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Isothermal plastic forming of high-carbon steel
ELSEVIER
Materials
Science
Isothermal
and Engineering
A234L236
(1997)
430-433
plastic forming of high-carbon
S. Rusz a,*, J. Sinczak b, W. Lapkowski a Faculty of Mechanical Engineering, Technical Uniwrsity~Ostrava, b Faculty of Metallurgy and Materials Engineering, Akademia Received
31 January
1997; received
steel
b
Yeronymova 425, Frydek-Mislek, Czech Republic G&niczo-Hutnicza, 30-059 Krakow, Poland
in revised
form
8 April
1997
Abstract The work deals with the problem of an unconventional plastic forming process-deformation of high melting temperature alloys in isothermal conditions. The tests were performed for alloyed steel containing O.S!!C. Upsetting the 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 to create the superplasticity effect, the samples were subjected to thermomechanical processing. The main experiment included axisymmetrical closed die forging. Very good filling of the groove and low loads were observed in all tests. In spite of relatively long time of deformation at high temperatures. the material maintained fine microstructure. 0 1997 Elsevier Science S.A. Keywords:
Isothermal
forging;
Superplastic
deformation
1. Introduction Isothermal forging involves hot plastic deformation of metals or alloys at constant temperature and with low strain rates. In order to maintain constant temperature of the sample, the upper tool is heated to the test temperature. Maintaining those conditions allows large plastic deformations to be obtained for a number of materials which are difficult to deform in conventional ways. Beyond this, introduction of proper microstructure prior to deformation (fine grains) leads to the superplastic effect [1,5]. Consequently this process has all the advantages of the superplastic effect, such as possibility of obtaining large deformations with small loads. The superplasticity effect can be easily obtained in the steels,which have austenitic-ferritic structure in the temperatures of deformation. It is difficult to maintain fine microstructure without the presence of the second phase. These conditions are often met by the eutectic and eutectoid steels.The fine grains can be obtained in this steels by special thermomechanical treatment. In hypereutectoid steels the superplastic conditions can be obtained due to complex structure of cementite with the * Corresponding 6916490; e-mail:
author. Tel.: + 42 69 6993315; [email protected]
0921-5093/97/$17.00 Prrso921-5093(97)ool55-X
Q 1997 Elsevier
Science
fax:
S.A. All rights
+ 42 69
reserved
grain size of about 0.1 pm located in the fine ferrite with the grain size of about 1 pm. 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 these steels too. An increase of the carbon contents up to 1.9% increasesthe fraction of cementite to 30% and limits of the grain growth and fosters superplastic effect. The investigation of the superplasticity in the conventional structural steels was also carried out. The present project deals with the 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 produced by the thermomechanical treatment [2]. Then, the sampleswith the diameter of 10 mm and height of 15 mm were compressed in the INSTRON machine with the maximum load capacity of 250 kN. According to the data given in [2], two velocities of the die were used, 1 and 2 mm min - I. The schematic illustration of the testing device is shown in Fig. 1 and the process parameters are given in Table 2. During the test, the temperature inside the sample was monitored with the accuracy of 1°C. The thermocou-
S. Ram et al. /Materials Table 1 Chemical
Science
and Engineering
A234-236
(1997)
430-433
431
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
ples NiCr-Ni embedded in the sample were used in the measurements. The second part of the experiment included axisymmetrical forging in 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 15 mm high and 25 mm in diameter. The schematic illustration of the die proper for forging in the high temperatures is presented in [4]. The continuous monitoring of specimen temperature and the force in the forging was performed.
3. Analysis
of the upsetting tests
The average strain rate in the sample during upsetting was 1.7 x 10W3 s-l for the die velocity 1 mm min-’ and 3.0 x lo- 3 s ~ ’ for the die velocity 2 mm s-i. The die velocity increases slightly during the test. An assumption was made that this increase does not affect the kinetics of deformation and phenomena connected 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 is stabilized after exceeding the strain of 0.15. In the investigated temperature range (720-900°C) the yield stress depends not only on the temperature but also on the strain rate. Calculated strain rate sensitivity parameter m varies between 0.25 and 0.43. Thus, it can be concluded that investigated steel in the considered parameters achieves superplastic properties at studied conditions. This phenomenon was described already, but in general there are several mechanisms of the superplasticity which have been considered. The model [1,6] of the superplastic deformation were developed on the basis of these mechanisms. The model differ by the description of the elementary processes,such as dislocation creep (first stage), slip along the grain boundaries (second stage), combined with the deformation inside the grains thrird stage). Physical interpretations of the developed relationships are based mainly on the dislocation mechanisms having variable character at different stages of the superplastic deformation. Three stagesof 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 the superplastic deformation at the second stage is then connected with the change of the deformation mode from deformation of grains for larger strain rates to the deformation by the slip along the boundaries in slow tests. Mathematical description of the stress-strain relationships in the superplastic conditions are usually given in the form of higher order polynomials or in the form of the rheological model [3]. The strain rates used in the industrial processeswere in particular interest for the Authors. These strain rates corresponds for the third stage of superplastic deformation, which involves mainly dislocation slip inside the grains and marginally the slip along the grain boundaries. The stress-strain curves obtained from the upsetting tests for two strain rates applied and temperatures (Fig. 3) were used in the computer simulation of forging of the shape shown in Fig. 2. Table 2 Process parameters Strain
Fig. 1. The block diagram of the research stand: R,, velocity measurement; R,, force measurement; Z, power supply; R,, sample temperature measurement; R,, furnace temperature control.
1.7 3.0
rate x 10-s
s-’
Temperature
(“C)
720, 750, 780, 860, 900 720, 750, 780, 860, 900
432
S. Rusz et al. /Materials
@56
i-
Science
and Engineering
(1997)
430-433
prevents achieving the superplastic conditions in the whole volume of the forging. Thus, the 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 refined microstructure before the deformation in isothermal condition is to be prepared, in the sameway as for the superplastic deformation. Long deformation time required for the superplastic process fosters the grain growth. Therefore, this time should be kept as short as possible, in particular for the materials which do not contain additions stabilizing the fine microstructure in high temperature. The microstructure of the investigated steel is shown in Fig. 4. The initial microstructure was martensitic with fine precipitates of carbides. After heat treatment involving heating to 800°C and quenching in water. Fig. 4a shows the microstructure prior to deformation and Fig. 4b the microstructure after deformation. It is seenin the figure that the fine grain microstructure was maintained in the forging.
I
Fig. 2. Model forging.
25r a v = 1 mmlmin . _ _ .. . _ . ., . - . . . . .__e_.
A2344236
.. . - .
.----
--.-._..-. -720
--750 . . . . ..7*0
lC
Temponlurs
-.
- .860.
-*--go0
-0
0.05
0.1
0.15
0.2
0.25 Relative
0.3
0.35
0.4
0.45
0.5
0.55
strain
b
v = 2 mmlmin 25
Temperalure
-7720 --750 . . . ...760
lC
- * - - 660 --.-go0 0
I 0
0.05
0.1
0.15
0.2
0.25 Relalive
0.3
0.35
0.4
0.45
0.5
0.55
strain
Fig. 3. Stress-strain curves for the steel for two deformation rates applied: (a) 1 mm min-‘, (b) 2 mm min-‘.
4. Description of results The process of isothermal plastic forming is characterized by large inhomogeneity of strain rates, what
Fig. 4. Structure (martensit with carbides) of the initial material (a) and the forging (b) obtained from the steel of the chemical composition given in Table 2.
S. Rusz et al. /Materials
Science
and Engineering
The maintenance of the fine microstructure during deformation can be obtained by the presence of the second phase. The second phase is effective when its properties in the temperature of superplastic deformation are similar to those of the matrix. The high carbon steel have this feature. The fine grains in this steel can be achieved by the proper thermomechanical treatment. The cementite grains measuring 0.2 nm are located in the ferrite matrix having the grains of 2 urn. The heat treatment applied to the steel containing 0.5%C was the most efficient in the case of the two phase structure CI+ y. Moreover, an introduction of the additives to this steel fosters the stabilization of the fine grain microstructure during deformation.
A234-236
(1997)
430-433
433
which require the super-plastic deformation. Their excellent formability, allowing to extend the range of the methods of plastic forming, is the main advantage of the superplastic alloys. Moreover, it gives very good filling of the groove and allows elimination of the grinding operations, which are required after conventional forging of complex parts. Low force is another advantage of the super-plastic deformations. Application of the optimum deformation conditions (strain rates and temperatures) allows to form the material under low stresses. Thus larger products can be forged on the same machine. References
5. Conclusions
The results of the investigation can be extended on the steels with the similar chemical composition. The tested steel, supple to superplastic conditions, can be used for the manufacturing of the products shaping of
[l] G.J. Gun, Kuzn. &tamp. Proizvodstvo 7 (1994) 9. [2] S. Rusz, Rudy i Metale 11 (1995) 454. [3] Shen Yuli et al., Proc. Asia-Pacific Symposium on Advances in Engineering Plasticity and its Application-AEPA’92, Hong Kong, 15-17 December, Elsevier (ed. W.B. Lee), 1993, 1041. [4] J. Sinczak, Z. Mahnowski, Stand. J. Metall. 16 (1987) 194. [5] S. Szczepanik, J. Sinczak, Metall. Foundry Eng. 4 (1994) 441. [6] S.W. Zehr, W.A. Backofen, Tram ASM 61 (1968) 300.
Materials
Science
Isothermal
and Engineering
A234L236
(1997)
430-433
plastic forming of high-carbon
S. Rusz a,*, J. Sinczak b, W. Lapkowski a Faculty of Mechanical Engineering, Technical Uniwrsity~Ostrava, b Faculty of Metallurgy and Materials Engineering, Akademia Received
31 January
1997; received
steel
b
Yeronymova 425, Frydek-Mislek, Czech Republic G&niczo-Hutnicza, 30-059 Krakow, Poland
in revised
form
8 April
1997
Abstract The work deals with the problem of an unconventional plastic forming process-deformation of high melting temperature alloys in isothermal conditions. The tests were performed for alloyed steel containing O.S!!C. Upsetting the 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 to create the superplasticity effect, the samples were subjected to thermomechanical processing. The main experiment included axisymmetrical closed die forging. Very good filling of the groove and low loads were observed in all tests. In spite of relatively long time of deformation at high temperatures. the material maintained fine microstructure. 0 1997 Elsevier Science S.A. Keywords:
Isothermal
forging;
Superplastic
deformation
1. Introduction Isothermal forging involves hot plastic deformation of metals or alloys at constant temperature and with low strain rates. In order to maintain constant temperature of the sample, the upper tool is heated to the test temperature. Maintaining those conditions allows large plastic deformations to be obtained for a number of materials which are difficult to deform in conventional ways. Beyond this, introduction of proper microstructure prior to deformation (fine grains) leads to the superplastic effect [1,5]. Consequently this process has all the advantages of the superplastic effect, such as possibility of obtaining large deformations with small loads. The superplasticity effect can be easily obtained in the steels,which have austenitic-ferritic structure in the temperatures of deformation. It is difficult to maintain fine microstructure without the presence of the second phase. These conditions are often met by the eutectic and eutectoid steels.The fine grains can be obtained in this steels by special thermomechanical treatment. In hypereutectoid steels the superplastic conditions can be obtained due to complex structure of cementite with the * Corresponding 6916490; e-mail:
author. Tel.: + 42 69 6993315; [email protected]
0921-5093/97/$17.00 Prrso921-5093(97)ool55-X
Q 1997 Elsevier
Science
fax:
S.A. All rights
+ 42 69
reserved
grain size of about 0.1 pm located in the fine ferrite with the grain size of about 1 pm. 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 these steels too. An increase of the carbon contents up to 1.9% increasesthe fraction of cementite to 30% and limits of the grain growth and fosters superplastic effect. The investigation of the superplasticity in the conventional structural steels was also carried out. The present project deals with the 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 produced by the thermomechanical treatment [2]. Then, the sampleswith the diameter of 10 mm and height of 15 mm were compressed in the INSTRON machine with the maximum load capacity of 250 kN. According to the data given in [2], two velocities of the die were used, 1 and 2 mm min - I. The schematic illustration of the testing device is shown in Fig. 1 and the process parameters are given in Table 2. During the test, the temperature inside the sample was monitored with the accuracy of 1°C. The thermocou-
S. Ram et al. /Materials Table 1 Chemical
Science
and Engineering
A234-236
(1997)
430-433
431
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
ples NiCr-Ni embedded in the sample were used in the measurements. The second part of the experiment included axisymmetrical forging in 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 15 mm high and 25 mm in diameter. The schematic illustration of the die proper for forging in the high temperatures is presented in [4]. The continuous monitoring of specimen temperature and the force in the forging was performed.
3. Analysis
of the upsetting tests
The average strain rate in the sample during upsetting was 1.7 x 10W3 s-l for the die velocity 1 mm min-’ and 3.0 x lo- 3 s ~ ’ for the die velocity 2 mm s-i. The die velocity increases slightly during the test. An assumption was made that this increase does not affect the kinetics of deformation and phenomena connected 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 is stabilized after exceeding the strain of 0.15. In the investigated temperature range (720-900°C) the yield stress depends not only on the temperature but also on the strain rate. Calculated strain rate sensitivity parameter m varies between 0.25 and 0.43. Thus, it can be concluded that investigated steel in the considered parameters achieves superplastic properties at studied conditions. This phenomenon was described already, but in general there are several mechanisms of the superplasticity which have been considered. The model [1,6] of the superplastic deformation were developed on the basis of these mechanisms. The model differ by the description of the elementary processes,such as dislocation creep (first stage), slip along the grain boundaries (second stage), combined with the deformation inside the grains thrird stage). Physical interpretations of the developed relationships are based mainly on the dislocation mechanisms having variable character at different stages of the superplastic deformation. Three stagesof 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 the superplastic deformation at the second stage is then connected with the change of the deformation mode from deformation of grains for larger strain rates to the deformation by the slip along the boundaries in slow tests. Mathematical description of the stress-strain relationships in the superplastic conditions are usually given in the form of higher order polynomials or in the form of the rheological model [3]. The strain rates used in the industrial processeswere in particular interest for the Authors. These strain rates corresponds for the third stage of superplastic deformation, which involves mainly dislocation slip inside the grains and marginally the slip along the grain boundaries. The stress-strain curves obtained from the upsetting tests for two strain rates applied and temperatures (Fig. 3) were used in the computer simulation of forging of the shape shown in Fig. 2. Table 2 Process parameters Strain
Fig. 1. The block diagram of the research stand: R,, velocity measurement; R,, force measurement; Z, power supply; R,, sample temperature measurement; R,, furnace temperature control.
1.7 3.0
rate x 10-s
s-’
Temperature
(“C)
720, 750, 780, 860, 900 720, 750, 780, 860, 900
432
S. Rusz et al. /Materials
@56
i-
Science
and Engineering
(1997)
430-433
prevents achieving the superplastic conditions in the whole volume of the forging. Thus, the 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 refined microstructure before the deformation in isothermal condition is to be prepared, in the sameway as for the superplastic deformation. Long deformation time required for the superplastic process fosters the grain growth. Therefore, this time should be kept as short as possible, in particular for the materials which do not contain additions stabilizing the fine microstructure in high temperature. The microstructure of the investigated steel is shown in Fig. 4. The initial microstructure was martensitic with fine precipitates of carbides. After heat treatment involving heating to 800°C and quenching in water. Fig. 4a shows the microstructure prior to deformation and Fig. 4b the microstructure after deformation. It is seenin the figure that the fine grain microstructure was maintained in the forging.
I
Fig. 2. Model forging.
25r a v = 1 mmlmin . _ _ .. . _ . ., . - . . . . .__e_.
A2344236
.. . - .
.----
--.-._..-. -720
--750 . . . . ..7*0
lC
Temponlurs
-.
- .860.
-*--go0
-0
0.05
0.1
0.15
0.2
0.25 Relative
0.3
0.35
0.4
0.45
0.5
0.55
strain
b
v = 2 mmlmin 25
Temperalure
-7720 --750 . . . ...760
lC
- * - - 660 --.-go0 0
I 0
0.05
0.1
0.15
0.2
0.25 Relalive
0.3
0.35
0.4
0.45
0.5
0.55
strain
Fig. 3. Stress-strain curves for the steel for two deformation rates applied: (a) 1 mm min-‘, (b) 2 mm min-‘.
4. Description of results The process of isothermal plastic forming is characterized by large inhomogeneity of strain rates, what
Fig. 4. Structure (martensit with carbides) of the initial material (a) and the forging (b) obtained from the steel of the chemical composition given in Table 2.
S. Rusz et al. /Materials
Science
and Engineering
The maintenance of the fine microstructure during deformation can be obtained by the presence of the second phase. The second phase is effective when its properties in the temperature of superplastic deformation are similar to those of the matrix. The high carbon steel have this feature. The fine grains in this steel can be achieved by the proper thermomechanical treatment. The cementite grains measuring 0.2 nm are located in the ferrite matrix having the grains of 2 urn. The heat treatment applied to the steel containing 0.5%C was the most efficient in the case of the two phase structure CI+ y. Moreover, an introduction of the additives to this steel fosters the stabilization of the fine grain microstructure during deformation.
A234-236
(1997)
430-433
433
which require the super-plastic deformation. Their excellent formability, allowing to extend the range of the methods of plastic forming, is the main advantage of the superplastic alloys. Moreover, it gives very good filling of the groove and allows elimination of the grinding operations, which are required after conventional forging of complex parts. Low force is another advantage of the super-plastic deformations. Application of the optimum deformation conditions (strain rates and temperatures) allows to form the material under low stresses. Thus larger products can be forged on the same machine. References
5. Conclusions
The results of the investigation can be extended on the steels with the similar chemical composition. The tested steel, supple to superplastic conditions, can be used for the manufacturing of the products shaping of
[l] G.J. Gun, Kuzn. &tamp. Proizvodstvo 7 (1994) 9. [2] S. Rusz, Rudy i Metale 11 (1995) 454. [3] Shen Yuli et al., Proc. Asia-Pacific Symposium on Advances in Engineering Plasticity and its Application-AEPA’92, Hong Kong, 15-17 December, Elsevier (ed. W.B. Lee), 1993, 1041. [4] J. Sinczak, Z. Mahnowski, Stand. J. Metall. 16 (1987) 194. [5] S. Szczepanik, J. Sinczak, Metall. Foundry Eng. 4 (1994) 441. [6] S.W. Zehr, W.A. Backofen, Tram ASM 61 (1968) 300.