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Microstructure, hardness, and thermal fatigue behavior of H21 steel processed by laser surface remelting
Applied Surface Science 276 (2013) 62–67
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Microstructure, hardness, and thermal fatigue behavior of H21 steel processed by laser surface remelting Zhihui Zhang a , Pengyu Lin a,∗ , 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, People’s Republic of 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
Article history: Received 1 September 2012 Received in revised form 1 March 2013 Accepted 2 March 2013 Available online 14 March 2013 Keywords: Steel SEM Laser Microstructure Microhardness
a b s t r a c t We investigate the effects of laser surface remelting on the resistance to thermal fatigue of a hot working die steel H21 in this work. Laser treatment was engineered to process samples with several treated morphologies. A dominantly martensitic microstructure, nano-sized carbide and high dislocation density in the treated area, were resulted. The average microhardness of the treated area is ∼780 HV. The thermal fatigue resistance of laser-treated samples is notably higher than their as-received counterpart. As the thermal fatigue cycle increases, the microhardness of treated area is reduced and then flatted. Crack initiation and propagation both are hampered by it. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Many studies pay attention to the hot working die steel H21 (3Cr2W8V) due to its application in engineering fields [1–3]. The resistance to thermal fatigue of H21 is of particular interest to researchers, because of its nature and its working environment. Via calculation, Li et al. [4] reported a new equation to express the thermal fatigue crack initial life (TFCL) from the viewpoint of engineering. The optimum heat treatment condition is thereby given in their study. In the meanwhile, the thermal fatigue resistance of H21 is also compared to that of H13 due to their respective chemical compositions, for better understanding its thermal fatigue behavior both theoretically and experimentally. The wear behavior of H21 is also of interest to researchers [2]. The hard phase particles in tool steels can considerably influence wear resistance. When the volume fraction of carbide in H21 is increased up to 5 vol.%, a microstructure with homogeneous distribution of (WTi)C is produced. The corresponding wear resistance is also improved. Note that both thermal fatigue and wear are related to surface characteristics. From this point of view, laser surface treatment has played a significant role in modifying alloy microstructure on
∗ Corresponding author. Tel.: +86 0431 8578 0434. E-mail address: [email protected] (P. Lin). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.03.009
surface and then improving properties [5–8]. It can produce a thin layer at surface with fine grains and small dendrites [7]. Therefore, laser processing recently has been widely used to harden, melt, or remelt the surface of steels [9–11]. Conde et al. [11] employed laser surface melting (LSM) to treat different steels and claimed that a modified surface layer is critical to achieve property improvement. In the meanwhile, they also suggested that laser processing parameters should be chosen to generate optimum experimental results. A few studies on laser processing and H21 steel have been carried out elsewhere [12–15]. Laser strengthening spots were engineered by Chen et al. [12] to form the non-smooth surface of Cr2W8V steel and to improve its hardness and thus wear resistance. They also claimed that the wear behavior is dependent on spacing between spots. Distribution of strengthening spots on sample surface is directly associated with wear resistance. In addition, another study [15] reported effects of strengthening stripes on surface of this material and showed that different parameters have different effects on modifying the surface microstructure. As compared with frequency and scanning speed of laser, pulse energy and duration have played major role in modifying the microstructure. In addition to these parameters, these studies investigated different laser-treated surface morphologies. Ref. [12] reported a processed surface with strengthening spots that increased microhardness. Several morphologies are engineered by Zhou et al. [13]. Zhang et al. [16] also studied mechanical properties, microstructures and
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thermal fatigue behavior with different surface morphologies. All the aforementioned studies are essential for developing good laser treatment for H21 steel. However, so far, there have been a few direct studies and many others are still needed. In order to further the study on laser processing of H21 die steel, in our study, we investigate the microstructural evolution, and the thermal fatigue behavior of this material processed by laser surface remelting (LSR) with different treated morphologies. Related mechanism is also discussed. 2. Materials and methods The chemical compositions of the hot working die steel H21 are 0.35 wt.% C, 0.30 wt.% Si, 0.40 wt.% Mn, 2.60 wt.% Cr, 8.02 wt.% W, 0.40 wt.% V and Fe in balance. Samples were cut in the dimension of 40 × 20 × 3 mm3 using an electro-discharge machine. An Nd: YAG pulsed laser with the wavelength of 1.06 m was employed to treat the surface of samples. Note that only part of surface was treated. This is different with some other studies [17,18]. It will be discussed
Fig. 2. SEM microstructure of samples: (a) microstructure of the as-received sample; (b) microstructure of laser-treated sample, and (b) provides comparison of the two areas.
later. On the other hand, effects of the relative laser-treated area to overall steel surface on resistance to thermal fatigue (area ratio, %) were also studied in this work. Thermal fatigue tests were carried out by self-restrained thermal fatigue testing machine. It should be noted that Zhang and co-workers [19] studied the laser-treated stripes and, however, by varying the stripe spacing, different thermal fatigue results were achieved. Therefore, in this work, in order to better study the effects of laser-treated morphologies on thermal fatigue behavior, three morphologies are designed: spot, network, and stripe. Three laser-treated surface morphologies are engineered in our study to compare with their experimental results. All samples with different laser-treated morphologies are shown in Fig. 1.
Fig. 1. The surface morphologies of laser-treated samples: (a) strengthening spots on sample surface, each spot represents one laser surface treatment; (b) strengthening stripes on sample surface; (c) strengthening network on sample surface.
Fig. 3. The XRD patterns of both as-received and laser-treated samples.
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The microstructure of samples after various thermal fatigue cycles was characterized to study the microstructural evolution during thermal fatigue. Microhardness was also examined using a Knoop and Vickers Hardness Table (USA), under a load of 25 g. In this study, a characteristic is discussed: the effective treatment depth, or the strengthening depth. This is because laser can only treat a surface layer. It is essential to measure and analyze this characteristic. As a matter of fact, the microhardness can be used to determine the effective treatment depth, because the laser-treated area possesses notably higher microhardness than its original counterpart. Plain-view and cross-sectional microstructures of the samples were given using scanning electron microscopy (SEM, JSM-5600LV, Japan) equipped with energy dispersive spectrometers (EDS). High resolution transmission electron microscopy (HRTEM, JEOL-2100F, Japan) was used to show microstructural details. X-ray diffraction (XRD) helped to examine sample phase. 3. Results Fig. 2 shows SEM morphologies of both as-received and lasertreated samples. The microstructure of the as-received sample comprises coarse grains and large size carbide and dendrites (Fig. 2a). Fig. 2b provides comparison of original and laser-treated areas. The laser-treated microstructure consists of dominant martensite and small-size precipitate in appearance of necklace. Many previous studies [16,19,20,21] reported the self-quenching effect of laser surface treatment. The very high cooling speed leads to formation of dominantly martensitic microstructure. When the original microstructure contains C and other alloying elements in high concentration, ultrafine or even nano-sized precipitates also are formed. XRD (Fig. 3) is employed to examine the two samples. Note that a new phase (Fe3 W3 C) is seen. It indicates that the precipitate is in the form of M6 C. Our EDS (Fig. 4) shows the elemental distribution of the lasertreated area. It is obvious that within this microstructure, a homogeneous elemental distribution was acquired after laser treatment. Apparent element aggregation is not seen in Fig. 4b. This characteristic is essential for improving mechanical properties and its effect will be discussed later. On the other hand, the effective treatment depth of our LSR is also found in Fig. 4a. Many studies have different treatment depth. For instance, Ref. [7] gives a depth of ∼80 m, and moreover another study [19] gives ∼40. This
Fig. 4. The cross-sectional SEM microstructure of the laser-treated area, and EDS line analysis of the elemental composition: (a) the EDS analysed area; (b) EDS of (a).
Fig. 5. HRTEM microstructure of laser-treated sample: (a) the overall HRTEM microstructure, the inserted image is the corresponding SAD pattern; (b) the nanosized carbides.
Fig. 6. the distribution of microhardness in the laser-hardened area: (a) in plainview direction; (b) in perpendicular direction.
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is because different laser parameters, such as power output and density, were pre-determined. In the present study, the effective treatment depth is ∼650 m. Fig. 5 shows the HRTEM microstructure of the laser-treated area. It is characterized as dominant martensite, nano-sized carbide and high dislocation density. Fig. 5b better demonstrates large amount of nano-sized carbide in appearance of necklace. This result coincides with Fig. 2. Fig. 6 gives microhardness curves of the laser-treated area. Like elemental distribution, the distribution of microhardness is also homogeneous. In the both plain and longitudinal directions to the sample surface, the laser-treated area has an average microhardness of ∼780 HV. Distribution of microhardness in the longitudinal direction, on the other hand, demonstrates the effective strengthening depth from the sample surface. Fig. 7. The relation of microhardness with thermal fatigue cycles in thermal fatigue test: both the as-received and laser-treated areas were investigated.
4. Discussion The microhardness of samples as the function of thermal fatigue cycle is given in Figure 7. A notable change of microhardness of the treated area was caused by thermal fatigue test. In the
Fig. 8. The microstructural evolution during thermal fatigue test: (a) the microstructure after 0 time, namely before the thermal fatigue test; (b) after 800 times; (c) after 1600 times; (d) after 2400 times; (e) after 3200 times; (f) after 4000 times.
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Table 1 The crack densities of both as-received and laser-treated samples. Number of cycles
Original Crack (mm)
600 600–800 800–1000 1000–1400 2400
– 1.2 2.2 3.4 6
Spots Number
Crack (mm)
6 15 22 56
– – 1.1 2.4 5.5
Stripes Number
Crack (mm)
4 13 42
– – 1 1.5 4.1
Network Number
Crack (mm)
Number
4 8 30
– – – Micro-crack 2.6
24
Fig. 9. The blocking effect of laser-hardened area: most cracks are hampered in front of the hardened area, irrespective of its shapes. (a) The strengthening stripes; (b) strengthening spots; I is laser-hardened area; II is crack.
early stage of thermal fatigue (≤1000 times), microhardness of the treated area was considerably decreased. In the late stage of thermal fatigue, the microhardness of the treated area is slightly decreased and almost constant. It is twice as high as that of the original surface. This microhardness curve is independent of the laser-treated morphologies because it is an instinctive nature from sample microstructures. Therefore studying the microstructural evolution during the thermal test is essential for better understanding the change of microhardness. Fig. 8 shows the microstructural modification with thermal fatigue. As aforementioned, the laser-hardened microstructure mainly comprises fine martensite. During the thermal fatigue, microstructure was also significantly modified. In the early stage of thermal fatigue test, grains grew. Simultaneously, large amount of C combined with other elements (Fe, W, etc.) and thus large size carbide precipitated at grain boundaries. These activities gave rise to the decrease in microhardness. A relatively stable microstructure was formed in the later stage of thermal fatigue. The corresponding microhardness is also stable, as shown in Fig. 7. The microhardness of original sample is constant, compared to its lasertreated counterpart. Note that in the early stage of thermal fatigue, microstructural evolution is obvious. It agrees well with the change of microhardness in this stage. Table 1 gives the crack densities of different samples after thermal fatigue. As compared with the as-received sample, all three samples with strengthening spots, stripes and network exhibit higher resistance to cracking. Strengthening network has the best resistance. Even in the late stage of thermal fatigue, only small size cracks were observed on this sample. Our analysis indicates that the TFCN lives of the three samples are all higher than that of the as-received one. In the meanwhile, the network gives the highest TFCN life of all. Because of different laser-treated morphologies, the area ratio is also different. The strengthening network has the highest area ratio of all. The blocking effect of hard structure on surface is summarized by Suresh [22], that, crack initiation and propagation both can be stopped, or at least curbed by hard structures, because more energy is required to penetrate them. Tsay and Tsay [23] reported a phenomenon that once cracks were developed
tortuously, the crack growth was slowed. The present experimental results are in good agreement with theirs. It should be noted that the partially treated surface contains soft phase like austenite in original area. Stratton [24], and Young and Bhadeshia [25] both report the benefit of presence of ductile phase like austenite in hard microstructure. A certain amount of austenite in microstructure can enhance the toughness and the ability of stopping, or at least hampering, microcrack that causes mechanical failure. It is conceivable that treatment of partial surface of samples can generate optimum experimental results. To better understand the blocking mechanism of laser-treated area, Cracks nearby different laser-treated morphologies are shown in Fig. 9. It is clear that cracks are blocked or bifurcated in front of strengthening spots, stripes and networks, due to their dominantly martensitic microstructure. However, one can anticipate that, since the strengthening network can block cracks in all directions, the corresponding sample possessed highest resistance of all. In comparison, spots can only hamper cracks toward them. 5. Conclusions A hot working die steel H21 was processed by means of laser surface remelting. Several treatment morphologies were engineered on the steel surface: strengthening spots, stripes and network. LSR can lead to a dominantly martensitic microstructure in the laser-treated area. Other notable characteristics such as nano-sized carbide and high dislocation density were also resulted. The effective treatment depth is ∼650 m. Inside the laser-treated area, both elemental and microhardness distributions are homogeneous. Moreover, the average microhardness of the laser-treated area is ∼780 HV. The thermal fatigue results show that as the experiment progressed, the microhardness of laser-treated area was considerably reduced, but it was still higher than that of the original surface. This change was caused by the microstructural evolution during thermal fatigue. The TFCN life of laser-treated samples is notably higher than their as-received counterpart. The crack size and crack density both are reduced due to the blocking effect of laser-treated
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area. Crack initiation and propagation both are hampered. Amongst all the three laser-treated morphologies, strengthening network led to best result of all, since it can curb cracks in all directions. Acknowledgements This work is supported by 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] M. Wei, S. Wang, L. Wang, K. Chen, Effect of microstructures on elevatedtemperature wear resistance of a hot working die steel, Journal of Iron and Steel Research, International 18 (2011) 47–53. [2] A.J. Leonard, W.M. Rainforth, Wear behaviour of tool steels with added (WTi)C particles, Wear 255 (2003) 517–526. [3] S.Q. Wang, M.X. Wei, F. Wang, X.H. Cui, K.M. Chen, Effect of morphology of oxide scale on oxidation wear in hot working die steels, Materials Science and Engineering A 505 (2009) 20–26. [4] G. Li, X. Li, J. Wu, Study of the thermal fatigue crack initial life of H13 and H21 steels, Journal of Materials Processing Technology 74 (1998) 23–26. [5] B.S. Yilbas, S. Akhtar, C. Karatas, Laser surface treatment of pre-prepared Rene 41 surface, Optics and Lasers in Engineering 50 (2012) 1533–1537. [6] V. Antonov, I. Iordanova, S. Gurkovsky, Investigation of surface oxidation of low carbon sheet steel during its treatment with Nd:glass pulsed laser, Surface and Coatings Technology 160 (2002) 44–53. [7] I. Iordanova, V. Antonov, S. Gurkovsky, Changes of microstructure and mechanical properties of cold-rolled low carbon steel due to its surface treatment by Nd:glass pulsed laser, Surface and Coatings Technology 153 (2002) 267–275. [8] T.M. Yue, L.J. Yan, C.P. Chan, C.F. Dong, H.C. Man, G.K.H Pang, Excimer laser surface treatment of aluminum alloy AA7075 to improve corrosion resistance, Surface and Coatings Technology 179 (2004) 158–164. [9] J. Lee, J. Jang, B. Joo, Y. Son, Y. Moon, Laser surface hardening of AISI H13 tool steel, The Transactions of Nonferrous Metals Society of China 19 (2009) 917–920. [10] B. Mahmoudi, M.J. Torkamany, A.R. Sabour Rouh Aghdam, J. Sabbaghzade, Laser surface hardening of AISI 420 stainless steel treated by pulsed Nd:YAG laser, Materials and Design 31 (2010) 2553–2560.
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[11] A. Conde, R. Colac¸o, R. Vilar, J. de Damborenea, Corrosion behaviour of steels after laser surface melting, Materials and Design 21 (2000) 441–445. [12] L. Chen, L.Q. Ren, Y. Zhao, H. Zhou, The wear-resistance of 3Cr2W8V steel with cave pit non-smooth surface processed by laser, Journal of Bionic Engineering 5 (2008) 34–39. [13] H. Zhou, Y. Cao, Z.H. Zhang, L.Q. Ren, X.Z. Li, Thermal fatigue behavior of 3Cr2W8V die steel with biomimetic non-smooth surface, Materials Science and Engineering A 433 (2006) 144–148. [14] H. Zhou, L. Chen, W. Wang, L.Q. Ren, H.Y. Shan, Z.H. Zhang, Abrasive particle wear behavior of 3Cr2W8V steel processed to bionic non-smooth surface by laser, Materials Science and Engineering A 412 (2005) 323–327. [15] Z. Zhang, H. Zhou, L. Ren, X. Tong, H. Shan, X. Li, Surface morphology of laser tracks used for forming the non-smooth biomimetic unit of 3Cr2W8V steel under different processing parameters, Applied Surface Science 254 (2008) 2548–2555. [16] Z. Zhang, L. Ren, H. Zhou, X. Tong, Biomimetic coupling effect of nonsmooth mechanical property and microstructural features on thermal fatigue behavior of medium carbon steel, Chinese Science Bulletin 54 (2009) 584–591. [17] C. Tassin, F. Laroudie, M. Pons, L. Lelait, Carbide-reinforced coatings on AISI 316 L stainless steel by laser surface alloying, Surface and Coatings Technology 76–77 (76) (1995) 450–460. [18] J. Dutta Majumdar, B. Ramesh Chandra, A.K. Nath, I. Manna, Studies on compositionally graded silicon carbide dispersed composite surface on mild steel developed by laser surface cladding, Journal of Materials Processing Technology 203 (2008) 505–512. [19] H. Zhou, Z.H. Zhang, L.Q. Ren, Q.F. Song, L. Chen, Thermal fatigue behavior of medium carbon steel with striated non-smooth surface, Surface and Coatings Technology 200 (2006) 6758–6764. [20] H. Shin, Y. Yoo, Microstructural and hardness investigation of hot-work tool steels by laser surface treatment, Journal of Materials Processing Technology 201 (2008) 342–347. [21] H. Pantsar, Relationship between processing parameters, alloy atom diffusion distance and surface hardness in laser hardening of tool steel, Journal of Materials Processing Technology 189 (2007) 435–440. [22] S. Suresh, Fatigue of Materials, Cambridge University Press, 1992. [23] L.W. Tsay, C.Y. Tsay, The effect of microstructures on the fatigue crack growth in Ti-6Al-4V laser welds, International Journal of Fatigue 10 (1997) 713–720. [24] P.F. Stratton, Optimising nano-carbide precipitation in tool steels, Materials Science and Engineering A 449 (2007) 809–812. [25] C.H. Young, H.K.D.H. Bhadeshia, The strength of mixtures of bainite and martensite, Materials Science and Technology 10 (1994) 209–214.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Microstructure, hardness, and thermal fatigue behavior of H21 steel processed by laser surface remelting Zhihui Zhang a , Pengyu Lin a,∗ , 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, People’s Republic of 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
Article history: Received 1 September 2012 Received in revised form 1 March 2013 Accepted 2 March 2013 Available online 14 March 2013 Keywords: Steel SEM Laser Microstructure Microhardness
a b s t r a c t We investigate the effects of laser surface remelting on the resistance to thermal fatigue of a hot working die steel H21 in this work. Laser treatment was engineered to process samples with several treated morphologies. A dominantly martensitic microstructure, nano-sized carbide and high dislocation density in the treated area, were resulted. The average microhardness of the treated area is ∼780 HV. The thermal fatigue resistance of laser-treated samples is notably higher than their as-received counterpart. As the thermal fatigue cycle increases, the microhardness of treated area is reduced and then flatted. Crack initiation and propagation both are hampered by it. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Many studies pay attention to the hot working die steel H21 (3Cr2W8V) due to its application in engineering fields [1–3]. The resistance to thermal fatigue of H21 is of particular interest to researchers, because of its nature and its working environment. Via calculation, Li et al. [4] reported a new equation to express the thermal fatigue crack initial life (TFCL) from the viewpoint of engineering. The optimum heat treatment condition is thereby given in their study. In the meanwhile, the thermal fatigue resistance of H21 is also compared to that of H13 due to their respective chemical compositions, for better understanding its thermal fatigue behavior both theoretically and experimentally. The wear behavior of H21 is also of interest to researchers [2]. The hard phase particles in tool steels can considerably influence wear resistance. When the volume fraction of carbide in H21 is increased up to 5 vol.%, a microstructure with homogeneous distribution of (WTi)C is produced. The corresponding wear resistance is also improved. Note that both thermal fatigue and wear are related to surface characteristics. From this point of view, laser surface treatment has played a significant role in modifying alloy microstructure on
∗ Corresponding author. Tel.: +86 0431 8578 0434. E-mail address: [email protected] (P. Lin). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.03.009
surface and then improving properties [5–8]. It can produce a thin layer at surface with fine grains and small dendrites [7]. Therefore, laser processing recently has been widely used to harden, melt, or remelt the surface of steels [9–11]. Conde et al. [11] employed laser surface melting (LSM) to treat different steels and claimed that a modified surface layer is critical to achieve property improvement. In the meanwhile, they also suggested that laser processing parameters should be chosen to generate optimum experimental results. A few studies on laser processing and H21 steel have been carried out elsewhere [12–15]. Laser strengthening spots were engineered by Chen et al. [12] to form the non-smooth surface of Cr2W8V steel and to improve its hardness and thus wear resistance. They also claimed that the wear behavior is dependent on spacing between spots. Distribution of strengthening spots on sample surface is directly associated with wear resistance. In addition, another study [15] reported effects of strengthening stripes on surface of this material and showed that different parameters have different effects on modifying the surface microstructure. As compared with frequency and scanning speed of laser, pulse energy and duration have played major role in modifying the microstructure. In addition to these parameters, these studies investigated different laser-treated surface morphologies. Ref. [12] reported a processed surface with strengthening spots that increased microhardness. Several morphologies are engineered by Zhou et al. [13]. Zhang et al. [16] also studied mechanical properties, microstructures and
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thermal fatigue behavior with different surface morphologies. All the aforementioned studies are essential for developing good laser treatment for H21 steel. However, so far, there have been a few direct studies and many others are still needed. In order to further the study on laser processing of H21 die steel, in our study, we investigate the microstructural evolution, and the thermal fatigue behavior of this material processed by laser surface remelting (LSR) with different treated morphologies. Related mechanism is also discussed. 2. Materials and methods The chemical compositions of the hot working die steel H21 are 0.35 wt.% C, 0.30 wt.% Si, 0.40 wt.% Mn, 2.60 wt.% Cr, 8.02 wt.% W, 0.40 wt.% V and Fe in balance. Samples were cut in the dimension of 40 × 20 × 3 mm3 using an electro-discharge machine. An Nd: YAG pulsed laser with the wavelength of 1.06 m was employed to treat the surface of samples. Note that only part of surface was treated. This is different with some other studies [17,18]. It will be discussed
Fig. 2. SEM microstructure of samples: (a) microstructure of the as-received sample; (b) microstructure of laser-treated sample, and (b) provides comparison of the two areas.
later. On the other hand, effects of the relative laser-treated area to overall steel surface on resistance to thermal fatigue (area ratio, %) were also studied in this work. Thermal fatigue tests were carried out by self-restrained thermal fatigue testing machine. It should be noted that Zhang and co-workers [19] studied the laser-treated stripes and, however, by varying the stripe spacing, different thermal fatigue results were achieved. Therefore, in this work, in order to better study the effects of laser-treated morphologies on thermal fatigue behavior, three morphologies are designed: spot, network, and stripe. Three laser-treated surface morphologies are engineered in our study to compare with their experimental results. All samples with different laser-treated morphologies are shown in Fig. 1.
Fig. 1. The surface morphologies of laser-treated samples: (a) strengthening spots on sample surface, each spot represents one laser surface treatment; (b) strengthening stripes on sample surface; (c) strengthening network on sample surface.
Fig. 3. The XRD patterns of both as-received and laser-treated samples.
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The microstructure of samples after various thermal fatigue cycles was characterized to study the microstructural evolution during thermal fatigue. Microhardness was also examined using a Knoop and Vickers Hardness Table (USA), under a load of 25 g. In this study, a characteristic is discussed: the effective treatment depth, or the strengthening depth. This is because laser can only treat a surface layer. It is essential to measure and analyze this characteristic. As a matter of fact, the microhardness can be used to determine the effective treatment depth, because the laser-treated area possesses notably higher microhardness than its original counterpart. Plain-view and cross-sectional microstructures of the samples were given using scanning electron microscopy (SEM, JSM-5600LV, Japan) equipped with energy dispersive spectrometers (EDS). High resolution transmission electron microscopy (HRTEM, JEOL-2100F, Japan) was used to show microstructural details. X-ray diffraction (XRD) helped to examine sample phase. 3. Results Fig. 2 shows SEM morphologies of both as-received and lasertreated samples. The microstructure of the as-received sample comprises coarse grains and large size carbide and dendrites (Fig. 2a). Fig. 2b provides comparison of original and laser-treated areas. The laser-treated microstructure consists of dominant martensite and small-size precipitate in appearance of necklace. Many previous studies [16,19,20,21] reported the self-quenching effect of laser surface treatment. The very high cooling speed leads to formation of dominantly martensitic microstructure. When the original microstructure contains C and other alloying elements in high concentration, ultrafine or even nano-sized precipitates also are formed. XRD (Fig. 3) is employed to examine the two samples. Note that a new phase (Fe3 W3 C) is seen. It indicates that the precipitate is in the form of M6 C. Our EDS (Fig. 4) shows the elemental distribution of the lasertreated area. It is obvious that within this microstructure, a homogeneous elemental distribution was acquired after laser treatment. Apparent element aggregation is not seen in Fig. 4b. This characteristic is essential for improving mechanical properties and its effect will be discussed later. On the other hand, the effective treatment depth of our LSR is also found in Fig. 4a. Many studies have different treatment depth. For instance, Ref. [7] gives a depth of ∼80 m, and moreover another study [19] gives ∼40. This
Fig. 4. The cross-sectional SEM microstructure of the laser-treated area, and EDS line analysis of the elemental composition: (a) the EDS analysed area; (b) EDS of (a).
Fig. 5. HRTEM microstructure of laser-treated sample: (a) the overall HRTEM microstructure, the inserted image is the corresponding SAD pattern; (b) the nanosized carbides.
Fig. 6. the distribution of microhardness in the laser-hardened area: (a) in plainview direction; (b) in perpendicular direction.
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is because different laser parameters, such as power output and density, were pre-determined. In the present study, the effective treatment depth is ∼650 m. Fig. 5 shows the HRTEM microstructure of the laser-treated area. It is characterized as dominant martensite, nano-sized carbide and high dislocation density. Fig. 5b better demonstrates large amount of nano-sized carbide in appearance of necklace. This result coincides with Fig. 2. Fig. 6 gives microhardness curves of the laser-treated area. Like elemental distribution, the distribution of microhardness is also homogeneous. In the both plain and longitudinal directions to the sample surface, the laser-treated area has an average microhardness of ∼780 HV. Distribution of microhardness in the longitudinal direction, on the other hand, demonstrates the effective strengthening depth from the sample surface. Fig. 7. The relation of microhardness with thermal fatigue cycles in thermal fatigue test: both the as-received and laser-treated areas were investigated.
4. Discussion The microhardness of samples as the function of thermal fatigue cycle is given in Figure 7. A notable change of microhardness of the treated area was caused by thermal fatigue test. In the
Fig. 8. The microstructural evolution during thermal fatigue test: (a) the microstructure after 0 time, namely before the thermal fatigue test; (b) after 800 times; (c) after 1600 times; (d) after 2400 times; (e) after 3200 times; (f) after 4000 times.
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Table 1 The crack densities of both as-received and laser-treated samples. Number of cycles
Original Crack (mm)
600 600–800 800–1000 1000–1400 2400
– 1.2 2.2 3.4 6
Spots Number
Crack (mm)
6 15 22 56
– – 1.1 2.4 5.5
Stripes Number
Crack (mm)
4 13 42
– – 1 1.5 4.1
Network Number
Crack (mm)
Number
4 8 30
– – – Micro-crack 2.6
24
Fig. 9. The blocking effect of laser-hardened area: most cracks are hampered in front of the hardened area, irrespective of its shapes. (a) The strengthening stripes; (b) strengthening spots; I is laser-hardened area; II is crack.
early stage of thermal fatigue (≤1000 times), microhardness of the treated area was considerably decreased. In the late stage of thermal fatigue, the microhardness of the treated area is slightly decreased and almost constant. It is twice as high as that of the original surface. This microhardness curve is independent of the laser-treated morphologies because it is an instinctive nature from sample microstructures. Therefore studying the microstructural evolution during the thermal test is essential for better understanding the change of microhardness. Fig. 8 shows the microstructural modification with thermal fatigue. As aforementioned, the laser-hardened microstructure mainly comprises fine martensite. During the thermal fatigue, microstructure was also significantly modified. In the early stage of thermal fatigue test, grains grew. Simultaneously, large amount of C combined with other elements (Fe, W, etc.) and thus large size carbide precipitated at grain boundaries. These activities gave rise to the decrease in microhardness. A relatively stable microstructure was formed in the later stage of thermal fatigue. The corresponding microhardness is also stable, as shown in Fig. 7. The microhardness of original sample is constant, compared to its lasertreated counterpart. Note that in the early stage of thermal fatigue, microstructural evolution is obvious. It agrees well with the change of microhardness in this stage. Table 1 gives the crack densities of different samples after thermal fatigue. As compared with the as-received sample, all three samples with strengthening spots, stripes and network exhibit higher resistance to cracking. Strengthening network has the best resistance. Even in the late stage of thermal fatigue, only small size cracks were observed on this sample. Our analysis indicates that the TFCN lives of the three samples are all higher than that of the as-received one. In the meanwhile, the network gives the highest TFCN life of all. Because of different laser-treated morphologies, the area ratio is also different. The strengthening network has the highest area ratio of all. The blocking effect of hard structure on surface is summarized by Suresh [22], that, crack initiation and propagation both can be stopped, or at least curbed by hard structures, because more energy is required to penetrate them. Tsay and Tsay [23] reported a phenomenon that once cracks were developed
tortuously, the crack growth was slowed. The present experimental results are in good agreement with theirs. It should be noted that the partially treated surface contains soft phase like austenite in original area. Stratton [24], and Young and Bhadeshia [25] both report the benefit of presence of ductile phase like austenite in hard microstructure. A certain amount of austenite in microstructure can enhance the toughness and the ability of stopping, or at least hampering, microcrack that causes mechanical failure. It is conceivable that treatment of partial surface of samples can generate optimum experimental results. To better understand the blocking mechanism of laser-treated area, Cracks nearby different laser-treated morphologies are shown in Fig. 9. It is clear that cracks are blocked or bifurcated in front of strengthening spots, stripes and networks, due to their dominantly martensitic microstructure. However, one can anticipate that, since the strengthening network can block cracks in all directions, the corresponding sample possessed highest resistance of all. In comparison, spots can only hamper cracks toward them. 5. Conclusions A hot working die steel H21 was processed by means of laser surface remelting. Several treatment morphologies were engineered on the steel surface: strengthening spots, stripes and network. LSR can lead to a dominantly martensitic microstructure in the laser-treated area. Other notable characteristics such as nano-sized carbide and high dislocation density were also resulted. The effective treatment depth is ∼650 m. Inside the laser-treated area, both elemental and microhardness distributions are homogeneous. Moreover, the average microhardness of the laser-treated area is ∼780 HV. The thermal fatigue results show that as the experiment progressed, the microhardness of laser-treated area was considerably reduced, but it was still higher than that of the original surface. This change was caused by the microstructural evolution during thermal fatigue. The TFCN life of laser-treated samples is notably higher than their as-received counterpart. The crack size and crack density both are reduced due to the blocking effect of laser-treated
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