detect using traditional methods of non-destructive testing. Moreover, since repeated heat treatment leads to over tempering and a degradation of properties, specific procedures for making and then lifing repair welds are required. The present paper summarizes examples of damage and discusses best option repairs. [Copyright &y& Elsevier]