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A novel crack healing technique in a low carbon steel by cyclic phase transformation heat treatment: The process and mechanism
Materials Science & Engineering A xxx (xxxx) xxx
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea
A novel crack healing technique in a low carbon steel by cyclic phase transformation heat treatment: The process and mechanism Meng He, Zheng Zhentai *, Fen Shi, Donghui Guo, Jinling Yu School of Materials Science and Engineering, Hebei University of Technology, No. 8, Guangrongdao Road, Hongqiao District, Tianjin, 300130, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Crack healing Cyclic phase transformation Healing mechanism Diffusion Recrystallization
Crack is one of the most important factors resulting in the failure of metallic material which may lead to huge engineering losses and even severe accidents. Not only in economically, but also in environmental protection, it is highly meaningful to heal cracks in metallic materials. However, the currently common healing techniques are subject to various disadvantage. Therefor we developed a novel crack healing technique by means of cyclic phase transformation heat treatment. The results show that cyclic phase heat treatment technique can heal internal crack. The process of internal crack healing can be divided into three stages, atomic diffusion or recrystallization is the mechanisms of crack healing. In the first stage of the healing process, crack healing depends on atomic diffusion and recrystallization. However, in other stages of the healing process, atomic diffusion controls the crack healing. Compared to isothermal heat treatment technique, this novel technique can avoid coarsening grains of the matrix effectively, and improve the healing efficiency and healing degree by accelerating atomic diffusion.
1. Introduction As is known, cracks in metals are not avertible during the process of use and manufacturing, which will severely deteriorate the mechanical properties and decrease the service life of these structures. And those defects even will result in the fracture which will cause serious safety accidents. If the cracks in metals can be healed, the service life and the mechanical properties of these metallic materials will be enhanced. This means that the crack healing can save energy and have huge economic and ecological benefits. Hence, the crack healing has become an indis pensable section in remanufacturing engineering and researchers have paid extensive attention of to crack healing. In the previous researches, several ways were used to heal the cracks in metal. And those methods can be divided into two types, material supply and energy supply. Li [1]et al. applied the gas nitrocarburizing technique to heal surface crack of metallic materials, and they found the crack healed gradually as the nitrides formed and grew. Zheng [2,3] et al. used the electro-chemical technique to heal the surface crack in nickel plates successfully. However, those material supply methods can heal the surface crack only and cannot heal internal crack. The internal crack in metals has to be healed by energy supply method.
The common energy supply methods include isothermal heat treat ment technique, thermal mechanical coupling technique and electropulsing technique. There is no doubt that the most effective method to heal crack is thermal mechanical coupling technique. Yu [4] et al. studied that the regular of internal crack healing in specimens under thermal mechanical coupling technique, and found the degree of crack healing increases with decreasing deformation passes and strain rate, and with increasing reduction ratio and heating temperature. Xin [5] et al. found hot pressure which is perpendicular to the crack faces can prevent grain coarsening of the base metal and accelerate the crack healing process. However, thermal mechanical coupling technique will make workpieces deformed. While workpieces cannot be further deformed after their forming, these cracks cannot be healed by thermal mechanical coupling technique. So, some researchers took electro-pulsing technique and isothermal heat treatment technique for crack healing in metals. Satapathy [6]et al. found that cavities and voids developed at the tip of the crack when the current exceeded the critical value. Hosoi [7,8]et al. reported that the process of the fatigue crack healing under high density electric current, and they found that the bridges were formed between the surfaces of the crack near the crack tip. Zhou [9,10]et al. studied the crack healing of the medium carbon steel
* Corresponding author. E-mail addresses: [email protected] (M. He), [email protected] (Z. Zhentai), [email protected] (F. Shi), [email protected] (D. Guo), 1716323540@ qq.com (J. Yu). https://doi.org/10.1016/j.msea.2019.138712 Received 13 September 2019; Received in revised form 16 November 2019; Accepted 18 November 2019 Available online 20 November 2019 0921-5093/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Meng He, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2019.138712
M. He et al.
Materials Science & Engineering A xxx (xxxx) xxx
by electro-pulsing technique, and found cracks were partly healed in very limited spans of time. Yu [11]et al. have analyzed the effect of multipass electro-pulsing treatment on healing crack in SUS304 stainless steel. Furthermore, they found that both sides of the crack had formed continuous healed regions after seven times of electro-pulsing treat ment. However, the electro-pulsing treatment can not heal the speci mens which have complex geometries and it is unclear that the microstruture of the healing region even if the cracks have been healed. So, isothermal heat treatment technique may be seen as the best way to heal crack. Song [12]et al. have reported that the healing behavior of submicron-scale voids implanted in a cold-rolled Al–Mg–Er alloy by in situ transmission, and found lattice diffusion played a leading role in healing the crack. Gao [13]et al. used the molecular dynamics way to simulate crack healing in Cu crystal by high temperature, and found the orientation of the crack plane determined the critical temperature necessary for a crack to heal. Zhang [14]et al. has presented the microstructure of 20 steel is represented primarily by ferrite in crack healing zone, and the ferrite in the region of healing of crack has higher hardness compared with the matrix metal. Xin [15–17]et al. have analyzed restoration of mechanical properties, mechanisms of crack healing and the microstructure in healing zone under high temperatures, and found atomic diffusion and recrystallization dominated the entire healing process. Although isothermal heat treatment healing technique has been used widely, it often needs higher temperatures and longer durations which will deteriorate the microstructure and properties of the matrix. Obviously, isothermal heat treatment technique also has shortcomings. Hence, we propose the cyclic phase transformation heat treatment to heal the cracks in metallic materials. And this technique not
only keeps the advantages of isothermal heat treatment technique, but also overcomes the shortcomings of it. In this study, a novel technique which was named as cyclic phase transformation heat treatment technique has been attempted to heal the cracks in a low carbon steel. The morphology evolution and micro structures of the crack healing zone were analyzed and the healing mechanism was discussed. The effect of healing was evaluated by the recovery of mechanical properties. In addition, the advantages and reasons for those advantages of the cyclic phase transformation heat treatment technique compared with the isothermal heat treatment were analyzed. 2. Experimental procedure 2.1. Specimen preparation A low carbon steel was used in this experiment, and the chemical composition (in wt%) of this low carbon steel is C 0.13, Si 0.28, Mn 0.55, P 0.008, and Fe balance. A drilling and compression method was adopted to introduce internal cracks into the specimens. As schemati cally shown in Fig. 1, Firstly, cylindrical specimens with height of 15 mm and diameter of 35 mm were machined. And electric discharge machining (EDM) was used to slice a through hole with a diameter of 2 mm which drilled through the cylinder in axial center along the radial direction. Ethanol and via acetone were used to clean up scraps inside the holes. Then, the specimens were compressed gently into a drum shape with 30 mm in height though universal testing machine (WDW300, Changchun, China) with a compressing rate of 0.5 mm/min under
Fig. 1. Schematic illustration of specimen preparation (unit in mm). 2
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ambient conditions. The holes were compressed as planes. Finally, the planes were closed by laser welding, which would be seen as internal cracks. Slices were cut into 1 mm from the drum specimen along its longitudinal axis and perpendicular to the original through hole, and tensile specimen were cut from the slices by using EDM.
3. Results and discussion 3.1. Pre-crack morphology As can be seen from Fig. 3 the internal pre-crack was in the center of specimen and typical morphology. The long pre-crack surface was rather irregular as a result of the machining process. The width of pre-crack decreased gradually from center to both sides of the crack. The width of the crack at the length range of 1 mm in the center of the pre-crack was about 2.5 μm.
2.2. Crack healing experiment The cyclic phase transformation heat treatment experiments were made in the Gleeble1500-D thermo-mechanical simulator. The Ni–Cr and Ni–Al thermo-electrical couples that were welded in the steel sur face about 1 mm away from the crack were used to control and monitor the healing temperature. The specimens were exposed to cyclic phase transformation heat treatment for a differing number of cycles; viz. 5-cy cles (7.5min), 10-cycles (15min), 20-cycles (30min), and 30-cycles (45min). Each cycle included that the specimens were cooled down from 950 � C to 650 � C at a cooling rate of 6 � C/s and held 5s, followed by heated to 950 � C at a heating rate of 10 � C/s and held 5s. The cyclic phase transformation heat treatment curves are shown in Fig. 2. After cyclic phase transformation heat treatment, specimens were air-cooled to room temperature. Besides, in order to prove the superiority of the cyclic phase transformation heat treatment, the isothermal heat treat ment experiments were carried out for comparison. The isothermal heat treatment experiments were also conducted using Gleeblle1500-D sys tem and the following parameters: holding temperature ¼ 950 � C, holding time ¼ 45 or 90min, the rate of heat ¼ 10 � C/s.
3.2. Morphology evolution of crack healing zone SEM micrographs of crack healing zone for different cycle numbers and different healing methods were shown in Fig. 4. The microstructure consists mainly of ferrite in the internal crack healing zone. Obviously, the original long crack had been segmented into short cracks after 5 cycles of phase transformation heat treatment, as shown in Fig. 4(a). And some bulges were formed in the middle position of short crack surface. This means some short cracks had tendency of further seg mentation. At the same time, the short crack surfaces tended to be smooth. As can be seen from Fig. 4(b), when the cycles increase to 10 numbers, the short cracks had gradually shrank and spheroidized to elliptic void and circular void, and those voids presented in the position of the original long crack. Fig. 4(c) showed that the short cracks dis appeared completely. Meanwhile, the sizes and numbers of the elliptic and circular void decreased. And the residual voids presented linear distribution along the longitudinal direction of the original long crack. As can be seen from Fig. 4(d), residual voids almost disappeared completely, and microstructural morphology in the healing zone resembled that in the matrix. Some grains had through the location of the original long crack at the same time. This suggested that the crack had been healed. It is obvious by comparing Fig. 4(d) and (e) that the level of the crack healing under cyclic phase transformation heat treatment is higher than the degree of crack healing by isothermal heat treatment for same healing time. Only when the healing time increased to 90min, could the original crack be healed under isothermal heat treatment, as can be seen from Fig. 4(f). This indicates that cyclic heat treatment has better effect on healing crack than isothermal heat treatment. And the particular of related mechanism is analyzed in Sec tion 3.4.2.
2.3. Morphology observation and mechanical property tests To test the crack healing effects, the sections were ground using progressively finer grades of SiC papers, and polished by buffing with diamond powder and ion beam. Subsequently, the sections were ultra sonically cleaned, and finally the ferrite grain boundaries were etching by using alcohol solution containing 4% of nitric acid (volume fraction). Morphologies of the crack and matrix both before and after healing were observed on metallographic microscope, and ferrite grain sizes of the matrix were measured by the line intercept method on account of Standard GB/T6394-2002. The morphological evolution was observed by Gemini scanning electron microscope (SEM). The Electron Back scattered Diffraction (EBSD) measurements were also applied for the healing specimen. At room temperature, Instron 5848 Micro Tester was used to perform tensile tests at the strain rate of 0.5 mm/min. At least 3 specimens were tested for each condition.
3.3. Microstructural of the matrix Fig. 5 presents that the microstructural of the matrix under different healing methods. The microstructure of the matrix mainly consists of
Fig. 2. Cyclic phase transformation heat treatment plots.
Fig. 3. Optical microscope images of the internal pre-crack. 3
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Fig. 4. SEM micrographs of crack healing zone for different cycle numbers and healing methods. (a)–(d) cyclic phase transformation heat treatment for different cycle numbers (a) 5 cycles (7.5min); (b) 10 cycles (15min); (c) 20 cycles (30min); (d) 30 cycles (45min); (e) isothermal heat treatment for 45min; (f) isothermal heat treatment for 90min.
ferrite. As exhibited in Fig. 5(a), prior average grain size of the matrix without heat treatment was about 18.64 μm. After cyclic phase trans formation heat treatment for 30 cycles (45min), the average grain size of the matrix had decreased to about 15.95 μm, as exhibited in Fig. 5(b). After isothermal heat treatment for 45min, the average grain size of the matrix had increased to about 23.82 μm, as exhibited in Fig. 5(c). Obviously, the average grain sizes of the matrix increased markedly after isothermal heat. However, the grains were refined after cyclic heat treatment. This observation can be explained through the following
theories. Because of the shorter time of the cyclic phase transformation heat treatment staying at high temperature level, the austenite grains had no enough time to grow. And the cementite dissolution remains incomplete at short high temperature holding time, undissolved cementite embedded in austenite hinder the growth of austenite grains [18]. Furthermore, the dislocations generated during cyclic phase transformation heat treatment could increase the nucleation rate of austenite. All of those resulted in formation of the fine austenite crystal grains. As we all know, the size of austenite grains can determine the 4
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Fig. 5. Microstructural of the matrix for different healing methods. (a) without heat treatment; (b) cyclic phase transformation heat treatment for 45min (30 cycles); (c) isothermal heat treatment for 45min.
ferrite grain size. The fine ferrite grains evolve from these fine austenite grains. So, the average grain size of the matrix under cyclic phase transformation heat treatment is the smallest. The smaller the grain size is, the greater the Mechanical properties of metallic material.
crack almost completely disappeared, and residual voids presented along the original long crack. And the microstructural morphology of crack healing zone was similar to that of the matrix. As shown in Fig. 7 (b), some grains grew through the original long crack, and new grain boundaries presented in the center of the crack healing zone. Obviously, both cyclic phase transformation heat treatment and isothermal heat treatment can heal crack. However, the time that cyclic phase trans formation heat treatment need is less than isothermal heat treatment. The reason of this phenomenon is that cyclic phase transformation heat treatment can accelerate the diffusion of atom. As can be seen from Fig. 5, the grains of matrix were refined after cyclic phase trans formation heat treatment. With the decrease of the size of grain, the grain boundary area increases gradually. Furthermore, cyclic phase transformation heat treatment could introduce dislocations into the matrix [18,19]. It is well known that grain boundary and dislocation are the great diffusion channel. In addition, cyclic phase transformation heat treatment results in a non-equilibrium state which reduces the diffusion activation energy of atoms [20]. So, cyclic phase trans formation heat treatment can accelerate the diffusion of atom, thus resulting in accelerated crack healing.
3.4. Mechanism analysis 3.4.1. Healing mechanism The diffusion microvoids of the crack healing zone under different cycle numbers are shown in Fig. 6. It shown that all specimen had an obvious diffusion microvoids region in the crack healing zone after cy clic phase transformation heat treatment. However, the position and size of the diffusion microvoids region in specimen for different cycles are different. As shown in Fig. 6(a), the diffusion microvoids region located in the matrix on the side of crack, whose width was about 11.6 μm. And partial healing region had no microvoid. With the cycles reaching to 20 numbers, as shown in Fig. 6(b), both matrix on the side of crack and crack healing zone had microvoids, and the width of the microvoids region decreased to about 7.5 μm. When the cycle raised to 30 numbers, the width of microvoids region had further decreased to about 3.9 μm, as can be seen from Fig. 6(c). The formation of the diffusion microvoids region indicated that atomic diffusion played an important role throughout crack healing process. As shown in Fig. 4(a), the fine grains were presented in healing zone. With the increase of the cyclic time, as shown in Fig. 4(b)(c)(d), the fine recrystallization grains gradually dis appeared, which was due to the growth of fine recrystallization grains and the annexation of larger matrix grains near the crack. This suggested that recrystallization played an important role in prophase of crack healing. Through the preceding analysis, the crack healing was dominated by atomic diffusion and recrystallization. Recrystallization splits crack into void rapidly. Atomic diffusion supplies material for recrystallization and leads to void healing.
3.5. Mechanical properties test In order to estimate the influences of crack healing on the strength recovery, the tensile tests were conducted. Fig. 8 shows the effects of cycle numbers on the restoration of tensile properties at room temper ature. As can be seen from Fig. 8, the ultimate tensile strength increased with the increase of the cycles. After 30 cycles, the ultimate tensile strength has increased to 525 MPa. However, the increasing rate of the tensile properties has gradually decreased. This means that the healing effect brought by each cyclic heat treatment showed the tendency of gradually weakening with the increase of the cycle numbers. The different mechanisms in different healing period caused this phenome non. Crack healing in prophase relied on recrystallization and atomic diffusion, while the healing mechanism in later period was mainly atomic diffusion. Fig. 9 shows the stress-strain curves and macro profile images of the without crack, cyclic phase transformation heat-treated and isothermal
3.4.2. Accelerating mechanism Fig. 7 shows that EBSD images of the cross-section of the crack healing zone after healing by different healing methods. As can be seen from Fig. 7(a), it is obvious that the crack has healed, the original long
Fig. 6. Diffusion microvoids of the crack healing zone for different cycle numbers. (a) 5 cycles; (b) 20 cycles; (c) 30 cycles. 5
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Fig. 7. Electron backscatter diffraction (EBSD) images of the cross-section of the crack healing zone after healing under different healing methods. (a) cyclic phase transformation heat treatment for 30 cycles (45min); (b) isothermal heat treatment for 90min.
specimens even if isothermal heat-treated and cyclic heat-treated spec imens had same crack healing area, which is related to the gain coars ening (see Fig. 5(b) and (c)). The ultimate tensile strength of specimen without crack is highest, which indicates that whether cyclic heat treatment or isothermal heat treatment can not totally recover the ul timate tensile strength. This is because of residual voids in the position of the original crack (see Fig. 4(d) and (f)). However, when cycle numbers reach to 30 times, the ultimate tensile strength of healing specimens has been recovered to 96% compared with that of specimens without crack. And the healing effect brought by each cyclic heat treatment decreases gradually with the increase of the cycle numbers. So, it is wasteful to increase the ultimate tensile strength of heat-treated specimens to the ultimate tensile strength of specimen without crack even though extending heat-treated time from the viewpoint of engi neering application and saving resources. As can be seen from Fig. 9(b), the fracture plane of the specimen without crack was far away the middle of the specimen. However, the fracture plane of isothermal heattreated specimens was near the original crack which located in the middle of specimens, but the part of the fracture of cyclic heat-treated specimen took place in the matrix, as shown in Fig. 9(c)(d)(e). This indicated that cyclic heat treatment had better effect than isothermal heat treatment on healing crack.
Fig. 8. Ultimate tensile strength for different cycle numbers.
heat-treated specimens. The ultimate tensile strength of cyclic heattreated is higher than that of isothermal heat-treated specimens after healing for 45 min, as shown in Fig. 9(a). This is because the crack healing area under cyclic phase transformation heat treatment is bigger than that by isothermal heat treatment after same healing time (see Fig. 4(d) and (e)). Furthermore, the ultimate tensile strength of isothermal heat-treated is also lower than that of cyclic heat-treated
4. Conclusion (a) A novel technique of crack healing by cyclic phase transformation heat treatment has been attempted. The result suggested that the
Fig. 9. The stress-strain curves and macro profile images of the without crack, cyclic phase transformation heat-treated and isothermal heat-treated specimens. (a) the stress-strain curves; (b) (c) (d) (e) macro profile images for different healing condition. 6
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internal crack in a low carbon steel can be successfully healed by cyclic phase transformation heat treatment. (b) The evolution of crack healing can be divided into three periods: (i) splitting long crack into ellipse voids by formation, growth and contact of bulges on the partial crack surface; (ii) evolution from ellipse crack to circular void; (iii) the decrease of the size and number of circular voids, crack healing. (c) The crack healing mechanisms is atomic diffusion and recrystal lization in the low carbon steel. Atomic diffusion took place throughout the healing process, but recrystallization only happened in first stage of the healing process. (d) The cyclic phase transformation heat treatment technique is su perior to the isothermal heat treatment technique. It can reduce the time of crack healing by accelerating atomic diffusion, pre vent matrix grains from coarsening and increase ultimate tensile strength.
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Author contributions section Meng He: Research concept and design, Writing the article, Collec tion and assembly of data, Date analysis and interpretation. Zhentai Zheng: Research concept and design, Critical revisions of the article. Fen Shi: Collection and assembly of data. Donghui Guo: acquisition of data. Jinling Yu: acquisition of data. Declaration of competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors gratefully acknowledge the financial supported by the Natural Science Foundation of Hebei Province, P. R. China under Grant No. E2017202011. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.msea.2019.138712. References [1] A. Li, X. Chen, C.S. Zhang, G.D. Cui, Hui Zhao, C. Yang, A novel crack healing in steels by gas nitrocarburizing, Appl. Surf. Sci. 442 (2018) 437–445, https://doi. org/10.1016/j.apsusc.2018.02.181.
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Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea
A novel crack healing technique in a low carbon steel by cyclic phase transformation heat treatment: The process and mechanism Meng He, Zheng Zhentai *, Fen Shi, Donghui Guo, Jinling Yu School of Materials Science and Engineering, Hebei University of Technology, No. 8, Guangrongdao Road, Hongqiao District, Tianjin, 300130, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Crack healing Cyclic phase transformation Healing mechanism Diffusion Recrystallization
Crack is one of the most important factors resulting in the failure of metallic material which may lead to huge engineering losses and even severe accidents. Not only in economically, but also in environmental protection, it is highly meaningful to heal cracks in metallic materials. However, the currently common healing techniques are subject to various disadvantage. Therefor we developed a novel crack healing technique by means of cyclic phase transformation heat treatment. The results show that cyclic phase heat treatment technique can heal internal crack. The process of internal crack healing can be divided into three stages, atomic diffusion or recrystallization is the mechanisms of crack healing. In the first stage of the healing process, crack healing depends on atomic diffusion and recrystallization. However, in other stages of the healing process, atomic diffusion controls the crack healing. Compared to isothermal heat treatment technique, this novel technique can avoid coarsening grains of the matrix effectively, and improve the healing efficiency and healing degree by accelerating atomic diffusion.
1. Introduction As is known, cracks in metals are not avertible during the process of use and manufacturing, which will severely deteriorate the mechanical properties and decrease the service life of these structures. And those defects even will result in the fracture which will cause serious safety accidents. If the cracks in metals can be healed, the service life and the mechanical properties of these metallic materials will be enhanced. This means that the crack healing can save energy and have huge economic and ecological benefits. Hence, the crack healing has become an indis pensable section in remanufacturing engineering and researchers have paid extensive attention of to crack healing. In the previous researches, several ways were used to heal the cracks in metal. And those methods can be divided into two types, material supply and energy supply. Li [1]et al. applied the gas nitrocarburizing technique to heal surface crack of metallic materials, and they found the crack healed gradually as the nitrides formed and grew. Zheng [2,3] et al. used the electro-chemical technique to heal the surface crack in nickel plates successfully. However, those material supply methods can heal the surface crack only and cannot heal internal crack. The internal crack in metals has to be healed by energy supply method.
The common energy supply methods include isothermal heat treat ment technique, thermal mechanical coupling technique and electropulsing technique. There is no doubt that the most effective method to heal crack is thermal mechanical coupling technique. Yu [4] et al. studied that the regular of internal crack healing in specimens under thermal mechanical coupling technique, and found the degree of crack healing increases with decreasing deformation passes and strain rate, and with increasing reduction ratio and heating temperature. Xin [5] et al. found hot pressure which is perpendicular to the crack faces can prevent grain coarsening of the base metal and accelerate the crack healing process. However, thermal mechanical coupling technique will make workpieces deformed. While workpieces cannot be further deformed after their forming, these cracks cannot be healed by thermal mechanical coupling technique. So, some researchers took electro-pulsing technique and isothermal heat treatment technique for crack healing in metals. Satapathy [6]et al. found that cavities and voids developed at the tip of the crack when the current exceeded the critical value. Hosoi [7,8]et al. reported that the process of the fatigue crack healing under high density electric current, and they found that the bridges were formed between the surfaces of the crack near the crack tip. Zhou [9,10]et al. studied the crack healing of the medium carbon steel
* Corresponding author. E-mail addresses: [email protected] (M. He), [email protected] (Z. Zhentai), [email protected] (F. Shi), [email protected] (D. Guo), 1716323540@ qq.com (J. Yu). https://doi.org/10.1016/j.msea.2019.138712 Received 13 September 2019; Received in revised form 16 November 2019; Accepted 18 November 2019 Available online 20 November 2019 0921-5093/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Meng He, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2019.138712
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by electro-pulsing technique, and found cracks were partly healed in very limited spans of time. Yu [11]et al. have analyzed the effect of multipass electro-pulsing treatment on healing crack in SUS304 stainless steel. Furthermore, they found that both sides of the crack had formed continuous healed regions after seven times of electro-pulsing treat ment. However, the electro-pulsing treatment can not heal the speci mens which have complex geometries and it is unclear that the microstruture of the healing region even if the cracks have been healed. So, isothermal heat treatment technique may be seen as the best way to heal crack. Song [12]et al. have reported that the healing behavior of submicron-scale voids implanted in a cold-rolled Al–Mg–Er alloy by in situ transmission, and found lattice diffusion played a leading role in healing the crack. Gao [13]et al. used the molecular dynamics way to simulate crack healing in Cu crystal by high temperature, and found the orientation of the crack plane determined the critical temperature necessary for a crack to heal. Zhang [14]et al. has presented the microstructure of 20 steel is represented primarily by ferrite in crack healing zone, and the ferrite in the region of healing of crack has higher hardness compared with the matrix metal. Xin [15–17]et al. have analyzed restoration of mechanical properties, mechanisms of crack healing and the microstructure in healing zone under high temperatures, and found atomic diffusion and recrystallization dominated the entire healing process. Although isothermal heat treatment healing technique has been used widely, it often needs higher temperatures and longer durations which will deteriorate the microstructure and properties of the matrix. Obviously, isothermal heat treatment technique also has shortcomings. Hence, we propose the cyclic phase transformation heat treatment to heal the cracks in metallic materials. And this technique not
only keeps the advantages of isothermal heat treatment technique, but also overcomes the shortcomings of it. In this study, a novel technique which was named as cyclic phase transformation heat treatment technique has been attempted to heal the cracks in a low carbon steel. The morphology evolution and micro structures of the crack healing zone were analyzed and the healing mechanism was discussed. The effect of healing was evaluated by the recovery of mechanical properties. In addition, the advantages and reasons for those advantages of the cyclic phase transformation heat treatment technique compared with the isothermal heat treatment were analyzed. 2. Experimental procedure 2.1. Specimen preparation A low carbon steel was used in this experiment, and the chemical composition (in wt%) of this low carbon steel is C 0.13, Si 0.28, Mn 0.55, P 0.008, and Fe balance. A drilling and compression method was adopted to introduce internal cracks into the specimens. As schemati cally shown in Fig. 1, Firstly, cylindrical specimens with height of 15 mm and diameter of 35 mm were machined. And electric discharge machining (EDM) was used to slice a through hole with a diameter of 2 mm which drilled through the cylinder in axial center along the radial direction. Ethanol and via acetone were used to clean up scraps inside the holes. Then, the specimens were compressed gently into a drum shape with 30 mm in height though universal testing machine (WDW300, Changchun, China) with a compressing rate of 0.5 mm/min under
Fig. 1. Schematic illustration of specimen preparation (unit in mm). 2
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ambient conditions. The holes were compressed as planes. Finally, the planes were closed by laser welding, which would be seen as internal cracks. Slices were cut into 1 mm from the drum specimen along its longitudinal axis and perpendicular to the original through hole, and tensile specimen were cut from the slices by using EDM.
3. Results and discussion 3.1. Pre-crack morphology As can be seen from Fig. 3 the internal pre-crack was in the center of specimen and typical morphology. The long pre-crack surface was rather irregular as a result of the machining process. The width of pre-crack decreased gradually from center to both sides of the crack. The width of the crack at the length range of 1 mm in the center of the pre-crack was about 2.5 μm.
2.2. Crack healing experiment The cyclic phase transformation heat treatment experiments were made in the Gleeble1500-D thermo-mechanical simulator. The Ni–Cr and Ni–Al thermo-electrical couples that were welded in the steel sur face about 1 mm away from the crack were used to control and monitor the healing temperature. The specimens were exposed to cyclic phase transformation heat treatment for a differing number of cycles; viz. 5-cy cles (7.5min), 10-cycles (15min), 20-cycles (30min), and 30-cycles (45min). Each cycle included that the specimens were cooled down from 950 � C to 650 � C at a cooling rate of 6 � C/s and held 5s, followed by heated to 950 � C at a heating rate of 10 � C/s and held 5s. The cyclic phase transformation heat treatment curves are shown in Fig. 2. After cyclic phase transformation heat treatment, specimens were air-cooled to room temperature. Besides, in order to prove the superiority of the cyclic phase transformation heat treatment, the isothermal heat treat ment experiments were carried out for comparison. The isothermal heat treatment experiments were also conducted using Gleeblle1500-D sys tem and the following parameters: holding temperature ¼ 950 � C, holding time ¼ 45 or 90min, the rate of heat ¼ 10 � C/s.
3.2. Morphology evolution of crack healing zone SEM micrographs of crack healing zone for different cycle numbers and different healing methods were shown in Fig. 4. The microstructure consists mainly of ferrite in the internal crack healing zone. Obviously, the original long crack had been segmented into short cracks after 5 cycles of phase transformation heat treatment, as shown in Fig. 4(a). And some bulges were formed in the middle position of short crack surface. This means some short cracks had tendency of further seg mentation. At the same time, the short crack surfaces tended to be smooth. As can be seen from Fig. 4(b), when the cycles increase to 10 numbers, the short cracks had gradually shrank and spheroidized to elliptic void and circular void, and those voids presented in the position of the original long crack. Fig. 4(c) showed that the short cracks dis appeared completely. Meanwhile, the sizes and numbers of the elliptic and circular void decreased. And the residual voids presented linear distribution along the longitudinal direction of the original long crack. As can be seen from Fig. 4(d), residual voids almost disappeared completely, and microstructural morphology in the healing zone resembled that in the matrix. Some grains had through the location of the original long crack at the same time. This suggested that the crack had been healed. It is obvious by comparing Fig. 4(d) and (e) that the level of the crack healing under cyclic phase transformation heat treatment is higher than the degree of crack healing by isothermal heat treatment for same healing time. Only when the healing time increased to 90min, could the original crack be healed under isothermal heat treatment, as can be seen from Fig. 4(f). This indicates that cyclic heat treatment has better effect on healing crack than isothermal heat treatment. And the particular of related mechanism is analyzed in Sec tion 3.4.2.
2.3. Morphology observation and mechanical property tests To test the crack healing effects, the sections were ground using progressively finer grades of SiC papers, and polished by buffing with diamond powder and ion beam. Subsequently, the sections were ultra sonically cleaned, and finally the ferrite grain boundaries were etching by using alcohol solution containing 4% of nitric acid (volume fraction). Morphologies of the crack and matrix both before and after healing were observed on metallographic microscope, and ferrite grain sizes of the matrix were measured by the line intercept method on account of Standard GB/T6394-2002. The morphological evolution was observed by Gemini scanning electron microscope (SEM). The Electron Back scattered Diffraction (EBSD) measurements were also applied for the healing specimen. At room temperature, Instron 5848 Micro Tester was used to perform tensile tests at the strain rate of 0.5 mm/min. At least 3 specimens were tested for each condition.
3.3. Microstructural of the matrix Fig. 5 presents that the microstructural of the matrix under different healing methods. The microstructure of the matrix mainly consists of
Fig. 2. Cyclic phase transformation heat treatment plots.
Fig. 3. Optical microscope images of the internal pre-crack. 3
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Fig. 4. SEM micrographs of crack healing zone for different cycle numbers and healing methods. (a)–(d) cyclic phase transformation heat treatment for different cycle numbers (a) 5 cycles (7.5min); (b) 10 cycles (15min); (c) 20 cycles (30min); (d) 30 cycles (45min); (e) isothermal heat treatment for 45min; (f) isothermal heat treatment for 90min.
ferrite. As exhibited in Fig. 5(a), prior average grain size of the matrix without heat treatment was about 18.64 μm. After cyclic phase trans formation heat treatment for 30 cycles (45min), the average grain size of the matrix had decreased to about 15.95 μm, as exhibited in Fig. 5(b). After isothermal heat treatment for 45min, the average grain size of the matrix had increased to about 23.82 μm, as exhibited in Fig. 5(c). Obviously, the average grain sizes of the matrix increased markedly after isothermal heat. However, the grains were refined after cyclic heat treatment. This observation can be explained through the following
theories. Because of the shorter time of the cyclic phase transformation heat treatment staying at high temperature level, the austenite grains had no enough time to grow. And the cementite dissolution remains incomplete at short high temperature holding time, undissolved cementite embedded in austenite hinder the growth of austenite grains [18]. Furthermore, the dislocations generated during cyclic phase transformation heat treatment could increase the nucleation rate of austenite. All of those resulted in formation of the fine austenite crystal grains. As we all know, the size of austenite grains can determine the 4
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Fig. 5. Microstructural of the matrix for different healing methods. (a) without heat treatment; (b) cyclic phase transformation heat treatment for 45min (30 cycles); (c) isothermal heat treatment for 45min.
ferrite grain size. The fine ferrite grains evolve from these fine austenite grains. So, the average grain size of the matrix under cyclic phase transformation heat treatment is the smallest. The smaller the grain size is, the greater the Mechanical properties of metallic material.
crack almost completely disappeared, and residual voids presented along the original long crack. And the microstructural morphology of crack healing zone was similar to that of the matrix. As shown in Fig. 7 (b), some grains grew through the original long crack, and new grain boundaries presented in the center of the crack healing zone. Obviously, both cyclic phase transformation heat treatment and isothermal heat treatment can heal crack. However, the time that cyclic phase trans formation heat treatment need is less than isothermal heat treatment. The reason of this phenomenon is that cyclic phase transformation heat treatment can accelerate the diffusion of atom. As can be seen from Fig. 5, the grains of matrix were refined after cyclic phase trans formation heat treatment. With the decrease of the size of grain, the grain boundary area increases gradually. Furthermore, cyclic phase transformation heat treatment could introduce dislocations into the matrix [18,19]. It is well known that grain boundary and dislocation are the great diffusion channel. In addition, cyclic phase transformation heat treatment results in a non-equilibrium state which reduces the diffusion activation energy of atoms [20]. So, cyclic phase trans formation heat treatment can accelerate the diffusion of atom, thus resulting in accelerated crack healing.
3.4. Mechanism analysis 3.4.1. Healing mechanism The diffusion microvoids of the crack healing zone under different cycle numbers are shown in Fig. 6. It shown that all specimen had an obvious diffusion microvoids region in the crack healing zone after cy clic phase transformation heat treatment. However, the position and size of the diffusion microvoids region in specimen for different cycles are different. As shown in Fig. 6(a), the diffusion microvoids region located in the matrix on the side of crack, whose width was about 11.6 μm. And partial healing region had no microvoid. With the cycles reaching to 20 numbers, as shown in Fig. 6(b), both matrix on the side of crack and crack healing zone had microvoids, and the width of the microvoids region decreased to about 7.5 μm. When the cycle raised to 30 numbers, the width of microvoids region had further decreased to about 3.9 μm, as can be seen from Fig. 6(c). The formation of the diffusion microvoids region indicated that atomic diffusion played an important role throughout crack healing process. As shown in Fig. 4(a), the fine grains were presented in healing zone. With the increase of the cyclic time, as shown in Fig. 4(b)(c)(d), the fine recrystallization grains gradually dis appeared, which was due to the growth of fine recrystallization grains and the annexation of larger matrix grains near the crack. This suggested that recrystallization played an important role in prophase of crack healing. Through the preceding analysis, the crack healing was dominated by atomic diffusion and recrystallization. Recrystallization splits crack into void rapidly. Atomic diffusion supplies material for recrystallization and leads to void healing.
3.5. Mechanical properties test In order to estimate the influences of crack healing on the strength recovery, the tensile tests were conducted. Fig. 8 shows the effects of cycle numbers on the restoration of tensile properties at room temper ature. As can be seen from Fig. 8, the ultimate tensile strength increased with the increase of the cycles. After 30 cycles, the ultimate tensile strength has increased to 525 MPa. However, the increasing rate of the tensile properties has gradually decreased. This means that the healing effect brought by each cyclic heat treatment showed the tendency of gradually weakening with the increase of the cycle numbers. The different mechanisms in different healing period caused this phenome non. Crack healing in prophase relied on recrystallization and atomic diffusion, while the healing mechanism in later period was mainly atomic diffusion. Fig. 9 shows the stress-strain curves and macro profile images of the without crack, cyclic phase transformation heat-treated and isothermal
3.4.2. Accelerating mechanism Fig. 7 shows that EBSD images of the cross-section of the crack healing zone after healing by different healing methods. As can be seen from Fig. 7(a), it is obvious that the crack has healed, the original long
Fig. 6. Diffusion microvoids of the crack healing zone for different cycle numbers. (a) 5 cycles; (b) 20 cycles; (c) 30 cycles. 5
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Fig. 7. Electron backscatter diffraction (EBSD) images of the cross-section of the crack healing zone after healing under different healing methods. (a) cyclic phase transformation heat treatment for 30 cycles (45min); (b) isothermal heat treatment for 90min.
specimens even if isothermal heat-treated and cyclic heat-treated spec imens had same crack healing area, which is related to the gain coars ening (see Fig. 5(b) and (c)). The ultimate tensile strength of specimen without crack is highest, which indicates that whether cyclic heat treatment or isothermal heat treatment can not totally recover the ul timate tensile strength. This is because of residual voids in the position of the original crack (see Fig. 4(d) and (f)). However, when cycle numbers reach to 30 times, the ultimate tensile strength of healing specimens has been recovered to 96% compared with that of specimens without crack. And the healing effect brought by each cyclic heat treatment decreases gradually with the increase of the cycle numbers. So, it is wasteful to increase the ultimate tensile strength of heat-treated specimens to the ultimate tensile strength of specimen without crack even though extending heat-treated time from the viewpoint of engi neering application and saving resources. As can be seen from Fig. 9(b), the fracture plane of the specimen without crack was far away the middle of the specimen. However, the fracture plane of isothermal heattreated specimens was near the original crack which located in the middle of specimens, but the part of the fracture of cyclic heat-treated specimen took place in the matrix, as shown in Fig. 9(c)(d)(e). This indicated that cyclic heat treatment had better effect than isothermal heat treatment on healing crack.
Fig. 8. Ultimate tensile strength for different cycle numbers.
heat-treated specimens. The ultimate tensile strength of cyclic heattreated is higher than that of isothermal heat-treated specimens after healing for 45 min, as shown in Fig. 9(a). This is because the crack healing area under cyclic phase transformation heat treatment is bigger than that by isothermal heat treatment after same healing time (see Fig. 4(d) and (e)). Furthermore, the ultimate tensile strength of isothermal heat-treated is also lower than that of cyclic heat-treated
4. Conclusion (a) A novel technique of crack healing by cyclic phase transformation heat treatment has been attempted. The result suggested that the
Fig. 9. The stress-strain curves and macro profile images of the without crack, cyclic phase transformation heat-treated and isothermal heat-treated specimens. (a) the stress-strain curves; (b) (c) (d) (e) macro profile images for different healing condition. 6
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internal crack in a low carbon steel can be successfully healed by cyclic phase transformation heat treatment. (b) The evolution of crack healing can be divided into three periods: (i) splitting long crack into ellipse voids by formation, growth and contact of bulges on the partial crack surface; (ii) evolution from ellipse crack to circular void; (iii) the decrease of the size and number of circular voids, crack healing. (c) The crack healing mechanisms is atomic diffusion and recrystal lization in the low carbon steel. Atomic diffusion took place throughout the healing process, but recrystallization only happened in first stage of the healing process. (d) The cyclic phase transformation heat treatment technique is su perior to the isothermal heat treatment technique. It can reduce the time of crack healing by accelerating atomic diffusion, pre vent matrix grains from coarsening and increase ultimate tensile strength.
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Author contributions section Meng He: Research concept and design, Writing the article, Collec tion and assembly of data, Date analysis and interpretation. Zhentai Zheng: Research concept and design, Critical revisions of the article. Fen Shi: Collection and assembly of data. Donghui Guo: acquisition of data. Jinling Yu: acquisition of data. Declaration of competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors gratefully acknowledge the financial supported by the Natural Science Foundation of Hebei Province, P. R. China under Grant No. E2017202011. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.msea.2019.138712. References [1] A. Li, X. Chen, C.S. Zhang, G.D. Cui, Hui Zhao, C. Yang, A novel crack healing in steels by gas nitrocarburizing, Appl. Surf. Sci. 442 (2018) 437–445, https://doi. org/10.1016/j.apsusc.2018.02.181.
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