UDK 669.14:620.18 Original scientific article/Izvirni znanstveni članek ISSN 1580-2949 MTAEC9. 41(6)301(2007) DEVELOPMENT OF MICROSTRUCTURE OF STEEL FOR THERMAL POWER GENERATION RAZVOJ MIKROSTRUKTURE JEKEL ZA TERMIČNO GENERACIJO ENERGIJE Kvackaj Tibor, Kuskulic T., Fujda M., Pokorny I., Weiss M.1, Bevilaqua T.1 Technical University in Kosice. Slovakia 1ZP-Podbrezova. a. s. tibor.kvackaj@tuke.sk Prejem rokopisa — received: 2006-10-05; sprejem za objavo - accepted for publication: 2007-10-18 The evolution of microstructure during the reheating and cooling of steel for thermal power generation was investigated. On the basis of the microstructure produced during cooling a CCT diagram is proposed. Key words: steel. thermal power generation. microstructure. CCT diagram Raziskan je bil razvoj mikrostrukture pri segrevanju in ohlajanju jekel za toplotno generacijo energije. Na podlagi mikrostrukture. ki je nastala pri ohlajanju. je bil predložen CCT-diagram. Ključne besede: jeklo. toplotna generacija energije. mikrostrukture. CCT diagram 1 INTRODUCTION Natural gas is now an important source of energy. However. especially in developing countries. lignite and anthracite will also be used for the generation of electrical energy. and power stations utilizing these energy resources for steam production will also play an important role in the future. The minimization of the capital costs required to build new steam-power stations leads to the extensive use of ferrite-martensite steels. or martensite steels for all the main components in boilers and turbines 1. For modern power stations the operating conditions are defined as follows: • 540 oC/18 MPa • 600 oC /30 MPa • 720 oC /37.5 MPa - expected for the future The applied materials were analyzed for their creep rupture strength under the following conditions: • short-term creep behavior up to t = 10 000 h • long-term creep test up to t = 100 000 h In Figure 12 the chemical analysis and the creep resistance is given for some classic and newly developed steels. The 1CrMoV steel has been used for a very long time and is considered as a classic steel. The steels with an increased content of Cr up to 9-12 % with addition of Nb and B are considered as newly developed types. The newly developed steels can be. in comparison to classic materials. applied in operating conditions with increased steam temperatures of 30-70 °C. The steels for Steel C Cr Mo W v Nb N B wft4 ICrMoV 0.20 1,0 0,9 - 0,30 . 12CrMoV 0.21 12.0 1.0 0,30 - - 1 OCrMoV 0.12 10.0 1.5 0.20 0.05 0.05 - 10CrMoWV 0.12 10.0 1.0 1.0 0.20 0.05 0.05 - SCrMoVB 0.13 9.0 1.5 0.25 0.05 0.02 0.010 100 COQ h creep rupture s trength 1%OMoV\/ \ 10« :>Mo[W)VNbN10-1(-1) 9%CrMoVNfciB 12%CrMoV apprax. 30 "C appro*. 70 "C 500 550 500 55£ Temperature. T/"C Figure 12: Creep rupture strength of the new 9-10 % Cr rotor materials applied in Europe Slika 1: Odpornost proti lezenju pri novih 9-10 % Cr jeklih. ki se uporabljajo v Evropi Table 13: Chemical composition and properties of two steels Tabela 13: Kemična sestava in lastnosti dveh jekel w C Cr Mo W Ni V Nb N B/ Mg/g Rp0.2/ MPa fatt50/ °C GX12CrMoVNbN9 1 0.12 9 1.0 — 0.4 0.2 0.06 0.05 80 633 +46 GX12CrMoWVNbN 10 1 1 0.12 10 1.0 1.0 0.7 0.2 0.06 0.05 10 730 +40 Materiali in tehnologije / Materials and technology 41 (2007) 5, 231-236 2 31 T. KVACKAJ ET AL.: DEVELOPMENT OF MICROSTRUCTURE OF STEEL FOR THERMAL POWER GENERATION steam-turbine rotors can be divided into three groups according to their chemical composition: • 10 % Cr + Mo • 10 % Cr + W+ Mo • 9 % Cr + Mo + B These steels are used for the manufacturing of rotors with diameters up to 1200 mm. The basic properties for two steel grades are given in Table 13. Besides the steels based on CrMoV or CrMoVW, steels based on CrMoNiV or CrMoNiVW have also been developed. The research work is currently oriented to improve steel technology with the aim to decrease the content of unwanted elements in the steel and to master the forging technology and heat-treating processes of these high-purity steels. The aim is to obtain good chemical and structural homogenity of the material, as well as forgings with minimum of defects 4. The properties of steel depend on the steam characteristics, since the turbines can operate in conditions of: • HP - high pressure • IP - intermediate pressure • LP - low pressure The specifications of the material should be defined by the following characteristics: 1. Static strength - failure strength 2. Creep rupture strength, high-temperature strength 3. Toughness - fracture toughness 4. Fatigue properties - low-cycle fatigue - high-cycle fatigue 5. Crack growth rate - static-creep (CG) - alternative - fatigue 6. Corrosion resistance - local corrosion - corrosion under pressure - corrosion fatigue 7. Erosion resistance 2 MATERIAL AND EXPERIMENTS For the experiments the CrNiMoV steel, which is the equivalent to STN 41 6537, with the chemical composition in Table 2, was used. The experiments were aimed at: - the evaluation of the influence of temperature and time on the austenite grain size. - the influence of cooling rate on the formation of the microstructure and the ARA diagram. The experimental methods were: - light microscopy - differential dilatometry Table 2: Chemical analysis of the experimental steel Tabela 2: Kemična sestava jekla za raziskavo 3 RESULTS AND DISCUSSION The diagrams showing the influence of reheating temperature and reheating time on the austenite grain size change are in Figure 2 and Figure 3. From the dependencies in these two figures it was concluded that: - the holding time at 1000 °C does not influence the austenite grain size. Thus it is possible to classify this temperature as "low-sensitive" to austenite grain size change. - the reheating temperature of 1050 °C had a great influence on the austenite grain size, while, for holding times of 15 min and 30 min the difference in the austenite grain size is on average 8 pm, but for holding times of 45 min and 60 min this difference increases on average to 45,5 pm. It is possible to classify this temperature as sensitive to austenite grain size change. - for reheating temperatures of (1100, 1150, 1200) °C, there is a slow increase in the austenite grain size for all the reheating times. It is possible to classify this 15 min 30 min 45 min 60 min 1100 T/°C Figure 2: Dependence of AGS on reheating time Slika 2: Odvisnost velikosti AGS avstenitnih zrn od temperature segrevanja 160 140 120 100 80 60 40 20 0 20 30 40 Time, t/min Figure 3: Dependence of AGS on reheating time Slika 3: Odvisnost AGS od trajanja segrevanja C Mn Si P S Cr Ni Cu Mo V Al As Sn Sb Ca H N O w/% w/(pg/g) 0.29 0.04 <0.01 0.003 0.003 1.57 2.84 0.010 0.39 0.11 0.004 11 8 <5 20 0.5 44 25 950 1000 1050 1150 1200 0 302 Materiali in tehnologije / Materials and technology 41 (2007) 6, 301-303 T. KVACKAJ ET AL.: DEVELOPMENT OF MICROSTRUCTURE OF STEEL FOR THERMAL POWER GENERATION 1 ID I« IOUA ] LHJtXI 140000 t/% Figure 4: CCT diagram Slika 4: CCT diagram temperature interval as low-sensitive to austenite grain size change. At the reheating temperature of 1050 °C a faster growth of austenite grain size is observed by increasing the reheating time. From 30 min to 45 min there is a loss of the hindering effect of carbide and nitride particles on the migration of austenite grain boundaries. The solubility of the VC and VN precipitates for given contents of vanadium and carbon is calculated from Equations (1) and (2): lg (w(V)4/3 ■ w(C)) = 7,06 - 10 800/T (1) lg (w(V) ■ w(N)) = 3,02 - 7 840/T (2) The solubility is reached at 940 °C for vanadium carbide and 970 °C for vanadium nitride. It is clear that the precipitates of vanadium carbide and nitride do not hinder the growth of austenite grains during the reheating time at 1050 °C. The influence of cooling rate on the formation of the final microstructures can be evaluated by considering the dilatometry curves, the hardness and the microstructures after different cooling rates from a constant reheating temperature. In this way the transformation CCT diagram can be obtained Figure 4. The obtained diagram shows that in the examined steel a real-time ferritic transformation of austenite does not occur and that only the formation of martensite, martensite + bainite, or bainite takes place. The investigated steel is thus a high-through-hardening steel. 4 CONCLUSION From the results of the experimental work on a CrNiMoV steel it is possible to draw the following conclusions: - for the reheating temperature T = 1000 °C, the effect of reheating time is negligible and the austenite grain size remains in the interval đ = 49-57 pm. This reheating temperature ensures the fine-grained austenitic structure, which is a good starting point for achieving a fine-grained secondary structure, also in cases without plastic deformation after reheating. This is also valid for 15 min or 30 min reheating at 1050 °C when the diameter d = 52-61 pm is obtained. - for the reheating temperature of 1050 °C and reheating time t = 45 min and 60 min an increased sensitivity to austenite grain growth is found and the size of ô = 103-109 pm is obtained. Similar characteristics are also observed for the reheating conditions T = 1100-1200 °C and t = 15 min or 60 min when the size of đ = 106-140 pm is obtained. If the reheating conditions are chosen considering the mentioned intervals, a controlled forging and controlled cooling regime will be required for achieving a fine-grained secondary structure. - the CCT diagram obtained shows that the investigated steel is self-hardening. Acknowledgement This research was carried out within the scope of the EUREKA E!3192 ENSTEEL project. 5 REFERENCES 'Kvackaj, T.: Entry opponency of project EUREKA E!3192 ENSTEEL 2 Sheng, S., Kern, T. U.: High strenght cast and forged materials for application in steam turbine design, In: PARSONS 2000 Advanced materials for 21s century turbines and power plant, 3-7 July 2000, Cambridge 3 Mayer, K. H., Kern, T. U., Staubli, M., Tolksdorf, E.: Long-term investigation of specimens of 24 production components manufactured from advanced martensitic 10 % Cr steels for 600 °C steam turbines, In: PARSONS 2000 Advanced materials for 21st century turbines and power plant, 3-7 July 2000, Cambridge 4 Scarlin, R. B.: Improved materials for high efficiency steam turbines, In: Advances in turbine materials, design and manufacturing, 4-6 November 1997, Newcastle upon Tyne 302 Materiali in tehnologije / Materials and technology 41 (2007) 6, 301-303