NiCoCrAlYHf coating evolution through multiple refurbishment processing on a single crystal nickel superalloy

A combination of creep tests, ex-service blade samples, thermodynamic equilibrium calculations, combined thermodynamic and kinetic calculations, image analysis, chemical composition mapping and heat treatments have been conducted on PWA1483 to determine if microstructural rejuvenation can be achieved when taking the presence of oxidation coatings into account as part of a blade refurbishment strategy. The work has shown that the γˊ morphology changes during creep testing, and that through subsequent heat treatments the γˊ microstructure can be altered to achieve a similar γˊ size and distribution to the original creep test starting condition. Thermodynamic equilibrium calculations have been shown to be helpful in determining the optimum temperatures to be used for the refurbishment heat treatments. The interaction of oxidation resistant coatings with the alloy substrate and refurbishment process have been explored with both experimental measurements and coupled thermodynamic and kinetic calculations. The predictive nature of the coupled thermodynamic and kinetic calculations was evaluated against an ex-service blade sample which had undergone refurbishment and further ageing. In general there was good agreement between the experimental observations and model predictions, and the modelling indicated that there were limited differences expected as a result of two different refurbishment methodologies. However, on closer inspection, there were some discrepancies occurring near the interface location between the coating and the base alloy. This comparison with experimental data provided an opportunity to refine the compositional predictions as a result of both processing methodologies and longer term exposure. The improved model has also been used to consider multiple processing cycles on a sample, and to evaluate the coating degradation between component service intervals and the consequences of rejuvenation of the blade with repeated engine exposure. The results from the experimental work and modelling studies potentially offer an assessment tool when considering a component for refurbishment

Assessment of surface hardening effects from shot peening on a Ni-based alloy using electron backscatter diffraction techniques

An electron backscatter diffraction (EBSD)-based tool is described to assess the depth of strain-hardening effects of shot-peening treatments applied to the Ni-based superalloy, Udimet© alloy 720Li. The method consists of a statistical analysis of a number of data points from each grain scanned based on the grain orientation spread and their relative position from the shot-peened edge. The output is a quantitative measure of the depth of strain-hardening effects. The tool is used at various shot-peening intensities to demonstrate the ability to distinguish between these changes, using a range of intensities from 4 to 10 Almen. An increase in shot-peening intensity is observed to increase the depth of strain-hardening effects in the alloy. A comparison with residual stress measurements using X-ray diffraction for the same material shows that the strain-hardened depth determined by EBSD extends to approximately half the distance of the residual stress present due to shot peening. A comparison is also made with predicted profiles from the PeenstressSM model and subsequent microhardness testing. A positive correlation is observed between strained hardened depth and surface roughness of the peened samples. In each case, the increases in surface roughness and strain-hardened depth diminish toward the upper end of the shot-peening intensity range studied for this alloy

The effect of pre-service treatments on the long term properties of 9Cr steels strengthened by boron and nitrogen

Martensitic 9Cr steels have been developed which are strengthened by boron in order to stabilise the microstructure and improve their long-term creep strength. Boron plays a key role in these steels by stabilising the martensitic laths by decreasing the coarsening rate of M23C6 carbides, which act as pinning points in the microstructure. In this work two modified FB2 steel forgings are compared. Both forgings have similar compositions but one underwent an additional remelting process during manufacture. Creep tests showed that this additional processing step resulted in a significant increase in time to failure. In order to investigate the effect of the processing route on microstructural evolution during aging and creep, a range of advanced electron microscopy techniques have been used including ion beam induced secondary electron imaging and High Angle Annular Dark Field (HAADF) imaging in the Scanning Transmission Electron Microscope. These techniques have enabled the particle population characteristics of all the second phase particles (M23C6, Laves phase, BN and MX) to be quantified for materials from both forging processes. These quantitative data have enabled a better understanding of how the processing route affects the microstructural evolution of FB2 steels

Microstructural characterisation of creep tested 9CR welds for MarBN steel

Creep properties of 9Cr heat resistant steels can be improved by the addition of boron and nitrogen to produce martensitic boron-nitrogen strengthened steels (MarBN). The joining of this material is a crucial consideration in the material design since welds can introduce relatively weak points in the structural material. In the present study, creep tests of a number of MarBN weld filler metals have been carried out to determine the effect of chemistry on the creep life of weld metal. The creep life of the weld metals was analysed, and the evolution of creep damage was investigated. Significant differences in the rupture life during creep have been observed as a function of boron, nitrogen and molybdenum concentrations in the weld consumable composition. Although the creep lives differed, the particle size and number in the failed creep tested specimens were similar, which indicates that there is a possible critical point for MarBN weld filler metal creep failure

Comparison of the effects of a conventional heat treatment between cast and selective laser melted IN939 alloy

Additive manufacturing (AM) is a process where, as the name suggests, material is added during production, in contrast to techniques such as machining, where material is removed. With metals, AM processes involve localised melting of a powder or wire in specific locations to produce a part, layer by layer. AM techniques have recently been applied to the repair of gas turbine blades. These components are often produced from nickel-based superalloys, a group of materials which possess excellent mechanical properties at high temperatures. However, although the microstructural and mechanical property evolution during the high temperature exposure of conventionally produced superalloy materials is reasonably well understood, the effects of prolonged high temperature exposure on AM material are less well known. This research is concerned with the microstructures of components produced using AM techniques and an examination of the effect of subsequent high temperature exposures. In particular, the paper will focus on the differences between cast and SLM IN939 as a function of heat treatment and subsequent ageing, including differences in grain structure and precipitate size, distribution and morphology, quantified using advanced electron microscopy techniques

The use of combined three-dimensional electron backscatter diffraction and energy dispersive X-ray analysis to assess the characteristics of the gamma/gamma-prime microstructure in alloy 720Li (TM)

Multiple three-dimensional reconstructions of a γ/γ′ phase structure in Alloy 720Li have been carried out by employing a serial milling technique with simultaneous electron backscatter diffraction (EBSD) and energy dispersive x-ray (EDX) analysis data collection. Combining EBSD data with EDX is critical in obtaining maps to distinguish between the chemically differing, but crystallographically similar γ and γ′ phases present in the alloy studied. EDX is shown to allow the differentiation of γ and γ′ phases, with EBSD providing increased grain shape accuracy. The combination of data sources also allowed identification of coherent γ/γ′ phase interfaces that would not be identified using solely EBSD or EDX. The study identifies a region of grain banding within the alloy, which provides the basis for a three-dimensional comparison and discussion of γ′ phase size between coarse and fine grain regions, whilst also identifying coherent γ′ phase interfaces, possible only using both EDX and EBSD systems simultaneously. The majority of the γ′ phase lies in the range of 1–10 μm in non-banded regions, with a detectable particle size limit of 500 nm being established. The validity of the reconstruction has been demonstrated using an electron interaction volumes model, and an assessment of the validity of EBSD and EDX data sources is discussed showing γ′ phase connectivity in all dimensions

Formation of diffusion zones in coated Ni-Al-X ternary alloys and Ni-based superalloys

Coatings are an essential part of the materials system to protect the turbine blades from oxidation and corrosive attack during service. Inter-diffusion of alloying elements between a turbine blade substrate and their coatings is a potential concern for coated turbine blades at ever increasing operating temperatures because this can cause the formation of undesirable Secondary Reaction Zones (SRZs), which may degrade the mechanical properties of coated Ni-based superalloys. Understanding the effects of each element on the SRZ formation is essential in order to understand both the mechanism and inter-diffusion behaviour between coatings and substrates. In this research, a number of simpler aluminized ternary Ni-Al-X (where X is Co, Cr, Re, Ru or Ta) alloys were investigated in order to elucidate the separate effects of each element on the microstructural evolution, especially at the coating/substrate interface. The aluminized ternary alloys developed distinctive diffusion zones, depending on the third alloy element, ‘X’. Specifically, it has been found that both Ni-Al-Re and Ni-Al-Ta alloys developed a continuous SRZ-like diffusion layer. This diffusion zone persisted in the Ni-Al-Re alloys after high temperature exposure, indicating that Re has a stronger effect on SRZ formation than Ta

The effect of simulated post weld heat treatment temperature overshoot on microstructural evolution in P91 and P92 power plant steels

Creep strength enhanced ferritic (CSEF) steels, in particular modified 9Cr steels Grade 91 and 92, are becoming more widely used in the electrical power generation industry for the construction of header and steam piping in advanced coal-fired power plants. They typically enter service having received a standard high temperature normalizing treatment following by a lower temperature tempering treatment designed to produce an optimum microstructural condition. However, situations may arise in practice, particularly during welding operations for example, whereby the component may receive an additional heat treatment which briefly exceeds the Ac, and possibly the Ac , temperature before stabilizing at the tempering temperature. In this research, simulated post weld heat treatments (PWHT) have been applied to Grade 91 and 92 materials using carefully controlled heating and cooling rates within a dilatometer. Peak temperatures applied were below Ac, between Ac and Ac, and above Ac, prior to a subsequent heat treatment at 750°C for 2 hours. Hardness measurements demonstrated a significant reduction once the Ac temperature was exceeded. Advanced electron microscopy has been carried out to investigate the effect of the PWHT excursions on subsequent microstructural evolution. Electron back scatter diffraction has been used to quantify the nature of the martensite laths and grain structure changes as a function of temperature. The detailed size distribution of carbides within the microstructure has also been determined using both scanning and transmission electron microscopy. These results are discussed in respect of the likely consequences of such a PWHT overshoot on subsequent mechanical properties during high temperature service. Copyright © 2011 Electric Power Research Institute Distributed by ASM International®. All rights reserved

An investigation on oxidation/carburisation of 9Cr-1Mo steel heat exchanger tube in an AGR environment

9Cr-1Mo steels have been used extensively in the power generation industry. In this study, a wide range of experimental samples exposed at different times and temperatures in a CO2 environment were analysed to look at the development of the metal and oxides over time. The main objective of this work was to obtain a better understanding of the carburisation and oxidation behaviour of 9Cr 1Mo steels as a function of temperature/time, with special attention paid to the transition from protective to breakaway oxidation. In addition, experiments were also carried out to investigate any links between oxidation transition and carburisation behaviour of these materials

Factors contributing to heat affected zone damage in Grade 91 steel feature type cross-weld tests

Grade 91 steel has been widely utilized in power plants over the last 20 years. Its specification worldwide has dramatically increased since the acceptance of ASME Code Case 1943 for this material in 1983. Recent evaluation of a combination of ex-service Grade 91 steel components and virgin material has provided a unique opportunity to revisit commonly stated factors which contribute to damage in cross-weld creep tests. The approach adopted here is grounded in the fundamental objective of linking metallurgical risk factors in Grade 91 steel to the cross-weld creep performance. Establishing metallurgical risk factors in 9%Cr steels is regarded as a key consideration in the integration of a well-engineered life management strategy for these complex materials. In this study, two heats of ex-service Grade 91 steel which exhibit a similar response to the deformation resistance (i.e. strength) but dramatic differences in the susceptibility to damage (i.e. creep ductility) were evaluated in the welded condition using large, feature type cross-weld creep samples. Heat affected zone damage was investigated from both a macro-damage and micro-damage perspective. The macro-damage evaluation provided a comprehensive understanding of the global damage distribution through the heat affected zone (HAZ). The damage was linked to extensive hardness mapping and calculated peak temperatures through the HAZ. The microdamage characterization included a number of local observations for cavities in each sample using scanning electron microscope techniques. General observations were made regarding the shape and size of creep cavities and association of damage with microstructural features. These observations were linked to the as-fabricated microstructure and as characterized by electron backscatter diffraction (EBSD) and energy dispersive x-ray spectroscopy (EDS) mapping