- Email: [email protected]
Studies on the adhesion of oxide layer formed on X10CrMoVNb9-1 steel
archives of civil and mechanical engineering 14 (2014) 335–341
Available online at www.sciencedirect.com
ScienceDirect journal homepage: http://www.elsevier.com/locate/acme
Original Research Article
Studies on the adhesion of oxide layer formed on X10CrMoVNb9-1 steel M. Gwoździk *, Z. Nitkiewicz 1 Czestochowa University of Technology, Institute of Materials Engineering, Al. Armii Krajowej 19, 42-201 Czestochowa, Poland
article info
abstract
Article history:
The paper contains results of studies on the formation of oxide layers on X10CrMoVNb9-1
Received 14 June 2013
(P91) steel long-term operated at an elevated temperature. X10CrMoVNb9-1 steel
Received in revised form
operated at the temperature of 595 8C for 54,144 h was the studied material. X-ray structural
7 October 2013
examinations (XRD) were carried out, microscope observations using an optical
Accepted 13 October 2013
and scanning microscope were performed. The native material chemical composition
Available online 2 December 2013
was analysed by means of emission spark spectroscopy, while that of oxide layers on a
Keywords:
were characterised on the basis of scratch test. The adhesion of oxide layers, friction
scanning microscope (EDS). Mechanical properties of the oxide layer – steel (substrate) Oxide layers
force, friction coefficient, scratching depth was determined as well as the force at which
SEM
the layer was delaminated. It has been found that the oxide layer formed under
LM
the influence of applied pressure is more degradeted in the areas where are porous and
XRD
cracks.
Scratch test
# 2013 Politechnika Wrocławska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved.
1.
Introduction
Steels for use at elevated temperatures find wide application in conventional and nuclear energy generation, in armaments industry, petrochemical industry, aviation, and space technology. In the power sector such steels are used mainly for boiler pipes, superheater coils, chambers, pipelines, parts of boilers, steam turbines, as well as other devices. Steels and alloys exposed to high temperatures are required to possess specific resistance properties at normal
and elevated temperatures, such as: guaranteed yield point in a determined temperature or, in parts functioning in creep conditions: creep resistance and creep limit. Both the creep limit and creep resistance are determined for ever longer times. In recent years the problem of extending service life of thermal–mechanical devices in Polish power plants has been gaining in importance. Poland, as a member state of the European Union, participates in an active way in developing energy policy of the Community. In the year 2009 the enactment (No. 202/2009) entitled ‘‘Energy Policy of Poland
* Corresponding author. Tel.: +48 34 3250736; fax: +48 34 3250721. E-mail addresses: [email protected] (M. Gwoździk), [email protected] (Z. Nitkiewicz). 1 Tel.: +48 34 3250716; fax: +48 34 3250721. 1644-9665/$ – see front matter # 2013 Politechnika Wrocławska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved. http://dx.doi.org/10.1016/j.acme.2013.10.005
336
archives of civil and mechanical engineering 14 (2014) 335–341
until the year 2030’’, where one of the aims is to increase the efficiency of power generation, through construction of highperformance power generating units. However, apart from construction of new power units, modernisation is planned for devices that have been in use in power plants for over 30 years, the purpose of such modernisation is to extend their service life for the next 20 years. For some devices, or to be more proper for elements there of, that may mean exceeding the threshold of 350,000 working hours [1,2]. Extension by some 20 years of the service life of 200 MW units is related to compliance with the environmental requirements, which are to be binding after 2016, as well as provision of sufficient durability, in particular to parts of critical importance in boilers, turbine sets, as well as main steam pipelines, which is indispensable for their safe operation, as well as the expected high availability level. Provision of durability of boiler drums, rotors, turbine cylinders, and pipelines, in the time horizon of up to 350,000 working hours should not pose too great technical difficulties or result in too huge investment outlays. The scope of necessary reconstruction and modernisation works will depend upon the technical condition of specific devices, which need to be correctly diagnosed using suitable methods for investigating technical conditions and forecasting the remaining service life. The main aim for proper revamping of units is to increase their reliability and efficiency, as well as to prolong their durability, at minimum cost. That can be achieved thanks to overhauls done in accordance with specified service life or technical condition of the device. A proper method, in accordance with Polish and international experiences, is the method of prolonging service life in line with the condition of the device. Technical assessment of the condition of elements of specific units consists of associated analysis of a variety of information, namely results of diagnostic, standard, and special studies, history of use, operation conditions, as well as effort in zones of potential occurrence of damages (assessment of the exhaustion degree and risk degree assessment). In recent years, as demonstrated by Polish studies, a substantial share in part failure frequency is that of corrosion. Among others, such parts as superheater coils of secondary steam boilers get damaged due to excessive reduction of wall thickness, due to corrosion, from the inside as well as from the outside. Such a reduction is due to the widely understanding of high temperature corrosion, a contribution to which is made by oxides formed on the surface (or absence of such oxides), which do not always perform a protective role. Protective layers of oxides, which are formed during the normal operation exert equally important influence upon the longevity of operation of parts of turbines and boiler, dependent on mechanical properties, including creep strength and creep limit. Their good adhesion is important, as well as very slow growth, and slight susceptibility to scaling. Excessive growth of oxide layer from the steam side (internal wall of pipe) has negative consequences during long term operation, because: it reduces the bore of pipes, especially in heavy wall tubes with small inside diameter,
the oxide layer causes reduction of wall thickness and increase of stresses, moreover – scaling of oxide layer may lead to erosion inside the turbine. Scaling of oxide layer is an extremely harmful phenomenon, as the scaled particles may get to the turbine, and lead to fatal consequences. Scaled oxide layers may clog the bore of superheater pipe, as well as other steam pipelines, causing local overheating, which often leads to pipe burst. On the flue gas end, besides oxide layers also ash deposits are frequently formed, which most often insulate. Steels for operation at elevated temperatures are the subject of interest of numerous scientific centres in Poland and worldwide [2–17], mainly due to their application in the power industry. Oxide layers formed on long-term operated steels fulfil as important role as the creep limit or the creep strength [18]. Co-combustion of coal and biomass causes deterioration in conditions of heated surfaces operation, which results in a reduced life and in an increased failure rate of components working long-term at elevated temperatures due to progressing corrosion processes. Corrosion processes in components operating long-term at elevated temperatures result both from oxidation of such steels as well as from aggressive action of chemical compounds contained in the flue gas on the protective layer and steel [12,13]. The protective layer stability and its composition depend mainly on temperature and on the environment, in which it is situated, and also on the chemical composition of steel. In turn, the flue gas composition depends on the fuel type and on conditions of its combustion. In the case of hard coal complete combustion the flue gas comprises such components (gaseous) as: N2, O2, CO2, SO2, H2O (water vapour). However, after the introduction of low-emission combustion techniques and biomass co-combustion the following components exist in the flue gas: unburnt carbon, CO as well as chlorides, sulphides and sulphates [12].
2.
Material and experimental methods
The material studied comprised specimens of X10CrMoVNb9-1 (P91) steel taken from an exhaust pipeline operated at the temperature of 595 8C during 54,144 h. Studies were carried out both on the fire and counter-fire side of the tube (Fig. 1). The oxide layer was studied on a surface and a crosssection at the outer surface of the tube wall. The analysis of steel chemical composition was carried out using spark emission spectroscopy on a Spectro spectrometer (Table 1). Thorough examinations of the oxide layer carried out on the outside surface of tube wall comprised: - microscopic examinations of the oxide layer were performed using an Olympus GX41 optical microscope, - thickness measurements of formed oxide layers, - chemical composition analysis of deposits/oxides using a Jeol JSM-6610LV scanning electron microscope (SEM) working with an Oxford EDS electron microprobe X-ray analyser, - X-ray (XRD) measurements; the layer was subject to measurements using a Seifert 3003T/T X-ray diffractometer and the radiation originating from a tube with a cobalt anode
337
archives of civil and mechanical engineering 14 (2014) 335–341
Fig. 2 – The scratch test method diagram.
Apart from the oxide layer adhesion determination the Scratch Test allowed also microscopic observations and the analysis of scratch line on the entire length of the pre-set force.
3.
Fig. 1 – Place of samples taking for tests.
(lCo = 0.17902 nm). X-ray studies were performed, comprising measurements in a symmetric Bragg–Brentano geometry (XRD). XRD measurements were performed in the 5–1208 range of angles with an angular step of 0.18 and the exposure time of 4 s. To interpret the results the diffractograms were described by a Pseudo Voight curve using the Analyse software. DHN PDS and PDF4 + 2009 computer software and crystallographic database were used for the phase identification, - studies on formed oxides adhesion, and - microscopic assessment of formed scratches. The oxide layer adhesion tests were carried out on an automated Revetest XPress Plus instrument using a diamond Rockwell indenter (Fig. 2). The test was carried out using the following parameters: preset load of 1–200 N, scratch 10 mm long.
Results of examinations
The steel microstructure consists of a tempered martensite with a recrystallised ferrite grain. The size of recrystallised ferrite grain is 10 acc. to standard [20]. Precipitations of fine carbides on grain boundaries and within martensite needles are visible (Fig. 3). Performed macroscopic and SEM studies (Fig. 4) have shown that the originating oxide layer is much more developed on the fire side (Fig. 4b, d), while on the counterfire side (Fig. 4a and c) the forming oxide layers on the studied steel are discontinuous in places. Surface damages are clearly visible in this area of layer surface, which then take a more developed form of irregular shapes, reaching deeper into the layer, down to the steel substrate. The oxide layer formed on the studied steel on the counterfire side is 105 mm thick, while on the fire side 160 mm (Fig. 5). A significant layer thickness on the fire side is caused by the formation of a larger amount of deposits. Performed EDS analysis of chemical composition (Fig. 6) combined with X-ray phase analysis (Fig. 7) has shown that deposits occur on the outside surface of tube. Based on DHN PDS and PDF4 + 2009 crystallographic database it has been found that the forming deposits are: NaFeAl2Si4O13 3H2O,
Table 1 – Chemical composition of examined steel, wt%. Acc.
Analysis PN-EN 10028-2:2010 [19]
Chemical composition, wt% C
Si
Mn
P
S
Cr
Mo
V
0.08 0.08–0.12
0.37 Max 0.50
0.49 0.30–0.60
0.008 Max 0.020
0.001 Max 0.005
8.15 8.00–9.50
0.93 0.85–1.05
0.20 0.18–0.25
338
archives of civil and mechanical engineering 14 (2014) 335–341
CaSO4, CaS, KAlSi2O6, Al2SiO5, SiO2, CaTi2O4, CaCO3 in accordance with the catalogue card: 00-026-1318, 30-0279, 08-0464, 15-0047, 13-0122, 18-1170, 11-0029, 05-0453, respectively. In the case of counter-fire side apart from the aforementioned compounds also magnesium based compounds exist, such as: Mg2Al4Si5O18 (12-0303) and MgSO4 (21-0546), while on the fire side additionally Na2TiSi4O11 (331297). Under the deposit layer iron based oxides exist, such as hematite (Fe2O3), magnetite (Fe3O4) and chromite (FeCr2O4), with catalogue card numbers: 01-079-0007, 01-089-0951, 01075-3312, respectively. Performed characteristics of oxide layer – native material mechanical properties (Fig. 8) have shown that in the case of counter-fire side the deposit layer exfoliates at the force of 40 N (Fig. 8a). Increasing the force to 100 N results in delamination of the iron oxides layer. Instead, on the fire side the deposits layer is damaged only at the force of 90 N and the iron oxides layer is delaminated at 140 N. On the counter-fire side during the scratch test on the length of 6 mm the oxide layer spalled (Fig. 9). Such delamination results from the morphology of formed deposits/oxides layer. Adhesion with the substrate material is broken on the counter-fire side, which causes oxide scaling and spalling. This is a result of much larger volume of oxide as compared with steel, on which it was formed. The scale forming on the fire side is porous, significant chipping occur in places, resulting from a smaller volume of oxide than of steel, on which it was formed. A thorough analysis of defected oxide layers on X10CrMoVNb9-1 steel operated at T = 535 8C has been presented by the author in paper [21]. Fig. 3 – Microstructure of P91 steel operated for 54,144 h at the temperature of 595 8C: (a) LM and (b) SEM.
4.
Summary of results
Performed studies have shown that the layer of deposits/ oxides on X10CrMoVNb9-1 steel operated at T = 595 8C during
Fig. 4 – Oxides formed on X10CrMoVNb9-1 steel operated at 595 8C during 54,144 h: (a, c) opposite fire wall and (b, d) fire wall.
archives of civil and mechanical engineering 14 (2014) 335–341
339
Fig. 5 – The thickness of oxides layer formed on the steel examined: (a) opposite fire wall – LM, (b) fire wall – LM, (c) opposite fire wall – SEM and (d) fire wall – SEM.
54,144 h directly on the flue gas side consists of deposits based on Na, Al, Si, Ca, Ti, mainly sulphates, which occur both on the counter-fire and fire side. MgSO4 and Mg2Al4Si5O18 occur additionally on the counter-fire side. Iron oxides such as Fe2O3,
Fig. 6 – Microstructure of the surface oxide layer and EDS point microanalysis: (a-b) opposite fire wall and (c, d) fire wall.
Fe3O4 and chromite FeCr2O4 occur under deposits. Such a laminar structure of oxide layers based on iron and chromium was presented in papers [22,23], although also bilayer [24] and monolayer [25] oxides exist, which are affected by both temperature of component operation and by the flue gas composition (outside surface of tube wall), the flowing medium composition (inside surface of tube wall) and by the chemical composition of steel. Tests performed on the scratch tester have shown that the layers of deposits/oxides originating on the counter-fire
Fig. 7 – Diffraction pattern of oxide layers obtained using the XRD technique: (a) opposite fire wall and (b) fire wall.
340
archives of civil and mechanical engineering 14 (2014) 335–341
Acknowledgements Financial support of Structural Funds in the Operational Programme – Innovative Economy (IE OP) financed from the European Regional Development Fund – Project. ‘‘Modern material technologies in aerospace industry’’, Nr POIG.01.01.02-00-015/08-00 is gratefully acknowledged.
references
Fig. 8 – Graph of data obtained from the scratch test for a preset force of 1–200 N: (a) opposite fire wall and (b) fire wall.
Fig. 9 – Microstructure of the scratch (a) and EDS point microanalysis (b).
and fire side behave differently. On the counter-fire side deposits delaminate at the force of 40 N, iron oxides at the force of 100 N, while on the fire side at 90 N and 140 N, respectively. Tests performed on the scratch tester for 10CrMo9-10 steel operated at T = 575 8C have shown [26] that at the load of 50 N the oxide layer was not totally damaged. Only load increased to 200 N caused that the layer was delaminated at the load of 112 N. Similar results were obtained for P91 steel operated at the temperature of 535 8C [27], where the oxide layer was delaminated from the substrate at the force of 100 N.
[1] Uchwała nr 202/2009 Rady Ministrów z dnia 10 listopada 2009 roku w sprawie ‘‘Polityki energetycznej Polski do 2030 roku’’. [2] J. Trzeszczyński, Ocena stanu technicznego i prognozowanie trwałości elementów krytycznych urządzeń cieplnomechanicznych przewidzianych do eksploatacji powyżej 300000 godzin, in: XII Sympozjum InformacyjnoSzkoleniowe: Diagnostyka i remonty urządzeń cieplnomechanicznych elektrowni. Modernizacje urządzeń energetycznych w celu przedłużenia ich eksploatacji powyżej 300000 godzin, Wisła, (2010), pp. 11–21. [3] J. Hald, Microstructure and long-term creep properties of 9– 12%Cr steels, International Journal of Pressure Vessels and Piping 85 (2008) 30–37. [4] J.C. Vaillant, B. Vandenberghe, B. Hahn, H. Heuser, C. Jochum, T/P23, 24, 911 and 92: new grades for advanced coal-fired power plants – Properties and experience, International Journal of Pressure Vesseles and Piping 85 (2008) 38–46. [5] V. Lepingle, G. Louis, D. Allue, B. Lefebvre, B. Vandenberghe, Steam oxidation resistance of new 12%Cr steels: Comparison with some other ferritic steels, Corrosion Science 50 (2008) 1011–1019. [6] P.J. Ennis, W.J. Quadakkers, Implication of steam oxidation for the service life of high-strength martensitic steel components in high temperature plant, International Journal of Pressure Vessels and Piping 84 (2007) 82–87. [7] P.J. Ennis, W.J. Quadakkers, Mechanism of steam oxidation in high strength martensitic steels, International Journal of Pressure Vessels and Piping 84 (2007) 75–81. [8] J. Żurek, E. Wessel, L. Niewolak, F. Schmitz, T.U. Kern, L. Singheiser, W.J. Quadakkers, Anomalous temperature dependence of oxidation kinetics during steam oxidation of ferritic steels in the temperature range 550–650 8C, Corrosion Science 46 (2004) 2301–2317. [9] D. Laverede, T. Gomez-Acebo, F. Castro, Continuous and cyclic oxidation of T91 ferritic steel under steam, Corrosion Science 46 (2004) 613–631. [10] A. Hernas, G. Moskal, B. Augustyniak, M. Chmielewski, Degradacja austenitycznej stali 321H podczas długotrwałej eksploatacji, Energetyka, Zeszyt tematyczny nr XIX, Projektowanie i innowacje remontowe w energetyce (2009) 65–69. [11] A. Hernas, G. Moskal, K. Przegaliński, Degradacja stali T91 oraz HCM12(A) po 70000 godzinach ekspozycji w kotle, Energetyka, Zeszyt tematyczny nr XIX, Projektowanie i innowacje remontowe w energetyce (2009) 69–72. [12] K. Klepacki, K. Panaś, R. Wojtkiewicz, Korozja przegrzewaczy kotłów współspalających biomasę, XIII Sympozjum Informacyjno-Szkoleniowe, Diagnostyka i remonty urządzeń cieplno-mechanicznych elektrowni, Katowice, 2011 127–132. [13] F. Klepacki, D. Wywrot, Trwałość wężownic przegrzewaczy wtórnych w warunkach niskoemisyjnego spalania, XII Sympozjum Informacyjno-Szkoleniowe, Diagnostyka i remonty urządzeń cieplno-mechanicznych elektrowni, Wisła, 201029–35.
archives of civil and mechanical engineering 14 (2014) 335–341
[14] M. Gwoździk, Characteristic of crystallite sizes and lattice deformations changes in the oxide layer formed on steel operated for a long time at an elevated temperature, Solid State Phenomena 203–204 (2013) 204–207. [15] M. Gwoździk, Z. Nitkiewicz, Texturing of magnetite forming during long-term operation of a pipeline of 10CrMo9–10 steel, Solid State Phenomena 203–204 (2013) 121–124. [16] M. Gwoździk, Utlenianie stali 10CrMo9–10 podczas eksploatacji w wysokiej temperaturze, Surface Engineering (1) (2013) 36–39. [17] M. Gwoździk, Z. Nitkiewicz, Analysis of crystallite size and lattice deformations changes in an oxide layer on P91 steel, Archives of Metallurgy and Materials 1 (58) (2013) 31–34. [18] S. Stachura, M. Gwoździk, Z. Stradomski, Charakterystyka stali nowej generacji dla bloków energetycznych, Energetyka (2) (2010) 94–102. [19] PN-EN 10028:2010, Wyroby płaskie ze stali na urządzenia ciśnieniowe. Część 2: Stale niestopowe i stopowe o określonych własnościach w podwyższonych temperaturach. [20] PN-EN ISO 643: Stal. Mikrograficzne określanie wielkości ziarna. [21] M. Gwoździk, Analiza zdefektowania warstw tlenkowych na stali X10CrMoVNb9-1 długotrwale eksploatowanej w
[22]
[23]
[24]
[25]
[26]
[27]
341
temperaturze 535 8C, Inżynieria Materiałowa 4 (182) (2011) 432–434. M. Gwoździk, Charakterystyka warstw tlenkowych powstałych na stali P91 długotrwale eksploatowanej w podwyższonej temperaturze, Inżynieria Materiałowa 2 (180) (2011) 128–131. M. Gwoździk, Utlenianie stali X10CrMoVNb9-1 podczas eksploatacji w wysokiej temperaturze, Inżynieria Materiałowa 5 (189) (2012) 409–412. M. Gwoździk, Charakterystyka warstw tlenkowych powstałych na stali 13CrMo4-5 długotrwale eksploatowanej w podwyższonej temperaturze, Hutnik – Wiadomości Hutnicze 79 (12) (2012) 932–935. M. Gwoździk, Charakterystyka warstw tlenkowych powstałych na stali 10CrMo9-10 długotrwale eksploatowanej w podwyższonej temperaturze, Hutnik – Wiadomości Hutnicze 78 (9) (2011) 781–783. M. Gwoździk, Z. Nitkiewicz, Badanie przyczepności warstwy tlenkowej powstałej na stali 10CrMo9-10, Hutnik – Wiadomości Hutnicze 79 (12) (2012) 924–927. M. Gwoździk, Z. Nitkiewicz, Charakterystyka odporności na zarysowanie warstwy tlenkowej powstałej na stali P91, Inżynieria Materiałowa 4 (182) (2011) 435–438.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: http://www.elsevier.com/locate/acme
Original Research Article
Studies on the adhesion of oxide layer formed on X10CrMoVNb9-1 steel M. Gwoździk *, Z. Nitkiewicz 1 Czestochowa University of Technology, Institute of Materials Engineering, Al. Armii Krajowej 19, 42-201 Czestochowa, Poland
article info
abstract
Article history:
The paper contains results of studies on the formation of oxide layers on X10CrMoVNb9-1
Received 14 June 2013
(P91) steel long-term operated at an elevated temperature. X10CrMoVNb9-1 steel
Received in revised form
operated at the temperature of 595 8C for 54,144 h was the studied material. X-ray structural
7 October 2013
examinations (XRD) were carried out, microscope observations using an optical
Accepted 13 October 2013
and scanning microscope were performed. The native material chemical composition
Available online 2 December 2013
was analysed by means of emission spark spectroscopy, while that of oxide layers on a
Keywords:
were characterised on the basis of scratch test. The adhesion of oxide layers, friction
scanning microscope (EDS). Mechanical properties of the oxide layer – steel (substrate) Oxide layers
force, friction coefficient, scratching depth was determined as well as the force at which
SEM
the layer was delaminated. It has been found that the oxide layer formed under
LM
the influence of applied pressure is more degradeted in the areas where are porous and
XRD
cracks.
Scratch test
# 2013 Politechnika Wrocławska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved.
1.
Introduction
Steels for use at elevated temperatures find wide application in conventional and nuclear energy generation, in armaments industry, petrochemical industry, aviation, and space technology. In the power sector such steels are used mainly for boiler pipes, superheater coils, chambers, pipelines, parts of boilers, steam turbines, as well as other devices. Steels and alloys exposed to high temperatures are required to possess specific resistance properties at normal
and elevated temperatures, such as: guaranteed yield point in a determined temperature or, in parts functioning in creep conditions: creep resistance and creep limit. Both the creep limit and creep resistance are determined for ever longer times. In recent years the problem of extending service life of thermal–mechanical devices in Polish power plants has been gaining in importance. Poland, as a member state of the European Union, participates in an active way in developing energy policy of the Community. In the year 2009 the enactment (No. 202/2009) entitled ‘‘Energy Policy of Poland
* Corresponding author. Tel.: +48 34 3250736; fax: +48 34 3250721. E-mail addresses: [email protected] (M. Gwoździk), [email protected] (Z. Nitkiewicz). 1 Tel.: +48 34 3250716; fax: +48 34 3250721. 1644-9665/$ – see front matter # 2013 Politechnika Wrocławska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved. http://dx.doi.org/10.1016/j.acme.2013.10.005
336
archives of civil and mechanical engineering 14 (2014) 335–341
until the year 2030’’, where one of the aims is to increase the efficiency of power generation, through construction of highperformance power generating units. However, apart from construction of new power units, modernisation is planned for devices that have been in use in power plants for over 30 years, the purpose of such modernisation is to extend their service life for the next 20 years. For some devices, or to be more proper for elements there of, that may mean exceeding the threshold of 350,000 working hours [1,2]. Extension by some 20 years of the service life of 200 MW units is related to compliance with the environmental requirements, which are to be binding after 2016, as well as provision of sufficient durability, in particular to parts of critical importance in boilers, turbine sets, as well as main steam pipelines, which is indispensable for their safe operation, as well as the expected high availability level. Provision of durability of boiler drums, rotors, turbine cylinders, and pipelines, in the time horizon of up to 350,000 working hours should not pose too great technical difficulties or result in too huge investment outlays. The scope of necessary reconstruction and modernisation works will depend upon the technical condition of specific devices, which need to be correctly diagnosed using suitable methods for investigating technical conditions and forecasting the remaining service life. The main aim for proper revamping of units is to increase their reliability and efficiency, as well as to prolong their durability, at minimum cost. That can be achieved thanks to overhauls done in accordance with specified service life or technical condition of the device. A proper method, in accordance with Polish and international experiences, is the method of prolonging service life in line with the condition of the device. Technical assessment of the condition of elements of specific units consists of associated analysis of a variety of information, namely results of diagnostic, standard, and special studies, history of use, operation conditions, as well as effort in zones of potential occurrence of damages (assessment of the exhaustion degree and risk degree assessment). In recent years, as demonstrated by Polish studies, a substantial share in part failure frequency is that of corrosion. Among others, such parts as superheater coils of secondary steam boilers get damaged due to excessive reduction of wall thickness, due to corrosion, from the inside as well as from the outside. Such a reduction is due to the widely understanding of high temperature corrosion, a contribution to which is made by oxides formed on the surface (or absence of such oxides), which do not always perform a protective role. Protective layers of oxides, which are formed during the normal operation exert equally important influence upon the longevity of operation of parts of turbines and boiler, dependent on mechanical properties, including creep strength and creep limit. Their good adhesion is important, as well as very slow growth, and slight susceptibility to scaling. Excessive growth of oxide layer from the steam side (internal wall of pipe) has negative consequences during long term operation, because: it reduces the bore of pipes, especially in heavy wall tubes with small inside diameter,
the oxide layer causes reduction of wall thickness and increase of stresses, moreover – scaling of oxide layer may lead to erosion inside the turbine. Scaling of oxide layer is an extremely harmful phenomenon, as the scaled particles may get to the turbine, and lead to fatal consequences. Scaled oxide layers may clog the bore of superheater pipe, as well as other steam pipelines, causing local overheating, which often leads to pipe burst. On the flue gas end, besides oxide layers also ash deposits are frequently formed, which most often insulate. Steels for operation at elevated temperatures are the subject of interest of numerous scientific centres in Poland and worldwide [2–17], mainly due to their application in the power industry. Oxide layers formed on long-term operated steels fulfil as important role as the creep limit or the creep strength [18]. Co-combustion of coal and biomass causes deterioration in conditions of heated surfaces operation, which results in a reduced life and in an increased failure rate of components working long-term at elevated temperatures due to progressing corrosion processes. Corrosion processes in components operating long-term at elevated temperatures result both from oxidation of such steels as well as from aggressive action of chemical compounds contained in the flue gas on the protective layer and steel [12,13]. The protective layer stability and its composition depend mainly on temperature and on the environment, in which it is situated, and also on the chemical composition of steel. In turn, the flue gas composition depends on the fuel type and on conditions of its combustion. In the case of hard coal complete combustion the flue gas comprises such components (gaseous) as: N2, O2, CO2, SO2, H2O (water vapour). However, after the introduction of low-emission combustion techniques and biomass co-combustion the following components exist in the flue gas: unburnt carbon, CO as well as chlorides, sulphides and sulphates [12].
2.
Material and experimental methods
The material studied comprised specimens of X10CrMoVNb9-1 (P91) steel taken from an exhaust pipeline operated at the temperature of 595 8C during 54,144 h. Studies were carried out both on the fire and counter-fire side of the tube (Fig. 1). The oxide layer was studied on a surface and a crosssection at the outer surface of the tube wall. The analysis of steel chemical composition was carried out using spark emission spectroscopy on a Spectro spectrometer (Table 1). Thorough examinations of the oxide layer carried out on the outside surface of tube wall comprised: - microscopic examinations of the oxide layer were performed using an Olympus GX41 optical microscope, - thickness measurements of formed oxide layers, - chemical composition analysis of deposits/oxides using a Jeol JSM-6610LV scanning electron microscope (SEM) working with an Oxford EDS electron microprobe X-ray analyser, - X-ray (XRD) measurements; the layer was subject to measurements using a Seifert 3003T/T X-ray diffractometer and the radiation originating from a tube with a cobalt anode
337
archives of civil and mechanical engineering 14 (2014) 335–341
Fig. 2 – The scratch test method diagram.
Apart from the oxide layer adhesion determination the Scratch Test allowed also microscopic observations and the analysis of scratch line on the entire length of the pre-set force.
3.
Fig. 1 – Place of samples taking for tests.
(lCo = 0.17902 nm). X-ray studies were performed, comprising measurements in a symmetric Bragg–Brentano geometry (XRD). XRD measurements were performed in the 5–1208 range of angles with an angular step of 0.18 and the exposure time of 4 s. To interpret the results the diffractograms were described by a Pseudo Voight curve using the Analyse software. DHN PDS and PDF4 + 2009 computer software and crystallographic database were used for the phase identification, - studies on formed oxides adhesion, and - microscopic assessment of formed scratches. The oxide layer adhesion tests were carried out on an automated Revetest XPress Plus instrument using a diamond Rockwell indenter (Fig. 2). The test was carried out using the following parameters: preset load of 1–200 N, scratch 10 mm long.
Results of examinations
The steel microstructure consists of a tempered martensite with a recrystallised ferrite grain. The size of recrystallised ferrite grain is 10 acc. to standard [20]. Precipitations of fine carbides on grain boundaries and within martensite needles are visible (Fig. 3). Performed macroscopic and SEM studies (Fig. 4) have shown that the originating oxide layer is much more developed on the fire side (Fig. 4b, d), while on the counterfire side (Fig. 4a and c) the forming oxide layers on the studied steel are discontinuous in places. Surface damages are clearly visible in this area of layer surface, which then take a more developed form of irregular shapes, reaching deeper into the layer, down to the steel substrate. The oxide layer formed on the studied steel on the counterfire side is 105 mm thick, while on the fire side 160 mm (Fig. 5). A significant layer thickness on the fire side is caused by the formation of a larger amount of deposits. Performed EDS analysis of chemical composition (Fig. 6) combined with X-ray phase analysis (Fig. 7) has shown that deposits occur on the outside surface of tube. Based on DHN PDS and PDF4 + 2009 crystallographic database it has been found that the forming deposits are: NaFeAl2Si4O13 3H2O,
Table 1 – Chemical composition of examined steel, wt%. Acc.
Analysis PN-EN 10028-2:2010 [19]
Chemical composition, wt% C
Si
Mn
P
S
Cr
Mo
V
0.08 0.08–0.12
0.37 Max 0.50
0.49 0.30–0.60
0.008 Max 0.020
0.001 Max 0.005
8.15 8.00–9.50
0.93 0.85–1.05
0.20 0.18–0.25
338
archives of civil and mechanical engineering 14 (2014) 335–341
CaSO4, CaS, KAlSi2O6, Al2SiO5, SiO2, CaTi2O4, CaCO3 in accordance with the catalogue card: 00-026-1318, 30-0279, 08-0464, 15-0047, 13-0122, 18-1170, 11-0029, 05-0453, respectively. In the case of counter-fire side apart from the aforementioned compounds also magnesium based compounds exist, such as: Mg2Al4Si5O18 (12-0303) and MgSO4 (21-0546), while on the fire side additionally Na2TiSi4O11 (331297). Under the deposit layer iron based oxides exist, such as hematite (Fe2O3), magnetite (Fe3O4) and chromite (FeCr2O4), with catalogue card numbers: 01-079-0007, 01-089-0951, 01075-3312, respectively. Performed characteristics of oxide layer – native material mechanical properties (Fig. 8) have shown that in the case of counter-fire side the deposit layer exfoliates at the force of 40 N (Fig. 8a). Increasing the force to 100 N results in delamination of the iron oxides layer. Instead, on the fire side the deposits layer is damaged only at the force of 90 N and the iron oxides layer is delaminated at 140 N. On the counter-fire side during the scratch test on the length of 6 mm the oxide layer spalled (Fig. 9). Such delamination results from the morphology of formed deposits/oxides layer. Adhesion with the substrate material is broken on the counter-fire side, which causes oxide scaling and spalling. This is a result of much larger volume of oxide as compared with steel, on which it was formed. The scale forming on the fire side is porous, significant chipping occur in places, resulting from a smaller volume of oxide than of steel, on which it was formed. A thorough analysis of defected oxide layers on X10CrMoVNb9-1 steel operated at T = 535 8C has been presented by the author in paper [21]. Fig. 3 – Microstructure of P91 steel operated for 54,144 h at the temperature of 595 8C: (a) LM and (b) SEM.
4.
Summary of results
Performed studies have shown that the layer of deposits/ oxides on X10CrMoVNb9-1 steel operated at T = 595 8C during
Fig. 4 – Oxides formed on X10CrMoVNb9-1 steel operated at 595 8C during 54,144 h: (a, c) opposite fire wall and (b, d) fire wall.
archives of civil and mechanical engineering 14 (2014) 335–341
339
Fig. 5 – The thickness of oxides layer formed on the steel examined: (a) opposite fire wall – LM, (b) fire wall – LM, (c) opposite fire wall – SEM and (d) fire wall – SEM.
54,144 h directly on the flue gas side consists of deposits based on Na, Al, Si, Ca, Ti, mainly sulphates, which occur both on the counter-fire and fire side. MgSO4 and Mg2Al4Si5O18 occur additionally on the counter-fire side. Iron oxides such as Fe2O3,
Fig. 6 – Microstructure of the surface oxide layer and EDS point microanalysis: (a-b) opposite fire wall and (c, d) fire wall.
Fe3O4 and chromite FeCr2O4 occur under deposits. Such a laminar structure of oxide layers based on iron and chromium was presented in papers [22,23], although also bilayer [24] and monolayer [25] oxides exist, which are affected by both temperature of component operation and by the flue gas composition (outside surface of tube wall), the flowing medium composition (inside surface of tube wall) and by the chemical composition of steel. Tests performed on the scratch tester have shown that the layers of deposits/oxides originating on the counter-fire
Fig. 7 – Diffraction pattern of oxide layers obtained using the XRD technique: (a) opposite fire wall and (b) fire wall.
340
archives of civil and mechanical engineering 14 (2014) 335–341
Acknowledgements Financial support of Structural Funds in the Operational Programme – Innovative Economy (IE OP) financed from the European Regional Development Fund – Project. ‘‘Modern material technologies in aerospace industry’’, Nr POIG.01.01.02-00-015/08-00 is gratefully acknowledged.
references
Fig. 8 – Graph of data obtained from the scratch test for a preset force of 1–200 N: (a) opposite fire wall and (b) fire wall.
Fig. 9 – Microstructure of the scratch (a) and EDS point microanalysis (b).
and fire side behave differently. On the counter-fire side deposits delaminate at the force of 40 N, iron oxides at the force of 100 N, while on the fire side at 90 N and 140 N, respectively. Tests performed on the scratch tester for 10CrMo9-10 steel operated at T = 575 8C have shown [26] that at the load of 50 N the oxide layer was not totally damaged. Only load increased to 200 N caused that the layer was delaminated at the load of 112 N. Similar results were obtained for P91 steel operated at the temperature of 535 8C [27], where the oxide layer was delaminated from the substrate at the force of 100 N.
[1] Uchwała nr 202/2009 Rady Ministrów z dnia 10 listopada 2009 roku w sprawie ‘‘Polityki energetycznej Polski do 2030 roku’’. [2] J. Trzeszczyński, Ocena stanu technicznego i prognozowanie trwałości elementów krytycznych urządzeń cieplnomechanicznych przewidzianych do eksploatacji powyżej 300000 godzin, in: XII Sympozjum InformacyjnoSzkoleniowe: Diagnostyka i remonty urządzeń cieplnomechanicznych elektrowni. Modernizacje urządzeń energetycznych w celu przedłużenia ich eksploatacji powyżej 300000 godzin, Wisła, (2010), pp. 11–21. [3] J. Hald, Microstructure and long-term creep properties of 9– 12%Cr steels, International Journal of Pressure Vessels and Piping 85 (2008) 30–37. [4] J.C. Vaillant, B. Vandenberghe, B. Hahn, H. Heuser, C. Jochum, T/P23, 24, 911 and 92: new grades for advanced coal-fired power plants – Properties and experience, International Journal of Pressure Vesseles and Piping 85 (2008) 38–46. [5] V. Lepingle, G. Louis, D. Allue, B. Lefebvre, B. Vandenberghe, Steam oxidation resistance of new 12%Cr steels: Comparison with some other ferritic steels, Corrosion Science 50 (2008) 1011–1019. [6] P.J. Ennis, W.J. Quadakkers, Implication of steam oxidation for the service life of high-strength martensitic steel components in high temperature plant, International Journal of Pressure Vessels and Piping 84 (2007) 82–87. [7] P.J. Ennis, W.J. Quadakkers, Mechanism of steam oxidation in high strength martensitic steels, International Journal of Pressure Vessels and Piping 84 (2007) 75–81. [8] J. Żurek, E. Wessel, L. Niewolak, F. Schmitz, T.U. Kern, L. Singheiser, W.J. Quadakkers, Anomalous temperature dependence of oxidation kinetics during steam oxidation of ferritic steels in the temperature range 550–650 8C, Corrosion Science 46 (2004) 2301–2317. [9] D. Laverede, T. Gomez-Acebo, F. Castro, Continuous and cyclic oxidation of T91 ferritic steel under steam, Corrosion Science 46 (2004) 613–631. [10] A. Hernas, G. Moskal, B. Augustyniak, M. Chmielewski, Degradacja austenitycznej stali 321H podczas długotrwałej eksploatacji, Energetyka, Zeszyt tematyczny nr XIX, Projektowanie i innowacje remontowe w energetyce (2009) 65–69. [11] A. Hernas, G. Moskal, K. Przegaliński, Degradacja stali T91 oraz HCM12(A) po 70000 godzinach ekspozycji w kotle, Energetyka, Zeszyt tematyczny nr XIX, Projektowanie i innowacje remontowe w energetyce (2009) 69–72. [12] K. Klepacki, K. Panaś, R. Wojtkiewicz, Korozja przegrzewaczy kotłów współspalających biomasę, XIII Sympozjum Informacyjno-Szkoleniowe, Diagnostyka i remonty urządzeń cieplno-mechanicznych elektrowni, Katowice, 2011 127–132. [13] F. Klepacki, D. Wywrot, Trwałość wężownic przegrzewaczy wtórnych w warunkach niskoemisyjnego spalania, XII Sympozjum Informacyjno-Szkoleniowe, Diagnostyka i remonty urządzeń cieplno-mechanicznych elektrowni, Wisła, 201029–35.
archives of civil and mechanical engineering 14 (2014) 335–341
[14] M. Gwoździk, Characteristic of crystallite sizes and lattice deformations changes in the oxide layer formed on steel operated for a long time at an elevated temperature, Solid State Phenomena 203–204 (2013) 204–207. [15] M. Gwoździk, Z. Nitkiewicz, Texturing of magnetite forming during long-term operation of a pipeline of 10CrMo9–10 steel, Solid State Phenomena 203–204 (2013) 121–124. [16] M. Gwoździk, Utlenianie stali 10CrMo9–10 podczas eksploatacji w wysokiej temperaturze, Surface Engineering (1) (2013) 36–39. [17] M. Gwoździk, Z. Nitkiewicz, Analysis of crystallite size and lattice deformations changes in an oxide layer on P91 steel, Archives of Metallurgy and Materials 1 (58) (2013) 31–34. [18] S. Stachura, M. Gwoździk, Z. Stradomski, Charakterystyka stali nowej generacji dla bloków energetycznych, Energetyka (2) (2010) 94–102. [19] PN-EN 10028:2010, Wyroby płaskie ze stali na urządzenia ciśnieniowe. Część 2: Stale niestopowe i stopowe o określonych własnościach w podwyższonych temperaturach. [20] PN-EN ISO 643: Stal. Mikrograficzne określanie wielkości ziarna. [21] M. Gwoździk, Analiza zdefektowania warstw tlenkowych na stali X10CrMoVNb9-1 długotrwale eksploatowanej w
[22]
[23]
[24]
[25]
[26]
[27]
341
temperaturze 535 8C, Inżynieria Materiałowa 4 (182) (2011) 432–434. M. Gwoździk, Charakterystyka warstw tlenkowych powstałych na stali P91 długotrwale eksploatowanej w podwyższonej temperaturze, Inżynieria Materiałowa 2 (180) (2011) 128–131. M. Gwoździk, Utlenianie stali X10CrMoVNb9-1 podczas eksploatacji w wysokiej temperaturze, Inżynieria Materiałowa 5 (189) (2012) 409–412. M. Gwoździk, Charakterystyka warstw tlenkowych powstałych na stali 13CrMo4-5 długotrwale eksploatowanej w podwyższonej temperaturze, Hutnik – Wiadomości Hutnicze 79 (12) (2012) 932–935. M. Gwoździk, Charakterystyka warstw tlenkowych powstałych na stali 10CrMo9-10 długotrwale eksploatowanej w podwyższonej temperaturze, Hutnik – Wiadomości Hutnicze 78 (9) (2011) 781–783. M. Gwoździk, Z. Nitkiewicz, Badanie przyczepności warstwy tlenkowej powstałej na stali 10CrMo9-10, Hutnik – Wiadomości Hutnicze 79 (12) (2012) 924–927. M. Gwoździk, Z. Nitkiewicz, Charakterystyka odporności na zarysowanie warstwy tlenkowej powstałej na stali P91, Inżynieria Materiałowa 4 (182) (2011) 435–438.