Elevated Temperature Low Cycle Fatigue Properties of Martensitic Cast Steel

Transcription

1 International Journal of Engineering & Technology IJET-IJENS Vol:13 No:01 86 Elevated Temperature Low Cycle Properties of Martensitic Cast Steel Stanisław Mroziński, Grzegorz Golański Abstract This work investigated the elevated temperature low cycle fatigue (LCF) properties of GX12CrMoVNbN9 1 (GP91) cast steel. tests were performed for five levels of total strain amplitude ξac and temperature of 400, 550 and 600oC. In addition, the preliminary results from tensile test were presented. Strong cyclic softening was observed in all fatigue tests at elevated temperatures without stabilization period of loop parameters. The plastic strain amplitudes during cyclic strain loading were measured and correlated with the fatigue lifetime using Coffin Mason Basquin plots at each test temperature. The fatigue lifetime decreased as the temperature test increased. The temperature effect on lifetime was more pronounced at low strain amplitudes. Index Term estimation, Cast., Mechanical properties, Lifetime I. INTRODUCTION Thermal mechanical fatigue occurring with the participation of elastic plastic strains is the basic mechanism of damage in many elements serviced at elevated temperatures. These elements include for example: electric power boilers, boiler pipes, superheaters, engine elements. Temperatures of service for the steels, of which these elements are made, reach C. The basis for forecasting the fatigue lifetime of such elements is the knowledge of low cycle properties of these materials, determined at elevated temperatures. Material characteristics are most often determined for the so-called period of stabilization of cyclic properties. If this period does not occur, they are determined from the period corresponding to half the fatigue lifetime. Therefore, they do not take account of mutual interactions of stress and temperature appearing during low cycle fatigue and their influence on the course of low cycle properties. This is the reason why the results of calculations and tests of fatigue lifetime of construction elements subject to changing load at elevated temperatures are characterized by a considerable scatter [1], [2]. The required reliability, necessary This paper was realized in the framework of the grant No. 1215/B/T02/2011/40 funded by Ministry of Science and Higher Education in the years Stanisław Mroziński is with the University of Technology and Life Sciences in Bydgoszcz, Kaliskiego 7, Bydgoszcz, Poland corresponding author (phone: ; fax: ; stmpkm@utp.edu.pl). Grzegorz Golański is with the Institute of Materials Engineering, Czestochowa University of Technology, Armii Krajowej 19, Czestochowa, Poland ( grisza@wip.pcz.pl). for these elements, is mostly achieved through selection of adequately high factor of safety. The fundamental purpose of this study is to determine the influence of elevated temperature on low cycle properties of a cast steel. Additional aim is experimental verification at elevated temperature of the analytical models used for the description of low cycle properties of steels at ambient temperature. II. EXPERIMENTAL PROCEDURE The research material was high-chromium GX12CrMoVNbN9-1 (GP91) cast steel of the following chemical composition (%mass): 0.12C; 0.47Mn; 0.31Si; 0.014P; 0.004S; 8.22Cr; 0.90Mo; 0.12V; 0.07Nb; 0.04N. The investigated GP91 cast steel was after heat treatment (as-received condition) with the following parameters of temperature and time: 1040 o C/12h/oil o C/12h/air o C/8h/furnace. The influence of heat treatment parameters on the properties and microstructure of the examined cast steel is presented inter alia in the work [3]. Low cycle tests were performed using testing machine, the 8502 Instron type, with strain control ( ac = const). The tests were carried out at elevated temperature: 400, 550 and 600 o C. tests were preceded by the static test of tension run at the abovementioned temperatures. The test samples prepared for research were round and threaded (Fig. 1). tests as well as the static test of tension at elevated temperature were realized using the heating chamber. The test pieces were resistance-heated, the temperature of test pieces was controlled using thermoelements Pt Rh/Pt. Loading applied during the tests was oscillating sinusoidally with the strain ratio R = - 1. The tests were carried out at five levels of total strain amplitude ac : 0.25; 0.30; 0.35; 0.50 and 0.60%. The frequency of load change f during the tests amounted to 0.2Hz. Assumed as the criterion for the end of a fatigue test and at the same time the fatigue lifetime N f at a given strain level, was the number of cycles N at which the occurrence of deformation on the hysteresis loop arm in the compression half-cycle was observed. The analysis of fatigue properties of GP91 cast steel under the conditions of changing loads was performed using the parameters of hysteresis loop which included: total strain amplitude - ac, plastic strain amplitude - ap, elastic strain amplitude - ae, stress amplitude - a. Their values were determined on the basis of the values of loading force and strains recorded during the fatigue tests. On the basis of the recorded values of stress amplitude a in the following stress cycles, the graphs of changes in the

2 International Journal of Engineering & Technology IJET-IJENS Vol:13 No:01 87 characteristic hysteresis loop parameters were plotted, as the function of the number of stress cycles N. The characteristic quantities for the loop, determined for half the number of cycles to failure (N/N f =0.5), were used while preparing the basic fatigue characteristics of the cast steel. The obtained data were also used to determine the slopes of regression lines applied for the description of dependence between stress a and strain ap (Manson s criterion). Fig. 1. Test sample for the low cycle fatigue tests III. RESEARCH RESULT AND ANALYSIS A. Initial microstructure The investigated cast steel in the as-received condition (after heat treatment) was characterized by the microstructure of high-temperature tempered martensite with elongated subgrains whose shape was inherited from the lath martensite with numerous precipitations. On the boundaries of prior austenite grain and on the boundaries of subgrains, M 23 C 6 carbides were precipitated. Inside the subgrains, many precipitations of the MX type were observed. Such a microstructure is a typical microstructure of quenched and tempered 9 12%Cr steels [4], [5]. Detailed information on the microstructure of high-chromium steels/cast steels is provided in the work [3], [6]. Example of the microstructure of the examined cast steel in the as-received condition is presented in Fig. 2. B. Tensile properties at elevated temperatures The low cycle fatigue tests of GP91 cast steel in the as - Fig. 2. Microstructure of tempered martensite of GX12CrMoVNbN9-1 cast steel in the as-received condition (after heat treatment ), SEM, etched with ferric chloride received condition were preceded by the static test of tension at elevated temperature. Table I includes the results obtained from the test of mechanical properties. T ABLE I Mechanical properties of GX12CrMoVNbN9 1 cast steel at elevated temperature Temperature o C YS TS An increase in the temperature of testing leads to a decrease in yield strength (YS) from the level of 419 to 303, as the temperature of testing increases from 400 to 600 o C. A similar dependence was observed for tensile strength (TS), where a significant decrease from 536 to 338 could be noticed. A decrease in the values of strength properties was accompanied by the growth of the values of plastic properties elongation and reduction of area. C. LCF properties at elevated temperatures The tests carried out have proved that the process of low cycle fatigue of GP91 cast steel is characterized by strong cyclic softening (an increase in the width of hysteresis loop ap and a strong decrease in the stress amplitude a ), whose intensity grows along with the growth of temperature of the fatigue test. Regardless of the fatigue test temperature, there was no period of stabilization of the hysteresis loop parameters observed in the following stages of cyclic strain. Cyclic softening of the examined cast steel continued until the occurrence of a crack in the test piece, which proves cyclic exhausting of fatigue lifetime of the cast steel. Example of changes in the hysteresis loop parameter stress a, as the function of the number of cycles N for three levels of strain ac, is presented in Fig. 3. Cyclic softening that occurred during low cycle fatigue was also observed in high-temperature creep resisting martensitic steels of 9 12%Cr grade. In these steels however, contrary to the investigated cast steel, there was a clear period of stabilization of the hysteresis loop parameters observable stress amplitude a and strain amplitude ac [7]. Due to the lack of a clear period of stabilization of the hysteresis loop, analytical description of fatigue properties of the examined cast steel is considerably difficult. Considering the changes observed in the parameters of hysteresis loop, in the function of the number of stress cycles, the values of hysteresis loop parameters necessary for analytical descriptions of characteristics of the examined cast steel were determined for the number of cycles N corresponding to El. % RA % E

3 International Journal of Engineering & Technology IJET-IJENS Vol:13 No:01 88 Fig. 3. Influence of temperature on the changes in stress a : a) ac =0.25%, b) ac =0.35%, c) ac =0.60% a) b) c) Fig. 4. Influence of the temperature of fatigue test on the loop shape: a) ac =0.25%, b) ac =0.35%, c) ac =0.60% 0.5N f (points 1, 2 and 3 in Fig. 3). Examples of hysteresis loop obtained at three levels of strain for this number of stress cycles is shown in Fig. 4. On the basis of comparative analysis of hysteresis loops obtained at the tested temperatures for five values of strain (Fig. 3 and 4), it can be concluded that the temperature has an influence on two basic parameters of the loop, i.e. ε ap (width of the loop) and σ (height of the loop). For the same values of strain, along with the temperature growth, the loop width increases and the loop height decreases. For the analytical description of dependence between stress σ a and strain ε ap, Morrow s equation was applied (1): lg lg K' n' lg (1) a ap where: K - cyclic strain hardening coefficient, ; n - cyclic strain hardening exponent. The graphs obtained as a result of approximation of hysteresis loop parameters ( a and ε ap ) from the periods corresponding to half the fatigue lifetime (N/N f =0.5) are shown in Fig. 5. While Table II includes the values of parameters of Morrow s equation (n and K ). Mathematical model of cyclic softening of GP91 cast steel, described with Morrow s equation (1), is given in Table II. Fig. 5. Influence of temperature on the characteristics of cyclic strain of GP91 cast steel

4 International Journal of Engineering & Technology IJET-IJENS Vol:13 No:01 89 T ABLE II Functions describing the course of cyclic strain of GP91 cast steel, described with Morrow s equation (1) Temperature, o C Regression function and correlation coefficient lg a = lgk + n lgε ap ; n cyclic strain hardening exponent; K cyclic strain hardening coefficient. 400 lg a = lg lgε ap ; R 2 = lg a = lg lgε ap ; R 2 = 0.96 steel for all temperatures ran with the dominant role of plastic strain component ap. T ABLE III Mathematical model of fatigue life of GP91 cast steel strength coefficient f ductility coefficient f strength exponent b ductility exponent c strain reversals 2N t 600 lg a = lg lgε ap ; R 2 = 0.96 Growth of the temperature of fatigue test leads to an evident decrease in the value of coefficient K and increase in the value of hardening coefficient n for the temperature of 600ºC and its decrease at the temperature of 550ºC. The values of parameters n and K the basic material characteristics used during the calculations of low cycle loads in the case of materials not revealing the period of stabilization, depend on the period of lifetime (N/N f ), for which they were determined. In the work [8], [9] it has been shown that the values of parameters n and K depend on the number of cycles of changing loads for which they were determined. The values of these parameters determined at half the lifetime (N/N f =0.5) are not the average values for the whole fatigue test, which in the Authors view shows the scale of simplification and can lead to considerable errors. This remark gains particular meaning when describing the fatigue properties of a cast steel at elevated temperatures in which the range of changes in these properties is bigger, compared to ambient temperatures. lifetime of the investigated cast steel is described using the equation of Manson-Coffin-Basquin (MCB) (2). ºC Therefore, it can be assumed that for these strain levels ac the cyclic strain resistance of the investigated cast steel mostly depends on its plastic properties. Similar dependence was also observed in the case of high-temperature creep resisting martensitic steels of the P91 and P92 type [10], as well as HB20 type cast stainless steel [11], however, the point of intersection N t for these steels at room temperature amounted to about 1000 cycles, while for the HB20 steel at the temperature of 600 o C 4012 cycles. ac 2 ae 2 ap 2 ' f E 2N f b ' 2N f f (2) where: b - fatigue strength exponent; c - fatigue ductility exponent; f - fatigue strength coefficient, ; f - fatigue ductility coefficient; E - Young s modulus,. For graphic illustration of the influence of temperature on lifetime, Fig. 6 shows the obtained results in the form of fatigue graphs, whilst Table III includes the parameters of MCB equation (2). Performed analysis of the obtained characteristics (Fig. 6) shows that the abscissa 2N t, the point of intersection of two curves: ae =f(2n f ) and ap =f(2n f ), in the analyzed cases, amounts to 4620 and around 5700 cycles, respectively, for the temperature of 400 o C and for temperatures of 550 o C and 600 o C. This proves that with the values of total strain ac applied in fatigue tests, the process of cyclic strain in the examined cast Fig. 6. Low cycle fatigue life of GP91 cast steel at 400 and 600 o C temperature Analysis of the performed fatigue graphs (Fig. 6) obtained at room temperature and elevated temperature allows to state that the influence of temperature on lifetime depends on the level of total strain amplitude. This influence is slight in the area of the largest strains realized in the research ( ac =0,60%) and increases as the value of strain ac falls (Fig. 7). Characteristics of low cycle fatigue of the examined cast steel is provided in Table IV. Due to the lack of a clear stabilization period of the fatigue characteristics (Fig. 3), the value of stress a was determined from the period corresponding to half the fatigue lifetime (N/N f =0.5) [12], [13].

5 Strain amplitude ε ac, % International Journal of Engineering & Technology IJET-IJENS Vol:13 No:01 90 T ABLE IV characteristics of GP91 cast steel in the as-received condition Temperature 400ºC Temperature 550ºC Temperature 600ºC cycles to failure N f Stress a (N/N f =0.5), cycles to failure N f Stress a (N/N f =0.5), cycles to failure N f Stress a (N/N f =0.5), cycles to failure Nf Strain ac, % Fig. 7. Low cycle fatigue life of GP91 cast steel in the as-received condition: 400 at the temperature of 400ºC; 550 at the temperature of 550ºC; 600 at the temperature of 600ºC Increase in the temperature of testing in the scope of low cycle fatigue, apart from reducing the fatigue lifetime of GP91 cast steel, also contributes to the straining of the investigated material while the values of stress are lower and lower σ a (Table IV). This has an influence on the reduction of low-cycle stress transfer capacity, along with the temperature growth. During the tests, regardless of the temperature, the changes in fatigue characteristics of cast steel were observed (changes in the hysteresis loops parameters). The changes in the loops parameters as the function of a number of stress cycles and the lack of a clear stabilization period, make it difficult to determine the representative material data, used further for the calculations of fatigue life. The results obtained while testing the cast steel confirm the results included inter alia in the work [2], where the difficulties in determining the period of stabilization of cast steel at elevated temperature as well as the influence of the extent of fatigue damage on material data were signaled. The changes in fatigue characteristics of steels/cast steels occurring at elevated temperatures are the reason why calculating the fatigue life of construction elements serviced at elevated temperatures using material data, determined e.g. at half the fatigue life, raise doubts. This data reflect only the instantaneous properties of the material. The work [14], proposes a method of calculating fatigue life that considers the changes in cyclic properties occurring during loading. A new calculation method requires special analysis of the results of low cycle fatigue tests. The proposal of method for analyzing the test results is provided inter alia in the work [8]. IV. CONCLUSION 1. Martensitic GX12CrMoVNbN9-1 cast steel during low cycle fatigue at elevated temperatures of 400, 550 and 600C is subject to cyclic softening and does not reveal a clear period of stabilization. 2. The extent of changes in cyclic properties is influenced by the level of strain ac and the temperature. At the temperature of 600C, the extent of changes in cyclic properties is definitely higher than at the temperature of 400C. At both temperatures, the extent of changes in fatigue properties decreases along with the growth of total strain ac. 3. lifetime of martensitic cast steel is influenced by the level of strain ac,, as well as the temperature of testing. Influence of the temperature on fatigue lifetime depends on the level of strain. It is slight in the area of very large strains and increases as the level of strain falls. 4. The changes appearing in the parameters of hysteresis loop and the lack of clear stabilization period at elevated temperatures of the examined cast steel makes it difficult to determine the basic material data. Their values depend on the period of fatigue lifetime assumed to determine them. Assuming them from the period corresponding to half the fatigue lifetime makes them reflect only the instantaneous cyclic properties of the cast steel from this period of fatigue lifetime. 5. During the service of power plant facilities, interactions connected with the changes in stress and temperature occur. The tests presented in this paper were carried out under the conditions of constant amplitude stress and constant temperatures. In order to formulate detailed conclusions on the fatigue properties of the cast steel, further studies should take into consideration the changes in stress and temperature occurring during the tests.

6 International Journal of Engineering & Technology IJET-IJENS Vol:13 No:01 91 REFERENCES [1] J. Byrne, and N. Y. K. Kan, I. W. Hussey, G. F. Harrison, Influence of sub-surface defects on low-cycle fatigue life in a gas turbine disc alloy at elevated temperature, Inter. J. of, vol. 21, 1999, pp [2] A. Nagesha, M. Valsan, R. Kannan, K. Bhanu Sankara Rao and S. L. Mannan, Influence of temperature on the low cycle fatigue behaviour of a modified 9Cr-1 Mo ferritic steel, Inter. J. of, vol. 24, 2002, pp [3] G. Golański and J. Słania, Mechanical properties of the GX12CrMoVNbN91 (GP91) cast steel after different heat treatments, Archives Metall. Mater., vol. 57/2, 2012, pp [4] A. Zielińska Lipiec, T. Kozieł and A. Czyrska Filemonowicz, Quantitative characterisation of the microstructure high chromium steel with boron for advanced steam power plants, J. Achiev. Mater. Manufact. Eng., vol. 43/1, 2010, pp [5] J. Hald, Microstructure and long-term creep properties of 9 12% Cr steels, Inter. J. 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Mroziński and K. Werner, Influence of Temperature on Low Cycle Properties of Martensitic Cast Steel, The Archive of Mech. Eng., 2012, vol. 59/3, pp [13] S. Mroziński and G. Golański, Low Cycle of GX12CrMoVNbN9-1 Cast Steel at Elevated Temperature, J. Achiev. Mater. Manufact. Eng., 2011, vol.49/1, pp [14] S. Mroziński, Stabilization of cyclic properties in metals and its influence on fatigue life, Publ. House of the University of Technology and Life Sciences, Bydgoszcz, No. 128, 2008 (in Polish) Stanisław Mroziński was born on 7th May 1961 in Bydgoszcz (Poland). In 1986 he graduated from University of Technology and Life Sciences (the UTP) in Bydgoszcz, Faculty of Mechanical Engineering. Since 1986 he has been employed in the UTP Faculty of Mechanical Engineering, where he got the following academic degrees: PhD degree in technical sciences (1995), post -doctoral degree in technical sciences (2008). Since 2010 he has been employed as an associate professor of the UTP. In the years he performed the function of a deputy dean in the Faculty of Mechanical Engineering for organization and development issues. He is an author or co-author of around 60 articles and scientific papers presented in conferences and seminars, home and abroad. At present he manages a research laboratory accredited in 2001 by the Polish Centre for Accreditation. His research work deals with the issues of fatigue of materials and constructions, as well as methods of experimental study on the structure and operation of machines. Since 2002 Stanisław Mroziński, Ph.D. (Eng) has been an expert auditor of the Polish Centre for Accreditation, assessing the research laboratories in Poland. He actively participates in the work of associations acting in the university as well as in the country: Polish Society of Mechanical Engineers and Technicians (SIMP), Polish Society of Theoretical and Applied Mechanics (PTMTiS), European Structural Integrity Society (ESIS). His achievements for the University were awarded several times, for instance with state medals (Silver Cross of Merit in 1996) and First-Class Rector Awards. Grzegorz Golański was born on 7th October 1973 in Wieluń (Poland). In 1998 he graduated from Czestochowa University of Technology in the Faculty of Metallurgy and Materials Engineering. Since 2001 he has been employed in this Faculty, first as an assistant, and next as an assistant professor. In 2003 he got a degree of PhD in technical sciences in the field of materials engineering. He is an author or co-author of around 110 articles and scientific papers presented in conferences and seminars, home and abroad. Either as a supervisor or executor, he was involved in realization of over 50 scientific research works connected with production problems. He took part in the realization of 7 research projects (in two of them as a supervisor). His research work deals with the issues related to the processes of degradation of high-temperature creep resisting materials, as well as methods of shaping of microstructure and properties of steels and cast steels through heat treatment. For his remarkable achievements in t he field of science he was awarded several times, for instance with Rector Awards.