Pamela Henderson

On rafting in a single crystal nickel-base superalloy after high and low temperature creep

Rafting (also known as directional coarsening) was first studied more than 25 years ago in SX Udimet 700. At intermediate temperatures, 700--800 C, no microstructural changes were seen during the creep of SX alloys in tests which lasted less than a few thousand hours. In a study of CMSX-4 crept at 750 C the immediate area of the fracture contained many cracks, but away from the fracture the microstructure looked identical to that of the uncrept material. It was not possible to see a change in the material with scanning electron microscopy. The aim of the work presented here was to find a way of imaging low and intermediate temperature creep damage using simple techniques and equipment readily available in most laboratories. This area is one of practical importance as SX alloys are being introduced into industrial gas turbines for power generation and principles for condition assessment need to be developed which are relevant to the temperature of usage. As a first step towards finding a solution it was necessary to study the conditions under which rafting occurred in other SX alloys and a brief summary of some important findings is given.

Growth of pores during the creep of a single crystal nickel-base superalloy

The use of single crystal (SX) nickel-base superalloys is set to increase in the future with the introduction of SX blades into land-based gas turbines for power generation. Cavities are pre-existing in SX alloys as interdendritic casting porosity, from which cracks nucleate and grow during the later stages of creep. It is generally assumed that no new cavities nucleate during creep. In this respect, cavities in SX alloys have been considered uninteresting and there have been no quantitative studies of cavities during the creep of SX alloys. However, cavities can be easily studied by optical microscopy, which could be readily developed into a remaining life assessment technique should the results prove interesting. This was the motivation for the work presented here.

Effect of sulphur containing additive on initial corrosion of superheater tubes in waste fired boiler

Abstract The major drawback to generating electricity from waste fired boilers is the rapid corrosion of superheaters which increases the maintenance costs. Within the last few years, it has been shown that additions of ammonium sulphate to biomass fired boilers decrease the corrosion tendencies. This paper reports on the effects of ammonium sulphate on corrosion in a waste fired CFB boiler. Air cooled probes were exposed at a position corresponding to the one of superheater tubes. The probe temperature was 500°C, corresponding to a steam temperature of ∼450°C. Both the austenitic steel EN1·4301 (Fe–18Cr–9Ni) and the low alloyed ferritic steel EN1·7380 (Fe–2·25Cr–1Mo) were tested. During exposure, the concentration of alkali chlorides in the flue gas was measured and a decrease was observed when adding ammonium sulphate. After 4 h of exposure, the probes were removed for detailed analysis with SEM-EDS, TOF-SIMS and XRD. The sides of the tubes facing the flue gas were covered with a calcium rich deposit, while relatively more sodium and potassium were present on the lee side. The results also show that ammonium sulphate shifted the deposit composition from chloride rich and highly corrosive, to one significantly less corrosive and dominated by sulphates of sodium, potassium and calcium. Metallography shows a marked difference in corrosion attack between the two steels. Iron chlorides accumulate at the metal/oxide interface of the ferritic steel, while the amounts of iron chlorides were significantly lower in the austenitic steel. These results indicate that ammonium sulphate has the potential to reduce corrosion in waste fired boilers and that austenitic stainless steels are more likely to resist corrosion in these environments than low alloyed ferritic steels.

High temperature creep in a 〈001〉 single crystal nickel-base superalloy

Creep data at 982 C and 950 C have been presented for the single crystal alloy CMSX-4. At these temperatures the creep rate is initially very low and 50% of the creep life is consumed in reaching {approximately}1% strain. Some steady-state creep occurred at 950 C. Low strains are important; plots of log, strain rate versus log, strain give more weight to low strains and also showed that tertiary creep is strain controlled. A linear strain softening approach described the increase in creep rate reasonably well. Creep curves, where time had been normalized with respect to the rupture life, could be superimposed at a given temperature, in spite of widely differing rupture lives. Normalized creep curves can only be superimposed if primary creep is negligible, but could be used as a means of estimating the rupture life of a long term test after a few millistrains have been reached.

The use of strain range partitioning in thermo-mechanical fatigue

The strain range partitioning (SRP) method was developed as an aid to fatigue lifetime prediction and works on the assumption that inelastic strain can be divided into two different types of strain, viz. plasticity and creep. The effects of creep depend upon where in the cycle it occurs and whether it is reversed by plasticity or creep. In practice the SRP method is seldom used because of the large amount of experimental data required to determine the basic relationships and methods which can be applied directly to a fatigue loop with a hold-time (for example, with the incorporation of a frequency factor) are more often preferred. However, the SRP method is well suited to describing types of strain occurring in fatigue at varying temperatures as shown by the examples of bithermal fatigue.

Corrosion of HR3C heat exchanger alloy in a biomass fired PF utility boiler

Detailed microscopic examinations have been conducted on two, temperature-regulated probes (commercial HR3C heat exchanger alloy) after being exposed to biomass flue gas inside a PF boiler for 3770 h at 600°C and 650°C respectively. Corrosion of the tube proceeds via scale formation and internal element depletion. Three characteristic types of internal corrosion have been identified depending on their position relative to the flue gas passage and deposit/flue gas chemistry. Severe, mainly internal corrosion occurs at down-stream locations where higher potassium chloride content exists within the deposit. Corrosion mechanisms corresponding to each type of internal corrosion have been proposed based on further laboratory tests and thermodynamic analysis. The increased temperature (650°C) causes slightly higher material wastage for the alloy. Korrosion der Warmetauscherlegierung HR3C in einem mit Biomasse gefeuerten Versorgungskessel Detaillierte mikroskopische Untersuchungen wurden an zwei, temperatur-regulierten Proben (kommerzielle Warmetauscherlegierung HR3C) nach 3770 h Auslagerung in Biomassenabgas in einem PF-Kessel bei 600 bzw. 650°C durchgefuhrt. Die Korrosion des Rohres erfolgt uber Zunderbildung und innere Elementverarmung. In Abhangigkeit von der relativen Position zum Abgasdurchtritt und von der Ablagerungs-/Abgas-Chemie wurden drei Typen von innerer Korrosion identifiziert. Starke, im wesentlichen innere Korrosion tritt an den stromabwartsgerichteten Stellen auf, wo hohe Kaliumchloridgehalte in den Ablagerungen vorhanden sind. Aufbauend auf weiteren Laborversuchen sowie thermodynamischer Analyse sind entsprechend den einzelnen Typen der inneren Korrosion Korrosionsmechanismen vorgeschlagen worden. Die erhohte Temperatur (650°C) verursacht einen etwas hoheren Materialverlust fur die Legierung.

Corrosion of furnace wall materials in waste-wood fired power plant

Abstract Corrosion tests were performed with four different materials exposed at the furnace wall in a power boiler burning recycled wood, with the aim of evaluating coatings to reduce the corrosion. The nickel base Alloy 625 and the iron–chromium–aluminium alloy Kanthal APMT had the lowest corrosion rates followed by the stainless steel 310S. The low alloy steel 16Mo3, from which the walls are constructed, had the highest rate. Different corrosion mechanisms were found to occur according to the alloy type. Thermodynamic modelling showed that chlorine gas exists at extremely low levels under the prevailing conditions and the hydrated form is thermodynamically favoured.

A Metallographic Technique for High Temperature Creep Damage Assessment in Single Crystal Alloys

The use of single crystal (SX) nickel-base superalloys will increase in the future with the introduction of SX blades into large gas turbines for base-load electricity production. Prolonged periods of use at high temperatures may cause creep deformation and the assessment of damage can give large financial savings. A number of techniques can be applied for life assessment, e.g. calculations based on operational data, non-destructive testing or material interrogation, but because of the uncertainties involved the techniques are often used in combination. This paper describes a material interrogation (metallographic) technique for creep strain assessment in SX alloys.Creep tests have been performed at 950°C on the SX alloy CMSX-4 and quantitative microstructural studies performed on specimens interrupted at various levels of strain. It was found that the strengthening γ′-particles, initially cuboidal in shape, coalesced to form large plates or rafts normal to the applied stress. The γ-matrix phase also formed plates. CMSX-4 contains ∼ 70 vol % γ′-particles and after creep deformation the microstructure turned itself inside out, i.e. the gamma “matrix” became the isolated phase surrounded by the γ′-“particles”. This can cause problems for computerised image analysis, which in this case, were overcome with the choice of a suitable measurement parameter.The rafts reached their maximum length before 2% strain, but continued to thicken with increasing strain. Although of different dimensions, the aspect ratios (length/thickness ratio) of the gamma-prime rafts and the gamma plates were similar at similar levels of strain, increasing from ∼1 at zero strain to a maximum of ∼3 at about 1–2 % strain.Analysis of microstructural measurements from rafting studies on SX alloys presented in the literature showed that the aspect ratios of the γ- and γ′-phases were similar and that at a temperature of 950–1000°C a maximum length/thickness ratio of about 2.5–3.5 is reached at 1 to 2% creep strain. Measurement of gamma-prime raft or (or gamma plate) dimensions on longitudinal sections of blades is thus a suitable method for high temperature creep damage assessment of SX alloys. This gives a considerable advantage over conventional Ni-base superalloys whose microstructures are usually very stable with respect to increasing creep strain.Copyright