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Thermal mechanical behavior and microstructure characteristic of microalloyed CrMo steel under cross deformation
Materials Science and Engineering A 527 (2010) 4702–4707
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Thermal mechanical behavior and microstructure characteristic of microalloyed CrMo steel under cross deformation Qingchao Tian ∗ , Xiaoming Dong, Jie Hong Baosteel Research Institute, Tube & Pipe Department, Shanghai 201900, China
a r t i c l e
i n f o
Article history: Received 7 July 2009 Received in revised form 23 March 2010 Accepted 2 April 2010
Keywords: Pipe making Cross deformation Equivalent strain Texture Collapse resistance
a b s t r a c t Pipes with a diameter of 177.8 mm were prepared by the cross deformation of an Nb, V and Ti microalloyed CrMo steel. Equivalent strains of pipe making process and their effect on microstructure evolution were investigated. A thermal mechanical simulator was employed to simulate the conditions of the pipe making at different elevated temperatures. Microstructure was studied using a transmission electron microscope and an X-ray diffractometer. It was found that the (1 1 1) fiber formed after the pipe making has been inherited in the steel pipes after heat treatment, and austenite recrystallization of the steel begins at 1223 K, in good accordance with the predicted recrystallization start temperature. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Oil country tube goods (OCTG) have been extensively applied to the exploitation of oil and gas. Casing is a kind of pipe goods used as a lining for an oil or gas well. In the case of the existence of saline rock, shale bed or gypsum-salt rock, where the geological conditions are very severe and complicated, and the external stress is abnormally high, the pipe used may fail in the radial direction, causing catastrophic economic loss. Therefore, super-high collapse resistance casing has been developed for using in the high-pressure or complicated geological stratum functioning as an effective resistance to external collapse. A super-high collapse resistance casing has been developed with collapse strength 60% higher than that specified by API 5C2 standard [1]. It has been proposed that both the existence of (1 1 1) fiber and the characteristic of precipitates distribution are responsible for the high collapse resistance. It is mentioned that the cross deformation process, of which deformation strain is much higher in comparison with that of longitudinal deformation, play a key role to the formation of that sort of microstructure. Large strain rate that characterizes pipe making process significantly influences microstructure as well as mechanical properties. Pipe making process usually contains piercing, rolling, reeling and sizing. During this process, dynamic restoration of deformed austenite occurs mainly due to two mechanisms: dynamic recovery
∗ Corresponding author. Tel.: +86 21 26649444. E-mail address: [email protected] (Q. Tian). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.04.007
and dynamic recrystallization (DRX). There were some studies on the dynamic restoration process of metals and alloys during plastic deformation under moderate strain rates less than 5/s [2–7]. However, how the pipe making process affects the microstructure formation is not clear. In this study, the deformation strains as well as the microstructure evolution corresponding to the pipe making process were analyzed, and the high temperature deformation process was simulated using a thermal mechanical simulator. Generally, the material for high collapse resistance pipe is CrMo steel. Elements such as Nb, V and Ti alloyed in steel assure enough precipitation of carbon nitride to pin grain boundary and to refine grain size at high temperatures. In the research, a Ti, Nb and V microallyed CrMo steel was designed to achieve super-high collapse resistance by making use of their strengthening effect, and during the pipe making process, cross deformation with large strain was adopted to obtain ideal texture components. 2. Experimental procedure The composition of the steel billet is shown in Table 1. The pipes were prepared by the following process: piercing (cross deformation)–rolling (cross deformation)–reeling (longitudinal deformation)–sizing (longitudinal deformation). The prepared pipes were then further quenched with the optimized austenization parameters using Box–Behnken design, and tempered at 963 K to attain the required strength [8], and thus super-high collapse resistance casing was prepared after sizing and straightening at temperatures higher than 773 K.
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Table 1 Chemical composition of the designed steel (wt.%). C
Cr
Mo
Ti + Nb + V
0.28
0.5
0.4
0.2
The specimens for grain size observation were firstly polished, and then etched with picric acid at temperatures from 323 K to 353 K. Prior austenite grains were observed using an optical metallographic microscope equipped with Mias grain evaluation software. In order to get the comparable measurement results, the texture of the outside surface of the pipe specimens before or after heat treatment was measured using a Rigaku Rint-2200/PC diffractometer. A JEM-2010 transmission electron microscopy (TEM) was used to observe the precipitates distribution. TEM specimens were first mechanically polished, and then prepared by a twin-jet electro-polishing apparatus. For better understanding the thermal mechanical process of pipe making, specimens with the dimension of 8 × 12 (mm × mm) were cut from the steel billet for hot compression to simulate high temperature deformation using a Gleeble 3800 thermal mechanical simulator. Specimens were firstly heated to 1523 K with a heating rate of 10 K/s, holding for 2 min, and then the temperature was lowered to varied temperature levels with a holding time of 1.5 min, as shown in Fig. 1. After the specimens were deformed at a large strain rate of 5/s, they were quenched to room temperature with a cooling rate of about 30 K/s. 3. Results and discussion 3.1. Mechanism of high-temperature deformation It is known that work hardening effect prevails at the initial stage of deformation at elevated temperatures, and sharply increases the flow stress. With the increase of deformation strain, work hardening and dynamic softening occur simultaneously. The dynamic softening effect is originated from DRX or dynamic recovery process. Generally under high strain rates, when deformation reaches a certain degree, dynamic softening occurs and compensates for the increase of flow stress contributed by hardening effect, thereafter hardening and softening reach a balance, as a result, the flow stress becomes stable after experiencing a maximum peak. While deforming under low strain rates, work hardening cannot keep up with softening process, so that flow stress shows a multi-peak characteristic. Fig. 2 shows the true stress–true strain curves of specimens at different elevated temperatures with a strain rate of 5/s. It can be seen that for the specific material, flow stresses exhibit multi-peak
Fig. 1. Schematic diagram of hot compression test: (1) specimens reheated to 1523 K at a rate of 10 K/s; (2) held for 5 min at 1283 K; (3) cooled to the deformation temperature at a rate of 5 K/s; (4) deformed at constant temperatures (1473 K, 1423 K, 1373 K, 1323 K, 1273 K, 1223 K, 1173 K, 1123 K, and 1073 K); (5) quenched rapidly after deformation at a cooling rate of about 30 K/s.
Fig. 2. Flow stress during hot deformation at elevated temperatures.
rather than a stable stress. However, for the deformation of 35CrMo steel under such a high strain rate, only one peak exists in the stress–strain curve [9]. The phenomenon was not only owed to the competing result of hardening and softening, but also contributed by the formation effect of precipitates discussed below. It can be seen from Fig. 2 that the flow stress increases with the decrease of deformation temperature, T. The maximum peak flow stresses are 87 MPa and 258 MPa when the temperatures are 1473 K and 1073 K, respectively, as shown in Fig. 3. Fig. 3 shows the maximum peak flow stress dependence of deformation temperature, indicating obviously two different variation tendencies. The two fitting lines, in fact, respectively represent the DRX process and the dynamic recovery process. The cross point means the beginning of DRX. At the present case, the austenite recrystallization is expected to begin at about 1223 K. 3.2. Thermal mechanical process at high temperatures Deformation resistance can be indicated by the peak flow stress during hot deformation. Zener and Hollomon [10] found that the stress–strain relation depended on the deformation temperature ˙ and can be denoted by a parameter, Z = T and the strain rate ε, n , where Z is the Zener–Hollomon parameter, R ε˙ exp(Q/RT ) = Am is the universal gas constant (R = 8.314 J/(mol K)), Q is the activation energy, for 35CrMo steel, whose value for DRX is 378.2 kJ/mol [9], is used for an estimation in this research, m is the maximum peak flow stress for the specific situation, and A and n are constant. The ln Z and ln m relationship is represented in Fig. 4, and it can be seen ln Z linearly increases with the increase of ln m . The deformation resistance during DRX can then be represented by
Fig. 3. Maximum peak flow stress dependence of deformation temperature.
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prior austenite grain boundaries in the specimen at temperature of 1223 K (Fig. 5b), in good accordance with the predicted recrystallization start temperature. Fine recrystallized austenite grains are found distributing along the deformed austenite grain boundaries at temperature of 1273 K (Fig. 5c). 3.3. Deformation strains of the pipe making process
Fig. 4. The ln Z dependence of ln m .
Zener–Hollomon parameter as follows: m = 1.46 Z 0.13
(1)
The microstructure of deformed specimen varies from the surface to the center due to varied distribution of deformation strains. For sound comparison, only the microstructure in the center part of the specimen, where the strain is relatively higher, is shown in Fig. 5. When deformation temperature is low, only dynamic recovery occurs and the elongated austenite grains are preserved as shown in Fig. 5a, while when temperature is high, say 1473 K, DRX occurs, and the deformed austenite grains recover to polycrystal grains (Fig. 5d). Besides the characteristic of dynamic recovery morphology, recrystallization nuclei can be identified along the
During longitudinal deformation process, longitudinal, radial, circumferential and overall equivalent strains are calculated by the following equations: ε1 = ln ((D0 − SZ ), ε2 = ln SZ /S0 , ε3 = ln DZ − SZ /D0 − S0 , and S0 )S0 )/((DZ − √ εeq = ( 2/3) (ε1 − ε2 )2 + (ε2 − ε3 )2 + (ε1 − ε3 )2 , where D0 , S0 , DZ and SZ are the outer diameter (D) and the wall thickness (S) before (subscript 0) and after (subscript Z) deformation, respectively [11]. Considering the additional strain introduced by cross deformation, the following experimental equations have been given [11]: As for cross piecing, εTeq = −2.69 + 2.59, and for cross rolling: εTeq = −1.83 + 2.06, where is the stretching ratio between the lengths before and after deformation. Taking the preparation of the API specification of 177.8 × 9.19 pipe for example, large deformation strains were assigned to cross piercing and cross rolling at high temperatures with a strain rate of about 4–6/s. The main parameters for the pipe making process are listed in Table 2. The equivalent strains for cross piercing, cross rolling, longitudinal reeling and longitudinal sizing are calculated to be 5.96, 0.89, 0.07 and 0.09, respectively. The equivalent strains for cross piercing (5.96) or cross rolling (0.89) is about 3 times that for longitudinal piercing (1.94) or rolling (0.33). It is reasonable that the pipes under cross deformation exhibit obviously different textures from that under longitudinal rolling [12].
Fig. 5. The deformed microstructures of specimens at (a) 1123 K, (b) 1223 K, (c) 1273 K and (d) 1473 K.
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Table 2 Equivalent strains calculated for the making process of a 177.8 × 9.19 pipe. T, K Cross piercing Cross rolling Longitudinal reeling Longitudinal sizing a
1513 1373 1223 1143
D0 , mm 178 195 192 192
S0 , mm 89 13 9.8 9.19
Dz , mm 195 192 192 177.8
Sz , mm 13 9.8 9.19 9.19
3.34 1.32 1.06 1.08
εeq
εTeq a
1.94 0.33a 0.07 0.09
5.96 0.89 – –
The data represents the equivalent strain of longitudinal deformation.
Fig. 6. Prior austenite grains under hot rolling state.
3.4. Microstructure characteristics during pipe making According to the calculation results of Table 2 and the above discussions, complete DRX occurs after cross piercing of billet (1513 K, εTeq = 5.96), and after cross rolling (1373 K, εTeq = 0.89) under the strain rate of 5/s. During the longitudinal reeling and sizing process at strains of less than 0.1 as indicated by the dot line in Fig. 2, the pipes only undergo work hardening process. The work-hardened microstructure should be kept if the pipes were quenched to room
Fig. 7. Precipitates of as-rolled specimen showing the (Cr, Fe)23 C6 , Mo2 C, and (Ti, Nb, V)C precipitate types.
temperature immediately. However, the pipes were air cooled in the industrial production process, therefore, there was enough time for the occurrence of static recrystallization under high temperature austenite state. As shown in Fig. 6, polygonal austenite grains, whose average size is about 20–40 m, have replaced the deformed grains. It is noted that the as-rolled specimen shows bainitic microstructure. Based on TEM observation of as-rolled pipe specimens, it is found that large amount of carbonitrides are formed along the
Fig. 8. Texture distribution in the = 0◦ and = 45◦ sections of Euler space along circumferential direction for pipes under the as-rolled state, showing (1 1 1) 1 1 0, intensity 3.3.
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Fig. 9. Prior austenite grains after heat treatment.
rolling direction, as shown in Fig. 7. It is believed that although deformed austenite grains have recovered to polygonal ones, precipitates distribution reflects the original direction of deformed austenite grains. It has been determined that massive and moderate sizes of (Cr, Fe)23 C6 and Mo2 C particles and fine M(C, N) (M is V or Ti and or Nb) precipitates exist in the investigated steel casing [1]. According to the thermodynamics of M(C, N) (M is V and or Ti and or Nb) formation, the precipitates of TiC, NbN, and NbC should be formed while V should still exist in the solid solution state during the piercing, rolling, reeling and sizing process of pipe making [13]. It is known that work hardening results from the increase of dislocation intensity. In the investigated steel, high intensity dislocation area provides nuclei site not only for recovery and recrystallization softening, but also for the precipitation of M(C, N) particles, and thus breaks the balance of work hardening and dynamic softening process. That is the reason why the fluctuations of flow stress exist in Fig. 2. On the other hand, the precipitates of these elements in the pipe making process can effectively change
Fig. 10. TEM observation of the microstructure of heat-treated specimen showing the (Cr, Fe)23 C6 , Mo2 C, and (Ti, Nb, V)C precipitate types.
the intensity and type of texture through refining austenite grain size, inhibiting the austenite recrystallization, and retarding ␥/␣ phase transformation. Fig. 8 shows the texture components in the = 0◦ and = 45◦ sections of Euler space for pipes under as-rolled state. The overall intensity of texture is not very strong. The main texture component is the (1 1 1) 1 1 0 fiber with an intensity of 3.3 grade. It has been proposed that the (1 1 1) fiber in the casing is beneficial to increase collapse resistance [12], nevertheless, it is interesting to find that the (1 1 1) fiber has already formed in the intermediate process of pipe preparation. 3.5. Microstructure evolution after heat treatment Heating the pipes to 1173 K and adopting proper holding time for austenization can optimize the prior austenite grain size to be about 10 m [8], as shown in Fig. 9. After quenching and tempering,
Fig. 11. Texture distribution in the = 0◦ and = 45◦ sections of Euler space along circumferential direction for pipes under quenching and tempering state, showing (1 1 1) 1 1 0, (1 1 1) 1 1 2, intensity 3.1.
Q. Tian et al. / Materials Science and Engineering A 527 (2010) 4702–4707
the precipitates distribution changes greatly in comparison with that of as-rolled state (Fig. 10). VC precipitates should be substantially formed during tempering process [13]. It has been confirmed that large amount of VC particles with sizes of less than 10 nm exist in the quenched and tempered pipes [1]. During collapse process under external pressure, these precipitates can effectively pin dislocation motion, hinder slip movement, and thus significantly increase collapse resistance. Fig. 11 shows the main texture is the (1 1 1) fiber with the components of (1 1 1) 1 1 0 and (1 1 1) 1 1 2. Obviously, the austenization process does not change the nature of the fiber texture formed in the pipe making process. It is certain that large strains have been assigned to cross piercing and cross rolling process, in which complete DRX occurs. Thus it is inferred that cross deformation is responsible for the formation of (1 1 1) fiber.
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(3) The precipitates distribution of as-rolled pipes still reflects the characteristic of deformed austenite, and after heat treatment, both the austenite grains and the precipitates are refined and the deformed characteristic is changed. (4) It is found that the (1 1 1) fiber is developed during the pipe making process. It is assured that cross deformation is responsible for the formation of (1 1 1) fiber. The followed heat treatment of the as-rolled pipes cannot change the (1 1 1) fiber nature. (5) Complete DRX occurs in the piercing and rolling stages of pipe making process. During the reeling and sizing process, only the work hardening process occurs. And the work-hardened microstructure vanishes owing to the occurrence of static recrystallization at high temperature austenite state. References
4. Conclusions (1) For making the API specification of 177.8 × 9.19 pipe, large deformation strains were assigned to the piercing and rolling process, and the equivalent strains for cross piercing, cross rolling, longitudinal reeling and longitudinal sizing were 5.96, 0.89, 0.07 and 0.09, respectively. (2) Ti, Nb and V microalloying of CrMo steel leads to the existence of multi-peak of flow stress in the stress–strain curve under high strain rate. The austenite recrystallization start temperature is found at 1223 K by microstructure observation, in accordance with the predicted value using the maximum peak flow stresses. The deformation resistance dependence of Zener–Hollomon parameter during DRX is found to be m = 1.46 Z0.13 .
[1] Q. Tian, X. Dong, Baosteel Tech. Res. 2 (2008) 21–25. [2] S. Torizuka, A. Ohmori, S.V.S. Narayana Murty, K. Nagai, Mater. Sci. Forum. 503–504 (2006) 329–334. [3] S.V.S. Narayana Murty, S. Torizuka, K. Nagai, N. Koseki, Y. Kogo, Scripta Mater. 52 (2005) 713–718. [4] H. Yang, Z. Li, Z. Zhang, J. Zhejiang Univ. Sci. A 7 (2006) 1453–1460. [5] S.I. Kim, Y.C. Yoo, Mater. Sci. Eng. A 311 (2001) 108–113. [6] A. Belyakov, H. Miura, T. Sakai, Mater. Sci. Eng. A 255 (1998) 139–147. [7] N. Fujita, R. Sahara, T. Narushima, C. Ouchi, ISIJ Int. 48 (2008) 1419–1428. [8] X. Dong, Q. Tian, Proceedings of The Third Baosteel Biennial Academic Conference, Shanghai, 2008, pp. 94–99. [9] J. Ye, Z. Xu, Steel Rolling 21 (2004) 23–27. [10] I. Tamura, H. Sekine, T. Tanaka, C. Ouchi, Thermomechanical Processing of Highstrength Low-alloying Steels, Butterworth, London, 1988. [11] J. Zhu, C. Fu, Y. Liu, Steel Pipe 29 (2002) 17–22. [12] Q. Tian, X. Dong, MS&T’08 Materials Science & Technology 2008, Pittsburgh, PA, October 5–9, 2008, pp. 2010–2016. [13] J. Strid, K.E. Easterling, Acta Metall. 33 (1985) 2057–2074.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Thermal mechanical behavior and microstructure characteristic of microalloyed CrMo steel under cross deformation Qingchao Tian ∗ , Xiaoming Dong, Jie Hong Baosteel Research Institute, Tube & Pipe Department, Shanghai 201900, China
a r t i c l e
i n f o
Article history: Received 7 July 2009 Received in revised form 23 March 2010 Accepted 2 April 2010
Keywords: Pipe making Cross deformation Equivalent strain Texture Collapse resistance
a b s t r a c t Pipes with a diameter of 177.8 mm were prepared by the cross deformation of an Nb, V and Ti microalloyed CrMo steel. Equivalent strains of pipe making process and their effect on microstructure evolution were investigated. A thermal mechanical simulator was employed to simulate the conditions of the pipe making at different elevated temperatures. Microstructure was studied using a transmission electron microscope and an X-ray diffractometer. It was found that the (1 1 1) fiber formed after the pipe making has been inherited in the steel pipes after heat treatment, and austenite recrystallization of the steel begins at 1223 K, in good accordance with the predicted recrystallization start temperature. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Oil country tube goods (OCTG) have been extensively applied to the exploitation of oil and gas. Casing is a kind of pipe goods used as a lining for an oil or gas well. In the case of the existence of saline rock, shale bed or gypsum-salt rock, where the geological conditions are very severe and complicated, and the external stress is abnormally high, the pipe used may fail in the radial direction, causing catastrophic economic loss. Therefore, super-high collapse resistance casing has been developed for using in the high-pressure or complicated geological stratum functioning as an effective resistance to external collapse. A super-high collapse resistance casing has been developed with collapse strength 60% higher than that specified by API 5C2 standard [1]. It has been proposed that both the existence of (1 1 1) fiber and the characteristic of precipitates distribution are responsible for the high collapse resistance. It is mentioned that the cross deformation process, of which deformation strain is much higher in comparison with that of longitudinal deformation, play a key role to the formation of that sort of microstructure. Large strain rate that characterizes pipe making process significantly influences microstructure as well as mechanical properties. Pipe making process usually contains piercing, rolling, reeling and sizing. During this process, dynamic restoration of deformed austenite occurs mainly due to two mechanisms: dynamic recovery
∗ Corresponding author. Tel.: +86 21 26649444. E-mail address: [email protected] (Q. Tian). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.04.007
and dynamic recrystallization (DRX). There were some studies on the dynamic restoration process of metals and alloys during plastic deformation under moderate strain rates less than 5/s [2–7]. However, how the pipe making process affects the microstructure formation is not clear. In this study, the deformation strains as well as the microstructure evolution corresponding to the pipe making process were analyzed, and the high temperature deformation process was simulated using a thermal mechanical simulator. Generally, the material for high collapse resistance pipe is CrMo steel. Elements such as Nb, V and Ti alloyed in steel assure enough precipitation of carbon nitride to pin grain boundary and to refine grain size at high temperatures. In the research, a Ti, Nb and V microallyed CrMo steel was designed to achieve super-high collapse resistance by making use of their strengthening effect, and during the pipe making process, cross deformation with large strain was adopted to obtain ideal texture components. 2. Experimental procedure The composition of the steel billet is shown in Table 1. The pipes were prepared by the following process: piercing (cross deformation)–rolling (cross deformation)–reeling (longitudinal deformation)–sizing (longitudinal deformation). The prepared pipes were then further quenched with the optimized austenization parameters using Box–Behnken design, and tempered at 963 K to attain the required strength [8], and thus super-high collapse resistance casing was prepared after sizing and straightening at temperatures higher than 773 K.
Q. Tian et al. / Materials Science and Engineering A 527 (2010) 4702–4707
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Table 1 Chemical composition of the designed steel (wt.%). C
Cr
Mo
Ti + Nb + V
0.28
0.5
0.4
0.2
The specimens for grain size observation were firstly polished, and then etched with picric acid at temperatures from 323 K to 353 K. Prior austenite grains were observed using an optical metallographic microscope equipped with Mias grain evaluation software. In order to get the comparable measurement results, the texture of the outside surface of the pipe specimens before or after heat treatment was measured using a Rigaku Rint-2200/PC diffractometer. A JEM-2010 transmission electron microscopy (TEM) was used to observe the precipitates distribution. TEM specimens were first mechanically polished, and then prepared by a twin-jet electro-polishing apparatus. For better understanding the thermal mechanical process of pipe making, specimens with the dimension of 8 × 12 (mm × mm) were cut from the steel billet for hot compression to simulate high temperature deformation using a Gleeble 3800 thermal mechanical simulator. Specimens were firstly heated to 1523 K with a heating rate of 10 K/s, holding for 2 min, and then the temperature was lowered to varied temperature levels with a holding time of 1.5 min, as shown in Fig. 1. After the specimens were deformed at a large strain rate of 5/s, they were quenched to room temperature with a cooling rate of about 30 K/s. 3. Results and discussion 3.1. Mechanism of high-temperature deformation It is known that work hardening effect prevails at the initial stage of deformation at elevated temperatures, and sharply increases the flow stress. With the increase of deformation strain, work hardening and dynamic softening occur simultaneously. The dynamic softening effect is originated from DRX or dynamic recovery process. Generally under high strain rates, when deformation reaches a certain degree, dynamic softening occurs and compensates for the increase of flow stress contributed by hardening effect, thereafter hardening and softening reach a balance, as a result, the flow stress becomes stable after experiencing a maximum peak. While deforming under low strain rates, work hardening cannot keep up with softening process, so that flow stress shows a multi-peak characteristic. Fig. 2 shows the true stress–true strain curves of specimens at different elevated temperatures with a strain rate of 5/s. It can be seen that for the specific material, flow stresses exhibit multi-peak
Fig. 1. Schematic diagram of hot compression test: (1) specimens reheated to 1523 K at a rate of 10 K/s; (2) held for 5 min at 1283 K; (3) cooled to the deformation temperature at a rate of 5 K/s; (4) deformed at constant temperatures (1473 K, 1423 K, 1373 K, 1323 K, 1273 K, 1223 K, 1173 K, 1123 K, and 1073 K); (5) quenched rapidly after deformation at a cooling rate of about 30 K/s.
Fig. 2. Flow stress during hot deformation at elevated temperatures.
rather than a stable stress. However, for the deformation of 35CrMo steel under such a high strain rate, only one peak exists in the stress–strain curve [9]. The phenomenon was not only owed to the competing result of hardening and softening, but also contributed by the formation effect of precipitates discussed below. It can be seen from Fig. 2 that the flow stress increases with the decrease of deformation temperature, T. The maximum peak flow stresses are 87 MPa and 258 MPa when the temperatures are 1473 K and 1073 K, respectively, as shown in Fig. 3. Fig. 3 shows the maximum peak flow stress dependence of deformation temperature, indicating obviously two different variation tendencies. The two fitting lines, in fact, respectively represent the DRX process and the dynamic recovery process. The cross point means the beginning of DRX. At the present case, the austenite recrystallization is expected to begin at about 1223 K. 3.2. Thermal mechanical process at high temperatures Deformation resistance can be indicated by the peak flow stress during hot deformation. Zener and Hollomon [10] found that the stress–strain relation depended on the deformation temperature ˙ and can be denoted by a parameter, Z = T and the strain rate ε, n , where Z is the Zener–Hollomon parameter, R ε˙ exp(Q/RT ) = Am is the universal gas constant (R = 8.314 J/(mol K)), Q is the activation energy, for 35CrMo steel, whose value for DRX is 378.2 kJ/mol [9], is used for an estimation in this research, m is the maximum peak flow stress for the specific situation, and A and n are constant. The ln Z and ln m relationship is represented in Fig. 4, and it can be seen ln Z linearly increases with the increase of ln m . The deformation resistance during DRX can then be represented by
Fig. 3. Maximum peak flow stress dependence of deformation temperature.
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prior austenite grain boundaries in the specimen at temperature of 1223 K (Fig. 5b), in good accordance with the predicted recrystallization start temperature. Fine recrystallized austenite grains are found distributing along the deformed austenite grain boundaries at temperature of 1273 K (Fig. 5c). 3.3. Deformation strains of the pipe making process
Fig. 4. The ln Z dependence of ln m .
Zener–Hollomon parameter as follows: m = 1.46 Z 0.13
(1)
The microstructure of deformed specimen varies from the surface to the center due to varied distribution of deformation strains. For sound comparison, only the microstructure in the center part of the specimen, where the strain is relatively higher, is shown in Fig. 5. When deformation temperature is low, only dynamic recovery occurs and the elongated austenite grains are preserved as shown in Fig. 5a, while when temperature is high, say 1473 K, DRX occurs, and the deformed austenite grains recover to polycrystal grains (Fig. 5d). Besides the characteristic of dynamic recovery morphology, recrystallization nuclei can be identified along the
During longitudinal deformation process, longitudinal, radial, circumferential and overall equivalent strains are calculated by the following equations: ε1 = ln ((D0 − SZ ), ε2 = ln SZ /S0 , ε3 = ln DZ − SZ /D0 − S0 , and S0 )S0 )/((DZ − √ εeq = ( 2/3) (ε1 − ε2 )2 + (ε2 − ε3 )2 + (ε1 − ε3 )2 , where D0 , S0 , DZ and SZ are the outer diameter (D) and the wall thickness (S) before (subscript 0) and after (subscript Z) deformation, respectively [11]. Considering the additional strain introduced by cross deformation, the following experimental equations have been given [11]: As for cross piecing, εTeq = −2.69 + 2.59, and for cross rolling: εTeq = −1.83 + 2.06, where is the stretching ratio between the lengths before and after deformation. Taking the preparation of the API specification of 177.8 × 9.19 pipe for example, large deformation strains were assigned to cross piercing and cross rolling at high temperatures with a strain rate of about 4–6/s. The main parameters for the pipe making process are listed in Table 2. The equivalent strains for cross piercing, cross rolling, longitudinal reeling and longitudinal sizing are calculated to be 5.96, 0.89, 0.07 and 0.09, respectively. The equivalent strains for cross piercing (5.96) or cross rolling (0.89) is about 3 times that for longitudinal piercing (1.94) or rolling (0.33). It is reasonable that the pipes under cross deformation exhibit obviously different textures from that under longitudinal rolling [12].
Fig. 5. The deformed microstructures of specimens at (a) 1123 K, (b) 1223 K, (c) 1273 K and (d) 1473 K.
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Table 2 Equivalent strains calculated for the making process of a 177.8 × 9.19 pipe. T, K Cross piercing Cross rolling Longitudinal reeling Longitudinal sizing a
1513 1373 1223 1143
D0 , mm 178 195 192 192
S0 , mm 89 13 9.8 9.19
Dz , mm 195 192 192 177.8
Sz , mm 13 9.8 9.19 9.19
3.34 1.32 1.06 1.08
εeq
εTeq a
1.94 0.33a 0.07 0.09
5.96 0.89 – –
The data represents the equivalent strain of longitudinal deformation.
Fig. 6. Prior austenite grains under hot rolling state.
3.4. Microstructure characteristics during pipe making According to the calculation results of Table 2 and the above discussions, complete DRX occurs after cross piercing of billet (1513 K, εTeq = 5.96), and after cross rolling (1373 K, εTeq = 0.89) under the strain rate of 5/s. During the longitudinal reeling and sizing process at strains of less than 0.1 as indicated by the dot line in Fig. 2, the pipes only undergo work hardening process. The work-hardened microstructure should be kept if the pipes were quenched to room
Fig. 7. Precipitates of as-rolled specimen showing the (Cr, Fe)23 C6 , Mo2 C, and (Ti, Nb, V)C precipitate types.
temperature immediately. However, the pipes were air cooled in the industrial production process, therefore, there was enough time for the occurrence of static recrystallization under high temperature austenite state. As shown in Fig. 6, polygonal austenite grains, whose average size is about 20–40 m, have replaced the deformed grains. It is noted that the as-rolled specimen shows bainitic microstructure. Based on TEM observation of as-rolled pipe specimens, it is found that large amount of carbonitrides are formed along the
Fig. 8. Texture distribution in the = 0◦ and = 45◦ sections of Euler space along circumferential direction for pipes under the as-rolled state, showing (1 1 1) 1 1 0, intensity 3.3.
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Fig. 9. Prior austenite grains after heat treatment.
rolling direction, as shown in Fig. 7. It is believed that although deformed austenite grains have recovered to polygonal ones, precipitates distribution reflects the original direction of deformed austenite grains. It has been determined that massive and moderate sizes of (Cr, Fe)23 C6 and Mo2 C particles and fine M(C, N) (M is V or Ti and or Nb) precipitates exist in the investigated steel casing [1]. According to the thermodynamics of M(C, N) (M is V and or Ti and or Nb) formation, the precipitates of TiC, NbN, and NbC should be formed while V should still exist in the solid solution state during the piercing, rolling, reeling and sizing process of pipe making [13]. It is known that work hardening results from the increase of dislocation intensity. In the investigated steel, high intensity dislocation area provides nuclei site not only for recovery and recrystallization softening, but also for the precipitation of M(C, N) particles, and thus breaks the balance of work hardening and dynamic softening process. That is the reason why the fluctuations of flow stress exist in Fig. 2. On the other hand, the precipitates of these elements in the pipe making process can effectively change
Fig. 10. TEM observation of the microstructure of heat-treated specimen showing the (Cr, Fe)23 C6 , Mo2 C, and (Ti, Nb, V)C precipitate types.
the intensity and type of texture through refining austenite grain size, inhibiting the austenite recrystallization, and retarding ␥/␣ phase transformation. Fig. 8 shows the texture components in the = 0◦ and = 45◦ sections of Euler space for pipes under as-rolled state. The overall intensity of texture is not very strong. The main texture component is the (1 1 1) 1 1 0 fiber with an intensity of 3.3 grade. It has been proposed that the (1 1 1) fiber in the casing is beneficial to increase collapse resistance [12], nevertheless, it is interesting to find that the (1 1 1) fiber has already formed in the intermediate process of pipe preparation. 3.5. Microstructure evolution after heat treatment Heating the pipes to 1173 K and adopting proper holding time for austenization can optimize the prior austenite grain size to be about 10 m [8], as shown in Fig. 9. After quenching and tempering,
Fig. 11. Texture distribution in the = 0◦ and = 45◦ sections of Euler space along circumferential direction for pipes under quenching and tempering state, showing (1 1 1) 1 1 0, (1 1 1) 1 1 2, intensity 3.1.
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the precipitates distribution changes greatly in comparison with that of as-rolled state (Fig. 10). VC precipitates should be substantially formed during tempering process [13]. It has been confirmed that large amount of VC particles with sizes of less than 10 nm exist in the quenched and tempered pipes [1]. During collapse process under external pressure, these precipitates can effectively pin dislocation motion, hinder slip movement, and thus significantly increase collapse resistance. Fig. 11 shows the main texture is the (1 1 1) fiber with the components of (1 1 1) 1 1 0 and (1 1 1) 1 1 2. Obviously, the austenization process does not change the nature of the fiber texture formed in the pipe making process. It is certain that large strains have been assigned to cross piercing and cross rolling process, in which complete DRX occurs. Thus it is inferred that cross deformation is responsible for the formation of (1 1 1) fiber.
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(3) The precipitates distribution of as-rolled pipes still reflects the characteristic of deformed austenite, and after heat treatment, both the austenite grains and the precipitates are refined and the deformed characteristic is changed. (4) It is found that the (1 1 1) fiber is developed during the pipe making process. It is assured that cross deformation is responsible for the formation of (1 1 1) fiber. The followed heat treatment of the as-rolled pipes cannot change the (1 1 1) fiber nature. (5) Complete DRX occurs in the piercing and rolling stages of pipe making process. During the reeling and sizing process, only the work hardening process occurs. And the work-hardened microstructure vanishes owing to the occurrence of static recrystallization at high temperature austenite state. References
4. Conclusions (1) For making the API specification of 177.8 × 9.19 pipe, large deformation strains were assigned to the piercing and rolling process, and the equivalent strains for cross piercing, cross rolling, longitudinal reeling and longitudinal sizing were 5.96, 0.89, 0.07 and 0.09, respectively. (2) Ti, Nb and V microalloying of CrMo steel leads to the existence of multi-peak of flow stress in the stress–strain curve under high strain rate. The austenite recrystallization start temperature is found at 1223 K by microstructure observation, in accordance with the predicted value using the maximum peak flow stresses. The deformation resistance dependence of Zener–Hollomon parameter during DRX is found to be m = 1.46 Z0.13 .
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