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Deformation induced martensitic transformation in stainless steels
Materials Science and Engineering A 378 (2004) 308–313
Deformation induced martensitic transformation in stainless steels E. Nagy∗ , V. Mertinger, F. Tranta, J. Sólyom Department of Physical Metallurgy, University of Miskolc, Miskolc-Egyetemváros H3515, Hungary Received 1 July 2003; received in revised form 5 November 2003
Abstract Deformation induced martensitic transformation was investigated in metastable austenitic stainless steel. This steel can present a microstructure of austenite (␥), ␣ martensite and non magnetic ε martensite. Uni-axial tensile test was used for loading at different temperatures below room temperature (from −120 to 20 ◦ C). During the deformation the transformation takes place at certain places in an anisotropic way and texture also develops. Quantitative phase analysis was done by X-ray diffraction (XRD) and magnetic methods while the texture was described by X-ray diffraction using a special inverse pole figure. The quantitative phase analysis has shown that the formation of ␣ and ε martensite from austenite is the function of deformation rate, and deformation temperature. The transformation of the textured austenite takes place in an anisotropic way and a well defined crystallographic relationship between the parent and ␣ martensite phase has been measured. © 2004 Elsevier B.V. All rights reserved. Keywords: TRIP steels; Martensitic transformation; Texture
1. Introduction In the austenitic stainless steels according to the chemical composition and the condition of thermomechanical treatment (deformation rate, temperature), different transformations take place, such as ␥ → ε, ε → ␣ , ␥ → ␣ , ␥ → ε → ␣ . The volume percent of the phases have a great effect on the mechanical (strength, strain) and other behaviours for example corrosion [1]. With the control of volume fraction of martensites a favourable big strain can be obtained even at low deformation temperature which is called transformation induced plasticity (TRIP) effect. It is generally true, that by decreasing the test temperature, the volume fraction of alpha martensite increases, the austenite decreases and epsilon shows a maximum [2,3]. To determine the volume percent of the phases, magnetic method (for alpha ), X-ray (XRD) or neutron diffraction (for alpha , epsilon, austenite,) and quantitative metallography (for alpha , epsilon, austenite,) are used [4,5]. The results obtained from the different methods include their error too. As the X-ray diffraction is suitable for determining all the phases and free from subjective error, it is the most frequently used method, but the most frequent mistake, i.e. the presence of texture is usually not taken into consideration. ∗
Corresponding author. Tel.: +36-46-565-201; fax: +36-46-565-201. E-mail address: [email protected] (E. Nagy).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.11.074
In these materials there is always a texture [6] due to the deformation that continuously changes owing to the thermomechanical treatments and the anisotrope transformation [7,8]. In this paper, experimental results including the phase analysis and texture measurements for the martensitic transformations at a temperature range below room temperature are presented. The aim of this investigation is to describe the transformation process in more detail by considering the deformation and transformation texture.
2. Experiments The thermomechanical treatment of stainless steels was performed by tensile tests over a wide temperature range below the room temperatures in the Laboratory of Department of Mechanical Technology at University of Miskolc using an MTS 458.20. type Microconsor test machine. The composition of the investigated materials is shown in Table 1. In the initial state the material was machined to cylinders with a diameter of 7 mm and a length of 100 mm (without the clamping heads). The cylindrical samples were loaded at 20, 0, −20, −40, −60, −80, −100 and −120 ◦ C temperatures up to the maximum uniform strain. To generate the low temperatures a climate chamber with controlled flow of liquid nitrogen gas was
E. Nagy et al. / Materials Science and Engineering A 378 (2004) 308–313 Table 1 The composition of investigated steel (KO36) Wt. (%) C Si Mn P S Cr Ni Ti Mo
0.04 0.45 1.42 0.042 0.01 18.93 10.77 0.16 0.18
used and the deformation was followed by a piston shift method. Phase analysis was performed by using X-ray diffraction and magnetisation measurements. To describe the texture view of deformed samples a special inverse pole figure was used based on the X-ray diffraction data. This method—described in detail in an earlier paper [7]—is very useful for quantitative comparison of changing during deformation. For X-ray diffraction measurements samples with 5 mm diameter and 10 mm height were cut out from the maximum uniform part of the tensile test probes in three typical directions. They are: L—longitudinal section (parallel to the direction of tensile), C—cross section (perpendicular to the direction of tensile), A—tilted section (at angle of 45◦ ). X-ray measurements were made on a Bruker Advance D8 diffractometer using Co K␣ radiation. During the tests a measuring range between 45 and 130◦ , a stepwidth of 0.1◦ and a collecting time of 20 s was used. To determine the amount of the phases APX63 software was applied in the calculations. Samples for magnetic measurements were prepared cylinders with 3 mm diameter and 3 mm height cut from signed maximum uniform part of the tensile test probes. The magnetic measurements were performed in a Quantum Design SQUID magnetometer (MPMS-5S) at the Insti-
309
tute of Van der Waals–Zeeman, University of Amsterdam. During the measurements an applied magnetic field was used to induce a magnetisation in the sample up to saturation magnetisation. This method has been described in detail in an earlier paper by Zhao et al. [4]. 3. Results 3.1. Tensile test The tensile test curves of the samples which were loaded up to the maximum uniform strain at eight different temperatures are shown in Fig. 1. It can be seen that by decreasing the test temperature, the elongation was strongly increased while the hardening was slightly increased (20 to −20 ◦ C). On the other hand below −40 ◦ C test temperature, the increase in strength is accompanied by a decrease in elongation. 3.2. Quantitative phase analysis Figs. 2–4 shows the results of quantitative phase analysis using X-ray diffraction. The results of the three different (longitudinal, cross and tilted) sections which are represented in separate curves show quite a great difference. It can be seen from the three figures that the formation of the epsilon and alpha martensites take place at the same time and by decreasing the test temperature, the amount of alpha martensite strongly increases on each section while the epsilon martensite only slightly increases except at −100 and −120 ◦ C temperatures on the longitudinal surface. Due to lack of pure alpha martensite standard, it is impossible to count the volume ratio from the results of the magnetisation measurements, but the value of saturation magnetisation is proportional to the amount of ferromagnetic alpha martensite phase in the non ferromagnetic epsilon and austenite matrix. Therefore the saturation magnetisation values divided by the density of samples have been normalised to the volume percent of ␣ martensite determined on the different sections,
Fig. 1. The true stress–strain curve of steel were loaded at different temperature up to maximum uniform elongation.
E. Nagy et al. / Materials Science and Engineering A 378 (2004) 308–313 100
100
90
90
80
80
Volume percent,%
Volume percent, %
310
70 60
alpha martensite epsilon martensite austenite
50 40 30 20
alpha martensite
60
epsilon martensite
50
austenite 40 30 20
10 0 -140
70
10
-120
-100
-80
-60
-40
-20
0
20
40
0 -140
Temperature, ˚C
-120
-100
-80
-60
-40
-20
0
20
40
Temperature,˚C Fig. 2. The volume fraction of phases on longitudinal section of samples as function of test temperature.
Fig. 4. The volume fraction of phases on tilted section of samples as function of test temperature.
so the tendency of volume-change of ␣ martensite as a function of the formation temperature has been compared on the basis of the magnetisation and X-ray data. In accordance with this the data of longitudinal section are calculated as follows:
100 90
Volume percent, %
80 70 60
alpha martensite epsilon martensite austenite
50 40
VM,T,L = Mρ,T
where VM,T ,L is the volume fraction of ␣ martensite at T temperature on the longitudinal section calculated on the basis of magnetisation, Mρ,−120 is the ratio between the saturation magnetisation of sample was loaded at −120 ◦ C up to the maximum uniform strain and density, Mρ,T are the same as the previous data loaded at any of the T temperatures, VXRD,−120,L is the ␣ martensite volume percent of sample loaded at −120 ◦ C up to the maximal uniform strain determined by XRD on the longitudinal section. Fig. 5 shows the
30 20 10 0 -140
-120
-100
-80
-60
-40
-20
0
20
40
Temperature, ˚C Fig. 3. The volume fraction of phases on cross-section of samples as function of test temperature.
longitudinal section by X ray cross section by X ray tilted section by X ray magnetic normalised to longitudinal section magnetic normalised to cross section magnetic normalised to tilted section
100 90 80
Volume percent,%
VXRD,−120,L Mρ,−120
70 60 50 40 30 20 10 0 -140
-120
-100
-80
-60
-40
-20
0
20
40
Temperature,˚C Fig. 5. The volume fraction of ␣ martensite determined by X-ray diffraction on different section of samples compared to magnetic results.
E. Nagy et al. / Materials Science and Engineering A 378 (2004) 308–313
311
2,00
2,00
1,80
1,80 1,60
1,60 1,40
1,00 0,80 -120 -100 -80 -60 -40 -20 0
0,60
Texture number
1,20
Texture number
1,40 1,20
1,00
0,80
0,60
0,40 0
0,40
0,20
-40 0,20 -80
0,00
initial {111}
{200}
{220}
{311}
{222}
Miller index of austenite Fig. 6. Texture numbers of austenite were calculated on the cross-section as function of test temperature.
comparison curves and it can be stated that the tendency of the two curves determined by two different methods are very similar both on the longitudinal and tilted sections but in the case of cross-section the XRD-data show a slower increase in the volume percent.
0,00
-120 {110}
{200}
{211}
{220}
Miller index of martensite
Fig. 8. Texture numbers of ␣ martensite were calculated on the cross-section as function of test temperature.
3.3. Texture analysis Because of the low volume of ε martensite, only two reflections could usually be measured so the texture analysis was done only for the austenite and ␣ martensite phases. The texture number (Thkl ) of austenite and ␣ martensite calculated on longitudinal and cross section as function of test temperature are shown in Figs. 6–9. The result on the tilted section shows an intermediate volume so they are not shown in our paper. The most important results can be summarised as follows: The initial sample shows an original texture. The texture view is independent of test temperature; these are the strongest T200 and weak T111 , T222 reflections for austenite and the strongest T200 and weak T110 , T220 for martensite on the longitudinal section while on the contrary, on the cross-section T200 of martensite and T220 of austenite is zero. The very strong T200 of martensite at room temperature is caused by the error of calculation, as only one reflection could be measured. The texture number for a given crystallographic plain is very similar as function of temperature on the cross-section. But on the longitudinal section the T200 of martensite slightly decreases by decreasing the temperature while the T220 of austenite shows a minimum value at −80 ◦ C. 4. Discussion
Fig. 7. Texture numbers of austenite were calculated on the longitudinal section as function of test temperature.
On the basis of the texture data it can be stated that the preferred orientation developing under the influence of ␣
312
E. Nagy et al. / Materials Science and Engineering A 378 (2004) 308–313
10,00 9,00 8,00
6,00 5,00 4,00
Texture number
7,00
formation, and the T220 values of austenite start increasing again. It can also be stated that by increasing the quantity of ␣ martensite phase its texture number decreases, and it indicates, that the transformation processes and not the deformation processes dominate in the texture of martensite. By comparing the transformations with the tensile curves it can be stated that the higher elongation of samples having a temperature of 0 ◦ C is caused by the appearance of ε and ␣ phases, and the hardening following it is caused by the intensive increase of martensite.
3,00
5. Summary
2,00
The deformation induced martensitic transformation was investigated in metastable austenitic stainless steels by X-ray diffraction and magnetic method. Uni-axial tensile test was used for loading at different temperatures between room temperature and −120 ◦ C. The results can be summarised as:
0 -40 1,00
-80 -120
0,00 {110}
{200}
{211}
{220}
Miller index of martensite Fig. 9. Texture numbers of ␣ martensite were calculated on the longitudinal section as function of test temperature.
martensite deformation is, when the plane diagonal of the elementary cells turn towards the direction of deformation. In this case the number of {2 0 0} planes increase on the longitudinal section and the number of {1 1 0} planes increase on the cross section. Contrary to it the texture of austenite deformation means the increase of texture number of {2 2 0} planes on longitudinal section and zero value on cross section. By investigating both phases at the same time it means that {1 1 0}␣ is parallel to {1 1 1}␥ on the cross section and {2 0 0}␣ is parallel to {2 2 0}␥ on the longitudinal section. This relationship can well be explained as the {1 1 0} plane is perpendicular to the {2 0 0} plane and the {1 1 1} plane is perpendicular to the {2 2 0} plane in the cubical lattice system, so the increase of the number of any of the planes in the longitudinal section is accompanied by increasing the number of the corresponding perpendicular ones in the cross-section. This measurement result is in accordance with the Kurdjumov–Sachs relationship, as well as with the results obtained earlier by Schumann [9]. It can be supposed, that the ␥ → ␣ transformation starts along the austenite {2 2 0} planes to its detriment. This can well be seen with the changes of the T220 of austenite measurement numbers by decreasing the temperature. There is less and less possibility for the formation of the preferred orientation of austenite by increasing the quantity of martensite, so the texture numbers decrease up to −80 ◦ C. The ␥ → ε transformation takes place parallel with this process. The ε → ␣ transformation process starts below −80 ◦ C, and it is shown by the fact that the ε phase quantity data decrease by increasing the extent of
• The ␥ → ␣ , ␥ → ε transfomations take place parallel at higher temperature range (above −80 ◦ C) while ε → ␣ transformation process starts below −80 ◦ C in this steel. • Due to the thermomechanical treatment a well defined texture view is formed in both austenite and ␣ martensite phases and it is independent on the test temperature. • The ␥ → ␣ transformation starts along the austenite {2 2 0} planes to its detriment, the Kurdjumov–Sachs relationship is valid. • The transformation processes and not the deformation processes dominate in the change of austenite texture view so the ␥ → ␣ , ε → ␣ transformations can be described by texture measurements too.
Acknowledgements The authors acknowledge the Hungarian research foundation FKFP (0096/2001) and Erzsebet Nagy would like to thank Dr. Lie Zhao, Department of Material Science and Technology, Delft University of Technology, The Netherlands for his assistance with the magnetisation measurements and Dr. Valeria Mertinger would like to acknowledge Bolyai Research Fellowship for their help.
References [1] C. Eckstein, A. Weiß, H.J. Eckstein, Mater. Corrosion 46 (1995) 76– 82. [2] A. Weiß, X. Fang, H.J. Eckstein, C. Eckstein, W. Dahl, Steel Res. 66 (11) (1995) 495–500. [3] P.L. Mangonon Jr., G. Thomas, Metall. Trans. 1 (1970) 1587– 1594.
E. Nagy et al. / Materials Science and Engineering A 378 (2004) 308–313 [4] L. Zhao, N.H. van Dijk, E. Brück, J. Sietsma, S. van der Zwaag, Mater. Sci. Eng. A 313 (2001) 145–152. [5] E. Nagy, V. Mertinger, F. Tranta, J. Sólyom, Strain Induced Martensitic Transformation in Austenitic Steel, Hungary, Miskolc MicroCAD 2000, International Scientific Conference, 2000. [6] M.R. Raymond, C.N. Tomé, M.A.M. Bourke, Acta Mater. 48 (2000) 553–564.
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[7] E. Nagy, V. Mertinger, F. Tranta, J. Sólyom, Mater. Sci. Forum 414–415 (2003) 281–288. [8] S. Kruijver, L. Zhao, J. Sietsma, E. Offerman, N. van Dijk, L. Margulies, E. Lauridsen, S. Grigull, H. Poulsen, Z. van der, Steel Res. 73 (6–7) (2002) 236–241. [9] H. Schumann, Arhiv. für. das. Eisenhüttenwesen, December 1969, pp. 1027–1037.
Deformation induced martensitic transformation in stainless steels E. Nagy∗ , V. Mertinger, F. Tranta, J. Sólyom Department of Physical Metallurgy, University of Miskolc, Miskolc-Egyetemváros H3515, Hungary Received 1 July 2003; received in revised form 5 November 2003
Abstract Deformation induced martensitic transformation was investigated in metastable austenitic stainless steel. This steel can present a microstructure of austenite (␥), ␣ martensite and non magnetic ε martensite. Uni-axial tensile test was used for loading at different temperatures below room temperature (from −120 to 20 ◦ C). During the deformation the transformation takes place at certain places in an anisotropic way and texture also develops. Quantitative phase analysis was done by X-ray diffraction (XRD) and magnetic methods while the texture was described by X-ray diffraction using a special inverse pole figure. The quantitative phase analysis has shown that the formation of ␣ and ε martensite from austenite is the function of deformation rate, and deformation temperature. The transformation of the textured austenite takes place in an anisotropic way and a well defined crystallographic relationship between the parent and ␣ martensite phase has been measured. © 2004 Elsevier B.V. All rights reserved. Keywords: TRIP steels; Martensitic transformation; Texture
1. Introduction In the austenitic stainless steels according to the chemical composition and the condition of thermomechanical treatment (deformation rate, temperature), different transformations take place, such as ␥ → ε, ε → ␣ , ␥ → ␣ , ␥ → ε → ␣ . The volume percent of the phases have a great effect on the mechanical (strength, strain) and other behaviours for example corrosion [1]. With the control of volume fraction of martensites a favourable big strain can be obtained even at low deformation temperature which is called transformation induced plasticity (TRIP) effect. It is generally true, that by decreasing the test temperature, the volume fraction of alpha martensite increases, the austenite decreases and epsilon shows a maximum [2,3]. To determine the volume percent of the phases, magnetic method (for alpha ), X-ray (XRD) or neutron diffraction (for alpha , epsilon, austenite,) and quantitative metallography (for alpha , epsilon, austenite,) are used [4,5]. The results obtained from the different methods include their error too. As the X-ray diffraction is suitable for determining all the phases and free from subjective error, it is the most frequently used method, but the most frequent mistake, i.e. the presence of texture is usually not taken into consideration. ∗
Corresponding author. Tel.: +36-46-565-201; fax: +36-46-565-201. E-mail address: [email protected] (E. Nagy).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.11.074
In these materials there is always a texture [6] due to the deformation that continuously changes owing to the thermomechanical treatments and the anisotrope transformation [7,8]. In this paper, experimental results including the phase analysis and texture measurements for the martensitic transformations at a temperature range below room temperature are presented. The aim of this investigation is to describe the transformation process in more detail by considering the deformation and transformation texture.
2. Experiments The thermomechanical treatment of stainless steels was performed by tensile tests over a wide temperature range below the room temperatures in the Laboratory of Department of Mechanical Technology at University of Miskolc using an MTS 458.20. type Microconsor test machine. The composition of the investigated materials is shown in Table 1. In the initial state the material was machined to cylinders with a diameter of 7 mm and a length of 100 mm (without the clamping heads). The cylindrical samples were loaded at 20, 0, −20, −40, −60, −80, −100 and −120 ◦ C temperatures up to the maximum uniform strain. To generate the low temperatures a climate chamber with controlled flow of liquid nitrogen gas was
E. Nagy et al. / Materials Science and Engineering A 378 (2004) 308–313 Table 1 The composition of investigated steel (KO36) Wt. (%) C Si Mn P S Cr Ni Ti Mo
0.04 0.45 1.42 0.042 0.01 18.93 10.77 0.16 0.18
used and the deformation was followed by a piston shift method. Phase analysis was performed by using X-ray diffraction and magnetisation measurements. To describe the texture view of deformed samples a special inverse pole figure was used based on the X-ray diffraction data. This method—described in detail in an earlier paper [7]—is very useful for quantitative comparison of changing during deformation. For X-ray diffraction measurements samples with 5 mm diameter and 10 mm height were cut out from the maximum uniform part of the tensile test probes in three typical directions. They are: L—longitudinal section (parallel to the direction of tensile), C—cross section (perpendicular to the direction of tensile), A—tilted section (at angle of 45◦ ). X-ray measurements were made on a Bruker Advance D8 diffractometer using Co K␣ radiation. During the tests a measuring range between 45 and 130◦ , a stepwidth of 0.1◦ and a collecting time of 20 s was used. To determine the amount of the phases APX63 software was applied in the calculations. Samples for magnetic measurements were prepared cylinders with 3 mm diameter and 3 mm height cut from signed maximum uniform part of the tensile test probes. The magnetic measurements were performed in a Quantum Design SQUID magnetometer (MPMS-5S) at the Insti-
309
tute of Van der Waals–Zeeman, University of Amsterdam. During the measurements an applied magnetic field was used to induce a magnetisation in the sample up to saturation magnetisation. This method has been described in detail in an earlier paper by Zhao et al. [4]. 3. Results 3.1. Tensile test The tensile test curves of the samples which were loaded up to the maximum uniform strain at eight different temperatures are shown in Fig. 1. It can be seen that by decreasing the test temperature, the elongation was strongly increased while the hardening was slightly increased (20 to −20 ◦ C). On the other hand below −40 ◦ C test temperature, the increase in strength is accompanied by a decrease in elongation. 3.2. Quantitative phase analysis Figs. 2–4 shows the results of quantitative phase analysis using X-ray diffraction. The results of the three different (longitudinal, cross and tilted) sections which are represented in separate curves show quite a great difference. It can be seen from the three figures that the formation of the epsilon and alpha martensites take place at the same time and by decreasing the test temperature, the amount of alpha martensite strongly increases on each section while the epsilon martensite only slightly increases except at −100 and −120 ◦ C temperatures on the longitudinal surface. Due to lack of pure alpha martensite standard, it is impossible to count the volume ratio from the results of the magnetisation measurements, but the value of saturation magnetisation is proportional to the amount of ferromagnetic alpha martensite phase in the non ferromagnetic epsilon and austenite matrix. Therefore the saturation magnetisation values divided by the density of samples have been normalised to the volume percent of ␣ martensite determined on the different sections,
Fig. 1. The true stress–strain curve of steel were loaded at different temperature up to maximum uniform elongation.
E. Nagy et al. / Materials Science and Engineering A 378 (2004) 308–313 100
100
90
90
80
80
Volume percent,%
Volume percent, %
310
70 60
alpha martensite epsilon martensite austenite
50 40 30 20
alpha martensite
60
epsilon martensite
50
austenite 40 30 20
10 0 -140
70
10
-120
-100
-80
-60
-40
-20
0
20
40
0 -140
Temperature, ˚C
-120
-100
-80
-60
-40
-20
0
20
40
Temperature,˚C Fig. 2. The volume fraction of phases on longitudinal section of samples as function of test temperature.
Fig. 4. The volume fraction of phases on tilted section of samples as function of test temperature.
so the tendency of volume-change of ␣ martensite as a function of the formation temperature has been compared on the basis of the magnetisation and X-ray data. In accordance with this the data of longitudinal section are calculated as follows:
100 90
Volume percent, %
80 70 60
alpha martensite epsilon martensite austenite
50 40
VM,T,L = Mρ,T
where VM,T ,L is the volume fraction of ␣ martensite at T temperature on the longitudinal section calculated on the basis of magnetisation, Mρ,−120 is the ratio between the saturation magnetisation of sample was loaded at −120 ◦ C up to the maximum uniform strain and density, Mρ,T are the same as the previous data loaded at any of the T temperatures, VXRD,−120,L is the ␣ martensite volume percent of sample loaded at −120 ◦ C up to the maximal uniform strain determined by XRD on the longitudinal section. Fig. 5 shows the
30 20 10 0 -140
-120
-100
-80
-60
-40
-20
0
20
40
Temperature, ˚C Fig. 3. The volume fraction of phases on cross-section of samples as function of test temperature.
longitudinal section by X ray cross section by X ray tilted section by X ray magnetic normalised to longitudinal section magnetic normalised to cross section magnetic normalised to tilted section
100 90 80
Volume percent,%
VXRD,−120,L Mρ,−120
70 60 50 40 30 20 10 0 -140
-120
-100
-80
-60
-40
-20
0
20
40
Temperature,˚C Fig. 5. The volume fraction of ␣ martensite determined by X-ray diffraction on different section of samples compared to magnetic results.
E. Nagy et al. / Materials Science and Engineering A 378 (2004) 308–313
311
2,00
2,00
1,80
1,80 1,60
1,60 1,40
1,00 0,80 -120 -100 -80 -60 -40 -20 0
0,60
Texture number
1,20
Texture number
1,40 1,20
1,00
0,80
0,60
0,40 0
0,40
0,20
-40 0,20 -80
0,00
initial {111}
{200}
{220}
{311}
{222}
Miller index of austenite Fig. 6. Texture numbers of austenite were calculated on the cross-section as function of test temperature.
comparison curves and it can be stated that the tendency of the two curves determined by two different methods are very similar both on the longitudinal and tilted sections but in the case of cross-section the XRD-data show a slower increase in the volume percent.
0,00
-120 {110}
{200}
{211}
{220}
Miller index of martensite
Fig. 8. Texture numbers of ␣ martensite were calculated on the cross-section as function of test temperature.
3.3. Texture analysis Because of the low volume of ε martensite, only two reflections could usually be measured so the texture analysis was done only for the austenite and ␣ martensite phases. The texture number (Thkl ) of austenite and ␣ martensite calculated on longitudinal and cross section as function of test temperature are shown in Figs. 6–9. The result on the tilted section shows an intermediate volume so they are not shown in our paper. The most important results can be summarised as follows: The initial sample shows an original texture. The texture view is independent of test temperature; these are the strongest T200 and weak T111 , T222 reflections for austenite and the strongest T200 and weak T110 , T220 for martensite on the longitudinal section while on the contrary, on the cross-section T200 of martensite and T220 of austenite is zero. The very strong T200 of martensite at room temperature is caused by the error of calculation, as only one reflection could be measured. The texture number for a given crystallographic plain is very similar as function of temperature on the cross-section. But on the longitudinal section the T200 of martensite slightly decreases by decreasing the temperature while the T220 of austenite shows a minimum value at −80 ◦ C. 4. Discussion
Fig. 7. Texture numbers of austenite were calculated on the longitudinal section as function of test temperature.
On the basis of the texture data it can be stated that the preferred orientation developing under the influence of ␣
312
E. Nagy et al. / Materials Science and Engineering A 378 (2004) 308–313
10,00 9,00 8,00
6,00 5,00 4,00
Texture number
7,00
formation, and the T220 values of austenite start increasing again. It can also be stated that by increasing the quantity of ␣ martensite phase its texture number decreases, and it indicates, that the transformation processes and not the deformation processes dominate in the texture of martensite. By comparing the transformations with the tensile curves it can be stated that the higher elongation of samples having a temperature of 0 ◦ C is caused by the appearance of ε and ␣ phases, and the hardening following it is caused by the intensive increase of martensite.
3,00
5. Summary
2,00
The deformation induced martensitic transformation was investigated in metastable austenitic stainless steels by X-ray diffraction and magnetic method. Uni-axial tensile test was used for loading at different temperatures between room temperature and −120 ◦ C. The results can be summarised as:
0 -40 1,00
-80 -120
0,00 {110}
{200}
{211}
{220}
Miller index of martensite Fig. 9. Texture numbers of ␣ martensite were calculated on the longitudinal section as function of test temperature.
martensite deformation is, when the plane diagonal of the elementary cells turn towards the direction of deformation. In this case the number of {2 0 0} planes increase on the longitudinal section and the number of {1 1 0} planes increase on the cross section. Contrary to it the texture of austenite deformation means the increase of texture number of {2 2 0} planes on longitudinal section and zero value on cross section. By investigating both phases at the same time it means that {1 1 0}␣ is parallel to {1 1 1}␥ on the cross section and {2 0 0}␣ is parallel to {2 2 0}␥ on the longitudinal section. This relationship can well be explained as the {1 1 0} plane is perpendicular to the {2 0 0} plane and the {1 1 1} plane is perpendicular to the {2 2 0} plane in the cubical lattice system, so the increase of the number of any of the planes in the longitudinal section is accompanied by increasing the number of the corresponding perpendicular ones in the cross-section. This measurement result is in accordance with the Kurdjumov–Sachs relationship, as well as with the results obtained earlier by Schumann [9]. It can be supposed, that the ␥ → ␣ transformation starts along the austenite {2 2 0} planes to its detriment. This can well be seen with the changes of the T220 of austenite measurement numbers by decreasing the temperature. There is less and less possibility for the formation of the preferred orientation of austenite by increasing the quantity of martensite, so the texture numbers decrease up to −80 ◦ C. The ␥ → ε transformation takes place parallel with this process. The ε → ␣ transformation process starts below −80 ◦ C, and it is shown by the fact that the ε phase quantity data decrease by increasing the extent of
• The ␥ → ␣ , ␥ → ε transfomations take place parallel at higher temperature range (above −80 ◦ C) while ε → ␣ transformation process starts below −80 ◦ C in this steel. • Due to the thermomechanical treatment a well defined texture view is formed in both austenite and ␣ martensite phases and it is independent on the test temperature. • The ␥ → ␣ transformation starts along the austenite {2 2 0} planes to its detriment, the Kurdjumov–Sachs relationship is valid. • The transformation processes and not the deformation processes dominate in the change of austenite texture view so the ␥ → ␣ , ε → ␣ transformations can be described by texture measurements too.
Acknowledgements The authors acknowledge the Hungarian research foundation FKFP (0096/2001) and Erzsebet Nagy would like to thank Dr. Lie Zhao, Department of Material Science and Technology, Delft University of Technology, The Netherlands for his assistance with the magnetisation measurements and Dr. Valeria Mertinger would like to acknowledge Bolyai Research Fellowship for their help.
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