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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]

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]

Creep strength-enhanced ferritic steels, such as Grade 91, are the preferred material for much of the high-energy boiler tubing and piping components used in modern power generating plants. Weld repair techniques that achieve the necessary performance without the need for high-temperature post weld heat treatment (PWHT) offer particular benefits for Grade 91 steel. These benefits arise since there are many examples of poor heat treatment control which have resulted in component microstructures with below the minimum properties expected by design codes. Furthermore, even a controlled PWHT at temperatures at around 760 °C will further temper the base material. This is significant because excessive base metal tempering is one suggested criterion requiring component replacement. Successful demonstration of controlled welding techniques linked to minimal or no PWHT would alleviate these problems. This article presents results from a major project which is aimed at considering options for designing a 'well-engineered' repair. In this project, the creep performance of candidate repair methods was evaluated using large, feature test-type specimens containing the entire weldment including both fusion lines and heat-affected zones. The results show that the cross-weld life in Grade 91 steels does not appear to be a function of whether or not the welding procedures include PWHT. The results offer the potential to qualify 'cold' weld repairs in these steels. [ABSTRACT FROM AUTHOR]