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The mechanical properties of medium carbon steel processed by a biomimetic laser technique
Materials Science & Engineering A 560 (2013) 627–632
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
The mechanical properties of medium carbon steel processed by a biomimetic laser technique Pengyu Lin a, Zhihui Zhang a,n, Hong Zhou b, Luquan Ren a a The Key Laboratory of Engineering Bionics (Ministry of Education, China) and the College of Biological and Agricultural Engineering, Jilin University (Nanling Campus), 5988 Renmin Street, Changchun 130025, PR China b The Key Laboratory of Automobile Materials of China Ministry of Education, School of Materials Science and Engineering, Nanling Campus of Jilin University, Changchun, Jilin Province 130025, People’s Republic of China
a r t i c l e i n f o
abstract
Article history: Received 19 July 2012 Received in revised form 1 October 2012 Accepted 4 October 2012 Available online 17 October 2012
We investigate the effects of biomimetic laser surface treatment on the mechanical properties of medium carbon steel. Steel surface was strengthened by laser-treated or -hardened area (the biomimetic strengthening spots). Microhardness, UTS and YS are simultaneously improved by the martensitic microstructure with nano-sized carbides due to the self-quenching effect of laser treatment. The ductility is also increased but not continuously. This is because the laser-induced martensite is detrimental to ductility. In addition, thermal fatigue behavior of medium carbon steel is improved. Fewer cracks are found after thermal fatigue test. Crack nucleation and propagation are hampered by the strengthening spots. The biomimetic sample, as compared with the as-received counterpart, has a slower crack growth. & 2012 Elsevier B.V. All rights reserved.
Keywords: Electron microscopy Martensite Steel Grain refinement Precipitation
1. Introduction Medium carbon steel has been considered as one of the most important structural materials because of its stable chemical properties and good mechanical properties [1–3]. It is mainly applied to the automobile, ship-making, and military industries due to its proper alloying concentrations that give this wide availability. As our requirements to the safety and service grow, studies [4–7] are carried out to better its service by improving relative properties. Hot or cold working is a good way for this purpose. Clayton and Jin [1] employed a cooling process to develop a microstructure of medium carbon steel, which comprises carbide-free bainite ferrite laths, with or without lath boundary retained austenite. This microstructure is the best of all samples in that work and exhibits a wear resistance comparable to that of Hadfield’s austenitic steel under severe rolling/sliding contact. In the meanwhile, another study [7] shows that an intercalated protective layer by co-precipitation provided additional resistance to corrosion as a corrosion inhibitor and reduced corrosion rate. We have developed a new method to improve the mechanical properties of steels and irons, namely the biomimetic strengthening technique [8–12]. This method is as effective as those aforementioned [1,4,7]. By mimicking the nature or structure of some creatures like pangolins and leaves that completely adapt to their environment, similar
n
Corresponding Author. Tel.: þ86 431 8578 0434; fax: þ 86 431 509 4699. E-mail address: [email protected] (Z. Zhang).
0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.10.006
surfaces on steels can be acquired. Notwithstanding some biomimetic techniques employed casting [13] and etc., most of them prefer laser surface process (melting or remelting) [12,14]. This is because laser can be easily operated and moreover it can mimick many complex surfaces, such as spots and networks. One of our previous studies [14] reported an improvement of mechanical properties of H13 die steel led by the surface with biomimetic strengthening spots (convex units). This structure mimicks the outer skin of some beetles and the convex structure gives them excellent resistance to wear conditions. Results show that the ultimate tensile strength, yield strength and elongation were increased due to it. Zhang et al. [8] studied the thermal fatigue properties of medium carbon steel using this method and related mechanisms were discussed but further studies are still needed with regard to the importance and potential of this material. In another study, Zhou et al. [15] also investigated the effects of biomimetic strengthening stripes on the thermal fatigue behavior of steel. It indicates that nevertheless the biomimetic laser technique can fabricate strengthening structure in different shapes; these structures benefit the surface properties of steel. Here we provide a detailed study on effects of biomimetic laser technique on the mechanical properties of medium carbon steel, in order to fulfill the vacancy of this field. The microstructure, mechanical properties and their relation were discussed.
2. Materials and methods The No. 45 steel was chosen to represent medium carbon steel. Its chemical compositions are 0.48 wt% C, 0.2 wt% Si, 0.45 wt% Mn,
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P. Lin et al. / Materials Science & Engineering A 560 (2013) 627–632
Fig. 1. The surface morphology of biomimetic samples: (a) Biomimetic strengthening spots on sample surface, each spot represents one laser surface treatment; (b) overview of the biomimetic samples for tensile test.
0.03 wt% S, 0.01 wt% P and Fe in balance. This material is the one studied in Ref. [8]. Samples were cut in the dimension of 40 20 3 mm3 using an electro-discharge machine. The biomimetic laser technique was described elsewhere [14–17]. So it is only briefly introduced here. An Nd:YAG laser with the wavelength of 1.06 mm was employed to prepare the biomimetic surfaces on samples. The surface morphology of the samples was engineered to mimick that of beetles covered by some convex spots. This feature is believed to provide excellent adaptability in their corresponding habitats [18]. The surface morphology of the samples is shown in Fig. 1a. Our previous study [17] shows that changing some key parameters, such as the current and the wavelength, led to the change of treatment effects like the effective strengthening depth. So in this study, the laser parameters are same for all samples. The diameter of diomimetic strengthening spots is 0.7 mm, and they are lined in two directions, along the tensile direction and perpendicular to it. The spacing between every two spots is 1.5 mm. The schematic of a tensile test sample is shown in Fig. 1b. In order to determine the relationship between mechanical properties and microstructures, tensile and thermal tests were carried out. The sample for tensile test is pictured in Fig. 1b. Microhardness was also examined using a Knoop and Vickers Hardness Table (USA), under a load of 25 g. The thermal fatigue tests were carried out on a self-controlled thermal fatigue testing machine. A complete cycle consists of heading for 80 s up to the maximum temperature of 650 1C in resistance furnace, and then water cooling for 3 s to the minimum temperature of 25 1C in water tank. Plain-view and cross-sectional microstructures of the samples were characterized by scanning electron microscopy (SEM, JSM5600LV, Japan) equipped with energy dispersive specstrometers (EDS). X-ray diffraction (XRD) was also used to examine samples.
3. Results Fig. 2 shows the SEM microstructures of the as-received No. 45 and laser-treated steels. Apparently the as-received microstructure comprises ferrite and austenite (see Fig. 2a). This characteristic was once reported elsewhere [8,15]. In comparison, the microstructure
Fig. 2. The plain-view SEM micro-structure of the as-received and laser-treated steels: (a) microstructure of as-received No. 45 steel; (b) laser-treated (a); part (c) shows the boundary of a and b with a transition zone between them.
of the strengthening spots mainly consists of martensite. The two microstructures were shown simultaneously in Fig. 2c and in addition, a transition area between them was acquired. XRD (Fig. 3) better identified phases in microstructure. The biomimetic strengthening spots exhibit a microstructure with reduced austenite (Fig. 3b), compared with its as-received counterpart. As inferred from both Figs. 2 and 3, the laser treatment gave rise to martensitic transformation [19] and the mechanism will be discussed later.
P. Lin et al. / Materials Science & Engineering A 560 (2013) 627–632
Our EDS gives the distribution of elements in Fig. 4. It is obvious that the chemical compositions of the laser-treated microstructure are homogeneous in both plain and perpendicular directions and in the meanwhile, elemental aggregation cannot be found. Moreover, the cross-sectional morphology of the strengthening spots is also illustrated in Fig. 4a and b. The microhardness of biomimetic strengthening spots is plotted in Fig. 5 as a function of physical distance in both plain and crosssectional directions. Distribution of the microhardness is also homogeneous, which is in excellent agreement with that of chemical
Fig. 3. The XRD patterns of the as-received and laser-treated steels: (a) the as-received material; (b) the laser-treated counterpart.
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composition. It indicates that a considerable improvement in microhardness was possessed due to the laser surface treatment.
4. Discussion Fig. 6 gives the relation between mechanical properties and the laser-treated area. We found that as the laser-treated relative area to overall surface of samples increases, namely the number of the biomimetic strengthening spots on surface increases, the ultimate tensile strength (UTS) and yield strength (YS) both are improved, with the increase of 14% and 17%, respectively at the area ratio of 25–30% (Fig. 6a). On the other hand, the increase in the number of strengthening spots on sample surface also caused considerable change to elongation (EL) but not like the UTS and YS which have the continuous improvement. A turning point is resulted at the ratio of 17%. The corresponding maximum EL is 40%. As a matter of fact, the biomimetic laser strengthening technique resulted in some similar effects with those led by other laser surface treatments, such as laser surface melting (LSM), laser surface remelting (LSR) and laser surface hardening (LSH) [19–22]. Basu et al. [23] studied the improvement in wear resistance improvement of 52100 steel treated with laser surface hardening (LSH) and an analytical modeling with experimental parameters. The laser surface technique generated a selfquenching effect on the surface of steel and gave rise to the microstructure with predominant martensite and carbides. Due to it, the hardness of the steel surface was improved. It should be noted that laser can only harden the surface of a certain depth with pre-determined parameters such as energy and power density. In the present study, the depth is 400 mm. In addition, Colaco and Vilar [24] reported the reduction of austenite because
Fig. 4. EDS line analysis of the elemental composition: (a) and (b) microstructures with two EDS analyzed zones; (c) EDS of (a); (d) EDS of b.
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Fig. 6. The relations of mechanical properties like UTS, YS and EL with the lasertreated area: (a) the UTS and YS increase with the laser-treated area; (b) the effect of laser-treated area on EL. Fig. 5. The microhardness of biomimetic strengthening spots: (a) in the crosssectional direction, the depth ‘‘0.0’’ represents the surface; (b) in the plain direction, the dotted line marks the geometric axis of the strengthening spot.
of laser surface melting, which also facilitates hardness improvement. As shown in Figs. 4 and 5, the microstructure closer to surface has higher microhardness. So it is conceivable that it contains less austenite. Fig. 7 shows that TEM microstructure of a steel sample processed by the laser technique. The microstructure comprises martensite and high dislocation-density and nano-sized carbides were shown in Fig. 7b. They are mainly the M7C3 or M23C6 carbides [24]. In a word, all the characteristics contribute to the resulted high hardness of the laser-strengthened spots. When the area ratio increased, or in other words, the number of strengthening spots on surface increased, the UTS and YS were better improved and this improvement was continuous with the area ratio (see Fig. 6a). Whereas a turning point on the EL curve intercepted this continuousness (Fig. 6b). At the area ratio of 17%, the EL of 40% was acquired. One can therefore anticipate that a predominantly martensitic microstructure leads to decreased ductility. What is more, some previous studies [19,25] claimed that the presence of ductile phases in the hard matrix should be beneficial for improving the mechanical properties. Fig. 6 demonstrates that proper combination of treated and as-received areas can optimize mechanical properties. Table 1 gives the crack density on the surface of samples. It is obvious that under the same thermal fatigue condition, the
biomimetic strengthened samples have fewer cracks than their as-received counterparts. In addition, cracks of both samples increase with thermal fatigue cycle. The surface morphology and cracks of samples are shown in Fig. 8. The as-received sample exhibits a surface with naturally developed cracks. In comparison, cracks on the surface of the laser-treated one, however, are curbed by the strengthening spots. As inferred from Fig. 8 [8] and as shown by other studies [15,26], cracks propagation can be stopped, deviated or bifurcated by the strengthening spots because crack nucleation, propagation or even penetration on the hardened area requires higher energy. This phenomenon is proposed as the blocking effect [10]. The crack-prohibiting mechanism was also given in the form of mathematical relation.
5. Conclusions We use laser surface treatment (remelting) to bring biomimetic strengthening effects to medium carbon steel. The strengthened surface is featured by the biomimetic strengthening spots. These strengthening spots are inspired by the skin of some beetles. The improvements of microhardness, UTS and YS of surface essentially lie in the self-quenching effect of laser surface remelting that generates a predominantly martensitic microstructure with nano-sized carbides. In the meanwhile, ductility of the samples is also increased.
P. Lin et al. / Materials Science & Engineering A 560 (2013) 627–632
Fig. 7. The TEM microstructure of a steel sample processed by the biomimetic laser technique: (a) the microstructure with martensite and dislocation; (b) The dark-field TEM shows the nano-sized carbides.
Table 1 Crack density on surfaces of both as-received and laser-treated samples., where L is the crack length (mm). In our fatigue test, the cracks are categorized as small sized crack (L o0.5 mm), medium sized crack (0.5 mmoL o1.5 mm) and large sized crack (L 41.5 mm). Both the as-received and laser-treated samples were tested under the same condition.
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Fig. 8. The surface morphologies of both as-received and laser-treated samples after thermal fatigue test: (a) the as-received sample; (b) the laser-treated sample.
are hampered by the spots. On the other hand, the numbers of cracks on both samples increase with the fatigue cycle, whereas, the biomimetic sample has a slower crack growth.
Acknowledgements Cycling number
As-received sample
L o0.5 (mm) r400 400–800 800– 1200 1200– 1600 Z2400
The sample with biomimetic strengthening
0.5r Lr 1.5 (mm)
L41.5 (mm)
Lo 0.5 (mm)
0.5 rL r1.5 (mm)
L41.5 (mm)
5 5
– – 3
– –
1 3
– – 1
– –
11
6
1
7
3
–
29
15
4
20
8
2
On the other hand, as the number of strengthening spots on surface increases, or namely the treated area is increased, both UTS and YS possess a continuous increase but not the elongation. This is because the laser-induced martensite is detrimental to ductility. Presence of ductile phases is beneficial for improving the mechanical properties. Biomimetic strengthening spots also better the thermal fatigue behavior of medium carbon steel. Fewer cracks are found on the strengthened surface. On one hand, crack nucleation and propagation
This work is supported by Project 985 -Bionic Engineering Science and Technology Innovation of Jilin University, the National Natural Science Foundation for Youths (No.51005097), and the Research Fund of young scholars for the Doctoral Program of Higher Education of China (No. 20100061120074). References [1] P. Clayton, N. Jin, Wear 200 (1996) 74–82. [2] B. Kurt, J. Mater. Proc. Technol. 190 (2007) 138–141. [3] P. Venkata Ramana, G. Madhusudhan Reddy, T. Mohandas, A.V.S.S.K.S. Gupta, Mater. Des. 31 (2010) 749–760. [4] A. Atta, O. El-Azabawy, H. Ismail, M. Hegazy, Corros. Sci. 53 (2011) 1680–1689. [5] M.A. Hegazy, A.S. El-Tabei, A.H. Bedair, M.A. Sadeq, Corros. Sci. 54 (2012) 219–230. [6] M. Barbalat, L. Lanarde, D. Caron, M. Meyer, J. Vittonato, F. Castillon, S. Fontaine, Ph. Refait, Corros. Sci. 55 (2012) 246–253. [7] Y. Wang, D. Zhang, Mater. Res. Bull. 46 (2011) 1963–1968. [8] Z. Zhang, L. Ren, H. Zhou, X. Tong, Chin. Sci. Bull. 54 (2009) 584–591. [9] L. Ren, Sci. China. Ser. E: Technol. Sci 52 (2009) 273–284. [10] Z. Zhang, H. Zhou, L. Ren, X. Tong, H. Shan, L. Liu, Int. J. Fatigue. 31 (2009) 468–475.
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[11] H. Zhou, L. Chen, W. Wang, L. Ren, H. Shan, Z. Zhang, Mater. Sci. Eng. A 412 (2005) 323–327. [12] L. Ren, S. Deng, J. Deng, Z. Han, J. Bionic. Eng. 1 (2004) 9–19. [13] H. Zhou, Q. Guo, P. Lin, W. Zhang, X. Zhang, L. Ren, Appl. Surf. Sci. 255 (2008) 3394–3399. [14] Z. Zhang, H. Zhou, L. Ren, X. Tong, H. Shan, Y. Cao, Appl. Surf. Sci. 253 (2007) 8939–8944. [15] H. Zhou, Z. Zhang, L. Ren, Q. Song, L. Chen, Surf. Coat. Tech. 200 (2006) 6758–6764. [16] Z. Zhang, H. Zhou, L. Ren, X. Tong, H. Shan, X. Li, Appl. Surf. Sci. 254 (2008) 2548–2555. [17] Z. Zhang, L. Ren, Ti Zhou2, Z. Han, H. Zhou, L. Chen, Y. Zhao, J. Bionic. Eng. 7 (2010) 67–76.
[18] R. Naik, M. Stone, Mater. Today 9 (2005) 18–26. [19] P. Lin, Y. Zhu, H. Zhou, C. Wang, L. Ren, Scr. Mater 63 (2010) 839–842. [20] C.P. Paul, H. Alemohammad, E. Toyserkani, A. Khajepour, S. Corbin, Mater. Sci. Eng. A 464 (2007) 170–176. [21] P. Cruz, M. Oden, T. Ericsson, Int. J. Fatigue 20 (1998) 389–398. [22] B. Mahmoudi, M.J. Torkamany, A.R.Sabour Rouh Aghdam, J. Sabbaghzade, Mater. Des. 31 (2010) 2553–2560. [23] A. Basu, J. Chakraborty, S.M. Shariff, G. Padmanabham, S.V. Joshi, G. Sundararajan, J. Dutta Majumdar, I. Manna, Scr. Mater. 56 (2007) 887–890. [24] R. Colaco, R. Vilar, Wear 258 (2005) 225–231. [25] C.H. Young, H.K.D.H. Bhadeshia, Mater. Sci. Technol. 10 (1994) 209–214. [26] S. Suresh, Fatigue of Materials, Cambridge University Press, 1992.
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
The mechanical properties of medium carbon steel processed by a biomimetic laser technique Pengyu Lin a, Zhihui Zhang a,n, Hong Zhou b, Luquan Ren a a The Key Laboratory of Engineering Bionics (Ministry of Education, China) and the College of Biological and Agricultural Engineering, Jilin University (Nanling Campus), 5988 Renmin Street, Changchun 130025, PR China b The Key Laboratory of Automobile Materials of China Ministry of Education, School of Materials Science and Engineering, Nanling Campus of Jilin University, Changchun, Jilin Province 130025, People’s Republic of China
a r t i c l e i n f o
abstract
Article history: Received 19 July 2012 Received in revised form 1 October 2012 Accepted 4 October 2012 Available online 17 October 2012
We investigate the effects of biomimetic laser surface treatment on the mechanical properties of medium carbon steel. Steel surface was strengthened by laser-treated or -hardened area (the biomimetic strengthening spots). Microhardness, UTS and YS are simultaneously improved by the martensitic microstructure with nano-sized carbides due to the self-quenching effect of laser treatment. The ductility is also increased but not continuously. This is because the laser-induced martensite is detrimental to ductility. In addition, thermal fatigue behavior of medium carbon steel is improved. Fewer cracks are found after thermal fatigue test. Crack nucleation and propagation are hampered by the strengthening spots. The biomimetic sample, as compared with the as-received counterpart, has a slower crack growth. & 2012 Elsevier B.V. All rights reserved.
Keywords: Electron microscopy Martensite Steel Grain refinement Precipitation
1. Introduction Medium carbon steel has been considered as one of the most important structural materials because of its stable chemical properties and good mechanical properties [1–3]. It is mainly applied to the automobile, ship-making, and military industries due to its proper alloying concentrations that give this wide availability. As our requirements to the safety and service grow, studies [4–7] are carried out to better its service by improving relative properties. Hot or cold working is a good way for this purpose. Clayton and Jin [1] employed a cooling process to develop a microstructure of medium carbon steel, which comprises carbide-free bainite ferrite laths, with or without lath boundary retained austenite. This microstructure is the best of all samples in that work and exhibits a wear resistance comparable to that of Hadfield’s austenitic steel under severe rolling/sliding contact. In the meanwhile, another study [7] shows that an intercalated protective layer by co-precipitation provided additional resistance to corrosion as a corrosion inhibitor and reduced corrosion rate. We have developed a new method to improve the mechanical properties of steels and irons, namely the biomimetic strengthening technique [8–12]. This method is as effective as those aforementioned [1,4,7]. By mimicking the nature or structure of some creatures like pangolins and leaves that completely adapt to their environment, similar
n
Corresponding Author. Tel.: þ86 431 8578 0434; fax: þ 86 431 509 4699. E-mail address: [email protected] (Z. Zhang).
0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.10.006
surfaces on steels can be acquired. Notwithstanding some biomimetic techniques employed casting [13] and etc., most of them prefer laser surface process (melting or remelting) [12,14]. This is because laser can be easily operated and moreover it can mimick many complex surfaces, such as spots and networks. One of our previous studies [14] reported an improvement of mechanical properties of H13 die steel led by the surface with biomimetic strengthening spots (convex units). This structure mimicks the outer skin of some beetles and the convex structure gives them excellent resistance to wear conditions. Results show that the ultimate tensile strength, yield strength and elongation were increased due to it. Zhang et al. [8] studied the thermal fatigue properties of medium carbon steel using this method and related mechanisms were discussed but further studies are still needed with regard to the importance and potential of this material. In another study, Zhou et al. [15] also investigated the effects of biomimetic strengthening stripes on the thermal fatigue behavior of steel. It indicates that nevertheless the biomimetic laser technique can fabricate strengthening structure in different shapes; these structures benefit the surface properties of steel. Here we provide a detailed study on effects of biomimetic laser technique on the mechanical properties of medium carbon steel, in order to fulfill the vacancy of this field. The microstructure, mechanical properties and their relation were discussed.
2. Materials and methods The No. 45 steel was chosen to represent medium carbon steel. Its chemical compositions are 0.48 wt% C, 0.2 wt% Si, 0.45 wt% Mn,
628
P. Lin et al. / Materials Science & Engineering A 560 (2013) 627–632
Fig. 1. The surface morphology of biomimetic samples: (a) Biomimetic strengthening spots on sample surface, each spot represents one laser surface treatment; (b) overview of the biomimetic samples for tensile test.
0.03 wt% S, 0.01 wt% P and Fe in balance. This material is the one studied in Ref. [8]. Samples were cut in the dimension of 40 20 3 mm3 using an electro-discharge machine. The biomimetic laser technique was described elsewhere [14–17]. So it is only briefly introduced here. An Nd:YAG laser with the wavelength of 1.06 mm was employed to prepare the biomimetic surfaces on samples. The surface morphology of the samples was engineered to mimick that of beetles covered by some convex spots. This feature is believed to provide excellent adaptability in their corresponding habitats [18]. The surface morphology of the samples is shown in Fig. 1a. Our previous study [17] shows that changing some key parameters, such as the current and the wavelength, led to the change of treatment effects like the effective strengthening depth. So in this study, the laser parameters are same for all samples. The diameter of diomimetic strengthening spots is 0.7 mm, and they are lined in two directions, along the tensile direction and perpendicular to it. The spacing between every two spots is 1.5 mm. The schematic of a tensile test sample is shown in Fig. 1b. In order to determine the relationship between mechanical properties and microstructures, tensile and thermal tests were carried out. The sample for tensile test is pictured in Fig. 1b. Microhardness was also examined using a Knoop and Vickers Hardness Table (USA), under a load of 25 g. The thermal fatigue tests were carried out on a self-controlled thermal fatigue testing machine. A complete cycle consists of heading for 80 s up to the maximum temperature of 650 1C in resistance furnace, and then water cooling for 3 s to the minimum temperature of 25 1C in water tank. Plain-view and cross-sectional microstructures of the samples were characterized by scanning electron microscopy (SEM, JSM5600LV, Japan) equipped with energy dispersive specstrometers (EDS). X-ray diffraction (XRD) was also used to examine samples.
3. Results Fig. 2 shows the SEM microstructures of the as-received No. 45 and laser-treated steels. Apparently the as-received microstructure comprises ferrite and austenite (see Fig. 2a). This characteristic was once reported elsewhere [8,15]. In comparison, the microstructure
Fig. 2. The plain-view SEM micro-structure of the as-received and laser-treated steels: (a) microstructure of as-received No. 45 steel; (b) laser-treated (a); part (c) shows the boundary of a and b with a transition zone between them.
of the strengthening spots mainly consists of martensite. The two microstructures were shown simultaneously in Fig. 2c and in addition, a transition area between them was acquired. XRD (Fig. 3) better identified phases in microstructure. The biomimetic strengthening spots exhibit a microstructure with reduced austenite (Fig. 3b), compared with its as-received counterpart. As inferred from both Figs. 2 and 3, the laser treatment gave rise to martensitic transformation [19] and the mechanism will be discussed later.
P. Lin et al. / Materials Science & Engineering A 560 (2013) 627–632
Our EDS gives the distribution of elements in Fig. 4. It is obvious that the chemical compositions of the laser-treated microstructure are homogeneous in both plain and perpendicular directions and in the meanwhile, elemental aggregation cannot be found. Moreover, the cross-sectional morphology of the strengthening spots is also illustrated in Fig. 4a and b. The microhardness of biomimetic strengthening spots is plotted in Fig. 5 as a function of physical distance in both plain and crosssectional directions. Distribution of the microhardness is also homogeneous, which is in excellent agreement with that of chemical
Fig. 3. The XRD patterns of the as-received and laser-treated steels: (a) the as-received material; (b) the laser-treated counterpart.
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composition. It indicates that a considerable improvement in microhardness was possessed due to the laser surface treatment.
4. Discussion Fig. 6 gives the relation between mechanical properties and the laser-treated area. We found that as the laser-treated relative area to overall surface of samples increases, namely the number of the biomimetic strengthening spots on surface increases, the ultimate tensile strength (UTS) and yield strength (YS) both are improved, with the increase of 14% and 17%, respectively at the area ratio of 25–30% (Fig. 6a). On the other hand, the increase in the number of strengthening spots on sample surface also caused considerable change to elongation (EL) but not like the UTS and YS which have the continuous improvement. A turning point is resulted at the ratio of 17%. The corresponding maximum EL is 40%. As a matter of fact, the biomimetic laser strengthening technique resulted in some similar effects with those led by other laser surface treatments, such as laser surface melting (LSM), laser surface remelting (LSR) and laser surface hardening (LSH) [19–22]. Basu et al. [23] studied the improvement in wear resistance improvement of 52100 steel treated with laser surface hardening (LSH) and an analytical modeling with experimental parameters. The laser surface technique generated a selfquenching effect on the surface of steel and gave rise to the microstructure with predominant martensite and carbides. Due to it, the hardness of the steel surface was improved. It should be noted that laser can only harden the surface of a certain depth with pre-determined parameters such as energy and power density. In the present study, the depth is 400 mm. In addition, Colaco and Vilar [24] reported the reduction of austenite because
Fig. 4. EDS line analysis of the elemental composition: (a) and (b) microstructures with two EDS analyzed zones; (c) EDS of (a); (d) EDS of b.
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Fig. 6. The relations of mechanical properties like UTS, YS and EL with the lasertreated area: (a) the UTS and YS increase with the laser-treated area; (b) the effect of laser-treated area on EL. Fig. 5. The microhardness of biomimetic strengthening spots: (a) in the crosssectional direction, the depth ‘‘0.0’’ represents the surface; (b) in the plain direction, the dotted line marks the geometric axis of the strengthening spot.
of laser surface melting, which also facilitates hardness improvement. As shown in Figs. 4 and 5, the microstructure closer to surface has higher microhardness. So it is conceivable that it contains less austenite. Fig. 7 shows that TEM microstructure of a steel sample processed by the laser technique. The microstructure comprises martensite and high dislocation-density and nano-sized carbides were shown in Fig. 7b. They are mainly the M7C3 or M23C6 carbides [24]. In a word, all the characteristics contribute to the resulted high hardness of the laser-strengthened spots. When the area ratio increased, or in other words, the number of strengthening spots on surface increased, the UTS and YS were better improved and this improvement was continuous with the area ratio (see Fig. 6a). Whereas a turning point on the EL curve intercepted this continuousness (Fig. 6b). At the area ratio of 17%, the EL of 40% was acquired. One can therefore anticipate that a predominantly martensitic microstructure leads to decreased ductility. What is more, some previous studies [19,25] claimed that the presence of ductile phases in the hard matrix should be beneficial for improving the mechanical properties. Fig. 6 demonstrates that proper combination of treated and as-received areas can optimize mechanical properties. Table 1 gives the crack density on the surface of samples. It is obvious that under the same thermal fatigue condition, the
biomimetic strengthened samples have fewer cracks than their as-received counterparts. In addition, cracks of both samples increase with thermal fatigue cycle. The surface morphology and cracks of samples are shown in Fig. 8. The as-received sample exhibits a surface with naturally developed cracks. In comparison, cracks on the surface of the laser-treated one, however, are curbed by the strengthening spots. As inferred from Fig. 8 [8] and as shown by other studies [15,26], cracks propagation can be stopped, deviated or bifurcated by the strengthening spots because crack nucleation, propagation or even penetration on the hardened area requires higher energy. This phenomenon is proposed as the blocking effect [10]. The crack-prohibiting mechanism was also given in the form of mathematical relation.
5. Conclusions We use laser surface treatment (remelting) to bring biomimetic strengthening effects to medium carbon steel. The strengthened surface is featured by the biomimetic strengthening spots. These strengthening spots are inspired by the skin of some beetles. The improvements of microhardness, UTS and YS of surface essentially lie in the self-quenching effect of laser surface remelting that generates a predominantly martensitic microstructure with nano-sized carbides. In the meanwhile, ductility of the samples is also increased.
P. Lin et al. / Materials Science & Engineering A 560 (2013) 627–632
Fig. 7. The TEM microstructure of a steel sample processed by the biomimetic laser technique: (a) the microstructure with martensite and dislocation; (b) The dark-field TEM shows the nano-sized carbides.
Table 1 Crack density on surfaces of both as-received and laser-treated samples., where L is the crack length (mm). In our fatigue test, the cracks are categorized as small sized crack (L o0.5 mm), medium sized crack (0.5 mmoL o1.5 mm) and large sized crack (L 41.5 mm). Both the as-received and laser-treated samples were tested under the same condition.
631
Fig. 8. The surface morphologies of both as-received and laser-treated samples after thermal fatigue test: (a) the as-received sample; (b) the laser-treated sample.
are hampered by the spots. On the other hand, the numbers of cracks on both samples increase with the fatigue cycle, whereas, the biomimetic sample has a slower crack growth.
Acknowledgements Cycling number
As-received sample
L o0.5 (mm) r400 400–800 800– 1200 1200– 1600 Z2400
The sample with biomimetic strengthening
0.5r Lr 1.5 (mm)
L41.5 (mm)
Lo 0.5 (mm)
0.5 rL r1.5 (mm)
L41.5 (mm)
5 5
– – 3
– –
1 3
– – 1
– –
11
6
1
7
3
–
29
15
4
20
8
2
On the other hand, as the number of strengthening spots on surface increases, or namely the treated area is increased, both UTS and YS possess a continuous increase but not the elongation. This is because the laser-induced martensite is detrimental to ductility. Presence of ductile phases is beneficial for improving the mechanical properties. Biomimetic strengthening spots also better the thermal fatigue behavior of medium carbon steel. Fewer cracks are found on the strengthened surface. On one hand, crack nucleation and propagation
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