- Email: [email protected]
Restoration of impact properties of internal crack healing in a low carbon steel
Materials Science & Engineering A 682 (2017) 433–440
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Restoration of impact properties of internal crack healing in a low carbon steel
MARK
⁎
Ruishan Xina,b, Qingxian Maa,b, , Dongdong Guoa,b, Weiqi Lia,b a b
Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China Key Laboratory for Advanced Materials Processing Technology of Ministry of Education, Tsinghua University, Beijing 100084, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Restoration Impact properties Crack healing Charpy fracture Brittle dimple fracture Cleavage and ductile dimple mixed fracture
In this paper, a new model of internal crack presetting was presented via a butt-joint and compression method. Restoration of impact properties of internal crack healing in a low carbon steel was investigated using a 300 J pendulum impact testing machine. The results show that impact properties of crack healing zone dramatically increase from 900 ℃ to 1000 ℃. Recrystallization in the crack healing zone promotes the rapid restoration of impact properties of internal crack healing. Percentage recovery of impact properties after crack healing treatment increases with increasing healing temperature and holding time in the temperature range 900– 1100 ℃, while impact properties deteriorate at 1200 ℃ due to the formation of reticular ferrite and grain coarsening near the crack healing zone. Additionally, a new phenomenon is discovered: when internal cracks are disappeared completely, tensile properties of crack healing zones can be restored completely but their impact properties are only partially achieved. Post-fracture analysis indicates that in the initial healing stage the Charpy fracture exhibits massive brittle dimples and unhealed regions, and the ultimate fracture mode of the center region presents cleavage and ductile dimple mixed fracture.
1. Introduction Cracks and voids in metallic materials are well known to severely deteriorate the mechanical properties and shorten the service life of these materials, and even result in serious safety accidents. These defects often occur in the workpieces during their manufacturing and usage. For instance, the results of ultrasonic flaw detection have indicated that such defects are the primary cause of rejects. With the increasing of steel ingots in size and weight, these problems will become more prominent. Furthermore, more and more demands on the quality of metallic workpieces are required. Thus, it is vital to heal cracks and eliminate these cavity-type defects, which will not only reduce reject ratio and improve their mechanical properties and service life, but also save large amounts of energy and have enormous potential economic and ecological benefits. Griffith [1] proposed that the cracking was not a reversible operation, but a very narrow crack could be healed by the heat treatment if the temperature was sufficient to bring the atoms on either side of the crack within mutual range by thermal agitation. A considerable amount of studies on cracking in metallic materials are conducted every year [2–5]. However, in recent two decades, some investigations have begun to be carried out on crack healing in metallic
⁎
materials. According to the existing researches, the common method of internal crack healing is energy supply including heat treatment healing technique [6–9], electropulsing technique [10–15] and thermal mechanical coupling technique (hot plastic deformation) [16–19]. Obviously, among these crack healing techniques, the thermal mechanical coupling technique is the most effective method, which directly reduces the crack width or even bring about crack closure and welding of the closed crack surfaces [20]. However, when workpieces can not be further deformed after their forming or cavity-type defects are generated during the use process of workpieces, these defects can only be healed by heat treatment or elctropulsing technique. Han et al. [21] discovered the phenomenon of crack healing in 20MnMo steel at elevated temperature, and pointed out that crack healing mainly depended on atomic diffusion and migration. Yu et al. [22] systematically analyzed the effects of different parameters on crack healing in a low-carbon steel under hot plastic deformation and found that the degree of crack healing increased with increasing heating temperature, reduction ratio and holding time duration, and with decreasing number of deformation passes and strain rate. Wei et al. [23] investigated the behavior of crack healing in 1045 steel using a quasi in situ observation under scanning electron microscope and ultrasonic scanning and found that the crack healing at crack tips was achieved at 1100 ℃ for
Corresponding author at: Department of Mechanical Engineering, Tsinghua University, Beijing 10084, China. E-mail address: [email protected] (Q. Ma).
http://dx.doi.org/10.1016/j.msea.2016.11.068 Received 8 September 2016; Received in revised form 9 November 2016; Accepted 19 November 2016 Available online 21 November 2016 0921-5093/ © 2016 Elsevier B.V. All rights reserved.
Materials Science & Engineering A 682 (2017) 433–440
R. Xin et al.
120 min. Zhang et al. [24] reported that microstructure of steel 20 in the range of healing of internal crack was represented primarily by ferrite. Additionally, in-situ TEM observation of crack healing in single crystal iron during heating was carried out by Gao et al. [25], the experimental results indicated that the nanometer scale crack in single crystal Fe could heal completely upon heating and under a low oxygen partial pressure. Song et al. [26] also studied the healing process of submicro-scale voids embedded in a cold-rolled Al-Mg-Er alloy using in-situ TEM, and found that voids were healed successfully at a relatively low temperature of 453 K and the healing process consisted of three stages: an initial fast-healing stage, then a slow-healing stage, and finally a rapid healing near the end. Recently, some studies have been conducted on crack healing mechanisms [27–29] in metallic materials using molecular dynamic simulations. However, crack healing in metallic materials is challenging and has not yet been completely understood. And so far, the quantitative studies have been seldom done on static mechanical properties after crack healing [8,11]. Moreover, existing researches on the restoration of impact properties of crack healing zone were never reported. Xin, in his previous study [8], has presented microstructure and static tensile properties of internal crack healing in a low carbon steel, and discussed the mechanisms of crack healing. This paper will provide an update of the research concerned with the restoration of impact properties after crack healing and quantitatively evaluate the crack healing degree of this steel under dynamic loads, following previously published results in this journal [8].
Fig. 2. SEM micrograph of the pre-crack zone before healing treatment.
quenching. With these steps, internal cracks were preset in the center of the drum samples, and the residual gaps after the deformation of the disk grooves formed the internal pre-cracks and voids, as shown in Figs. 1 and 2. Crack healing tests were performed using a box resistor-stove (HMF1400-30) in the temperature range of 900–1200 ℃( in steps of 100 ℃) for 30 min, 60 min, 120 min, 180 min and 240 min, respectively. After healing treatment, these samples were rapidly quenched in water to freeze the microstructure of crack healing zone. Subsequently, the samples were cut along their longitudinal axis, and the sections were polished progressively finer grades of silicon carbide paper, ultrasonically cleaned and finally etched in a 4% nital solution. Restoration of impact properties of internal crack healing was quantitatively evaluated by the ratios between the impact absorbed energy of crack healing zone and the defect-free matrix material. Charpy U-notch specimens with dimensions of 10 mm×10 mm×55 mm were cut from the center of the drum samples and along the longitudinal axis, as shown in Fig. 1. And the crack healing zones were confirmed to lie in the middlemost part of the Unotch specimens. Room- temperature impact tests were performed on standard Charpy U-notch specimens at least three times for each data point using a 300 J pendulum impact testing machine, and the datum of Charpy absorbed energy obtained by averaging three values at each data point were listed in Table 1. The microstructure of crack healing zone was observed using an OLYMPUS-BX51 optical microscope (OM). Microstructural morphology of crack healing zone and Charpy impact fracture surfaces were examined using a JSM-7100F scanning electron microscope (FEG-SEM).
2. Experimental procedure The material used in this study was a low carbon steel, having a composition (in wt%) of 0.20 C, 0.29 Si, 0.58 Mn, 0.009 P. In order to prepare the Charpy impact specimens which conformed to the standard dimensional requirement (10 mm×10 mm×55 mm), a new model of internal crack presetting was presented and internal cracks were introduced into samples via the butt-joint and compression method. The samples were prepared as shown in Fig. 1: (1) the steel was fabricated into the same amount of workpieces A and B, with 40 mm in diameter and 37.5 mm in height; (2) the chamfers were processed at one end of workpieces for the subsequent welding, and a disk groove of 16 mm in diameter and 0.5 mm in depth was cut on the chamfering end face of workpiece B; (3) after cleaning out the machining oil on the end faces, workpieces A and B were welded along the chamfers to make up the samples of 40 mm in diameter and 75 mm in height; (4) the cylindrical samples were subjected to 20% height reduction at 800 ℃ with nominal strain rate of 0.1 s-1 using a 1000 kN hydraulic press (WAW-1000) under ambient conditions, followed by rapid water
Fig. 1. Schematic illustration of the processes of crack presetting, crack healing and impact specimen manufacturing.
434
Materials Science & Engineering A 682 (2017) 433–440
R. Xin et al.
as shown in Fig. 3d and e. It's well-known that reticular ferrite distributes along the original austenite grain boundary. Therefore, the size of the original austenite grains can be identified in Fig. 3d and e.
Table 1 Results of dynamic impact tests at room temperature. Healing temperature [°C]
Holding time [min]
Charpy absorbed energy [J]a
Standard deviation [J]
900
30 60 120 180 240
– 4.0 6.2 7.2 8.0
– 0.458 0.872 0.600 0.557
1000
30 60 120 180 240
8.0 12.0 13.2 14.0 15.9
0.361 0.557 1.082 0.721 0.854
1100
30 60 120 180 240
9.0 13.0 14.2 15.9 18.1
0.520 0.800 0.624 0.493 0.557
30 60 120 180 240
8.5 10.8 11.6 13.0 12.1
1.082 0.721 0.608 0.794 0.600
1200
a
3.3. SEM morphology evolution of crack zone Fig. 4 shows SEM micrographs of crack healing zone at different healing temperatures for 240 min. As shown in Fig. 4, after healing at 1000 ℃ and 1200, respectively, for 240 min, the intermittent short cracks had disappeared. It was clearly seen that lots of residual microvoids still existed in the crack healing zone. Grains in the crack healing zone were much finer than those in the matrix after healing at 1000 ℃ (Fig. 4a and b). When healing temperature reached 1200 ℃, as shown in Fig. 4c and d, the pre-crack had been completely healed and microvoids had been barely visible. 3.4. Restoration of Charpy impact properties Table 1 gives the Charpy impact testing results of crack healing zone under various healing conditions. The datum of Charpy absorbed energy were the average values of three specimens at each data point. The Charpy impact specimens were not achieved after healing at 900 ℃ for 30 min because the upper and lower parts of the specimens directly disconnected during their manufacturing from the drum samples. Therefore, there was no data at this healing condition. This indicated that the bonding strength between the both surfaces and the degree of crack healing were extremely low. At other healing conditions, the specimens did not fracture before the impact tests. From the data shown in this table, at the same healing temperature, the Charpy absorbed energy increased gradually with increasing holding times. When the cracks were healed at 1000 ℃, 1100 ℃ and 1200 ℃, respectively, for 30 min, their Charpy absorbed energy were nearly equal. The characteristic was similar to that of static tensile properties at 1000 ℃ and 1100 ℃ for 30 min [8]. This was mainly attributed to that they were all in the initial stage of recrystallization in the crack zone [8]. It can also be noticed from the table that the maximum Charpy absorbed energy obtained was 18.1 J after crack healing at 1100 ℃ for 240 min. However, for the same holding time, the Charpy absorbed energy at 1200 ℃ was lower than that at 1100 ℃, as shown in Table 1. This characteristic was obviously distinct from the restoration of static tensile properties. The static tensile properties at 1200 ℃ for 90 min and 120 min were completely restored and the fractures did not occur at the middle of the tensile samples [8]. This difference was mainly due to that the the formation of reticular ferrite and grain coarsening near the crack healing zone deteriorated impact properties, as shown in Fig. 3d and e. It is well known that coarsening grain and intercrystalline reticular ferrite depress crack growth resistance of the materials strongly and result in unstable propagation of crack very easily. Additionally, the Charpy absorbed energy of the experimental steel was measured three times and the average value was 28.8 J. The percentage recovery of Charpy absorbed energy was defined as the following expression.
The table listed average values of three specimens at each data point.
3. Results and discussion 3.1. Microstructural morphology of pre-crack zone Fig. 2 presents typically microstructural morphology of the precrack zone before healing treatment. In order to make workpieces A and B joint together and prevent Charpy impact specimens directly disconnecting during their manufacturing, 20% proper reduction was determined. In Fig. 2, it can be clearly seen that the local contact and healing of both crack surfaces occurred, and lots of irregular voids existed in the crack zone and presented linear distribution. According to the results of our previous work [8], the current state of the precrack zone was in the second stage of linear crack healing: crack surface contact and long crack segmentation. 3.2. Microstructural evolution of crack zone Fig. 3 presents the microstructural evolution of crack healing zone under different healing temperatures and holding times. After healing at 900 ℃ for 240 min, as shown in Fig. 3a, some intermittent short cracks were clearly visible in the crack zone. Only ferrite grains were found on both sides of these intermittent cracks compared to the matrix structure. Previous studies [8,21,22,24] on crack healing in low carbon steel have confirmed the microstructure in the crack healing zone consisted primarily of ferrite and iron atoms in the matrix migrated towards the crack zone. The crack healing mechanism was atomic diffusion at lower temperatures (≤900 ℃) [8]. When the crack was healed at 1000 ℃ for 240 min (Fig. 3b), intermittent short cracks had disappeared completely, massive fine ferrite generated in the crack healing zone. Recrystallization took place in the crack healing zone [8], and a certain amount of microvoids remained in the crack healing zone, as shown in Fig. 3b and Fig. 4b. After healing at 1100 ℃ for 240 min (Fig. 3c), the sizes of ferrite grains in the crack healing zone was bigger than those at 1000 ℃ for 240 min. Simultaneously, the number of microvoids decreased with increasing healing temperatures and holding times [8]. When healing temperature reached 1200 ℃ (Fig. 3d and e), fine granular ferrite in the crack healing zone had disappeared totally and evolved into reticular ferrite. Meanwhile, the original austenite grain coarsening took place with increasing holding times,
η=
AKU ×100% AKU 0
(1)
Where η is the percentage recovery of Charpy absorbed energy, AKU is the Charpy absorbed energy under various healing conditions, AKU 0 is the Charpy absorbed energy of the experimental steel. Fig. 5 shows the percentage recovery of room-temperature Charpy absorbed energy under various healing conditions. It can be clearly seen that Charpy absorbed energy was not completely restored under any healing condition. Under the pre-crack condition in this work, the maximum percentage recovery of Charpy absorbed energy was 62.85% after crack healing at 1100 ℃ for 240 min, as shown in Fig. 5. For the 435
Materials Science & Engineering A 682 (2017) 433–440
R. Xin et al.
Fig. 3. Microstructural evolution of crack healing zone under different healing temperatures and holding times: (a) 900 ℃×240 min, (b) 1000 ℃×240 min, (c) 1100 ℃×240 min, (d) 1200 ℃×120 min, (e) 1200 ℃×240 min.
holding time ranged from 30 min to 60 min, the recovery rates were much bigger than those in the holding time range from 60 min to 240 min. The recovery rate was the greatest in the initial healing stage. Xin [8] also showed that when the healing temperatures exceeded 1000 ℃, recrystallization occurred in the crack healing zone and fine recrystallized grains rapidly filled the crack gap. The rapid decrease of crack gap led to the high recovery rate of impact properties. In other words, in the initial healing stage, the occurrence of recrystallization and the formation of massive fine recrystallized grains facilitated the rapid recovery of Charpy impact properties. In our previous work [8], the pre-crack width was about 4 μm at the length range of 20 mm in the center region of the pre-crack, it was a long crack. After healing at 1200 ℃ for above 90 min, the percentage recovery of static tensile properties reached 100%. In this work, the pre-crack width was smaller than that in our previous work.
same holding time, the percentage recovery of Charpy absorbed energy at 900 ℃ was significantly lower than that at above 1000 ℃. The similar characteristic of the restoration of tensile properties was analyzed in the previous work. This was mainly attributed to the different mechanisms of crack healing at 900 ℃ and above 1000 ℃ [8]. After healing at 1200 ℃ for 240 min, the crack had been completely healed (Figs. 3e and 4d), while the percentage recovery of crack healing zone at 1200 ℃ for 240 min was only about 42%. That was mainly attributed to the formation of reticular ferrite and grain coarsening near the crack healing zone (Fig. 3d and e). It can be also noticed from Fig. 5 that the percentage recovery of Charpy absorbed energy increased gradually with increasing holding times, except for those at 1200 ℃. Nevertheless, the recovery rate gradually decreased with increasing holding times. When the healing temperatures were 1000 ℃, 1100 ℃ and 1200 ℃, respectively, and
436
Materials Science & Engineering A 682 (2017) 433–440
R. Xin et al.
Fig. 4. SEM micrographs of crack healing zone at different healing temperatures for 240 min: (a) 1000 ℃, (b) magnified image of the selected region in Fig. 4a showing crack healing zone, (c) 1200 ℃, (d) magnified image of the selected region in Fig. 4c showing crack healing zone.
3.5. Charpy fracture morphology Fig. 6 shows SEM fractographs of the pre-crack zone before healing treatment. The upper and lower parts fractured directly during the fabricating of Charpy impact specimens from the drum samples before healing treatment. The fracture surface was flat and massive corrugated machining marks were clearly identified, as shown in Fig. 6a. In order to further examine the fracture surface, the magnified images of selected regions b and c were achieved. It can be obviously seen that the fracture of the pre-crack zone exhibited massive dimples and unhealed regions, as shown in Fig. 6b and c. This indicated that local healing between the upper and lower end surfaces occurred. It can be known from Xin [8] that these dimples were brittle fractures, which were formed by the existing microvoids in the crack healing zone and without macroscopic plastic deformation. Simultaneously, there were lots of second precipitated particles in the unhealed regions, as shown in Fig. 6c. Fig. 7 shows the room-temperature Charpy impact macroscopic fractographs of crack healing zone under various healing conditions. After healing at 900 ℃ for 30 min, as shown in Fig. 7a, the fracture was similar to the fractograph of the pre-crack zone (Fig. 6a). Compared with Fig. 6a, the fracture exhibited a small quantity of cleavage fractures. Furthermore, the area of cleavage fractures increased gradually with increasing healing temperatures and holding times. Correspondingly, the degree of crack healing enhanced progressively. The fracture surfaces of the notch tip and center regions of the Charpy impact specimens fractured at room temperature were examined because their fracture modes might be different, as shown in Fig. 8. The variation of the areas of different fracture modes represented different crack healing degree. After healing at 900 ℃ for 240 min, the fracture near the notch tip presented brittle dimples and unhealed regions (Fig. 8a). The fracture of the center region
Fig. 5. Percentage recovery of room-temperature Charpy absorbed energy under various healing conditions.
Consequently, the tensile properties of crack healing zone in this work should be restored completely after healing at 1200 ℃ for above 90 min, while its impact properties was far from fully recovered. Based on the above analysis, by comparing the results from Xin [8] with the results in this work, it can be concluded that when the crack has completely disappeared, the static tensile properties can be restored completely but their impact properties are only partially achieved. As a result, the quantitative evaluation of the crack healing degree should rely on the operational environment and loads of workpieces.
437
Materials Science & Engineering A 682 (2017) 433–440
R. Xin et al.
Fig. 6. SEM fractographs of pre-crack zone before healing treatment (a) overall, (b) magnified image of the selected region b in Fig. 6a showing the unhealed region and dimple fracture region, (c) magnified image of selected region c in Fig. 6b showing the unhealed regions (yellow dashed regions). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
absorbed energy after healing at 1200 ℃ was lower than that at 1100 ℃. In view of the above analysis of macroscopic and microscopic fractographs of crack healing zone (Figs. 7 and 8), in the initial healing stage, the Charpy impact fracture presented massive brittle dimples and unhealed regions. With increasing the microstructural healing degree of crack zone, the area of brittle dimples and unhealed regions gradually decreased, corresponding to that the area of cleavage fracture gradually increased. When the pre-crack was healed at 1100 ℃ and 1200 ℃, respectively, for 240 min, the unhealed regions completely disappeared, and the fractographs of the notch tip regions were mostly composed of brittle dimples while the center regions exhibited cleavage and ductile dimple mixed fractures.
occurred in a cleavage mode together with a brittle dimpled mode, as shown in Fig. 8b. And some unhealed regions were also seen in Fig. 8b. When healing temperature reached 1000 ℃, the fracture of the notch tip region exhibited massive brittle dimples with uneven size and a small quantity of microvoids, and unhealed regions nearly disappeared, as shown in Fig. 8c. The center region fractured in a cleavage mode which had typical river patterns (Fig. 8d). At the same time, brittle dimples have disappeared in the center region. When the pre-crack was healed at 1100 ℃ for 240 min, the fracture of the notch tip region showed complete brittle dimples, and there have been no microvoids and unhealed regions, as shown in Fig. 8e. At this point, the center region exhibited cleavage and ductile dimple mixed fracture, as shown in Fig. 8f. The existence of ductile dimples at 1100 ℃ was corresponding to the higher Charpy absorbed energy. When the healing temperature further raised to 1200 ℃, the fracture of the notch tip region occurred mostly in a brittle dimpled mode and a small quantity of cleavage mode. as shown in Fig. 8g. The occurrence of cleavage facets in the notch tip region indicated the microstructural healing degree of crack healing zone further enhanced. Meanwhile, the fracture of the center region was similar to that at 1100 ℃ and presented massive cleavage facets and a small quantity of ductile dimples, as shown in Fig. 8g. The fracture exhibited typical fan-shaped river pattern. Compared with Fig. 8f and h, the size of cleavage facet at 1200 ℃ was greater than that at 1100 ℃. Correspondingly, the Charpy
4. Conclusions 1. Impact properties of internal crack healing in this low carbon steel dramatically increase from 900 ℃ to 1000 ℃ due to two different mechanisms of crack healing, atomic diffusion at 900 ℃, and recrystallization and grain growth at above 1000 ℃. Recrystallization in the crack healing zone promotes the rapid restoration of impact properties of internal crack healing 2. Percentage recovery of impact properties increases with increasing healing temperature and holding time in the temperature range
Fig. 7. Room-temperature Charpy impact macroscopic fractographs of crack healing zone under various healing conditions: (a) 900 ℃×30 min, (b) 900 ℃×240 min, (c) 1000 ℃×240 min, (d) 1100 ℃×240 min, (e) 1200 ℃×240 min.
438
Materials Science & Engineering A 682 (2017) 433–440
R. Xin et al.
Fig. 8. SEM fractographs of the U-notch tip and center regions of Charpy impact fracture surfaces of crack healing zone under different healing conditions: (a, b) 900 ℃×240 min, (c, d) 1000 ℃×240 min, (e, f) 1100 ℃×240 min, (g, h) 1200 ℃×240 min.
439
Materials Science & Engineering A 682 (2017) 433–440
R. Xin et al.
900–1100 ℃, while impact properties deteriorate at 1200 ℃ due to the formation of reticular ferrite and grain coarsening near the crack healing zone. 3. In this work, when internal cracks are disappeared completely, tensile properties of crack healing zones can be restored completely but their impact properties are only partially achieved. 4. In the initial stage of crack healing, the Charpy fracture exhibits massive brittle dimples and unhealed regions. With the increase of healing temperature and holding time, cleavage fracture regions increase progressively. The ultimate fracture of the notch tip region presents brittle dimples and the fracture surface of the center region are mainly composed of cleavage facets with a small quantity of ductile dimples.
[9] R.S. Xin, Q.X. Ma, Z.D. Shan, Microstructure evolution of internal crack healing in a low-carbon steel at elevated temperatures, J. Tsinghua Univ. (Sci. Technol.) 55 (2015) 304–309. [10] A. Hosoi, T. Nagahama, Y. Ju, Fatigue crack healing by a controlled high density electric current field, Mater. Sci. Eng. A 533 (2012) 38–42. [11] H. Song, Z.J. Wang, Microcrack healing and local recrystallization in pre-deformed sheet by high density electropulsing, Mater. Sci. Eng. A 490 (2008) 1–6. [12] Y.Z. Zhou, J.D. Guo, M. Gao, G.H. He, Crack healing in a steel by using electropulsing technique, Mater. Lett. 58 (2004) 1732–1736. [13] X.G. Zheng, Y.N. Shi, K. Lu, Electro-healing cracks in nickel, Mater. Sci. Eng. A 561 (2013) 52–59. [14] Y.P. Tang, A. Hosoi, Y. Morita, Y. Ju, Restoration of fatigue damage in stainless steel by high-density electric current, Int. J. Fatigue 56 (2013) 69–74. [15] Y.Z. Zhou, Y. Zeng, G.H. He, B.L. Zhou, The healing of quenched crack in 1045 steel under electropulsing, J. Mater. Res. 16 (2001) 17–19. [16] X.M. Zhao, X. Lin, J. Chen, L. Xue, W.D. Huang, The effect of hot isostatic pressing on crack healing, microstructure, mechanical properties of Rene88DT superalloy prepared by laser solid forming, Mater. Sci. Eng. A 504 (2009) 129–134. [17] H.L. Yu, X.H. Liu, Thermal-mechanical finite element analysis of evolution of surface cracks during slab rolling, Mater. Manuf. Process. 24 (2009) 570–578. [18] H.L. Yu, A.K. Tieu, C. Lu, A. Godbole, Investigation of closure of internal cracks during rolling by FE model considering crack surface roughness, Int. J. Adv. Manuf. Technol. 75 (2014) 1633–1640. [19] H.T. Gao, Z.R. Ai, H.L. Yu, H.Y. Wu, X.H. Liu, Analysis of internal crack healing mechanism under rolling deformation, Plos One 9 (2014) e101907. [20] C.Y. Park, D.Y. Yang, Modelling of void crushing for large-ingot hot forging, J. Mater. Process. Technol. 67 (1997) 195–200. [21] J.T. Han, G. Zhao, Q.X. Cao, Discovery of inner crack recovery and its structure change in 20MnMo steel, Acta Metall. Sin. 32 (1996) 723–729. [22] H.L. Yu, X.H. Liu, X.W. Li, Crack healing in a low-carbon steel under hot plastic deformation, Metall, Mater. Trans. A 45A (2014) 1001–1009. [23] D.B. Wei, J.T. Han, Z.Y. Jiang, C. Lu, A.K. Tieu, A study on crack healing in 1045 steel, J. Mater. Process. Technol. 177 (2006) 233–237. [24] Y.J. Zhang, J.T. Han, Analysis of microstructure of steel 20 in the range of healing of internal crack, Met. Sci. Heat Treat. 58 (2012) 526–528. [25] K.W. Gao, L.J. Qiao, W.Y. Chu, In situ TEM observation of crack healing in alphaFe, Scripta Mater. 44 (2001) 1055–1059. [26] M. Song, K. Du, S.P. Wen, Z.R. Nie, H.Q. Ye, In situ electron microscopy investigation of void healing in an Al-Mg-Er alloy at at a low temperature, Acta Mater. 69 (2014) 236–245. [27] G.Q. Xu, M.J. Demkowicz, Healing of nanocrakcs by disclinations, Phys. Rev. Lett. 111 (2013) 145501. [28] J. Li, Q.H. Fang, B. Liu, Y. Liu, Y.W. Liu, P.H. Wen, Mechanism of crack healing at room temperature revealed by atomistic simulations, Acta Mater. 95 (2015) 291–301. [29] D.B. Wei, J.T. Han, A.K. Tieu, Z.Y. Jiang, Simulation of crack healing in BCC Fe, Scripta Mater. 51 (2004) 583–587.
Acknowledgments The authors acknowledge the financial support from the National Basic Research Program (“973” Program) of China (Grant No. 2011CB012903). References [1] A.A. Griffith, The phenomena of rupture and flow in solids, Philos. Trans. R. Soc. London, Ser. A 221 (1920) 163–198. [2] D.C. Tung, J.C. Lippold, Self-restrained testing for residual stress driven cracking in nick-based alloys, Mater. Sci. Eng. A 673 (2016) 158–166. [3] C. Örnek, D.L. Engelberg, Towards understanding the effect of deformation mode on stress corrosion cracking susceptibility of grade 2205 duplex stainless, Mater. Sci. Eng. A 666 (2016) 269–279. [4] Z.Y. Liu, X.Z. Wang, C.W. Du, J.K. Li, X.G. Li, Effect of hydrogen-induced plasticity on the stress corrosion cracking of X70 pipeline steel in simulated soil environments, Mater. Sci. Eng. A 658 (2016) 348–354. [5] J. Goebel, T. Ghidini, A.J. Graham, Stress-corrosion cracking characterisation of the advanced aerospace Al-Li 2099-T86 alloy, Mater. Sci. Eng. A 673 (2016) 16–23. [6] C.F. Dong, X.G. Li, Z.S. Shen, W.Y. Chu, Study on crack healing of hydrogen attack in carbon steel by heat treatment, Corros 59 (2003) 401–406. [7] H.L. Zhang, P.Z. Huang, J. Sun, Morphological healing evolution of penny-shaped fatigue microcracks in pure iron at elevated temperatures, Appl. Phys. Lett. 87 (2004) 1143–1145. [8] R.S. Xin, Q.X. Ma, W.Q. Li, Microstructure and mechanical properties of internal crack healing in a low carbon steel, Mater. Sci. Eng. A 662 (2016) 65–71.
440
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Restoration of impact properties of internal crack healing in a low carbon steel
MARK
⁎
Ruishan Xina,b, Qingxian Maa,b, , Dongdong Guoa,b, Weiqi Lia,b a b
Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China Key Laboratory for Advanced Materials Processing Technology of Ministry of Education, Tsinghua University, Beijing 100084, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Restoration Impact properties Crack healing Charpy fracture Brittle dimple fracture Cleavage and ductile dimple mixed fracture
In this paper, a new model of internal crack presetting was presented via a butt-joint and compression method. Restoration of impact properties of internal crack healing in a low carbon steel was investigated using a 300 J pendulum impact testing machine. The results show that impact properties of crack healing zone dramatically increase from 900 ℃ to 1000 ℃. Recrystallization in the crack healing zone promotes the rapid restoration of impact properties of internal crack healing. Percentage recovery of impact properties after crack healing treatment increases with increasing healing temperature and holding time in the temperature range 900– 1100 ℃, while impact properties deteriorate at 1200 ℃ due to the formation of reticular ferrite and grain coarsening near the crack healing zone. Additionally, a new phenomenon is discovered: when internal cracks are disappeared completely, tensile properties of crack healing zones can be restored completely but their impact properties are only partially achieved. Post-fracture analysis indicates that in the initial healing stage the Charpy fracture exhibits massive brittle dimples and unhealed regions, and the ultimate fracture mode of the center region presents cleavage and ductile dimple mixed fracture.
1. Introduction Cracks and voids in metallic materials are well known to severely deteriorate the mechanical properties and shorten the service life of these materials, and even result in serious safety accidents. These defects often occur in the workpieces during their manufacturing and usage. For instance, the results of ultrasonic flaw detection have indicated that such defects are the primary cause of rejects. With the increasing of steel ingots in size and weight, these problems will become more prominent. Furthermore, more and more demands on the quality of metallic workpieces are required. Thus, it is vital to heal cracks and eliminate these cavity-type defects, which will not only reduce reject ratio and improve their mechanical properties and service life, but also save large amounts of energy and have enormous potential economic and ecological benefits. Griffith [1] proposed that the cracking was not a reversible operation, but a very narrow crack could be healed by the heat treatment if the temperature was sufficient to bring the atoms on either side of the crack within mutual range by thermal agitation. A considerable amount of studies on cracking in metallic materials are conducted every year [2–5]. However, in recent two decades, some investigations have begun to be carried out on crack healing in metallic
⁎
materials. According to the existing researches, the common method of internal crack healing is energy supply including heat treatment healing technique [6–9], electropulsing technique [10–15] and thermal mechanical coupling technique (hot plastic deformation) [16–19]. Obviously, among these crack healing techniques, the thermal mechanical coupling technique is the most effective method, which directly reduces the crack width or even bring about crack closure and welding of the closed crack surfaces [20]. However, when workpieces can not be further deformed after their forming or cavity-type defects are generated during the use process of workpieces, these defects can only be healed by heat treatment or elctropulsing technique. Han et al. [21] discovered the phenomenon of crack healing in 20MnMo steel at elevated temperature, and pointed out that crack healing mainly depended on atomic diffusion and migration. Yu et al. [22] systematically analyzed the effects of different parameters on crack healing in a low-carbon steel under hot plastic deformation and found that the degree of crack healing increased with increasing heating temperature, reduction ratio and holding time duration, and with decreasing number of deformation passes and strain rate. Wei et al. [23] investigated the behavior of crack healing in 1045 steel using a quasi in situ observation under scanning electron microscope and ultrasonic scanning and found that the crack healing at crack tips was achieved at 1100 ℃ for
Corresponding author at: Department of Mechanical Engineering, Tsinghua University, Beijing 10084, China. E-mail address: [email protected] (Q. Ma).
http://dx.doi.org/10.1016/j.msea.2016.11.068 Received 8 September 2016; Received in revised form 9 November 2016; Accepted 19 November 2016 Available online 21 November 2016 0921-5093/ © 2016 Elsevier B.V. All rights reserved.
Materials Science & Engineering A 682 (2017) 433–440
R. Xin et al.
120 min. Zhang et al. [24] reported that microstructure of steel 20 in the range of healing of internal crack was represented primarily by ferrite. Additionally, in-situ TEM observation of crack healing in single crystal iron during heating was carried out by Gao et al. [25], the experimental results indicated that the nanometer scale crack in single crystal Fe could heal completely upon heating and under a low oxygen partial pressure. Song et al. [26] also studied the healing process of submicro-scale voids embedded in a cold-rolled Al-Mg-Er alloy using in-situ TEM, and found that voids were healed successfully at a relatively low temperature of 453 K and the healing process consisted of three stages: an initial fast-healing stage, then a slow-healing stage, and finally a rapid healing near the end. Recently, some studies have been conducted on crack healing mechanisms [27–29] in metallic materials using molecular dynamic simulations. However, crack healing in metallic materials is challenging and has not yet been completely understood. And so far, the quantitative studies have been seldom done on static mechanical properties after crack healing [8,11]. Moreover, existing researches on the restoration of impact properties of crack healing zone were never reported. Xin, in his previous study [8], has presented microstructure and static tensile properties of internal crack healing in a low carbon steel, and discussed the mechanisms of crack healing. This paper will provide an update of the research concerned with the restoration of impact properties after crack healing and quantitatively evaluate the crack healing degree of this steel under dynamic loads, following previously published results in this journal [8].
Fig. 2. SEM micrograph of the pre-crack zone before healing treatment.
quenching. With these steps, internal cracks were preset in the center of the drum samples, and the residual gaps after the deformation of the disk grooves formed the internal pre-cracks and voids, as shown in Figs. 1 and 2. Crack healing tests were performed using a box resistor-stove (HMF1400-30) in the temperature range of 900–1200 ℃( in steps of 100 ℃) for 30 min, 60 min, 120 min, 180 min and 240 min, respectively. After healing treatment, these samples were rapidly quenched in water to freeze the microstructure of crack healing zone. Subsequently, the samples were cut along their longitudinal axis, and the sections were polished progressively finer grades of silicon carbide paper, ultrasonically cleaned and finally etched in a 4% nital solution. Restoration of impact properties of internal crack healing was quantitatively evaluated by the ratios between the impact absorbed energy of crack healing zone and the defect-free matrix material. Charpy U-notch specimens with dimensions of 10 mm×10 mm×55 mm were cut from the center of the drum samples and along the longitudinal axis, as shown in Fig. 1. And the crack healing zones were confirmed to lie in the middlemost part of the Unotch specimens. Room- temperature impact tests were performed on standard Charpy U-notch specimens at least three times for each data point using a 300 J pendulum impact testing machine, and the datum of Charpy absorbed energy obtained by averaging three values at each data point were listed in Table 1. The microstructure of crack healing zone was observed using an OLYMPUS-BX51 optical microscope (OM). Microstructural morphology of crack healing zone and Charpy impact fracture surfaces were examined using a JSM-7100F scanning electron microscope (FEG-SEM).
2. Experimental procedure The material used in this study was a low carbon steel, having a composition (in wt%) of 0.20 C, 0.29 Si, 0.58 Mn, 0.009 P. In order to prepare the Charpy impact specimens which conformed to the standard dimensional requirement (10 mm×10 mm×55 mm), a new model of internal crack presetting was presented and internal cracks were introduced into samples via the butt-joint and compression method. The samples were prepared as shown in Fig. 1: (1) the steel was fabricated into the same amount of workpieces A and B, with 40 mm in diameter and 37.5 mm in height; (2) the chamfers were processed at one end of workpieces for the subsequent welding, and a disk groove of 16 mm in diameter and 0.5 mm in depth was cut on the chamfering end face of workpiece B; (3) after cleaning out the machining oil on the end faces, workpieces A and B were welded along the chamfers to make up the samples of 40 mm in diameter and 75 mm in height; (4) the cylindrical samples were subjected to 20% height reduction at 800 ℃ with nominal strain rate of 0.1 s-1 using a 1000 kN hydraulic press (WAW-1000) under ambient conditions, followed by rapid water
Fig. 1. Schematic illustration of the processes of crack presetting, crack healing and impact specimen manufacturing.
434
Materials Science & Engineering A 682 (2017) 433–440
R. Xin et al.
as shown in Fig. 3d and e. It's well-known that reticular ferrite distributes along the original austenite grain boundary. Therefore, the size of the original austenite grains can be identified in Fig. 3d and e.
Table 1 Results of dynamic impact tests at room temperature. Healing temperature [°C]
Holding time [min]
Charpy absorbed energy [J]a
Standard deviation [J]
900
30 60 120 180 240
– 4.0 6.2 7.2 8.0
– 0.458 0.872 0.600 0.557
1000
30 60 120 180 240
8.0 12.0 13.2 14.0 15.9
0.361 0.557 1.082 0.721 0.854
1100
30 60 120 180 240
9.0 13.0 14.2 15.9 18.1
0.520 0.800 0.624 0.493 0.557
30 60 120 180 240
8.5 10.8 11.6 13.0 12.1
1.082 0.721 0.608 0.794 0.600
1200
a
3.3. SEM morphology evolution of crack zone Fig. 4 shows SEM micrographs of crack healing zone at different healing temperatures for 240 min. As shown in Fig. 4, after healing at 1000 ℃ and 1200, respectively, for 240 min, the intermittent short cracks had disappeared. It was clearly seen that lots of residual microvoids still existed in the crack healing zone. Grains in the crack healing zone were much finer than those in the matrix after healing at 1000 ℃ (Fig. 4a and b). When healing temperature reached 1200 ℃, as shown in Fig. 4c and d, the pre-crack had been completely healed and microvoids had been barely visible. 3.4. Restoration of Charpy impact properties Table 1 gives the Charpy impact testing results of crack healing zone under various healing conditions. The datum of Charpy absorbed energy were the average values of three specimens at each data point. The Charpy impact specimens were not achieved after healing at 900 ℃ for 30 min because the upper and lower parts of the specimens directly disconnected during their manufacturing from the drum samples. Therefore, there was no data at this healing condition. This indicated that the bonding strength between the both surfaces and the degree of crack healing were extremely low. At other healing conditions, the specimens did not fracture before the impact tests. From the data shown in this table, at the same healing temperature, the Charpy absorbed energy increased gradually with increasing holding times. When the cracks were healed at 1000 ℃, 1100 ℃ and 1200 ℃, respectively, for 30 min, their Charpy absorbed energy were nearly equal. The characteristic was similar to that of static tensile properties at 1000 ℃ and 1100 ℃ for 30 min [8]. This was mainly attributed to that they were all in the initial stage of recrystallization in the crack zone [8]. It can also be noticed from the table that the maximum Charpy absorbed energy obtained was 18.1 J after crack healing at 1100 ℃ for 240 min. However, for the same holding time, the Charpy absorbed energy at 1200 ℃ was lower than that at 1100 ℃, as shown in Table 1. This characteristic was obviously distinct from the restoration of static tensile properties. The static tensile properties at 1200 ℃ for 90 min and 120 min were completely restored and the fractures did not occur at the middle of the tensile samples [8]. This difference was mainly due to that the the formation of reticular ferrite and grain coarsening near the crack healing zone deteriorated impact properties, as shown in Fig. 3d and e. It is well known that coarsening grain and intercrystalline reticular ferrite depress crack growth resistance of the materials strongly and result in unstable propagation of crack very easily. Additionally, the Charpy absorbed energy of the experimental steel was measured three times and the average value was 28.8 J. The percentage recovery of Charpy absorbed energy was defined as the following expression.
The table listed average values of three specimens at each data point.
3. Results and discussion 3.1. Microstructural morphology of pre-crack zone Fig. 2 presents typically microstructural morphology of the precrack zone before healing treatment. In order to make workpieces A and B joint together and prevent Charpy impact specimens directly disconnecting during their manufacturing, 20% proper reduction was determined. In Fig. 2, it can be clearly seen that the local contact and healing of both crack surfaces occurred, and lots of irregular voids existed in the crack zone and presented linear distribution. According to the results of our previous work [8], the current state of the precrack zone was in the second stage of linear crack healing: crack surface contact and long crack segmentation. 3.2. Microstructural evolution of crack zone Fig. 3 presents the microstructural evolution of crack healing zone under different healing temperatures and holding times. After healing at 900 ℃ for 240 min, as shown in Fig. 3a, some intermittent short cracks were clearly visible in the crack zone. Only ferrite grains were found on both sides of these intermittent cracks compared to the matrix structure. Previous studies [8,21,22,24] on crack healing in low carbon steel have confirmed the microstructure in the crack healing zone consisted primarily of ferrite and iron atoms in the matrix migrated towards the crack zone. The crack healing mechanism was atomic diffusion at lower temperatures (≤900 ℃) [8]. When the crack was healed at 1000 ℃ for 240 min (Fig. 3b), intermittent short cracks had disappeared completely, massive fine ferrite generated in the crack healing zone. Recrystallization took place in the crack healing zone [8], and a certain amount of microvoids remained in the crack healing zone, as shown in Fig. 3b and Fig. 4b. After healing at 1100 ℃ for 240 min (Fig. 3c), the sizes of ferrite grains in the crack healing zone was bigger than those at 1000 ℃ for 240 min. Simultaneously, the number of microvoids decreased with increasing healing temperatures and holding times [8]. When healing temperature reached 1200 ℃ (Fig. 3d and e), fine granular ferrite in the crack healing zone had disappeared totally and evolved into reticular ferrite. Meanwhile, the original austenite grain coarsening took place with increasing holding times,
η=
AKU ×100% AKU 0
(1)
Where η is the percentage recovery of Charpy absorbed energy, AKU is the Charpy absorbed energy under various healing conditions, AKU 0 is the Charpy absorbed energy of the experimental steel. Fig. 5 shows the percentage recovery of room-temperature Charpy absorbed energy under various healing conditions. It can be clearly seen that Charpy absorbed energy was not completely restored under any healing condition. Under the pre-crack condition in this work, the maximum percentage recovery of Charpy absorbed energy was 62.85% after crack healing at 1100 ℃ for 240 min, as shown in Fig. 5. For the 435
Materials Science & Engineering A 682 (2017) 433–440
R. Xin et al.
Fig. 3. Microstructural evolution of crack healing zone under different healing temperatures and holding times: (a) 900 ℃×240 min, (b) 1000 ℃×240 min, (c) 1100 ℃×240 min, (d) 1200 ℃×120 min, (e) 1200 ℃×240 min.
holding time ranged from 30 min to 60 min, the recovery rates were much bigger than those in the holding time range from 60 min to 240 min. The recovery rate was the greatest in the initial healing stage. Xin [8] also showed that when the healing temperatures exceeded 1000 ℃, recrystallization occurred in the crack healing zone and fine recrystallized grains rapidly filled the crack gap. The rapid decrease of crack gap led to the high recovery rate of impact properties. In other words, in the initial healing stage, the occurrence of recrystallization and the formation of massive fine recrystallized grains facilitated the rapid recovery of Charpy impact properties. In our previous work [8], the pre-crack width was about 4 μm at the length range of 20 mm in the center region of the pre-crack, it was a long crack. After healing at 1200 ℃ for above 90 min, the percentage recovery of static tensile properties reached 100%. In this work, the pre-crack width was smaller than that in our previous work.
same holding time, the percentage recovery of Charpy absorbed energy at 900 ℃ was significantly lower than that at above 1000 ℃. The similar characteristic of the restoration of tensile properties was analyzed in the previous work. This was mainly attributed to the different mechanisms of crack healing at 900 ℃ and above 1000 ℃ [8]. After healing at 1200 ℃ for 240 min, the crack had been completely healed (Figs. 3e and 4d), while the percentage recovery of crack healing zone at 1200 ℃ for 240 min was only about 42%. That was mainly attributed to the formation of reticular ferrite and grain coarsening near the crack healing zone (Fig. 3d and e). It can be also noticed from Fig. 5 that the percentage recovery of Charpy absorbed energy increased gradually with increasing holding times, except for those at 1200 ℃. Nevertheless, the recovery rate gradually decreased with increasing holding times. When the healing temperatures were 1000 ℃, 1100 ℃ and 1200 ℃, respectively, and
436
Materials Science & Engineering A 682 (2017) 433–440
R. Xin et al.
Fig. 4. SEM micrographs of crack healing zone at different healing temperatures for 240 min: (a) 1000 ℃, (b) magnified image of the selected region in Fig. 4a showing crack healing zone, (c) 1200 ℃, (d) magnified image of the selected region in Fig. 4c showing crack healing zone.
3.5. Charpy fracture morphology Fig. 6 shows SEM fractographs of the pre-crack zone before healing treatment. The upper and lower parts fractured directly during the fabricating of Charpy impact specimens from the drum samples before healing treatment. The fracture surface was flat and massive corrugated machining marks were clearly identified, as shown in Fig. 6a. In order to further examine the fracture surface, the magnified images of selected regions b and c were achieved. It can be obviously seen that the fracture of the pre-crack zone exhibited massive dimples and unhealed regions, as shown in Fig. 6b and c. This indicated that local healing between the upper and lower end surfaces occurred. It can be known from Xin [8] that these dimples were brittle fractures, which were formed by the existing microvoids in the crack healing zone and without macroscopic plastic deformation. Simultaneously, there were lots of second precipitated particles in the unhealed regions, as shown in Fig. 6c. Fig. 7 shows the room-temperature Charpy impact macroscopic fractographs of crack healing zone under various healing conditions. After healing at 900 ℃ for 30 min, as shown in Fig. 7a, the fracture was similar to the fractograph of the pre-crack zone (Fig. 6a). Compared with Fig. 6a, the fracture exhibited a small quantity of cleavage fractures. Furthermore, the area of cleavage fractures increased gradually with increasing healing temperatures and holding times. Correspondingly, the degree of crack healing enhanced progressively. The fracture surfaces of the notch tip and center regions of the Charpy impact specimens fractured at room temperature were examined because their fracture modes might be different, as shown in Fig. 8. The variation of the areas of different fracture modes represented different crack healing degree. After healing at 900 ℃ for 240 min, the fracture near the notch tip presented brittle dimples and unhealed regions (Fig. 8a). The fracture of the center region
Fig. 5. Percentage recovery of room-temperature Charpy absorbed energy under various healing conditions.
Consequently, the tensile properties of crack healing zone in this work should be restored completely after healing at 1200 ℃ for above 90 min, while its impact properties was far from fully recovered. Based on the above analysis, by comparing the results from Xin [8] with the results in this work, it can be concluded that when the crack has completely disappeared, the static tensile properties can be restored completely but their impact properties are only partially achieved. As a result, the quantitative evaluation of the crack healing degree should rely on the operational environment and loads of workpieces.
437
Materials Science & Engineering A 682 (2017) 433–440
R. Xin et al.
Fig. 6. SEM fractographs of pre-crack zone before healing treatment (a) overall, (b) magnified image of the selected region b in Fig. 6a showing the unhealed region and dimple fracture region, (c) magnified image of selected region c in Fig. 6b showing the unhealed regions (yellow dashed regions). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
absorbed energy after healing at 1200 ℃ was lower than that at 1100 ℃. In view of the above analysis of macroscopic and microscopic fractographs of crack healing zone (Figs. 7 and 8), in the initial healing stage, the Charpy impact fracture presented massive brittle dimples and unhealed regions. With increasing the microstructural healing degree of crack zone, the area of brittle dimples and unhealed regions gradually decreased, corresponding to that the area of cleavage fracture gradually increased. When the pre-crack was healed at 1100 ℃ and 1200 ℃, respectively, for 240 min, the unhealed regions completely disappeared, and the fractographs of the notch tip regions were mostly composed of brittle dimples while the center regions exhibited cleavage and ductile dimple mixed fractures.
occurred in a cleavage mode together with a brittle dimpled mode, as shown in Fig. 8b. And some unhealed regions were also seen in Fig. 8b. When healing temperature reached 1000 ℃, the fracture of the notch tip region exhibited massive brittle dimples with uneven size and a small quantity of microvoids, and unhealed regions nearly disappeared, as shown in Fig. 8c. The center region fractured in a cleavage mode which had typical river patterns (Fig. 8d). At the same time, brittle dimples have disappeared in the center region. When the pre-crack was healed at 1100 ℃ for 240 min, the fracture of the notch tip region showed complete brittle dimples, and there have been no microvoids and unhealed regions, as shown in Fig. 8e. At this point, the center region exhibited cleavage and ductile dimple mixed fracture, as shown in Fig. 8f. The existence of ductile dimples at 1100 ℃ was corresponding to the higher Charpy absorbed energy. When the healing temperature further raised to 1200 ℃, the fracture of the notch tip region occurred mostly in a brittle dimpled mode and a small quantity of cleavage mode. as shown in Fig. 8g. The occurrence of cleavage facets in the notch tip region indicated the microstructural healing degree of crack healing zone further enhanced. Meanwhile, the fracture of the center region was similar to that at 1100 ℃ and presented massive cleavage facets and a small quantity of ductile dimples, as shown in Fig. 8g. The fracture exhibited typical fan-shaped river pattern. Compared with Fig. 8f and h, the size of cleavage facet at 1200 ℃ was greater than that at 1100 ℃. Correspondingly, the Charpy
4. Conclusions 1. Impact properties of internal crack healing in this low carbon steel dramatically increase from 900 ℃ to 1000 ℃ due to two different mechanisms of crack healing, atomic diffusion at 900 ℃, and recrystallization and grain growth at above 1000 ℃. Recrystallization in the crack healing zone promotes the rapid restoration of impact properties of internal crack healing 2. Percentage recovery of impact properties increases with increasing healing temperature and holding time in the temperature range
Fig. 7. Room-temperature Charpy impact macroscopic fractographs of crack healing zone under various healing conditions: (a) 900 ℃×30 min, (b) 900 ℃×240 min, (c) 1000 ℃×240 min, (d) 1100 ℃×240 min, (e) 1200 ℃×240 min.
438
Materials Science & Engineering A 682 (2017) 433–440
R. Xin et al.
Fig. 8. SEM fractographs of the U-notch tip and center regions of Charpy impact fracture surfaces of crack healing zone under different healing conditions: (a, b) 900 ℃×240 min, (c, d) 1000 ℃×240 min, (e, f) 1100 ℃×240 min, (g, h) 1200 ℃×240 min.
439
Materials Science & Engineering A 682 (2017) 433–440
R. Xin et al.
900–1100 ℃, while impact properties deteriorate at 1200 ℃ due to the formation of reticular ferrite and grain coarsening near the crack healing zone. 3. In this work, when internal cracks are disappeared completely, tensile properties of crack healing zones can be restored completely but their impact properties are only partially achieved. 4. In the initial stage of crack healing, the Charpy fracture exhibits massive brittle dimples and unhealed regions. With the increase of healing temperature and holding time, cleavage fracture regions increase progressively. The ultimate fracture of the notch tip region presents brittle dimples and the fracture surface of the center region are mainly composed of cleavage facets with a small quantity of ductile dimples.
[9] R.S. Xin, Q.X. Ma, Z.D. Shan, Microstructure evolution of internal crack healing in a low-carbon steel at elevated temperatures, J. Tsinghua Univ. (Sci. Technol.) 55 (2015) 304–309. [10] A. Hosoi, T. Nagahama, Y. Ju, Fatigue crack healing by a controlled high density electric current field, Mater. Sci. Eng. A 533 (2012) 38–42. [11] H. Song, Z.J. Wang, Microcrack healing and local recrystallization in pre-deformed sheet by high density electropulsing, Mater. Sci. Eng. A 490 (2008) 1–6. [12] Y.Z. Zhou, J.D. Guo, M. Gao, G.H. He, Crack healing in a steel by using electropulsing technique, Mater. Lett. 58 (2004) 1732–1736. [13] X.G. Zheng, Y.N. Shi, K. Lu, Electro-healing cracks in nickel, Mater. Sci. Eng. A 561 (2013) 52–59. [14] Y.P. Tang, A. Hosoi, Y. Morita, Y. Ju, Restoration of fatigue damage in stainless steel by high-density electric current, Int. J. Fatigue 56 (2013) 69–74. [15] Y.Z. Zhou, Y. Zeng, G.H. He, B.L. Zhou, The healing of quenched crack in 1045 steel under electropulsing, J. Mater. Res. 16 (2001) 17–19. [16] X.M. Zhao, X. Lin, J. Chen, L. Xue, W.D. Huang, The effect of hot isostatic pressing on crack healing, microstructure, mechanical properties of Rene88DT superalloy prepared by laser solid forming, Mater. Sci. Eng. A 504 (2009) 129–134. [17] H.L. Yu, X.H. Liu, Thermal-mechanical finite element analysis of evolution of surface cracks during slab rolling, Mater. Manuf. Process. 24 (2009) 570–578. [18] H.L. Yu, A.K. Tieu, C. Lu, A. Godbole, Investigation of closure of internal cracks during rolling by FE model considering crack surface roughness, Int. J. Adv. Manuf. Technol. 75 (2014) 1633–1640. [19] H.T. Gao, Z.R. Ai, H.L. Yu, H.Y. Wu, X.H. Liu, Analysis of internal crack healing mechanism under rolling deformation, Plos One 9 (2014) e101907. [20] C.Y. Park, D.Y. Yang, Modelling of void crushing for large-ingot hot forging, J. Mater. Process. Technol. 67 (1997) 195–200. [21] J.T. Han, G. Zhao, Q.X. Cao, Discovery of inner crack recovery and its structure change in 20MnMo steel, Acta Metall. Sin. 32 (1996) 723–729. [22] H.L. Yu, X.H. Liu, X.W. Li, Crack healing in a low-carbon steel under hot plastic deformation, Metall, Mater. Trans. A 45A (2014) 1001–1009. [23] D.B. Wei, J.T. Han, Z.Y. Jiang, C. Lu, A.K. Tieu, A study on crack healing in 1045 steel, J. Mater. Process. Technol. 177 (2006) 233–237. [24] Y.J. Zhang, J.T. Han, Analysis of microstructure of steel 20 in the range of healing of internal crack, Met. Sci. Heat Treat. 58 (2012) 526–528. [25] K.W. Gao, L.J. Qiao, W.Y. Chu, In situ TEM observation of crack healing in alphaFe, Scripta Mater. 44 (2001) 1055–1059. [26] M. Song, K. Du, S.P. Wen, Z.R. Nie, H.Q. Ye, In situ electron microscopy investigation of void healing in an Al-Mg-Er alloy at at a low temperature, Acta Mater. 69 (2014) 236–245. [27] G.Q. Xu, M.J. Demkowicz, Healing of nanocrakcs by disclinations, Phys. Rev. Lett. 111 (2013) 145501. [28] J. Li, Q.H. Fang, B. Liu, Y. Liu, Y.W. Liu, P.H. Wen, Mechanism of crack healing at room temperature revealed by atomistic simulations, Acta Mater. 95 (2015) 291–301. [29] D.B. Wei, J.T. Han, A.K. Tieu, Z.Y. Jiang, Simulation of crack healing in BCC Fe, Scripta Mater. 51 (2004) 583–587.
Acknowledgments The authors acknowledge the financial support from the National Basic Research Program (“973” Program) of China (Grant No. 2011CB012903). References [1] A.A. Griffith, The phenomena of rupture and flow in solids, Philos. Trans. R. Soc. London, Ser. A 221 (1920) 163–198. [2] D.C. Tung, J.C. Lippold, Self-restrained testing for residual stress driven cracking in nick-based alloys, Mater. Sci. Eng. A 673 (2016) 158–166. [3] C. Örnek, D.L. Engelberg, Towards understanding the effect of deformation mode on stress corrosion cracking susceptibility of grade 2205 duplex stainless, Mater. Sci. Eng. A 666 (2016) 269–279. [4] Z.Y. Liu, X.Z. Wang, C.W. Du, J.K. Li, X.G. Li, Effect of hydrogen-induced plasticity on the stress corrosion cracking of X70 pipeline steel in simulated soil environments, Mater. Sci. Eng. A 658 (2016) 348–354. [5] J. Goebel, T. Ghidini, A.J. Graham, Stress-corrosion cracking characterisation of the advanced aerospace Al-Li 2099-T86 alloy, Mater. Sci. Eng. A 673 (2016) 16–23. [6] C.F. Dong, X.G. Li, Z.S. Shen, W.Y. Chu, Study on crack healing of hydrogen attack in carbon steel by heat treatment, Corros 59 (2003) 401–406. [7] H.L. Zhang, P.Z. Huang, J. Sun, Morphological healing evolution of penny-shaped fatigue microcracks in pure iron at elevated temperatures, Appl. Phys. Lett. 87 (2004) 1143–1145. [8] R.S. Xin, Q.X. Ma, W.Q. Li, Microstructure and mechanical properties of internal crack healing in a low carbon steel, Mater. Sci. Eng. A 662 (2016) 65–71.
440