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Influence of hardness of substrates on properties of surface layer formed by fine particle bombarding
Materials Science & Engineering A 574 (2013) 197–204
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Influence of hardness of substrates on properties of surface layer formed by fine particle bombarding Tatsuro Morita a,n, Sho Noda a, Chuji Kagaya b a b
Department of Mechanical and System Engineering, Graduate School of Science and Technology, Kyoto Institute of Technology, Gosyokaido-cho, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan Department of Mechanical Engineering, Chubu University, 1200 Matsumoto-cho, Kasugai, Aichi 487-8501, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 24 November 2012 Received in revised form 21 February 2013 Accepted 12 March 2013 Available online 22 March 2013
This study investigated the influence that the hardness of substrates has on the properties of surface layers formed by fine particle bombarding (FPB). For this purpose, the hardness of mild steel JIS S45C was changed by quenching and tempering, and then FPB was performed under the same conditions. The properties of the formed surface layers, such as the microstructure, surface hardness, residual stress and hardness distribution, were systematically examined. The surface layers were nano-crystallized through severe plastic deformation induced by FPB, regardless of the hardness of the substrates. The grain size of nano-crystals in the surface layers decreased with increasing hardness of the substrates, so that the surface hardness markedly increased. FPB introduced compressive residual stress and its absolute value became high with increasing hardness of the substrates. Since plastic deformation induced by FPB reached deeper positions when the hardness of the substrates was reduced, the thickness of hardened regions increased. & 2013 Elsevier B.V. All rights reserved.
Keywords: Fine particle bombarding Mild steel Microstructure Surface hardness Residual stress Hardness distribution
1. Introduction Fine particle bombarding (FPB) is a surface modification method in which fine collision particles (maximum diameter of particles o200 μm) are accelerated by compressed air and then used to bombard the surfaces of materials [1]. In FPB, since the kinetic energy of one particle is very low, plastic deformation cannot reach deep positions from the surface. In contrast, numerous fine particles collide with the surface one after another, and all kinetic energy is applied only to the vicinity of the surface. In consequence, the applied energy density (total kinetic energy per unit volume per time) of FPB becomes much higher than that of conventional shot peening [2]. FPB generates severely deformed thin layers along the surfaces of metals. Due to striking plastic deformation, the surface hardness increases through nano-crystallization of the microstructure [3–6]. At the same time, high compressive residual stress is introduced without marked deterioration of surface roughness. As a result, FPB greatly improves wear resistance [7] and fatigue strength [2,8–10], which are closely related to the surface properties. According to our studies, FPB has a more significant effect on the fatigue strength of titanium alloys when combined with pretreatments such as plasma carburizing and nitriding [11,12]. Moreover, FPB can provide desirable functionalities through surface alloying because the elements of collision particles diffuse from the surface during treatment. For example, the corrosion
resistance of aluminum alloy and mild steel to salt water was improved by using fine particles of pure titanium [13,14]. Nano-crystallization of the surface microstructure is also achieved by other methods, such as surface mechanical attrition [15,16]; however, FPB is easily applicable for various machine parts because it can be performed in air at room temperature and the shapes of the parts are basically unrestricted. Although FPB has accordingly progressed from the viewpoint of engineering applications, accumulation of its fundamental results is insufficient. From the above background, the previous study comprehensively investigated the influences of particle size and air pressure on the surface properties, mechanical properties and fatigue strength of commercial pure titanium [2]. This study further investigated the influence of the hardness of the substrates on the properties of the surface layers formed by FPB. Mild steel JIS S45C was used as the model material and its hardness was changed by quenching and tempering. FPB was performed under the same condition for all substrates possessing different hardness. The properties of each formed layer, such as the microstructure, surface hardness, residual stress and hardness distribution, were systematically examined. In particular, the surface microstructures were observed through transmission electron microscopy (TEM), and the conditions of plastic deformation under the surface layers were investigated by electron back scattered diffraction pattern (EBSD) analysis. 2. Materials and experimental procedures
n
Corresponding author. Tel.: þ81 75 724 7326; fax: þ 81 75 724 7300. E-mail address: [email protected] (T. Morita).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.03.026
Table 1 shows the chemical composition of mild steel JIS S45C used in this study. The material was supplied as round bars with
198
T. Morita et al. / Materials Science & Engineering A 574 (2013) 197–204
a diameter of 12 mm and was machined to the shapes of the specimens shown in Fig. 1. After the specimens had been fully annealed to eliminate their process history, they were quenched and tempered to change their hardness under the conditions Table 1 Chemical composition of mild steel JIS S45C used in this study (mass%). C
Si
Mn
P
S
Cu
Ni
Cr
0.44
0.20
0.70
0.022
0.014
0.11
0.072
0.166
Fig. 1. Shapes of specimens (mm): (a) button-type specimen, (b) tensile specimen (JIS Z 2241, no. 14).
Table 2 Conditions of the heat treatments and FPB. Abbreviation Annealing
Quenching Tempering
FPB
AN
–
–
–
Particles: high-speed steel (1000 Hv), maximum diameter: 34 μm (#400), air pressure: 0.5 MPa, treatment time: 10 s and incidence angle: direction normal to surface
Q
1083 K, 3.6 ks, FC m
T473
m
1118 K, 300 s, Q m
T673
m
m
T873
m
m
473 K, 3.6 ks, AC 673 K, 3.6 ks, AC 873 K, 3.6 ks, AC
FC: furnace cooled, Q: quenched, AC: air cooled.
shown in Table 2. Test sections of button specimens (Fig. 1(a)) and tensile specimens (Fig. 1(b)) were polished with emery papers and alumina powders to mirror surfaces. Hereafter, the abbreviations shown in Table 2 are used to distinguish the materials. The microstructure, hardness and mechanical properties of each material were investigated to obtain basic data. Microstructures were optically observed on cross sections of button specimens after etching with 1% etchant of nitric acid and alcohol. Hardness of the materials was measured by a micro-Vickers hardness tester under the test force of 490 mN (50gf) and the dwell time of 15 s, based on JIS Z 2244. The hardness measurement was conducted five times on the materials and their mean values were used as data. A tensile test was performed for three specimens of each material under the initial force rate of 200 N/s at room temperature in air, based on JIS Z 2241, and their mean values were used as data of mechanical properties of the materials. FPB was conducted on the polished test sections of button specimens under the condition shown in Table 2. The surface features were observed on the FPBed surfaces. The microstructures near the surfaces were observed on their etched cross sections. For the above observations, scanning electron microscopy (SEM) was used. The surface microstructures were investigated by TEM observation and electron diffraction in detail. For these experiments, small samples with a diameter of 3 mm were cut from the FPBed surfaces. To avoid damage to the FPBed surface, each sample was carefully polished and ion-milled from the substrate side. The TEM observation was conducted in the direction normal to the FPBed surfaces. The grain size of nano-crystals in the surface layers was obtained from the results of TEM observation. Surface hardness was measured on the FPBed surfaces by the same method mentioned above and hardness distributions were obtained on the cross sections. In the measurements of the surface hardness and hardness distributions, hardness was measured at each position five times and their averages were used as data. X-ray residual stress measurement was performed on the FPBed surfaces of button specimens. The conditions of the measurement were as follows: X-ray: Cr Kα, diffraction plane: (211), diffraction angle: 2θ¼156.41, sin2 ψ method (ψ¼101, 201, 301, 351 and 401), oscillation: 731, and stress constant: K ¼−317.9 MPa/deg. Usually, it is difficult to detect diffraction peaks of FPBed materials because the surface microstructures are nano-crystallized; however, relatively clear peaks were detected in this study
Fig. 2. Microstructures of the substrates observed optically.
T. Morita et al. / Materials Science & Engineering A 574 (2013) 197–204
so that the peak search was conducted in terms of the width at half maximum. The conditions of plastic deformation under the surface layers were investigated on cross sections by EBSD analysis. Using EBSD
199
analysis, inverse pole figure (IPF) maps were obtained under the confidence index (CI) of 0.1 without data cleaning.
3. Results and discussion 3.1. Properties of substrates
Table 3 Mechanical properties and hardness of the substrates before FPB.
AN Q T473 T673 T873
Young's modulus (GPa)
Yield strength (MPa)
Tensile strength (MPa)
Elongation (%)
Reduction Hardness in area (%) (Hv)
211 204 209 214 210
376 – 1707 1294 746
649 1739 2025 1362 832
24.4 0.3 12.0 14.8 18.7
56.8 4.3 33.6 57.5 60.1
191 752 606 460 296
Fig. 2 shows the microstructures of the substrates. As seen in the figure, the microstructure of the AN material was composed of ferrite and perlite phases. The microstructure of the Q material was acicular martensite. The microstructures of the tempered materials (T473–T873) were typical tempered martensite depending on the treatment temperature. Table 3 shows the mechanical properties and hardness of the substrates. The tensile strength of the Q material was lower than that of the T473 because of the marked reduction in ductility.
Fig. 3. Features of FPBed materials observed by SEM on the surfaces and etched cross sections.
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In the tempered materials (T473–T873), the yield strength and tensile strength decreased and ductility increased with increasing the tempering temperature. Hardness increased through quenching and decreased by increasing tempering temperature. The above results showed that a gradable change in the hardness of the substrates was suitably achieved by the heat treatments performed in this study. 3.2. Formation mechanism of surface layers Fig. 3 shows the surface features of the FPBed materials together with the microstructures observed on the cross sections. To ease comparison, the observation results were arranged in order of the hardness of the substrates. The collision of fine particles formed many craters on the surfaces. Such craters became large and deep as the hardness of the substrates decreased. The distinguishable layers (hereafter, “surface layers”), shown by dotted lines in the figure, were observed on the cross sections. The surface layers corresponded
to the regions in which the microstructures were nano-crystallized, as explained in the next section. A previous study [17] reported that FPB generated surface layers through folding projections generated around craters when the substrate was steel with relatively low hardness (372 Hv). In titanium alloys, another study [18] showed that FPB formed isolated surface regions which possessed a fine microstructure through a similar mechanism to that mentioned above. Hereafter, this formation mechanism of the surface layers is called the “folding mechanism”. The traces of plastic deformation observed in the AN material actually suggested that projections were mashed and folded into neighbor craters one after another, as shown in the illustration (Fig. 3). A previous study [17] further reported that the folding mechanism did not work in steel possessing high hardness (587 Hv). In this study, however, half-folded projections (arrows in Fig. 3) were found on the cross sections of Q and T473 materials with high hardness, although the observed projections were small. The existence of such projections meant that the folding
Fig. 4. Surface microstructures observed by TEM and diffraction patterns of FPBed materials.
T. Morita et al. / Materials Science & Engineering A 574 (2013) 197–204
201
Fig. 4 shows the microstructures of the surface layers observed in both bright and dark fields by TEM. This figure includes the diffraction patterns obtained in the observed regions.
As seen in the figure, the microstructures of the surface layers were nano-crystallized by FPB. The grain size of nano-crystals decreased with increasing hardness of the substrates. The diffraction pattern tended to form a ring-like pattern, corresponding to the refinement of the microstructure. The degree of completeness of the diffraction ring corresponds to the degree of fineness of the grains captured in the selected area diffraction. Thus, as the grain size decreased, the diffraction patterns approached a more complete ring. In FPB, nano-crystallization in the surface layers occurs through dynamic recrystallization due to marked plastic deformation induced by the collision of fine particles [3–6]. Accordingly, the grain size of nano-crystals will depend on the degree of plastic deformation. The surface layers were formed on FPBed materials through the folding mechanism, as mentioned in the previous section. This means that the degree of plastic deformation near the surface is closely related to the conditions of folded projections. In AN material with low hardness, the trace of plastic deformation showed that folded projections were relatively large (Fig. 3). On the other hand, the folded projections became smaller with increasing hardness of the substrate (for example, Q and T473 materials) because plastic deformation was restricted. In the case of substrates with high hardness, plastic deformation cannot extend to deep positions; therefore, the kinetic energy of collision particles was intensively applied to shallow surface
Fig. 5. Relationship between hardness of the substrates and the surface hardness after FPB.
Fig. 7. sin2 ψ plots obtained on FPBed surfaces.
Fig. 6. Relationship between grain size of the surface layers and the surface hardness after FPB.
Fig. 8. Relationship between hardness of the substrates and residual stress measured on FPBed surfaces.
mechanism can generate surface layers, regardless of the hardness of the substrates. On the other hand, the average thickness of the surface layers increased with decreasing hardness of the substrates (Fig. 3); however, the thickness markedly fluctuated. From an engineering viewpoint, the fluctuation of the thickness should be considered because the effects of surface layers on wear resistance and fatigue strength are usually related to their minimum thickness. The boundaries between the surface layers and the substrates consisted of a series of circular arcs, as seen in Fig. 3. It is thought that such arcs were the traces of particles that collided with the surfaces in the initial stage of FPB as follows: initial craters reached deep positions with decreasing hardness of the substrates. The formed projections around the initial craters were folded into the dents. The folded projections played role of buffering against later particle collision until they were sufficiently packed. As a result, the traces of initial craters remained and the thickness of the surface layers became uneven, especially in materials with low hardness.
3.3. Microstructures of surface layers
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regions. As a result, small projections were made and folded repeatedly, so that severe plastic deformation was induced near the surface. Moreover, it is conventional to assume that the dislocation density of the Q material would be highest compared for all the substrates and decreased with increasing tempering temperature. In addition to this, when the initial dislocation density of the substrates is high, nano-crystallization can more easily occur through dynamic recrystallization. Namely, substrates with high hardness can easily be nano-crystallized by FPB. The above consideration suggested that the grain size of nanocrystals in the surface layers reduced with increasing hardness
of the substrates because both the degree of plastic deformation near the surface and dislocation density of the substrates became high. 3.4. Surface hardness Fig. 5 shows the relationship between the hardness of the substrates and the surface hardness after FPB. In Fig. 6, the results shown in Fig. 5 have been rearranged to show the relationship between the grain size of nano-crystals in the surface layers and the surface hardness. FPB increased surface hardness regardless of the hardness of the substrates, as understood by comparing the hardness of the
Fig. 9. IPF maps of FPBed materials obtained by EBSD analysis.
T. Morita et al. / Materials Science & Engineering A 574 (2013) 197–204
substrates (dotted line) and the surface hardness after FPB (plots and solid line) in Fig. 5. Many studies concerning nano-crystalline materials have reported that hardness continued to increase with decreasing grain size even in the region of very small grains of less than 100 nm (for example [19–23]). Corresponding to these studies, the surface hardness of FPBed materials depended on the grain size of nano-crystals in the surface layers (Fig. 6). In FPBed materials, although dislocation hardening also can result in increased surface hardness, it is difficult to distinguish the effect of dislocation hardening from that of grain refinement; however, Kimura, et al., reported that the hardness of severely plastic-deformed metals was controlled by grain refinement rather
203
than dislocation hardening when the hardness was beyond about 500 Hv [20]. Based on this paper, the nano-crystallization of surface microstructures is the main factor controlling the surface hardness of FPBed materials. In the case of nano-crystalline materials, there are other factors affecting their hardness; that is, Coble creep and the distribution of grain size. Coble creep is mass flow due to grain boundary diffusion or atomic diffusion dominant at the grain boundary and affects the hardness state in nano-crystalline materials even at room temperature. A previous study reported that Coble creep decreases the hardness of nano-crystalline materials at the grain size of or below 10–30 nm [21,22]. Moreover, the distribution of grain size affects the hardness of nano-crystalline materials as well as the average grain size [23]. Nevertheless, the result obtained in this study (Fig. 6) showed that surface hardness increased with the decreasing grain size of nano-crystals. In consequence, it is probable that grain refinement is the factor most strongly controlling the surface hardness of FPBed materials. 3.5. Residual stress
Fig. 10. Hardness distributions of FPBed materials.
Fig. 7 shows the sin2 ψ plots of the FPBed materials. In Fig. 8, the results obtained from Fig. 7 have been rearranged to show the relationship between the hardness of the substrates and the residual stress measured on the surfaces of FPBed materials. As shown in Fig. 8, FPB placed high compressive residual stress on the surfaces. Compressive residual stresses are often dominant in peened or FPBed surfaces because the plastically deformed region near the surface tends to expand beyond the overall component volume during treatment but is effectively constrained by the material volume
Fig. 11. Schematic illustration to explain the influence of the hardness of the substrates on the surface properties.
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below it. At the end of the surface treatment, the compressive residual stresses remain dominant at the treated surfaces. Since compressive residual stress is generated by plastic deformation induced on the FPBed surfaces, its absolute value depends on the degree of plastic deformation. On the other hand, the degree of plastic deformation induced on the FPBed surfaces became higher with increasing hardness of the substrates (Section 3.3). As a result, the absolute value of compressive residual stress increased with increasing hardness of the substrates (Fig. 8). Based on the constraining regime imposed by the material volume below the treated surface, the value of residual stress is also strongly influenced by the mechanical compliance of the material below the treated layer, as well as the degree of plastic deformation. Therefore, the lower mechanical compliance of the substrate results in a higher value of compressive residual stress, as is the case for the Q material. This is another reason why compressive residual stress increased with increasing hardness of the substrates. 3.6. Deformation under surface layers and hardness distributions Fig. 9 reveals that plastic deformation had extended to material regions below the grain refined surface layers (i.e. the material below the dotted lines). In addition, no data on the surface layers were obtained because the microstructures were nano-crystallized and the grains were smaller than the measurement limit of the used equipment. Fig. 10 shows the hardness distributions measured using cross sections of FPBed materials, including the surface hardness. In this figure, the thickness of the surface layers is shown by dotted lines and arrows. As understood from Fig. 9, plastic deformation extended to beneath the surface layers. For example, crushed microstructures were clearly seen under the surface layers in T673 and T873 and AN materials. Such deformed regions reached deep positions with decreasing hardness of the substrates. The above plastic deformation formed hardened regions under the surface layers (Fig. 10). The hardened regions became thick with decreasing hardness of the substrates, corresponding to the reached depth of plastic deformation. The increased hardness in the above regions is different from the increased hardness of the surface layers and is mainly caused by dislocation hardening and not by grain refinement, which is dominant within the surface near layers. Finally, Fig. 11 shows an illustration to summarize the effects of FPB on the surface properties. The surface microstructure was nano-crystallized by FPB. Since the grain size of nano-crystals decreased with increasing hardness of the substrates, the surface hardness markedly increased. At the same time, higher compressive residual stress was introduced although the hardened layers became thin. 4. Conclusions 1. FPB formed distinguishable surface layers which were nanocrystallized through the folding mechanism, regardless of the hardness of the substrates. 2. The grain size of nano-crystals in the surface layers decreased with increasing hardness of the substrates. This change was
caused by the increase in both the degree of plastic deformation induced on the surface and inital dislocation density of the substrates before FPB because high dislocation density facilitates dynamic recrystallization. 3. The surface hardness of FPBed materials increased mainly by grain refinement when hardness of the substrates increased. At the same time, the amount of compressive residual stress in the surface layer became high. 4. Since plastic deformation by FPB reached deep positions with decreasing hardness of the substrates, the thickness of the hardened layers increased.
Acknowledgment The authors sincerely thank Fujikihan Co. Ltd. for all FPB treatments conducted in this study.
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[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
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Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Influence of hardness of substrates on properties of surface layer formed by fine particle bombarding Tatsuro Morita a,n, Sho Noda a, Chuji Kagaya b a b
Department of Mechanical and System Engineering, Graduate School of Science and Technology, Kyoto Institute of Technology, Gosyokaido-cho, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan Department of Mechanical Engineering, Chubu University, 1200 Matsumoto-cho, Kasugai, Aichi 487-8501, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 24 November 2012 Received in revised form 21 February 2013 Accepted 12 March 2013 Available online 22 March 2013
This study investigated the influence that the hardness of substrates has on the properties of surface layers formed by fine particle bombarding (FPB). For this purpose, the hardness of mild steel JIS S45C was changed by quenching and tempering, and then FPB was performed under the same conditions. The properties of the formed surface layers, such as the microstructure, surface hardness, residual stress and hardness distribution, were systematically examined. The surface layers were nano-crystallized through severe plastic deformation induced by FPB, regardless of the hardness of the substrates. The grain size of nano-crystals in the surface layers decreased with increasing hardness of the substrates, so that the surface hardness markedly increased. FPB introduced compressive residual stress and its absolute value became high with increasing hardness of the substrates. Since plastic deformation induced by FPB reached deeper positions when the hardness of the substrates was reduced, the thickness of hardened regions increased. & 2013 Elsevier B.V. All rights reserved.
Keywords: Fine particle bombarding Mild steel Microstructure Surface hardness Residual stress Hardness distribution
1. Introduction Fine particle bombarding (FPB) is a surface modification method in which fine collision particles (maximum diameter of particles o200 μm) are accelerated by compressed air and then used to bombard the surfaces of materials [1]. In FPB, since the kinetic energy of one particle is very low, plastic deformation cannot reach deep positions from the surface. In contrast, numerous fine particles collide with the surface one after another, and all kinetic energy is applied only to the vicinity of the surface. In consequence, the applied energy density (total kinetic energy per unit volume per time) of FPB becomes much higher than that of conventional shot peening [2]. FPB generates severely deformed thin layers along the surfaces of metals. Due to striking plastic deformation, the surface hardness increases through nano-crystallization of the microstructure [3–6]. At the same time, high compressive residual stress is introduced without marked deterioration of surface roughness. As a result, FPB greatly improves wear resistance [7] and fatigue strength [2,8–10], which are closely related to the surface properties. According to our studies, FPB has a more significant effect on the fatigue strength of titanium alloys when combined with pretreatments such as plasma carburizing and nitriding [11,12]. Moreover, FPB can provide desirable functionalities through surface alloying because the elements of collision particles diffuse from the surface during treatment. For example, the corrosion
resistance of aluminum alloy and mild steel to salt water was improved by using fine particles of pure titanium [13,14]. Nano-crystallization of the surface microstructure is also achieved by other methods, such as surface mechanical attrition [15,16]; however, FPB is easily applicable for various machine parts because it can be performed in air at room temperature and the shapes of the parts are basically unrestricted. Although FPB has accordingly progressed from the viewpoint of engineering applications, accumulation of its fundamental results is insufficient. From the above background, the previous study comprehensively investigated the influences of particle size and air pressure on the surface properties, mechanical properties and fatigue strength of commercial pure titanium [2]. This study further investigated the influence of the hardness of the substrates on the properties of the surface layers formed by FPB. Mild steel JIS S45C was used as the model material and its hardness was changed by quenching and tempering. FPB was performed under the same condition for all substrates possessing different hardness. The properties of each formed layer, such as the microstructure, surface hardness, residual stress and hardness distribution, were systematically examined. In particular, the surface microstructures were observed through transmission electron microscopy (TEM), and the conditions of plastic deformation under the surface layers were investigated by electron back scattered diffraction pattern (EBSD) analysis. 2. Materials and experimental procedures
n
Corresponding author. Tel.: þ81 75 724 7326; fax: þ 81 75 724 7300. E-mail address: [email protected] (T. Morita).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.03.026
Table 1 shows the chemical composition of mild steel JIS S45C used in this study. The material was supplied as round bars with
198
T. Morita et al. / Materials Science & Engineering A 574 (2013) 197–204
a diameter of 12 mm and was machined to the shapes of the specimens shown in Fig. 1. After the specimens had been fully annealed to eliminate their process history, they were quenched and tempered to change their hardness under the conditions Table 1 Chemical composition of mild steel JIS S45C used in this study (mass%). C
Si
Mn
P
S
Cu
Ni
Cr
0.44
0.20
0.70
0.022
0.014
0.11
0.072
0.166
Fig. 1. Shapes of specimens (mm): (a) button-type specimen, (b) tensile specimen (JIS Z 2241, no. 14).
Table 2 Conditions of the heat treatments and FPB. Abbreviation Annealing
Quenching Tempering
FPB
AN
–
–
–
Particles: high-speed steel (1000 Hv), maximum diameter: 34 μm (#400), air pressure: 0.5 MPa, treatment time: 10 s and incidence angle: direction normal to surface
Q
1083 K, 3.6 ks, FC m
T473
m
1118 K, 300 s, Q m
T673
m
m
T873
m
m
473 K, 3.6 ks, AC 673 K, 3.6 ks, AC 873 K, 3.6 ks, AC
FC: furnace cooled, Q: quenched, AC: air cooled.
shown in Table 2. Test sections of button specimens (Fig. 1(a)) and tensile specimens (Fig. 1(b)) were polished with emery papers and alumina powders to mirror surfaces. Hereafter, the abbreviations shown in Table 2 are used to distinguish the materials. The microstructure, hardness and mechanical properties of each material were investigated to obtain basic data. Microstructures were optically observed on cross sections of button specimens after etching with 1% etchant of nitric acid and alcohol. Hardness of the materials was measured by a micro-Vickers hardness tester under the test force of 490 mN (50gf) and the dwell time of 15 s, based on JIS Z 2244. The hardness measurement was conducted five times on the materials and their mean values were used as data. A tensile test was performed for three specimens of each material under the initial force rate of 200 N/s at room temperature in air, based on JIS Z 2241, and their mean values were used as data of mechanical properties of the materials. FPB was conducted on the polished test sections of button specimens under the condition shown in Table 2. The surface features were observed on the FPBed surfaces. The microstructures near the surfaces were observed on their etched cross sections. For the above observations, scanning electron microscopy (SEM) was used. The surface microstructures were investigated by TEM observation and electron diffraction in detail. For these experiments, small samples with a diameter of 3 mm were cut from the FPBed surfaces. To avoid damage to the FPBed surface, each sample was carefully polished and ion-milled from the substrate side. The TEM observation was conducted in the direction normal to the FPBed surfaces. The grain size of nano-crystals in the surface layers was obtained from the results of TEM observation. Surface hardness was measured on the FPBed surfaces by the same method mentioned above and hardness distributions were obtained on the cross sections. In the measurements of the surface hardness and hardness distributions, hardness was measured at each position five times and their averages were used as data. X-ray residual stress measurement was performed on the FPBed surfaces of button specimens. The conditions of the measurement were as follows: X-ray: Cr Kα, diffraction plane: (211), diffraction angle: 2θ¼156.41, sin2 ψ method (ψ¼101, 201, 301, 351 and 401), oscillation: 731, and stress constant: K ¼−317.9 MPa/deg. Usually, it is difficult to detect diffraction peaks of FPBed materials because the surface microstructures are nano-crystallized; however, relatively clear peaks were detected in this study
Fig. 2. Microstructures of the substrates observed optically.
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so that the peak search was conducted in terms of the width at half maximum. The conditions of plastic deformation under the surface layers were investigated on cross sections by EBSD analysis. Using EBSD
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analysis, inverse pole figure (IPF) maps were obtained under the confidence index (CI) of 0.1 without data cleaning.
3. Results and discussion 3.1. Properties of substrates
Table 3 Mechanical properties and hardness of the substrates before FPB.
AN Q T473 T673 T873
Young's modulus (GPa)
Yield strength (MPa)
Tensile strength (MPa)
Elongation (%)
Reduction Hardness in area (%) (Hv)
211 204 209 214 210
376 – 1707 1294 746
649 1739 2025 1362 832
24.4 0.3 12.0 14.8 18.7
56.8 4.3 33.6 57.5 60.1
191 752 606 460 296
Fig. 2 shows the microstructures of the substrates. As seen in the figure, the microstructure of the AN material was composed of ferrite and perlite phases. The microstructure of the Q material was acicular martensite. The microstructures of the tempered materials (T473–T873) were typical tempered martensite depending on the treatment temperature. Table 3 shows the mechanical properties and hardness of the substrates. The tensile strength of the Q material was lower than that of the T473 because of the marked reduction in ductility.
Fig. 3. Features of FPBed materials observed by SEM on the surfaces and etched cross sections.
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In the tempered materials (T473–T873), the yield strength and tensile strength decreased and ductility increased with increasing the tempering temperature. Hardness increased through quenching and decreased by increasing tempering temperature. The above results showed that a gradable change in the hardness of the substrates was suitably achieved by the heat treatments performed in this study. 3.2. Formation mechanism of surface layers Fig. 3 shows the surface features of the FPBed materials together with the microstructures observed on the cross sections. To ease comparison, the observation results were arranged in order of the hardness of the substrates. The collision of fine particles formed many craters on the surfaces. Such craters became large and deep as the hardness of the substrates decreased. The distinguishable layers (hereafter, “surface layers”), shown by dotted lines in the figure, were observed on the cross sections. The surface layers corresponded
to the regions in which the microstructures were nano-crystallized, as explained in the next section. A previous study [17] reported that FPB generated surface layers through folding projections generated around craters when the substrate was steel with relatively low hardness (372 Hv). In titanium alloys, another study [18] showed that FPB formed isolated surface regions which possessed a fine microstructure through a similar mechanism to that mentioned above. Hereafter, this formation mechanism of the surface layers is called the “folding mechanism”. The traces of plastic deformation observed in the AN material actually suggested that projections were mashed and folded into neighbor craters one after another, as shown in the illustration (Fig. 3). A previous study [17] further reported that the folding mechanism did not work in steel possessing high hardness (587 Hv). In this study, however, half-folded projections (arrows in Fig. 3) were found on the cross sections of Q and T473 materials with high hardness, although the observed projections were small. The existence of such projections meant that the folding
Fig. 4. Surface microstructures observed by TEM and diffraction patterns of FPBed materials.
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Fig. 4 shows the microstructures of the surface layers observed in both bright and dark fields by TEM. This figure includes the diffraction patterns obtained in the observed regions.
As seen in the figure, the microstructures of the surface layers were nano-crystallized by FPB. The grain size of nano-crystals decreased with increasing hardness of the substrates. The diffraction pattern tended to form a ring-like pattern, corresponding to the refinement of the microstructure. The degree of completeness of the diffraction ring corresponds to the degree of fineness of the grains captured in the selected area diffraction. Thus, as the grain size decreased, the diffraction patterns approached a more complete ring. In FPB, nano-crystallization in the surface layers occurs through dynamic recrystallization due to marked plastic deformation induced by the collision of fine particles [3–6]. Accordingly, the grain size of nano-crystals will depend on the degree of plastic deformation. The surface layers were formed on FPBed materials through the folding mechanism, as mentioned in the previous section. This means that the degree of plastic deformation near the surface is closely related to the conditions of folded projections. In AN material with low hardness, the trace of plastic deformation showed that folded projections were relatively large (Fig. 3). On the other hand, the folded projections became smaller with increasing hardness of the substrate (for example, Q and T473 materials) because plastic deformation was restricted. In the case of substrates with high hardness, plastic deformation cannot extend to deep positions; therefore, the kinetic energy of collision particles was intensively applied to shallow surface
Fig. 5. Relationship between hardness of the substrates and the surface hardness after FPB.
Fig. 7. sin2 ψ plots obtained on FPBed surfaces.
Fig. 6. Relationship between grain size of the surface layers and the surface hardness after FPB.
Fig. 8. Relationship between hardness of the substrates and residual stress measured on FPBed surfaces.
mechanism can generate surface layers, regardless of the hardness of the substrates. On the other hand, the average thickness of the surface layers increased with decreasing hardness of the substrates (Fig. 3); however, the thickness markedly fluctuated. From an engineering viewpoint, the fluctuation of the thickness should be considered because the effects of surface layers on wear resistance and fatigue strength are usually related to their minimum thickness. The boundaries between the surface layers and the substrates consisted of a series of circular arcs, as seen in Fig. 3. It is thought that such arcs were the traces of particles that collided with the surfaces in the initial stage of FPB as follows: initial craters reached deep positions with decreasing hardness of the substrates. The formed projections around the initial craters were folded into the dents. The folded projections played role of buffering against later particle collision until they were sufficiently packed. As a result, the traces of initial craters remained and the thickness of the surface layers became uneven, especially in materials with low hardness.
3.3. Microstructures of surface layers
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regions. As a result, small projections were made and folded repeatedly, so that severe plastic deformation was induced near the surface. Moreover, it is conventional to assume that the dislocation density of the Q material would be highest compared for all the substrates and decreased with increasing tempering temperature. In addition to this, when the initial dislocation density of the substrates is high, nano-crystallization can more easily occur through dynamic recrystallization. Namely, substrates with high hardness can easily be nano-crystallized by FPB. The above consideration suggested that the grain size of nanocrystals in the surface layers reduced with increasing hardness
of the substrates because both the degree of plastic deformation near the surface and dislocation density of the substrates became high. 3.4. Surface hardness Fig. 5 shows the relationship between the hardness of the substrates and the surface hardness after FPB. In Fig. 6, the results shown in Fig. 5 have been rearranged to show the relationship between the grain size of nano-crystals in the surface layers and the surface hardness. FPB increased surface hardness regardless of the hardness of the substrates, as understood by comparing the hardness of the
Fig. 9. IPF maps of FPBed materials obtained by EBSD analysis.
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substrates (dotted line) and the surface hardness after FPB (plots and solid line) in Fig. 5. Many studies concerning nano-crystalline materials have reported that hardness continued to increase with decreasing grain size even in the region of very small grains of less than 100 nm (for example [19–23]). Corresponding to these studies, the surface hardness of FPBed materials depended on the grain size of nano-crystals in the surface layers (Fig. 6). In FPBed materials, although dislocation hardening also can result in increased surface hardness, it is difficult to distinguish the effect of dislocation hardening from that of grain refinement; however, Kimura, et al., reported that the hardness of severely plastic-deformed metals was controlled by grain refinement rather
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than dislocation hardening when the hardness was beyond about 500 Hv [20]. Based on this paper, the nano-crystallization of surface microstructures is the main factor controlling the surface hardness of FPBed materials. In the case of nano-crystalline materials, there are other factors affecting their hardness; that is, Coble creep and the distribution of grain size. Coble creep is mass flow due to grain boundary diffusion or atomic diffusion dominant at the grain boundary and affects the hardness state in nano-crystalline materials even at room temperature. A previous study reported that Coble creep decreases the hardness of nano-crystalline materials at the grain size of or below 10–30 nm [21,22]. Moreover, the distribution of grain size affects the hardness of nano-crystalline materials as well as the average grain size [23]. Nevertheless, the result obtained in this study (Fig. 6) showed that surface hardness increased with the decreasing grain size of nano-crystals. In consequence, it is probable that grain refinement is the factor most strongly controlling the surface hardness of FPBed materials. 3.5. Residual stress
Fig. 10. Hardness distributions of FPBed materials.
Fig. 7 shows the sin2 ψ plots of the FPBed materials. In Fig. 8, the results obtained from Fig. 7 have been rearranged to show the relationship between the hardness of the substrates and the residual stress measured on the surfaces of FPBed materials. As shown in Fig. 8, FPB placed high compressive residual stress on the surfaces. Compressive residual stresses are often dominant in peened or FPBed surfaces because the plastically deformed region near the surface tends to expand beyond the overall component volume during treatment but is effectively constrained by the material volume
Fig. 11. Schematic illustration to explain the influence of the hardness of the substrates on the surface properties.
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below it. At the end of the surface treatment, the compressive residual stresses remain dominant at the treated surfaces. Since compressive residual stress is generated by plastic deformation induced on the FPBed surfaces, its absolute value depends on the degree of plastic deformation. On the other hand, the degree of plastic deformation induced on the FPBed surfaces became higher with increasing hardness of the substrates (Section 3.3). As a result, the absolute value of compressive residual stress increased with increasing hardness of the substrates (Fig. 8). Based on the constraining regime imposed by the material volume below the treated surface, the value of residual stress is also strongly influenced by the mechanical compliance of the material below the treated layer, as well as the degree of plastic deformation. Therefore, the lower mechanical compliance of the substrate results in a higher value of compressive residual stress, as is the case for the Q material. This is another reason why compressive residual stress increased with increasing hardness of the substrates. 3.6. Deformation under surface layers and hardness distributions Fig. 9 reveals that plastic deformation had extended to material regions below the grain refined surface layers (i.e. the material below the dotted lines). In addition, no data on the surface layers were obtained because the microstructures were nano-crystallized and the grains were smaller than the measurement limit of the used equipment. Fig. 10 shows the hardness distributions measured using cross sections of FPBed materials, including the surface hardness. In this figure, the thickness of the surface layers is shown by dotted lines and arrows. As understood from Fig. 9, plastic deformation extended to beneath the surface layers. For example, crushed microstructures were clearly seen under the surface layers in T673 and T873 and AN materials. Such deformed regions reached deep positions with decreasing hardness of the substrates. The above plastic deformation formed hardened regions under the surface layers (Fig. 10). The hardened regions became thick with decreasing hardness of the substrates, corresponding to the reached depth of plastic deformation. The increased hardness in the above regions is different from the increased hardness of the surface layers and is mainly caused by dislocation hardening and not by grain refinement, which is dominant within the surface near layers. Finally, Fig. 11 shows an illustration to summarize the effects of FPB on the surface properties. The surface microstructure was nano-crystallized by FPB. Since the grain size of nano-crystals decreased with increasing hardness of the substrates, the surface hardness markedly increased. At the same time, higher compressive residual stress was introduced although the hardened layers became thin. 4. Conclusions 1. FPB formed distinguishable surface layers which were nanocrystallized through the folding mechanism, regardless of the hardness of the substrates. 2. The grain size of nano-crystals in the surface layers decreased with increasing hardness of the substrates. This change was
caused by the increase in both the degree of plastic deformation induced on the surface and inital dislocation density of the substrates before FPB because high dislocation density facilitates dynamic recrystallization. 3. The surface hardness of FPBed materials increased mainly by grain refinement when hardness of the substrates increased. At the same time, the amount of compressive residual stress in the surface layer became high. 4. Since plastic deformation by FPB reached deep positions with decreasing hardness of the substrates, the thickness of the hardened layers increased.
Acknowledgment The authors sincerely thank Fujikihan Co. Ltd. for all FPB treatments conducted in this study.
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