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Warm deformation of carbon steel
Journal of Materials Processing Technology 106 (2000) 123±130
Warm deformation of carbon steel A. Niechajowicz*, A. Tobota Institute of Mechanical Engineering and Automation, Technical University of Wroclaw, 50-371 Wroclaw, Poland
Abstract The stress and structure of carbon steels during deformation at elevated temperature was investigated. Below the austenite range, concurrent cementite dissolution, ferrite oversaturation, heteromorphous cementite precipitation and coagulation together with dynamic recovery and recrystallization of the matrix determine the structure and stress level during deformation and, ®nally, the structure and properties of the steel after deformation. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Carbon steel; Warm deformation; Stress; Structure
1. Introduction Modern methods of analysis and design of metalworking process, based on mathematical modeling or expert systems, enable the quick designing of the processes quickly and make it, to some degree, independent of the experience and intuition of designers. The accuracy and reliability of designed processes depend on access to proper data such as the stress±strain curves, the limit strains, the effect of the deformation conditions on the properties and structure of the materials, and so on. Therefore, a large part of the research work is devoted to the researching, categorizing and description of the behavior of the deformed material. The predominant role of hot and cold working ensures that for these processes, the phenomena accompanying deformation are known and well described [1±4]. The development of warm metal forming processes requires access to data enabling process design: this is especially essential for steel metalworking. Deformation at a temperature close to the transformation temperature brings about considerable changes of the structure and properties of the deformed steel. Therefore, extension of the knowledge in this area is important for the creation of a data base used directly for the design of the processes and, more important, for the creation of the precise model of the deformed materials. The aim of the paper was an investigation of the stress and structure during the deformation of carbon steels under warm metal forming conditions.
*
Corresponding author.
2. Effect of the temperature and the carbon content on the stress The experimental results presented by authors were obtained in torsion tests [4±7]. The torsional moment M and the angle of torsion a was transformed to stress s and strain e in accordance with the expressions p 3 3M ra ; e p s 2pr 3 3L where r is the radius of the sample, and L the gauge length of the sample. The stress- and strain-values are not exactly equal to the ¯ow stress and the true strain but they are convenient for the representation of M±a curves in relative, dimensionless form. Typical s±e curves for plain carbon steel (0.43% C), obtained from torsion tests, are shown in Fig. 1. The curve shape is dependent on the deformation conditions and results from the phenomena occurring during deformation. For a lower strain rate _e 0:03 sÿ1 the change of the temperature was less than 58C, so that the deformation was practically isothermal and all changes in the stress result from structural phenomena only. With a strain rate greater than 0.5 sÿ1, the temperature increases more, thus the stress is also affected by changes of the temperature during deformation. Strain hardening, dynamic recovery, recrystallization and strain ageing are well known and described for deformation. For temperatures below the austenite range, rebuilding of the pearlite occurs. This appears at the temperatures above 4008C, when the velocity of diffusion is suf®ciently high and it can be seen as a reduction of stress with strain at
0924-0136/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 0 ) 0 0 6 0 2 - 6
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A. Niechajowicz, A. Tobota / Journal of Materials Processing Technology 106 (2000) 123±130
Fig. 1. Stress±strain curves of normalized medium-carbon steel.
the temperature, as recrystallization cannot take place (Fig. 1). The stress level and stress±strain curves depend not only on the deformation conditions, but also on the carbon content. The in¯uence of the carbon content of initially normalized steels on the stress is visible in Fig. 2. The stress tends with deformation to approach the same level, independently of the carbon content, except for steel with a
carbon content of less then 0.1% C, where the stresses are distinctly different. Because there are no mathematical models of steel describing the ¯ow stress by the phenomena occurring during deformation, published data in graphical or numerical form is mainly used, and equations obtained by approximation of the experimental results can also be useful. A comparison of predicted and experimental results for low-
Fig. 2. Stress±strain curves of normalized-carbon steel.
A. Niechajowicz, A. Tobota / Journal of Materials Processing Technology 106 (2000) 123±130
125
3. Effect of the initial structure on the stress during warm deformation The effect of the initial structure on deformation under warm conditions is frequently omitted, although it can be signi®cant. This effect was tested for different carbon steels applying distinctly differentiated structures, from very stable (spheroidized) to the most unstable after quenching. The results are shown for low carbon, medium carbon and perlite steels in Figs. 5±7. The stresses are very different for small strains but generally tend to those for initially spheroidized. The in¯uence of the structure decreases with increase of the temperature and it almost disappears above 6508C. The effect decreases with decrease of carbon content [4].
Fig. 3. Approximated and experimental stress±strain curves of low-carbon steel _e 0:03 sÿ1 .
carbon steel is shown in Fig. 3. For isothermal deformation (_e 0:03ÿ0:5 sÿ1 , T500±9008C) the stress±strain curves for these steels can be described with good accuracy by the equation (Fig. 3) [4]: s 0:5649e0:34 13:0e244:9=T ÿ 16:67 16:7eÿ1:024e 6:782e 2:924ÿ106:7=T c6:782 e_ 0:0893 Frequently, when only simpli®ed process analysis is used, knowledge of the maximum stress can be suf®cient. The maximum stress for initially normalized steel (c0.02± 1.12% C) deformed at T400±9008C with a strain rate of e_ 0:03ÿ6:0 sÿ1 can be described with good accuracy by the following equation (Fig. 4) [6]: smax 37300 0:074 c0:329 e 0:064_eÿ0:0037Tÿ688=T
Fig. 4. Maximum stress of normalized-carbon steel _e 0:03 sÿ1 .
4. Structure during warm deformation The pearlite rebuilt during deformation is generally regarded as spheroidizing, coagulation and also spreading of the cementite. In reality this process is more complex, but research of the steel structure during warm deformation has made it possible to know more about the processes occurring during and after deformation that determine the deformation run, the structure changes and the properties after deformation. The cementite in the steel is generally regarded as a stable phase, which can spheroidize and coagulate when diffusion is possible. However, it is known that the carbon concentration directly in the vicinity of the cementite particles is greater than that in the long distance ferrite and depends on the curvature of the cementite. Also, the bonding power of the carbon in cementite is related closely to the bonding power of the carbon in a complex dislocation-carbon. During deformation at elevated temperature, in the state of strong thermodynamic unbalance, bonding of the carbon with dislocations can be locally more thermodynamically probable than in cementite. Then, in the presence of the large number of mobile dislocations and vacancies, carbon can be dissolved easily and displaced in the matrix, effecting its local oversaturation and in turn cementite can be precipitated. These cementite particles, depending upon the local conditions, can precipitate in differentiated elongated forms Ð needle, lamellar, or ellipsoidal Ð that are quite unstable [4]. This process can differ from typical precipitation, when only change of temperature takes place. Generally, transformation of the cementite during warm deformation can occur by: (i) the mechanical breaking of elongated cementite particles; (ii) the dissolution of cementite particles and oversaturation of ferrite; (iii) the precipitation of cementite from supersaturated ferrite; and (iv) the spheroidizing and coagulation of cementite. These phenomena, together with recovery and recrystallization, occur concurrently and interact during deformation, their velocity and share in the changes of the structure depending on the
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Fig. 5. The effect of the initial structure on stress±strain curves of low-carbon steel _e 0:03 sÿ1 .
chemical composition of the steel, the initial structure, the temperature and the strain rate. Based on these processes, it is possible to explain the changes of the observed structure and the stress versus strain characteristics. For initially spheroidized material, the most stable eutectoid steel deformed below 6008C, mainly recovery of the ferrite takes place, therefore the stress±strain curve is very ¯at; change of the cementite morphology was not observed. At 6508C the stress changes more, which can
result from recrystallization of the ferrite and changes of the cementite morphology. The microstructure of the test sample during deformation, obtained by its quenching immediately after the cessation of deformation, is shown in Fig. 8. As for deformation at 5008C, there is no visible change of the structure; but during deformation at 6508C there are distinct changes, elongated particles of the cementite being observed. After air cooling, not a great number of elongated particles were visible.
Fig. 6. The effect of the initial structure on stress±strain curves of medium-carbon steel.
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Fig. 7. The effect of the initial structure on stress±strain curves of pearlite steel _e 0:03 sÿ1 .
More distinct changes of the stress occur for initially quenched material, the most unstable eutectoid steel. During heating before deformation, the tempering process is started and it is accelerated with deformation, causing the stress to quickly tend to descend to the level for spheroidized steel; the descent is faster with increase of the temperature. During deformation, the changes of the structure are very complex. There are visible globular and elongated cementite particles predominant at temperatures of less than 6508C, whilst at 6508C and above cementite can precipitate from super-
saturated ferrite in lamellar shape also (Fig. 9). The lamellar cementite is very unstable and quickly transforms to more globular form. Air-cooling after deformation causes the cementite particles to be less elongated, and mainly ellipsoidal, with distinct size differentiation. After deformation at temperatures higher than 6008C, regions with lamellar cementite can remain and appear as typical pearlite. Such structures were reported by other authors [8] and were attributed to local increase of the temperature. The results presented for low strain rates exclude this possibility and
Fig. 8. Structure of initially spheroidized pearlitic steel quenched after deformation (6508C, e_ 0:03 sÿ1 ).
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Fig. 9. Initially-quenched pearlitic steel quenched after deformation (6508C, e_ 0:03 sÿ1 ).
allow to their attribution to cementite transformation below the transformation temperature. Other changes of the stress and structure can be observed for initially normalized steel. Initial breaking of the cementite accelerates its transformation. Regions with lamellar pearlite, partially spheroidized pearlite, globular cementite and newly precipitated cementite, coexist and transform. This differentiation of the structure stays after deformation also. Above 6008C it is hard to say whether pearlite regions are newly created or are the remainder of the initial structure. For steel with different carbon content, behavioral differences result from the distribution of the strain between the softer ferrite matrix, and the harder pearlite. For low-carbon steel (Fig. 5) a large part of the deformation takes place in the ferrite, the changes of the cementite being slower. The lamellar pearlite in normalized steel undergoes breaking and transformation to globular form within clusters of pearlite at temperature up to 6008C. At 5008C, only for strain greater than 1.5, these changes in¯uence the stress. At 6508C there are zones with clusters of cementite and ones with more uniformly distributed cementite in the matrix, but the size and shape of the cementite is very different. After the deformation of spheroidized low-carbon steel up to 6508C only slight changes in the distribution and size of the cementite were observed. In quenched steel, similar Ð but with greater size differentiation Ð particles were visible. Independent of the initial structure, above 6508C, cementite starts to come together, forming cementite clusters in the ferrite, similar to grains of pearlite, with differentiated carbide morphology (Fig. 10). Medium-carbon steel (0.44% C) behaves in an intermediate manner between high- and low-carbon steel (Fig. 5). The
existence of lamellar pearlite existence decreases the deformation of the pearlite and delays cementite transformation. The second-hand deformation of the pearlite is suf®cient to disperse all of the cementite in all of the matrix at 600± 7008C. The structure during and after deformation is differentiated, with spheroidized, elongated and lamellar particles of cementite being visible, although less than in pearlite steel. At 7208C for initially quenched steel, cementite begins to cluster, similarly to pearlite with a very differentiated shape of the cementite (Fig. 11) noticed also after the deformation of low-carbon steel at a temperature close to that of pearlite reaction. For normalized and spheroidized steel at this temperature, such grouping is less distinct. The deformation of low- and medium-carbon steels at a temperature below but close to the pearlitic reaction gives as a result a structure that is very similar to the structure after the deformation of these steels in the austenite range. During the deformation of steel at high strain rate (6.0 sÿ1) changes of the stress and structure are similar but faster than those for low strain rate. However, since the conditions are not isothermal, thermal instability appears, along with inhomogeneity of the strain and temperature. Therefore the stress and structure cannot be assigned to real conditions. The observed changes of the stress allow the statement that warm deformation of the carbon steel leads to convergence of the stress to the same level for large strain, independently of the initial structure. This results from the formation of some kind of complex structure in dynamic equilibrium, that changes continuously, but generally includes supersaturated ferrite and dissolved and precipitated cementite particles that have very different sizes and shapes.
A. Niechajowicz, A. Tobota / Journal of Materials Processing Technology 106 (2000) 123±130
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Fig. 10. Initially-quenched low-carbon steel after deformation (6508C, e_ 0:03 sÿ1 ).
The temperature 6508C is distinguished for warm deformation. At this temperature the structure is the most differentiated and elongated and lamellar cementite is observed during and also after deformation. It is important that the stresses converged not only for different initial structure, but that they converged also for different steel with different carbon contents (Fig. 2.). At 6508C with a strain of 0.8, there are no differences in steel with a carbon content in the range
of 0.11±0.8% C. This strain is smaller at higher temperatures and is greater at up to about 5008C. At lower temperatures, the stresses tend to one level but do not achieve this completely. Steel with a lower carbon content than 0.11% C has distinctly lower stress at all test temperatures, which can be seen for maximum stress in Fig. 2. This means that at temperatures higher than 4008C the stress for large deformation is controlled mainly by the state of the matrix and
Fig. 11. Initially-quenched medium-carbon steel after deformation (7208C, e_ 0:03 sÿ1 ).
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A. Niechajowicz, A. Tobota / Journal of Materials Processing Technology 106 (2000) 123±130
Fig. 12. The effect of the initial structure on the mechanical properties of medium-carbon steel (ultimate tensile stress and elongation: solid lines; yield stress and reduction of area: broken lines).
less by the numbers of precipitations. With deformation, ferrite is supersaturated to a value of about 0.1% C this value being weakly dependent on the carbon content. When the carbon content of steel is lower, the oversaturation of ferrite is also lower and results in lower stresses. Deformation at a temperature close to the austenite range has the effect that the cementite distribution comes near to unity after pearlite reaction. The structure during deformation in¯uences the structure after deformation. The cementite particles are characterized by a very great difference in size and shape. This determines the properties of medium- and high-carbon steel after deformation (Fig. 12). By combination of the initial structure and the deformation conditions, it is possible to control the properties better than by heat treatment.
References [1] K.P. Rao, Y.K. Prasad, E. Hawbolt, in: Proceedings of the International Conference on Advances in Materials and Processing Technologies, Dublin, 1993, 1517 pp. [2] B. Hadarik, J. Boruta, A. Ploch, I Schindler, in: Proceedings of Mat. Konf. Plast'94, Wisla, 1994, 55 pp. [3] M. Pietrzyk, J. Kusiak, Mat. Konf. Zastosowanie komputeroÂw w zakladach przetwoÂrstwa metali, Koninki, 1996, p. 49. [4] A. Niechajowicz, A. Tobota, Raport ITMiA PWr., Wroclaw, 1995. [5] A. Niechajowicz, A. Tobota, L. Nakonieczny, Mechanik 12 (1988) 577. [6] A. Niechajowicz, A. Tobota, ObroÂbka Plastyczna Metali 1 (1995) 15. [7] A. Niechajowicz, A. Tobota, ObroÂbka Plastyczna Metali 1 (1995) 7. [8] J. Baer, A. Wachowiak, in: Proceedings of Mat. Konf. Plast'94, Wisla, 1994, p. 119.
Warm deformation of carbon steel A. Niechajowicz*, A. Tobota Institute of Mechanical Engineering and Automation, Technical University of Wroclaw, 50-371 Wroclaw, Poland
Abstract The stress and structure of carbon steels during deformation at elevated temperature was investigated. Below the austenite range, concurrent cementite dissolution, ferrite oversaturation, heteromorphous cementite precipitation and coagulation together with dynamic recovery and recrystallization of the matrix determine the structure and stress level during deformation and, ®nally, the structure and properties of the steel after deformation. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Carbon steel; Warm deformation; Stress; Structure
1. Introduction Modern methods of analysis and design of metalworking process, based on mathematical modeling or expert systems, enable the quick designing of the processes quickly and make it, to some degree, independent of the experience and intuition of designers. The accuracy and reliability of designed processes depend on access to proper data such as the stress±strain curves, the limit strains, the effect of the deformation conditions on the properties and structure of the materials, and so on. Therefore, a large part of the research work is devoted to the researching, categorizing and description of the behavior of the deformed material. The predominant role of hot and cold working ensures that for these processes, the phenomena accompanying deformation are known and well described [1±4]. The development of warm metal forming processes requires access to data enabling process design: this is especially essential for steel metalworking. Deformation at a temperature close to the transformation temperature brings about considerable changes of the structure and properties of the deformed steel. Therefore, extension of the knowledge in this area is important for the creation of a data base used directly for the design of the processes and, more important, for the creation of the precise model of the deformed materials. The aim of the paper was an investigation of the stress and structure during the deformation of carbon steels under warm metal forming conditions.
*
Corresponding author.
2. Effect of the temperature and the carbon content on the stress The experimental results presented by authors were obtained in torsion tests [4±7]. The torsional moment M and the angle of torsion a was transformed to stress s and strain e in accordance with the expressions p 3 3M ra ; e p s 2pr 3 3L where r is the radius of the sample, and L the gauge length of the sample. The stress- and strain-values are not exactly equal to the ¯ow stress and the true strain but they are convenient for the representation of M±a curves in relative, dimensionless form. Typical s±e curves for plain carbon steel (0.43% C), obtained from torsion tests, are shown in Fig. 1. The curve shape is dependent on the deformation conditions and results from the phenomena occurring during deformation. For a lower strain rate _e 0:03 sÿ1 the change of the temperature was less than 58C, so that the deformation was practically isothermal and all changes in the stress result from structural phenomena only. With a strain rate greater than 0.5 sÿ1, the temperature increases more, thus the stress is also affected by changes of the temperature during deformation. Strain hardening, dynamic recovery, recrystallization and strain ageing are well known and described for deformation. For temperatures below the austenite range, rebuilding of the pearlite occurs. This appears at the temperatures above 4008C, when the velocity of diffusion is suf®ciently high and it can be seen as a reduction of stress with strain at
0924-0136/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 0 ) 0 0 6 0 2 - 6
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A. Niechajowicz, A. Tobota / Journal of Materials Processing Technology 106 (2000) 123±130
Fig. 1. Stress±strain curves of normalized medium-carbon steel.
the temperature, as recrystallization cannot take place (Fig. 1). The stress level and stress±strain curves depend not only on the deformation conditions, but also on the carbon content. The in¯uence of the carbon content of initially normalized steels on the stress is visible in Fig. 2. The stress tends with deformation to approach the same level, independently of the carbon content, except for steel with a
carbon content of less then 0.1% C, where the stresses are distinctly different. Because there are no mathematical models of steel describing the ¯ow stress by the phenomena occurring during deformation, published data in graphical or numerical form is mainly used, and equations obtained by approximation of the experimental results can also be useful. A comparison of predicted and experimental results for low-
Fig. 2. Stress±strain curves of normalized-carbon steel.
A. Niechajowicz, A. Tobota / Journal of Materials Processing Technology 106 (2000) 123±130
125
3. Effect of the initial structure on the stress during warm deformation The effect of the initial structure on deformation under warm conditions is frequently omitted, although it can be signi®cant. This effect was tested for different carbon steels applying distinctly differentiated structures, from very stable (spheroidized) to the most unstable after quenching. The results are shown for low carbon, medium carbon and perlite steels in Figs. 5±7. The stresses are very different for small strains but generally tend to those for initially spheroidized. The in¯uence of the structure decreases with increase of the temperature and it almost disappears above 6508C. The effect decreases with decrease of carbon content [4].
Fig. 3. Approximated and experimental stress±strain curves of low-carbon steel _e 0:03 sÿ1 .
carbon steel is shown in Fig. 3. For isothermal deformation (_e 0:03ÿ0:5 sÿ1 , T500±9008C) the stress±strain curves for these steels can be described with good accuracy by the equation (Fig. 3) [4]: s 0:5649e0:34 13:0e244:9=T ÿ 16:67 16:7eÿ1:024e 6:782e 2:924ÿ106:7=T c6:782 e_ 0:0893 Frequently, when only simpli®ed process analysis is used, knowledge of the maximum stress can be suf®cient. The maximum stress for initially normalized steel (c0.02± 1.12% C) deformed at T400±9008C with a strain rate of e_ 0:03ÿ6:0 sÿ1 can be described with good accuracy by the following equation (Fig. 4) [6]: smax 37300 0:074 c0:329 e 0:064_eÿ0:0037Tÿ688=T
Fig. 4. Maximum stress of normalized-carbon steel _e 0:03 sÿ1 .
4. Structure during warm deformation The pearlite rebuilt during deformation is generally regarded as spheroidizing, coagulation and also spreading of the cementite. In reality this process is more complex, but research of the steel structure during warm deformation has made it possible to know more about the processes occurring during and after deformation that determine the deformation run, the structure changes and the properties after deformation. The cementite in the steel is generally regarded as a stable phase, which can spheroidize and coagulate when diffusion is possible. However, it is known that the carbon concentration directly in the vicinity of the cementite particles is greater than that in the long distance ferrite and depends on the curvature of the cementite. Also, the bonding power of the carbon in cementite is related closely to the bonding power of the carbon in a complex dislocation-carbon. During deformation at elevated temperature, in the state of strong thermodynamic unbalance, bonding of the carbon with dislocations can be locally more thermodynamically probable than in cementite. Then, in the presence of the large number of mobile dislocations and vacancies, carbon can be dissolved easily and displaced in the matrix, effecting its local oversaturation and in turn cementite can be precipitated. These cementite particles, depending upon the local conditions, can precipitate in differentiated elongated forms Ð needle, lamellar, or ellipsoidal Ð that are quite unstable [4]. This process can differ from typical precipitation, when only change of temperature takes place. Generally, transformation of the cementite during warm deformation can occur by: (i) the mechanical breaking of elongated cementite particles; (ii) the dissolution of cementite particles and oversaturation of ferrite; (iii) the precipitation of cementite from supersaturated ferrite; and (iv) the spheroidizing and coagulation of cementite. These phenomena, together with recovery and recrystallization, occur concurrently and interact during deformation, their velocity and share in the changes of the structure depending on the
126
A. Niechajowicz, A. Tobota / Journal of Materials Processing Technology 106 (2000) 123±130
Fig. 5. The effect of the initial structure on stress±strain curves of low-carbon steel _e 0:03 sÿ1 .
chemical composition of the steel, the initial structure, the temperature and the strain rate. Based on these processes, it is possible to explain the changes of the observed structure and the stress versus strain characteristics. For initially spheroidized material, the most stable eutectoid steel deformed below 6008C, mainly recovery of the ferrite takes place, therefore the stress±strain curve is very ¯at; change of the cementite morphology was not observed. At 6508C the stress changes more, which can
result from recrystallization of the ferrite and changes of the cementite morphology. The microstructure of the test sample during deformation, obtained by its quenching immediately after the cessation of deformation, is shown in Fig. 8. As for deformation at 5008C, there is no visible change of the structure; but during deformation at 6508C there are distinct changes, elongated particles of the cementite being observed. After air cooling, not a great number of elongated particles were visible.
Fig. 6. The effect of the initial structure on stress±strain curves of medium-carbon steel.
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Fig. 7. The effect of the initial structure on stress±strain curves of pearlite steel _e 0:03 sÿ1 .
More distinct changes of the stress occur for initially quenched material, the most unstable eutectoid steel. During heating before deformation, the tempering process is started and it is accelerated with deformation, causing the stress to quickly tend to descend to the level for spheroidized steel; the descent is faster with increase of the temperature. During deformation, the changes of the structure are very complex. There are visible globular and elongated cementite particles predominant at temperatures of less than 6508C, whilst at 6508C and above cementite can precipitate from super-
saturated ferrite in lamellar shape also (Fig. 9). The lamellar cementite is very unstable and quickly transforms to more globular form. Air-cooling after deformation causes the cementite particles to be less elongated, and mainly ellipsoidal, with distinct size differentiation. After deformation at temperatures higher than 6008C, regions with lamellar cementite can remain and appear as typical pearlite. Such structures were reported by other authors [8] and were attributed to local increase of the temperature. The results presented for low strain rates exclude this possibility and
Fig. 8. Structure of initially spheroidized pearlitic steel quenched after deformation (6508C, e_ 0:03 sÿ1 ).
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Fig. 9. Initially-quenched pearlitic steel quenched after deformation (6508C, e_ 0:03 sÿ1 ).
allow to their attribution to cementite transformation below the transformation temperature. Other changes of the stress and structure can be observed for initially normalized steel. Initial breaking of the cementite accelerates its transformation. Regions with lamellar pearlite, partially spheroidized pearlite, globular cementite and newly precipitated cementite, coexist and transform. This differentiation of the structure stays after deformation also. Above 6008C it is hard to say whether pearlite regions are newly created or are the remainder of the initial structure. For steel with different carbon content, behavioral differences result from the distribution of the strain between the softer ferrite matrix, and the harder pearlite. For low-carbon steel (Fig. 5) a large part of the deformation takes place in the ferrite, the changes of the cementite being slower. The lamellar pearlite in normalized steel undergoes breaking and transformation to globular form within clusters of pearlite at temperature up to 6008C. At 5008C, only for strain greater than 1.5, these changes in¯uence the stress. At 6508C there are zones with clusters of cementite and ones with more uniformly distributed cementite in the matrix, but the size and shape of the cementite is very different. After the deformation of spheroidized low-carbon steel up to 6508C only slight changes in the distribution and size of the cementite were observed. In quenched steel, similar Ð but with greater size differentiation Ð particles were visible. Independent of the initial structure, above 6508C, cementite starts to come together, forming cementite clusters in the ferrite, similar to grains of pearlite, with differentiated carbide morphology (Fig. 10). Medium-carbon steel (0.44% C) behaves in an intermediate manner between high- and low-carbon steel (Fig. 5). The
existence of lamellar pearlite existence decreases the deformation of the pearlite and delays cementite transformation. The second-hand deformation of the pearlite is suf®cient to disperse all of the cementite in all of the matrix at 600± 7008C. The structure during and after deformation is differentiated, with spheroidized, elongated and lamellar particles of cementite being visible, although less than in pearlite steel. At 7208C for initially quenched steel, cementite begins to cluster, similarly to pearlite with a very differentiated shape of the cementite (Fig. 11) noticed also after the deformation of low-carbon steel at a temperature close to that of pearlite reaction. For normalized and spheroidized steel at this temperature, such grouping is less distinct. The deformation of low- and medium-carbon steels at a temperature below but close to the pearlitic reaction gives as a result a structure that is very similar to the structure after the deformation of these steels in the austenite range. During the deformation of steel at high strain rate (6.0 sÿ1) changes of the stress and structure are similar but faster than those for low strain rate. However, since the conditions are not isothermal, thermal instability appears, along with inhomogeneity of the strain and temperature. Therefore the stress and structure cannot be assigned to real conditions. The observed changes of the stress allow the statement that warm deformation of the carbon steel leads to convergence of the stress to the same level for large strain, independently of the initial structure. This results from the formation of some kind of complex structure in dynamic equilibrium, that changes continuously, but generally includes supersaturated ferrite and dissolved and precipitated cementite particles that have very different sizes and shapes.
A. Niechajowicz, A. Tobota / Journal of Materials Processing Technology 106 (2000) 123±130
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Fig. 10. Initially-quenched low-carbon steel after deformation (6508C, e_ 0:03 sÿ1 ).
The temperature 6508C is distinguished for warm deformation. At this temperature the structure is the most differentiated and elongated and lamellar cementite is observed during and also after deformation. It is important that the stresses converged not only for different initial structure, but that they converged also for different steel with different carbon contents (Fig. 2.). At 6508C with a strain of 0.8, there are no differences in steel with a carbon content in the range
of 0.11±0.8% C. This strain is smaller at higher temperatures and is greater at up to about 5008C. At lower temperatures, the stresses tend to one level but do not achieve this completely. Steel with a lower carbon content than 0.11% C has distinctly lower stress at all test temperatures, which can be seen for maximum stress in Fig. 2. This means that at temperatures higher than 4008C the stress for large deformation is controlled mainly by the state of the matrix and
Fig. 11. Initially-quenched medium-carbon steel after deformation (7208C, e_ 0:03 sÿ1 ).
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Fig. 12. The effect of the initial structure on the mechanical properties of medium-carbon steel (ultimate tensile stress and elongation: solid lines; yield stress and reduction of area: broken lines).
less by the numbers of precipitations. With deformation, ferrite is supersaturated to a value of about 0.1% C this value being weakly dependent on the carbon content. When the carbon content of steel is lower, the oversaturation of ferrite is also lower and results in lower stresses. Deformation at a temperature close to the austenite range has the effect that the cementite distribution comes near to unity after pearlite reaction. The structure during deformation in¯uences the structure after deformation. The cementite particles are characterized by a very great difference in size and shape. This determines the properties of medium- and high-carbon steel after deformation (Fig. 12). By combination of the initial structure and the deformation conditions, it is possible to control the properties better than by heat treatment.
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