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Decarburization and grain growth kinetics during the annealing of electrical steels
Scripta Materialia,
pp.1253-1257,1996 Elsevier Science Ltd Cowrkht 0 1996 Acta Metahrcica Inc. k&&d in the USA. All rightskerved 1359-6462196 h2.00 + .OO Vol. 35, No. 11,
PI1 S1359-6462(96)00309-O
DECARBURIZATION AND GRAIN GROWTH KINETICS DURING THE ANNEALING OF ELECTRICAL STEELS Carlos R. Oldani Centro de Investigacih de Materiales y Metrologia Avda.Velez Sarsfield 156 1 - 5000 Cordoba - Argentina (Received October 4, 1995) (Accepted June 10,1996) Introduction Electrical steels are generally described as thin steel sheets of variable thickness (from 0.27 to 0.76 mm), whose function is to efficiently transport the magnetic flux in electrical equipments (1). The electromagnetic properties expected from these materials are a low magnetic losses and a high permeability. It can be said that a cyclically magnetized-demagnetized material is not free of energy losses because a portion of the power, the loss, is irreversibly transformed into heat. According to commercial terminology, steels known as laminations or low carbon steel, are the ones with non-oriented grain, partially processed and with very low silicon content (CO.5 “A). Laminations were developed in the 50s as a low cost alternative to the silicon steels, to be employed in low and medium output machines like fractional power motors, ballasts and small transformers. These steels are usually produced in a partially processed condition and they reach their maximum magnetic potential during the final steps of manufacture at the user’s plant. Efficient control of the operations by which the sheets are submitted is essential to obtain the optimum steel yield in the magnetic circuit they are made for. In these operations a decarburization annealing heat treatment produces important effects such as removing punching residual tensions, decarburization to very low carbon content, ferritic grain growth and a favourable magnetic crystallographic texture. Experimental Procedure For the experimental work on the decarburization kinetic study, a tubular furnace with controlled temperature and atmosphere was used. The samples were obtained from commercial Al-killed steel laminations, with initial chemical composition as shown in Table 1. The samples that were previously degreased in a sequential wash in an ultrasonic agitation bath, composed of alkaline detergent [6%]; Na(OH) [3%] and isopropilic alcohol, were introduced in the furnace in open steel capsules. The furnace atmosphere was cleaned with a NZ flow and then with a mixture of Nz+H2 flow, saturated in water at 3O“C.The treatment was conducted under the latter mixture. The sample cooling was made under a subsequent Nz flow. The time required by the sample to
1253
1254
DECARBURIZATION
TABLE 1 Initial Chemical Composition SZNllplC
D
Vol. 35, No. 11
AND GRAIN GROWTH KINETICS
of the Sample
Thickness mm
%C
%Mn
%Si
%S
%I’
%Al
0.56
0.062
0.30
<0.016
0.01I
0.021
0.047
%cJ
%N
0.0033 0.0053
reach the temperature test was recorded, so that, the zero time was set as beginning immediately after that period. The processed samples were analyzed for carbon content in a LECO G-44 M762-300 equipment. For the grain growth kinetics study, lamination sheets without decarburization presenting two different levels of decarburization but similar grain size (obtained asper the conditions shown in Table 2) were used. Samples of electrolytic iron (sample F in Table 2) were also introduced. The heat treatments were conducted at temperatures between 600°C to 900°C and treatment times of up to 120 minutes. These isothermal heat treatments of the samples were performed in a vertical resistance furnace with an isothermal lead bath. The samples obtained were metalographically prepared and their microstructure revealed with Nital 2%. The ferritic grain size was measured, asper ASTM El 12, that is, by matching each sample with the pertaining ASTM number. When the samples showed duplex microstructure, the corresponding ASTM number was assigned as a pondered grain size as calculated by the relative percentages of each one size. Results and Discussion The results of decarburation treatment kinetics are presented in Figures 1 to 4. As it is shown in Figure 1 for the 600°C results a slow tendency to decarburization is observed. In this case, the diffusion take place in the ferritic phase because the samples were treated at a temperature below the eutectoid temperature and no structural changes occured. To analyze the theoretical decarburation times, the diffusion-controlled process equations for moving interface with two sides carbon extraction (2) were solved. The metastable Fe-CFe3 phase diagram (3) was also used and the obtained curves, are shown in the same figure. The separation of experimental points from the theoretical curve, may be explained by extra kinetics phenomena that retard the process (4) for instance, the heating cycle, the homogenization time at temperature, the CO pressure near the sample, the sample surface condition, and the internal oxidation. In Figure 2, the theoretical and experimental results at 700°C are drawn. As expected, the diffusion process was accelerated by the temperature. At SOO”C,Figure 3, the material must present some austenite in its microstructure (( OZ+ y ) field of the Fe-CFe, diagram). At this temperature the
TABLE 2 Treated Samples Characteristics
Vol. 35, No. 11
DECAREXJRIZATION
Figure 1. Decarburization at 600°C.
AND GRAIN GROWTH KINETICS
Figure 2. Decarburization at 700°C.
1255
Figure 3. Decarburization at SOOT.
austenite formation is sufficiently low (< 1%) and has no influence in the diffusion process, so it is produced fundamentally in the ferritic phase. At 900°C (Figure 4), the process is more complex. When the material reaches the experimental temperature it is totally in austenitic phase. When decarburization proceeds the formation of ferrite begins, in the ( a + y ) field, until the carbon content is very low and all the material is transformed into ferrite. Figure 4 displays the calculated carbon diffusion curves (4) in austenitic and ferritic phases. The position of experimental points demonstrates that the diffusion process was run between these two extremes. On the other hand, it takes to the material a relatively long time period to heat from 700“ to 900°C while the diffusion process is very rapid. The theoretical initial carbon content at the experimental temperature must be lower, due to the partial decarburization in the (a + ‘j’ ) field during heating, but its magnitude, as relative to the total experiment time, is not evaluated in this paper. It can be observed that, the required time to obtain very low carbon contents is considerably larger than the necessary time at 800°C. This difference can be directly correlated with the diffusion coefficients for the ferrite (5) and the austenite (6), respectively: -20100
D,=2~10-~e
RT
-27000
Dy=1x10-2e
RT
Evaluation of these coefftcients at 900°C results in a value 39 times smaller for the y phase. Therefore, the diffusion process in that phase would be slowed in the same proportion. The control Iofgrain growth is the main factor in the improvement of the magnetic materials properties. Grain growth increases the magnetic domains size so that fewer histeresis losses are obtained as result of the existence of fewer domains walls to move. When the domains are larger than an optimum value, the losses increase, basically in their anomalous component, because the domains walls must move over longer distances at a constant frequency. The final grain size, that must be approximately 150 pm, depends on various factors. The most important are the initial stress produced by the skin pass (7) the annealing time and temperature (8) and the inclusions and precipitates morphology. The steel must have initial stress (over about 2% of cold deformation) to drive grain growth. If no previous stress is present, the grain growth is only a function of temperature and time of treatment (8). The inclusions/precipitates forming elements, such
Figure 4. Decarburization at 900°C.
Figure 5. Grain growth at 600°C.
Figure 6. Grain growth at 700°C
1256
DECARBURIZATIONAND GRAINGROWTHKINETICS
Figure7. Graingrowth at 800°C.
Vol. 35, No. 11
Figure 8. Grain growth at 900°C
as C, Al and Mn in the steel, may affect the final grain size by growth inhibition (9). This effect depends on where these second fase particles appear in the microstructure and also on their size. Precipitates larger than a critical size do not anchor the grain boundary, while smaller ones, particularly those between 0.01 to 0.10 pm, must be avoided because they may stop the domains walls motion during the magnetization process (7). The results of gram growth experiments are shown in Figures 5 to 8. It can be seen in Figures 5 and 6 that the steel grain size (samples A, M and B) did not experiment any growth even at 700°C. On the other hand, the electrolytic iron grain size increases with higher temperatures in shorter times. That is explained by the fact that electrolytic iron, even if it has no previous stress, it does not contain impurities in the grain boundary. Therefore, the grain grows only as a function of temperature and time of treatment. At 800°C Figure 7, sample B had an explosive grain growth, reaching an homogeneous ASTM 00 number (grain size = 510 l.tm) in the whole sample. In 15 minutes a small percentage of duplex grains was observed. This kind of growth in the carbon-free sample indicates that the previous stress (approximately 5% of skin pass) is, with the exception of the temperature, the main factor that drives grain boundary motion. CFe3 present in sample M inhibited the previous growth and duplex grains are obtained throughout the specimen, even during long treatment times. The same phenomenon was observed in sample A with various grain sizes. Sample F showed a lower grain growth than the other samples because it had no previous stress to drive the growth. The austenite in the (a + y ) field acts as a pinning point and prevents growth but the most deleterious effect is the decarburization delay which takes place, because of the austenite very low diffusion coefficient. Anchors remotion is due to the decarburization process where austenite transforms into ferrite. In Figure 8, it is observed that grain growth occur in shorter times in sample B, than in sample M because cementite does not permit the grain boundaries movement. On the contrary, sample A underwent an strong grain size reduction in the experimental times due to austenitic grain nucleation and initial growth (grain size = 4 pm). Finally, the different behavior between samples B and F show the skin pass influence. Conclusions
Experimental work were carried out to determine the decarburization and gram growth kinetics conditions in electrical laminations, during the decarburization annealing heat treatment. It could be concluded that: .
. .
Since the carbon content was the only factor that differed among the analized steel samples, it may be said that carbon, as CFe3 precipitates, retards the movement of the grain boundaries. It is thermodynamically and kinetically possible to eliminate this obstacle with an appropriate decarburization annealing heat treatment. The decarburization process can be suitably described with the diffusion equations for moving interface with two sides carbon extraction, for temperatures ranging from 600” to 800°C.
Vol. 35, No. 11
l
DECARBURIZATIONAND GRAIN GROWTHKINETICS
1257
At 900°C the decarburization process trayectory in a or y phases and during the heating cycle must be considered.
Future work should address the analysis of the decarburization kinetics during the heating of samples, until the test temperature is reached and the characterization of the grain boundary in order to predict its growth. Acknowledgments The author thanks CONICOR for supporting this work under Grant No. 3304/94 and CIMM for the provision of laboratory facilities. Very helpful discussions of the results and conclusions were carried on with Eng. F. Actis, to whom the author is very grateful. References 1. F. Actis, C. Ctldani, H. Moyano, G. Cohen, Jornadas de la SAM y I Taller Argentino de Materiales MagnCticm, CIMM, CQdoba, Argentina (1995). 2. G.H. Geiger, D.R. Poirier, Transportphenomena in metallurgy, p.490, Addison-Wesley Publishing Company (1973). 3. D.W. Taylor; Hard & Sofr Mag.Mat. with Appl., Proc.Conf.ASM, Ed.J.A.Salsgiver et al, 65 (1987). 4. IA. Tomilin, let al., Stai in English, (6), 586 (1969). 5. R.P. Smith, TranxAlME, 224, 105 (1962). 6. Metals Reference Book - SMITHELES, C.; 3e’ Ed., p.594, London Butterworths (1962). 7. G. Lyudkowsky, P.D. Southwick, Met.Trans.A, 17A, 1267 (1986). 8. L. Rabet, L. Kestens, et al, Mat.Sci.Forum, 94-96,611 (1992). 9. G. Lyudkowsky, J.M. Shapiro, J.Appl.Phys., 57 (1) 4235 (1985).
pp.1253-1257,1996 Elsevier Science Ltd Cowrkht 0 1996 Acta Metahrcica Inc. k&&d in the USA. All rightskerved 1359-6462196 h2.00 + .OO Vol. 35, No. 11,
PI1 S1359-6462(96)00309-O
DECARBURIZATION AND GRAIN GROWTH KINETICS DURING THE ANNEALING OF ELECTRICAL STEELS Carlos R. Oldani Centro de Investigacih de Materiales y Metrologia Avda.Velez Sarsfield 156 1 - 5000 Cordoba - Argentina (Received October 4, 1995) (Accepted June 10,1996) Introduction Electrical steels are generally described as thin steel sheets of variable thickness (from 0.27 to 0.76 mm), whose function is to efficiently transport the magnetic flux in electrical equipments (1). The electromagnetic properties expected from these materials are a low magnetic losses and a high permeability. It can be said that a cyclically magnetized-demagnetized material is not free of energy losses because a portion of the power, the loss, is irreversibly transformed into heat. According to commercial terminology, steels known as laminations or low carbon steel, are the ones with non-oriented grain, partially processed and with very low silicon content (CO.5 “A). Laminations were developed in the 50s as a low cost alternative to the silicon steels, to be employed in low and medium output machines like fractional power motors, ballasts and small transformers. These steels are usually produced in a partially processed condition and they reach their maximum magnetic potential during the final steps of manufacture at the user’s plant. Efficient control of the operations by which the sheets are submitted is essential to obtain the optimum steel yield in the magnetic circuit they are made for. In these operations a decarburization annealing heat treatment produces important effects such as removing punching residual tensions, decarburization to very low carbon content, ferritic grain growth and a favourable magnetic crystallographic texture. Experimental Procedure For the experimental work on the decarburization kinetic study, a tubular furnace with controlled temperature and atmosphere was used. The samples were obtained from commercial Al-killed steel laminations, with initial chemical composition as shown in Table 1. The samples that were previously degreased in a sequential wash in an ultrasonic agitation bath, composed of alkaline detergent [6%]; Na(OH) [3%] and isopropilic alcohol, were introduced in the furnace in open steel capsules. The furnace atmosphere was cleaned with a NZ flow and then with a mixture of Nz+H2 flow, saturated in water at 3O“C.The treatment was conducted under the latter mixture. The sample cooling was made under a subsequent Nz flow. The time required by the sample to
1253
1254
DECARBURIZATION
TABLE 1 Initial Chemical Composition SZNllplC
D
Vol. 35, No. 11
AND GRAIN GROWTH KINETICS
of the Sample
Thickness mm
%C
%Mn
%Si
%S
%I’
%Al
0.56
0.062
0.30
<0.016
0.01I
0.021
0.047
%cJ
%N
0.0033 0.0053
reach the temperature test was recorded, so that, the zero time was set as beginning immediately after that period. The processed samples were analyzed for carbon content in a LECO G-44 M762-300 equipment. For the grain growth kinetics study, lamination sheets without decarburization presenting two different levels of decarburization but similar grain size (obtained asper the conditions shown in Table 2) were used. Samples of electrolytic iron (sample F in Table 2) were also introduced. The heat treatments were conducted at temperatures between 600°C to 900°C and treatment times of up to 120 minutes. These isothermal heat treatments of the samples were performed in a vertical resistance furnace with an isothermal lead bath. The samples obtained were metalographically prepared and their microstructure revealed with Nital 2%. The ferritic grain size was measured, asper ASTM El 12, that is, by matching each sample with the pertaining ASTM number. When the samples showed duplex microstructure, the corresponding ASTM number was assigned as a pondered grain size as calculated by the relative percentages of each one size. Results and Discussion The results of decarburation treatment kinetics are presented in Figures 1 to 4. As it is shown in Figure 1 for the 600°C results a slow tendency to decarburization is observed. In this case, the diffusion take place in the ferritic phase because the samples were treated at a temperature below the eutectoid temperature and no structural changes occured. To analyze the theoretical decarburation times, the diffusion-controlled process equations for moving interface with two sides carbon extraction (2) were solved. The metastable Fe-CFe3 phase diagram (3) was also used and the obtained curves, are shown in the same figure. The separation of experimental points from the theoretical curve, may be explained by extra kinetics phenomena that retard the process (4) for instance, the heating cycle, the homogenization time at temperature, the CO pressure near the sample, the sample surface condition, and the internal oxidation. In Figure 2, the theoretical and experimental results at 700°C are drawn. As expected, the diffusion process was accelerated by the temperature. At SOO”C,Figure 3, the material must present some austenite in its microstructure (( OZ+ y ) field of the Fe-CFe, diagram). At this temperature the
TABLE 2 Treated Samples Characteristics
Vol. 35, No. 11
DECAREXJRIZATION
Figure 1. Decarburization at 600°C.
AND GRAIN GROWTH KINETICS
Figure 2. Decarburization at 700°C.
1255
Figure 3. Decarburization at SOOT.
austenite formation is sufficiently low (< 1%) and has no influence in the diffusion process, so it is produced fundamentally in the ferritic phase. At 900°C (Figure 4), the process is more complex. When the material reaches the experimental temperature it is totally in austenitic phase. When decarburization proceeds the formation of ferrite begins, in the ( a + y ) field, until the carbon content is very low and all the material is transformed into ferrite. Figure 4 displays the calculated carbon diffusion curves (4) in austenitic and ferritic phases. The position of experimental points demonstrates that the diffusion process was run between these two extremes. On the other hand, it takes to the material a relatively long time period to heat from 700“ to 900°C while the diffusion process is very rapid. The theoretical initial carbon content at the experimental temperature must be lower, due to the partial decarburization in the (a + ‘j’ ) field during heating, but its magnitude, as relative to the total experiment time, is not evaluated in this paper. It can be observed that, the required time to obtain very low carbon contents is considerably larger than the necessary time at 800°C. This difference can be directly correlated with the diffusion coefficients for the ferrite (5) and the austenite (6), respectively: -20100
D,=2~10-~e
RT
-27000
Dy=1x10-2e
RT
Evaluation of these coefftcients at 900°C results in a value 39 times smaller for the y phase. Therefore, the diffusion process in that phase would be slowed in the same proportion. The control Iofgrain growth is the main factor in the improvement of the magnetic materials properties. Grain growth increases the magnetic domains size so that fewer histeresis losses are obtained as result of the existence of fewer domains walls to move. When the domains are larger than an optimum value, the losses increase, basically in their anomalous component, because the domains walls must move over longer distances at a constant frequency. The final grain size, that must be approximately 150 pm, depends on various factors. The most important are the initial stress produced by the skin pass (7) the annealing time and temperature (8) and the inclusions and precipitates morphology. The steel must have initial stress (over about 2% of cold deformation) to drive grain growth. If no previous stress is present, the grain growth is only a function of temperature and time of treatment (8). The inclusions/precipitates forming elements, such
Figure 4. Decarburization at 900°C.
Figure 5. Grain growth at 600°C.
Figure 6. Grain growth at 700°C
1256
DECARBURIZATIONAND GRAINGROWTHKINETICS
Figure7. Graingrowth at 800°C.
Vol. 35, No. 11
Figure 8. Grain growth at 900°C
as C, Al and Mn in the steel, may affect the final grain size by growth inhibition (9). This effect depends on where these second fase particles appear in the microstructure and also on their size. Precipitates larger than a critical size do not anchor the grain boundary, while smaller ones, particularly those between 0.01 to 0.10 pm, must be avoided because they may stop the domains walls motion during the magnetization process (7). The results of gram growth experiments are shown in Figures 5 to 8. It can be seen in Figures 5 and 6 that the steel grain size (samples A, M and B) did not experiment any growth even at 700°C. On the other hand, the electrolytic iron grain size increases with higher temperatures in shorter times. That is explained by the fact that electrolytic iron, even if it has no previous stress, it does not contain impurities in the grain boundary. Therefore, the grain grows only as a function of temperature and time of treatment. At 800°C Figure 7, sample B had an explosive grain growth, reaching an homogeneous ASTM 00 number (grain size = 510 l.tm) in the whole sample. In 15 minutes a small percentage of duplex grains was observed. This kind of growth in the carbon-free sample indicates that the previous stress (approximately 5% of skin pass) is, with the exception of the temperature, the main factor that drives grain boundary motion. CFe3 present in sample M inhibited the previous growth and duplex grains are obtained throughout the specimen, even during long treatment times. The same phenomenon was observed in sample A with various grain sizes. Sample F showed a lower grain growth than the other samples because it had no previous stress to drive the growth. The austenite in the (a + y ) field acts as a pinning point and prevents growth but the most deleterious effect is the decarburization delay which takes place, because of the austenite very low diffusion coefficient. Anchors remotion is due to the decarburization process where austenite transforms into ferrite. In Figure 8, it is observed that grain growth occur in shorter times in sample B, than in sample M because cementite does not permit the grain boundaries movement. On the contrary, sample A underwent an strong grain size reduction in the experimental times due to austenitic grain nucleation and initial growth (grain size = 4 pm). Finally, the different behavior between samples B and F show the skin pass influence. Conclusions
Experimental work were carried out to determine the decarburization and gram growth kinetics conditions in electrical laminations, during the decarburization annealing heat treatment. It could be concluded that: .
. .
Since the carbon content was the only factor that differed among the analized steel samples, it may be said that carbon, as CFe3 precipitates, retards the movement of the grain boundaries. It is thermodynamically and kinetically possible to eliminate this obstacle with an appropriate decarburization annealing heat treatment. The decarburization process can be suitably described with the diffusion equations for moving interface with two sides carbon extraction, for temperatures ranging from 600” to 800°C.
Vol. 35, No. 11
l
DECARBURIZATIONAND GRAIN GROWTHKINETICS
1257
At 900°C the decarburization process trayectory in a or y phases and during the heating cycle must be considered.
Future work should address the analysis of the decarburization kinetics during the heating of samples, until the test temperature is reached and the characterization of the grain boundary in order to predict its growth. Acknowledgments The author thanks CONICOR for supporting this work under Grant No. 3304/94 and CIMM for the provision of laboratory facilities. Very helpful discussions of the results and conclusions were carried on with Eng. F. Actis, to whom the author is very grateful. References 1. F. Actis, C. Ctldani, H. Moyano, G. Cohen, Jornadas de la SAM y I Taller Argentino de Materiales MagnCticm, CIMM, CQdoba, Argentina (1995). 2. G.H. Geiger, D.R. Poirier, Transportphenomena in metallurgy, p.490, Addison-Wesley Publishing Company (1973). 3. D.W. Taylor; Hard & Sofr Mag.Mat. with Appl., Proc.Conf.ASM, Ed.J.A.Salsgiver et al, 65 (1987). 4. IA. Tomilin, let al., Stai in English, (6), 586 (1969). 5. R.P. Smith, TranxAlME, 224, 105 (1962). 6. Metals Reference Book - SMITHELES, C.; 3e’ Ed., p.594, London Butterworths (1962). 7. G. Lyudkowsky, P.D. Southwick, Met.Trans.A, 17A, 1267 (1986). 8. L. Rabet, L. Kestens, et al, Mat.Sci.Forum, 94-96,611 (1992). 9. G. Lyudkowsky, J.M. Shapiro, J.Appl.Phys., 57 (1) 4235 (1985).