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On the fatigue crack growth behaviour of two ferrite–pearlite microalloyed steels
November 2000
Materials Letters 46 Ž2000. 185–188 www.elsevier.comrlocatermatlet
On the fatigue crack growth behaviour of two ferrite–pearlite microalloyed steels V. Subramanya Sarma a,) , K.A. Padmanabhan a , G. Jaeger b, A. Koethe b, M. Schaper c a
b
Indian Institute of Technology, Kanpur 208 016 India Institut f ur und Werkstofforschung (IFW), Dresden, Germany ¨ Festkorper¨ c Technische UniÕersitat, ¨ Dresden, Germany Received 24 March 2000; accepted 19 May 2000
Abstract Fatigue crack growth rates ŽFCGR. in the Paris regime of two ferrite–pearlite microalloyed ŽMA. steels are shown to be in good agreement with those predicted using a model of crack tip element failure by low cycle fatigue ŽLCF.. Fatigue striations of correct spacing Ž0.1–0.2 mm. dimensions were seen in the fractured specimens of both the steels. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Microalloyed steels; Fatigue crack growth; Paris region; Low cycle fatigue
Fatigue crack growth ŽFCG. in metallic materials has been studied extensively, for a review, see Ref. w1x. The fatigue crack growth rate ŽFCGR., Žd ard N ., is usually plotted against the stress intensity range, Ž D K ., Ža log–log plot.. Of the three regions, regions I Žnear threshold. and II Žsteady state ŽParis. regime. are of special interest. Control processed, ferrite–pearlite microalloyed ŽMA. steels are replacing quenched and tempered ŽQ and T. steels in many applications w2x. The present study is part of a detailed investigation on two such MA steels w3–6x, a pearlite-dominant medium carbon Ž49MnVS3. and another ferrite-dominant low carbon ŽE-38. grade. 49MnVS3 was received as 17-mm thick slabs and E-38 in the form of 7-mm thick plates. The chemical compositions are given in Table
1. Cyclic stress-strain ŽCSS. and fatigue properties, as well as the microstructural parameters, are presented in Table 2. The FCG tests were carried out with a computer-controlled dynamic compliance ŽDYNACOMP. equipment w7,8x, in which the specimen represents the decisive compliance of a mechanical resonator ŽFig. 1.. Exciting the resonator at its resonance frequency enables the determination of the effective crack length by high-resolution period mea-
Table 1 Chemical compositions Žwt.%. of the MA steels Material C 49Mn VS3 E-38
)
Si
Mn P
S
V
0.49 0.27 0.98 0.02 0.05 0.09
Nb
N
Fe
–
0.016 Balance
0.09 0.03 0.81 0.02 0.03 0.007 0.015 0.007 Balance
Corresponding author.
00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 1 6 5 - 8
V. Subramanya Sarma et al.r Materials Letters 46 (2000) 185–188
186
Table 2 Mechanical properties and microstructural details Material
sys ŽMPa.
scysŽMPa.
d Žmm.
l Žmm.
n
X
c
´ fX
49MnVS3
530
510
16 " 2.2
0.25 " 0.03
E-38
371
318
9"1
ND
0.14 0.078 0.23
y0.64 y0.82 y0.6
0.6 3.35 0.3
ND — Not determined. sys , 0.2% monotonic yield strength; scys , 0.2% cyclic yield strength; d, pearlite colony sizergrain size; l, interlamellar spacing; nX , cyclic X strain hardening exponent; c, Coffin–Manson exponent; ´ f Coffin–Manson coefficient.
surements. Resonance vibrations at high amplitudes serve to load the specimen and monitor the crack length. All measurements were carried out on single edge notched ŽSEN. specimens of cross-section 6.5 = 20 mm2 , which were notched for 4 mm by spark
erosion Žnotch root radius 0.12 mm.. Crack propagation was perpendicular to the slab forging direction in 49MnVS3 but was along the plate rolling direction in E-38. The FCG tests were carried out using the D K, increasing the Žconstant load. procedure at a load ratio Žminimum loadrmaximum load., R, of 0.25.
Fig. 1. A schematic of the DYNACOMP machine.
Fig. 2. d ard N vs. D K plots for MA steels 49MnVS3 and E-38.
Fig. 3. Computed and experimental FCGR plots for Ža. 49MnVS3, and Žb. E-38.
V. Subramanya Sarma et al.r Materials Letters 46 (2000) 185–188
187
Fig. 4. Striations in Ža. 49MnVS3, and Žb. E-38.
Despite the vastly different microstructures, the two steels exhibited a similar FCG response ŽFig. 2.. It is well known that FCG in the Paris region is not influenced by the microstructure. The lack of influence is generally attributed to the cyclic plastic zone size being larger than the characteristic microstructural unit size Žcontinuum behaviour.. Using the approach of crack tip element failure by LCF, it has been shown that w9x da
1r c
Ž1y1rc .
DK
2rc
sj Ž2 s. Ž 1. dN E where j Žs Ž Esr4scys ´ fX d .. is a dimensionless parameter, E is the Young’s modulus, scys is the cyclic yield stress, ´ fX is the cyclic ductility coefficient, d is the dislocation barrier spacing Žgrain size or interlamellar spacing., c is the cyclic ductility exponent and s is the striation spacing. It is reasoned that the lack of influence of microstructure is due to the increase in the cyclic yield strength ‘scys ’ and the fatigue ductility exponent ‘c’ being compensated for by a corresponding decrease in the dislocation barrier spacing ‘d’, as this leads to a very narrow range for the ‘j ’ values and the crack growth rates w9x. From Eq. Ž1. and the Paris relation Žd ard N s CD K m ., ‘m’s 2rc and the Paris coefficient, C, is a function of j . Tomkins w10x has shown that at low stresses, c s 1rŽ2 nX q 1.. Therefore, m s 2 Ž 2 nX q 1 . Ž 2. The calculated and the experimental crack growth plots are shown in Fig. 3a and b, respectively. For 49MnVS3, the dislocation barrier spacing, d, was
taken as 2 1r2 Ž t q l., where t is the thickness of the cementite plate and l, the interlamellar spacing, was taken as 0.5 mm Žthe factor 2 1r2 is due to cementite deforming at approximately 458 to the loading axis w4,11x.. For E-38, the dislocation barrier spacing was taken to be equal to the average grain size Ž9 mm.. The striation spacing ‘s’ was taken as 0.1 mm for 49MnVS3 and 0.2 mm for E-38. For 49MnVS3, c at low strain amplitude was obtained experimentally ŽTable 2.. Due to experimental limitations, LCF data for E-38 could not be generated at low strain amplitudes. And so, Eq. Ž2. was used for calculating m. In both the steels, a very good correlation between the calculated and the experimental growth rates ŽFig. 3. was obtained. Fractography revealed evidence for striated crack growth in both the steels ŽFig. 4.. The striation spacings were indeed of the order 0.1–0.2 mm. Acknowledgements The authors thank the Volkswagen Stiftung for financial assistance. Drs. J.J. Irani and O.N. Mohanty of Tata Iron and Steel, Jamshedpur, India, supplied the microalloyed steels. References w1x D.L. Davidson, J. Lankford, Int. Mater. Rev. 32 Ž1992. 45. w2x M. Korchynsky, in: M. Korchynsky, .A.J. De Ardo ŽEds.., Proc. of Int. Conf. ‘Microalloying 95’, Iron and Steel Society, Pittsburgh, USA, 1995, p. 3.
188
V. Subramanya Sarma et al.r Materials Letters 46 (2000) 185–188
w3x V. Subramanya Sarma, K.A. Padmanabhan, Int. J. Fatigue 17 Ž1997. 135. w4x V. Subramanya Sarma, K.A. Padmanabhan, Siddhartha Das, J. Mater. Sci. Lett. 16 Ž1997. 1495. w5x V. Subramanya Sarma, K.A. Padmanabhan, A. Gueth, A. Koethe, Mater. Sci. Technol. 15 Ž1999. 260. w6x V. Subramanya Sarma, K.A. Padmanabhan, G. Jaeger, A. Koethe, in: K.T. Rie, P.D. Portella ŽEds.., Proc. Low Cycle Fatigue Elasto-Plast. Behav. Mater., wInt. Conf.x, 4th,
w7x w8x
w9x w10x w11x
Garmisch-Partenkirchen, Germany Elsevier, Amsterdam, 1998, p. 697. F. Schlat, Int. J. Fract. 19 Ž1982. R37. M. Schaper, A. Boehm, in: K.H. Schwalbe, C. Berger ŽEds.., Eur. Conf. Fract. ŽECF 10., Berlin, vol. II, Verlag EMAS, 1994, p. 1451. K.S. Chan, Metall. Trans. A 24 Ž1993. 2473. B. Tomkins, Philos. Mag. 18 Ž1968. 1041. L.E. Miller, G.C. Smith, 208 JISI Ž1970. 998.
Materials Letters 46 Ž2000. 185–188 www.elsevier.comrlocatermatlet
On the fatigue crack growth behaviour of two ferrite–pearlite microalloyed steels V. Subramanya Sarma a,) , K.A. Padmanabhan a , G. Jaeger b, A. Koethe b, M. Schaper c a
b
Indian Institute of Technology, Kanpur 208 016 India Institut f ur und Werkstofforschung (IFW), Dresden, Germany ¨ Festkorper¨ c Technische UniÕersitat, ¨ Dresden, Germany Received 24 March 2000; accepted 19 May 2000
Abstract Fatigue crack growth rates ŽFCGR. in the Paris regime of two ferrite–pearlite microalloyed ŽMA. steels are shown to be in good agreement with those predicted using a model of crack tip element failure by low cycle fatigue ŽLCF.. Fatigue striations of correct spacing Ž0.1–0.2 mm. dimensions were seen in the fractured specimens of both the steels. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Microalloyed steels; Fatigue crack growth; Paris region; Low cycle fatigue
Fatigue crack growth ŽFCG. in metallic materials has been studied extensively, for a review, see Ref. w1x. The fatigue crack growth rate ŽFCGR., Žd ard N ., is usually plotted against the stress intensity range, Ž D K ., Ža log–log plot.. Of the three regions, regions I Žnear threshold. and II Žsteady state ŽParis. regime. are of special interest. Control processed, ferrite–pearlite microalloyed ŽMA. steels are replacing quenched and tempered ŽQ and T. steels in many applications w2x. The present study is part of a detailed investigation on two such MA steels w3–6x, a pearlite-dominant medium carbon Ž49MnVS3. and another ferrite-dominant low carbon ŽE-38. grade. 49MnVS3 was received as 17-mm thick slabs and E-38 in the form of 7-mm thick plates. The chemical compositions are given in Table
1. Cyclic stress-strain ŽCSS. and fatigue properties, as well as the microstructural parameters, are presented in Table 2. The FCG tests were carried out with a computer-controlled dynamic compliance ŽDYNACOMP. equipment w7,8x, in which the specimen represents the decisive compliance of a mechanical resonator ŽFig. 1.. Exciting the resonator at its resonance frequency enables the determination of the effective crack length by high-resolution period mea-
Table 1 Chemical compositions Žwt.%. of the MA steels Material C 49Mn VS3 E-38
)
Si
Mn P
S
V
0.49 0.27 0.98 0.02 0.05 0.09
Nb
N
Fe
–
0.016 Balance
0.09 0.03 0.81 0.02 0.03 0.007 0.015 0.007 Balance
Corresponding author.
00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 1 6 5 - 8
V. Subramanya Sarma et al.r Materials Letters 46 (2000) 185–188
186
Table 2 Mechanical properties and microstructural details Material
sys ŽMPa.
scysŽMPa.
d Žmm.
l Žmm.
n
X
c
´ fX
49MnVS3
530
510
16 " 2.2
0.25 " 0.03
E-38
371
318
9"1
ND
0.14 0.078 0.23
y0.64 y0.82 y0.6
0.6 3.35 0.3
ND — Not determined. sys , 0.2% monotonic yield strength; scys , 0.2% cyclic yield strength; d, pearlite colony sizergrain size; l, interlamellar spacing; nX , cyclic X strain hardening exponent; c, Coffin–Manson exponent; ´ f Coffin–Manson coefficient.
surements. Resonance vibrations at high amplitudes serve to load the specimen and monitor the crack length. All measurements were carried out on single edge notched ŽSEN. specimens of cross-section 6.5 = 20 mm2 , which were notched for 4 mm by spark
erosion Žnotch root radius 0.12 mm.. Crack propagation was perpendicular to the slab forging direction in 49MnVS3 but was along the plate rolling direction in E-38. The FCG tests were carried out using the D K, increasing the Žconstant load. procedure at a load ratio Žminimum loadrmaximum load., R, of 0.25.
Fig. 1. A schematic of the DYNACOMP machine.
Fig. 2. d ard N vs. D K plots for MA steels 49MnVS3 and E-38.
Fig. 3. Computed and experimental FCGR plots for Ža. 49MnVS3, and Žb. E-38.
V. Subramanya Sarma et al.r Materials Letters 46 (2000) 185–188
187
Fig. 4. Striations in Ža. 49MnVS3, and Žb. E-38.
Despite the vastly different microstructures, the two steels exhibited a similar FCG response ŽFig. 2.. It is well known that FCG in the Paris region is not influenced by the microstructure. The lack of influence is generally attributed to the cyclic plastic zone size being larger than the characteristic microstructural unit size Žcontinuum behaviour.. Using the approach of crack tip element failure by LCF, it has been shown that w9x da
1r c
Ž1y1rc .
DK
2rc
sj Ž2 s. Ž 1. dN E where j Žs Ž Esr4scys ´ fX d .. is a dimensionless parameter, E is the Young’s modulus, scys is the cyclic yield stress, ´ fX is the cyclic ductility coefficient, d is the dislocation barrier spacing Žgrain size or interlamellar spacing., c is the cyclic ductility exponent and s is the striation spacing. It is reasoned that the lack of influence of microstructure is due to the increase in the cyclic yield strength ‘scys ’ and the fatigue ductility exponent ‘c’ being compensated for by a corresponding decrease in the dislocation barrier spacing ‘d’, as this leads to a very narrow range for the ‘j ’ values and the crack growth rates w9x. From Eq. Ž1. and the Paris relation Žd ard N s CD K m ., ‘m’s 2rc and the Paris coefficient, C, is a function of j . Tomkins w10x has shown that at low stresses, c s 1rŽ2 nX q 1.. Therefore, m s 2 Ž 2 nX q 1 . Ž 2. The calculated and the experimental crack growth plots are shown in Fig. 3a and b, respectively. For 49MnVS3, the dislocation barrier spacing, d, was
taken as 2 1r2 Ž t q l., where t is the thickness of the cementite plate and l, the interlamellar spacing, was taken as 0.5 mm Žthe factor 2 1r2 is due to cementite deforming at approximately 458 to the loading axis w4,11x.. For E-38, the dislocation barrier spacing was taken to be equal to the average grain size Ž9 mm.. The striation spacing ‘s’ was taken as 0.1 mm for 49MnVS3 and 0.2 mm for E-38. For 49MnVS3, c at low strain amplitude was obtained experimentally ŽTable 2.. Due to experimental limitations, LCF data for E-38 could not be generated at low strain amplitudes. And so, Eq. Ž2. was used for calculating m. In both the steels, a very good correlation between the calculated and the experimental growth rates ŽFig. 3. was obtained. Fractography revealed evidence for striated crack growth in both the steels ŽFig. 4.. The striation spacings were indeed of the order 0.1–0.2 mm. Acknowledgements The authors thank the Volkswagen Stiftung for financial assistance. Drs. J.J. Irani and O.N. Mohanty of Tata Iron and Steel, Jamshedpur, India, supplied the microalloyed steels. References w1x D.L. Davidson, J. Lankford, Int. Mater. Rev. 32 Ž1992. 45. w2x M. Korchynsky, in: M. Korchynsky, .A.J. De Ardo ŽEds.., Proc. of Int. Conf. ‘Microalloying 95’, Iron and Steel Society, Pittsburgh, USA, 1995, p. 3.
188
V. Subramanya Sarma et al.r Materials Letters 46 (2000) 185–188
w3x V. Subramanya Sarma, K.A. Padmanabhan, Int. J. Fatigue 17 Ž1997. 135. w4x V. Subramanya Sarma, K.A. Padmanabhan, Siddhartha Das, J. Mater. Sci. Lett. 16 Ž1997. 1495. w5x V. Subramanya Sarma, K.A. Padmanabhan, A. Gueth, A. Koethe, Mater. Sci. Technol. 15 Ž1999. 260. w6x V. Subramanya Sarma, K.A. Padmanabhan, G. Jaeger, A. Koethe, in: K.T. Rie, P.D. Portella ŽEds.., Proc. Low Cycle Fatigue Elasto-Plast. Behav. Mater., wInt. Conf.x, 4th,
w7x w8x
w9x w10x w11x
Garmisch-Partenkirchen, Germany Elsevier, Amsterdam, 1998, p. 697. F. Schlat, Int. J. Fract. 19 Ž1982. R37. M. Schaper, A. Boehm, in: K.H. Schwalbe, C. Berger ŽEds.., Eur. Conf. Fract. ŽECF 10., Berlin, vol. II, Verlag EMAS, 1994, p. 1451. K.S. Chan, Metall. Trans. A 24 Ž1993. 2473. B. Tomkins, Philos. Mag. 18 Ž1968. 1041. L.E. Miller, G.C. Smith, 208 JISI Ž1970. 998.