Recent Developments in Niobium Containing Austenitic Stainless Steels for Thermal Power Plants. Abstract

Transcription

1 Recent Developments in Niobium Containing Austenitic Stainless Steels for Thermal Power Plants Mariana Perez de Oliveira 1, Wei Zhang 2, Hongyao Yu 3, Hansheng Bao 3, Xishan Xie 4 1 Companhia Brasileira de Metalurgia e Mineração 12901, Av das Nações Unidas, 23o andar, São Paulo, Brazil 2 CITIC Metal, Capital Mansion 1903, Beijing , China 3 China Iron & Steel Research Institute Group, Beijing , China 4 University of Science & Technology Beijing, Beijing , China Keywords: niobium, power generation, creep, high-temperature corrosion resistance Abstract The challenge of growing continuously in a sustainable way is the main driver to improve efficiency in the use of natural resources. The increasing demand for energy has made thermal power based countries to set audacious programs to increase efficiency of thermal power generation. In China, coal-burning accounts nowadays for approximately 65% of the total primary energy supply being responsible for around 25% of the countries CO2 emission, this coal-based energy supply scenario is believed to continue until Therefore, the country has invested strongly in the last years in the construction of more efficient power plants. To attend higher operating temperatures and steam pressures, the application of higher performance materials is mandatory, presenting improved mechanical resistance - to stand the higher pressures applied - and having sufficient high temperature and corrosion resistance with the best cost-benefit relation possible. The present work addresses some research developments made in niobium containing austenitic stainless steels for super heaters and re-heater tubes in the past years as a joint effort between industry and academia to understand mechanisms and optimize the steel chemical composition, improving its performance. Niobium role has been studied in detail in heat resistant stainless steels TP347H, Super 304 and HR3C, a summary of such studies is presented in this paper. Niobium improves high temperature properties as it precipitates as nano-size MX and NbCrN, well dispersed in the matrix, hindering dislocation movement, increasing precipitation strengthening and creep resistance. Introduction The world primary energy supply matrix relies 30% on coal burning [1], reflecting the importance of thermal based power plants for the continuous development of the planet s economy. The burning of coal, natural gas, and oil for electricity and heat also calls the attention on the sustainability side as it represents the largest single source of global greenhouse gas emissions, accounting for 26% of total global greenhouse gas emissions in 2004 [2]. Despite the fact that more than 60% of the global power plant investments over are spent on renewables sources of energy, fossil-fuelled plants will still play an important role in the future energy supply matrix, accounting for almost 30% of the investments [2]. The basic principle of a thermal power generation plant is to rotate a turbine, which is connected to a generator, with the use of water vapor. The efficiency of this process is influenced by different factors, but the most representative ones are the vapor temperature and pressure.

2 Improvements of 1% in the efficiency of an 800MW vapor turbine would lower CO2 emissions by 1 million tons during its lifetime (around 20 years) [3]. Ultra Super Critical (USC) power plants are the latest generation of thermal power plants and can achieve efficiency of 45%; they use water vapor pressures above 24MPa and temperatures above 566 o C and represent a challenge for the choice of materials, as the higher temperature and pressure demand higher corrosion and heat resistant materials. The most critical components in terms of corrosion and temperature resistance are the super heater and reheater tubes, they can achieve temperatures up to 700 o C. For temperatures above 600 o C, austenitic stainless steel and nickel alloys are the materials to be used, as they present the highest corrosion and creep resistance [4]. This paper will highlight some of the austenitic grades recently developed for this type of components and the role of niobium in such steels, mainly with regards to improving high temperature resistance. Austenitic Stainless Steels The austenitic heat-resistant grades TP347H, Super304H and HR3C are steels widely used as superheater/reheater components for 600 USC fossil power plants. They are characterized by chromium contents in the range of 18 to 25% and nickel above 5%, especially to increase corrosion and oxidation resistance, respectively. Niobium is added in levels that can reach up to 1% to improve the strength at high temperatures through the precipitation of carbides and carbonitrides. The data presented in this paper is a summary of research projects on austenitic heat resistant steels for USC plants conducted by the University of Beijing (USTB) and China Iron and Steel Research Institute Group (CISRI). The chemical compositions of the steels analyzed in this work are shown in Table 1. Table 1 Chemical composition of TP347H, Super304H and HR3C (in wt%) Steel C Si Mn P S Cr Ni Nb N Others Fe TP347H ppm Bal. Super ppm Cu 3.00 Bal. 304H HR3C <67ppm < 81 ppm V Bal. Hongyao et al. [5-7] have conducted studies on the high temperature strength of steels TP347H and Super 304. Steel. The steels have been solution heat treated at 1160 o C and 1150 o C, respectively, and water cooled. Wang et al. [8-9] have studied the high temperature performance of steel HR3C, solution heat treated at 1250 o C and water cooled. Long time aging heat treatments and creep rupture tests have been performed together with detailed microstructure characterization. TP347H TP347H steel contains up to 18%Cr, 10%Ni and 0.8%Nb, as shown in Table 1. Niobium, chromium, carbon and nitrogen are considered the main responsible elements for maintenance of the creep strength during long exposure times at high temperatures, due to the during service formation of nanometer MX type precipitates, which cause strengthening and results in mechanical properties stability during long exposure periods.

3 Hongyao et. al [10, 11] have performed long time aging treatments at 650 o C in a TP347H steel until 10,000hrs. As it can be seen in Figure 1, during the initial 1,000 hours of thermal treatment the micro-hardness of the steel studied increases quickly with the aging time, reaching values close to 220HV, which are kept at such levels until 10,000 hours at 650 o C. Figure 1: Micro-hardness of TP347H steel as a function of aging time at 650. [10, 11] Samples were analyzed by scanning electron microscope (SEM) and high-resolution transmission electron microscopy (HRTEM), at several different stages of the aging treatment. Figure 2 a., b. and c. [10, 11] are SEM pictures that show the precipitate evolution in the sample at the as heat treatment condition and after 1,000h and 5,000 hours aging respectively. It is possible to observe the increase in the volume fraction of precipitates, which are nano-size and homogeneously distributed in ɣ-matrix. The main precipitated phases in the grains were identified as Nb-rich MX type precipitates. The precipitates keep its nano-size and have slow coarsening kinetics, maintaining good strengthening effect. Figure 2:SEM images of TP347H steel after long-time aging at 650 : a. initial state, b. after 1,000h treatment, c. after 5,000h treatment [10,11]. Figure 3 [10, 11] shows HRTEM results of MX phase after 650 /1,000h aging in TP347H. MX phase characterizes with thin plate and in rhomboid morphology. Its crystal structure is NaCl type and the lattice parameter a = nm. This kind of MX phase is rich in Nb, as shown in the EDS analysis. According to HRTEM results, the MX precipitate is not coherent with γ- matrix. According to Thermo-Calc. calculations made in the study, the main equilibrium phases in TP347H steel at 650 are MX, M23C6 and σ. The amount of MX phase increases with increasing of C and Nb contents and the amount of σ phase decreases with increasing of C content. Therefore, C content should be controlled between 0.06 to 0.10% to optimize MX precipitation and to hinder σ phase formation. By adding N element, MX phase becomes a complex carbonnitride, which is very stable for strengthening effect in TP347H.

4 Figure 3: MX phase in TP347H steel after 1,000h aging at 650. a. HRTEM image, b. diffraction patterns of the precipitate and c. EDS result showing the presence of niobium and chromium in the precipitate. [10, 11] Super 304 H This grade also contains about 18%Cr and 9%Ni, as seen in Table 1. However, Cu is added for further precipitation strengthening besides Nb, Cr, C and N. The research made by Hongyao et al [12-14]. has also analyzed the behavior of this steel in long time thermal aging treatment at 650 o C, as shown in Figure 4. Similar to the TP347H steel, but with higher average values of 250 HV; the micro-hardness increases rapidly and reaches a maximum at 1,000h, after that, values were stable during the entire treatment. Figure 4: Micro-hardness measurements during 10,000h long time aging tests at 650 o C with steel Super 304H.[12-14]. Figure 5 shows SEM images of this steel in the initial state and after long-time aging at 650 o C for 500h and for 1,000h, respectively. Before the aging treatment there is only a small fraction of primary MX precipitates (Figure 5a.). However, after long-time aging, Figure 5b. and c., intensive precipitation happens inside the grains and at grain boundaries. Figure 5: SEM images of Super304H after long-time aging at 650 ; a. initial state, b. after 500hrs and c. after 1,000h;[12-14] As the precipitates are nanometric, further studies have been carried on by TEM and HRTEM. Figure 6 shows TEM images of Super304H at the initial state and after long-time aging at 650 o C

5 for 500h. At the initial state, it is possible to observe that the grain boundary is smooth and there are no precipitated particles (Figure 6 a.). After long-time aging for 500 hours, there are many precipitates not only inside the grains but also at grain boundaries (Figure 6 b.). The precipitates inside the grains are Cu-rich phase and MX phase, determined by SAED and EDS analyses. The Cu-rich phase precipitates distribute homogeneously in austenitic matrix and their average size is very about 10nm (Figure 6 c.). Figure 6:TEM images of Super304H after aging at 650 a. initial condition; b. and c. after500h[12-14] Therefore, the main strengthening phases are Cu-rich phase and Nb-rich MX precipitated inside the grains and Cr-rich M23C6 precipitated at grain boundaries. Cu-rich precipitates in Super 304 steels can be considered an additional mechanism to obtain the higher performance that this grade presented in relation to the TP347H grade during long-term aging treatment at 650 o C. The combination of these two nano-size Cu-rich and Nb-rich (MX) precipitates constitute a great contribution to the high strengthening effect in the grade Super304H. HR3C This type of austenitic stainless steel grade, also known as UNS S31042, has the highest performance in terms of steam oxidation and fireside corrosion, as it contains increased amounts of chromium and nickel, usually in the range of 25 and 20%, respectively. [15] Besides niobium addition in the range of 0.4%, the steel has also nitrogen in the level of 0.2% and, the composition studied, 0.06% vanadium, used as a precipitation-strengthening element at ambient temperature. Stabilizing elements such as Nb, Ti, and V greatly improve the creep strength of austenitic stainless steels, mainly by precipitating fine carbides intragranularly. [16] The effect of Nb content on the precipitation in HR3C steel has been studied by Wang et al., which will have some points briefly highlighted here [8,9]. Figure 7 [8] shows the effect of different Nb content on the equilibrium phases in HR3C steel at 700 o C, calculated using Thermo-Calc software. As it can be observed, Nb has little effect on the content of M23C6. The fraction of MX type precipitate do not present an increase with the increase in the Nb content as could be expected but a decrease instead. According with the calculation, the increase in the niobium content caused a slight increase in the Z-phase fraction present and in the sigma phase. Therefore, the analysis shows that the optimum amount of niobium to be added to HR3C steel must be determined in order to achieve the highest dispersion strengthening effect possible through the formation of nano-size and well dispersed MX and NbCrN precipitates. In order determine the optimum niobium content in this grade, vacuum induction laboratory heats of HR3C were made with 0.23wt%, 0.40wt% and 0.56wt% Nb. The ingots produced have been forged and heat treated. Creep test were performed at 700. Figure 8 (a) [8] shows that HR3C steel with 0.40 wt% Nb has the longest lasting fracture life in all the four different creep stresses used in the tests. The creep fracture life of the sample with 0.56 wt% Nb was slightly higher than the sample containing 0.23wt% Nb only at 245 MPa,

6 being lower in the other three conditions studied. As the test stress is increased, fracture life of the three steels tended to converge, but when the test stress was 135 MPa, the relationship between the fracture life and Nb content is more apparent. HR3C steel has the best performance with Nb content of 0.40wt%. Nb precipitates are The hardness variation with aging time at 700 o C for the different steels tested can be seen at Figure 8 (b) [8]. It is possible to infer that the best performance along the test is also seen for the grade with 0.40wt% of niobium. Figure 9 (a)-(d) [9] shows TEM pictures of the interaction between NbC precipitate and dislocations at an aged sample at 700 o C for 6,000 hours from the steel with 0.40wt%Nb. The presence of nano-scale particles distributed homogeneously in the matrix interacts with the dislocations, bringing the strengthening at high temperatures. Mole fraction of phses MX M23C6 σ Z 700 o C Nb content, wt% Figure 7:.Thermodinamic calculation showing the effect of niobium content on the phase equilibrium in HR3C steel at different niobium contents: 0.20%Nb, 0.40%Nb and 0.60% at 700.[8] Figure 8: (a) Effect of niobium content and test stress on creep fracture life of HR3C steel (700 ) and (b) evolution of the hardness as a function of aging time at 700 o C for the different niobium contents studied.[8] (a) (b) (c) Figure 9: TEM pictures of the Interaction of a NbC precipitate and dislocation in aging test at 700oC for the HR3C steel with 0.40wt%Nb. HR3C steel. (a) bright field; (b) dark field; (c) diffraction pattern; (d) identification of the diffraction pattern.[9] (d)

7 Conclusions Despite the increased investments in more sustainable energy resources, thermal power generation through coal burning will still be an important share of the primary world energy supply. Therefore, there is still a strong need for the improvement of energy efficiency, which is obtained in USC power plants through higher vapor temperatures and pressures. Austenitic stainless steels have been used at temperatures above 600 o C, being the grades TP347H, Super304H and HR3C the ones with the highest performance. The use of niobium has an important role in improving such high temperature properties as it precipitates as nano-size MX and NbCrN, well dispersed in the matrix, hindering dislocation movement, increasing precipitation strengthening and creep resistance. References [1] Environmental Protection Agency (EPA), United States, [2] International Energy Agency (IEA), Key World Energy Statistics, International Energy Agency (IEA), pdf. [3] THORNTON, D.V. and MEYER, K.H.. Proceedings of 2000 International Joint Power Generation Conference, Florida, July 2000 [4] OAKEY, J.E., PINDER, L.W., VANSTONE, R., HENDERSON, M., OSGERBY, S. Department of Trade and Industry s Cleaner Coal Technology Transfer Programme Report, [5] HONGYAO, Y., CHENGYU, C., JIANXIN D., et al. Journal of University of Science and Technology Beijing, 2010, 32(7): [6] CHENGYU, C., HONGYAO.Y,JIANXIN D., et al. Progress in Natural Science: Materials International, 2012, 22(3): [7] HONGYAO, Y., JIANXIN D, XIE, X.. Chinese Journal of Materials Research, 2010, 24(5): [8] WANG J. Study on Microstructure and Mechanical Properties of Austenitic Heat-resistant S31042 Steel. Doctoral Dissertation (2011). [9] WANG J., LIU Z., BAO H.S., CHENG S, WANG B.. Journal of Iron and Steel Research International. Vol. 20 (2013), No.4, p [10] HONGYAO, Y., CHENGYU, C., JIANXIN D. Materials Science, 2011, 1: [11] HONGYAO, Y., CHENGYU, C., JIANXIN D. Heat-Resistant Steel. Advanced Materials Research, 2012, : [12] HONGYAO, Y., JIANXIN D., XISHAN, X. Materials Science Forum, 2010, : [13] CHENGYU, C, HONGYAO, Y., XISHAN X.. Alloy Steel: Properties and Use, InTech, 2011 [14] CHENGYU, C, HONGYAO, Y, JIANXIN D. Acta Metallurgica Sinica, 2011, 24(2): [15] MASUYAMA, F. ISIJ International, Vol. 41 (2001), No. 6, pp , December [16] SOURMAIL, T. Materials Science and Technology, vol.17, January 2001.