Analysis of High Temperature Steam Oxidation of Superheater Steels Used in Coal Fired Boilers
T. Dudziak 0 1 2 3 4 5
M. Łukaszewicz 0 1 2 3 4 5
N. Simms 0 1 2 3 4 5
J. Nicholls 0 1 2 3 4 5
N. Simms 0 1 2 3 4 5
0 M. Łukaszewicz
1 & T. Dudziak
2 Finally, the work was performed in Cranfield University, School of Apllied Science, Centre for Energy and Resource Technology , MK43 0AL Cranfield , United Kingdom
3 School of Applied Science, Cranfield University , Cranfield, Bedfordshire MK43 0AL , UK
4 Exova (UK) Ltd , 12 High March, Daventry, Northamptonshire NN11 4HB , UK
5 Foundry Research Institute, Centre for High Temperature Studies , Zakopian ́ska 73, 30-418 Krako ́w , Poland
The present work compares the behaviour of four steels: (T23, T92, T347HFG, Super304H) in the temperature range 600-750 C. This study focuses on the analysis of the oxidation kinetics in terms of mass change, metal loss and thickness change of the selected materials. In order to understand the differences in oxidation rates between the selected steels, the impact of chromium and the alloying elements were considered in this work. The obtained results show that the impact of alloying elements differs with exposure conditions and importance of the synergy effect.
Austenitic steels; High temperature
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Raising environmental awareness is driving the global economy towards the
reduction of CO2 emissions and fuel consumption [1]. The energy sector is
contributing to these goals by increasing power generation efficiency; for
conventional systems this is a strong function of temperature and pressure of
steam entering the steam turbine [2–4]. Higher operational temperatures can cause
serious problems for boiler components due to their effects in reducing component
lifetime. Steam oxidation of high temperature resistant steels has an important
impact on the power plant lifetime and efficiency. Higher operational temperatures
significantly accelerate oxidation processes, which in turn lead to the development
of thick, non-protective oxides especially in low alloyed steels. The important
concern resulting from fast oxide growth is reduction of heat transfer and the pipe
ability to withstand the high steam pressures due to the metal loss [5]. Material
strength at high temperature is the main concern, the creep resistance of steels under
harsh conditions (temperature, stress) limits the maximum application temperature
[6]. Especially, the steels with relatively low Cr addition like T23 or T92 exposed at
high temperatures (590–620 C). Nevertheless, creep strength of those steels is
considerably higher than that for T22 and T91 respectively due to addition of Nb, V
and substitution of Mo for W [7].
Finally, the stresses generated during the oxide growth and high temperature
exposure changes the scale morphology and leads to exfoliation and finally
materials failures [8]. Therefore, understanding the steam oxidation processes and
factors influencing them is crucial for improvement of power plant efficiency [9].
Currently research for the power plant components concentrates on understanding
the steam oxidation behaviour of the materials which can withstand steam
temperature up to 650, 700 and 760 C for European COST-522, European Thermie
and US Department of Energy’s Vision 21 programmes respectively, therefore those
materials have to exhibit good creep rapture strength and oxidation resistance [1].
Three groups of the materials are considered as the prospective candidates for the
ultra supercritical (USC) boiler tubing [8–10]:
Ferritic steels;
Austenitic steels;
Nickel –based steels.
The materials are characterised by different conductivity, temperature, steam
oxidation behaviour, and creep resistance [11–14]. These properties have significant
impact on their performance in steam conditions [9].
The last decade has brought the extensive research and development of the
highstrength ferritic steels [15], which have good mechanical and physical properties
and significantly reduce cost of the power plant construction [2]. The two ferritic
steels which are successfully applied for boilers components in existing power
plants are T23 (7CrWMoVNb9-6) and T92 (X10CrWMoVNb9-2), therefore steam
oxidation of these two steels is analysed and compared. Steam oxidation of T23 and
T92 steels at low temperature is well documented in the literature therefore
particular research aims to investigate their performance under significantly higher
conditions.
Austenitic steels are commonly used for the final superheater (SH) tubing due to
their good oxidation resistance [16]. In particular research two types of 18 wt% Cr
austenitic steels are tested—T347HFG (X6CrNiNb18-10) and Super 304H
(X5CrNi18-10), both steels are already used in the final stages of the SH in
coalfired plants [17, 18].
This article aims to investigate and compare high temperature steam oxidation
behaviour of ferritic and austenitic steels as well as study the differences within
these two groups. Furthermore the selection of the tested conditions allows
investigation and understanding of (...truncated)