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A study on crack healing in 1045 steel
Journal of Materials Processing Technology 177 (2006) 233–237
A study on crack healing in 1045 steel D. Wei a,∗ , J. Han b,1 , Z.Y. Jiang a,2 , C. Lu a,3 , A.K. Tieu a,4 a
School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, Wollongong, NSW 2522, Australia b School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China
Abstract This study aims to investigate the healing method of internal cracks in 1045 steel. Real internal cracks were produced in 1045 steel specimens using plate impact technology successfully. A quasi in situ observation under scanning electron microscope and ultrasonic scanning was performed to investigate the behaviour of crack healing. After 120 min heat treatment at 1100 ◦ C, the crack healing at crack tips was achieved. However, micro-voids were left in crack healing area and the edges of the remnant micro-voids were very smooth. © 2006 Elsevier B.V. All rights reserved. Keywords: Crack; Healing; Steel; Plate impact
1. Introduction In metal forming process, internal cracks occur in workpieces because of inhomogeneous deformation. Although the propagation of cracks is restrained in hot forming because there exists a healing process while cracks initiate [1], some cracks are still left in the formed workpieces. The situation is usually more severe in cold-formed workpieces. These internal cracks in metal product decrease the strength, rigidity, toughness, plasticity and residual life. Heat treatment may heal the internal cracks existing in the formed metal materials. Griffith [2] proposed that the cracking is not a “reversible” operation, but a very small crack may be healed if the maximum temperature of the heat-treatment is sufficient to bring the atoms on either side of the crack to within a mutual range by thermal agitation. Kumanin et al. [3] analyzed the damage evolution in metals, and indicated that many types of heat treatment methods can be used to decrease the amount of defects in metals after a longterm operation at high temperature. Han et al. [4] investigated the crack healing in 20MnMo steel. Wei et al. [5] studied the
crack healing behaviour in plain carbon steel at elevated temperature. For studying crack healing in metals, several kinds of precracks had been obtained by different methods. However, some cracks obtained are not internal but surface cracks [6,7] or not real internal cracks [4]. The real internal cracks were obtained by hydrogen attack in 304 austenite steel [8], but their positions could not be determined. On the other hand, in situ observation on crack healing is very significant for understanding its mechanism. In situ observations have been operated on crack healing in some transparent non-metallic materials. But it is difficult to do in situ observation of internal crack healing in metals because they are not transparent. In this study, the technology of plate impact was applied to produce internal cracks at a predictable position in 1045 steel sample at high strain rates [9]. The internal cracks were tested non-destructively by ultrasonic scanning. A quasi in situ observation under scanning electron microscope (SEM) was designed to study crack healing at elevated temperature. 2. Experiments
∗
Corresponding author. Tel.: +61 2 42214549; fax: +61 2 42213101. E-mail addresses: [email protected] (D. Wei), [email protected] (J. Han), [email protected] (Z.Y. Jiang), [email protected] (C. Lu), [email protected] (A.K. Tieu). 1 Tel.: +86 10 62332572; fax: +86 10 62332572. 2 Tel.: +61 2 42214545; fax: +61 2 42213101. 3 Tel.: +61 2 42214639; fax: +61 2 42213101. 4 Tel.: +61 2 42213061; fax: +61 2 42213101. 0924-0136/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2006.04.067
2.1. Sample preparation 2.1.1. Principle In the experiment of plate impact, when the flyer impacts the target at high speed (Fig. 1(a)), compression waves are produced in the target and are reflected to form rare-faction waves at the interfaces adjacent to low impedance media (usually the free surface), and the rare-faction waves interact with each other to
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D. Wei et al. / Journal of Materials Processing Technology 177 (2006) 233–237
Fig. 1. Flyer and target: (a) position of spallation plane in target; (b) interaction of stress waves in plate impact (hp : thickness of flyer; ht : thickness of target; d1 : diameter of target; d2 : diameter of spallation area; V: impact speed; t: impact time). form tensile waves (Fig. 1(b)) [9]. If the tensile strength resulted from the tensile waves is high enough, it can cause spallation in target [10]. 2.1.2. Specimen design The specimens were taken from the rolled bars of 1045 steel (wt.% C 0.43%, Mn 0.67%, Si 0.29%, S 0.0079%, P 0.012%). In order to obtain several specimens that have the same state of damage for comparing the effect of healing under different conditions, the number of designed targets was three. The size of flyer and target are Φ 99.0 mm × 5.0 mm and Φ 32.0 mm × 10.0 mm, respectively. Three targets of Φ 32 mm contact each other tangently to constitute a large circle of Φ 70 mm that is concentric with the flyer of Φ 99 mm (Fig. 2(a)). Experiments were carried out with a single-stage light gas gun. The speeds of flyers were 73.1, 128.8, 176.1, 186.7 and 191.1 m/s, respectively, in each plate impact. As hp /ht was 0.5, the spallation occurred at the position of half thickness of the targets. No spallation happened at the impact speeds of 73.1 or 128.8 m/s, and the extent of spallation at 176.1 m/s was obviously less than that at 191.1 m/s.
2.2. Experimental procedure of heat treatment Phase transformation is significant for the inner damage healing in steel [3]. In this study, the healing temperature 1100 ◦ C was set above AC3 in Fe–C phase diagram. According to the equation regressed from the data of 20MnMo steel, initial healing speed is high and the following healing process will be at a relatively low speed [4]. As crack size in this study was large, the cracks could not be healed completely even after a long holding time at elevated temperature.
Fig. 2. Plate impact specimens: (a) relative position of flyer and three targets; (b) split of target plate.
In order to obtain a distinct comparison, the holding time was 120 min in this experiment.
2.3. Quasi in situ observation Quasi in situ observation was designed to study the crack healing in metals. Each specimen was split into four parts, as shown in Fig. 2(b). The opposite two sections on part 0 and part 1 or 3 share the same crack morphology. Part 0 was not subjected to any heat treatment and was used to obtain an original
Fig. 3. Ultrasonic scanning results of specimen 186-1 before and after heat treatment. (a) Original crack; (b) after 120 min heat treatment at 1100 ◦ C.
D. Wei et al. / Journal of Materials Processing Technology 177 (2006) 233–237
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Fig. 4. Morphology of crack tips in specimen 186-2 before and after heat treatment. (a) Original crack tip on the section of 186-20 (left: SE; right: BSD); (b) crack tip on the section of 186-23 after 120 min heat treatment at 1100 ◦ C (left: SE; right: BSD).
crack morphology while part 1 or 3 were used to obtain a crack morphology after heat treatment at elevated temperature in vacuum. Table 1 summarizes the experimental parameters. After cleaned in ultrasonic environment, specimen 186-23 was sealed in the vacuum of 133 × 10−4 to 133 × 10−5 Pa in a quartz glass tube. It was taken out from quartz tubes after heat treatment at 1100 ◦ C. After ground and polished, the opposite two sections on 186–20 and 186–23 were eroded with alcohol nitric acid solution (4 ml/100 ml), and then observed under the SEM for comparison. Specimen 191-21 was ground, polished and eroded with alcohol nitric acid solution (4 ml/100 ml) after split. Vickers hardness marks were made along the crack for positioning. After obtaining original crack morphology on one section under SEM, it was sealed in the vacuum of 133 × 10−4 to 133 × 10−5 Pa in a quartz glass tube. After heat treatment, specimen 191-21 was taken out from the quartz tube and the new crack morphology on the same section was obtained under SEM. The advantage of the above observation is that the crack morphology in one certain specimen before and after heat treatment can be contrasted. However, the internal cracks became surface cracks after split. On the other hand, grinding and polishing might change crack morphology, which sometimes makes it difficult
to find the same position on the opposite two sections on part 0 and part 1 or 3. Because of these disadvantages, the proposed method has been called “quasi” in situ observation. Specimen 186-1 was not split. An USIP20 C ultrasonic scanning system was used to inspect the internal cracks in this specimen before and after heat treatment and to obtain the damage distribution map [10]. After process by a PC, the pseudo 3D images which display the depth of damage by different colors can be obtained.
Table 1 Experimental parameters Specimen number
Impact speed (m/s)
Heat treatment temperature (◦ C)
Holding time (min)
186-23 191-21 186-1
186.7 191.1 186.7
1100 1100 1100
120 120 120
Fig. 5. Original crack tip in 191-21 and the Vickers hardness mark.
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Fig. 6. Morphology of the crack tip in 191-21 after 120 min heat treatment at 1100 ◦ C. (a) SE; (b) BSD.
3. Results and discussion Fig. 3 shows the result of ultrasonic scanning on specimen 186-1 before and after 120 min heat treatment at 1100 ◦ C. The scanning parameters before and after heat treatment were the same. For describing the damage quantitatively, the ratio between the internal crack area and the whole specimen area on the projection was calculated. The ratio of crack area to sample area was 33.5% and 28.5% before and after heat treatment, respectively. Fig. 4 shows the quasi in situ observation of the internal crack in split specimen 186-2 before and after heat treatment. From the SEM images, the original crack tip on the section of 186-20 was very clear, as shown in Fig. 4(a) (left), while this crack tip could not be found on the corresponding position on the section of specimen 186-23 after 120 min heat treatment at 1100 ◦ C in vacuum, as shown in Fig. 4(b) (left). But this did not mean that the crack tip has been healed completely. Switching from second electron (SE) mode to back scattered dispersion (BSD) mode, the crack tip which was invisible in Fig. 4(b) (left) was found again, as shown in Fig. 4(b) (right). Comparing to the original morphology shown in Fig. 4(a) (right), the crack was filled after heat treatment. Although this was not a complete healing, it could not be detected by SE mode. However, micro-voids could be found in the healing area around the crack tips. For convenience, 14 Vickers hardness marks were made on the section of specimen 191-21, one of which is shown in Fig. 5 (BSD). After 120 min heat treatment at 1100 ◦ C in vacuum, the SE image of the above field is shown in Fig. 6(a). The reason of switching from BSD to SE mode was that it was impossible to position the above Vickers hardness mark with BSD mode after heat treatment, as shown in Fig. 6(b). Both white circles in Fig. 6(a) and (b) show the position of the Vickers hardness mark after heat treatment. The edge of the Vickers hardness mark was very clear before heat treatment, while only a very obscure mark could be found after heat treatment. Before heat treatment, the crack tip shown in Fig. 5 can be divided into three short sub-cracks “A”, “B” and “C”, the widths of which are less than 10 m. Between these three sub-cracks are some micro-cracks with the width less than 1 m. After heat treatment, these micro-cracks were
healed completely but several remnant micro-voids were left. In the meantime, sub-cracks “A”, “B” and “C” evolved into many micro-voids that distributed along the original crack line, as shown in Fig. 6(a). Taking sub-crack “A” as an example, the fine tip evolved into isolated small size micro-voids, while the wide middle part evolved into the big size micro-voids. The edges of all these micro-voids are very smooth. 4. Conclusion Real internal cracks can be produced in 1045 steel specimens using plate impact technology successfully. A quasi in situ observation under scanning electron microscope and ultrasonic scanning can be adopted to study the in situ crack healing process. After 120 min heat treatment at 1100 ◦ C, the crack healing at crack tips can be achieved. Micro-voids are left in crack healing area and the edges of the remnant micro-voids were smooth. Acknowledgments The authors acknowledge support of the Australia Research Council and the support of National Natural Science Foundation of China. References [1] E. Shapiro, G. Dieter, High temperature-high strain rate fracture of inconel 600, Metall. Trans. 1 (6) (1970) 1711–1719. [2] A. Griffith, Phenomena of rupture and flow in solids, Philos. Trans. R. Soc. Lond. Ser. A 221 (4) (1920) 163–198. [3] V.I. Kumanin, L.A. Kovaleva, M.L. Sokolova, The use of recovery heat treatment to eliminate damage in metallic materials, Metalloved. Term. Obrab. Met. (4) (1995) 7–12. [4] J. Han, G. Zhao, Q. Cao, Study on internal crack recovery of 20MnMo steel, Sci. China (Ser. E) 27 (1) (1997) 23–27. [5] D. Wei, J. Han, A.K. Tieu, Z.Y. Jiang, An analysis on the inhomogeneous microstructure in steel crack healing area, Key Eng. Mater. 274–276 (2004) 1053–1058. [6] K.W. Gao, L.J. Qiao, W.Y. Chu, In situ TEM observation of crack healing in alpha-Fe, Scripta Mater. 44 (2001) 1055–1059. [7] Y. Zhou, Y. Zeng, G. He, B. Zhou, The healing of quenched crack in 1045 steel under electropulsing, J. Mater. Res. 16 (2001) 17–19.
D. Wei et al. / Journal of Materials Processing Technology 177 (2006) 233–237 [8] X. Li, C. Dong, H. Chen, Healing of hydrogen attack crack in austenite stainless steel under heat treatment, Acta Metall. Sin. (Engl. Lett.) 15 (2002) 385–390. [9] D. Wei, J. Han, A.K. Tieu, Z.Y. Jiang, Experimental study and FEM analysis of dynamic response in metals during plate impact, in: S. Ghosh, J.M. Castro, J.K. Lee (Eds.), Proceedings of the 8th
237
International Conference on Numerical Methods in Industrial Forming Processes, Ohio, USA, June 13–17, Am. Inst. Phys. (2004) 498– 503. [10] L. Shen, S. Zhao, Y. Bai, L. Luo, Experimental study on the criteria and mechanism of spallation in an aluminium alloy, Int. J. Impact Eng. 12 (1992) 9–19.
A study on crack healing in 1045 steel D. Wei a,∗ , J. Han b,1 , Z.Y. Jiang a,2 , C. Lu a,3 , A.K. Tieu a,4 a
School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, Wollongong, NSW 2522, Australia b School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China
Abstract This study aims to investigate the healing method of internal cracks in 1045 steel. Real internal cracks were produced in 1045 steel specimens using plate impact technology successfully. A quasi in situ observation under scanning electron microscope and ultrasonic scanning was performed to investigate the behaviour of crack healing. After 120 min heat treatment at 1100 ◦ C, the crack healing at crack tips was achieved. However, micro-voids were left in crack healing area and the edges of the remnant micro-voids were very smooth. © 2006 Elsevier B.V. All rights reserved. Keywords: Crack; Healing; Steel; Plate impact
1. Introduction In metal forming process, internal cracks occur in workpieces because of inhomogeneous deformation. Although the propagation of cracks is restrained in hot forming because there exists a healing process while cracks initiate [1], some cracks are still left in the formed workpieces. The situation is usually more severe in cold-formed workpieces. These internal cracks in metal product decrease the strength, rigidity, toughness, plasticity and residual life. Heat treatment may heal the internal cracks existing in the formed metal materials. Griffith [2] proposed that the cracking is not a “reversible” operation, but a very small crack may be healed if the maximum temperature of the heat-treatment is sufficient to bring the atoms on either side of the crack to within a mutual range by thermal agitation. Kumanin et al. [3] analyzed the damage evolution in metals, and indicated that many types of heat treatment methods can be used to decrease the amount of defects in metals after a longterm operation at high temperature. Han et al. [4] investigated the crack healing in 20MnMo steel. Wei et al. [5] studied the
crack healing behaviour in plain carbon steel at elevated temperature. For studying crack healing in metals, several kinds of precracks had been obtained by different methods. However, some cracks obtained are not internal but surface cracks [6,7] or not real internal cracks [4]. The real internal cracks were obtained by hydrogen attack in 304 austenite steel [8], but their positions could not be determined. On the other hand, in situ observation on crack healing is very significant for understanding its mechanism. In situ observations have been operated on crack healing in some transparent non-metallic materials. But it is difficult to do in situ observation of internal crack healing in metals because they are not transparent. In this study, the technology of plate impact was applied to produce internal cracks at a predictable position in 1045 steel sample at high strain rates [9]. The internal cracks were tested non-destructively by ultrasonic scanning. A quasi in situ observation under scanning electron microscope (SEM) was designed to study crack healing at elevated temperature. 2. Experiments
∗
Corresponding author. Tel.: +61 2 42214549; fax: +61 2 42213101. E-mail addresses: [email protected] (D. Wei), [email protected] (J. Han), [email protected] (Z.Y. Jiang), [email protected] (C. Lu), [email protected] (A.K. Tieu). 1 Tel.: +86 10 62332572; fax: +86 10 62332572. 2 Tel.: +61 2 42214545; fax: +61 2 42213101. 3 Tel.: +61 2 42214639; fax: +61 2 42213101. 4 Tel.: +61 2 42213061; fax: +61 2 42213101. 0924-0136/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2006.04.067
2.1. Sample preparation 2.1.1. Principle In the experiment of plate impact, when the flyer impacts the target at high speed (Fig. 1(a)), compression waves are produced in the target and are reflected to form rare-faction waves at the interfaces adjacent to low impedance media (usually the free surface), and the rare-faction waves interact with each other to
234
D. Wei et al. / Journal of Materials Processing Technology 177 (2006) 233–237
Fig. 1. Flyer and target: (a) position of spallation plane in target; (b) interaction of stress waves in plate impact (hp : thickness of flyer; ht : thickness of target; d1 : diameter of target; d2 : diameter of spallation area; V: impact speed; t: impact time). form tensile waves (Fig. 1(b)) [9]. If the tensile strength resulted from the tensile waves is high enough, it can cause spallation in target [10]. 2.1.2. Specimen design The specimens were taken from the rolled bars of 1045 steel (wt.% C 0.43%, Mn 0.67%, Si 0.29%, S 0.0079%, P 0.012%). In order to obtain several specimens that have the same state of damage for comparing the effect of healing under different conditions, the number of designed targets was three. The size of flyer and target are Φ 99.0 mm × 5.0 mm and Φ 32.0 mm × 10.0 mm, respectively. Three targets of Φ 32 mm contact each other tangently to constitute a large circle of Φ 70 mm that is concentric with the flyer of Φ 99 mm (Fig. 2(a)). Experiments were carried out with a single-stage light gas gun. The speeds of flyers were 73.1, 128.8, 176.1, 186.7 and 191.1 m/s, respectively, in each plate impact. As hp /ht was 0.5, the spallation occurred at the position of half thickness of the targets. No spallation happened at the impact speeds of 73.1 or 128.8 m/s, and the extent of spallation at 176.1 m/s was obviously less than that at 191.1 m/s.
2.2. Experimental procedure of heat treatment Phase transformation is significant for the inner damage healing in steel [3]. In this study, the healing temperature 1100 ◦ C was set above AC3 in Fe–C phase diagram. According to the equation regressed from the data of 20MnMo steel, initial healing speed is high and the following healing process will be at a relatively low speed [4]. As crack size in this study was large, the cracks could not be healed completely even after a long holding time at elevated temperature.
Fig. 2. Plate impact specimens: (a) relative position of flyer and three targets; (b) split of target plate.
In order to obtain a distinct comparison, the holding time was 120 min in this experiment.
2.3. Quasi in situ observation Quasi in situ observation was designed to study the crack healing in metals. Each specimen was split into four parts, as shown in Fig. 2(b). The opposite two sections on part 0 and part 1 or 3 share the same crack morphology. Part 0 was not subjected to any heat treatment and was used to obtain an original
Fig. 3. Ultrasonic scanning results of specimen 186-1 before and after heat treatment. (a) Original crack; (b) after 120 min heat treatment at 1100 ◦ C.
D. Wei et al. / Journal of Materials Processing Technology 177 (2006) 233–237
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Fig. 4. Morphology of crack tips in specimen 186-2 before and after heat treatment. (a) Original crack tip on the section of 186-20 (left: SE; right: BSD); (b) crack tip on the section of 186-23 after 120 min heat treatment at 1100 ◦ C (left: SE; right: BSD).
crack morphology while part 1 or 3 were used to obtain a crack morphology after heat treatment at elevated temperature in vacuum. Table 1 summarizes the experimental parameters. After cleaned in ultrasonic environment, specimen 186-23 was sealed in the vacuum of 133 × 10−4 to 133 × 10−5 Pa in a quartz glass tube. It was taken out from quartz tubes after heat treatment at 1100 ◦ C. After ground and polished, the opposite two sections on 186–20 and 186–23 were eroded with alcohol nitric acid solution (4 ml/100 ml), and then observed under the SEM for comparison. Specimen 191-21 was ground, polished and eroded with alcohol nitric acid solution (4 ml/100 ml) after split. Vickers hardness marks were made along the crack for positioning. After obtaining original crack morphology on one section under SEM, it was sealed in the vacuum of 133 × 10−4 to 133 × 10−5 Pa in a quartz glass tube. After heat treatment, specimen 191-21 was taken out from the quartz tube and the new crack morphology on the same section was obtained under SEM. The advantage of the above observation is that the crack morphology in one certain specimen before and after heat treatment can be contrasted. However, the internal cracks became surface cracks after split. On the other hand, grinding and polishing might change crack morphology, which sometimes makes it difficult
to find the same position on the opposite two sections on part 0 and part 1 or 3. Because of these disadvantages, the proposed method has been called “quasi” in situ observation. Specimen 186-1 was not split. An USIP20 C ultrasonic scanning system was used to inspect the internal cracks in this specimen before and after heat treatment and to obtain the damage distribution map [10]. After process by a PC, the pseudo 3D images which display the depth of damage by different colors can be obtained.
Table 1 Experimental parameters Specimen number
Impact speed (m/s)
Heat treatment temperature (◦ C)
Holding time (min)
186-23 191-21 186-1
186.7 191.1 186.7
1100 1100 1100
120 120 120
Fig. 5. Original crack tip in 191-21 and the Vickers hardness mark.
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D. Wei et al. / Journal of Materials Processing Technology 177 (2006) 233–237
Fig. 6. Morphology of the crack tip in 191-21 after 120 min heat treatment at 1100 ◦ C. (a) SE; (b) BSD.
3. Results and discussion Fig. 3 shows the result of ultrasonic scanning on specimen 186-1 before and after 120 min heat treatment at 1100 ◦ C. The scanning parameters before and after heat treatment were the same. For describing the damage quantitatively, the ratio between the internal crack area and the whole specimen area on the projection was calculated. The ratio of crack area to sample area was 33.5% and 28.5% before and after heat treatment, respectively. Fig. 4 shows the quasi in situ observation of the internal crack in split specimen 186-2 before and after heat treatment. From the SEM images, the original crack tip on the section of 186-20 was very clear, as shown in Fig. 4(a) (left), while this crack tip could not be found on the corresponding position on the section of specimen 186-23 after 120 min heat treatment at 1100 ◦ C in vacuum, as shown in Fig. 4(b) (left). But this did not mean that the crack tip has been healed completely. Switching from second electron (SE) mode to back scattered dispersion (BSD) mode, the crack tip which was invisible in Fig. 4(b) (left) was found again, as shown in Fig. 4(b) (right). Comparing to the original morphology shown in Fig. 4(a) (right), the crack was filled after heat treatment. Although this was not a complete healing, it could not be detected by SE mode. However, micro-voids could be found in the healing area around the crack tips. For convenience, 14 Vickers hardness marks were made on the section of specimen 191-21, one of which is shown in Fig. 5 (BSD). After 120 min heat treatment at 1100 ◦ C in vacuum, the SE image of the above field is shown in Fig. 6(a). The reason of switching from BSD to SE mode was that it was impossible to position the above Vickers hardness mark with BSD mode after heat treatment, as shown in Fig. 6(b). Both white circles in Fig. 6(a) and (b) show the position of the Vickers hardness mark after heat treatment. The edge of the Vickers hardness mark was very clear before heat treatment, while only a very obscure mark could be found after heat treatment. Before heat treatment, the crack tip shown in Fig. 5 can be divided into three short sub-cracks “A”, “B” and “C”, the widths of which are less than 10 m. Between these three sub-cracks are some micro-cracks with the width less than 1 m. After heat treatment, these micro-cracks were
healed completely but several remnant micro-voids were left. In the meantime, sub-cracks “A”, “B” and “C” evolved into many micro-voids that distributed along the original crack line, as shown in Fig. 6(a). Taking sub-crack “A” as an example, the fine tip evolved into isolated small size micro-voids, while the wide middle part evolved into the big size micro-voids. The edges of all these micro-voids are very smooth. 4. Conclusion Real internal cracks can be produced in 1045 steel specimens using plate impact technology successfully. A quasi in situ observation under scanning electron microscope and ultrasonic scanning can be adopted to study the in situ crack healing process. After 120 min heat treatment at 1100 ◦ C, the crack healing at crack tips can be achieved. Micro-voids are left in crack healing area and the edges of the remnant micro-voids were smooth. Acknowledgments The authors acknowledge support of the Australia Research Council and the support of National Natural Science Foundation of China. References [1] E. Shapiro, G. Dieter, High temperature-high strain rate fracture of inconel 600, Metall. Trans. 1 (6) (1970) 1711–1719. [2] A. Griffith, Phenomena of rupture and flow in solids, Philos. Trans. R. Soc. Lond. Ser. A 221 (4) (1920) 163–198. [3] V.I. Kumanin, L.A. Kovaleva, M.L. Sokolova, The use of recovery heat treatment to eliminate damage in metallic materials, Metalloved. Term. Obrab. Met. (4) (1995) 7–12. [4] J. Han, G. Zhao, Q. Cao, Study on internal crack recovery of 20MnMo steel, Sci. China (Ser. E) 27 (1) (1997) 23–27. [5] D. Wei, J. Han, A.K. Tieu, Z.Y. Jiang, An analysis on the inhomogeneous microstructure in steel crack healing area, Key Eng. Mater. 274–276 (2004) 1053–1058. [6] K.W. Gao, L.J. Qiao, W.Y. Chu, In situ TEM observation of crack healing in alpha-Fe, Scripta Mater. 44 (2001) 1055–1059. [7] Y. Zhou, Y. Zeng, G. He, B. Zhou, The healing of quenched crack in 1045 steel under electropulsing, J. Mater. Res. 16 (2001) 17–19.
D. Wei et al. / Journal of Materials Processing Technology 177 (2006) 233–237 [8] X. Li, C. Dong, H. Chen, Healing of hydrogen attack crack in austenite stainless steel under heat treatment, Acta Metall. Sin. (Engl. Lett.) 15 (2002) 385–390. [9] D. Wei, J. Han, A.K. Tieu, Z.Y. Jiang, Experimental study and FEM analysis of dynamic response in metals during plate impact, in: S. Ghosh, J.M. Castro, J.K. Lee (Eds.), Proceedings of the 8th
237
International Conference on Numerical Methods in Industrial Forming Processes, Ohio, USA, June 13–17, Am. Inst. Phys. (2004) 498– 503. [10] L. Shen, S. Zhao, Y. Bai, L. Luo, Experimental study on the criteria and mechanism of spallation in an aluminium alloy, Int. J. Impact Eng. 12 (1992) 9–19.