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Thermal fatigue characteristics of plasma duplex treated nodular cast irons
Surface & Coatings Technology 205 (2010) 896–901
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Thermal fatigue characteristics of plasma duplex treated nodular cast irons Bong-Yong Jeong a,⁎, Jeong-Ho Chang a, Myung-Ho Kim b a b
Bio-IT Convergence Center, Korea Institute of Ceramic Engineering and Technology, 103 Fashion Danji-gil, Geumcheon-gu, Seoul 153-801, South Korea Department of Materials Science and Engineering, Inha University, Yonghyun-dong, Namgu, Inchon 402-751, South Korea
a r t i c l e
i n f o
Article history: Received 13 April 2010 Accepted in revised form 10 August 2010 Available online 14 August 2010 Keywords: Thermal fatigue Plasma diffusion treatment Nodular cast iron Plasma nitriding Oxidation
a b s t r a c t The correlation between the thermal fatigue resistance and microstructure of untreated and plasma surface engineered nodular cast irons was investigated. Both the ferrite and pearlite matrix nodular cast irons were evaluated. During the tests, 60 mm long cylinders with 20 mm diameter were subjected to 1000 high frequency induction heating and water cooling cycles, lasting 7 and 5 s, respectively. The thermal fatigue damage was evaluated by analyzing the crack dimensions and distributions. The results showed that the plasma surface treatment increased thermal fatigue resistance. When thermal cracking occurred, the cracks always nucleated at the surface of the specimen. In addition, the nodular cast iron with a pearlite matrix had a better resistance to thermal fatigue than that of the ferrite matrix. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Thermal fatigue, which is a factor that determines the life span of a material, is a significant issue in turbine blades, train wheels, and forged and extruded molds [1–4]. In thermal fatigue, if the thermal stress exceeds the yield stress, plastic deformation or cracks may initiate and grow until the material is destroyed. When the fatigue occurred, the damages usually occur during thermal cycling. The cracking phenomena from the heat checking are produced by thermal fatigue, such as the periodical variation of temperature, strain, and stress. The length of the cracks induced by heat checks is short in the beginning of the test and becomes longer during prolonged exposure to thermal cycling. Usually these cracks do not cause the catastrophic failure of a component, but deteriorate its surface finishing and limit its service life. On the other hand, gross cracking is the cracking phenomena produced by thermal shock. Depending on the toughness of the materials, such as the presence of graphite and non metallic inclusions, these cracks can propagate very quickly even causing the catastrophic failure of a material. Nodular cast irons are inexpensive to manufacture and have outstanding wear resistance characteristics. For this reason, they are widely used as materials for wear plates on continuous casting machines, air exhauster parts, and ingot mold, as well as on brake drums of large-sized trucks, pistons, and piston rings of engine parts in ships [5]. Since these parts are used in high temperature environments on a periodic basis, knowing the thermal fatigue characteristics is important. Thermal fatigue occurs through the
⁎ Corresponding author. Tel.: + 82 2 3282 2469; fax: + 82 2 3282 7811. E-mail address: [email protected] (B.-Y. Jeong). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.08.040
formation of periodic stress/strain from being repeatedly heated and cooled on a periodic basis [6]. These characteristics are known to be influenced by microstructural and chemical elements, as well as the properties of materials, such as seal intensity, thermal expansion factors, and thermal conductivity [7–9]. In the cases of low alloy steels, die casting alloys, and nodular cast irons, a hard coating layer with a high gradient of hardness may be formed on the surface through the application of nitriding treatment or Plasma Assisted Chemical Vapor Deposition (PACVD). In addition, some research results have shown that if hard coating and nitriding treatment are performed with TiN and CrN on hot wall tool steels, the low frequency fatigue resistance, as well as the thermal fatigue characteristics, can be improved [10]. However, no precise mechanism has yet been found that can explain this, although it is generally believed that the formation and growth of cracks is significantly reduced due to the high strength gradient and compression residual stress, which is generated as a dense layer that forms on the surface [11,12]. On the other hand, the surface characteristics due to coating can deteriorate through increased surface roughness and the partial exfoliation of the nitride layer due to the graphite that exists on the surface of the nodular cast iron. In order to compensate for this, there has been a rising interest in the plasma duplex treatment, which combines plasma nitriding and oxidation treatment, and then maximizes the characteristics possessed by each surface layer [13– 15]. This study has closely investigated the microstructure and thermal fatigue characteristics after plasma nitriding and duplex treatment on a nodular cast iron with pearlitic and ferritic matrix, respectively. In this work, the nitriding and oxidation process conditions are combined and applied to the duplex treatment, while plasma oxidation etching [16] is introduced as a preceding treatment that etches the graphite exposed on the surface.
B.-Y. Jeong et al. / Surface & Coatings Technology 205 (2010) 896–901 Table 1 Constitution and thickness of the studied nodular cast iron surface layers. Specimen
Untreated N O–S–N O–S–N–O
Compound layer
Oxide layer
Phase
Thickness[μm]
Phase
Thickness[μm]
– ε-Fe2,3N ε-Fe2,3N γ′-Fe4N
– 9.0 7.5 5.4
– – – Fe3O4 + Fe2O3
– – – 1.5
897
and immediately dipped into a water bath. Then, it was examined after a thermal fatigue test with a 5-second cooling cycle that was repeated 1000 times. Optical and scanning electron microscopies (SEM) were used to observe cross-sectional microstructure and thermal fatigue cracks. Atomic force microscopy (AFM) and X-ray diffraction (XRD) were used to analyze three-dimensional surface topography and the oxides, respectively. For the evaluation of the thermal fatigue damage of the samples that went through plasma nitriding and duplex treatment, the number, entire length, maximum length and average length of the cracks were measured.
2. Experimental details 3. Results and discussion Nodular cast iron FCD 70, with a fine pearlite matrix structure (3.70 wt.%C, 2.80 wt.%Si, 0.5 wt.%Mn, 0.06 wt.%P, 0.004 wt.%S, 0.035 wt.%Mg, 0.48 wt.%Cu, 0.19 wt.%Mo, 0.04 wt.%Sn, and 0.18 wt.% Ni) and FCD 40, which has a ferrite matrix (3.40 wt.%C, 2.80 wt.%Si, 0.35 wt.%Mn, 0.06 wt.%P, 0.004 wt.%S, and 0.03 wt.%Mg) were used for this study. Plasma duplex treatment was performed with the insitu process after polishing. Treatment conditions of 550 °C, 10 h, −540 V, 3 Torr, N2:H2 = 4:1, and duty factor = 0.5 (pulse on time/off time = 100 μs/100 μs) were used for nitriding. For the duplex treatment, nitriding and oxidation treatment were performed in order after the preceding treatment of plasma oxidation and sputtering. The same conditions (500 °C, 3 h, − 600 V, 0.4 Torr, O2 100%, DC) were used for both preceding and following oxidation treatment. Sputtering was performed under the conditions of 500 °C, 5 h, 90%Ar + 10%H2, DC, to eliminate the oxide layer formed on the matrix along with the graphite etching during the plasma oxidation. Etching was performed to prevent degrade of the surface characteristics due to the graphite on the surface. It was confirmed by analysis of microstructure and XRD after the preliminary experiment that the oxide layer was completely eliminated. The specimen identification and treatment condition is shown in Table 1. A schematic representation of the thermal fatigue test configuration used in the present study is shown in Fig. 1. A cylindrical specimen of diameter 20 mm and length 60 mm was used for the cycling test after plasma treatment. Each specimen, 7 samples per test conditions, according to treatment conditions was heated by a high frequency induction coil, maintained for 7 s at 500 °C
3.1. Microstructure The plasma diffusion treatment involves sputtering/deposition reactions caused by ion bombardment of plasma active species and mass transfer. The surface structure passes through many stages during the plasma nitriding and oxidation treatment of the nodular cast iron. Fig. 2 shows the surface morphology after plasma nitriding and oxidation, respectively. Fine nitride and solid projections of oxide are distributed on the surface layer and the nitrides and oxides cover the spheroidal graphite. Fine nitride and solid projections of oxide are distributed on the surface layer, and the spheroidal graphite is covered by the nitrides. In general, because most oxidized layers are denser than the nitride layers that have a number of micro pores, they have better corrosion resistance. Therefore, there has been a good deal of interest on attempts to improve corrosion resistance, as well as wear resistance, by forming a duplex oxide layer on the nitride layer [17,18]. Fig. 3(a) and (b) shows the cross-sectional microstructure of the plasma nitriding and duplex treated specimen. Also, Fig. 4 shows the results of the XRD analysis on the surface layer according to each of the process conditions. In the case of nitriding for 10 h at 550 °C (Fig. 3(a)), the single phase nitride layer that was formed was a 9 μm thick layer of ε-Fe2–3N (Fig. 4(a)). Fig. 3(b) shows a duplex layer in which an approximately 1.5 μm thick oxide layer was formed on a nitride layer of about 5.4 μm. By XRD analysis, this oxide layer was found to have contained a mixture of Fe2O3 and Fe3O4 (Fig. 4(c)). Fig. 4(b) shows that
Fig. 1. A schematic representation of the thermal fatigue test equipment.
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B.-Y. Jeong et al. / Surface & Coatings Technology 205 (2010) 896–901
Fig. 2. SEM surface images of nodular cast iron samples after (a) plasma nitriding and (b) plasma oxidation treatment.
Fig. 3. SEM cross-section images of nodular cast iron samples after (a) plasma nitriding treatment at 550 °C for 10 h and (b) plasma duplex treatment (oxidation–sputtering– nitriding–oxidation).
nitriding was performed after oxidation and sputtering. A α-Fe peak was detected along with oxide and ε-Fe2–3N. It is clear that the complete elimination of the oxide layer during the Ar/H2 plasma sputtering was performed successfully [14,16]. Fig. 4(c) shows the XRD result of the duplex treated sample (oxidation–sputtering– nitriding–oxidation), and it is clear that the oxide, nitride and α-Fe were intermixed. The nitride formed here is γ′-Fe4N, not the ε-Fe2–3N. It is believed that the hcp structure of ε-Fe2–3N was changed to a fcc structure of γ′-Fe4N by ion bombardment of oxygen active species after nitriding. Clearly, more work is required to characterize and analyze the effect of the active species.
3.2. Topography and hardness Fig. 5 shows the change in the surface topography with process parameters. Compared to the flat untreated specimen (a), the nitrided one (b) shows more irregular morphology, such as peaks-and-valleys. The duplex treated (c) shows a higher irregularity level than that of (b). The maximum heights of the peak of (b) and (c) are 892 nm and 2153 nm, respectively. On the other hand, the peak height of the oxidized, sputtered, nitrided and oxidized state (d) is lower than that of (c) due to the oxidation. It is believed that the irregularity is etched by the sputtering of the oxygen active species (O and O+ 2 ) during the plasma oxidation. The surface roughness average (Ra) was calculated and the result is shown in Fig. 6. The specimen that went through the oxidation– sputtering–nitride process shows the highest roughness value. When oxidation was added to this sequence, the Ra value decreased significantly. This is believed to be a result of the ion bombardment
Fig. 4. XRD results with the surface treatment conditions: (a) nitriding, (b) oxidation– sputtering–nitriding and (c) oxidation–sputtering–nitriding–oxidation.
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Fig. 5. Three-dimensional AFM surface images with treatment conditions of (a) untreated, (b) nitrided, (c) oxidation–sputtering–nitrided and (d) oxidation–sputtering–nitriding– oxidation treated nodular cast iron samples.
of the high-speed neutral particles and ion active species during the plasma electric discharge. Generally, a thermal fatigue test carried out where the tool steel and duplex were treated with hard coatings of TiN and CrN after plasma nitriding and reported that the cracking density can partially be influenced by the change in values of the surface roughness. In other words, the larger the value of roughness, the higher the possibility that a surface crack can occur. However, it is believed that such research results cannot be generally realized. Moreover, no consistent correlation has been observed in this research that could
Fig. 6. Surface roughness behavior with treatment conditions. (O–S–N means oxidation–sputtering–nitrided and O–S–N–O means oxidation–sputtering–nitriding– oxidation treated nodular cast iron samples).
prove the thermal fatigue cracking density is directly influenced by the change in the values of surface roughness. Instead, it has been shown that the formation behavior of the thermal fatigue crack is influenced by various factors, such as process parameters and the thickness and hardness of the surface layers. Basically, the reason for the improvement of thermal fatigue through nitriding or hard coating is that the frequency of cracking occurrence is suppressed by high compression residual stress and the high hardness of the surface layer. Also, the effective case depth of nitride samples, which represent the
Fig. 7. Variation of the layer hardness with treatment conditions after thermal cycling test: marks 1, 2, 3 and 4 mean untreated, nitrided, oxidation–sputtering–nitrided and oxidation–sputtering–nitrided–oxidation samples, respectively. Marks p and f mean the nodular cast iron samples with pearlite and ferrite matrix, respectively.
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Fig. 8. Cross-sectional optical micrographs of the crack after a thermal cycling test at 500 °C.
degree of surface hardening, is strictly correlated with the nucleation and propagation behavior of thermal cracks [19]. Therefore, in order to investigate more the correlation of the following thermal fatigue tests with hardness properties, the hardness of the matrix and surface hardened layers with treatment conditions were measured. The result is shown in Fig. 7. There was almost no change in the hardness of the matrix with treatment conditions, but the hardness of the hardened layers significantly increased and showed similar trends in both the pearlite and ferrite matrix. In other words, the nitrided specimen showed the highest value, and the hardness of the duplex treated specimens (4p and 4f) was the lowest. Thus, it is expected that the thermal fatigue resistance will be influenced by these hardness characteristics, and the next section will look into their correlation.
cracks depending on the shape of the spheroidal graphite. Fatigue cracks nucleate at nodules, microscopic shrinkage cavities, and large graphite nodules that are an incomplete shape or near the specimen surface. Fatigue cracks also occur in graphite nodule clusters that are an irregular shape. If the graphite nodule is large and has an irregular shape in the thermal fatigue cracking of a cast iron, the cracks are long and narrow, while the nearby nodules speed up the crack propagation. In Fig. 10(a), the cracks are larger than that of (b) due to the large nodule size. In addition, the comparison of cracks, (3) and (4) in Fig. 10(b) shows that the cracks propagate easily from nodule to nodule. To quantify the results, the total sum of the number and length of the cracks size of the investigated area of each specimen is shown in Fig. 11. Fig. 12 shows the average and maximum crack
3.3. Formation behavior of thermal cracking Fig. 8 shows a cross-sectional optical microstructure, which shows the thermal cracking of the untreated nodular cast iron on which thermal cycling was performed 1000 times. According to the XRD analysis (Fig. 9), the thin layer on the surface is an oxide layer made of Fe2O3 and Fe3O4. As seen in Fig. 8, the crack starts on the surface and spreads toward the center, showing a cracking pattern that has a very irregular path rather than a straight one. Because the cast iron is a complex material made of graphite, pearlite and cementite, its thermal fatigue cracks are complicated in comparison to those of steels or other materials. In addition, the thermal fatigue characteristic is influenced significantly by the shape of the graphite. Fig. 10 shows the microstructure and the formation behavior of thermal
Fig. 9. XRD results of untreated samples after thermal cycling test.
Fig. 10. Light micrographs of cross-section of thermal cycled samples. No etching. (a) Nodular cast iron with ferrite matrix and (b) nodular cast iron with pearlite matrix.
B.-Y. Jeong et al. / Surface & Coatings Technology 205 (2010) 896–901
901
4. Conclusions
Fig. 11. The crack count and length of the samples after thermal cycling test. Marks 1, 2, 3 and 4 mean untreated, nitrided, oxidation–sputtering–nitrided and oxidation– sputtering–nitrided–oxidation samples, respectively. Marks p and f mean the nodular cast iron samples with pearlite and ferrite matrix, respectively.
The thermal fatigue characteristic of the nodular cast iron was shown to be influenced by the size and shape of the graphite nodules. The cracks quickly propagated toward the center due to the notch effect, in which stress was concentrated in the large, irregular graphite nodules. In addition, if the matrix structure was ferrite, the crack would propagate along the grain boundaries and the interfaces between the graphite nodules. Regardless of the matrix structure, the thermal fatigue crack in the nodular graphite cast iron showed an irregular cracking pattern. However, there is no consistent correlation which can prove the thermal fatigue cracking density is directly influenced by the change in the values of surface roughness. The untreated specimen showed the highest crack density. In the case of the plasma surface treatment, the nodular cast iron of the pearlite matrix showed better thermal fatigue resistance than that of the ferrite matrix. The thermal fatigue characteristics were influenced by the change in the hardness of the surface hardened layer. The nitrided samples showed the highest hardness and thermal crack resistance. It was hard to find the influence of the surface roughness. Acknowledgement
length of each specimen. In general, thermal fatigue resistance is evaluated with the measuring of the crack size and crack density, i.e. Pmax (maximum crack length), ΣP (total sum of crack length), Pmed (average value of crack length) and ρ (number of cracks per unit of length) [6]. In Fig. 11, p and f stand for the pearlite and ferrite matrix, respectively. The untreated specimen is 1, the nitrided specimen is 2, the oxidation–sputtering–nitriding treated specimen is 3, and the oxidation–sputtering–nitriding–oxidation treated specimen is shown as 4. The untreated specimens (1p and 1f) displayed the highest damage while the nitrided specimen (2p and 2f) showed the minimum level of the crack count. Fig. 12 shows the average and maximum values of crack length. The pattern is very similar to Fig. 11. These results can be correlated to the surface hardness characteristics shown in Fig. 7. The untreated specimens (1p and 1f) with the smallest layer hardness values had the lowest thermal fatigue crack resistance. Also, the nitrided specimens (2p and 2f) with the maximum layer hardness values had the best thermal fatigue crack resistance.
Fig. 12. The crack length of the samples after thermal cycling test. Marks 1, 2, 3 and 4 mean untreated, nitrided, oxidation–sputtering–nitrided and oxidation–sputtering– nitrided–oxidation samples, respectively. Marks p and f mean the nodular cast iron samples with pearlite and ferrite matrix, respectively.
This research was supported by a grant from the Fundamental R;D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Korea. References [1] H. Fredriksson, P.A. Sunnerkrantz, P. Ljubinkovic, Mater. Sci. Technol. 4 (1988) 222. [2] Y.J. Park, R.B. Gundlach, J.F. Janowak, AFS Trans. 98 (1990) 267. [3] K. RÖhrig, AFS Trans. 86 (1978) 75. [4] R. Hallstein, D. Lohe, E. Macherauch, D. Eifler, Proc. Third Int. Conf. On Low Cycle Fatigue, Berlin, 1992, p. 182. [5] G.S. Cho, K.H. Choe, K.K. Lee, A. Ikenaga, Mater. Sci. Technol. 23 (2007) 97. [6] G.E. Dieter, Mechanical Metallurgy3rd ed., , 1986, p. 390. [7] S.C. Lee, L.C. Weng, Metall. Trans. A 22A (1991) 1821. [8] M.C. Rukadikar, G.P. Reddy, AFS Trans. 96 (1988) 351. [9] K.R. Ziegler, J.F. Wallace, AFS Trans. 92 (1984) 735. [10] C.M.D. Starling, J.R.T. Branco, Thin Solid Films 308–309 (1997) 436. [11] M.M. Tosic, R. Gligorijevic, Mater. Sci. Eng. 140 (1991) 469. [12] A. Celik, S. Kardeniz, Surf. Coat. Technol. 72 (1995) 169. [13] S. Hoppe, Surf. Coat. Technol. 98 (1998) 1199. [14] Bong-Yong Jeong, Myung-Ho Kim, J. Kor. Inst. Met. & Mater. 38 (2000) 1069. [15] C. Dawes, D.F. Tranter, Nitrotee, Surf.Treat. Techno., Heat Treatment Metal 3 (1985) 70. [16] Bong-Yong Jeong, Min-Sun Hwang, Chongmu Lee, Myung-Ho Kim, J. Kor. Inst. Met. & Mater. 38 (2000) 823. [17] P. Mas, B. Greller, Le sursulf oxynit, Rev, Prat. Metall. 28 (1986) 105. [18] D.A. Jones, Principles and Prevention of Corrosion, 2nd Ed, Prentice Hall, Inc, 1996, p. 403. [19] M. Pellizzari, A. Molinari, G. Straffelini, Mater. Sci. Eng. A352 (2003) 186.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Thermal fatigue characteristics of plasma duplex treated nodular cast irons Bong-Yong Jeong a,⁎, Jeong-Ho Chang a, Myung-Ho Kim b a b
Bio-IT Convergence Center, Korea Institute of Ceramic Engineering and Technology, 103 Fashion Danji-gil, Geumcheon-gu, Seoul 153-801, South Korea Department of Materials Science and Engineering, Inha University, Yonghyun-dong, Namgu, Inchon 402-751, South Korea
a r t i c l e
i n f o
Article history: Received 13 April 2010 Accepted in revised form 10 August 2010 Available online 14 August 2010 Keywords: Thermal fatigue Plasma diffusion treatment Nodular cast iron Plasma nitriding Oxidation
a b s t r a c t The correlation between the thermal fatigue resistance and microstructure of untreated and plasma surface engineered nodular cast irons was investigated. Both the ferrite and pearlite matrix nodular cast irons were evaluated. During the tests, 60 mm long cylinders with 20 mm diameter were subjected to 1000 high frequency induction heating and water cooling cycles, lasting 7 and 5 s, respectively. The thermal fatigue damage was evaluated by analyzing the crack dimensions and distributions. The results showed that the plasma surface treatment increased thermal fatigue resistance. When thermal cracking occurred, the cracks always nucleated at the surface of the specimen. In addition, the nodular cast iron with a pearlite matrix had a better resistance to thermal fatigue than that of the ferrite matrix. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Thermal fatigue, which is a factor that determines the life span of a material, is a significant issue in turbine blades, train wheels, and forged and extruded molds [1–4]. In thermal fatigue, if the thermal stress exceeds the yield stress, plastic deformation or cracks may initiate and grow until the material is destroyed. When the fatigue occurred, the damages usually occur during thermal cycling. The cracking phenomena from the heat checking are produced by thermal fatigue, such as the periodical variation of temperature, strain, and stress. The length of the cracks induced by heat checks is short in the beginning of the test and becomes longer during prolonged exposure to thermal cycling. Usually these cracks do not cause the catastrophic failure of a component, but deteriorate its surface finishing and limit its service life. On the other hand, gross cracking is the cracking phenomena produced by thermal shock. Depending on the toughness of the materials, such as the presence of graphite and non metallic inclusions, these cracks can propagate very quickly even causing the catastrophic failure of a material. Nodular cast irons are inexpensive to manufacture and have outstanding wear resistance characteristics. For this reason, they are widely used as materials for wear plates on continuous casting machines, air exhauster parts, and ingot mold, as well as on brake drums of large-sized trucks, pistons, and piston rings of engine parts in ships [5]. Since these parts are used in high temperature environments on a periodic basis, knowing the thermal fatigue characteristics is important. Thermal fatigue occurs through the
⁎ Corresponding author. Tel.: + 82 2 3282 2469; fax: + 82 2 3282 7811. E-mail address: [email protected] (B.-Y. Jeong). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.08.040
formation of periodic stress/strain from being repeatedly heated and cooled on a periodic basis [6]. These characteristics are known to be influenced by microstructural and chemical elements, as well as the properties of materials, such as seal intensity, thermal expansion factors, and thermal conductivity [7–9]. In the cases of low alloy steels, die casting alloys, and nodular cast irons, a hard coating layer with a high gradient of hardness may be formed on the surface through the application of nitriding treatment or Plasma Assisted Chemical Vapor Deposition (PACVD). In addition, some research results have shown that if hard coating and nitriding treatment are performed with TiN and CrN on hot wall tool steels, the low frequency fatigue resistance, as well as the thermal fatigue characteristics, can be improved [10]. However, no precise mechanism has yet been found that can explain this, although it is generally believed that the formation and growth of cracks is significantly reduced due to the high strength gradient and compression residual stress, which is generated as a dense layer that forms on the surface [11,12]. On the other hand, the surface characteristics due to coating can deteriorate through increased surface roughness and the partial exfoliation of the nitride layer due to the graphite that exists on the surface of the nodular cast iron. In order to compensate for this, there has been a rising interest in the plasma duplex treatment, which combines plasma nitriding and oxidation treatment, and then maximizes the characteristics possessed by each surface layer [13– 15]. This study has closely investigated the microstructure and thermal fatigue characteristics after plasma nitriding and duplex treatment on a nodular cast iron with pearlitic and ferritic matrix, respectively. In this work, the nitriding and oxidation process conditions are combined and applied to the duplex treatment, while plasma oxidation etching [16] is introduced as a preceding treatment that etches the graphite exposed on the surface.
B.-Y. Jeong et al. / Surface & Coatings Technology 205 (2010) 896–901 Table 1 Constitution and thickness of the studied nodular cast iron surface layers. Specimen
Untreated N O–S–N O–S–N–O
Compound layer
Oxide layer
Phase
Thickness[μm]
Phase
Thickness[μm]
– ε-Fe2,3N ε-Fe2,3N γ′-Fe4N
– 9.0 7.5 5.4
– – – Fe3O4 + Fe2O3
– – – 1.5
897
and immediately dipped into a water bath. Then, it was examined after a thermal fatigue test with a 5-second cooling cycle that was repeated 1000 times. Optical and scanning electron microscopies (SEM) were used to observe cross-sectional microstructure and thermal fatigue cracks. Atomic force microscopy (AFM) and X-ray diffraction (XRD) were used to analyze three-dimensional surface topography and the oxides, respectively. For the evaluation of the thermal fatigue damage of the samples that went through plasma nitriding and duplex treatment, the number, entire length, maximum length and average length of the cracks were measured.
2. Experimental details 3. Results and discussion Nodular cast iron FCD 70, with a fine pearlite matrix structure (3.70 wt.%C, 2.80 wt.%Si, 0.5 wt.%Mn, 0.06 wt.%P, 0.004 wt.%S, 0.035 wt.%Mg, 0.48 wt.%Cu, 0.19 wt.%Mo, 0.04 wt.%Sn, and 0.18 wt.% Ni) and FCD 40, which has a ferrite matrix (3.40 wt.%C, 2.80 wt.%Si, 0.35 wt.%Mn, 0.06 wt.%P, 0.004 wt.%S, and 0.03 wt.%Mg) were used for this study. Plasma duplex treatment was performed with the insitu process after polishing. Treatment conditions of 550 °C, 10 h, −540 V, 3 Torr, N2:H2 = 4:1, and duty factor = 0.5 (pulse on time/off time = 100 μs/100 μs) were used for nitriding. For the duplex treatment, nitriding and oxidation treatment were performed in order after the preceding treatment of plasma oxidation and sputtering. The same conditions (500 °C, 3 h, − 600 V, 0.4 Torr, O2 100%, DC) were used for both preceding and following oxidation treatment. Sputtering was performed under the conditions of 500 °C, 5 h, 90%Ar + 10%H2, DC, to eliminate the oxide layer formed on the matrix along with the graphite etching during the plasma oxidation. Etching was performed to prevent degrade of the surface characteristics due to the graphite on the surface. It was confirmed by analysis of microstructure and XRD after the preliminary experiment that the oxide layer was completely eliminated. The specimen identification and treatment condition is shown in Table 1. A schematic representation of the thermal fatigue test configuration used in the present study is shown in Fig. 1. A cylindrical specimen of diameter 20 mm and length 60 mm was used for the cycling test after plasma treatment. Each specimen, 7 samples per test conditions, according to treatment conditions was heated by a high frequency induction coil, maintained for 7 s at 500 °C
3.1. Microstructure The plasma diffusion treatment involves sputtering/deposition reactions caused by ion bombardment of plasma active species and mass transfer. The surface structure passes through many stages during the plasma nitriding and oxidation treatment of the nodular cast iron. Fig. 2 shows the surface morphology after plasma nitriding and oxidation, respectively. Fine nitride and solid projections of oxide are distributed on the surface layer and the nitrides and oxides cover the spheroidal graphite. Fine nitride and solid projections of oxide are distributed on the surface layer, and the spheroidal graphite is covered by the nitrides. In general, because most oxidized layers are denser than the nitride layers that have a number of micro pores, they have better corrosion resistance. Therefore, there has been a good deal of interest on attempts to improve corrosion resistance, as well as wear resistance, by forming a duplex oxide layer on the nitride layer [17,18]. Fig. 3(a) and (b) shows the cross-sectional microstructure of the plasma nitriding and duplex treated specimen. Also, Fig. 4 shows the results of the XRD analysis on the surface layer according to each of the process conditions. In the case of nitriding for 10 h at 550 °C (Fig. 3(a)), the single phase nitride layer that was formed was a 9 μm thick layer of ε-Fe2–3N (Fig. 4(a)). Fig. 3(b) shows a duplex layer in which an approximately 1.5 μm thick oxide layer was formed on a nitride layer of about 5.4 μm. By XRD analysis, this oxide layer was found to have contained a mixture of Fe2O3 and Fe3O4 (Fig. 4(c)). Fig. 4(b) shows that
Fig. 1. A schematic representation of the thermal fatigue test equipment.
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B.-Y. Jeong et al. / Surface & Coatings Technology 205 (2010) 896–901
Fig. 2. SEM surface images of nodular cast iron samples after (a) plasma nitriding and (b) plasma oxidation treatment.
Fig. 3. SEM cross-section images of nodular cast iron samples after (a) plasma nitriding treatment at 550 °C for 10 h and (b) plasma duplex treatment (oxidation–sputtering– nitriding–oxidation).
nitriding was performed after oxidation and sputtering. A α-Fe peak was detected along with oxide and ε-Fe2–3N. It is clear that the complete elimination of the oxide layer during the Ar/H2 plasma sputtering was performed successfully [14,16]. Fig. 4(c) shows the XRD result of the duplex treated sample (oxidation–sputtering– nitriding–oxidation), and it is clear that the oxide, nitride and α-Fe were intermixed. The nitride formed here is γ′-Fe4N, not the ε-Fe2–3N. It is believed that the hcp structure of ε-Fe2–3N was changed to a fcc structure of γ′-Fe4N by ion bombardment of oxygen active species after nitriding. Clearly, more work is required to characterize and analyze the effect of the active species.
3.2. Topography and hardness Fig. 5 shows the change in the surface topography with process parameters. Compared to the flat untreated specimen (a), the nitrided one (b) shows more irregular morphology, such as peaks-and-valleys. The duplex treated (c) shows a higher irregularity level than that of (b). The maximum heights of the peak of (b) and (c) are 892 nm and 2153 nm, respectively. On the other hand, the peak height of the oxidized, sputtered, nitrided and oxidized state (d) is lower than that of (c) due to the oxidation. It is believed that the irregularity is etched by the sputtering of the oxygen active species (O and O+ 2 ) during the plasma oxidation. The surface roughness average (Ra) was calculated and the result is shown in Fig. 6. The specimen that went through the oxidation– sputtering–nitride process shows the highest roughness value. When oxidation was added to this sequence, the Ra value decreased significantly. This is believed to be a result of the ion bombardment
Fig. 4. XRD results with the surface treatment conditions: (a) nitriding, (b) oxidation– sputtering–nitriding and (c) oxidation–sputtering–nitriding–oxidation.
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Fig. 5. Three-dimensional AFM surface images with treatment conditions of (a) untreated, (b) nitrided, (c) oxidation–sputtering–nitrided and (d) oxidation–sputtering–nitriding– oxidation treated nodular cast iron samples.
of the high-speed neutral particles and ion active species during the plasma electric discharge. Generally, a thermal fatigue test carried out where the tool steel and duplex were treated with hard coatings of TiN and CrN after plasma nitriding and reported that the cracking density can partially be influenced by the change in values of the surface roughness. In other words, the larger the value of roughness, the higher the possibility that a surface crack can occur. However, it is believed that such research results cannot be generally realized. Moreover, no consistent correlation has been observed in this research that could
Fig. 6. Surface roughness behavior with treatment conditions. (O–S–N means oxidation–sputtering–nitrided and O–S–N–O means oxidation–sputtering–nitriding– oxidation treated nodular cast iron samples).
prove the thermal fatigue cracking density is directly influenced by the change in the values of surface roughness. Instead, it has been shown that the formation behavior of the thermal fatigue crack is influenced by various factors, such as process parameters and the thickness and hardness of the surface layers. Basically, the reason for the improvement of thermal fatigue through nitriding or hard coating is that the frequency of cracking occurrence is suppressed by high compression residual stress and the high hardness of the surface layer. Also, the effective case depth of nitride samples, which represent the
Fig. 7. Variation of the layer hardness with treatment conditions after thermal cycling test: marks 1, 2, 3 and 4 mean untreated, nitrided, oxidation–sputtering–nitrided and oxidation–sputtering–nitrided–oxidation samples, respectively. Marks p and f mean the nodular cast iron samples with pearlite and ferrite matrix, respectively.
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Fig. 8. Cross-sectional optical micrographs of the crack after a thermal cycling test at 500 °C.
degree of surface hardening, is strictly correlated with the nucleation and propagation behavior of thermal cracks [19]. Therefore, in order to investigate more the correlation of the following thermal fatigue tests with hardness properties, the hardness of the matrix and surface hardened layers with treatment conditions were measured. The result is shown in Fig. 7. There was almost no change in the hardness of the matrix with treatment conditions, but the hardness of the hardened layers significantly increased and showed similar trends in both the pearlite and ferrite matrix. In other words, the nitrided specimen showed the highest value, and the hardness of the duplex treated specimens (4p and 4f) was the lowest. Thus, it is expected that the thermal fatigue resistance will be influenced by these hardness characteristics, and the next section will look into their correlation.
cracks depending on the shape of the spheroidal graphite. Fatigue cracks nucleate at nodules, microscopic shrinkage cavities, and large graphite nodules that are an incomplete shape or near the specimen surface. Fatigue cracks also occur in graphite nodule clusters that are an irregular shape. If the graphite nodule is large and has an irregular shape in the thermal fatigue cracking of a cast iron, the cracks are long and narrow, while the nearby nodules speed up the crack propagation. In Fig. 10(a), the cracks are larger than that of (b) due to the large nodule size. In addition, the comparison of cracks, (3) and (4) in Fig. 10(b) shows that the cracks propagate easily from nodule to nodule. To quantify the results, the total sum of the number and length of the cracks size of the investigated area of each specimen is shown in Fig. 11. Fig. 12 shows the average and maximum crack
3.3. Formation behavior of thermal cracking Fig. 8 shows a cross-sectional optical microstructure, which shows the thermal cracking of the untreated nodular cast iron on which thermal cycling was performed 1000 times. According to the XRD analysis (Fig. 9), the thin layer on the surface is an oxide layer made of Fe2O3 and Fe3O4. As seen in Fig. 8, the crack starts on the surface and spreads toward the center, showing a cracking pattern that has a very irregular path rather than a straight one. Because the cast iron is a complex material made of graphite, pearlite and cementite, its thermal fatigue cracks are complicated in comparison to those of steels or other materials. In addition, the thermal fatigue characteristic is influenced significantly by the shape of the graphite. Fig. 10 shows the microstructure and the formation behavior of thermal
Fig. 9. XRD results of untreated samples after thermal cycling test.
Fig. 10. Light micrographs of cross-section of thermal cycled samples. No etching. (a) Nodular cast iron with ferrite matrix and (b) nodular cast iron with pearlite matrix.
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4. Conclusions
Fig. 11. The crack count and length of the samples after thermal cycling test. Marks 1, 2, 3 and 4 mean untreated, nitrided, oxidation–sputtering–nitrided and oxidation– sputtering–nitrided–oxidation samples, respectively. Marks p and f mean the nodular cast iron samples with pearlite and ferrite matrix, respectively.
The thermal fatigue characteristic of the nodular cast iron was shown to be influenced by the size and shape of the graphite nodules. The cracks quickly propagated toward the center due to the notch effect, in which stress was concentrated in the large, irregular graphite nodules. In addition, if the matrix structure was ferrite, the crack would propagate along the grain boundaries and the interfaces between the graphite nodules. Regardless of the matrix structure, the thermal fatigue crack in the nodular graphite cast iron showed an irregular cracking pattern. However, there is no consistent correlation which can prove the thermal fatigue cracking density is directly influenced by the change in the values of surface roughness. The untreated specimen showed the highest crack density. In the case of the plasma surface treatment, the nodular cast iron of the pearlite matrix showed better thermal fatigue resistance than that of the ferrite matrix. The thermal fatigue characteristics were influenced by the change in the hardness of the surface hardened layer. The nitrided samples showed the highest hardness and thermal crack resistance. It was hard to find the influence of the surface roughness. Acknowledgement
length of each specimen. In general, thermal fatigue resistance is evaluated with the measuring of the crack size and crack density, i.e. Pmax (maximum crack length), ΣP (total sum of crack length), Pmed (average value of crack length) and ρ (number of cracks per unit of length) [6]. In Fig. 11, p and f stand for the pearlite and ferrite matrix, respectively. The untreated specimen is 1, the nitrided specimen is 2, the oxidation–sputtering–nitriding treated specimen is 3, and the oxidation–sputtering–nitriding–oxidation treated specimen is shown as 4. The untreated specimens (1p and 1f) displayed the highest damage while the nitrided specimen (2p and 2f) showed the minimum level of the crack count. Fig. 12 shows the average and maximum values of crack length. The pattern is very similar to Fig. 11. These results can be correlated to the surface hardness characteristics shown in Fig. 7. The untreated specimens (1p and 1f) with the smallest layer hardness values had the lowest thermal fatigue crack resistance. Also, the nitrided specimens (2p and 2f) with the maximum layer hardness values had the best thermal fatigue crack resistance.
Fig. 12. The crack length of the samples after thermal cycling test. Marks 1, 2, 3 and 4 mean untreated, nitrided, oxidation–sputtering–nitrided and oxidation–sputtering– nitrided–oxidation samples, respectively. Marks p and f mean the nodular cast iron samples with pearlite and ferrite matrix, respectively.
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