- Email: [email protected]
Influence of heat treatment on microstructure and mechanical properties of iron-based coating
Materials and Design 32 (2011) 3004–3007
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Influence of heat treatment on microstructure and mechanical properties of iron-based coating Li Hua-yi a,b, Li Guo-lu a, Wang Hai-dou b,⇑, Xu Bin-shi b a b
School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China National Key Lab for Remanufacturing, Academy of Armored Forces Engineering, Beijing 100072, China
a r t i c l e
i n f o
Article history: Received 15 October 2010 Accepted 23 December 2010 Available online 30 December 2010
a b s t r a c t Iron-based alloys were deposited on the low carbon steel by plasma cladding process. Furnace annealing was conducted at 600 °C for 40 min. Resulting microstructure and phases were observed and investigated by scanning electron microscopy (SEM), energy-dispersive spectrometer (EDS) and X-ray diffraction (XRD). Effect of post heat treatment on the mechanical properties of coatings was also studied by instrumented indentation technique. It was found that solid solution c-(Fe, Ni, Cr) and carbide reinforced phases Cr7C3 were the main phases of as-cladding coatings while iron carbide became the main carbide reinforced phase for annealed coatings. For all coatings, hardness and reduced elastic modulus showed obvious load dependence, namely decreased with the indentation load increasing. It was found that calculated values of annealed coatings were generally lower than those of as-cladding coatings as a result of the dissolution of the eutectic structure which decreased the effect of dispersion strengthening. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Surface wear and corrosion have become the main reasons for equipment failure. The need for improving the qualities of surface leads to the development of surface modification techniques such as spraying [1,2], electroplating [3,4] and laser or plasma cladding [5]. In recent years, plasma cladding technique becomes an active field due to its sufficient rapid cooling rate, good performance and low cost [6–8]. Post heat treatment is now widely used to further improve the required properties of bulk materials, such as furnace heat treatment [9], quenching [10] and laser remelting [11], of which furnace annealing is the most fundamental one. By adjusting the annealing temperature, it can reach the achievement of refining crystal, relieving residual stresses and even changing the microstructure and phases to enhance the mechanical properties [9], such as fracture toughness and wear resistance. However relative to bulk materials, little work have been done to analysis the effect of annealing on materials at small scale, such as films and coatings [7]. In this study, microscopy was used to investigate the effect of furnace annealing on phases and microstructure of iron-based alloy coating produced by plasma cladding process. At the same time, effect on mechanical properties was studied by an almost non-destructive method – instrument indentation test. To avoid ⇑ Corresponding author. Tel.: +86 10 66718541; fax: +86 10 66717144. E-mail address: [email protected] (W. Hai-dou). 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.12.055
the influences of pile-up on the calculating precision of properties, a new method based on the energy of indentation process proposed by Giannakopoulos and Suresh was applied [12,13] instead of the traditional Oliver and Pharr method [14,15]. 2. Experimental details 2.1. Material and preparation Commercial low carbon steel containing 0.1 wt.% carbon was taken as the substrate and degreased in acetone solution. No previous heat treatment was taken. Iron-based alloy powder with particles ranges from 150 to 180 lm was taken as the plasma cladding powder. The compositions of the powder are listed in Table 1. The experimental equipment adopted in the study was plasma cladding equipment produced by Academy of Armored Forces Engineering (China). Argon was used as the plasma gas. The parameters of plasma cladding process are shown in Table 2. After plasma cladding, the specimens were divided into two groups. One group remained the original status as cladding, others were annealed at 600 °C for 40 min. 2.2. Characterization Microstructures of the clad coatings were observed by scanning electron microscopy (SEM) (LEO1530VP) with energy dispersive spectroscopy (EDS) attachment, and phases were characterized by X-ray diffraction (XRD) (D8 ADVANCE, BRUKER/AKS, Germany).
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L. Hua-yi et al. / Materials and Design 32 (2011) 3004–3007 Table 1 Chemical composition of Fe-based alloy.
We ¼
Z
hmax
PðhÞdh ¼ hr
Element
C
Cr
Ni
B
Si
Fe
Mass fraction (%)
0.1
15
10
1
1
Bal.
A
120 (A) 22 (V) 1.8 (mm/s) Ar 2.2 (g/min) 5.0 (L/min)
2
hmax
The mechanical properties of coatings were investigated by nanoindentation equipment (Nano indenter XP) which can simultaneously record the applied indentation loads and penetrating depths during an indentation loading–unloading cycle. A Berkovich three-sided pyramidal indenter was used. Prior to indentation tests, surface of all specimens was grinded to a roughness of 0.09 lm. For both groups loads were applied by a linear ramp up to certain maximum load 20 mN, 50 mN, 100 mN, 150 mN and 200 mN respectively. The dwelling time at each maximum load was 10 s. For each load, at least five indents were made and then the average was used. The spacing between indents was at least 10 lm. All experiments were performed at room temperature. 2.3. Nano-indentation theory The calculation of mechanical properties in this study was based on the theory proposed by A. E. Giannakopoulos and S. Suresh [12,13]. The key feature of the method is that it takes into account the phenomenon of pile-up and sink-in and at the same time avoids applying contact depth to get the true contact area, which may be more applicable to materials that pile-up in comparison with the traditional O&P method [14,15]. Fig. 1 is the schematic of load-depth curve for a sharp indenter, where Pmax is the maximum indentation load which makes the maximum penetrating depth hmax, hr is the residual depth, namely the permanent depth of penetration after the indenter is fully unloaded. As shown in Fig. 1, the total work Wt can be decomposed into elastic and plastic parts, i.e. Wt = We + Wp. The hardness, H and the reduced elastic modulus, Er then can be calculated by the following equations [12], where m is the power law fitting constants.
Z 0
P max
PðhÞdh ¼
Z 0
P max
3
2
Ch dh ¼
aðh hr Þm dh ¼
hr
aðhmax hr Þmþ1 mþ1
Pmax ðhmax hr Þ; ¼ mþ1
H¼
Wt ¼
hmax
Wt We H ¼ 1 4:678 ; Er Wt
Table 2 Parameters of plasma cladding. Current Voltage Scanning velocity Shielding gas Feeding powder flow Plasma gas flow
Z
Chmax Pmax hmax ¼ ; 3 3
ð1Þ
2 H H þ 105:42 1 ¼ 9:96 12:64 1 Er Er 3 4 H H 229:57 1 þ 157:67 1 ; Er Er pmax : A
ð2Þ
ð3Þ
ð4Þ
ð5Þ
3. Results and discussion 3.1. Microstructure of the coating Microstructures of both the as-cladding coatings and the annealed coatings are shown in Fig. 1. Typical microstructure of the untreated original coating (as shown in Fig. 2a) consists of light phase and darker blocky phase which intercrosses each other and forms a kind of netlike structure and in agreement with the observation of Liming Zhang et al. [5] and Ying Tang et al. [7]. EDS analysis results, as shown in Table 3, reveal that the light phase is relatively rich in Fe, Cr and Ni while contains a few of carbon element. As can be seen from the micrograph of XRD analysis results (as shown in Fig. 3), the main phase of the light area is identified as c-(Fe, Ni, Cr) solid solution, while Cr dissolved in the solid solution and partly formed intermetallic compound particles such as (Cr, Fe)7C3 as the XRD results proved. In darker area dendritic c-(Fe, Ni) exists and dopes with an extended solid solution of Cr and a small amount of Si. According to the investigation of J.B. Cheng et al. [6], the eutectic of c-Fe/(Cr, Fe)7C3 formed when the temperature of molten pool falls at eutectic point as a result of the enriching of Fe in residual liquid. It is evident from Table 3 that compared to the light area; the darker area has obviously higher Cr content (21.08% Cr). Consequently the effect of solid solution strengthening is strengthened. Many tiny pits also found centralizing in the darker area which may due to the non-compacting of solid solution phases and the high velocity impact between the surface and arc plasma [8]. As can be seen in Fig. 2, both the groups have the darker islands that are embedded in the matrix. However, it can be seen that after annealed at 600 °C the distribution of the darker area is much more uniform than that of the as-cladding coating and the number of tiny pits obviously decreases with the distribution more uniform in the whole field of microscopy. The range in diffraction peak strength indicates the variation in qualities of corresponding phases as well. Compared to the annealed coatings; the untreated coatings has higher Cr7C3 which is the primary intermetallic compound. When treated at 600 °C, the energy of Cr ions was enhanced which resulted in the dissolution of Cr7C3 and the formation of more solid solutions.. As shown in Fig. 3, more carbon took part in the reaction with iron which promoted the formation of iron carbide at the same time. 3.2. Analysis of load-depth (p–h) curves
Fig. 1. Associated nomenclature in typical p–h curve.
Typical indentation load-depth curves as a function of indentation load are shown in Fig. 4. As shown in the figure, with the increasing indentation load, the depth for all coatings increases
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L. Hua-yi et al. / Materials and Design 32 (2011) 3004–3007
Fig. 3. XRD spectrum of the as-cladding and the annealed coatings.
Fig. 4. Load-depth curves of coatings by nanoindentation at different loads. Fig. 2. Microstructure of the coatings taken by SEM: (a) as cladding; (b) annealed at 600 °C.
3.3. Hardness Table 3 Results of the EDS test of different area in the coatings.
As-cladding coatings Annealed coatings
Light area (wt.%) Dark area (wt.%) Light area (wt.%) Dark area (wt.%)
Si
Cr
Fe
Ni
C
0.96
13.13
76.08
7.82
2.01
0.71 1.02
21.08 10.62
70.8 75.31
7.41 6.57
– 6.48
–
28.4
68.21
3.17
–
uniformly. For each group, the slope of loading curves at different loads is in substantial agreement. However, when choose the same indentation load, slope of loading curves for the as-cladding coatings is obviously greater than that of the annealed specimens which means to reach the same indentation depth, more load is required for the as-cladding coatings. This phenomenon proves that residual stress exists in the as-cladding coatings and the manipulation of annealing at 600 °C can achieve the effect of stress-relief. In addition, unloading curves for both groups at 50 mN, 100 mN, 150 mN show similar slope in appearance which indicates, according to the theory of Oliver & Pharr, the calculated elastic modulus based on the unloading curves may be range in a very small scope.
Variation in nanohardness of the coatings are given in Fig. 5 which calculated by the method that proposed by Giannakopoulos and Suresh. As shown in the figure, for both as-cladding coatings
Fig. 5. Nanohardness as a function of indentation load of Fe-based coatings by nanoindentation.
L. Hua-yi et al. / Materials and Design 32 (2011) 3004–3007
3007
iron carbide becomes the main carbide reinforced phase of the annealed coatings. For all coatings the hardness decreases with the indentation load increasing. Hardnesss of as-cladding coatings range from 5.5– 4.5 GPa, while the corresponding values decreased to 4.3–3.3 GPa for annealed coatings. This change may due to the dissolution of the eutectic structure which decreases the effect of dispersion strengthening. The distribution of reduced modulus for all coatings shows a similar trend as the hardness given. An obvious load dependence of reduced modulus can be found as well. The values of annealed coatings are a little lower (<30%) than that of as-cladding coatings, which attributed to the variation of the ration of We to Wt and more uniform distribution of austenite. However as what is shown, with the load increasing, a same value of reduced modulus will be achieved finally. Fig. 6. Reduced modulus as a function of indentation load of Fe-based coatings by nanoindentation.
and annealed coatings, with the indentation load increasing, the nanohardness decreases and for both curves the decrements are almost the same. This phenomenon is called as ‘‘indentation size effect (ISE)’’, namely the hardness measured usually increases with the decreasing depth of penetration. Much work has been done to investigate this phenomenon. Recent investigations proved that the change of pile-up ratio with the variation of penetration depth [16] and extra energy dissipation [17] may have significant effects on the ISE. Chicot [18] also proposed a hardness length-scale factor to model nano- and micro-indentation size effects which proportional to both the shear modulus and the Burgers vector depending on the dislocation spacing. Compared to the values of as-cladding coatings, low nanohardness values of the annealed coatings can be attributed to the dissolution of the eutectic structure which decreases the effect of dispersion strengthening. 3.4. Elastic modulus In Fig. 6, reduced elastic modulus of as-cladding coatings and annealed coatings are given, respectively. Similar to Fig. 5, distribution of reduced modulus shows obviously indentation load dependence. It can be seen reduced modulus of both groups decreases with the indentation load increasing just like the distributing regulation of nanohardness which can be attributed to the phenomenon of ISE as well [16–18]. However, with the indentation load increasing to 100 mN, values of reduced modulus for both groups trends to be stable. The average modulus values for annealed coatings are slightly lower than those measured for as-cladding coatings which can be attributed to the change of ratio of We to Wt and more uniform distribution of austenite. It can be inferred according to the tendency shown in Fig. 6, that modulus for both groups will achieve the same value when the load increases to a certain extent which coinciding with the results based on the Oliver & Pharr theory. 4. Conclusions In this study, influence of heat treatment on microstructure and mechanical properties of iron-based coatings is investigated. Mechanical properties of all coating were calculated by the method proposed by Giannakopoulos and Suresh. Relevant conclusions are shown as follows: For the as-cladding coatings, solid solution c-(Fe, Ni, Cr) and carbide reinforced phases Cr7C3 are the main phases. However, with the resolving of eutectic structure at annealing temperature,
Acknowledgements The paper was financially supported by Advanced Research Project (9140A270304090C8501), NSFC (50875053, 50735006) and Equipment Research Project. References [1] Hasan M, Stokes J, Looney L, Hashmi MSJ. Effect of spray parameters on residual stresses build-up of HVOF sprayed aluminium/tool-steel functionally graded coatings. Surf Coat Technol 2008;202(16):4006–10. [2] Binyou Fu, Dingyong He, Lidong Zhao. Effect of heat treatment on the microstructure and mechanical properties of Fe-based amorphous coatings. J Alloys Compd 2009;480(2):422–7. [3] Zhaoxia Ping, Yedong He, Changdong Gu, Tong-Yi Zhang. Mechanically assisted electroplating of Ni–P coatings on carbon steel. Surf Coat Technol 2008;202(24): 6023–8. [4] Aoki Hidemistsu, Hara Makoto, Masuzumi Takuro, Mazumder Motaharu K, Ooi Naoki, Watanabe Daisuke, et al. Influence of Cu electroplating solutions on boron carbon nitride (BCN) film. Appl Surf Sci 2007;255(6):3719–22. [5] Zhang Limin, Sun Dongbai, Yu Hongying, Li Huiqi. Characteristics of Fe-based alloy coating produced by plasma cladding process. Mater Sci Eng A 2007;457: 319–24. [6] Cheng JB, Xu BS, Liang XB, Wu YX. Microstructure and mechanical characteristics of iron-based coating prepared by plasma transferred arc cladding process. Mater Sci Eng, A 2008;492:407–12. [7] Tang Ying, Lin Yin, Zhao Naiqin, Chen Shanshan. Microstructure of high temperature treated iron alloy coating by plasma arc. Heat Treat Met 2009;33: 32–5. [8] Zhuqin Peng, Quanyi Shang, Zhaojun Zhang, Jinbin Lu. Microstructure and strengthening mechanism of Fe-based alloy powder cladding layer by plasma beam. J Zhongyuan Univ Technol 2007;18:6–9. [9] Xia Pengcheng, Yu Jingjiang, Sun Xiaofeng, Guan Hengrong, Hu Zhuangqi. Influence of heat treatment on the microstructure and mechanical properties of DZ951 alloy. Rare Met 2008;27(2):216–22. [10] Huiping Li, Guoqun zhao, Lianfang He. Finite element method based simulation of stress–strain field in the quenching process. Mater Sci Eng A 2008;478:276–90. [11] Satyajit Chatterjee SM, Shariff G, Padmanabham J, Dutta Majumdar A, Choudhury Roy. Study on the effect of laser post-treatment on the properties of nano structured Al2O3–TiB2–TiN based coatings developed by combined SHS and laser surface alloying. Surf Coat Technol 2010;205(1): 131–8. [12] Giannakopoulos AE, Suresh S. Determination of elastoplastic properties by instrumented sharp indentation. Scripta Mater 1999;40(10):1191–8. [13] Venkatesh TA, Van Vliet KJ, Giannakopoulos AE, Suresh S. Determination of elasto-plastic properties by instrumented sharp indentation: guidelines for property extraction. Scripta Mater 2000;42:833–9. [14] Pharr GM. Measurement of mechanical properties by ultra-low load indentation. Mater Sci Eng A 1998;253:151–9. [15] Oliver WC, Pharr GM. Measurement of hardness and elastic modulus by instrumented indentation: advances in understanding and refinements to methodology. J Mater Res 2004;19:3–20. [16] Huang YJ, Shen J, Sun Y, Sun JF. Indentation size effect of hardness of metallic glasses. Mater Des 2010;31:1563–6. [17] Gong JH, Wu JJ, Guan ZD. Analysis of the indentation size effect on the apparent hardness for ceramics. Mater Lett 1999;38:197–201. [18] Chicot D. Hardness length-scale factor to model nano- and micro-indentation size effects. Mater Sci Eng A 2009;499:454–61.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Influence of heat treatment on microstructure and mechanical properties of iron-based coating Li Hua-yi a,b, Li Guo-lu a, Wang Hai-dou b,⇑, Xu Bin-shi b a b
School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China National Key Lab for Remanufacturing, Academy of Armored Forces Engineering, Beijing 100072, China
a r t i c l e
i n f o
Article history: Received 15 October 2010 Accepted 23 December 2010 Available online 30 December 2010
a b s t r a c t Iron-based alloys were deposited on the low carbon steel by plasma cladding process. Furnace annealing was conducted at 600 °C for 40 min. Resulting microstructure and phases were observed and investigated by scanning electron microscopy (SEM), energy-dispersive spectrometer (EDS) and X-ray diffraction (XRD). Effect of post heat treatment on the mechanical properties of coatings was also studied by instrumented indentation technique. It was found that solid solution c-(Fe, Ni, Cr) and carbide reinforced phases Cr7C3 were the main phases of as-cladding coatings while iron carbide became the main carbide reinforced phase for annealed coatings. For all coatings, hardness and reduced elastic modulus showed obvious load dependence, namely decreased with the indentation load increasing. It was found that calculated values of annealed coatings were generally lower than those of as-cladding coatings as a result of the dissolution of the eutectic structure which decreased the effect of dispersion strengthening. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Surface wear and corrosion have become the main reasons for equipment failure. The need for improving the qualities of surface leads to the development of surface modification techniques such as spraying [1,2], electroplating [3,4] and laser or plasma cladding [5]. In recent years, plasma cladding technique becomes an active field due to its sufficient rapid cooling rate, good performance and low cost [6–8]. Post heat treatment is now widely used to further improve the required properties of bulk materials, such as furnace heat treatment [9], quenching [10] and laser remelting [11], of which furnace annealing is the most fundamental one. By adjusting the annealing temperature, it can reach the achievement of refining crystal, relieving residual stresses and even changing the microstructure and phases to enhance the mechanical properties [9], such as fracture toughness and wear resistance. However relative to bulk materials, little work have been done to analysis the effect of annealing on materials at small scale, such as films and coatings [7]. In this study, microscopy was used to investigate the effect of furnace annealing on phases and microstructure of iron-based alloy coating produced by plasma cladding process. At the same time, effect on mechanical properties was studied by an almost non-destructive method – instrument indentation test. To avoid ⇑ Corresponding author. Tel.: +86 10 66718541; fax: +86 10 66717144. E-mail address: [email protected] (W. Hai-dou). 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.12.055
the influences of pile-up on the calculating precision of properties, a new method based on the energy of indentation process proposed by Giannakopoulos and Suresh was applied [12,13] instead of the traditional Oliver and Pharr method [14,15]. 2. Experimental details 2.1. Material and preparation Commercial low carbon steel containing 0.1 wt.% carbon was taken as the substrate and degreased in acetone solution. No previous heat treatment was taken. Iron-based alloy powder with particles ranges from 150 to 180 lm was taken as the plasma cladding powder. The compositions of the powder are listed in Table 1. The experimental equipment adopted in the study was plasma cladding equipment produced by Academy of Armored Forces Engineering (China). Argon was used as the plasma gas. The parameters of plasma cladding process are shown in Table 2. After plasma cladding, the specimens were divided into two groups. One group remained the original status as cladding, others were annealed at 600 °C for 40 min. 2.2. Characterization Microstructures of the clad coatings were observed by scanning electron microscopy (SEM) (LEO1530VP) with energy dispersive spectroscopy (EDS) attachment, and phases were characterized by X-ray diffraction (XRD) (D8 ADVANCE, BRUKER/AKS, Germany).
3005
L. Hua-yi et al. / Materials and Design 32 (2011) 3004–3007 Table 1 Chemical composition of Fe-based alloy.
We ¼
Z
hmax
PðhÞdh ¼ hr
Element
C
Cr
Ni
B
Si
Fe
Mass fraction (%)
0.1
15
10
1
1
Bal.
A
120 (A) 22 (V) 1.8 (mm/s) Ar 2.2 (g/min) 5.0 (L/min)
2
hmax
The mechanical properties of coatings were investigated by nanoindentation equipment (Nano indenter XP) which can simultaneously record the applied indentation loads and penetrating depths during an indentation loading–unloading cycle. A Berkovich three-sided pyramidal indenter was used. Prior to indentation tests, surface of all specimens was grinded to a roughness of 0.09 lm. For both groups loads were applied by a linear ramp up to certain maximum load 20 mN, 50 mN, 100 mN, 150 mN and 200 mN respectively. The dwelling time at each maximum load was 10 s. For each load, at least five indents were made and then the average was used. The spacing between indents was at least 10 lm. All experiments were performed at room temperature. 2.3. Nano-indentation theory The calculation of mechanical properties in this study was based on the theory proposed by A. E. Giannakopoulos and S. Suresh [12,13]. The key feature of the method is that it takes into account the phenomenon of pile-up and sink-in and at the same time avoids applying contact depth to get the true contact area, which may be more applicable to materials that pile-up in comparison with the traditional O&P method [14,15]. Fig. 1 is the schematic of load-depth curve for a sharp indenter, where Pmax is the maximum indentation load which makes the maximum penetrating depth hmax, hr is the residual depth, namely the permanent depth of penetration after the indenter is fully unloaded. As shown in Fig. 1, the total work Wt can be decomposed into elastic and plastic parts, i.e. Wt = We + Wp. The hardness, H and the reduced elastic modulus, Er then can be calculated by the following equations [12], where m is the power law fitting constants.
Z 0
P max
PðhÞdh ¼
Z 0
P max
3
2
Ch dh ¼
aðh hr Þm dh ¼
hr
aðhmax hr Þmþ1 mþ1
Pmax ðhmax hr Þ; ¼ mþ1
H¼
Wt ¼
hmax
Wt We H ¼ 1 4:678 ; Er Wt
Table 2 Parameters of plasma cladding. Current Voltage Scanning velocity Shielding gas Feeding powder flow Plasma gas flow
Z
Chmax Pmax hmax ¼ ; 3 3
ð1Þ
2 H H þ 105:42 1 ¼ 9:96 12:64 1 Er Er 3 4 H H 229:57 1 þ 157:67 1 ; Er Er pmax : A
ð2Þ
ð3Þ
ð4Þ
ð5Þ
3. Results and discussion 3.1. Microstructure of the coating Microstructures of both the as-cladding coatings and the annealed coatings are shown in Fig. 1. Typical microstructure of the untreated original coating (as shown in Fig. 2a) consists of light phase and darker blocky phase which intercrosses each other and forms a kind of netlike structure and in agreement with the observation of Liming Zhang et al. [5] and Ying Tang et al. [7]. EDS analysis results, as shown in Table 3, reveal that the light phase is relatively rich in Fe, Cr and Ni while contains a few of carbon element. As can be seen from the micrograph of XRD analysis results (as shown in Fig. 3), the main phase of the light area is identified as c-(Fe, Ni, Cr) solid solution, while Cr dissolved in the solid solution and partly formed intermetallic compound particles such as (Cr, Fe)7C3 as the XRD results proved. In darker area dendritic c-(Fe, Ni) exists and dopes with an extended solid solution of Cr and a small amount of Si. According to the investigation of J.B. Cheng et al. [6], the eutectic of c-Fe/(Cr, Fe)7C3 formed when the temperature of molten pool falls at eutectic point as a result of the enriching of Fe in residual liquid. It is evident from Table 3 that compared to the light area; the darker area has obviously higher Cr content (21.08% Cr). Consequently the effect of solid solution strengthening is strengthened. Many tiny pits also found centralizing in the darker area which may due to the non-compacting of solid solution phases and the high velocity impact between the surface and arc plasma [8]. As can be seen in Fig. 2, both the groups have the darker islands that are embedded in the matrix. However, it can be seen that after annealed at 600 °C the distribution of the darker area is much more uniform than that of the as-cladding coating and the number of tiny pits obviously decreases with the distribution more uniform in the whole field of microscopy. The range in diffraction peak strength indicates the variation in qualities of corresponding phases as well. Compared to the annealed coatings; the untreated coatings has higher Cr7C3 which is the primary intermetallic compound. When treated at 600 °C, the energy of Cr ions was enhanced which resulted in the dissolution of Cr7C3 and the formation of more solid solutions.. As shown in Fig. 3, more carbon took part in the reaction with iron which promoted the formation of iron carbide at the same time. 3.2. Analysis of load-depth (p–h) curves
Fig. 1. Associated nomenclature in typical p–h curve.
Typical indentation load-depth curves as a function of indentation load are shown in Fig. 4. As shown in the figure, with the increasing indentation load, the depth for all coatings increases
3006
L. Hua-yi et al. / Materials and Design 32 (2011) 3004–3007
Fig. 3. XRD spectrum of the as-cladding and the annealed coatings.
Fig. 4. Load-depth curves of coatings by nanoindentation at different loads. Fig. 2. Microstructure of the coatings taken by SEM: (a) as cladding; (b) annealed at 600 °C.
3.3. Hardness Table 3 Results of the EDS test of different area in the coatings.
As-cladding coatings Annealed coatings
Light area (wt.%) Dark area (wt.%) Light area (wt.%) Dark area (wt.%)
Si
Cr
Fe
Ni
C
0.96
13.13
76.08
7.82
2.01
0.71 1.02
21.08 10.62
70.8 75.31
7.41 6.57
– 6.48
–
28.4
68.21
3.17
–
uniformly. For each group, the slope of loading curves at different loads is in substantial agreement. However, when choose the same indentation load, slope of loading curves for the as-cladding coatings is obviously greater than that of the annealed specimens which means to reach the same indentation depth, more load is required for the as-cladding coatings. This phenomenon proves that residual stress exists in the as-cladding coatings and the manipulation of annealing at 600 °C can achieve the effect of stress-relief. In addition, unloading curves for both groups at 50 mN, 100 mN, 150 mN show similar slope in appearance which indicates, according to the theory of Oliver & Pharr, the calculated elastic modulus based on the unloading curves may be range in a very small scope.
Variation in nanohardness of the coatings are given in Fig. 5 which calculated by the method that proposed by Giannakopoulos and Suresh. As shown in the figure, for both as-cladding coatings
Fig. 5. Nanohardness as a function of indentation load of Fe-based coatings by nanoindentation.
L. Hua-yi et al. / Materials and Design 32 (2011) 3004–3007
3007
iron carbide becomes the main carbide reinforced phase of the annealed coatings. For all coatings the hardness decreases with the indentation load increasing. Hardnesss of as-cladding coatings range from 5.5– 4.5 GPa, while the corresponding values decreased to 4.3–3.3 GPa for annealed coatings. This change may due to the dissolution of the eutectic structure which decreases the effect of dispersion strengthening. The distribution of reduced modulus for all coatings shows a similar trend as the hardness given. An obvious load dependence of reduced modulus can be found as well. The values of annealed coatings are a little lower (<30%) than that of as-cladding coatings, which attributed to the variation of the ration of We to Wt and more uniform distribution of austenite. However as what is shown, with the load increasing, a same value of reduced modulus will be achieved finally. Fig. 6. Reduced modulus as a function of indentation load of Fe-based coatings by nanoindentation.
and annealed coatings, with the indentation load increasing, the nanohardness decreases and for both curves the decrements are almost the same. This phenomenon is called as ‘‘indentation size effect (ISE)’’, namely the hardness measured usually increases with the decreasing depth of penetration. Much work has been done to investigate this phenomenon. Recent investigations proved that the change of pile-up ratio with the variation of penetration depth [16] and extra energy dissipation [17] may have significant effects on the ISE. Chicot [18] also proposed a hardness length-scale factor to model nano- and micro-indentation size effects which proportional to both the shear modulus and the Burgers vector depending on the dislocation spacing. Compared to the values of as-cladding coatings, low nanohardness values of the annealed coatings can be attributed to the dissolution of the eutectic structure which decreases the effect of dispersion strengthening. 3.4. Elastic modulus In Fig. 6, reduced elastic modulus of as-cladding coatings and annealed coatings are given, respectively. Similar to Fig. 5, distribution of reduced modulus shows obviously indentation load dependence. It can be seen reduced modulus of both groups decreases with the indentation load increasing just like the distributing regulation of nanohardness which can be attributed to the phenomenon of ISE as well [16–18]. However, with the indentation load increasing to 100 mN, values of reduced modulus for both groups trends to be stable. The average modulus values for annealed coatings are slightly lower than those measured for as-cladding coatings which can be attributed to the change of ratio of We to Wt and more uniform distribution of austenite. It can be inferred according to the tendency shown in Fig. 6, that modulus for both groups will achieve the same value when the load increases to a certain extent which coinciding with the results based on the Oliver & Pharr theory. 4. Conclusions In this study, influence of heat treatment on microstructure and mechanical properties of iron-based coatings is investigated. Mechanical properties of all coating were calculated by the method proposed by Giannakopoulos and Suresh. Relevant conclusions are shown as follows: For the as-cladding coatings, solid solution c-(Fe, Ni, Cr) and carbide reinforced phases Cr7C3 are the main phases. However, with the resolving of eutectic structure at annealing temperature,
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