- Email: [email protected]
Influence of carbon on creep crack growth of a cast HP alloy
MaterialsScience and Engineering, Al27 (1990) L7-L10
L7
Letter
Influence of carbon on creep crack growth of a cast HP alloy S. J. ZHU Laboratory of Fatigue and Fracture for Materials,Instituteof Metal Research, Academia Sinica, Shenyang I10015 (China) Y. WANG, S. G. XU and F. G. WANG MaterialsEngineering Department, Dalian Universityof Technology, Dalian 116024 (China) (Received December 11,1989; in revised form March 20, 1990)
Abstract The influence of carbon on the creep crack growth of a cast HP alloy has been investigated. Crack growth rates decreased with increasing carbon content from 0.35 to 0.56 wt.%. This is because the greater percentage of eutectic carbides resulting from the higher carbon content provides greater resistance to crack growth. Centrifugally cast HK40 alloy tubes have dominated the petrochemical reforming and cracking market for several decades. The development of this alloy resulted from the discovery of the potential of carbon as a strengthening resource [ 11. High carbon heat-resistant alloys possess a much higher level of creep rupture properties than do their wrought counterparts [l], which is further enhanced by the superior structures of centrifugally cast tubes. Although the use of the thick-walled HK40 centrifugal tubes has been widespread in the petrochemical industry, economic pressures necessitated a search for alloys with superior high temperature properties. HP alloy has a higher creep rupture strength than HK40 alloy. The influence of carbon on the stress rupture properties of HP alloy has been studied [2]. However, the final fracture of the reformer tubes occurs by the relatively brittle growth of a few intergranular 0921-5093/90/$3.50
cracks [3]. The precise micromechanism of fracture during creep of cast high temperature alloys has been a controversial issue [4-61. Since the 1970s there has been widespread interest in the creep crack growth behaviour of structural materials owing to the recognition of its importance in the life prediction and design of high temperature structures, and in the development of new high temperature materials. It has been shown that a material with a high stress rupture strength does not necessarily have a high resistance to crack growth [7, 81. Therefore, it is important to study the creep crack growth resistance of a material for high temperature applications. This Letter is concerned with the creep crack growth of HP alloy with different carbon contents. The aim of the investigation is to determine the carbon content which gives the optimum creep and creep crack growth properties. The materials tested consist of three centrifugally cast tubes. The chemical compositions are given in Table 1. Constant-load creep crack growth tests were conducted at 871 “C. The geometry-of-specimen and crack-length-measurement techniques used have been described previously [7,9]. The microstructures of the material containing 0.35 wt.% C aged for 3 h and the material containing 0.56 wt.% C aged for 16 h are shown in Fig. 1. Electron and X-ray diffraction tests showed that the Chinese script-like eutectic carbides were NbC and that the white skeleton-
TABLE 1 Element
Chemical compositions
of materials tested
Amount (wt.%)
C Si Mll P s Cr
0.35 0.84 0.79 0.013 0.008 25.06
El MO
35.70 0.89 0.08
0.44 0.50 0.82 0.017 0.010 24.25 34.00 0.89 0.08
8 Elsevier Sequoia/Printed
0.56 0.98 0.96 0.014 0.008 25.00 34.91 1.03 0.08
in The Netherlands
TABLE 2 Volume per cent fraction of eutectic carbides in the as-cast condition Carbon content (wt.%)
0.35 0.44 0.56
Volumeper centfraction MA
NbC
M7C3 + NbC
3.5 6.2 7.9
4.8 5.1 5.0
8.3 11.3 12.9
:
10-z-
Y
10-I 0.9
I 1.1
I 1 I 1.3 1.5 1.7 LogK (MPafi)
1
Fig. 3. Creep crack growth rates as a function of stress intensity K at 871 “C: 0, 0.35 wt.% C, X, 0.44 wt.% C; 0, 0.56 wt.% C.
Fig. 1. Microstructures of (a) the material containing 0.35 wt.% C aged for 3 h at 871 “C and (b) the material containing 0.56 wt.% C aged for 16 h at 871 “C.
r----I cl a
a
NbC
b
M7C3
b b
-J I
30
40
50 60 70 Diffraction Angie
I
80
90
Fig. 2. X-ray diffraction spectra from electrolytically extracted residuals of the material containing 0.44 wt.% C in the as-cast condition.
shaped eutectic carbides were M,C, [lo]. Figure 2 shows a typical example of X-ray diffraction spectra from electrolytically extracted residuals. Table 2 shows the volume per cent fraction of eutectic carbides in the as-cast condition. It can
be seen that the amount of NbC is almost the same while the amount of M,C3 increases with increasing carbon content. During aging, the eutectic carbides transformed from M,C3 to M,,C, [lo] and the secondary carbides M& and NbC [lo] precipitated in the matrix as shown in Fig. 1. Creep crack growth rates as a function of stress intensity are shown in Fig. 3. At a constant stress intensity, creep crack growth rates decrease with increasing carbon content. Figure 4 shows the cavities and cracks in the interrupted specimens. The cavities are irregular in shape and form most frequently at the interfaces of the austenite and intergranular carbide. The crack growth process includes the nucleation, growth and linkage of cavities. From Fig. 4, it can also be seen that the aspect ratio of the microcrack ahead of the main crack of the material containing 56 wt.% C is larger than that of the material containing 0.35 wt.% C. This means that the greater percentage of eutectic carbides present in the material containing 56 wt.% C provides higher resistance to the linkage of cavities or cracks.
L9
Table 3 shows the Vickers hardnesses of three materials in the as-cast and aged conditions at 871 “C for 100 h. As the carbon content increases, the corresponding increase in the Vickers hardness in the as-cast condition indicates the increase in eutectic carbides. Since the amount of eutectic carbides is almost constant during aging at 871 “C for 100 h [lo], the difference AHV in the Vickers hardness of the as-cast and aged samples mainly reflects the strenghtening by the secondary carbides. For an alloy made by the same casting technique, there should exist a maximum carbon content dissolved in austenite. When the maximum carbon content is reached, any further increase in carbon content leads only to an increase in the eutectic carbides. AHV increases with increasing carbon content from 0.35 to 0.44 wt.% C and remains the same from 0.44 to 0.56 wt.% C as can be seen in Table 3. This means that 0.44 wt.%
C is about the maximum carbon content of the material and that the increase in carbon content from 0.44 to 0.56 wt.% C does not increase the strengthening by the secondary carbides. Therefore, the decrease in creep crack growth rates with increasing carbon content from 0.35 to 0.44 wt.% C can be explained by the incremental strengthening associated with the increased amounts of skeleton-shaped eutectic carbides and the secondary carbide precipitates. As the carbon content increases from 0.44 to 0.56 wt.% C, the decrease in the creep crack growth rates is mainly because of the increased amounts of the skeletonshaped eutectic carbides. Creep crack growth rates as a function of pathindependent integral C* are shown in Fig. 5. It can be seen that the variation of the creep crack growth rates with C* does not seem to depend on the carbon content. This means that creep crack growth occurs purely by a deformation process [l l] and that the carbon-content dependence
TABLE 3 Vickers hardnesses and aged conditions
of materials
in the as-cast
Vickers hardness Carbon confent (wt.%)
As cast Aged at 871 “C for 100 h AHV
0.35
0.44
0.56
170 194 24
179 211 32
180 212 32
10'
X . xx609 x
“0’
x-
1te-
1o-3 0
Fig. 4. Cavities and cracks in the interrupted 0.35 wt.% C;(b) 0.56 wt.% C.
specimens: (a)
5
o
x
0.31
0.61
0.91
1.2I 1.51.8 Loge* : J/&h
:
Fig. 5. Creep crack growth rates as a function of the pathindependent integral C* at 871 “C: 0, 0.35 wt.% C; X, 0.44 wt.% C; l, 0.56 wt.% C.
L10
appears to be already included in C*. Comparison of the resistance to crack growth with creep strength shows that the influence of carbon content on creep crack growth rates and secondary creep rates are similar [10]. This similarity is reflected in the correlation of creep crack growth rates with C* as shown in Fig. 5.
References 1 H. S. Avery and C. R. Wilks, Trans. Am. Soc. Met., 40 (1948) 529. 2 D. B. Roach and J. A. Van Echo, A S T M Spec. Tech. Publ., 756 (1982) 275.
3 G. L. Dunlop, R. J. Twigg and D. M. R. Taplin, Scand. J. Metall., 7(1978) 152. 4 G. B. Thomas and T. B. Gibbons, Mater. Sci. Eng., 67 (1984) 13. 5 H. R. Tipler, in D. Francois (ed.), Advances in Fracture Research, Pergamon, Oxford, 1981, p. 1687. 6 S. Osgerby and T. B. Gibbons, Mater. Sci. Eng., 59(1983) Lll. 7 S.J. Zhu, P. E. Li, J. Zhao and Z. B. Cao, Mater. Sci. Eng., A l l 4 (1989) 7. 8 S.J. Zhu, J. Zhao, E G. Wang and Z. B. Cao, Chin. J. Met. Sci. Technol., 5(1989) 421. 9 Z. B. Cao, E E. Li, J. S. Zhang and S. J. Zhu, J. Mater. Sci., 23(1988) 3692. 10 Y. Wang, M. Eng. Thesis, Dalian University of Technology, 1989. 11 K. Sadananda and E Shahinian, MetaU. Trans. A, 14 (1983) 1467.
L7
Letter
Influence of carbon on creep crack growth of a cast HP alloy S. J. ZHU Laboratory of Fatigue and Fracture for Materials,Instituteof Metal Research, Academia Sinica, Shenyang I10015 (China) Y. WANG, S. G. XU and F. G. WANG MaterialsEngineering Department, Dalian Universityof Technology, Dalian 116024 (China) (Received December 11,1989; in revised form March 20, 1990)
Abstract The influence of carbon on the creep crack growth of a cast HP alloy has been investigated. Crack growth rates decreased with increasing carbon content from 0.35 to 0.56 wt.%. This is because the greater percentage of eutectic carbides resulting from the higher carbon content provides greater resistance to crack growth. Centrifugally cast HK40 alloy tubes have dominated the petrochemical reforming and cracking market for several decades. The development of this alloy resulted from the discovery of the potential of carbon as a strengthening resource [ 11. High carbon heat-resistant alloys possess a much higher level of creep rupture properties than do their wrought counterparts [l], which is further enhanced by the superior structures of centrifugally cast tubes. Although the use of the thick-walled HK40 centrifugal tubes has been widespread in the petrochemical industry, economic pressures necessitated a search for alloys with superior high temperature properties. HP alloy has a higher creep rupture strength than HK40 alloy. The influence of carbon on the stress rupture properties of HP alloy has been studied [2]. However, the final fracture of the reformer tubes occurs by the relatively brittle growth of a few intergranular 0921-5093/90/$3.50
cracks [3]. The precise micromechanism of fracture during creep of cast high temperature alloys has been a controversial issue [4-61. Since the 1970s there has been widespread interest in the creep crack growth behaviour of structural materials owing to the recognition of its importance in the life prediction and design of high temperature structures, and in the development of new high temperature materials. It has been shown that a material with a high stress rupture strength does not necessarily have a high resistance to crack growth [7, 81. Therefore, it is important to study the creep crack growth resistance of a material for high temperature applications. This Letter is concerned with the creep crack growth of HP alloy with different carbon contents. The aim of the investigation is to determine the carbon content which gives the optimum creep and creep crack growth properties. The materials tested consist of three centrifugally cast tubes. The chemical compositions are given in Table 1. Constant-load creep crack growth tests were conducted at 871 “C. The geometry-of-specimen and crack-length-measurement techniques used have been described previously [7,9]. The microstructures of the material containing 0.35 wt.% C aged for 3 h and the material containing 0.56 wt.% C aged for 16 h are shown in Fig. 1. Electron and X-ray diffraction tests showed that the Chinese script-like eutectic carbides were NbC and that the white skeleton-
TABLE 1 Element
Chemical compositions
of materials tested
Amount (wt.%)
C Si Mll P s Cr
0.35 0.84 0.79 0.013 0.008 25.06
El MO
35.70 0.89 0.08
0.44 0.50 0.82 0.017 0.010 24.25 34.00 0.89 0.08
8 Elsevier Sequoia/Printed
0.56 0.98 0.96 0.014 0.008 25.00 34.91 1.03 0.08
in The Netherlands
TABLE 2 Volume per cent fraction of eutectic carbides in the as-cast condition Carbon content (wt.%)
0.35 0.44 0.56
Volumeper centfraction MA
NbC
M7C3 + NbC
3.5 6.2 7.9
4.8 5.1 5.0
8.3 11.3 12.9
:
10-z-
Y
10-I 0.9
I 1.1
I 1 I 1.3 1.5 1.7 LogK (MPafi)
1
Fig. 3. Creep crack growth rates as a function of stress intensity K at 871 “C: 0, 0.35 wt.% C, X, 0.44 wt.% C; 0, 0.56 wt.% C.
Fig. 1. Microstructures of (a) the material containing 0.35 wt.% C aged for 3 h at 871 “C and (b) the material containing 0.56 wt.% C aged for 16 h at 871 “C.
r----I cl a
a
NbC
b
M7C3
b b
-J I
30
40
50 60 70 Diffraction Angie
I
80
90
Fig. 2. X-ray diffraction spectra from electrolytically extracted residuals of the material containing 0.44 wt.% C in the as-cast condition.
shaped eutectic carbides were M,C, [lo]. Figure 2 shows a typical example of X-ray diffraction spectra from electrolytically extracted residuals. Table 2 shows the volume per cent fraction of eutectic carbides in the as-cast condition. It can
be seen that the amount of NbC is almost the same while the amount of M,C3 increases with increasing carbon content. During aging, the eutectic carbides transformed from M,C3 to M,,C, [lo] and the secondary carbides M& and NbC [lo] precipitated in the matrix as shown in Fig. 1. Creep crack growth rates as a function of stress intensity are shown in Fig. 3. At a constant stress intensity, creep crack growth rates decrease with increasing carbon content. Figure 4 shows the cavities and cracks in the interrupted specimens. The cavities are irregular in shape and form most frequently at the interfaces of the austenite and intergranular carbide. The crack growth process includes the nucleation, growth and linkage of cavities. From Fig. 4, it can also be seen that the aspect ratio of the microcrack ahead of the main crack of the material containing 56 wt.% C is larger than that of the material containing 0.35 wt.% C. This means that the greater percentage of eutectic carbides present in the material containing 56 wt.% C provides higher resistance to the linkage of cavities or cracks.
L9
Table 3 shows the Vickers hardnesses of three materials in the as-cast and aged conditions at 871 “C for 100 h. As the carbon content increases, the corresponding increase in the Vickers hardness in the as-cast condition indicates the increase in eutectic carbides. Since the amount of eutectic carbides is almost constant during aging at 871 “C for 100 h [lo], the difference AHV in the Vickers hardness of the as-cast and aged samples mainly reflects the strenghtening by the secondary carbides. For an alloy made by the same casting technique, there should exist a maximum carbon content dissolved in austenite. When the maximum carbon content is reached, any further increase in carbon content leads only to an increase in the eutectic carbides. AHV increases with increasing carbon content from 0.35 to 0.44 wt.% C and remains the same from 0.44 to 0.56 wt.% C as can be seen in Table 3. This means that 0.44 wt.%
C is about the maximum carbon content of the material and that the increase in carbon content from 0.44 to 0.56 wt.% C does not increase the strengthening by the secondary carbides. Therefore, the decrease in creep crack growth rates with increasing carbon content from 0.35 to 0.44 wt.% C can be explained by the incremental strengthening associated with the increased amounts of skeleton-shaped eutectic carbides and the secondary carbide precipitates. As the carbon content increases from 0.44 to 0.56 wt.% C, the decrease in the creep crack growth rates is mainly because of the increased amounts of the skeletonshaped eutectic carbides. Creep crack growth rates as a function of pathindependent integral C* are shown in Fig. 5. It can be seen that the variation of the creep crack growth rates with C* does not seem to depend on the carbon content. This means that creep crack growth occurs purely by a deformation process [l l] and that the carbon-content dependence
TABLE 3 Vickers hardnesses and aged conditions
of materials
in the as-cast
Vickers hardness Carbon confent (wt.%)
As cast Aged at 871 “C for 100 h AHV
0.35
0.44
0.56
170 194 24
179 211 32
180 212 32
10'
X . xx609 x
“0’
x-
1te-
1o-3 0
Fig. 4. Cavities and cracks in the interrupted 0.35 wt.% C;(b) 0.56 wt.% C.
specimens: (a)
5
o
x
0.31
0.61
0.91
1.2I 1.51.8 Loge* : J/&h
:
Fig. 5. Creep crack growth rates as a function of the pathindependent integral C* at 871 “C: 0, 0.35 wt.% C; X, 0.44 wt.% C; l, 0.56 wt.% C.
L10
appears to be already included in C*. Comparison of the resistance to crack growth with creep strength shows that the influence of carbon content on creep crack growth rates and secondary creep rates are similar [10]. This similarity is reflected in the correlation of creep crack growth rates with C* as shown in Fig. 5.
References 1 H. S. Avery and C. R. Wilks, Trans. Am. Soc. Met., 40 (1948) 529. 2 D. B. Roach and J. A. Van Echo, A S T M Spec. Tech. Publ., 756 (1982) 275.
3 G. L. Dunlop, R. J. Twigg and D. M. R. Taplin, Scand. J. Metall., 7(1978) 152. 4 G. B. Thomas and T. B. Gibbons, Mater. Sci. Eng., 67 (1984) 13. 5 H. R. Tipler, in D. Francois (ed.), Advances in Fracture Research, Pergamon, Oxford, 1981, p. 1687. 6 S. Osgerby and T. B. Gibbons, Mater. Sci. Eng., 59(1983) Lll. 7 S.J. Zhu, P. E. Li, J. Zhao and Z. B. Cao, Mater. Sci. Eng., A l l 4 (1989) 7. 8 S.J. Zhu, J. Zhao, E G. Wang and Z. B. Cao, Chin. J. Met. Sci. Technol., 5(1989) 421. 9 Z. B. Cao, E E. Li, J. S. Zhang and S. J. Zhu, J. Mater. Sci., 23(1988) 3692. 10 Y. Wang, M. Eng. Thesis, Dalian University of Technology, 1989. 11 K. Sadananda and E Shahinian, MetaU. Trans. A, 14 (1983) 1467.