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Isothermal plastic forming of high melting temperature alloys
Journal of Materials Processing Technology 72 (1997) 429 – 433
Isothermal plastic forming of high melting temperature alloys J. Sinczak a, W. Lapkowski a, S. Rusz b,* a
b
Faculty of Metallurgy and Materials Engineering, Akademia Go´rniczo-Hutnicza, 30 -059 Krakow, Poland Faculty of Mechanical Engineering, Vysoka Skola Banska, Technical Uni6ersity, 4 trida 17 listopadu 15, 708 33 Ostra6a, Czech Republic Received 1 June 1996
Abstract This paper deals with the problem of an unconventional plastic forming process — deformation of high melting temperature alloys under isothermal conditions. The test were performed for alloyed steel containing 0.5% C. The upsetting process at various temperatures and strain rates was used to determine the optimum parameters of deformation. In order to obtain fine grains prior to deformation and create the superplasticity effect, the samples were subjected to thermomechanical processing. The main experiment included axisymmetrical closed-die forging. Very good filling of the grove and low loads were observed in all tests. Despite the relatively long time of deformation at high temperatures, the material maintained a fine microstructure. © 1997 Elsevier Science S.A. Keywords: Isothermal forging; Superplastic deformation
1. Introduction Isothermal forging involves the hot plastic deformation of metals or alloys at constant temperature and at low strain rates. In order to maintain constant temperature of the sample, the upper tool is heated to the test temperature. Maintaining these conditions allows large plastic deformations to be obtained for a number of materials that are difficult to deform in conventional ways. Beyond this, the introduction of a proper microstructure prior to deformation (fine grains) leads to the superplastic effect [5,6]. Consequently, this process has all of the advantages of the superplastic effect, such as the possibility of obtaining large deformations with small loads. The superplasticity effect can be obtained easily in steels, that have an austenitic – ferritic structure at the temperatures of deformation. It is difficult to maintain a fine microstructure without the presence of the second phase. The second phase is effective when its properties of the temperature of deformation are similar to those of the matrix. These conditions are often met by eutectic and eutectoid steels. Fine grains can be obtained in * Corresponding author. Tel.: +42 69 6991111; fax: + 42 69 6918647. 0924-0136/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 4 - 0 1 3 6 ( 9 7 ) 0 0 2 0 6 - 9
these steels by special thermomechanical treatment. In super-eutectoidal steels the superplastic conditions can be obtained with a complex structure of cementite with a grain size of about 1 mm. The eutectoid steel contains about 12% of cementite, which is not enough to constrain the grain growth. Nevertheless, the superplastic effect can be achieved in such steels also. An increase of the carbon contents to 1.9% increases the fraction of cementite to 30% and limits the grain growth and fosters the superplastic effect. The investigation of superplasticity in conventional steels using the technique was also carried out. The present project deals with low-alloyed medium carbon steel with the chemical composition given in Table 1.
2. Experiment The fine microstructure required to obtain the superplasticity effect was achieved by thermomechanical treatment [2]. Then, samples with a diameter of 10 mm and a height of 15 mm were compressed on an Instron machine with a capacity of 250 kN. According to the data given in [2], two velocities of the die were used, 1 and 2 mm min − 1. The schematic illustration of the testing device is shown in Fig. 1 and the process
J. Sinczak et al. / Journal of Materials Processing Technology 72 (1997) 429–433
430
Table 1 Chemical composition of examined steel (wt.%) C
Mn
Si
P
S
Cu
Ni
Cr
V
Al
0.5
0.85
0.35
0.015
0.006
0.02
0.03
1.2
0.15
0.02
parameters are given in Table 2. During the test, the temperature inside the sample was monitored to an accuracy of 1°C. NiCr – Ni thermocouples embedded in the sample were used in the measurements. The second part of the experiment included axisymmetrical forging under the conditions determined in the upsetting test. The model forging (Fig. 2) was forged with the parameters given in Table 2. The stock material was of 15 mm height and 25 mm diameter. A schematic illustration of a die suitable for forging at high temperatures is presented in [4]. Continuous monitoring of the temperature in the forging and the force was carried out.
3. Analysis of the upsetting tests The average strain rate in the sample during upsetting was 1.7× 10 − 3 s − 1 for the die velocity of 1 mm min − 1 and 3.0×10 − 3 s − 1 for the die velocity of 2 mm s − 1. The die velocity increases slightly during the test. An assumption was made that this increase does not affect the kinetics of deformation and the phenomena associated with the mechanism of deformation. The stress–strain curves were plotted on the basis of the measured relationship between the die displacement
and the upsetting force (Fig. 3). The results show that the level of the yield stress stabilizes after exceeding a strain of 0.15. Within the temperature range investigated (720–900°C) the yield stress depends not only on the temperature but also on the strain rate. The calculated strain-rate sensitivity parameter m varies between 0.25 and 0.43. Thus, it can be concluded that for this steel achieves superplastic conditions for the parameters considered. This phenomenon has been described several times, but in general there are several mechanisms of superplasticity that can be considered [7]. Models of superplastic deformation have been developed on the basis of these mechanisms. The models differ in the description of the elementary processes, such as dislocation creep and slip along the grain boundaries, combined with the deformation inside the grains. Physical interpretations of the developed relationships are based mainly on the dislocation mechanisms having various character at various stages of the superplastic deformation. Three stages of this deformation can be distinguished. The second stage involves usually the largest strain-rate sensitivity. The dislocation slide mechanism is common for all three stages, but it develops along the grain boundaries at stage 1 and inside the grains at stage 3. The appearance of superplastic deformation at the second stage is the associated with the change of the deformation mode from the deformation of grains for larger strain rates to deformation by slip along the boundaries in slow tests. Mathematical description of the stress–strain relationships under superplastic conditions are usually given in the form of higher-order polynomials or in the form of rheological model [3]. The strain rates used in the industrial processes were of particular interest to the authors. These strain rates are adequate for the third stage of superplastic deformation, which involves mainly dislocation slip inside the grains and marginally slip along the grain boundaries. The stress–strain curves obtained from the upsetting tests for various strain rates and temperatures (Fig. 3) were used in the computer simulation of forging of the shape shown in Fig. 2. Table 2 The range of experimental research performed
Fig. 1. The block diagram of the research stand: RV, velocity measurement; RF, force measurement; Z, power supply; RP, sample temperature measurement; RZ, furnace temperature control.
Strain rate (×10−3 s−1)
Temperature (°C)
1.7 3.0
720, 750, 780, 860, 900 720, 750, 780, 860, 900
J. Sinczak et al. / Journal of Materials Processing Technology 72 (1997) 429–433
431
when the strains concentrate in the flange (Fig. 4(b)). The strain rates at the beginning of that stage are close to those used in industrial practice on mechanical presses. An increase of the yield stress caused by the significant increase of the strain rate results in an increase of the force by several times during a short displacement of the die. The results of calculations and measurements coincide. The characteristic change of the force between 7 and 9 mm of the die displacement is observed (Fig. 5), which corresponds to 80% of the assumed total deformation.
5. Conclusions
4. Computer simulation of forging process
The process investigated is characterized by large inhomogeneity of strain rates, that prevents achieving superplastic conditions over the whole volume of the forging. Thus, isothermal conditions are created in some selected parts of the sample. The process has several advantages and is used in the plastic forming of hard-to-deform materials. The microstructure be-
Simulation was performed using code FORM-2D [1] based on the finite-element approach. Thermomechanical analysis is carried out in triangular six node elements using a viscoplastic model of the deforming body. A second-order approximation of the nodal velocities and linear approximation of average stress and temperature are employed. The program also enables an analysis of non-steady state and non-isothermal processes, assuming constant or varying volume, accounting for mixed boundary conditions in stresses and strains and the possibility of losing the contact between the tool and the workpiece. The program enables the introduction of active friction on the selected contact surfaces. It has an automatic mesh generator and an adaptive remeshing procedure. Calculations for the forging investigated, in which the symmetry is associated with the 60° angle, were carried out in two planes perpendicular to the main axis and inclined to each other at an angle of 30°. The results include the distributions of strains in the planes of the stable deformation, in the web and in the plane of the smallest longitudinal cross-section, results being presented in Fig. 4. Two characteristic stages of the precess are considered, which are distinguished by the change of the forging force, as illustrated in Fig. 5. The first stage of deformation is characterized by a slow increase of load. It ends when the sample makes contact with the horizontal part of the upper die. The strain distribution is uniform then (Fig. 4(a)) and the strain rates do not exceed 0.05 s − 1. Thus, a minimum increase of the yield stress and in consequence, of the force, could be expected. The second characteristic stage of deformation begins
Fig. 3. The stress – strain curves for the steel for various speeds of deformation (upsetting): (a) 1 mm min − 1; (b) 2 mm min − 1.
Fig. 2. The model forging (dimensions: mm).
432
J. Sinczak et al. / Journal of Materials Processing Technology 72 (1997) 429–433
Fig. 4. Grid distribution and distribution of the strain rate for two stages of forging for the 1 mm min − 1 deformation speed: (a) strain 70%; (b) strain 100%.
fore deformation under isothermal conditions is prepared in the same way as for superplastic deformation, by refining the microstructure. The long deformation time required for the super-plastic process fosters grain
Fig. 5. Forging force versus strain.
growth. Therefore, this time should be kept as short as possible, in particular for materials that do not contain additions to stabilizing the fine microstructure at high temperature. The microstructure of the steel investigated is shown in Fig. 6. The initial microstructure was martensitic with fine precipitates of carbides. After heat treatment involving heating to 800°C and quenching in water, Fig. 6(a) shows the microstructure prior to deformation and Fig. 6(b) the microstructure after deformation. It is seen from the figures that the fine-grain microstructure was maintained in the forging. The maintenance of the fine microstructure during deformation can be obtained by the presence of the second phase, the latter being effective when its properties at the temperature of superplastic deformation are similar to those of the matrix. High-carbon steels have this feature. Fine grains in such steels can be achieved by proper thermomechanical treatment. Cementite grains measuring 0.2 mm are located in the ferrite matrix having grains of 2 mm. The heat treatment applied to steel containing 0.5% C was the most efficient in the case of the two-phase structure a+g.
J. Sinczak et al. / Journal of Materials Processing Technology 72 (1997) 429–433
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Moreover, the introduction of additives to this steel fosters the stabilization of the fine-grain microstructure during deformation. The results of the present investigation can be extended to steels with similar chemical composition. The steels superplastic conditions can be used for the manufacturing of products, the shaping of which requires superplastic deformation. Their excellent formability, enabling an extension of the range of the methods of plastic forming, is the main advantage of superplastic alloys. Moreover, it gives very good filling of grooves and allows the elimination of the grinding operations that are required after the conventional forging of complex parts. A low force is another advantage of superplastic deformations. The application of the optimum deformation conditions (strain rates and temperatures) allows material formation under low stresses. Thus, larger products can be forged on the same machine.
References
Fig. 6. (a) The structure of the initial material; and (b) that of the forging obtained; for steel of the chemical composition given in Table 2.
.
[1] G.J. Gun, et al. Kuzn. Sztamp. Proizv. 7 (1994) 9. [2] S. Rusz, Rudy Metale 11 (1995) 454. [3] Y. Shen et al., in: W.B. Lee (Ed.), Advances in Engineering Plasticity and its Application, Elsevier, Amsterdam, 1993, p. 1041. [4] J. Sin´czak, Z. Malinowski, Scand. J. Metall. 16 (1987) 194. [5] J. Sin´czak, Z N. AGH Metalurgia i Odlewnictwo 144 (1992) 5. [6] S. Szczepanik, J. Sin´czak, Metall. Foundry Eng. 4 (1994) 441. [7] S.W. Zehr, W.A. Backofen, Trans. Am. Soc. Met. 61 (1968) 300.
Isothermal plastic forming of high melting temperature alloys J. Sinczak a, W. Lapkowski a, S. Rusz b,* a
b
Faculty of Metallurgy and Materials Engineering, Akademia Go´rniczo-Hutnicza, 30 -059 Krakow, Poland Faculty of Mechanical Engineering, Vysoka Skola Banska, Technical Uni6ersity, 4 trida 17 listopadu 15, 708 33 Ostra6a, Czech Republic Received 1 June 1996
Abstract This paper deals with the problem of an unconventional plastic forming process — deformation of high melting temperature alloys under isothermal conditions. The test were performed for alloyed steel containing 0.5% C. The upsetting process at various temperatures and strain rates was used to determine the optimum parameters of deformation. In order to obtain fine grains prior to deformation and create the superplasticity effect, the samples were subjected to thermomechanical processing. The main experiment included axisymmetrical closed-die forging. Very good filling of the grove and low loads were observed in all tests. Despite the relatively long time of deformation at high temperatures, the material maintained a fine microstructure. © 1997 Elsevier Science S.A. Keywords: Isothermal forging; Superplastic deformation
1. Introduction Isothermal forging involves the hot plastic deformation of metals or alloys at constant temperature and at low strain rates. In order to maintain constant temperature of the sample, the upper tool is heated to the test temperature. Maintaining these conditions allows large plastic deformations to be obtained for a number of materials that are difficult to deform in conventional ways. Beyond this, the introduction of a proper microstructure prior to deformation (fine grains) leads to the superplastic effect [5,6]. Consequently, this process has all of the advantages of the superplastic effect, such as the possibility of obtaining large deformations with small loads. The superplasticity effect can be obtained easily in steels, that have an austenitic – ferritic structure at the temperatures of deformation. It is difficult to maintain a fine microstructure without the presence of the second phase. The second phase is effective when its properties of the temperature of deformation are similar to those of the matrix. These conditions are often met by eutectic and eutectoid steels. Fine grains can be obtained in * Corresponding author. Tel.: +42 69 6991111; fax: + 42 69 6918647. 0924-0136/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 4 - 0 1 3 6 ( 9 7 ) 0 0 2 0 6 - 9
these steels by special thermomechanical treatment. In super-eutectoidal steels the superplastic conditions can be obtained with a complex structure of cementite with a grain size of about 1 mm. The eutectoid steel contains about 12% of cementite, which is not enough to constrain the grain growth. Nevertheless, the superplastic effect can be achieved in such steels also. An increase of the carbon contents to 1.9% increases the fraction of cementite to 30% and limits the grain growth and fosters the superplastic effect. The investigation of superplasticity in conventional steels using the technique was also carried out. The present project deals with low-alloyed medium carbon steel with the chemical composition given in Table 1.
2. Experiment The fine microstructure required to obtain the superplasticity effect was achieved by thermomechanical treatment [2]. Then, samples with a diameter of 10 mm and a height of 15 mm were compressed on an Instron machine with a capacity of 250 kN. According to the data given in [2], two velocities of the die were used, 1 and 2 mm min − 1. The schematic illustration of the testing device is shown in Fig. 1 and the process
J. Sinczak et al. / Journal of Materials Processing Technology 72 (1997) 429–433
430
Table 1 Chemical composition of examined steel (wt.%) C
Mn
Si
P
S
Cu
Ni
Cr
V
Al
0.5
0.85
0.35
0.015
0.006
0.02
0.03
1.2
0.15
0.02
parameters are given in Table 2. During the test, the temperature inside the sample was monitored to an accuracy of 1°C. NiCr – Ni thermocouples embedded in the sample were used in the measurements. The second part of the experiment included axisymmetrical forging under the conditions determined in the upsetting test. The model forging (Fig. 2) was forged with the parameters given in Table 2. The stock material was of 15 mm height and 25 mm diameter. A schematic illustration of a die suitable for forging at high temperatures is presented in [4]. Continuous monitoring of the temperature in the forging and the force was carried out.
3. Analysis of the upsetting tests The average strain rate in the sample during upsetting was 1.7× 10 − 3 s − 1 for the die velocity of 1 mm min − 1 and 3.0×10 − 3 s − 1 for the die velocity of 2 mm s − 1. The die velocity increases slightly during the test. An assumption was made that this increase does not affect the kinetics of deformation and the phenomena associated with the mechanism of deformation. The stress–strain curves were plotted on the basis of the measured relationship between the die displacement
and the upsetting force (Fig. 3). The results show that the level of the yield stress stabilizes after exceeding a strain of 0.15. Within the temperature range investigated (720–900°C) the yield stress depends not only on the temperature but also on the strain rate. The calculated strain-rate sensitivity parameter m varies between 0.25 and 0.43. Thus, it can be concluded that for this steel achieves superplastic conditions for the parameters considered. This phenomenon has been described several times, but in general there are several mechanisms of superplasticity that can be considered [7]. Models of superplastic deformation have been developed on the basis of these mechanisms. The models differ in the description of the elementary processes, such as dislocation creep and slip along the grain boundaries, combined with the deformation inside the grains. Physical interpretations of the developed relationships are based mainly on the dislocation mechanisms having various character at various stages of the superplastic deformation. Three stages of this deformation can be distinguished. The second stage involves usually the largest strain-rate sensitivity. The dislocation slide mechanism is common for all three stages, but it develops along the grain boundaries at stage 1 and inside the grains at stage 3. The appearance of superplastic deformation at the second stage is the associated with the change of the deformation mode from the deformation of grains for larger strain rates to deformation by slip along the boundaries in slow tests. Mathematical description of the stress–strain relationships under superplastic conditions are usually given in the form of higher-order polynomials or in the form of rheological model [3]. The strain rates used in the industrial processes were of particular interest to the authors. These strain rates are adequate for the third stage of superplastic deformation, which involves mainly dislocation slip inside the grains and marginally slip along the grain boundaries. The stress–strain curves obtained from the upsetting tests for various strain rates and temperatures (Fig. 3) were used in the computer simulation of forging of the shape shown in Fig. 2. Table 2 The range of experimental research performed
Fig. 1. The block diagram of the research stand: RV, velocity measurement; RF, force measurement; Z, power supply; RP, sample temperature measurement; RZ, furnace temperature control.
Strain rate (×10−3 s−1)
Temperature (°C)
1.7 3.0
720, 750, 780, 860, 900 720, 750, 780, 860, 900
J. Sinczak et al. / Journal of Materials Processing Technology 72 (1997) 429–433
431
when the strains concentrate in the flange (Fig. 4(b)). The strain rates at the beginning of that stage are close to those used in industrial practice on mechanical presses. An increase of the yield stress caused by the significant increase of the strain rate results in an increase of the force by several times during a short displacement of the die. The results of calculations and measurements coincide. The characteristic change of the force between 7 and 9 mm of the die displacement is observed (Fig. 5), which corresponds to 80% of the assumed total deformation.
5. Conclusions
4. Computer simulation of forging process
The process investigated is characterized by large inhomogeneity of strain rates, that prevents achieving superplastic conditions over the whole volume of the forging. Thus, isothermal conditions are created in some selected parts of the sample. The process has several advantages and is used in the plastic forming of hard-to-deform materials. The microstructure be-
Simulation was performed using code FORM-2D [1] based on the finite-element approach. Thermomechanical analysis is carried out in triangular six node elements using a viscoplastic model of the deforming body. A second-order approximation of the nodal velocities and linear approximation of average stress and temperature are employed. The program also enables an analysis of non-steady state and non-isothermal processes, assuming constant or varying volume, accounting for mixed boundary conditions in stresses and strains and the possibility of losing the contact between the tool and the workpiece. The program enables the introduction of active friction on the selected contact surfaces. It has an automatic mesh generator and an adaptive remeshing procedure. Calculations for the forging investigated, in which the symmetry is associated with the 60° angle, were carried out in two planes perpendicular to the main axis and inclined to each other at an angle of 30°. The results include the distributions of strains in the planes of the stable deformation, in the web and in the plane of the smallest longitudinal cross-section, results being presented in Fig. 4. Two characteristic stages of the precess are considered, which are distinguished by the change of the forging force, as illustrated in Fig. 5. The first stage of deformation is characterized by a slow increase of load. It ends when the sample makes contact with the horizontal part of the upper die. The strain distribution is uniform then (Fig. 4(a)) and the strain rates do not exceed 0.05 s − 1. Thus, a minimum increase of the yield stress and in consequence, of the force, could be expected. The second characteristic stage of deformation begins
Fig. 3. The stress – strain curves for the steel for various speeds of deformation (upsetting): (a) 1 mm min − 1; (b) 2 mm min − 1.
Fig. 2. The model forging (dimensions: mm).
432
J. Sinczak et al. / Journal of Materials Processing Technology 72 (1997) 429–433
Fig. 4. Grid distribution and distribution of the strain rate for two stages of forging for the 1 mm min − 1 deformation speed: (a) strain 70%; (b) strain 100%.
fore deformation under isothermal conditions is prepared in the same way as for superplastic deformation, by refining the microstructure. The long deformation time required for the super-plastic process fosters grain
Fig. 5. Forging force versus strain.
growth. Therefore, this time should be kept as short as possible, in particular for materials that do not contain additions to stabilizing the fine microstructure at high temperature. The microstructure of the steel investigated is shown in Fig. 6. The initial microstructure was martensitic with fine precipitates of carbides. After heat treatment involving heating to 800°C and quenching in water, Fig. 6(a) shows the microstructure prior to deformation and Fig. 6(b) the microstructure after deformation. It is seen from the figures that the fine-grain microstructure was maintained in the forging. The maintenance of the fine microstructure during deformation can be obtained by the presence of the second phase, the latter being effective when its properties at the temperature of superplastic deformation are similar to those of the matrix. High-carbon steels have this feature. Fine grains in such steels can be achieved by proper thermomechanical treatment. Cementite grains measuring 0.2 mm are located in the ferrite matrix having grains of 2 mm. The heat treatment applied to steel containing 0.5% C was the most efficient in the case of the two-phase structure a+g.
J. Sinczak et al. / Journal of Materials Processing Technology 72 (1997) 429–433
433
Moreover, the introduction of additives to this steel fosters the stabilization of the fine-grain microstructure during deformation. The results of the present investigation can be extended to steels with similar chemical composition. The steels superplastic conditions can be used for the manufacturing of products, the shaping of which requires superplastic deformation. Their excellent formability, enabling an extension of the range of the methods of plastic forming, is the main advantage of superplastic alloys. Moreover, it gives very good filling of grooves and allows the elimination of the grinding operations that are required after the conventional forging of complex parts. A low force is another advantage of superplastic deformations. The application of the optimum deformation conditions (strain rates and temperatures) allows material formation under low stresses. Thus, larger products can be forged on the same machine.
References
Fig. 6. (a) The structure of the initial material; and (b) that of the forging obtained; for steel of the chemical composition given in Table 2.
.
[1] G.J. Gun, et al. Kuzn. Sztamp. Proizv. 7 (1994) 9. [2] S. Rusz, Rudy Metale 11 (1995) 454. [3] Y. Shen et al., in: W.B. Lee (Ed.), Advances in Engineering Plasticity and its Application, Elsevier, Amsterdam, 1993, p. 1041. [4] J. Sin´czak, Z. Malinowski, Scand. J. Metall. 16 (1987) 194. [5] J. Sin´czak, Z N. AGH Metalurgia i Odlewnictwo 144 (1992) 5. [6] S. Szczepanik, J. Sin´czak, Metall. Foundry Eng. 4 (1994) 441. [7] S.W. Zehr, W.A. Backofen, Trans. Am. Soc. Met. 61 (1968) 300.