Search

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]

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]

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]