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Crack healing in a steel by using electropulsing technique
Materials Letters 58 (2004) 1732 – 1736 www.elsevier.com/locate/matlet
Crack healing in a steel by using electropulsing technique Yizhou Zhou a,*, Jingdong Guo b, Ming Gao, Guanhu He b a
WTM Institute, Department of Materials Science, University of Erlangen-Nu¨rnberg, Martensstrasse 5, D-91058 Erlangen, Germany b Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China Received 15 July 2003; received in revised form 9 October 2003; accepted 16 October 2003
Abstract Samples of steels with precracks were treated by electropulsing. As a result of electropulsing, the crack is partly healed. The healing process occurs with a very short duration pulse and the original microstructure in the area without a crack was maintained during the ‘‘healing’’ treatment. D 2004 Elsevier B.V. All rights reserved. Keywords: Metals and alloys; Defects; Crack; Healing; Microstructure; Electropulsing
1. Introduction In the past 50 years, research has indicated that an electric current can influence the behavior of materials, such as electromigration [1– 3], electroplasticity [4,5], the solidification of metals and alloys [6], the structural relaxation in amorphous solids and the amorphous crystallization [7,8], and so on. Although these studies have been fruitful, they have seldom been concerned with crack healing in metals as affected by electropulsing. Some recent experiments have shown that a crack in a metallic material can be healed in a high temperature environment [9 – 12]. However, it is very clear that the original microstructure and performances of the material will be changed under high temperature, which is undesirable in some cases. In our previous work [13], we found that a quench crack in a steel could be partly healed by electropulsing treatment, and the method has the important characteristic that the effect of healing on the uninjured part is very small. Thus, electropulsing is a promising tool for crack healing. In the previous work [13], the aim of using a quench crack was to show the above-mentioned important characteristic of electropulsing technique by the fact that the martensite in steel
* Corresponding author. Tel.: +49-9131-8527523; fax: +49-91318527515. E-mail address: [email protected] (Y. Zhou). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2003.10.049
samples could remain unchanged after a healing. However, the quench crack also has some shortcomings for the study about crack healing. For example, the position and the number of cracks are hard to control, and it is difficult to judge if a quench crack runs through thickness of sample completely. Sometimes, the second shortcoming is very detrimental for the study. It may lead one to think that the crack is partly healed during the observation after a healing treatment. In the present work, a different method is employed to obtain crack, the above-mentioned shortcoming about quench crack can be avoided by the method.
2. Experimental The material used in the present investigation was a medium carbon steel, having a composition (in wt.%) of 0.46 C, 0.05 S, 0.045 P, 0.30 Si and 0.60 Mn. The steel was annealed at 950 jC for 15 min and then cooled in air. Fig. 1 shows the schematic illustration for the preparation of samples with precracks through the thickness of samples. The annealed steel was machined to a column sample of 19 mm in diameter and 45 mm in height. A hole of 2 mm in diameter was drilled through the diameter of the column sample in its middlemost part. After cleaning out the machining oil in the hole, the column sample was pressed into the shape of a drum sample of 22 mm in the maximum diameter and 35 mm in height by a pressure machine under ambient conditions. After pressing, the hole through diam-
Y. Zhou et al. / Materials Letters 58 (2004) 1732–1736
Fig. 1. Schematic illustration for the preparation of through thickness precracked samples.
eter was changed to a crack through diameter. Sheet samples with precracks were cut from the drum sample along its axial plane, and the size of the sheet samples was 1 mm thick, 12 mm wide and 35 mm long. The precrack was in the middlemost part and through thickness of each sheet sample. Before the healing (electropulsing) treatment, one sample was polished and etched to examine the microstructure and two samples were just polished to examine the morphology of crack by optical microscope. After recording the morphology of crack, electropulsing was applied to the two polished samples with precracks under ambient conditions, by capacitor banks discharge. Each sample was only treated once by one electropulsing. The waveform of electropulsing was detected to be a damped oscillation wave. The pulse duration is about 800 As, the period of electropulsing tp = 130 As and the maximum current density jm = 2.4 kA/ mm2. The temperature of the matrix due to Joule heating was measured to be about 40 jC by a K-type thermocouple (diameter, 80 Am) soldered to a sample at the part without crack. The surfaces of the two samples were polished lightly to examine the morphology of crack after electropulsing by optical microscope. Subsequently, samples were etched to examine the microstructure by optical microscope and JSM6301F (JEOL, Tokyo, Japan) scanning electron microscopy (SEM). The chemical composition of micro-areas in samples was determined by the electron probe and the microhardness of samples was measured.
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from the figure that the crack is healed at the area close to the crack tip but not at the crack tip. The length of the healed area is about 350 Am. The microstructure in the healed area is different from that in the matrix and the black pearlite cannot be seen in the healed area. Fig. 3(b) shows a SEM micrograph of a part at the healed area, the bright microstructure in the figure is pearlite. The newly formed grains cover the original crack according to Fig. 3(b), although there are some little holes in the area, which means that the area of crack is effectively healed by electropulsing treatment. The hardness of the microstructure marked by M in Fig. 3(b) is very high, HV = 644, which is the hardness of martensite. However, the hardness of the microstructure marked by F in Fig. 3(b) is not high, HV = 252, which is a little higher than the hardness of ferrite in the matrix (HV = 180). The analyses of electron probe show that the carbon content in the microstructure marked by M is high, 0.589 wt.%, and it is close to the corresponding value of the pearlite in the matrix, 0.613 wt.%; the carbon content in the microstructure marked by F is low, 0.221 wt.%, and it is close to the corresponding value of the ferrite in the matrix, 0.197 wt.%. According to these experimental results, it is concluded that the microstructure marked by M is martensite and the microstructure marked by F is ferrite. In addition, microstructure observations show that the microstructure of the matrix (the part without crack) has no change after electropulsing. The reason is that the temperature of the matrix is low, which is
3. Results As a result of electropulsing, the crack is partly healed at the areas close to the crack tips. Fig. 2 shows the optical morphology of the left portion of a crack before and after electropulsing treatment. One can see that before electropulsing the crack is continuous in Fig. 2(a). After electropulsing, the crack is partly healed at the area marked by arrow A in Fig. 2(b). The optical micrograph of the microstructure in the healed area is shown in Fig. 3(a). It is clear
Fig. 2. Optical micrograph of the left portion of a crack before (a) and after (b) electropulsing.
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Fig. 3. Optical micrograph (a) and SEM micrograph (b) of a crack partly healed by electropulsing.
a distinguishing characteristic of electropulsing compared with the other healing methods [9 –12].
potential surfaces around the crack. Fig. 4 shows a schematic illustration for the distribution of the current lines (dashed lines) and the potential surfaces (solid lines) around a center crack. It is obvious from the figure that the current would flow turning around the crack tip, if the current could not flow through the crack. Since the equipotential surfaces are at right angles to the current lines, they are deformed to be arc surfaces around the crack. Arc line ABC and arc line DEF are two different equipotential surfaces in the figure. It can be seen that the distance between the two equipotential surfaces is small at the middle part of crack and large at the crack tip. Since the electric field E is the ratio between of the electrical potential and the distance, E is high at the middle part of crack and low at the crack tip. Thus, the crack is hardly broken down at its tip because of the low local E, although the width of crack is smallest there. Apart from the effect of electric field E, the width of crack is another important factor for breakdown of crack. For the cracks prepared in the present work, because the width of crack is always the widest at the middle part of crack, the crack is hardly broken down at that part, although E is the highest there. In other words, the breakdown should occur in the weakest part of crack, in the present study the crack tip and the middle part of crack are not the weakest part for breakdown by the combination of electric field E and the width of crack, while the area close to the crack tip seems to be the weakest part. The distribution of the current lines and potential surfaces around a crack will change because of breakdown; namely, the current does not flow turning around the crack tip, because the current can flow through the crack in the part of breakdown, and then the current density at crack tip decreases and the temperature due to Joule heating is not so high to cause the microstructure change. In the case without breakdown, since the current cannot flow through the crack, the current has to flow turning around the crack tip, and then the current density at crack tip increases significantly. When the current density is excessively high,
4. Discussion The mechanism about the crack healing caused by electropulsing is not very clear now, but we think that the possible reasons are the generation of low-temperature plasma and the motion of effective atoms toward the crack. The generation of low-temperature plasma depends on the electrical breakdown. We know that the breakdown voltage of air is about 800 V/mm [14]. In this case, if a typical width (1 Am) of the crack is considered, the breakdown of the crack can occur provided that the voltage difference between the two surfaces of the crack is 0.8 V. For the processes of applying high current density electropulsing, it is possible that the voltage difference between the two surfaces of the crack is more than 0.8 V, so the breakdown of the crack is possible here. However, the position of breakdown is related to the shape of crack and the deformation of electrical
Fig. 4. Schematic illustration for the distribution of the current lines (dashed lines) and the potential surfaces (solid lines) around a center crack.
Y. Zhou et al. / Materials Letters 58 (2004) 1732–1736
Fig. 5. SEM micrograph of a solidified microstructure after local melting, which is distinctly different from that in Fig. 3. From Zhou et al. [13].
local melting occurs at the crack tip due to Joule heating. However, the local melting at crack tip cannot lead to the healing of crack tip, see Ref. [13]. The solidified microstructure after local melting at the crack tip is shown in Fig. 5 for comparison with the microstructure in Fig. 3, which is distinctly different [13]. The characteristics of melting cannot be found in the microstructure in the healed part in Fig. 3. Based on the difference, we think a possible low-temperature plasma state may be created in the healed area because of electrical breakdown and the generation of plasma is necessary for crack healing, similar to electrical discharge compaction and field-activated sintering, which indicated that a good grain-to-grain contact could be achieved by plasma in the case of no melting [15 –18]. Due to the generation of plasma, the local temperature at the area can be high. Thus, the microstructure in the area may be different from the microstructure in the matrix. And we think this is the reason for the microstructure change in the healed part. The course of the microstructure change in the healed part is proposed to be as follows. During the passing of electropulsing, the original microstructure transformed to austenite due to the high local temperature, but the chemical composition of austenite was not uniform, since there was no significant long-range diffusion within such short treating duration when the current density of electropulsing was not so high. (The long-range diffusion of atoms is significant, when the current density of electropulsing is high, even with a very short treating duration [19].) Thus, the carbon content in the areas transformed from pearlite was high and the carbon content in the areas transformed from ferrite was low. During the cooling after electropulsing, the cooling rate of austenite in the healed area was very high due to the cooling effect of the metallic matrix, then the high carbon austenite from initial pearlite would transform to martensite, but the low carbon austenite from initial ferrite would transform to ferrite due to a low carbon content. And because of the rapid cooling, the hardness of ferrite in the healed part is higher than that in the matrix.
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Apart from the generation of plasma, it is another necessary factor for crack healing that the effective atoms move into the crack. Here, the way that the effective atoms move into the crack depends on the thermal expansion resulting from the following two aspects. (a) At the area that the current flows through the crack, the thermal expansion of the material is large due to the high local temperature. Since it is restricted by the cool matrix, the direction of expansion is to the inside of crack. (b) The sample was put into two fixed copper electrodes during the electropulsing treatment; the expansion of sample is not free, so the thermal expansion of matrix due to Joule heating also can cause the effective atoms to move into the crack. When the crack face separation distance is small enough due to the thermal expansion, the crack can be healed in combination with the effect of plasma. In brief, the generation of plasma and the motion of effective atoms toward crack are the possible reasons for crack healing. The generation of plasma results from electrical breakdown. For the crack in the present study, the area close to the crack tip is the weakest part for breakdown; therefore, the crack is healed in part but not at the crack tip. The motion of effective atoms toward crack depends on the thermal expansion.
5. Conclusions A through thickness precrack in a sample of steel can be partly healed by applying electropulsing. The healing can be finished within a very short duration. It is not necessary to detect the location, size and shape of crack during the healing process by using electropulsing technique. The original microstructure in the area without crack remains unchanged. Electropulsing is a promising method for further development of a crack healing technique.
Acknowledgements Financial supports by The National Natural Science Foundation of China (No. 90206044) and National Basic Research Development Program Item of China (G1999065009) are acknowledged. References [1] [2] [3] [4] [5]
H. Von Wever, W. Seith, Elektrochem. Z. 59 (1955) 942. C. Bosvieux, J. Friedel, J. Phys. Chem. Solids 23 (1962) 123. P.S. Ho, T. Kwok, Rep. Prog. Phys. 52 (1989) 301. A.F. Sprecher, S.L. Mannan, H. Conrad, Acta Metall. 34 (1986) 1145. H. Conrad, A.F. Sprecher, in: F.R.N. Nabarro (Ed.), Dislocations in Solids, Elsevier, Amsterdam, 1989, p. 497. [6] A.K. Misra, Metall. Trans. 16A (1985) 1354. [7] H. Mizubayashi, S. Okuda, Phys. Rev., B 40 (1989) 8057. [8] Z.H. Lai, H. Conrad, G.Q. Teng, Y.S. Chao, Mater. Sci. Eng. Abstr. 287 (2000) 238.
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[9] K.W. Gao, L.J. Qiao, W.Y. Chu, Scripta Mater. 44 (2001) 1055. [10] X.G. Li, C.F. Dong, H. Chen, W.Y. Chu, Acta Metall. Sin. 37 (2001) 1093. [11] D.B. Wei, J.T. Han, J.X. Xie, C.G. Fu, L.Z. Wang, Y.X. He, Acta Metall. Sin. 36 (2000) 622. [12] D.B. Wei, J.T. Han, J.X. Xie, C.G. Fu, L.Z. Wang, Y.X. He, Acta Metall. Sin. 36 (2000) 713. [13] Y.Z. Zhou, Y. Zeng, G.H. He, B.L. Zhou, J. Mater. Res. 16 (2001) 17.
[14] D. Halliday, R. Resnick, Physics, John Wiley and Sons, New York, 1978, p. 659. [15] J.R. Groza, M. Garcia, J.A. Schneider, J. Mater. Res. 16 (2001) 286. [16] J.R. Groza, A. Zavaliangos, Mater. Sci. Eng. Abstr. 287 (2000) 171. [17] S.H. Risbud, J.R. Groza, M.J. Kim, Phil. Mag. B 69 (1994) 525. [18] Z.A. Munir, Mater. Sci. Eng. Abstr. 287 (2000) 125. [19] Y.Z. Zhou, R.S. Qin, S.H. Xiao, G.H. He, B.L. Zhou, J. Mater. Res. 15 (2000) 1056.
Crack healing in a steel by using electropulsing technique Yizhou Zhou a,*, Jingdong Guo b, Ming Gao, Guanhu He b a
WTM Institute, Department of Materials Science, University of Erlangen-Nu¨rnberg, Martensstrasse 5, D-91058 Erlangen, Germany b Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China Received 15 July 2003; received in revised form 9 October 2003; accepted 16 October 2003
Abstract Samples of steels with precracks were treated by electropulsing. As a result of electropulsing, the crack is partly healed. The healing process occurs with a very short duration pulse and the original microstructure in the area without a crack was maintained during the ‘‘healing’’ treatment. D 2004 Elsevier B.V. All rights reserved. Keywords: Metals and alloys; Defects; Crack; Healing; Microstructure; Electropulsing
1. Introduction In the past 50 years, research has indicated that an electric current can influence the behavior of materials, such as electromigration [1– 3], electroplasticity [4,5], the solidification of metals and alloys [6], the structural relaxation in amorphous solids and the amorphous crystallization [7,8], and so on. Although these studies have been fruitful, they have seldom been concerned with crack healing in metals as affected by electropulsing. Some recent experiments have shown that a crack in a metallic material can be healed in a high temperature environment [9 – 12]. However, it is very clear that the original microstructure and performances of the material will be changed under high temperature, which is undesirable in some cases. In our previous work [13], we found that a quench crack in a steel could be partly healed by electropulsing treatment, and the method has the important characteristic that the effect of healing on the uninjured part is very small. Thus, electropulsing is a promising tool for crack healing. In the previous work [13], the aim of using a quench crack was to show the above-mentioned important characteristic of electropulsing technique by the fact that the martensite in steel
* Corresponding author. Tel.: +49-9131-8527523; fax: +49-91318527515. E-mail address: [email protected] (Y. Zhou). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2003.10.049
samples could remain unchanged after a healing. However, the quench crack also has some shortcomings for the study about crack healing. For example, the position and the number of cracks are hard to control, and it is difficult to judge if a quench crack runs through thickness of sample completely. Sometimes, the second shortcoming is very detrimental for the study. It may lead one to think that the crack is partly healed during the observation after a healing treatment. In the present work, a different method is employed to obtain crack, the above-mentioned shortcoming about quench crack can be avoided by the method.
2. Experimental The material used in the present investigation was a medium carbon steel, having a composition (in wt.%) of 0.46 C, 0.05 S, 0.045 P, 0.30 Si and 0.60 Mn. The steel was annealed at 950 jC for 15 min and then cooled in air. Fig. 1 shows the schematic illustration for the preparation of samples with precracks through the thickness of samples. The annealed steel was machined to a column sample of 19 mm in diameter and 45 mm in height. A hole of 2 mm in diameter was drilled through the diameter of the column sample in its middlemost part. After cleaning out the machining oil in the hole, the column sample was pressed into the shape of a drum sample of 22 mm in the maximum diameter and 35 mm in height by a pressure machine under ambient conditions. After pressing, the hole through diam-
Y. Zhou et al. / Materials Letters 58 (2004) 1732–1736
Fig. 1. Schematic illustration for the preparation of through thickness precracked samples.
eter was changed to a crack through diameter. Sheet samples with precracks were cut from the drum sample along its axial plane, and the size of the sheet samples was 1 mm thick, 12 mm wide and 35 mm long. The precrack was in the middlemost part and through thickness of each sheet sample. Before the healing (electropulsing) treatment, one sample was polished and etched to examine the microstructure and two samples were just polished to examine the morphology of crack by optical microscope. After recording the morphology of crack, electropulsing was applied to the two polished samples with precracks under ambient conditions, by capacitor banks discharge. Each sample was only treated once by one electropulsing. The waveform of electropulsing was detected to be a damped oscillation wave. The pulse duration is about 800 As, the period of electropulsing tp = 130 As and the maximum current density jm = 2.4 kA/ mm2. The temperature of the matrix due to Joule heating was measured to be about 40 jC by a K-type thermocouple (diameter, 80 Am) soldered to a sample at the part without crack. The surfaces of the two samples were polished lightly to examine the morphology of crack after electropulsing by optical microscope. Subsequently, samples were etched to examine the microstructure by optical microscope and JSM6301F (JEOL, Tokyo, Japan) scanning electron microscopy (SEM). The chemical composition of micro-areas in samples was determined by the electron probe and the microhardness of samples was measured.
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from the figure that the crack is healed at the area close to the crack tip but not at the crack tip. The length of the healed area is about 350 Am. The microstructure in the healed area is different from that in the matrix and the black pearlite cannot be seen in the healed area. Fig. 3(b) shows a SEM micrograph of a part at the healed area, the bright microstructure in the figure is pearlite. The newly formed grains cover the original crack according to Fig. 3(b), although there are some little holes in the area, which means that the area of crack is effectively healed by electropulsing treatment. The hardness of the microstructure marked by M in Fig. 3(b) is very high, HV = 644, which is the hardness of martensite. However, the hardness of the microstructure marked by F in Fig. 3(b) is not high, HV = 252, which is a little higher than the hardness of ferrite in the matrix (HV = 180). The analyses of electron probe show that the carbon content in the microstructure marked by M is high, 0.589 wt.%, and it is close to the corresponding value of the pearlite in the matrix, 0.613 wt.%; the carbon content in the microstructure marked by F is low, 0.221 wt.%, and it is close to the corresponding value of the ferrite in the matrix, 0.197 wt.%. According to these experimental results, it is concluded that the microstructure marked by M is martensite and the microstructure marked by F is ferrite. In addition, microstructure observations show that the microstructure of the matrix (the part without crack) has no change after electropulsing. The reason is that the temperature of the matrix is low, which is
3. Results As a result of electropulsing, the crack is partly healed at the areas close to the crack tips. Fig. 2 shows the optical morphology of the left portion of a crack before and after electropulsing treatment. One can see that before electropulsing the crack is continuous in Fig. 2(a). After electropulsing, the crack is partly healed at the area marked by arrow A in Fig. 2(b). The optical micrograph of the microstructure in the healed area is shown in Fig. 3(a). It is clear
Fig. 2. Optical micrograph of the left portion of a crack before (a) and after (b) electropulsing.
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Fig. 3. Optical micrograph (a) and SEM micrograph (b) of a crack partly healed by electropulsing.
a distinguishing characteristic of electropulsing compared with the other healing methods [9 –12].
potential surfaces around the crack. Fig. 4 shows a schematic illustration for the distribution of the current lines (dashed lines) and the potential surfaces (solid lines) around a center crack. It is obvious from the figure that the current would flow turning around the crack tip, if the current could not flow through the crack. Since the equipotential surfaces are at right angles to the current lines, they are deformed to be arc surfaces around the crack. Arc line ABC and arc line DEF are two different equipotential surfaces in the figure. It can be seen that the distance between the two equipotential surfaces is small at the middle part of crack and large at the crack tip. Since the electric field E is the ratio between of the electrical potential and the distance, E is high at the middle part of crack and low at the crack tip. Thus, the crack is hardly broken down at its tip because of the low local E, although the width of crack is smallest there. Apart from the effect of electric field E, the width of crack is another important factor for breakdown of crack. For the cracks prepared in the present work, because the width of crack is always the widest at the middle part of crack, the crack is hardly broken down at that part, although E is the highest there. In other words, the breakdown should occur in the weakest part of crack, in the present study the crack tip and the middle part of crack are not the weakest part for breakdown by the combination of electric field E and the width of crack, while the area close to the crack tip seems to be the weakest part. The distribution of the current lines and potential surfaces around a crack will change because of breakdown; namely, the current does not flow turning around the crack tip, because the current can flow through the crack in the part of breakdown, and then the current density at crack tip decreases and the temperature due to Joule heating is not so high to cause the microstructure change. In the case without breakdown, since the current cannot flow through the crack, the current has to flow turning around the crack tip, and then the current density at crack tip increases significantly. When the current density is excessively high,
4. Discussion The mechanism about the crack healing caused by electropulsing is not very clear now, but we think that the possible reasons are the generation of low-temperature plasma and the motion of effective atoms toward the crack. The generation of low-temperature plasma depends on the electrical breakdown. We know that the breakdown voltage of air is about 800 V/mm [14]. In this case, if a typical width (1 Am) of the crack is considered, the breakdown of the crack can occur provided that the voltage difference between the two surfaces of the crack is 0.8 V. For the processes of applying high current density electropulsing, it is possible that the voltage difference between the two surfaces of the crack is more than 0.8 V, so the breakdown of the crack is possible here. However, the position of breakdown is related to the shape of crack and the deformation of electrical
Fig. 4. Schematic illustration for the distribution of the current lines (dashed lines) and the potential surfaces (solid lines) around a center crack.
Y. Zhou et al. / Materials Letters 58 (2004) 1732–1736
Fig. 5. SEM micrograph of a solidified microstructure after local melting, which is distinctly different from that in Fig. 3. From Zhou et al. [13].
local melting occurs at the crack tip due to Joule heating. However, the local melting at crack tip cannot lead to the healing of crack tip, see Ref. [13]. The solidified microstructure after local melting at the crack tip is shown in Fig. 5 for comparison with the microstructure in Fig. 3, which is distinctly different [13]. The characteristics of melting cannot be found in the microstructure in the healed part in Fig. 3. Based on the difference, we think a possible low-temperature plasma state may be created in the healed area because of electrical breakdown and the generation of plasma is necessary for crack healing, similar to electrical discharge compaction and field-activated sintering, which indicated that a good grain-to-grain contact could be achieved by plasma in the case of no melting [15 –18]. Due to the generation of plasma, the local temperature at the area can be high. Thus, the microstructure in the area may be different from the microstructure in the matrix. And we think this is the reason for the microstructure change in the healed part. The course of the microstructure change in the healed part is proposed to be as follows. During the passing of electropulsing, the original microstructure transformed to austenite due to the high local temperature, but the chemical composition of austenite was not uniform, since there was no significant long-range diffusion within such short treating duration when the current density of electropulsing was not so high. (The long-range diffusion of atoms is significant, when the current density of electropulsing is high, even with a very short treating duration [19].) Thus, the carbon content in the areas transformed from pearlite was high and the carbon content in the areas transformed from ferrite was low. During the cooling after electropulsing, the cooling rate of austenite in the healed area was very high due to the cooling effect of the metallic matrix, then the high carbon austenite from initial pearlite would transform to martensite, but the low carbon austenite from initial ferrite would transform to ferrite due to a low carbon content. And because of the rapid cooling, the hardness of ferrite in the healed part is higher than that in the matrix.
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Apart from the generation of plasma, it is another necessary factor for crack healing that the effective atoms move into the crack. Here, the way that the effective atoms move into the crack depends on the thermal expansion resulting from the following two aspects. (a) At the area that the current flows through the crack, the thermal expansion of the material is large due to the high local temperature. Since it is restricted by the cool matrix, the direction of expansion is to the inside of crack. (b) The sample was put into two fixed copper electrodes during the electropulsing treatment; the expansion of sample is not free, so the thermal expansion of matrix due to Joule heating also can cause the effective atoms to move into the crack. When the crack face separation distance is small enough due to the thermal expansion, the crack can be healed in combination with the effect of plasma. In brief, the generation of plasma and the motion of effective atoms toward crack are the possible reasons for crack healing. The generation of plasma results from electrical breakdown. For the crack in the present study, the area close to the crack tip is the weakest part for breakdown; therefore, the crack is healed in part but not at the crack tip. The motion of effective atoms toward crack depends on the thermal expansion.
5. Conclusions A through thickness precrack in a sample of steel can be partly healed by applying electropulsing. The healing can be finished within a very short duration. It is not necessary to detect the location, size and shape of crack during the healing process by using electropulsing technique. The original microstructure in the area without crack remains unchanged. Electropulsing is a promising method for further development of a crack healing technique.
Acknowledgements Financial supports by The National Natural Science Foundation of China (No. 90206044) and National Basic Research Development Program Item of China (G1999065009) are acknowledged. References [1] [2] [3] [4] [5]
H. Von Wever, W. Seith, Elektrochem. Z. 59 (1955) 942. C. Bosvieux, J. Friedel, J. Phys. Chem. Solids 23 (1962) 123. P.S. Ho, T. Kwok, Rep. Prog. Phys. 52 (1989) 301. A.F. Sprecher, S.L. Mannan, H. Conrad, Acta Metall. 34 (1986) 1145. H. Conrad, A.F. Sprecher, in: F.R.N. Nabarro (Ed.), Dislocations in Solids, Elsevier, Amsterdam, 1989, p. 497. [6] A.K. Misra, Metall. Trans. 16A (1985) 1354. [7] H. Mizubayashi, S. Okuda, Phys. Rev., B 40 (1989) 8057. [8] Z.H. Lai, H. Conrad, G.Q. Teng, Y.S. Chao, Mater. Sci. Eng. Abstr. 287 (2000) 238.
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[9] K.W. Gao, L.J. Qiao, W.Y. Chu, Scripta Mater. 44 (2001) 1055. [10] X.G. Li, C.F. Dong, H. Chen, W.Y. Chu, Acta Metall. Sin. 37 (2001) 1093. [11] D.B. Wei, J.T. Han, J.X. Xie, C.G. Fu, L.Z. Wang, Y.X. He, Acta Metall. Sin. 36 (2000) 622. [12] D.B. Wei, J.T. Han, J.X. Xie, C.G. Fu, L.Z. Wang, Y.X. He, Acta Metall. Sin. 36 (2000) 713. [13] Y.Z. Zhou, Y. Zeng, G.H. He, B.L. Zhou, J. Mater. Res. 16 (2001) 17.
[14] D. Halliday, R. Resnick, Physics, John Wiley and Sons, New York, 1978, p. 659. [15] J.R. Groza, M. Garcia, J.A. Schneider, J. Mater. Res. 16 (2001) 286. [16] J.R. Groza, A. Zavaliangos, Mater. Sci. Eng. Abstr. 287 (2000) 171. [17] S.H. Risbud, J.R. Groza, M.J. Kim, Phil. Mag. B 69 (1994) 525. [18] Z.A. Munir, Mater. Sci. Eng. Abstr. 287 (2000) 125. [19] Y.Z. Zhou, R.S. Qin, S.H. Xiao, G.H. He, B.L. Zhou, J. Mater. Res. 15 (2000) 1056.