Microstructural investigation of the oxide formed on TP 347H FG during long-term steam oxidation (original) (raw)
Related papers
Investigation on steam oxidation behaviour of TP347H FG Part 1: Exposure at 256 bar
Materials and Corrosion, 2005
The stainless steel TP347H FG is a candidate material for the final stage tubing of superheater and reheater sections of ultra supercritical boilers operated at steam temperatures up to 620 8C in the mild corrosion environments of coal-firing. A series of field tests has been conducted with the aforementioned steel in coal-fired boilers and this paper focuses on the steam oxidation behaviour for specimens tested at various metal temperatures for exposure times of 7700, 23000 and 30000 hours as investigated by light optical and scanning electron microscopy. The oxide present on the specimens is a duplex oxide, where the outer layer consists of two sub-layers, an iron oxide layer and an iron-nickel oxide layer; the inner layer is chromium rich chromium-iron-nickel oxide. Microstructure examination showed that for all these samples the varying grain size of subsurface metal affected the oxide thickness, where the larger the metal grain size, the thicker the oxidation scale. This gave the appearance of uneven inner oxides with a varying pit thickness. Comparison of the pit thickness measurement and oxide composition reveals that the oxidation rate is fast during the initial oxidation stage, but the subsequent growth of oxide from further exposure is slower due to the formation of a healing layer consisting of chromium rich oxide near original alloy grain boundaries. At a temperature region above 600 8C a thin oxide rich in chromium and manganese is sometimes formed. In addition precipitation of secondary carbides in the bulk metal also occurs at this temperature region.
Investigation of steam oxidation behaviour of TP347H FG. Part 2: Exposure at 91 bar
Materials and Corrosion, 2005
Tube specimens of TP347FG were exposed in a test superheater loop in a biomass plant in Denmark. The specimens were exposed to surface metal temperatures in the range of 455–568°C, steam pressure of 91 bar and exposure duration of 3500 and 8700 hours. The oxide thickness and morphology was investigated using light optical and scanning electron microscopy. The oxide present on the specimens is a duplex oxide with an inner chromium rich oxide and an outer iron rich oxide. The inner oxide consisted of a primary iron chromium nickel oxide in the original alloy grain and a chromium rich oxide, “healing layer”, at the grain boundaries. This gave the appearance of uneven inner oxide and it was clear that the varying subsurface grain size affected inner oxide thickness, especially after longer exposure times. Longer exposure times from 3500 to 8700 hours resulted in increased pit thickness. Comparison of pit thickness revealed that increase of temperature from 455 to 525°C increases the oxi...
Development of the inner oxide zone upon steam oxidation of an austenitic stainless steel
Materials at High …, 2009
The oxidation behaviour of TP 347H FG in mixtures of water, oxygen, and hydrogen was investigated in the temperature range 500 -700 C for a fixed oxidation time of 336 h. The samples were characterised using reflective light and electron microscopy methods. Thin discontinuous double-layered oxide scales developed during oxidation at 500 C, whereas continuous doublelayered oxide scales covered the entire sample surface after oxidation at 600 and 700 C. The major part of the scale grew into the former alloy grains, whereas Fe2 2Cr spinel grew along the former alloy grain boundaries. TEM and EELS investigations revealed that the part of the scale that grows into the alloy grains consists of particles of Fe2 2Cr spinel embedded in a metallic Fe2 2Ni matrix, which indicates that this part of the scale grows by an internal oxidation mechanism. Growth of the internal oxidation zone at high humidity (46%) is not significantly affected by the type of carrier gas used.
Coatings
The efficiency of ultra-supercritical (USC) steam power plants is limited by the materials properties, in particular, the steam oxidation resistance of the currently used steels at temperatures higher than 600 °C. Under these conditions, steam oxidation results in the development of thick oxide scales which spall and can accumulate in tube bends leading to blockage, overheating and premature creep rupture, as well as erosion of downstream components such as steam valves and turbine blades. Most published work related to oxidation testing is carried out at atmospheric pressure, with significantly less testing of austenitic steels in supercritical steam, and rarely including protective coatings. Indeed, the effect of high-pressure steam in the oxidation process is not quite understood at present. This paper covers a comparison of the behaviour of TP347HFG stainless steel at 700 °C under atmospheric pressure and 25 MPa, with and without slurry-applied diffusion aluminide coatings. The ...
Microstructure Evolution During Steam Oxidation of a Nb Stabilized Austenitic Stainless Steel
Oxidation of Metals, 2010
The oxidation behaviour of TP 347H FG in mixtures of water, oxygen, and hydrogen was investigated at 500, 600, and 700°C for a fixed oxidation time of 336 h. The samples were characterised using X-ray diffraction, reflective light and electron microscopy methods. Thin discontinuous double-layered oxide scales developed during oxidation at 500°C, whereas continuous double-layered oxide scales covered the entire sample surface after oxidation at 600 and 700°C. The major part of the inner oxide layer developed within the former alloy grains, whereas a Fe-Cr spinel formed along the former alloy grain boundaries. Transmission electron microscopy and electron energy loss spectroscopy investigations revealed that the part of the scale that grows into the alloy grains consists of particles of Fe-Cr spinel embedded in a metallic Fe-Ni matrix, which indicates that this part of the scale grows by an internal oxidation mechanism. The thickness of the inner oxide zone at high humidity (46%) is not significantly affected by the type of carrier gas used, whereas this thickness at low humidity (8% H 2 O) is sensitive for the carrier gas and increases in the following order: air \ Ar?7% H 2 \ Ar, indicating that the presence of oxygen or hydrogen in addition to a relatively low content of water vapour counteracts the effect of water vapour on the development of the inner oxide zone.
Most of the electricity produced throughout the world is from thermal energy generating plant such as fossil fuel, nuclear, hydroelectric , geo – thermal , solar etc.,.but in the case of fossil fuel power plant high temperature steam oxidation is one of the important factor which will reduce the efficiency of the power plant. In steam power plant the high temperature steam oxidation takes place in super heaters and reheaters where the high temperature steam is passed through. This high temperature steam oxidation of metal limiting the steam temperature which is available for power generation. But increment of steam temperature is needed to increase the efficiency of the power plant. For that high temperature oxidation resistant material is needed. So in this research paper oxidation behavior of ferritic steel, martensitic steel and austenitic steel are reviewed from various research papers, which are all published by reputed journals
Morphologies of oxide growth and exfoliation in superheater and reheater tubing of steam boilers
Materials at High Temperatures, 2011
The intent of this paper is to present the current state of knowledge concerning the development of the oxide morphologies associated with scales grown in steam on alloys used in the superheater and reheater circuits of conventional fossil and heat recovery steam generator plants, and the influence of those morphologies on the scale exfoliation behaviour. Scale development is described for three classes of alloy: standard 1-9 wt%Cr ferritic steels, using T22 as an example; the ferritic-martensitic steel T91; and the 300-series austenitic steels, represented by TP304 and TP347, for which coarseand fine-grained versions are examined. Detailed information on oxidation mechanisms is combined with extensive knowledge of alloy behaviour in boiler service in an effort to allow engineers and materials scientists to gain an understanding of similarities and differences, as well as to appreciate the likely contributions of major parameters that influence oxide growth and exfoliation behaviour.
Continuous and cyclic oxidation of T91 ferritic steel under steam
Corrosion Science, 2004
The oxidation behaviour of T91 ferritic steel in steam has been studied under isothermal and non-isothermal conditions within a temperature range between 575 and 700°C. Isothermal treatments resulted in parabolic oxidation kinetics. Three clearly defined oxide layers constituted the oxide scales. The innermost layer was a (Fe,Cr) 3 O 4. The intermediate layer was porous magnetite (Fe 3 O 4) followed by a compact thinner layer of hematite (Fe 2 O 3). Under non-isothermal conditions the oxide scales were irregular and evidently cracked. An increase of the oxidation temperature produces an acceleration of the oxidation process, causing an increase of the oxide scale thickness that depends on the temperature increase and the exposure time.
Microscopical investigation of steamside oxide on X20CrMoV121 superheater tubes
Materials at High Temperatures, 2011
X20CrMoV121 is a 12%Cr martensitic steel which has been used in power plants in Europe for many decades. Superheater tubes exposed for various durations up to 135,000 h in power plants in Denmark at steam temperatures varying from 450 to 575 C were investigated. Light optical and scanning electron microscopy was used to investigate steamside oxide morphologies. At all temperatures there is a double layered oxide, however, the inner:outer oxide thickness is not always equal. At the lower steam temperature range of 5500 C, there is an internal oxidation zone at the oxidation front indicating that chromium is less mobile at these temperatures. At a higher steam temperature range of 540-575 C the inner oxide consists of chromium rich and chromium poor oxide running parallel to the oxidation front indicating that the chromium is more mobile within the steel. Both types of morphology have been observed in the laboratory, however, the internal oxidation is observed up to 600 C and the chromium rich oxide striation are observed at 700 C.
Corrosion of a T91 steel tube, used in subcritical conditions in an oil power plant for 157,000 hours was characterized 7 mainly through SEM, TEM, EDX and DRX analyses. Severe oxidation and carburization took place in both the outer (boiler) 8 and inner (steam) wall sides. The nature and morphology of the oxide scale multi-layer structure (hematite, magnetite and spinel) 9 depended on the environment exposure. Specific attention was given to the internal oxidation zone (IOZ) at the oxide/metal 10 interface. Diffusion of chromium during the oxidation process was determined, and was proposed to be responsible for the 11 continuous advancement of oxidation to the core of material, which eventually gave rise to the spinel. Finally, in the bulk 12 material, coarsening of the secondary carbides (M 23 C 6) was the main form of microstructural evolution. 13