D.J. Stephenson

Particle-surface interactions during the erosion of a gas turbine material (MarM002) by pyrolytic carbon particles

Abstract A single-impact technique was used to study particle-surf ace interactions during the erosion of a typical turbine blade material, MarM002, by pyrolytic carbon particles. It was shown that carbon particles as small as 50 μn can cause severe erosive damage and that the predominant mode of material loss is by the removal of surface oxide. The erosive response of MarM002 was considered at 700, 750, 850 and 950 °C and is shown to be a function of the temperature and the oxide thickness, with lower temperatures and thicker oxide scales favouring brittle erosion behaviour. This behaviour can account for the reported increase in corrosion of superalloys at lower temperatures, provided that an erosive component is present in the system, and suggests that optimum conditions for erosion-corrosion resistance are established at a temperature in the region of 850 °C for the conditions under investigation.

The sliding wear behaviour of reactively hot pressed nickel aluminides

Abstract The sliding wear behaviour of reactively hot pressed nickel aluminides has been studied. The influence of load and intermetallic stoichiometry on wear rate has been considered using a block-on-ring test method with 440C as the counterface material. The results shows that wear rate increases linearly with load and decreases as the nickel content increases. Stoichiometric NiAl exhibits wear rates similar to 440C, with material removal proceeding by the formation of sub-surface voids and cracks and the generation of plate-like wear debris typical of a delamination mechanism. The higher nickel containing materials such as Ni-40% Al, undergo severe deformation with the formation of an ultra-fine grained sub-surface region. The hardness of this region exceeds 750 Hv. Metal transfer from the 440C counterface is followed by oxidation of the transferred film to form a FeO rich layer resulting in mild wear and an order of magnitude reduction in wear rate compared with NiAl and 440C.

Particle-surface interactions during the erosion of aluminide-coated MarM002

Abstract Particle-surface interactions during the erosion of a nickel aluminide coating were assessed using a single-impact technique. It is shown that the erosive response is a function of the surface scale thickness and the temperature, with the temperature not only influencing the surface scale plasticity but also determining the contribution of the coating substrate to the impact process. In this respect the ductile-to-brittle transition temperature of the coating is of particular importance. Under a wide range of conditions typical of those found in gas turbines the erosion of aluminide coatings is shown to be controlled by the formation and removal of surface scales. This implies that the use of aluminide coatings will increase the erosion resistance of typical turbine blade materials because of the superior oxidation and corrosion resistance of this coating. This increase in erosion resistance will be particularly significant at higher operating temperatures, above 900 °C.

Modelling the influence of surface oxidation on high temperature erosion

Abstract The interaction between a growing surface oxide and impacting particles is of importance in many high temperature applications such as gas turbines and combined cycle power generation systems and may often result in accelerated rates of metal wastage. In this paper the interaction between a growing oxide film and an impacting particle is considered. The impact conditions are analysed to predict the impact damage morphology and the importance of parameters such as oxidation rate, oxide mechanical properties, impact velocity and particle size and distribution, highlighted. The analysis has been used to extend the range of application of predictive erosion models using a Monte Carlo simulation approach. The modelling has been used to produce high temperature erosion mechanism maps which define the conditions over which a particular material removal mechanism will predominate. For many of the high temperature erosion conditions encountered in practice, this modelling approach suggests that several material removal mechanisms may co-exist locally across a surface and it is therefore essential that these factors are included when life-prediction models are developed.

Monte Carlo modelling of erosion processes

Abstract Many models are available in the literature that describe ductile and brittle erosion processes and the influence of scale formation on the erosion process. It is clear that each of these models has limited applicability restricted by the mode of material removal being modelled. Hence, if erosion processes over a wide range of dynamic conditions expected in service are to be modelled then a new approach unifying the individual mechanistic models must be adopted. This paper presents such a unified approach. Monte Carlo simulation techniques are used to model the stochastic nature of erosion processes. Particle properties, material surface condition and the local dynamic impact environment are individually considered and permit a suitable mechanistic erosion model to be selected for the particular impact conditions. Variation in particle size, impact velocity and material properties are accommodated by using statistical distributions to describe each condition. Using Monte Carlo methods discrete impact conditions are selected and the amount of damage is calculated. The final erosion rate is given by summing all of the discrete damage events. Predictions using this modelling approach are compared with examples taken from laboratory studies aimed at simulating gas turbine service and coal combined cycle environments are used to demonstrate the versatility of this Monte Carlo approach.

Investigation of solid particle erosion in components of complex geometry

Many of the factors which control the rate of erosion, such as particle velocity, number of particles impacting a surface and their angle of impingement can be largely determined by the flow conditions of the system. In fact, many practical examples may be found when a change in the flow conditions has greatly increased or decreased erosion. In general where the flow direction changes rapidly (turbine blades, valves, pipe bends, etc.), erosion is usually considerably more severe than in straight pipes, though it has also been reported that local turbulence due to a roughened surface or misalignment can increase the rate of erosion damage. This paper presents experimental data on the dynamic behaviour of solid particles entrained within a gas phase in components of complex geometry. Flow conditions and local impact dynamics are quantified in order to determine areas susceptible to erosion and the probable metal loss rates. A combination of experimental techniques has been developed in order to pursue this goal. This includes a novel multi-layer paint erosion indication technique used to generate a three dimensional map of erosion damage, flow and particle visualisation, computational fluid dynamics (CFD) and metallic component erosion validation experiments. Results from the study of typical well head geometries used for oil and gas production are considered, and the benefits of using a range of complimentary techniques to study the solid particle erosion process are highlighted.

Erosion of material used in petroleum production

Abstract Erosion-corrosion arising from sand production is increasingly recognised as a significant problem in petroleum production. When erosion and corrosion interact, they do so in such a complex manner that it is difficult to determine the rate of metal loss with sufficient accuracy for reliable prediction of equipment lifetimes. An experimental programme was carried out to study the interaction between the erosion and corrosion under typical petroleum production conditions. A C-Mn steel has been exposed to environments simulating wet and dry CO 2 conditions. Erosion has been simulated by the introduction of sand particles (50–300 μm) and the influence of impact angle, velocity, particle loading and temperature has been investigated. The results demonstrated that for C-Mn steels there is a significant interaction between erosion and corrosion with the rate of metal loss from pure corrosion to erosion/corrosion increasing by 2 orders of magnitude. The use of wet CO 2 increases the rate of metal loss by factor of 2–4. It has been shown that the metal recession rate at low velocity is dominated by the formation and removal of surface corrosion products.

Paint layer erosion resistance behaviour for use in a multilayer paint erosion indication technique

Abstract A multilayer paint erosion indication technique to produce a highly visual and accelerated map of erosion damage occurring in a three-dimensional component model has been developed. An investigation to obtain an understanding of how the paint layers eroded as a function of a number of erosion variables was performed. It was observed that the erosion rate behaviour of paint layers as a function of angle of particle impact, velocity, time and particle loading was in good agreement with previously reported material erosion behaviour of steels and other engineering materials. These results indicate that it is possible to use the multilayer paint erosion indication technique to provide a highly visual representation of erosion damage to complex component geometries. Such baseline information on the paint layer erosion behaviour should provide an opportunity to relate erosion data from geometry based erosion maps, such as those shown in Fig. 1, to realistic engineering situations.

Modelling low stress abrasive wear

Abstract Low stress abrasive wear tests have been undertaken using a modified ASTM G65 procedure. Wear scar and wear debris morphologies indicate that the principal mechanism of material removal is throuhg the sliding of abrasive particles which form shallow grooves, from which thin platelets of material are removed. It is shown that there is a correlation between the abrasive particle size, wear debris size and wear groove size distributions. From a knowledge of the particle flux, the particle size distribution and the loading conditions, metal recession rates have been predicted based on a low cycle fatigue model. Wear rates for a wide range of materials and test conditions have been predicted to better than a factor of two. The proposed approach to modelling low stress abrasive wear should be appropriate when the degree of penetration of the abrasive is very low, well within the ploughing regime. In the present work, the degree of penetration was less than 0.1.

Erosion-corrosion modelling of gas turbine materials for coal-fired combined cycle power generation

Abstract The development of coal-fired combined cycle power generation systems is receiving considerable worldwide interest. The successful development and commercialisation of these new systems require that all the component parts are manufactured from appropriate materials and that these materials give predictable in-service performance. Corrosion and erosion-corrosion, resulting from coal derived particulates, deposition and gaseous species, have been identified as potential life limiting factors for these systems. Models to predict these modes of materials degradation are under active development. This paper outlines the development and testing of models suitable for use in gas turbine environments. The complexity of the corrosion processes means that an empirical approach to model development is required whereas a more mechanistic approach can be applied to erosion processes. For hot corrosion conditions, statistically based corrosion models have been produced using laboratory tests for two coatings and a base alloy at typical type I and type II hot corrosion temperatures (900 and 700°C). These models use the parameters of alkali sulphate deposition flux and SO x partial pressure (at each temperature and for set HCl partial pressures), to predict the rate of the most likely localised damage associated with hot corrosion reactions. For erosion-corrosion modelling, a series of laboratory tests have been carried out to investigate erosion behaviour in corrosive conditions appropriate to coal-fired gas turbines. Materials performance data have been obtained from samples located in the hot gas path of the Grimethorpe PFBC pilot plant, under well characterised conditions, for testing the corrosion and erosion-corrosion models. The models successfully predict the materials damage observed in the pilot plant environments.