UDK 669.14:620.18:620.17:621.73 Original scientific article/Izvirni znanstveni ~lanek ISSN 1580-2949 MTAEC9, 42(3)121(2008) DEVELOPMENT OF MICROSTRUCTURE DURING THE HOT PLASTIC DEFORMATION OF HIGH CLEAN STEELS FOR POWER PLANTS RAZVOJ MIKROSTRUKTURE MED VROČO PLASTIČNO DEFORMACIJO VISOKO ČISTEGA JEKLA ZA ENERGETSKE NAPRAVE Kuskulic, Tomas1, Kvackaj, Tibor1, Fujda, M.1, Pokorny, I.1, Bacso J.1, Molnarova, M.1 Kocisko, R.1, Weiss, Michael2, Bevilaqua, Tomas2 1 Technical university of Košice, Letna 9, 04200 Košice, Slovakia 2 ZP-Podbrezova a.s., Kockaren 35, 97681 Podbrezova, Slovakia Prejem rokopisa — received: 2007-09-28; sprejem za objavo - accepted for publication: 2008-01-11 The effect of the deformation and the deformation temperature on the primary austenite grain size, the recrystallisation and the mechanical properties of high clean steel STN 41 6537 (STN standard) were investigated. This steel grade is used for the forged rotors of steam turbines in power-generation facilities. Key words: steel, power generation, forging, recristallisation, mechanical properties Raziskan je bil vpliv deformacije in deformacijske temperature na velikost avstenitnih zrn, rekristalizacijo in mehanske lastnosti zelo čistega jekla STN 416537 (STN standard). To jeklo se uporablja za kovane rotorje parnih turbin pri proizvodnji električne energije. Ključne besede: jeklo, proizvodnja energije, kovanje, rekristalizacija, mehanske lastnosti 1 INTRODUCTION The world is characterised by a constantly increasing population and a demand for improved living conditions. The industrialisation of a country depends strongly on the availability of electrical energy, which plays a key role in the rate of development1. Modern high-pressure steam turbines operate under high working loads and at high temperatures. For this reason, much concern has been paid to the fatigue and creep behaviours of turbine materials. The rotor of a steam turbine also operates at high temperatures, and during complex stressing some cracks are likely to initiate. In addition to fatigue, creep damage plays an important role in rotor damage. Generally speaking, low-cycle fatigue uses up seventy percent of the life of the rotor, and creep accounts for the remaining thirty percent2. However, fatigue and creep always occur together, and the coupling of fatigue and creep must be considered in the lifetime prediction of a steam-turbine rotor3. Much effort has been spent on developing a new, high-strength ferritic resistant steel, which would also be used for large components of fossil-fuel-fired power plants4. Because of the degradation during long-term service at elevated temperatures that causes changes in the microstructure5, an improvement in microstructural stability is a prerequisite for achieving excellent long-term creep strength. New steels have been widely used in modern fossil-fuel-fired power plants and in many investigations the correlations between the microstructure and the mechanical properties have been reported6. The hot-working behaviour, in conjunction with the changes in the microstructure and the degradation during long-term creep deformation, is investigated and discussed in this article. 2 MATERIAL AND EXPERIMENTS For the experiments a steel based on CrNiMoV was used; this is equivalent to STN 41 6537, with the chemical composition described in Table 1. Table 1: Chemical composition Tabela 1: Kemična sestava C Mn Si P S Cr Ni Cu Mo V Al As Sn Sb Ca H N O w/% Mg/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 Materiali in tehnologije / Materials and technology 42 (2008) 3, 121-124 121 T. KUSKULIC ET AL.: DEVELOPMENT OF MICROSTRUCTURE DURING THE HOT PLASTIC DEFORMATION The experiments were designed to determine the influence of temperature and deformation on the austenite grain size. For the tests on the influence of the finish forging temperature and the final amount of deformation on the evolution of austenite grain size, the hot-working schedule in Figure 1 was applied. The experimental samples of size (26 x 30 x 55) mm were heated under controlled conditions in air and cooled down to the first deformation temperature T i = 1100 °C in fi,hold= 22 s and then deformed for £i = 50 %. The first plastic deformation was followed by holding the samples at the second deformation temperature T2 in a chain conveyer furnace for f2,hold = 80/100/120/150 s. Afterwards, the forgings were cooled from T1 to T2 = (800/850/900/950) °C and then submitted to a second plastic deformation £2 = (10/20/30/40/50/60) % and finally quenched in a KOH water solution. Next the samples were ground and polished and annealed at 550 °C for 48 h. After annealing the continuous layer of scale was removed with fine grinding and the grain boundaries revealed with stringers of oxide particles. Finally, the specimens were etched in a water solution of picric acid with the addition of CuCl and the microstructure was investigated with optical microscopy. The size of the statically recrystallised austenite grains was assessed with a linear method as an average of 20 measurements. In cases of the absence of static recrystallization of the austenite the average corrected austenite grain diameter was calculated from equations based on an assessment of the average effective nucleation area. 2000 d. Y, kor Sv(gb + db) Sv(gb + db) = 1000 • [0,429 • (d Y ,0 )+l,57l(d Y, ± )] = 1000 Y 0,429 • (1 - £ 0) + 1,571 (1 - £,) (2) d^kor/pm is the corrected average diameter of an austenite grain, dö/pm is the recrystallised diameter of an austenite grain before the thermal area of the inhibited austenite recrystallisation. Sv(gb+db) /(1/mm) is the average effective nucleation area of the grain boundaries and deformation bands, Sv(gb) is the effective nucleation area of the grain boundaries, Sv(db) is the area of the deformation bands inside the austenite grains, £2/% is the relative deformation. In the case of static recrystallization of the austenite, the austenite grain diameter was deduced by applying the equation: (1,68 • d kr) dy = ( n • zv) (3) where: where: dy/pm is the austenite grain diameter, dkr/mm is the diameter of the circumference, n is the number of intersected grain boundaries, zv is the magnification. The determined and deduced values for the grain size were then processed using non-linear numerical statistical methods and correlation equations for a description of the investigated dependences were generated. 3 RESULTS AND DISCUSSION The influence of the final deformation and temperature on the austenite grain size is shown in Figure 2 and 3. The obtained results show that with an increase in the amount of deformation £2 above 10 % and deformation temperature T2 above 850 °C the austenite grain diameter decreased from the original size di = 43 pm after the first plastic deformation to d/2 = 7.7 pm for T2^2 = Figure 1: Temperature and deformation regime Slika 1 : Režim temperature in deformacije Figure 2: Dependence of AGS on deformation Slika 2: Odvisnost AGS (velikost avstenitnih zrn) od deformacije 122 Materiali in tehnologije / Materials and technology 42 (2008) 3, 121-124 T. KUSKULIC ET AL.: DEVELOPMENT OF MICROSTRUCTURE DURING THE HOT PLASTIC DEFORMATION = — 1 2 i i - -c- TD2-80Û-C -O-TDJ-S50-C TD2-900'C -o~ ÎDZ-950-C U 10 20 M 40 50 60 Ufl1oim.*foii î-_j E -1 Figure 4: Dependence of Xsr on deformation Slika 4: Odvisnost Xsr (deleža rekristalizacije) od deformacije sf : r. k T V 20 - -4.^,2 -10 % -M-i2 -20 S i2 -30 % — X— 1 / —JO V, i2 -50 —o— (2 -00 % ; / ; / / 4 { 1 700 750 «on dsn soo »so I m I ^ 1111MJ lief Ol ' ' I. Il I ■ I. I ■ 111| I "P ill IH c I inn Figure 3: Dependence of AGS on deformation temperature Slika 3: Odvisnost AGS od temperature deformacije 950 °C/60 %, or to the size of dy2 = 15.0 pm for T2^2 = 850 °C/60 %. - for £2/% G <10;30> and the investigated temperature T2/°C G <800;950> the diameter of the austenite grain achieved coarse values of dy2/pm G <2.2;39.3>. For £2/% G <40;60> and T2/°C G <800;950> the diameter of the austenite grains achieved bottom values of G <7.7; 20.9> pm. For an explanation of the occurrence of two areas of austenite grain size it was necessary to determine the share of static recrystallised austenite after the second deformation. The influence of the deformation £2 and of the deformation temperature T2 on the share of the static recrystallisation of austenite is shown in Figures 4 and 5. - For the deformation £2/% G <10;30> and the temperature T2/°C G <800;950> the share of statically recrystallised austenite was of Xsr/% G <0;20>. - For the deformation £2/% G <40;60> and the temperatures T2 = 800 °C and 950 °C the share of statically recrystallised austenite was in the range of Xsr/% G <50;100> and it was classified as partly or completely recrystallised. Figure 5: Dependence of Xsr on temperature Slika 5: Odvisnost Xsr od temperature - For the deformation £2/% G <50;60> and the deformation temperatures T2 = 900 °C and 950 °C the share of statically recrystallised austenite was of XSR = 100 % and it was classified as completely recry-stallised. - For the deformation of £2 = 40 % and the temperature of T2 = 950 °C the share of statically recrystallised austenite was of Xsr = 90 %, and it is also classified as completely recrystallised. It can be concluded that the best conditions for attaining a completely statically recrystallised austenite are a deformation of £2 = 50 and 60 % at T2 = 900 °C and 950 °C. In this case the size of the austenite grain is in the range d>2 = 7.7-11.0 pm. Also, the static recrystallisation of austenite of (Xsr > 90 %) is achieved for a deformation of £2 = 40 % and a temperature of T2 = 950 °C; however, the attained size of the austenite grain is dy2 = 10 pm. The experimental numerical data were processed using linear and non-linear statistical methods, and the dependence of the final temperature, the amount of deformation and the austenite grain diameter were determined. Two equations were obtained for the dependence d 2 = f (£2;?d2). 1) Equation dY 2 = A0 x rDA2 x £ a 2 (4) where: TD2/°C is the second deformation temperature, £2/% is the relative deformation, A0 = 2.54585x107 A1 = -2.2021262 A2 = -0.697277 A graphical comparison of the calculated and measured values is shown in Figure 6. For the deformation of £2 = 30 % a strong deviation is found. 2) Equation dY2 = B0 x ln[1 /(1 - £)]B1 x ZB (5) Materiali in tehnologije / Materials and technology 42 (2008) 3, 121-124 123 T. KUSKULIC ET AL.: DEVELOPMENT OF MICROSTRUCTURE DURING THE HOT PLASTIC DEFORMATION Figure 6: Comparasion of measurement and calculated values for equation Slika 6: Primerjava meritev in izra~unov z ena~bo (4) Figure 7: Comparasion of measurement and calculated values for equation Slika 7: Primerjava meritev in izra~unov z ena~bo (6) where: e/% is the relative deformation, Z/s-1 is Zener-Holomon's parameter, B0 = 0.4913 B1 = -0.6347 B2 = 0.07653 Equation (6) was deduced for describing the dependence share of the recrystallisation versus the extent of the deformation and the temperature [ai (f 2 -0.1 )a2 (TD2 -800 )a3 ] X SR = 100 -<|1 - e (6) where: XSR/% is the amount of statically recrystallised austenite, £2/% is the relative deformation, Td2 /°C is the second deformation temperature, a1= -3.49-10-22 a2 = 18.1048 a3 = 15.38 A graphical comparison of the calculated and measured values is shown in Figure 7. 4 CONCLUSION We performed experiments to investigate the effect of changes of the final deformation and deformation temperature on the austenite grain size and on the share of static austenite recrystallisation. From the results of these experiments the conditions necessary to design the technology for crankshaft smith forging were established: the best conditions leading to a completely static recrystallisation of austenite (Xsr = 100 %) with forging are £2 = 50 and 60 % at T2 = 900 °C and 950 °C. In these conditions an austenite grain size of ^2 = 7.7-11.0 pm is achieved, - acceptable conditions leading to the static recrystallization of austenite (Xsr = 90 %) are £2 = 40 % at T2 = 950 °C, which ensure an austenite grain size of ^2=10 pm; - with the application of numerical statistical methods for the processing of experimental data, equations for the influence of temperature on the extent of the deformations on the size of the austenite grains were derived, together with the austenite static recrystallization. Acknowledgements The authors acknowledge the support given by the EUREKA E!3192 ENSTEEL. 5 REFERENCES 1 Feldmüller, A., Kern, T. 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