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Mechanical properties and residual stresses changing on 00Cr12 alloy by nanoseconds laser shock processing at high temperatures
Materials Science and Engineering A 528 (2011) 1949–1953
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Mechanical properties and residual stresses changing on 00Cr12 alloy by nanoseconds laser shock processing at high temperatures X.D. Ren ∗ , T. Zhang, Y.K. Zhang, D.W. Jiang, H.F. Yongzhuo, H.B. Guan, X.M. Qian School of Mechanical Engineering, Jiangsu University, Xuefu Road 301, Jingkou District, Zhenjiang 212013, PR China
a r t i c l e
i n f o
Article history: Received 20 May 2010 Received in revised form 27 September 2010 Accepted 28 October 2010
Keywords: Laser shock processing 00Cr12 alloy High temperature relaxation Residual stress Tensile property
a b s t r a c t Effects of laser shock processing (LSP) on 00Cr12 alloy’s mechanical properties at high temperatures were analyzed by investigating the changing of the residual stress and tensile properties. The results showed that the compressive residual stress produced by LSP, which makes the 00Cr12 alloy’s yield strength and tensile strength increase, still played an important role in the high temperature tensile test. The tensile fracture showed that the break position and expand direction were changed because of the residual compressive stress existing, and the residual compressive stress was greatly released with the increasing temperature. The results demonstrated that the cycle, stress amplitude and temperature-dependent relaxation of compressive residual stresses were more pronounced than the decrease of near-surface work strengthening. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental procedures
00Cr12 alloy has a broaden application in automotive exhaust treatment device, boiler furnace and nozzle which work in the hightemperature environment, in which 00Cr12 alloy is usually broken by fatigue fracture. Laser shock processing (LSP) is a new surface modification technology which generates high strength impact wave and then introduces compressive residual stress of several hundreds MPa by exposing metallic samples to high power density (GW/cm2 ), short pulse (ns level) laser beam [1–6], as shown in Fig. 1. The increasing compressive residual stress value and depth would significantly improve material’s properties [7–9], which serve to enhance fatigue properties by improving the resistance against fatigue crack initiation and propagation [10–14]. These beneficial effects prevail only when the near-surface compressive residual stresses or work hardenings remain stable during mechanical loading or during exposure to elevated temperature. Several investigations [5,6] have suggested that LSP induced near-surface microstructures, and residual stress states would exhibit higher thermal stability. We therefore explore the residual stress stability and tensile properties of 00Cr12 alloy by LSP in this study, and tensile testes were carried out at different temperatures after LSP. As a consequence, we aim to obtain whether LSP is still suitable at high temperature-fatigue conditions.
The compositions of 00Cr12 alloy are shown in Table 1, and its standard stretches fatigue specimen is shown in Fig. 2. 00Cr12 specimens were performed by Q-switched high power neodymium-doped glass laser with a wavelength of 1.054 m, pulse duration of 20 ns, and power density of 1.54 GW/cm2 in the strong laser laboratory of Jiangsu University. Large-diameter quarter-wave plate and high-strength large-diameter film polarizer were adopted. The laser beam spot size was maintained at a diameter of 6 mm, and the overlapping rate of the laser spot was 50%. Water was used as the transparent overlay, and aluminum foil with 0.1 mm thickness was used as the absorbent coating. After LSP, pulling test was performed on specimens by MTS809 Axial/Torsional Test System at different temperatures, and a threezone resistance type furnace, which could control temperature within a variation of ±1 ◦ C (±3 ◦ C for room temperature) at steadystate, was used for temperature control. A high temperature extensometer (MTS model No. 632-13F-20, gauge length 25 mm) was used to measure the strain and control the strain signal. Each specimen was heated at the constant temperature for 5 min and then pulled until broken with the speed of 3 mm/min, and the value of yield strength by 0.2% strains was recorded. The residual stresses in the laser impact zone of the specimen were determined by using X-350A X-ray stress tester with sin 2 method, and the tensile fracture was observed by a JEOL-6700 scanning electron microscope (SEM).
∗ Corresponding author. Tel.: +86 511 88797898; fax: +86 511 88780241. E-mail address: [email protected] (X.D. Ren). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.10.098
1950
X.D. Ren et al. / Materials Science and Engineering A 528 (2011) 1949–1953
Fig. 1. Schematic of principle of the laser shock processing.
Table 1 Chemical compositions of 00Cr12 alloy. Composition
Percent (wt.%)
C
Si
Mn
S
P
Cr
Ni
≤0.030
≤1.00
≤1.00
≤0.030
≤0.035
11.00–13.00
≤0.60
Fig. 2. Dimensions and schematic diagrams of the standard tensile fatigue specimen of 00Cr12 alloy after LSP.
3. Analysis and discussion 3.1. Residual stress Generally, the components should avoid high temperature that would release the residual compressive stress after LSP, which would weaken the effect of LSP. According to AMS 2546 “Laser shock processing standard (2004.8)”, the heating temperatures of stainless steel should not exceed 750 ◦ F (399 ◦ C) [15], but there is no clear specification to the heat-resistant steel. Fig. 3 shows the residual stress relaxation of 00Cr12 alloy at different temperatures. Surface average compressive residual stress of 00Cr12 alloy after two LSP impacts is about −382 MPa. It could be found that the increase in amplitude of stress relaxation was large when the temperature was lower than 200 ◦ C, and the increase in amplitude tended to slow down between 200 ◦ C and 500 ◦ C, while increased after 500 ◦ C. When temperature surpassed to 600 ◦ C, the compressive residual stress reduced in a large scale. The average residual compressive stress dropped to −212.5 MPa, decreasing about 64% at the temperature of 600 ◦ C. It could be concluded that the working temperature of 00Cr12 alloy should not exceed 600 ◦ C after LSP. And the low-temperature tempering treatment could be carried out before the tensile test in order to avoid the residual compressive stress significant relaxation. Residual compressive stress plays a dominant role in enhancing the anti-fatigue properties of the specimen, and the anti-fatigue properties would be significantly
reduced in high temperature conditions after LSP. Monotonic and cyclic relaxations are the reasons that residual stresses are not stable at high temperature. In addition, compressive stresses are not stable in the elevated temperatures and plastic deformations of service conditions. However, the residual compressive stress does not have enough time to release during the high-temperature tensile
Fig. 3. Redistribution diagram of the residual stress relaxation of 00Cr12 alloy at different temperatures.
X.D. Ren et al. / Materials Science and Engineering A 528 (2011) 1949–1953
1951
Fig. 4. Tensile stress–strain curves of 00Cr12 specimens at different temperatures. (a) 400 ◦ C; (b) 600 ◦ C.
Fig. 5. Schematic of mechanical properties changing at different temperatures. Relationship of the (a) elastic modulus and the temperatures, and (b) stresses and the temperatures.
process, so it will balance the tension in the process of tensile stress [1,2]. Therefore, LSP will be beneficial to enhance the yield strength and the tensile strength of 00Cr12 alloy. 3.2. Stress and strain The tensile stress–strain curves of 00Cr12 alloy at 400 ◦ C and 600 ◦ C are shown in Fig. 4(a) and (b), which indicated that the testing temperature had no significant effect on the elongation of the 00Cr12 alloy. Even at the testing temperature of 600 ◦ C (Fig. 4(b)), the fracture elongation was a little reduced for the 00Cr12 alloy. It was found that the shape of the stress–strain curves changes with increasing temperature. Fig. 5(a) shows that the elastic modulus and the cross-section shrinkage rate increase obviously, while the yield strength and tensile strength increase slightly with the increasing temperature after LSP. This could be ascribed to the surface micro-hardness increasing after LSP [16,17]. The plasticity of 00Cr12 alloy is enhanced at high temperatures, which could be found in Fig. 5(b). The thermal activation increases with the temperature elevating, and the dislocation
Fig. 6. Fracture morphology of specimens at the temperatures of 25 ◦ C, 400 ◦ C, 500 ◦ C and 600 ◦ C, respectively.
1952
X.D. Ren et al. / Materials Science and Engineering A 528 (2011) 1949–1953
Fig. 7. SEM of 00Cr12 alloy under different temperatures. (a) Fracture cracks originate at the center region at the room temperature. (b) SEM of the middle region at 25 ◦ C which has formed typical dimples. (c) The crack initiation at the 600 ◦ C. (d) SEM of the middle region at 600 ◦ C.
slips become easier. But the tensile process is so short (within the ns progress) that the residual compressive stresses have little time to release. The residual compressive stress would counteract the tensile stress partially in the stretch process which is equivalent to reduce the load of the material, so the tensile strength is improved. 3.3. Fracture morphology Fig. 6 displays the fracture morphology of specimens at different temperatures. It could be clearly found that the fracture morphology of specimens is greatly different from each other at the temperatures of 25 ◦ C, 400 ◦ C, 500 ◦ C and 600 ◦ C, respectively. Compared to room temperature, the sample surface color is slightly deeper than that at the temperature of 400 ◦ C, and the fracture face exhibits gray color and slightly metallic luster. While at the temperatures of 500 ◦ C and 600 ◦ C, the specimen surface color is obviously deepened and presents grayish-black, and the fracture face color is blue black without metallic luster. The “neck shrinks” phenomenon is not very obvious when the sample approaches to be broken, and it could be obtained that 00Cr12 is one kind of ferritic heat-resistant steel which has high structure stability and high temperature oxidation resistance working long-term at high temperatures.
intense planar sliding. Fig. 7(b) shows the SEM of the middle region at 25 ◦ C which has formed typical dimples with a few inclusions, and composition analysis indicates that the inclusions are manganese sulfide and some carbon. Fig. 7(c) shows that the sulfide inclusion which is bigger in size and distributed in clusters is crack initiation. Cracks occur easily in those defects, and expand along its cleavage plane and produce axial cracks, which indicate that residual compressive stresses do not release completely during the high temperature tensile test, indicating that residual compressive stresses still influence the expansion path of cracks in early stage [18–20]. It could also be found that the dimple is more homogeneous at 600 ◦ C than at 25 ◦ C, as shown in Fig. 7(d). At the room temperature, the cleavage plane and tearing ridge, which were usually taken as typical brittle fracture features, could be seen on the tensile fracture surfaces of the 00Cr12 alloy. At the elevated temperatures, however, the tensile fracture surface of the 00Cr12 alloy was composed of the cleavage plane, tearing ridge and dimple with small size. It implied that the 00Cr12 alloy showed a mixed ductile and brittle fracture feature. It is important to note that this beneficial effect of the mechanical surface treatment is not restricted solely to ambient temperatures, but it is also clearly in evidence at 600 ◦ C. This reveals that precipitates are more likely to nucleate around the dislocations during dynamic aging in LSP.
3.4. Microstructure morphology 4. Conclusion LSP also results in fatigue properties changing at high temperatures. It could be found that the precipitates and dislocation are entangled after LSP even at high temperature. The morphologies of 00Cr12 alloy surface with different temperatures are shown in Fig. 7. Fig. 7(a) shows that the fracture cracks originate at the center region at the room temperature, and then extend radiated to the edge in the form of radiation with a few pores, showing the
LSP is considered to be one of the most promising techniques in terms of its ability to induce compressive residual stresses, which improves the mechanical performance of the materials. The effect of LSP on the compressive residual stress and microstructure morphology of the 00Cr12 alloy under high temperature is experimentally investigated. The obtained results are as follows:
X.D. Ren et al. / Materials Science and Engineering A 528 (2011) 1949–1953
(1) LSP introduces a compressive residual stress filed at the surface of 00Cr12 alloy (up to −382 MPa). The residual compressive stress will be relaxed under high temperature. Monotonic and cyclic relaxations are the reasons that residual stresses are not stable at high temperature. The average residual compressive stress dropped to −212.5 MPa, decreasing about 64% at the temperature of 600 ◦ C. (2) LSP successfully enhances the yield strength and tensile strength of the 00Cr12 alloy in this study. The residual compressive stress would counter-balance the tensile stress partially in the stretch process which is equivalent to reduce the load of the material, so the tensile strength is improved. (3) Residual compressive stresses induced by LSP still influence the expansion path of cracks in early stage at the high temperature tensile test, and the results imply that LSP has the greatest effect on the 00Cr12 at less than 600 ◦ C.
Acknowledgements The authors are grateful to the Project supported by the National Natural Science Foundation of China (Grant Nos. 50905080 and 50735001), China Postdoctoral Science Foundation funded project (Grant No. 20100471385) and the Advanced Talent Foundation of Jiangsu University of China (Grant No. 10JDG066).
1953
References [1] C.S. Montross, T. Wei, L. Ye, G. Clark, Y.W. Mai, Int. J. Fatigue 24 (2002) 1021–1036. [2] Y.K. Zhang, C.L. Hu, L. Cai, J.C. Yang, X.R. Zhang, Appl. Phys. A 72 (2001) 113–116. [3] H. Zhang, C.Y. Yu, Mater. Sci. Eng. A 257 (1998) 322–327. [4] X.D. Ren, Y.K. Zhang, J.Z. Zhou, J.Z. Lu, L.C. Zhou, Mater. Des. 30 (2009) 3512–3517. [5] K. Ding, L. Ye, J. Mater. Process. Technol. 178 (2006) 162–169. ˜ J. Porro, M. [6] U. Sánchez-Santana, C. Rubio-González, G. Gomez-Rosas, J.L. Ocana, Morales, Wear 260 (2006) 847–854. [7] C.H. Yang, P.D. Hodgson, Q. Liu, L. Ye, J. Mater. Process. Technol. 201 (2008) 303–309. [8] X.C. Zhang, B.S. Xu, H.D. Wang, Y.X. Wu, Y. Jiang, Surf. Coat. Technol. 201 (2007) 6660–6662. [9] X.C. Zhang, B.S. Xu, H.D. Wang, Y.X. Wu, Appl. Surf. Sci. 254 (2008) 1881–1889. [10] C. Rubio-Gonzalez, J.L. Ocana, G. Gomez-Rosas, C. Molpeceres, M. Paredes, A. Banderas, J. Porro, M. Morales, Mater. Sci. Eng. A 386 (2004) 291–295. [11] P. Peyre, R. Fabbro, P. Merrien, H.P. Lieurade, Mater. Sci. Eng. A 210 (1996) 102–113. [12] B. Alfredsson, M. Olsson, Fatigue Fract. Eng. Mater. Struct. 22 (1999) 225–237. [13] Y.K. Zhang, X.D. Ren, J.Z. Zhou, J.Z. Lu, L.C. Zhou, Mater. Des. 30 (2009) 2769–2773. [14] H. Tada, P.C. Paris, The Stress Analysis of Cracks Handbook, ASME Press, New York, 2001, pp. 54–59. [15] SAE AMS 2546-2004 www.sae.org. [16] Y.K. Zhang, J.Z. Lu, X.D. Ren, H.B. Yao, H.X. Yao, Mater. Des. 30 (2009) 1697–1703. [17] F. Wang, Z.Q. Yao, Q.L. Deng, Materials 14 (2007) 529–532. [18] S.P. Wang, Y.J. Li, M. Yao, R.Z. Wang, J. Mater. Process. Technol. 73 (1998) 64–73. [19] G.H. Farrahi, J.L. Lebrun, D. Couratin, Fatigue Fract. Eng. Mater. Struct. 18 (1995) 211–220. [20] M. Guagliano, L. Vergani, Eng. Fract. Mech. 71 (2004) 501–512.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Mechanical properties and residual stresses changing on 00Cr12 alloy by nanoseconds laser shock processing at high temperatures X.D. Ren ∗ , T. Zhang, Y.K. Zhang, D.W. Jiang, H.F. Yongzhuo, H.B. Guan, X.M. Qian School of Mechanical Engineering, Jiangsu University, Xuefu Road 301, Jingkou District, Zhenjiang 212013, PR China
a r t i c l e
i n f o
Article history: Received 20 May 2010 Received in revised form 27 September 2010 Accepted 28 October 2010
Keywords: Laser shock processing 00Cr12 alloy High temperature relaxation Residual stress Tensile property
a b s t r a c t Effects of laser shock processing (LSP) on 00Cr12 alloy’s mechanical properties at high temperatures were analyzed by investigating the changing of the residual stress and tensile properties. The results showed that the compressive residual stress produced by LSP, which makes the 00Cr12 alloy’s yield strength and tensile strength increase, still played an important role in the high temperature tensile test. The tensile fracture showed that the break position and expand direction were changed because of the residual compressive stress existing, and the residual compressive stress was greatly released with the increasing temperature. The results demonstrated that the cycle, stress amplitude and temperature-dependent relaxation of compressive residual stresses were more pronounced than the decrease of near-surface work strengthening. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental procedures
00Cr12 alloy has a broaden application in automotive exhaust treatment device, boiler furnace and nozzle which work in the hightemperature environment, in which 00Cr12 alloy is usually broken by fatigue fracture. Laser shock processing (LSP) is a new surface modification technology which generates high strength impact wave and then introduces compressive residual stress of several hundreds MPa by exposing metallic samples to high power density (GW/cm2 ), short pulse (ns level) laser beam [1–6], as shown in Fig. 1. The increasing compressive residual stress value and depth would significantly improve material’s properties [7–9], which serve to enhance fatigue properties by improving the resistance against fatigue crack initiation and propagation [10–14]. These beneficial effects prevail only when the near-surface compressive residual stresses or work hardenings remain stable during mechanical loading or during exposure to elevated temperature. Several investigations [5,6] have suggested that LSP induced near-surface microstructures, and residual stress states would exhibit higher thermal stability. We therefore explore the residual stress stability and tensile properties of 00Cr12 alloy by LSP in this study, and tensile testes were carried out at different temperatures after LSP. As a consequence, we aim to obtain whether LSP is still suitable at high temperature-fatigue conditions.
The compositions of 00Cr12 alloy are shown in Table 1, and its standard stretches fatigue specimen is shown in Fig. 2. 00Cr12 specimens were performed by Q-switched high power neodymium-doped glass laser with a wavelength of 1.054 m, pulse duration of 20 ns, and power density of 1.54 GW/cm2 in the strong laser laboratory of Jiangsu University. Large-diameter quarter-wave plate and high-strength large-diameter film polarizer were adopted. The laser beam spot size was maintained at a diameter of 6 mm, and the overlapping rate of the laser spot was 50%. Water was used as the transparent overlay, and aluminum foil with 0.1 mm thickness was used as the absorbent coating. After LSP, pulling test was performed on specimens by MTS809 Axial/Torsional Test System at different temperatures, and a threezone resistance type furnace, which could control temperature within a variation of ±1 ◦ C (±3 ◦ C for room temperature) at steadystate, was used for temperature control. A high temperature extensometer (MTS model No. 632-13F-20, gauge length 25 mm) was used to measure the strain and control the strain signal. Each specimen was heated at the constant temperature for 5 min and then pulled until broken with the speed of 3 mm/min, and the value of yield strength by 0.2% strains was recorded. The residual stresses in the laser impact zone of the specimen were determined by using X-350A X-ray stress tester with sin 2 method, and the tensile fracture was observed by a JEOL-6700 scanning electron microscope (SEM).
∗ Corresponding author. Tel.: +86 511 88797898; fax: +86 511 88780241. E-mail address: [email protected] (X.D. Ren). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.10.098
1950
X.D. Ren et al. / Materials Science and Engineering A 528 (2011) 1949–1953
Fig. 1. Schematic of principle of the laser shock processing.
Table 1 Chemical compositions of 00Cr12 alloy. Composition
Percent (wt.%)
C
Si
Mn
S
P
Cr
Ni
≤0.030
≤1.00
≤1.00
≤0.030
≤0.035
11.00–13.00
≤0.60
Fig. 2. Dimensions and schematic diagrams of the standard tensile fatigue specimen of 00Cr12 alloy after LSP.
3. Analysis and discussion 3.1. Residual stress Generally, the components should avoid high temperature that would release the residual compressive stress after LSP, which would weaken the effect of LSP. According to AMS 2546 “Laser shock processing standard (2004.8)”, the heating temperatures of stainless steel should not exceed 750 ◦ F (399 ◦ C) [15], but there is no clear specification to the heat-resistant steel. Fig. 3 shows the residual stress relaxation of 00Cr12 alloy at different temperatures. Surface average compressive residual stress of 00Cr12 alloy after two LSP impacts is about −382 MPa. It could be found that the increase in amplitude of stress relaxation was large when the temperature was lower than 200 ◦ C, and the increase in amplitude tended to slow down between 200 ◦ C and 500 ◦ C, while increased after 500 ◦ C. When temperature surpassed to 600 ◦ C, the compressive residual stress reduced in a large scale. The average residual compressive stress dropped to −212.5 MPa, decreasing about 64% at the temperature of 600 ◦ C. It could be concluded that the working temperature of 00Cr12 alloy should not exceed 600 ◦ C after LSP. And the low-temperature tempering treatment could be carried out before the tensile test in order to avoid the residual compressive stress significant relaxation. Residual compressive stress plays a dominant role in enhancing the anti-fatigue properties of the specimen, and the anti-fatigue properties would be significantly
reduced in high temperature conditions after LSP. Monotonic and cyclic relaxations are the reasons that residual stresses are not stable at high temperature. In addition, compressive stresses are not stable in the elevated temperatures and plastic deformations of service conditions. However, the residual compressive stress does not have enough time to release during the high-temperature tensile
Fig. 3. Redistribution diagram of the residual stress relaxation of 00Cr12 alloy at different temperatures.
X.D. Ren et al. / Materials Science and Engineering A 528 (2011) 1949–1953
1951
Fig. 4. Tensile stress–strain curves of 00Cr12 specimens at different temperatures. (a) 400 ◦ C; (b) 600 ◦ C.
Fig. 5. Schematic of mechanical properties changing at different temperatures. Relationship of the (a) elastic modulus and the temperatures, and (b) stresses and the temperatures.
process, so it will balance the tension in the process of tensile stress [1,2]. Therefore, LSP will be beneficial to enhance the yield strength and the tensile strength of 00Cr12 alloy. 3.2. Stress and strain The tensile stress–strain curves of 00Cr12 alloy at 400 ◦ C and 600 ◦ C are shown in Fig. 4(a) and (b), which indicated that the testing temperature had no significant effect on the elongation of the 00Cr12 alloy. Even at the testing temperature of 600 ◦ C (Fig. 4(b)), the fracture elongation was a little reduced for the 00Cr12 alloy. It was found that the shape of the stress–strain curves changes with increasing temperature. Fig. 5(a) shows that the elastic modulus and the cross-section shrinkage rate increase obviously, while the yield strength and tensile strength increase slightly with the increasing temperature after LSP. This could be ascribed to the surface micro-hardness increasing after LSP [16,17]. The plasticity of 00Cr12 alloy is enhanced at high temperatures, which could be found in Fig. 5(b). The thermal activation increases with the temperature elevating, and the dislocation
Fig. 6. Fracture morphology of specimens at the temperatures of 25 ◦ C, 400 ◦ C, 500 ◦ C and 600 ◦ C, respectively.
1952
X.D. Ren et al. / Materials Science and Engineering A 528 (2011) 1949–1953
Fig. 7. SEM of 00Cr12 alloy under different temperatures. (a) Fracture cracks originate at the center region at the room temperature. (b) SEM of the middle region at 25 ◦ C which has formed typical dimples. (c) The crack initiation at the 600 ◦ C. (d) SEM of the middle region at 600 ◦ C.
slips become easier. But the tensile process is so short (within the ns progress) that the residual compressive stresses have little time to release. The residual compressive stress would counteract the tensile stress partially in the stretch process which is equivalent to reduce the load of the material, so the tensile strength is improved. 3.3. Fracture morphology Fig. 6 displays the fracture morphology of specimens at different temperatures. It could be clearly found that the fracture morphology of specimens is greatly different from each other at the temperatures of 25 ◦ C, 400 ◦ C, 500 ◦ C and 600 ◦ C, respectively. Compared to room temperature, the sample surface color is slightly deeper than that at the temperature of 400 ◦ C, and the fracture face exhibits gray color and slightly metallic luster. While at the temperatures of 500 ◦ C and 600 ◦ C, the specimen surface color is obviously deepened and presents grayish-black, and the fracture face color is blue black without metallic luster. The “neck shrinks” phenomenon is not very obvious when the sample approaches to be broken, and it could be obtained that 00Cr12 is one kind of ferritic heat-resistant steel which has high structure stability and high temperature oxidation resistance working long-term at high temperatures.
intense planar sliding. Fig. 7(b) shows the SEM of the middle region at 25 ◦ C which has formed typical dimples with a few inclusions, and composition analysis indicates that the inclusions are manganese sulfide and some carbon. Fig. 7(c) shows that the sulfide inclusion which is bigger in size and distributed in clusters is crack initiation. Cracks occur easily in those defects, and expand along its cleavage plane and produce axial cracks, which indicate that residual compressive stresses do not release completely during the high temperature tensile test, indicating that residual compressive stresses still influence the expansion path of cracks in early stage [18–20]. It could also be found that the dimple is more homogeneous at 600 ◦ C than at 25 ◦ C, as shown in Fig. 7(d). At the room temperature, the cleavage plane and tearing ridge, which were usually taken as typical brittle fracture features, could be seen on the tensile fracture surfaces of the 00Cr12 alloy. At the elevated temperatures, however, the tensile fracture surface of the 00Cr12 alloy was composed of the cleavage plane, tearing ridge and dimple with small size. It implied that the 00Cr12 alloy showed a mixed ductile and brittle fracture feature. It is important to note that this beneficial effect of the mechanical surface treatment is not restricted solely to ambient temperatures, but it is also clearly in evidence at 600 ◦ C. This reveals that precipitates are more likely to nucleate around the dislocations during dynamic aging in LSP.
3.4. Microstructure morphology 4. Conclusion LSP also results in fatigue properties changing at high temperatures. It could be found that the precipitates and dislocation are entangled after LSP even at high temperature. The morphologies of 00Cr12 alloy surface with different temperatures are shown in Fig. 7. Fig. 7(a) shows that the fracture cracks originate at the center region at the room temperature, and then extend radiated to the edge in the form of radiation with a few pores, showing the
LSP is considered to be one of the most promising techniques in terms of its ability to induce compressive residual stresses, which improves the mechanical performance of the materials. The effect of LSP on the compressive residual stress and microstructure morphology of the 00Cr12 alloy under high temperature is experimentally investigated. The obtained results are as follows:
X.D. Ren et al. / Materials Science and Engineering A 528 (2011) 1949–1953
(1) LSP introduces a compressive residual stress filed at the surface of 00Cr12 alloy (up to −382 MPa). The residual compressive stress will be relaxed under high temperature. Monotonic and cyclic relaxations are the reasons that residual stresses are not stable at high temperature. The average residual compressive stress dropped to −212.5 MPa, decreasing about 64% at the temperature of 600 ◦ C. (2) LSP successfully enhances the yield strength and tensile strength of the 00Cr12 alloy in this study. The residual compressive stress would counter-balance the tensile stress partially in the stretch process which is equivalent to reduce the load of the material, so the tensile strength is improved. (3) Residual compressive stresses induced by LSP still influence the expansion path of cracks in early stage at the high temperature tensile test, and the results imply that LSP has the greatest effect on the 00Cr12 at less than 600 ◦ C.
Acknowledgements The authors are grateful to the Project supported by the National Natural Science Foundation of China (Grant Nos. 50905080 and 50735001), China Postdoctoral Science Foundation funded project (Grant No. 20100471385) and the Advanced Talent Foundation of Jiangsu University of China (Grant No. 10JDG066).
1953
References [1] C.S. Montross, T. Wei, L. Ye, G. Clark, Y.W. Mai, Int. J. Fatigue 24 (2002) 1021–1036. [2] Y.K. Zhang, C.L. Hu, L. Cai, J.C. Yang, X.R. Zhang, Appl. Phys. A 72 (2001) 113–116. [3] H. Zhang, C.Y. Yu, Mater. Sci. Eng. A 257 (1998) 322–327. [4] X.D. Ren, Y.K. Zhang, J.Z. Zhou, J.Z. Lu, L.C. Zhou, Mater. Des. 30 (2009) 3512–3517. [5] K. Ding, L. Ye, J. Mater. Process. Technol. 178 (2006) 162–169. ˜ J. Porro, M. [6] U. Sánchez-Santana, C. Rubio-González, G. Gomez-Rosas, J.L. Ocana, Morales, Wear 260 (2006) 847–854. [7] C.H. Yang, P.D. Hodgson, Q. Liu, L. Ye, J. Mater. Process. Technol. 201 (2008) 303–309. [8] X.C. Zhang, B.S. Xu, H.D. Wang, Y.X. Wu, Y. Jiang, Surf. Coat. Technol. 201 (2007) 6660–6662. [9] X.C. Zhang, B.S. Xu, H.D. Wang, Y.X. Wu, Appl. Surf. Sci. 254 (2008) 1881–1889. [10] C. Rubio-Gonzalez, J.L. Ocana, G. Gomez-Rosas, C. Molpeceres, M. Paredes, A. Banderas, J. Porro, M. Morales, Mater. Sci. Eng. A 386 (2004) 291–295. [11] P. Peyre, R. Fabbro, P. Merrien, H.P. Lieurade, Mater. Sci. Eng. A 210 (1996) 102–113. [12] B. Alfredsson, M. Olsson, Fatigue Fract. Eng. Mater. Struct. 22 (1999) 225–237. [13] Y.K. Zhang, X.D. Ren, J.Z. Zhou, J.Z. Lu, L.C. Zhou, Mater. Des. 30 (2009) 2769–2773. [14] H. Tada, P.C. Paris, The Stress Analysis of Cracks Handbook, ASME Press, New York, 2001, pp. 54–59. [15] SAE AMS 2546-2004 www.sae.org. [16] Y.K. Zhang, J.Z. Lu, X.D. Ren, H.B. Yao, H.X. Yao, Mater. Des. 30 (2009) 1697–1703. [17] F. Wang, Z.Q. Yao, Q.L. Deng, Materials 14 (2007) 529–532. [18] S.P. Wang, Y.J. Li, M. Yao, R.Z. Wang, J. Mater. Process. Technol. 73 (1998) 64–73. [19] G.H. Farrahi, J.L. Lebrun, D. Couratin, Fatigue Fract. Eng. Mater. Struct. 18 (1995) 211–220. [20] M. Guagliano, L. Vergani, Eng. Fract. Mech. 71 (2004) 501–512.