RECRYSTELLIZATION BEHAVIOUR OF STEELS SA-508 AND 3,5Ni-1,5Cr

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1 RECRYSTELLIZATION BEHAVIOUR OF STEELS SA-508 AND 3,5Ni-1,5Cr Jakub HORNÍK a, Petr ZUNA a, Evgeniy ANISIMOV a, František JANDOŠ b a CTU in Prague, Prague, Czech Republic, EU, jakub.hornik@fs.cvut.cz b Research and Testing Institute Plzeň s.r.o., Pilsen, Czech Republic, EU Abstract Steel SA-508 (20MnMoNi5-5) and 3,5Ni-1,5Cr are bainitic weldable steels using for many applications, especially for heavy forgings like pressure vessels for nuclear reactors or big shafts. Austenite graingrowth and recrystallization behavior were evaluated in typical interval of forging temperatures ( ) C. The grain coarsening is evident above the temperature of 1100 C. Recrystallization inhibition is evident under the temperature of 900 C. Undesirable extreme grain coarsening in case of sufficient deformation was not observed. Keywords: microstructure, grainsize, recrystallization 1. INTRODUCTION This work is a part of a research which purpose is to optimize the processing conditions in order to obtain fine-grained structure during the production of wind turbine rotors by free forging. The work deals with the influence of heating conditions and deformation on the kinetics of austenite grain growth and recrystallization behavior of steel SA-508 commonly used in United States. Research is complemented by results from initial study on steel 3,5Ni-1,5Cr. Production of heavy forgings is specific and very expensive. The common defects occurring during the forming of rotors include unwanted local grain coarsening, which can occur as the presence of casting defects in the ingot, the temperature distribution differences and the inhomogeneity of deformation. The paper deals with the issue of the deformed state of the material, recrystallization processes and factors which influence them and possibly occurs during hot forming [1-4]. Objective of the experiment focuses on the laboratory evaluation of kinetics of austenite grain growth depending on the temperature of forming and recrystallization behavior of steels in the range of typical forging temperatures. 2. EXPERIMENTAL Tab. 1 Chemical composition of evaluated steels [wt. %] SA-508 C Mn Si P S Cr Ni Mo 0,18 1,27 0,27 0,005 0,001 0,07 0,64 0,48 Cu Co Ti V Al Nb B N 0,03 0,005 0,002 0,010 0,029-0,0003 0,004 3,5Ni-1,5Cr Experimental material was supplied by PILSEN STEEL Ltd. in the form of machined sections from forgings made from steel SA-508 Grade C1.1 and second steel signed as 3,5Ni-1,5Cr. Chemical composition of evaluated steels is shown in Tab. 1. To evaluate the kinetics of C Mn Si P S Cr Ni Mo austenite grain growth, the 0,21 0,31 0,10 0,004 0,002 1,75 3,58 0,46 temperature range corresponding Cu Co Ti V Al Sb As Sn to the typical range of 0, ,08 0,0078 0,0013 0,0039 0,005 temperatures including final forging steps (850, 900, 1000, 1100, 1200 and 1250 C) was selected. Samples of dimensions of 15 x 15 x 15 mm were cooled in water after the

2 treatment allowing austenitic grain evaluation. Heat treating was carried out in the filling of corundum and crushed coke, restricting decarburization. Holding times at the temperature were 15 and 60 minutes. Dwell time of 15 min was selected for laboratory processing with minimal decarburization, whereas 60 minutes dwell time better simulates real forging conditions. The laboratory upsetting was applied using gravity falling hammer weighing 11.6 kg and with a lines height of 3500 mm. Ram impact velocity is 8.3 m.s -1. Cylinders with a diameter of 8 mm and a height of 15 mm were used for the purpose of recrystallization behavior evaluation. In case of a sample with a height of 15 mm the equivalent strain rate is approximately s -1. All the cylinders were deformed to 60 % of their original height. The required deformation of the samples was achieved by inserting a stopper. Manipulation time from final deformation to cooling sample in water was 3 seconds. Samples were processed using the same austenitizing temperature of 1200 C for 15 min and air cooling at the deformation temperature afterwards. Subsequently, 15 minutes dwell was performed at the deformation temperature. The samples were deformed by upsetting. Part of them were immediately cooled down in water for the purpose dynamic, respectively metadynamic recrystallization evaluation. The other samples were put back into the furnace either for 60 s, 180 s, 300 s or 600 s and then cooled down in water. These processing were used for the purpose of static recrystallization evaluation. The deformation temperature was 850 C, 900 C, 1000 C or 1100 C. In case of steel 3,5Ni 1,5Cr the deformation experiment at temperature 850 C was performed. The microstructure was evaluated by light microscopy and scanning electron microscopy combined with EDS microanalysis. Samples were etched in NITAL and Villela-Bain agents or in hot picric acid reagent in solution with surfactant. 3. RESULTS AND DISCUSSION 3.1 As delivered state The microstructure of forgings in as-delivered state is homogeneous, predominantly bainitic (Fig. 1) with equiaxed prior austenite grain. In case of SA-508 steel grain size corresponds to (50 70) m. Grainsize of steel 3,5Ni-1,5Cr is in rage (70 90) m. Distribution of carbides was evaluated using SEM. Precipitates are located mainly at the interfaces between ferritic needles and on grain boundaries of prior austenite. a) SA-508 b) 3,5Ni-1,5Cr Fig. 1 Microstructure- as delivered state, LM

3 3.2 Kinetics of austenite grain growth The results of grain size measurements on both steels after annealing at the defined temperatures are graphically presented in Fig. 2. The dwell time at the temperature was 15 or 60 minutes. It is evident from the results that the grain size of SA-508 steel does not change significantly up to temperature of 1000 C. In case of dwell time 15 minutes, grain growth does not take place even at a temperature of 1100 C. Significant grain coarsening was observed between the temperatures of 1100 C and 1250 C. At a temperature of 1250 C the grain sizes of both holding times are similar. Grain of steel 3,5Ni-1,5Cr coarsens already at temperature of 1000 C. At temperature interval of ( ) C locally coarsened grains are evident. Monitored difference in grainsize is doubled. At temperature of 1100 C grain coarsens significantly in comparison with SA-805 at booth holding times. Increasing temperature decreases disparity in grainsize of both steels. Microstructure of samples austenitized at 800 C and cooled in water contains a considerable amount of carbide particles at grain boundaries and between the martensite needles (Fig. 3). The grain of both steels does not coarsen at this temperature. The Grainsize [ m] min SA min 3,5Ni-1,5Cr 60 min SA min 3,5Ni-1,5Cr Temperature [ C] Fig. 2 Kinetics of austenitic grain growth particles may effectively inhibit its growth. After austenitizing at the temperature of 1200 C, the quantity and size of particles are reduced considerably in both steels (Fig. 4). Their inhibiting effect decreases, the boundaries can migrate more easily and austenitic grain becomes coarser. At the temperature of 850 C number of particles is comparable in both steels microstructure. At higher temperatures particles are dissolved more intensively in case of steel 3,5Ni-1,5Cr. Grain coarsening of this steel is more significant in interval of annealing temperatures ( ) C. a) SA-508 b) 3,5Ni-1,5Cr Fig. 3 Microstructure after annealing at 850 C / 15 min / water, SEM

4 a) SA-508 b) 3,5Ni-1,5Cr Fig. 4 Microstructure after annealing at 1200 C / 15 min / water, SEM Analysis and identification of particles was performed by transmission electron microscopy on extraction replicas in case of steel SA-805. The majority of carbides of M 2 C and M 3 C (Fe, Mn, Mo) were identified. 3.3 Recrystallization behavior Kinetics of recrystallization after deformation at temperatures of 850 C, 900 C, 1000 C and 1100 C (for steel SA-508) plotted in Avrami s coordinates is shown in Fig. 5. The deformation temperature of steel 3,5Ni-1,5Cr was 850 C. In case of steel SA-508, the dynamic or postdynamic recrystallization of 70 % of the volume takes place at a deformation temperature of 1100 C. After the dwelltime of 180 s at deformation temperature of 1100 C the process of recrystallization is completed. Decreasing deformation temperatures lead to reduction of dynamically (postdynamically) recrystallized fraction. Significant retarding of recrystallization process is evident below the temperature of 1000 C. Deformation temperature of 900 C and 850 C leads to a dynamic (postdynamic) recrystallization only in 6 % resp. 4 % of volume. The recrystallization rate increases after 60 s hold with a typical value of kinetic exponent for Fig 5 Recrystallization kinetics after austenitization at temperature 1200 C (deformation at 850, 900, 1000 and 1100 C) - Avrami s scale. static recrystallization n 2. After the dwell of 180 s at the deformation temperature 850 C the recrystallized volume fraction reaches 60 %, at the deformation temperature of 900 C the recrystallized fraction reaches 66 %. Further dwell of 600 s at the deformation temperature leads to recrystallization of 88 % volume and 96 % at 900 C.

5 Differences in microstructure of SA-805 steel after the deformation at temperature of 850 C and different holding times are documented using BSD imaging in Fig C / 850 C / 3s / water 1200 C / 850 C / 600 s / water Fig. 6 Recrystallized structure of steel SA-508 after deformation 65 %, EBSD Recrystallization process of steel 3,5Ni-1,5Cr at the deformation temperature of 850 C has similar character as process monitored in steel SA-508 (Fig 5). Mutual effect of coarser initial austenite grain and present particles is assumed for observed delay in start of static recrystallization. Detail evaluation of precipitate and their effect on recrystallization rate was not finished yet. After then, the grain size was evaluated in regions of possible critical deformation near the cylinders ends (Table 2). The real deformation (60-65) % was measured in the central homogeneous region. Deformation of (7-10) % was measured in the area of the hindered deformation (dead-metal zone) [5]. This value is close to the point where critical deformation occurs and may lead to the grain coarsening. In case of applied austenitization condition (temperature of 1200 C / 15 min) the estimated grain size is 380 μm for steel SA-508 and 560 μm for steel 3,5Ni-1,5Cr. In the area of critical dead zone with deformation (7-10) % the recrystallized grainsize reaches 350 m in case of SA-508 steel and 396 m in steel 3,5Ni-1,5Cr. Generally the monitored grain size is comparable or finer then austenite grain before deformation. Extreme grain coarsening in both steels at evaluated condition was not observed. Tab. 2 Austenite grain size after laboratory deformation in the area of inhibited deformation [μm] Steel Annealing temperature [ C] SA Deformation temperature [ C] Dwell time at def. temperature [s] ,5Ni-1,5Cr Transmission electron microscopy was used for essential evaluation of particles size and distribution in steel SA-508. Particles distributed in the microstructure have predominantly spherical or cubic shape. The particle size generally does not exceed 0.5 m. Precipitates are located predominantly at the austenitic grain boundaries or between martenzitic needles in lower quantity. The quantities of fine particles present on the grain boundaries decreases with an increased deformation temperature. Consequent detail particle analysis necessary for description of recrystallization behavior is planned.

6 4. CONCLUSIONS Microstructure of both steel in as-delivered state is predominantly bainitic. In the temperature range 850 C C the significant austenite grain coarsening is evident above the temperature of 1100 C in case of steel SA-508. Austenitic grain of steel 3,5Ni-1,5Cr coarsens from temperature of 1000 C. The austenitization temperature of 1200 C and deformation 65 % at temperatures of 850 C, 900 C, 1000 C and 1100 C lead in all cases to recrystallization. At regions of dead zone, where the deformation reaches (7 10) % (close to the critical deformation value), the austenitic grain coarsening take effect. Grain size corresponds to initial austenitic grain before deformation or is smaller. In case of applied small deformation, the undesirable grain coarsening may take effect. After the austenitization of steel SA-508 at the temperature of 1200 C and consequent deformation at the temperature of 1100 C, 70 % of volume was dynamically, respectively metadynamically, recrystallized. Subsequent static recrystallization is completed after 180 s dwell at deformation temperature. Dynamically recrystallized volume fraction and following static recrystallization rate decreases with lower deformation temperature. The deformation at 850 C after austenitizing at the temperature of 1200 C leads to a dynamic, respectively metadynamic recrystallization only in 4 % of volume in case of SA-508 steel. After the dwell of 10 min at the deformation temperature 88 % of volume is recrystallized. Microstructure of steel 3,5Ni-1,5Cr is recrystallized dynamically in 5 % of volume, static recrystallization at the temperature of 850 C is slightly lower in comparison with SA-508. Transmission electron microscopy showed that microstructure of steel SA-508 contains precipitates in all states. The particles are mainly located at the boundaries of austenite grains and between some martensite needles. The number of particles decreases significantly with increasing processing temperature. The particle size decreases slightly with temperature. Following experimental program will be focused on a detail analysis of the minor phases in a range of forging temperatures for the purpose of complete conclusions about the influence of deformation conditions on the final austenite grain size, which significantly affects the secondary structure and mechanical properties. Interest will be focused on complete evaluation of the recrystallization kinetics of 3,5Ni-1,5Cr steel, interaction of recrystallization and precipitation and assessment of conditions on the grain boundaries. Structural characteristics will be correlated with the mechanical properties of the final state of evaluated steel. ACKNOWLEDGEMENTS The problematic was solved using laboratories of Innovation Centre for Diagnostics and Application of Materials at CTU in Prague as a part of the Ministry of Industry and Trade TI2/132 project. LITERATURE [1] Žídek, M. Dědek, V. Sommer, B.: Tváření oceli. SNTL/ALFA, Praha, s. [2] Sedláček, V. a kol.:zotavení a rekrystalizace. Academia, Praha, s. [3] Žídek, M.: Metalurgická tvařitelnost ocelí zatepla a zastudena. ALEKO, Praha, s. ISBN X. [4] Berns, H.-Theisen, W.:Ferrous Materials:Steels and Cast Iron. Springer Verlag Berlin Heidelberg, 2008 [5] Dieter, G. E. Kuhn, H. A. Semiatin, S. L.: Handbook of workability and process design, ASM int., 2003