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A new method for improving the resistance of high strength steel wires to room temperature creep and low cycle fatigue
Journal of Materials Processing Technology 89 – 90 (1999) 569 – 573
A new method for improving the resistance of high strength steel wires to room temperature creep and low cycle fatigue Zhenbo Zhao a, Derek O. Northwood a,*,1, Cheng Liu b, Yunxu Liu b a b
Mechanical and Materials Engineering, Uni6ersity of Windsor, Windsor, Ont., N9B 3P4, Canada Department of Materials Engineering, Jilin Institute of Technology, Changchun, 130012 PR China Received 21 October 1998
Abstract The effects of warm deformation treatments on the room temperature creep and the low cycle fatigue resistance of high carbon patented steel wires and high strength low carbon low alloy (HSLCLA) steel wires were studied. The low temperature creep strains of high carbon patented steel wires and high strength low carbon low alloy steel were decreased 85 and 65%, respectively, by the warm working treatments for the optimum warm deformation parameters (3% axial tensile plastic deformation at 300°C for 5 min). The low cycle fatigue lives of both steels were increased by 30 – 35% after warm deformation at the optimum deformation parameters. It is shown that the warm deformation treatment only affected the micro states such as lattice distortion, internal stress, dislocation density, sub-grain size and the amount of solute atoms in solid solution but did not change the overall microstructure or strength of the steel wires. Internal friction studies at different temperatures showed that the amount of solute atoms re-dissolved is different for different warm deformation temperatures and reaches a maximum at 300°C. It is considered that the improvement in the resistance of high strength steel wires to room temperature creep and low cycle fatigue can be attributed to the reduction of the amount of mobile dislocations through the rearrangement of dislocations and strengthening of matrix by the re-dissolution of solute atoms and dislocation pinning. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Warm deformation treatment; Room temperature creep; Low cycle fatigue; High strength steel wire
1. Introduction High strength, low stress relaxation steel wires are attractive for building applications since they can reduce the structure’s weight. Room temperature creep and low cycle fatigue are two types of failure found in these steel wires. Room temperature creep leads to a reduction in the level of the prestress and the low cycle fatigue results in crack initiation and propagation and finally fracture. Thus, there is an incentive to improve the resistance of these steels to these types of failure and thus ensure the reliability and service life of the
* Corresponding author. E-mail address: [email protected] (D.O. Northwood) 1 Also Faculty of Engineering and Applied Science, Ryerson Polytechnic University, 350 Victoria Street, Toronto, Ontario, Canada M5B 2K3.
resulting structures. Although considerable research [1– 5] has been carried out on high strength, low stress relaxation steel wires and commercial products are available [6], comparatively little work has been done on the low cycle fatigue behavior of these steels. The aim of the present investigation was to find an effective method(s) to improve the resistance of two high strength steels to both room temperature creep and low cycle fatigue.
2. Experimental details The chemical composition of the two steels is given in Table 1. The high carbon steel wires with a diameter of ¥= 1.77 mm, were subjected to the following treatment cycle: hot rolling, stretching and patenting at room temperature. HSLA steel wires were made with
0924-0136/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 9 9 ) 0 0 0 1 7 - 5
Z. Zhao et al. / Journal of Materials Processing Technology 89–90 (1999) 569–573
570
Table 1 Chemical composition of steel (wt%) Steel
C
Mn
Si
W
Nb
B
High carbon steel HSLCLA steel
0.64 0.20
0.50 2.34
0.22 0.42
– 0.05
– 0.03
– 0.005
the same diameter, 1.77 mm, by hot rolling, quenching and low temperature tempering, and patenting at room temperature. The warm deformation treatments were performed in a self-manufactured equipment which consisted of power control systems, heat control systems and load control systems for the plastic deformation. The effective length of the steel wire heated in an electric tube furnace is 200 mm. At a given temperature, the 3% homogeneous axial plastic deformation was controlled by the different linear velocity between the gathering wire machine and the releasing wire machine. Deformations were carried out within 5 min and the wires were unloaded after cooling to room temperature. Room temperature creep tests of the steel wires were performed on a R-100 creep test machine, with a load of 80% st (tensile strength) and a temperature control of 9 3°C. In the tests, the steel wires are first subjected to 10% of the total load for 5 min, then to 100% of the total load for 15 min. This is the start point for the strain measurements. The strains were measured by an extensometer. The samples used in the low cycle fatigue test were high carbon patented steel wires (st =1660 MPa, ¥ =1.0 mm) and HSLA steel wires (st =2260 MPa, ¥ =1.0 mm). The experiments were carried out on a PWS-20A fatigue testing machine. The load was chosen such that smax = 0.7st and the load ratio R =smin/ smax =0.57, i.e. for the high carbon patented steel
Fig. 1. Room temperature creep strain of high carbon patented steel wires after warm deformation treatment at different temperatures.
wires: smax = 1160 MPa, smin = 660 MPa and for the HSLA steel wires: smax = 1600 MPa, smin = 915 MPa. The frequency, f, was 2 Hz and a sine wave form was adopted in this experiment. The steel wires were first installed in cross heads of the testing machine using a purpose built fixture and then loaded according to the above-mentioned schedules. Thus, control of constant strain amplitude (here Do = omax − omin) can be realized by means of a transformation from load signal into
Table 2 Mechanical properties of two steels before the warm deformation treatmentsa Steel
st (MPa)
s0.2 (MPa)
c (%)
Number of bending cycles to fracture (bend angle = 180°C)
High carbon patented steel wire (¥ 1.77 mm) HSLCLA steel wire (¥ 1.77 mm)
1660
1560
40.5
19
2100
1970
18.7
13
a
st, tensile strength; s0.2, yield strength at 0.2% offset; c, reduction in area.
Table 3 Mechanical properties of two steels after the warm deformation treatment (300°C)a Steel
st (MPa)
s0.2 (MPa)
c (%)
Number of bending cycles to fracture (bend angle = 180°C)
High carbon patented steel wire (¥ 1.77 mm) HSLCLA steel wire (¥ 1.77 mm)
1663
1571
38.2
18
2119
1988
17.4
12
a
st, tensile strength; s0.2, yield strength at 0.2% offset; c, reduction in area.
Z. Zhao et al. / Journal of Materials Processing Technology 89–90 (1999) 569–573
571
control of constant displacement amplitude (where DV = Vmax −Vmin, V is displacement). The dislocation density and sub-grain size of the high strength low carbon low alloy (HSLCLA) steel wire after warm deformation treatments at various temperature were obtained by X-ray step scanning (the software used to calculate the dislocation density and sub-grain size was developed at Jilin University). Internal friction measurements of HSLCLA steel wire were carried out by a resonance method at temperatures from ambient to 400°C.
Fig. 4. Relationship between number of cycles and cyclic stress amplitude at constant strain amplitude for HSLCLA steel wires. Table 4 Dislocation density and the subgrain size of HSLCLA steel wires after warm deformation treatments at various temperatures Warm deforma- 100 tion temperature (°C) Dislocation density (mm/ mm2) Subgrain size (nm)
1.74E+11
26.8
300
1.26E+11
29.4
400
4.27E+10
46.9
Fig. 2. Room temperature creep strain of HSLCLA steel wires after warm deformation treatment at different temperatures.
Fig. 5. Internal friction spectra Q − 1 (T) and resonance frequency, f 20, as function of temperature for HSLCLA steel wire.
3. Results and discussion
Fig. 3. Relationship between number of cycles and cyclic stress amplitude at constant strain amplitude for high carbon patenting steel wires.
The mechanical properties of the two steels before and after the warm deformation treatments are given in Tables 2 and 3, respectively. The room temperature creep curves of the high carbon patented steels wires and HSLCLA steel wires are shown in Figs. 1 and 2, respectively. The effect of warm deformation tempera-
572
Z. Zhao et al. / Journal of Materials Processing Technology 89–90 (1999) 569–573
Table 5 Room temperature creep strains of high carbon patented steel wires and HSLCLA steel wires before and after warm deformation treatments (300°C) at different axial plastic deformation strains Steel
3% axial plastic deformation 5% axial plastic deformation Without the warm deformation treatments
High carbon patented steel wire HSLCLA steel wire
3.55×10−5
3.1×10−5
2.4×10−4
5.0×10−5
4.7×10−5
1.44×10−4
ture on the room temperature creep behavior of both types of steel wires are similar, in that, the creep strain is gradually reduced as the warm deformation temperature increases, reaches a minimum at 300°C, and then increases as the warm deformation temperature is increased above 300°C. The room temperature creep strains of the high carbon patented steel wires and high strength low carbon low alloy steel were decreased 85 and 65%, respectively, by the warm working treatments for the optimum warm deformation parameters (3% axial tensile plastic deformation at 300°C for 5 min). In addition, it can be seen from Fig. 1 that the room creep strain of steel wires without an axial plastic deformation at 300°C for 5min, is larger than that of steel wires without warm deformation treatments. From the low cycle fatigue experiments, the relationship between number of cycles and cyclic stress amplitude at constant strain amplitude for high carbon patented steel wires and for HSLCLA steel wires have been obtained for different treatment processes, as shown in Fig. 3 and Fig. 4, respectively. It can be seen that the low cycle fatigue lives of both steels were increased by as much as 30 – 35% after the optimum warm deformation treatment. The dislocation density and the sub-grain size of HSLCLA steel wires after warm deformation treatments at various temperatures are shown in Table 4. An internal friction peak for the HSLCLA steel wire is observed around 300°C, Fig. 5. It is well known that the internal friction is related to the interaction between dislocations and the Cottrell atmosphere of solute atoms. The breakaway from the solute atom cloud [7], as well as the concurrent motion of dislocations and the solute atom clouds [8] will all cause the appearance of the internal friction. Thus the internal friction peak round 300°C implies significant solute atom re-dissolution at this temperature. This temperature corresponds exactly to the optimum temperature (300°C) for the warm deformation treatments. When the temperature increases further, the effects of strengthening on the steel wires will be decreased due to recovery effects, such as a reduction in the dislocation density and a rapid growth of the sub -grains (as shown in Table 4). Carbonitrides also will be formed as a result of the precipitation of solute atoms.
Optical and SEM metallography has not provided any direct evidence of crack initiation by inclusions, suggesting that this initiation mechanism is not as dominant as reported for other high-strength steels [9,10]. Moreover, optical and SEM metallography has indicated that the warm deformation treatments did not significantly change the microstructures of the treated steel wires. The microstructural influence on fatigue properties resulting from the redistribution of precipitates, together with the structural uniformity proposed by Lee et al. [11] can not therefore be used to explain the increase in fatigue lives in our research. In addition, since the strength of the steel wires remains constant before and after the warm deformation treatments (see Tables 2 and 3), this is also indirect evidence that there are no significant changes in microstructure. However, it is believed that the micro-states of treated steel wires, such as lattice distortion, internal stress, dislocation density, sub-grain size and the amount of solute atoms in solid solution are changed by the warm deformation treatments. In general, the room temperature creep is a process of micro-amounts of continuous plastic deformation for a long time under a constant stress: this cumulative deformation is the time-dependent part of the strain resulting from stress. Low cycle fatigue is also a process of micro-amounts of cumulative plastic deformation, followed by crack initiation and formation, then crack propagation resulting in fracture after a long time Table 6 Room temperature creep strains of high carbon patented steel wires and HSLCLA steel wires after 3% axial plastic deformation treatments at different time and temperatures Steel
Temperature (°C)
Time (min)
Creep strain
High carbon patented steel wire HSLCLA steel wire High carbon patented steel wire HSLCLA steel wire
300
5
3.55×10−5
300
5
5.0×10−5
380
3
3.76×10−5
380
3
5.63×10−5
Z. Zhao et al. / Journal of Materials Processing Technology 89–90 (1999) 569–573
under a cyclic stress. The difference is that the material is subjected to either a constant or repeated (or fluctuating) loads. Therefore, both creep and low cycle fatigue are closely related in terms of physical mechanisms and that is the reason why we can improve the resistance to both processes at the same time by the warm deformation treatments. The motion of mobile dislocations can be initiated by the axial tensile stress and warm plastic deformation and then the dislocations are re-arranged along the axis and tend to stay at stable states in term of mutual interactions. A recent study [12] has shown that a relatively lower dislocation density (similar to the results after the warm deformation treatments, as shown in Table 4) will have a large effect on the multiplication rate of the mobile fraction of dislocations during stressing of the materials and this will result in a decreased mobile dislocation density at similar percentages of the yield strength. The effects of plastic deformation on the multiplication of dislocations, re-dissolution of solute atoms, formation of Cottrell atmospheres and dislocation pinning will promote further strengthening of the matrix. A comparison of results for creep of materials treated by different percentages of deformation are shown in Table 5. It shows that the application of warm deformation strains is useful in increasing the creep resistance. Of course, necking of the steel wires should be avoided when larger amounts of plastic deformation are applied. The results of Fig. 1 showed that the stabilizing temperature (tempering temperature) is one factor in the combined effects both of temperature and plastic deformation. (see the two curves treated at 300°C, with or without 3% axial plastic deformation). The time of treatment should be matched with the applied temperature in the warm deformation treatment. A comparison of the effects of time and temperature on creep is shown in Table 6. The time must be correspondingly reduced as the temperature is increased. The directional micro-states achieved through warm deformation treatments can only be obtained when the tensile stress and homogeneous plastic deformation are carried out at temperature. Once the tensile stress is unloaded, the micro-states will quickly recover to normal. Therefore, steel wires treated by warm de-
.
573
formation treatments should be unloaded after cooling to room temperature in order to keep a large axial residual compressive stress. In terms of the change of micro-states resulting from the warm deformation treatment, the optimum treatment conditions are not a low temperature for short time, nor a high temperature for long time, because increasing the temperature leads to a dramatic reduction in the dislocation density and a rapid growth of the sub-grains which will decrease the creep resistance.
4. Conclusions The room temperature creep strains of high carbon patented steel wires and high strength low carbon low alloy steel wires were decreased by up to 85 and 65%, respectively, by a warm deformation treatment comprising 3% axial tensile plastic deformation at 300°C for 5 min. For the same warm deformation parameters, the low cycle fatigue lives of both steel wires were increased by 30–35%. It is considered that the improvement in the resistance of high strength steel wires to room temperature creep and low cycle fatigue can be attributed to the reduction of the amount of mobile dislocations through the rearrangement of dislocations and the strengthening of matrix by the re-dissolution of solute atoms and dislocation pinning.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Y.X. Liu, Q.H. Zhu, C. Liu, Steel Struct. 11 (1996) 25. B. Sengupta, Indian Concr. J. 21 (1976) 56. N. Tsuyosh, J. Mater. Eng. 11 (1980) 253. D.G. Evans, R. Bhattacharya, Wire Ind. 39 (1972) 34. L.D. Martin, L. Scoll, ACI Mater. J. 8 (1976) 112. L.R. Chen, Steel Struct. 1 (1986) 132. A.V. Granato, K. Lucke, J. Appl. Phys. 52 (1981) 7136. G. Schoeck, Acta Metall. 11 (1963) 617. Y. Murakami, S. Kodama, S. Konuma, Int. J. Fatigue 11 (1989) 291. [10] S.R. Collins, G.M. Michael, Metall. Trans. A 24 (1993) 2701. [11] C.S. Lee, K.A. Lee, D.M. Li, S.J. Yoo, W.J. Nam, Mater. Sci. Eng. A241 (1998) 30 – 37. [12] A. Oehlert, A. Aterns, Acta Metall. Mater. 42 (1994) 1493–1508.
A new method for improving the resistance of high strength steel wires to room temperature creep and low cycle fatigue Zhenbo Zhao a, Derek O. Northwood a,*,1, Cheng Liu b, Yunxu Liu b a b
Mechanical and Materials Engineering, Uni6ersity of Windsor, Windsor, Ont., N9B 3P4, Canada Department of Materials Engineering, Jilin Institute of Technology, Changchun, 130012 PR China Received 21 October 1998
Abstract The effects of warm deformation treatments on the room temperature creep and the low cycle fatigue resistance of high carbon patented steel wires and high strength low carbon low alloy (HSLCLA) steel wires were studied. The low temperature creep strains of high carbon patented steel wires and high strength low carbon low alloy steel were decreased 85 and 65%, respectively, by the warm working treatments for the optimum warm deformation parameters (3% axial tensile plastic deformation at 300°C for 5 min). The low cycle fatigue lives of both steels were increased by 30 – 35% after warm deformation at the optimum deformation parameters. It is shown that the warm deformation treatment only affected the micro states such as lattice distortion, internal stress, dislocation density, sub-grain size and the amount of solute atoms in solid solution but did not change the overall microstructure or strength of the steel wires. Internal friction studies at different temperatures showed that the amount of solute atoms re-dissolved is different for different warm deformation temperatures and reaches a maximum at 300°C. It is considered that the improvement in the resistance of high strength steel wires to room temperature creep and low cycle fatigue can be attributed to the reduction of the amount of mobile dislocations through the rearrangement of dislocations and strengthening of matrix by the re-dissolution of solute atoms and dislocation pinning. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Warm deformation treatment; Room temperature creep; Low cycle fatigue; High strength steel wire
1. Introduction High strength, low stress relaxation steel wires are attractive for building applications since they can reduce the structure’s weight. Room temperature creep and low cycle fatigue are two types of failure found in these steel wires. Room temperature creep leads to a reduction in the level of the prestress and the low cycle fatigue results in crack initiation and propagation and finally fracture. Thus, there is an incentive to improve the resistance of these steels to these types of failure and thus ensure the reliability and service life of the
* Corresponding author. E-mail address: [email protected] (D.O. Northwood) 1 Also Faculty of Engineering and Applied Science, Ryerson Polytechnic University, 350 Victoria Street, Toronto, Ontario, Canada M5B 2K3.
resulting structures. Although considerable research [1– 5] has been carried out on high strength, low stress relaxation steel wires and commercial products are available [6], comparatively little work has been done on the low cycle fatigue behavior of these steels. The aim of the present investigation was to find an effective method(s) to improve the resistance of two high strength steels to both room temperature creep and low cycle fatigue.
2. Experimental details The chemical composition of the two steels is given in Table 1. The high carbon steel wires with a diameter of ¥= 1.77 mm, were subjected to the following treatment cycle: hot rolling, stretching and patenting at room temperature. HSLA steel wires were made with
0924-0136/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 9 9 ) 0 0 0 1 7 - 5
Z. Zhao et al. / Journal of Materials Processing Technology 89–90 (1999) 569–573
570
Table 1 Chemical composition of steel (wt%) Steel
C
Mn
Si
W
Nb
B
High carbon steel HSLCLA steel
0.64 0.20
0.50 2.34
0.22 0.42
– 0.05
– 0.03
– 0.005
the same diameter, 1.77 mm, by hot rolling, quenching and low temperature tempering, and patenting at room temperature. The warm deformation treatments were performed in a self-manufactured equipment which consisted of power control systems, heat control systems and load control systems for the plastic deformation. The effective length of the steel wire heated in an electric tube furnace is 200 mm. At a given temperature, the 3% homogeneous axial plastic deformation was controlled by the different linear velocity between the gathering wire machine and the releasing wire machine. Deformations were carried out within 5 min and the wires were unloaded after cooling to room temperature. Room temperature creep tests of the steel wires were performed on a R-100 creep test machine, with a load of 80% st (tensile strength) and a temperature control of 9 3°C. In the tests, the steel wires are first subjected to 10% of the total load for 5 min, then to 100% of the total load for 15 min. This is the start point for the strain measurements. The strains were measured by an extensometer. The samples used in the low cycle fatigue test were high carbon patented steel wires (st =1660 MPa, ¥ =1.0 mm) and HSLA steel wires (st =2260 MPa, ¥ =1.0 mm). The experiments were carried out on a PWS-20A fatigue testing machine. The load was chosen such that smax = 0.7st and the load ratio R =smin/ smax =0.57, i.e. for the high carbon patented steel
Fig. 1. Room temperature creep strain of high carbon patented steel wires after warm deformation treatment at different temperatures.
wires: smax = 1160 MPa, smin = 660 MPa and for the HSLA steel wires: smax = 1600 MPa, smin = 915 MPa. The frequency, f, was 2 Hz and a sine wave form was adopted in this experiment. The steel wires were first installed in cross heads of the testing machine using a purpose built fixture and then loaded according to the above-mentioned schedules. Thus, control of constant strain amplitude (here Do = omax − omin) can be realized by means of a transformation from load signal into
Table 2 Mechanical properties of two steels before the warm deformation treatmentsa Steel
st (MPa)
s0.2 (MPa)
c (%)
Number of bending cycles to fracture (bend angle = 180°C)
High carbon patented steel wire (¥ 1.77 mm) HSLCLA steel wire (¥ 1.77 mm)
1660
1560
40.5
19
2100
1970
18.7
13
a
st, tensile strength; s0.2, yield strength at 0.2% offset; c, reduction in area.
Table 3 Mechanical properties of two steels after the warm deformation treatment (300°C)a Steel
st (MPa)
s0.2 (MPa)
c (%)
Number of bending cycles to fracture (bend angle = 180°C)
High carbon patented steel wire (¥ 1.77 mm) HSLCLA steel wire (¥ 1.77 mm)
1663
1571
38.2
18
2119
1988
17.4
12
a
st, tensile strength; s0.2, yield strength at 0.2% offset; c, reduction in area.
Z. Zhao et al. / Journal of Materials Processing Technology 89–90 (1999) 569–573
571
control of constant displacement amplitude (where DV = Vmax −Vmin, V is displacement). The dislocation density and sub-grain size of the high strength low carbon low alloy (HSLCLA) steel wire after warm deformation treatments at various temperature were obtained by X-ray step scanning (the software used to calculate the dislocation density and sub-grain size was developed at Jilin University). Internal friction measurements of HSLCLA steel wire were carried out by a resonance method at temperatures from ambient to 400°C.
Fig. 4. Relationship between number of cycles and cyclic stress amplitude at constant strain amplitude for HSLCLA steel wires. Table 4 Dislocation density and the subgrain size of HSLCLA steel wires after warm deformation treatments at various temperatures Warm deforma- 100 tion temperature (°C) Dislocation density (mm/ mm2) Subgrain size (nm)
1.74E+11
26.8
300
1.26E+11
29.4
400
4.27E+10
46.9
Fig. 2. Room temperature creep strain of HSLCLA steel wires after warm deformation treatment at different temperatures.
Fig. 5. Internal friction spectra Q − 1 (T) and resonance frequency, f 20, as function of temperature for HSLCLA steel wire.
3. Results and discussion
Fig. 3. Relationship between number of cycles and cyclic stress amplitude at constant strain amplitude for high carbon patenting steel wires.
The mechanical properties of the two steels before and after the warm deformation treatments are given in Tables 2 and 3, respectively. The room temperature creep curves of the high carbon patented steels wires and HSLCLA steel wires are shown in Figs. 1 and 2, respectively. The effect of warm deformation tempera-
572
Z. Zhao et al. / Journal of Materials Processing Technology 89–90 (1999) 569–573
Table 5 Room temperature creep strains of high carbon patented steel wires and HSLCLA steel wires before and after warm deformation treatments (300°C) at different axial plastic deformation strains Steel
3% axial plastic deformation 5% axial plastic deformation Without the warm deformation treatments
High carbon patented steel wire HSLCLA steel wire
3.55×10−5
3.1×10−5
2.4×10−4
5.0×10−5
4.7×10−5
1.44×10−4
ture on the room temperature creep behavior of both types of steel wires are similar, in that, the creep strain is gradually reduced as the warm deformation temperature increases, reaches a minimum at 300°C, and then increases as the warm deformation temperature is increased above 300°C. The room temperature creep strains of the high carbon patented steel wires and high strength low carbon low alloy steel were decreased 85 and 65%, respectively, by the warm working treatments for the optimum warm deformation parameters (3% axial tensile plastic deformation at 300°C for 5 min). In addition, it can be seen from Fig. 1 that the room creep strain of steel wires without an axial plastic deformation at 300°C for 5min, is larger than that of steel wires without warm deformation treatments. From the low cycle fatigue experiments, the relationship between number of cycles and cyclic stress amplitude at constant strain amplitude for high carbon patented steel wires and for HSLCLA steel wires have been obtained for different treatment processes, as shown in Fig. 3 and Fig. 4, respectively. It can be seen that the low cycle fatigue lives of both steels were increased by as much as 30 – 35% after the optimum warm deformation treatment. The dislocation density and the sub-grain size of HSLCLA steel wires after warm deformation treatments at various temperatures are shown in Table 4. An internal friction peak for the HSLCLA steel wire is observed around 300°C, Fig. 5. It is well known that the internal friction is related to the interaction between dislocations and the Cottrell atmosphere of solute atoms. The breakaway from the solute atom cloud [7], as well as the concurrent motion of dislocations and the solute atom clouds [8] will all cause the appearance of the internal friction. Thus the internal friction peak round 300°C implies significant solute atom re-dissolution at this temperature. This temperature corresponds exactly to the optimum temperature (300°C) for the warm deformation treatments. When the temperature increases further, the effects of strengthening on the steel wires will be decreased due to recovery effects, such as a reduction in the dislocation density and a rapid growth of the sub -grains (as shown in Table 4). Carbonitrides also will be formed as a result of the precipitation of solute atoms.
Optical and SEM metallography has not provided any direct evidence of crack initiation by inclusions, suggesting that this initiation mechanism is not as dominant as reported for other high-strength steels [9,10]. Moreover, optical and SEM metallography has indicated that the warm deformation treatments did not significantly change the microstructures of the treated steel wires. The microstructural influence on fatigue properties resulting from the redistribution of precipitates, together with the structural uniformity proposed by Lee et al. [11] can not therefore be used to explain the increase in fatigue lives in our research. In addition, since the strength of the steel wires remains constant before and after the warm deformation treatments (see Tables 2 and 3), this is also indirect evidence that there are no significant changes in microstructure. However, it is believed that the micro-states of treated steel wires, such as lattice distortion, internal stress, dislocation density, sub-grain size and the amount of solute atoms in solid solution are changed by the warm deformation treatments. In general, the room temperature creep is a process of micro-amounts of continuous plastic deformation for a long time under a constant stress: this cumulative deformation is the time-dependent part of the strain resulting from stress. Low cycle fatigue is also a process of micro-amounts of cumulative plastic deformation, followed by crack initiation and formation, then crack propagation resulting in fracture after a long time Table 6 Room temperature creep strains of high carbon patented steel wires and HSLCLA steel wires after 3% axial plastic deformation treatments at different time and temperatures Steel
Temperature (°C)
Time (min)
Creep strain
High carbon patented steel wire HSLCLA steel wire High carbon patented steel wire HSLCLA steel wire
300
5
3.55×10−5
300
5
5.0×10−5
380
3
3.76×10−5
380
3
5.63×10−5
Z. Zhao et al. / Journal of Materials Processing Technology 89–90 (1999) 569–573
under a cyclic stress. The difference is that the material is subjected to either a constant or repeated (or fluctuating) loads. Therefore, both creep and low cycle fatigue are closely related in terms of physical mechanisms and that is the reason why we can improve the resistance to both processes at the same time by the warm deformation treatments. The motion of mobile dislocations can be initiated by the axial tensile stress and warm plastic deformation and then the dislocations are re-arranged along the axis and tend to stay at stable states in term of mutual interactions. A recent study [12] has shown that a relatively lower dislocation density (similar to the results after the warm deformation treatments, as shown in Table 4) will have a large effect on the multiplication rate of the mobile fraction of dislocations during stressing of the materials and this will result in a decreased mobile dislocation density at similar percentages of the yield strength. The effects of plastic deformation on the multiplication of dislocations, re-dissolution of solute atoms, formation of Cottrell atmospheres and dislocation pinning will promote further strengthening of the matrix. A comparison of results for creep of materials treated by different percentages of deformation are shown in Table 5. It shows that the application of warm deformation strains is useful in increasing the creep resistance. Of course, necking of the steel wires should be avoided when larger amounts of plastic deformation are applied. The results of Fig. 1 showed that the stabilizing temperature (tempering temperature) is one factor in the combined effects both of temperature and plastic deformation. (see the two curves treated at 300°C, with or without 3% axial plastic deformation). The time of treatment should be matched with the applied temperature in the warm deformation treatment. A comparison of the effects of time and temperature on creep is shown in Table 6. The time must be correspondingly reduced as the temperature is increased. The directional micro-states achieved through warm deformation treatments can only be obtained when the tensile stress and homogeneous plastic deformation are carried out at temperature. Once the tensile stress is unloaded, the micro-states will quickly recover to normal. Therefore, steel wires treated by warm de-
.
573
formation treatments should be unloaded after cooling to room temperature in order to keep a large axial residual compressive stress. In terms of the change of micro-states resulting from the warm deformation treatment, the optimum treatment conditions are not a low temperature for short time, nor a high temperature for long time, because increasing the temperature leads to a dramatic reduction in the dislocation density and a rapid growth of the sub-grains which will decrease the creep resistance.
4. Conclusions The room temperature creep strains of high carbon patented steel wires and high strength low carbon low alloy steel wires were decreased by up to 85 and 65%, respectively, by a warm deformation treatment comprising 3% axial tensile plastic deformation at 300°C for 5 min. For the same warm deformation parameters, the low cycle fatigue lives of both steel wires were increased by 30–35%. It is considered that the improvement in the resistance of high strength steel wires to room temperature creep and low cycle fatigue can be attributed to the reduction of the amount of mobile dislocations through the rearrangement of dislocations and the strengthening of matrix by the re-dissolution of solute atoms and dislocation pinning.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Y.X. Liu, Q.H. Zhu, C. Liu, Steel Struct. 11 (1996) 25. B. Sengupta, Indian Concr. J. 21 (1976) 56. N. Tsuyosh, J. Mater. Eng. 11 (1980) 253. D.G. Evans, R. Bhattacharya, Wire Ind. 39 (1972) 34. L.D. Martin, L. Scoll, ACI Mater. J. 8 (1976) 112. L.R. Chen, Steel Struct. 1 (1986) 132. A.V. Granato, K. Lucke, J. Appl. Phys. 52 (1981) 7136. G. Schoeck, Acta Metall. 11 (1963) 617. Y. Murakami, S. Kodama, S. Konuma, Int. J. Fatigue 11 (1989) 291. [10] S.R. Collins, G.M. Michael, Metall. Trans. A 24 (1993) 2701. [11] C.S. Lee, K.A. Lee, D.M. Li, S.J. Yoo, W.J. Nam, Mater. Sci. Eng. A241 (1998) 30 – 37. [12] A. Oehlert, A. Aterns, Acta Metall. Mater. 42 (1994) 1493–1508.