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Superior fatigue crack growth properties in newly developed weld metal
International Journal of Fatigue 21 (1999) S113–S118 www.elsevier.com/locate/ijfatigue
Superior fatigue crack growth properties in newly developed weld metal Akihiko Ohta *, Naoyuki Suzuki, Yoshio Maeda, Kazuo Hiraoka, Teruyoshi Nakamura National Research Institute for Metals, 1-2-1 Sengen, Tsukuba-shi, Ibaraki 305-0047, Japan
Abstract The fatigue threshold and high growth rate region properties of conventional welded joints were improved by using newly developed low transformation temperature welding wire. The developed weld metal which contains 10 wt% nickel and 10 wt% chromium begins to transform from austenite to martensite at about 180°C and finishes it at room temperature. During the transformation the weld metal expands. This expansion induces a compressive residual stress around the welded part. The stress ratio effect due to this compressive residual stress makes the fatigue crack growth properties of the developed weld metal superior by intensifying the fatigue crack closure. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Fatigue crack growth; Residual stress; Transformation; Steel welded joint
1. Introduction The fatigue crack growth properties of a conventional welded joint become unique [1,2] due to the disappearance of the fatigue crack closure by the effect of tensile residual stress. That is, fatigue crack closure is avoided by the critically high stress ratio condition as the tensile welding stress works as the mean stress. In this condition, the fatigue crack growth properties do not vary in spite of the variation of type of steels, welding methods, heat inputs and stress ratio. In the case of base metal, it is well known that the fatigue crack growth rate increases with increasing stress ratio [3,4]. This is well explained by fatigue crack closure effects [5–8]. The fatigue crack growth properties of conventional welded joints coincide with those of the base metal in the fatigue crack closure free condition at very high stress ratio [1,2]. Even in the case of welded joints, the fatigue crack closure is expected to occur when compressive welding residual stresses could be induced. The compressive welding residual stress has been successfully induced by using low transformation temperature welding wire [9].
* Corresponding author. Tel.: +81-298-59-2244; fax: +81-298-592201.
The fatigue limit of box welds was increased to values about twice that of the conventional one. In this paper, the fatigue crack growth properties of center notch type transverse butt welded joints in which the compressive welding residual stress is induced by using low transformation temperature welding wire are investigated. Thus, fatigue crack closure is intensified by the compressive residual stress, and superior fatigue crack growth properties are obtained.
2. Experimental procedure The material used in this investigation was 20 mm thick ferrite–pearlite steel of JIS SPV490 steel. The chemical composition and mechanical properties of this steel and the welding wires are given in Tables 1 and 2, respectively. The transverse butt welded joints were made by gas metal arc welding. The deformation of welding wire with cooling is shown in Fig. 1. In Fig. 1 Ms is the temperature at which transformation from austenite to martensite begins to occur, and ef is the amount of expansion or shrinkage from temperature Ms to room temperature. In the case of conventional wire, the shrinkage is dominant even though it expands around 500°C with the transformation from austenite to martensite. However, in the case of low transformation temperature
0142-1123/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 1 1 2 3 ( 9 9 ) 0 0 0 6 2 - 6
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A. Ohta et al. / International Journal of Fatigue 21 (1999) S113–S118
Table 1 Chemical composition of materials (wt%)
SPV490 Conventional wire Low transformation temperature wire
C
Si
Mn
P
S
Ni
0.12 0.13 0.025
0.28 0.03 0.32
1.22 1.86 0.70
0.019 0.007
0.004 0.015
Cr
0.02 10.0
0.01 10.0
Mo
V
0.11 0.53 0.13
0.03
Table 2 Mechanical properties of SPV490 steel
SPV490
Fig. 1.
Shrinkage of welding wire with cooling.
Fig. 2. Mechanism of inducing welding residual stress. (a) conventional welded specimen; (b) welded specimen with low transformation temperature welding wire.
Upper yield Tensile strength strength (MPa) (MPa)
Elongation (%)
Vikers hardness (HV 98)
579
40
207
628
welding wire, the expansion becomes important near room temperature as the transformation from austenite to martensite occurs around this temperature. This expansion at the final stage of welding induces compressive residual stress around the center of the specimen where the fatigue crack grows. The mechanism of inducing welding residual stress is shown in Fig. 2. If one considers a welded plate cut along the center line of the weld, each half deforms either by shrinkage or by expansion of weld line as shown in the left hand diagrams of Fig. 2. These imaginary deformations are constrained by the generation of residual stress as shown in the right hand diagrams. In the case of a welded joint with low transformation welding wire, the angular distortion was large in order to get a plane plate. Therefore, plastic deformation was
Fig. 3.
Dimensions of fatigue specimen.
A. Ohta et al. / International Journal of Fatigue 21 (1999) S113–S118
Fig. 4.
Aspect of clip on gage attached to specimen. Fig. 5.
Fig. 6.
Macrostructures of welded joints.
Residual stress distribution along the width of the specimen.
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A. Ohta et al. / International Journal of Fatigue 21 (1999) S113–S118
applied to straighten the angular distortion. After deformation, the joint was heated in a furnace at 720°C and cooled in air. The fatigue specimens were made from the welded plate as shown in Fig. 3. The center notch was machined in the middle of the weld metal. The residual stress distribution along the width of the specimen was measured with rosette type strain gages by releasing the restriction stress with metal saw cut around gages. The following equation was used to calculate the residual stress. sr⫽
−E (e ⫹nex) 1−n2 y
crack length, a, was calculated as the average of four values from front and back surfaces and left and right sides. The fatigue crack growth rates, da/dn, were obtained by directly dividing the increment of crack length, ⌬a, with the number of cycles elapsed for it, ⌬n. The fatigue crack closure was observed by using a clip on gage straddling the center notch as shown in Fig. 4. The relationship between load and modified crack opening displacement was recorded by the unloading elastic compliance method [7]. These measurements were done at 0.2 Hz using an x–y recorder.
(1) 3. Results and discussion
where E is the Young’s modulus, n is the Poisson’s ratio, ey and ex are longitudinal strain and transverse strain measured after the cut around gages, respectively. The fatigue crack growth tests were performed on these specimens using an electrohydraulic fatigue testing machine at room temperature in ambient air. The waveform was sinusoidal and the test frequency was 10–50 Hz. The fatigue crack tips were observed with two sets of travelling microscopes of ×20 magnification. The half
Fig. 7.
The macroscopic photographs are shown in Fig. 5. The weld metal shows a dendritic structure. The fatigue crack grew in the weld metal. The residual stress distribution along the width of the specimen is shown in Fig. 6. The tensile residual stress exists around the middle of the specimen in the conventional welded joint. Meanwhile, the compressive residual stress is successfully introduced around the middle of
Fatigue crack growth properties.
A. Ohta et al. / International Journal of Fatigue 21 (1999) S113–S118
S117
Fig. 8. Records of fatigue crack closure. (a) Conventional welded specimen; (b) welded specimen with low transformation temperature welding wire.
the specimen in the welded joints made with low transformation temperature welding wire. The fatigue crack growth properties of this joint made with low transformation temperature welding wire are shown in Fig. 7 with the fatigue crack growth behavior of conventional welded joints indicated by bands. The plot for the joints made with low transformation temperature welding wire is located at the right side of these bands. The fatigue threshold of this joint is about twice that for conventional welded joints. Fig. 8 shows an example of the records of fatigue crack closure. In this figure, d is the crack opening displacement, and d⬘ is the modified crack opening displacement with the unloading elastic compliance method. That is, the inverse propotional signal of the unloading line was subtracted from d by using load signal, P, as d⬘=d⫺aP, where a is the compliance of the cracked specimen. The vertical line means that the fatigue crack is opened. The deviated part of the record from the vertical line indicates the crack closure. It is clear that fatigue crack closure occurs in a very wide range of loading as compared with conventional welded joints. The effective range of this joint is about one third that of conventional welded joints.
4. Conclusions The fatigue crack growth properties of a welded joint made with low transformation temperature welding wire were investigated. Compressive residual stresses were successfully introduced around the middle of the specimen. The stress ratio effect produced by the compressive residual stress made the properties superior when compared to conventional weld metal.
References [1] Ohta A, Sasaki E, Kamakura M, Nihei M, Kosuge M, Kanao M, Inagaki M. Effect of residual tensile stresses on threshold level for fatigue crack propagation in welded joints of SM50B steel. Trans Japan Weld Soc 1981;12:31–8. [2] Ohta A, Suzuki N, Maeda Y. Unique fatigue threshold and growth properties of welded joints in a tensile residual stress field. Int J Fat 1998;19:S303–10. [3] Paris P, Erdogan F. A critical analysis of crack propagation laws. J Basic Engng, Trans ASME 1963;85:528–34. [4] Klesnil K, Lukas P. Effect of stress cycle asymmertry on fatigue crack growth. Mater Sci Engng 1972;9:231–40. [5] Elber W. The significance of fatigue crack closure. ASTM STP 1971;486:230–42.
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A. Ohta et al. / International Journal of Fatigue 21 (1999) S113–S118
[6] Elber W. Fatigue crack closure under cyclic tension. Engng Fract Mech 1970;2:37–45. [7] Kikukawa M, Jono M, Tanaka KI. Fatigue crack closure behavior at low stress intensity level. In: Proceedings of 2nd International Conference in Mechanical Behavior of Materials, 1976:254–77. [8] Ohta A, Kosuge M, Mawari T, Nishijima S. Fatigue crack propagation in tensile residual stress field of welded joints under fully compressive cycling. Int J Fat 1988;10:237–42.
[9] Ohta A, Suzuki N, Maeda Y. Effect of residual stress on fatigue of weldment. In: IIW 50th Annual Assembly Conference, International Conference on Performance of Dynamically Loaded Weld Structures. New York: Welding Research Council 1997:108–22.
Superior fatigue crack growth properties in newly developed weld metal Akihiko Ohta *, Naoyuki Suzuki, Yoshio Maeda, Kazuo Hiraoka, Teruyoshi Nakamura National Research Institute for Metals, 1-2-1 Sengen, Tsukuba-shi, Ibaraki 305-0047, Japan
Abstract The fatigue threshold and high growth rate region properties of conventional welded joints were improved by using newly developed low transformation temperature welding wire. The developed weld metal which contains 10 wt% nickel and 10 wt% chromium begins to transform from austenite to martensite at about 180°C and finishes it at room temperature. During the transformation the weld metal expands. This expansion induces a compressive residual stress around the welded part. The stress ratio effect due to this compressive residual stress makes the fatigue crack growth properties of the developed weld metal superior by intensifying the fatigue crack closure. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Fatigue crack growth; Residual stress; Transformation; Steel welded joint
1. Introduction The fatigue crack growth properties of a conventional welded joint become unique [1,2] due to the disappearance of the fatigue crack closure by the effect of tensile residual stress. That is, fatigue crack closure is avoided by the critically high stress ratio condition as the tensile welding stress works as the mean stress. In this condition, the fatigue crack growth properties do not vary in spite of the variation of type of steels, welding methods, heat inputs and stress ratio. In the case of base metal, it is well known that the fatigue crack growth rate increases with increasing stress ratio [3,4]. This is well explained by fatigue crack closure effects [5–8]. The fatigue crack growth properties of conventional welded joints coincide with those of the base metal in the fatigue crack closure free condition at very high stress ratio [1,2]. Even in the case of welded joints, the fatigue crack closure is expected to occur when compressive welding residual stresses could be induced. The compressive welding residual stress has been successfully induced by using low transformation temperature welding wire [9].
* Corresponding author. Tel.: +81-298-59-2244; fax: +81-298-592201.
The fatigue limit of box welds was increased to values about twice that of the conventional one. In this paper, the fatigue crack growth properties of center notch type transverse butt welded joints in which the compressive welding residual stress is induced by using low transformation temperature welding wire are investigated. Thus, fatigue crack closure is intensified by the compressive residual stress, and superior fatigue crack growth properties are obtained.
2. Experimental procedure The material used in this investigation was 20 mm thick ferrite–pearlite steel of JIS SPV490 steel. The chemical composition and mechanical properties of this steel and the welding wires are given in Tables 1 and 2, respectively. The transverse butt welded joints were made by gas metal arc welding. The deformation of welding wire with cooling is shown in Fig. 1. In Fig. 1 Ms is the temperature at which transformation from austenite to martensite begins to occur, and ef is the amount of expansion or shrinkage from temperature Ms to room temperature. In the case of conventional wire, the shrinkage is dominant even though it expands around 500°C with the transformation from austenite to martensite. However, in the case of low transformation temperature
0142-1123/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 1 1 2 3 ( 9 9 ) 0 0 0 6 2 - 6
S114
A. Ohta et al. / International Journal of Fatigue 21 (1999) S113–S118
Table 1 Chemical composition of materials (wt%)
SPV490 Conventional wire Low transformation temperature wire
C
Si
Mn
P
S
Ni
0.12 0.13 0.025
0.28 0.03 0.32
1.22 1.86 0.70
0.019 0.007
0.004 0.015
Cr
0.02 10.0
0.01 10.0
Mo
V
0.11 0.53 0.13
0.03
Table 2 Mechanical properties of SPV490 steel
SPV490
Fig. 1.
Shrinkage of welding wire with cooling.
Fig. 2. Mechanism of inducing welding residual stress. (a) conventional welded specimen; (b) welded specimen with low transformation temperature welding wire.
Upper yield Tensile strength strength (MPa) (MPa)
Elongation (%)
Vikers hardness (HV 98)
579
40
207
628
welding wire, the expansion becomes important near room temperature as the transformation from austenite to martensite occurs around this temperature. This expansion at the final stage of welding induces compressive residual stress around the center of the specimen where the fatigue crack grows. The mechanism of inducing welding residual stress is shown in Fig. 2. If one considers a welded plate cut along the center line of the weld, each half deforms either by shrinkage or by expansion of weld line as shown in the left hand diagrams of Fig. 2. These imaginary deformations are constrained by the generation of residual stress as shown in the right hand diagrams. In the case of a welded joint with low transformation welding wire, the angular distortion was large in order to get a plane plate. Therefore, plastic deformation was
Fig. 3.
Dimensions of fatigue specimen.
A. Ohta et al. / International Journal of Fatigue 21 (1999) S113–S118
Fig. 4.
Aspect of clip on gage attached to specimen. Fig. 5.
Fig. 6.
Macrostructures of welded joints.
Residual stress distribution along the width of the specimen.
S115
S116
A. Ohta et al. / International Journal of Fatigue 21 (1999) S113–S118
applied to straighten the angular distortion. After deformation, the joint was heated in a furnace at 720°C and cooled in air. The fatigue specimens were made from the welded plate as shown in Fig. 3. The center notch was machined in the middle of the weld metal. The residual stress distribution along the width of the specimen was measured with rosette type strain gages by releasing the restriction stress with metal saw cut around gages. The following equation was used to calculate the residual stress. sr⫽
−E (e ⫹nex) 1−n2 y
crack length, a, was calculated as the average of four values from front and back surfaces and left and right sides. The fatigue crack growth rates, da/dn, were obtained by directly dividing the increment of crack length, ⌬a, with the number of cycles elapsed for it, ⌬n. The fatigue crack closure was observed by using a clip on gage straddling the center notch as shown in Fig. 4. The relationship between load and modified crack opening displacement was recorded by the unloading elastic compliance method [7]. These measurements were done at 0.2 Hz using an x–y recorder.
(1) 3. Results and discussion
where E is the Young’s modulus, n is the Poisson’s ratio, ey and ex are longitudinal strain and transverse strain measured after the cut around gages, respectively. The fatigue crack growth tests were performed on these specimens using an electrohydraulic fatigue testing machine at room temperature in ambient air. The waveform was sinusoidal and the test frequency was 10–50 Hz. The fatigue crack tips were observed with two sets of travelling microscopes of ×20 magnification. The half
Fig. 7.
The macroscopic photographs are shown in Fig. 5. The weld metal shows a dendritic structure. The fatigue crack grew in the weld metal. The residual stress distribution along the width of the specimen is shown in Fig. 6. The tensile residual stress exists around the middle of the specimen in the conventional welded joint. Meanwhile, the compressive residual stress is successfully introduced around the middle of
Fatigue crack growth properties.
A. Ohta et al. / International Journal of Fatigue 21 (1999) S113–S118
S117
Fig. 8. Records of fatigue crack closure. (a) Conventional welded specimen; (b) welded specimen with low transformation temperature welding wire.
the specimen in the welded joints made with low transformation temperature welding wire. The fatigue crack growth properties of this joint made with low transformation temperature welding wire are shown in Fig. 7 with the fatigue crack growth behavior of conventional welded joints indicated by bands. The plot for the joints made with low transformation temperature welding wire is located at the right side of these bands. The fatigue threshold of this joint is about twice that for conventional welded joints. Fig. 8 shows an example of the records of fatigue crack closure. In this figure, d is the crack opening displacement, and d⬘ is the modified crack opening displacement with the unloading elastic compliance method. That is, the inverse propotional signal of the unloading line was subtracted from d by using load signal, P, as d⬘=d⫺aP, where a is the compliance of the cracked specimen. The vertical line means that the fatigue crack is opened. The deviated part of the record from the vertical line indicates the crack closure. It is clear that fatigue crack closure occurs in a very wide range of loading as compared with conventional welded joints. The effective range of this joint is about one third that of conventional welded joints.
4. Conclusions The fatigue crack growth properties of a welded joint made with low transformation temperature welding wire were investigated. Compressive residual stresses were successfully introduced around the middle of the specimen. The stress ratio effect produced by the compressive residual stress made the properties superior when compared to conventional weld metal.
References [1] Ohta A, Sasaki E, Kamakura M, Nihei M, Kosuge M, Kanao M, Inagaki M. Effect of residual tensile stresses on threshold level for fatigue crack propagation in welded joints of SM50B steel. Trans Japan Weld Soc 1981;12:31–8. [2] Ohta A, Suzuki N, Maeda Y. Unique fatigue threshold and growth properties of welded joints in a tensile residual stress field. Int J Fat 1998;19:S303–10. [3] Paris P, Erdogan F. A critical analysis of crack propagation laws. J Basic Engng, Trans ASME 1963;85:528–34. [4] Klesnil K, Lukas P. Effect of stress cycle asymmertry on fatigue crack growth. Mater Sci Engng 1972;9:231–40. [5] Elber W. The significance of fatigue crack closure. ASTM STP 1971;486:230–42.
S118
A. Ohta et al. / International Journal of Fatigue 21 (1999) S113–S118
[6] Elber W. Fatigue crack closure under cyclic tension. Engng Fract Mech 1970;2:37–45. [7] Kikukawa M, Jono M, Tanaka KI. Fatigue crack closure behavior at low stress intensity level. In: Proceedings of 2nd International Conference in Mechanical Behavior of Materials, 1976:254–77. [8] Ohta A, Kosuge M, Mawari T, Nishijima S. Fatigue crack propagation in tensile residual stress field of welded joints under fully compressive cycling. Int J Fat 1988;10:237–42.
[9] Ohta A, Suzuki N, Maeda Y. Effect of residual stress on fatigue of weldment. In: IIW 50th Annual Assembly Conference, International Conference on Performance of Dynamically Loaded Weld Structures. New York: Welding Research Council 1997:108–22.