metals Article
Tensile Fracture Behavior of Progressively-Drawn Pearlitic Steels Jesús Toribio *, Francisco-Javier Ayaso, Beatriz González, Juan-Carlos Matos, Diego Vergara and Miguel Lorenzo Fracture & Structural Integrity Research Group, University of Salamanca, E.P.S., Campus Viriato, Avda. Requejo 33, Zamora 49022, Spain;
[email protected] (F.-J.A.);
[email protected] (B.G.);
[email protected] (J.-C.M.);
[email protected] (D.V.);
[email protected] (M.L.) * Correspondence:
[email protected]; Tel.: +34-980-545-000 (ext. 3673); Fax: +34-980-545-002 Academic Editor: Isabel Gutierrez Received: 31 March 2016; Accepted: 10 May 2016; Published: 17 May 2016
Abstract: In this paper a study is presented of the tensile fracture behavior of progressively-drawn pearlitic steels obtained from five different cold-drawing chains, including each drawing step from the initial hot-rolled bar (not cold-drawn at all) to the final commercial product (pre-stressing steel wire). To this end, samples of the different wires were tested up to fracture by means of standard tension tests, and later, all of the fracture surfaces were analyzed by scanning electron microscopy (SEM). Micro-fracture maps (MFMs) were assembled to characterize the different fractographic modes and to study their evolution with the level of cumulative plastic strain during cold drawing. Keywords: cold drawing; pearlitic steel; steel wires; mechanical properties; tensile fracture; fractographic analysis; anisotropic behavior
1. Introduction Pearlitic steels are widely used in a high variety of applications in engineering, e.g., pre-stressing steel wires [1,2], railway rails [3–5], rock bolts [6], steel cord wires (tire reinforcement) [7], music wires [8], etc. The wide use of these types of steels is due to their excellent mechanical properties induced in the raw material during the mechanical conforming process, e.g., hot rolling for complex shapes or wire drawing in the case of cylindrical bars. As a result of the plastic strains undergone by the material during the latter process, the yield stress is high enough [9] for considering it as a high strength steel. The today importance of these steels in industry can be quantified in terms of the world production of drawn wires of pearlitic steels which can be estimated in 25 million tons per year [10]. In addition, the interest of the scientific community on these research areas is also very high, as indicated by the number of research papers published up to date in multiple journals indexed in the SCOPUS database (Figure 1). Consequently, from a pure scientific point of view, increasing the knowledge about pearlitic steel wires is a quite interesting issue. In a common commercial wire drawing chain, wires undergo a progressive reduction of their cross-sectional area during several steps (usually 6–8 passes) [11]. As a consequence of material hardening due to huge plastic strains in the wire, key microstructural changes are caused in the steels as was reported in many studies [12–19]. The analyses of such changes revealed a progressive increment of the microstructural anisotropy as the drawing degree is increased. These microstructural changes also affect the macroscopic mechanical behavior of the material [20–22], as in their fatigue and fracture behavior [23–27]. This paper goes further in the research, so that a study is presented of the tensile fracture behavior of progressively-drawn pearlitic steels obtained from five different real cold drawing chains, including each drawing step from the initial hot-rolled bar (not cold-drawn at all) to the final commercial product (pre-stressing steel wire). Metals 2016, 6, 114; doi:10.3390/met6050114
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Number of papers
chains, including each drawing step from the initial hot‐rolled bar (not cold‐drawn at all) to the final commercial product (pre‐stressing steel wire). 8 10
3
7 10
3
6 10
3
5 10
3
4 10
3
3 10
3
2 10
3
1 10
3
0
Pearlitic Steel
Cold Drawing
Wire Drawing
Pearlite
Keyword
Figure 1. Number of research papers indexed in Scopus related with the following keywords: pearlitic Figure 1. Number of research papers indexed in Scopus related with the following keywords: pearlitic steel, cold drawing, wire drawing, pearlite. (Data collected on February 2016). steel, cold drawing, wire drawing, pearlite. (Data collected on February 2016).
For this purpose, standard tension tests up to final fracture were carried out for each one of the For this purpose, standard tension tests up to final fracture were carried out for each one of wires corresponding to each step of the five commercial wire drawing chains considered in this study. the wires corresponding to each step of the five commercial wire drawing chains considered in this On one hand, results of testing allow one to obtain the mechanical properties—material yield strength study. On one hand, results of testing allow one to obtain the mechanical properties—material yield (σY) and ultimate tensile strength (UTS, σR)—of each one of the wires tested from the material strength (σY ) and ultimate tensile strength (UTS, σR )—of each one of the wires tested from the material constitutive law (stress‐strain curve). This mechanical characterization presents a high interest not constitutive law (stress-strain curve). This mechanical characterization presents a high interest not only only from the scientific point of view, but also from the educational viewpoint [28]. On the other hand, from the scientific point of view, but also from the educational viewpoint [28]. On the other hand, the the quantitative fractographic analysis (post mortem) of the fracture surfaces of tested samples, quantitative fractographic analysis (post mortem) of the fracture surfaces of tested samples, obtained obtained by scanning electron microscopy (SEM), allows one to reveal the microscopic fracture maps by scanning electron microscopy (SEM), allows one to reveal the microscopic fracture maps (MFMs) (MFMs) and the fracture micro‐mechanisms. From these analyses, the evolution of both mechanical and the fracture micro-mechanisms. From these analyses, the evolution of both mechanical properties properties and fracture behavior with drawing will be revealed for different wire drawing processes, and fracture behavior with drawing will be revealed for different wire drawing processes, thereby thereby achieving a better understanding of the mechanical behavior of these key types of steels. achieving a better understanding of the mechanical behavior of these key types of steels. 2. Experimental Program 2. Experimental Program Cylindrical samples of the wires obtained at the end of each drawing step of the five commercial Cylindrical samples of the wires obtained at the end of each drawing step of the five commercial manufacturing processes were used for testing. Each one of the families of progressively‐drawn steels manufacturing processes were used for testing. Each one of the families of progressively-drawn steels was labelled with a capital letter (A, B, C, D and E). In addition, to identify each particular steel was labelled with a capital letter (A, B, C, D and E). In addition, to identify each particular steel (member of the family), a number was added representing the number of drawing steps undergone (member of the family), a number was added representing the number of drawing steps undergone by by the specific wire. Thus, as a way of example, steel E4 corresponds to a wire obtained after the the specific wire. Thus, as a way of example, steel E4 corresponds to a wire obtained after the fourth fourth drawing step of family E. According to Table 1, the chemical composition of the five families drawing step of family E. According to Table 1, the chemical composition of the five families of steels of steels is similar with slight variations. is similar with slight variations. Table 1. Chemical composition of the five families of steels. Table 1. Chemical composition of the five families of steels.
Element
Element
% C %C % Mn % Mn % Si % Si % P %P % S %S % Al % Al % Cr % Cr %V % V
A A 0.800 0.800 0.690 0.690 0.230 0.230 0.012 0.012 0.009 0.009 0.004 0.004 0.265 0.265 0.060 0.060
Family of Steels B C D E B C 0.789 0.790 D0.795 E 0.789 0.789 0.698 0.790 0.670 0.795 0.624 0.7890.681 0.698 0.226 0.670 0.200 0.624 0.224 0.6810.210 0.226 0.200 0.224 0.210 0.011 0.009 0.011 0.010 0.011 0.009 0.011 0.010 0.005 0.009 0.009 0.008 0.008 0.0080.008 0.005 0.003 0.003 0.003 0.003 0.003 0.0030.003 0.003 0.071 0.071 0.187 0.187 0.164 0.164 0.2180.218 0.078 0.053 0.064 0.078 0.053 0.064 0.0610.061 Family of Steels
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Each one of the raw materials for each family undergoes different straining paths during the manufacturing process by cold drawing. Family A is forced to pass through six drawing dies, whereas the wire drawing of the other families (B, C, D and E) is divided into seven drawing steps. Table 2 summarizes each one of the five cold drawing procedures (straining paths or yielding histories) in terms of the wire diameter at the end of each drawing step. Table 2. Diameter of the wires at the end of each drawing step for the five families. Wire Diameter (mm)
Drawing Step 0 1 2 3 4 5 6 7
Family A
Family B
Family C
Family D
Family E
12.11 10.80 9.81 8.94 8.22 7.56 6.98 -
12.10 11.23 10.45 9.68 9.02 8.54 8.18 7.00
10.44 9.52 8.49 7.68 6.95 6.36 5.86 5.03
8.56 7.78 6.82 6.17 5.61 5.08 4.63 3.97
11.03 9.90 8.95 8.21 7.49 6.80 6.26 5.04
The cumulative plastic strain εP represents the drawing degree [23], and is defined as follows: εP “ 2ln
φ0 φi
(1)
where φ0 is the hot-rolled bar diameter (not cold drawn at all) and φi is the diameter of a wire undergoing i drawing steps. Results for the different drawn wires are given in Table 3. Table 3. Cumulative plastic strain of the progressively drawn steels. εP
Drawing Step 0 1 2 3 4 5 6 7
Family A
Family B
Family C
Family D
Family E
0 0.229 0.421 0.607 0.775 0.942 1.102 -
0 0.149 0.293 0.446 0.588 0.697 0.800 1.095
0 0.184 0.414 0.614 0.814 0.991 1.155 1.460
0 0.191 0.454 0.655 0.845 1.044 1.229 1.537
0 0.216 0.418 0.591 0.774 0.967 1.133 1.566
Standard tension tests were performed up to final fracture. Three tests were made for each drawing step (thus, as many as 117 standard tension tests were performed in the mechanical characterization, a huge collection of statistical quantitative data). 3. Microstructure of the Progressively-Drawn Wires The microstructure of the progressively-drawn wires is given in Figures 2–5 (the vertical side of longitudinal sections always corresponds to the wire axis). They show a progressive slenderizing of the pearlitic colonies and an increase of packing closeness (with decrease of pearlite interlamellar spacing). In addition, a progressive orientation (in the drawing direction) of both pearlitic colonies (first microstructural level) and ferrite/cementite lamellae (second microstructural level) can be observed. All these observations are fully consistent with previous research in similar steels [12–15].
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(B0)
(B4)
(B1)
(B5)
(B2)
(B6)
(B3)
(B7)
Figure 2. Metallographic analysis of the longitudinal section of family B for diverse drawing steps. Figure 2. Metallographic analysis of the longitudinal section of family B for diverse drawing steps.
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(B0)
(B4)
(B1)
(B5)
(B2)
(B6)
(B3)
(B7)
Figure 3. Metallographic analysis of the transverse section of family B for diverse drawing steps. Figure 3. Metallographic analysis of the transverse section of family B for diverse drawing steps.
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(E0)
(E4)
(E1)
(E5)
(E2)
(E6)
(E3)
(E7)
Figure 4. Metallographic analysis of the longitudinal section of family E for diverse drawing steps. Figure 4. Metallographic analysis of the longitudinal section of family E for diverse drawing steps.
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(E0)
(E4)
(E1)
(E5)
(E2)
(E6)
(E3)
(E7)
Figure 5. Metallographic analysis of the transverse section of family E for diverse drawing steps. Figure 5. Metallographic analysis of the transverse section of family E for diverse drawing steps.
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Metals 2016, 6, 114 4. Mechanical Behavior of the Progressively-Drawn Wires
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Conventional standard tension tests up to final fracture under a constant displacement rate of 4. Mechanical Behavior of the Progressively‐Drawn Wires 2 mm/minConventional standard tension tests up to final fracture under a constant displacement rate of 2 were carried out using cylindrical samples of 300 mm length for each one of the drawing steps and the five families of steels used in the experimental program. Such a length was selected mm/min were carried out using cylindrical samples of 300 mm length for each one of the drawing following the recommendation of a previous study [29]. Figure 6 shows the obtained master curves steps and the five families of steels used in the experimental program. Such a length was selected (stressfollowing the recommendation of a previous study [29]. Figure 6 shows the obtained master curves vs. strain curves) as results of testing for each drawing step of the considered steels. Furthermore, (stress strain curves) results of testing properties for each drawing step of the considered steels. Table 4 showsvs. the values of theas main mechanical of the previous steels. Furthermore, Table 4 shows the values of the main mechanical properties of the previous steels. 2.0
2.0
1.5
1.5
1.0
(GPa)
(GPa)
A6 A5 A4 A3 A2 A1 A0
0.5
B7 B6 B5 B4 B3 B2 B1 B0
1.0
0.5
0.0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08
0.0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08
(a)
2.0
2.0
1.5 (GPa)
1.5 (GPa)
(b)
C7 C6 C5 C4 C3 C2 C1 C0
1.0
0.5
D7 D6 D5 D4 D3 D2 D1 D0
1.0
0.5
0.0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08
0.0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08
(c)
(d) 2.0
(GPa)
1.5
E7 E6 E5 E4 E3 E2 E1 E0
1.0
0.5
0.0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08
(e)
Figure 6. Stress-strain curves of the progressively-drawn steels: (a) family A with six drawing degrees; (b) family B with seven drawing degrees; (c) family C with seven drawing degrees; (d) family D with seven drawing degrees; and (e) family E with seven drawing degrees.
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Table 4. Main mechanical properties for each drawing step of the considered families. Steel
E (GPa)
σY (GPa)
σR (GPa)
εR
1.27 1.29 1.45 1.52 1.58 1.65 1.84
0.076 0.018 0.030 0.028 0.026 0.022 0.054
1.27 1.34 1.37 1.43 1.49 1.55 1.58 1.84
0.066 0.057 0.061 0.052 0.042 0.038 0.035 0.052
1.23 1.27 1.36 1.42 1.50 1.58 1.64 1.91
0.066 0.061 0.046 0.047 0.043 0.045 0.042 0.051
1.23 1.32 1.42 1.49 1.55 1.63 1.69 1.88
0.072 0.056 0.038 0.045 0.048 0.051 0.047 0.057
1.23 1.28 1.36 1.41 1.50 1.60 1.62 1.83
0.068 0.056 0.049 0.055 0.049 0.048 0.043 0.059
Family A A0 A1 A2 A3 A4 A5 A6
194 201 187 190 190 195 207
0.72 1.10 1.12 1.18 1.26 1.33 1.57 Family B
B0 B1 B2 B3 B4 B5 B6 B7
202 204 204 203 203 201 201 205
0.72 0.84 0.88 0.95 1.01 1.09 1.12 1.58 Family C
C0 C1 C2 C3 C4 C5 C6 C7
203 199 201 204 204 204 204 208
0.69 0.78 0.90 0.97 1.06 1.14 1.23 1.65 Family D
D0 D1 D2 D3 D4 D5 D6 D7
194 192 189 194 200 202 202 206
0.68 0.84 0.99 1.00 1.07 1.16 1.25 1.65 Family E
E0 E1 E2 E3 E4 E5 E6 E7
199 192 194 192 196 199 200 208
0.72 0.83 0.91 0.93 1.02 1.13 1.16 1.49
E: Young modulus; σY : yield strength; σR : ultimate tensile strength (UTS); εR : strain at maximum load.
From such curves (Figure 6), the mechanical properties can be determined and, thus, the evolution of yield strength σY (Figure 7a) and ultimate tensile strength σR (Figure 7b) with the drawing degree is revealed in terms of cumulative plastic strain (εP ).
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From such curves (Figure 6), the mechanical properties can be determined and, thus, the 10 of 18 10 of 18 evolution of yield strength σY (Figure 7a) and ultimate tensile strength σR (Figure 7b) with the P). drawing degree is revealed in terms of cumulative plastic strain (ε From such curves (Figure 6), the mechanical properties can be determined and, thus, the
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evolution of yield strength σY (Figure 7a) and ultimate tensile strength σR (Figure 7b) with the drawing degree is revealed in terms of cumulative plastic strain (εP).
(a)
(b)
Figure 7. Mechanical properties evolution with cold drawing, for each one of the five pearlitic families Figure 7. Mechanical (a) properties evolution with cold drawing, for each one of the (b) five pearlitic families of steels: (a) yield strength; (b) ultimate tensile strength (UTS). of steels: (a) yield strength; (b) ultimate tensile strength (UTS). Figure 7. Mechanical properties evolution with cold drawing, for each one of the five pearlitic families As Figure 7a,b clearly show, a quasi‐linear growing trend does exist in the yield strength and of steels: (a) yield strength; (b) ultimate tensile strength (UTS).
As Figure 7a,b clearly show, a quasi-linear growing trend does exist in the yield strength and UTS UTS with cumulative plastic strain during drawing. Therefore, a linear dependence seems to exist with cumulative strainand during linear to dependence seems to exist between between both plastic parameters the drawing. drawing Therefore, degree. In a order reveal such a dependence, a As Figure 7a,b clearly show, a quasi‐linear growing trend does exist in the yield strength and mathematical fitting (Figure 8) was applied to the data included in Figure 7a,b. The last step of wire both parameters and the drawing degree. In order to reveal such a dependence, a mathematical UTS with cumulative plastic strain during drawing. Therefore, a linear dependence seems to exist drawing was not considered, since manufacturing companies used to apply a special heat treatment fitting (Figure 8) was applied and to the data included in Figure 7a,b. to Thereveal last step of a wire drawing was between both parameters the drawing degree. In order such dependence, a notafter the last drawing step for relieving the residual stresses induced by manufacturing. Thus, these considered, since manufacturing companies used to apply a special heat treatment after the last mathematical fitting (Figure 8) was applied to the data included in Figure 7a,b. The last step of wire equations mathematically represent the increment of yield strength and the UTS of pearlitic steels drawing was not considered, since manufacturing companies used to apply a special heat treatment drawing step for relieving the residual stresses induced by manufacturing. Thus, these equations due to the strain hardening mechanism. could useful for obtaining simple after the last drawing step for relieving the residual stresses induced by manufacturing. Thus, these mathematically represent the increment ofThese yield equations strength and thebe UTS of pearlitic steelsa due to the estimation of the material strength after a given straining process. equations mathematically represent the increment of yield strength and the UTS of pearlitic steels strain hardening mechanism. These equations could be useful for obtaining a simple estimation of the due to strength the strain hardening mechanism. These equations could be useful for obtaining a simple material after a given straining process. estimation of the material strength after a given straining process.
Figure 8. Mathematical fitting of the evolution of the mechanical properties (yield strength and UTS) with cumulative plastic strain of cold‐drawn wires. Figure 8. Mathematical fitting of the evolution of the mechanical properties (yield strength and UTS) Figure 8. Mathematical fitting of the evolution of the mechanical properties (yield strength and UTS) with cumulative plastic strain of cold-drawn wires.
5. Fractographic Analysis with cumulative plastic strain of cold‐drawn wires.
Fracture surfaces were observed by means of SEM. The fracture surface obtained after tensile 5. Fractographic Analysis testing of each drawing step of steel families B and E are shown in Figures 9 and 10, respectively. For 5. Fractographic Analysis each set of three tests (of the total amount of 117) a representative fracture surface was considered in Fracture surfaces were observed by means of SEM. The fracture surface obtained after tensile Fracture surfaces were observed of SEM. The fracture surface obtained tensile the fractographic analysis. According by to means the MFMs, the slightly‐drawn steels exhibit an after isotropic testing of each drawing step of steel families B and E are shown in Figures 9 and 10, respectively. For testing of each drawing step of steel families B and E are shown in Figures 9 and 10 respectively. fracture behavior and a smooth fracture surface at the macroscopic scale. As the drawing degree each set of three tests (of the total amount of 117) a representative fracture surface was considered in Forthe each set of three analysis. tests (of the total amount 117) a the representative fracture surface was fractographic According to the ofMFMs, slightly‐drawn steels exhibit an considered isotropic in the fractographic analysis. According tosurface the MFMs, the slightly-drawn exhibit an isotropic fracture behavior and a smooth fracture at the macroscopic scale. steels As the drawing degree fracture behavior and a smooth fracture surface at the macroscopic scale. As the drawing degree increases the mechanical behavior is modified (σY and σR increase, Figure 8, and the fracture behavior,
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too, showing a more irregular fractography with numerous peaks and valleys (strength anisotropy) as a consequence of the progressive microstructural orientation after manufacture. Metals 2016, 6, 114 11 of 18
(B0)
(B4)
(B1)
(B5)
(B2)
(B6)
(B3)
(B7)
Figure 9. Evolution with drawing of MFMs of drawn wires corresponding to the family B. Figure 9. Evolution with drawing of MFMs of drawn wires corresponding to the family B.
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(E0)
(E4)
(E1)
(E5)
(E2)
(E6)
(E3)
(E7)
Figure 10. Evolution with drawing of MFMs of drawn wires corresponding to the family E. Figure 10. Evolution with drawing of MFMs of drawn wires corresponding to the family E.
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In all steels, fracture initiates in a central fibrous region formed by micro-void coalescence (MVC), Metals 2016, 6, 114 13 of 18 this domain representing the fracture process zone (FPZ). In addition to the central zone of the wire all steels, fracture initiates in appears a central region formed micro‐void coalescence with In a fibrous aspect, ductile fracture infibrous the form of MVC at theby periphery of all the wires (MVC), this domain representing the fracture process zone (FPZ). In addition to the central zone of tested as an external ring (shear lip). The micro-void size at the central zone is higher than that the wire with a fibrous aspect, ductile fracture appears in the form of MVC at the periphery of all the observed in the external ring. In moderately-drawn wires, an intermediate zone between the external wires tested as an external ring (shear lip). The micro‐void size at the central zone is higher than that ring and the central zone was identified with a mixed fractography of MVC and cleavage (C). Radial observed in the external ring. In moderately‐drawn wires, an intermediate zone between the external marks in the direction from the central zone to the external ring are observed. Therefore, fracture was ring and the central zone was identified with a mixed fractography of MVC and cleavage (C). Radial initiated at the wire centre (central fibrous zone by MVC) and later it propagated in radial direction marks in the direction from the central zone to the external ring are observed. Therefore, fracture was towards the wire periphery. As the drawing degree increases, the area covered by brittle fracture initiated at the wire centre (central fibrous zone by MVC) and later it propagated in radial direction (cleavage zone) decreases, appearing MVC instead of C. Thus, in heavily drawn wires the observed towards the wire periphery. As the degree increases, the area covered by brittle fracture MFM is constituted in the majority ofdrawing cases only by MVC. (cleavage zone) decreases, appearing MVC instead of C. Thus, in heavily drawn wires the observed Attention should be paid to certain exceptions to the previously depicted common trend. MFM is constituted in the majority of cases only by MVC. For instance, the fibrous zone in the specimen E0 (Figure 10) is placed close to the external ring Attention should be paid to certain exceptions to the previously depicted common trend. For (peripheral fracture origin). Probably this behavior is caused by surface damage generated in the instance, the in the specimen E0 (Figure 10) is placed close to the external ring hot-rolled steelfibrous during zone storage. (peripheral fracture origin). Probably this behavior is caused by surface damage generated in the hot‐ MFMs were assembled from the SEM micrographs. Later, a quantitative fractographic analysis of rolled steel during storage. the fracture surfaces was carried out by a commercial image analysis software, paying special attention to theMFMs were assembled from the SEM micrographs. Later, a quantitative fractographic analysis following fracture features (Figure 11): of the fracture surfaces was carried out by a commercial image analysis software, paying special ‚attention to the following fracture features (Figure 11): Total fracture surface (SF ). ‚ Surface of the FPZ (SFPZ ). Total fracture surface (S F). ‚ Radius of the FPZ (r ). Surface of the FPZ (SFPZ FPZ). Radius of the FPZ (r FPZcrown ). ‚ Surface of the external formed by MVC (SEC ). ‚ Surface of the external crown formed by MVC (S EC). Mean or average depth (xm ), maximum depth (xmax ) and minimum depth (xmin ) of the external Mean or average depth (x crown formed by MVC. m), maximum depth (xmax) and minimum depth (xmin) of the external crown formed by MVC.
xmin
SFPZ
SEC
rFPZ
R
xmax
Figure 11. 11. Scheme fracture surfaces surfaces in in the the quantitative quantitative Figure Scheme of of the the parameters parameters used used for for defining defining the the fracture fractographic analysis. fractographic analysis.
An indicator of the steel ductility (maximum plastic strain undergone by the material up to An indicator of the steel ductility (maximum plastic strain undergone by the material up to fracture) is the reduction of the cross sectional area Z. This parameter can be obtained from the initial fracture) is the reduction of the cross sectional area Z. This parameter can be obtained from the initial area of the specimens (S0) and the final area after fracture (SF) as follows: area of the specimens (S0 ) and the final area after fracture (SF ) as follows: ˆ S0 SF˙ Z(%) S0 ´ (2) S 100 Zp%q “ S0 F ˆ 100 (2) S0 The value of the areas of the FPZ (SFPZ) and the external crown (SEC) were measured in terms of the whole fracture area SF. Accordingly, the radius of the FPZ (rFPZ) and the average depth of the external ring (xm) were obtained in terms of the radius R of the fractured area by means of the following equations:
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The value of the areas of the FPZ (SFPZ ) and the crown (SEC ) were measured in terms S external SFPZ (%) FPZ 100 of the whole fracture area SF . Accordingly, the radius of the FPZ (rFPZ ) and the average depth(3) of SF the external ring (xm ) were obtained in terms of the radius R of the fractured area by means of the following equations: ˆ SEC ˙ SEC (%) SFPZ (4) 100 SFPZ p%q “ SF ˆ 100 (3) SF ˆ ˙ SEC r ˆ 100 SEC p%q “ (4) rFPZ (%) SFPZ (5) F 100 R ¯ ´r (5) rFPZ p%q “ FPZ ˆ 100 R x ¯ ´ xm (%) xmm 100 (6) (6) xm p%q “ R ˆ 100 R 6. Discussion 6. Discussion The evolution of the reduction of the cross sectional area Z with cumulative plastic strain caused The evolution of the reduction of the cross sectional area Z with cumulative plastic strain caused by wire drawing for the five drawn steels is shown in Figure 12. Two trends with the drawing degree by wire drawing for the five drawn steels is shown in Figure 12. Two trends with the drawing degree are observed. On one hand, Z increases with the drawing degree for slightly drawn steels. On the are observed. On one hand, Z increases with the drawing degree for slightly drawn steels. On the other hand, for heavily-drawn steels the trend is the opposite, i.e., Z decreases with the drawing degree. other hand, for trend the drawing opposite, even i.e., Z with drawing Surprisingly, forheavily‐drawn the steel type A,steels Z stillthe rises withis cold in decreases the last step, justthe where such degree. Surprisingly, for the steel type A, Z still rises with cold drawing even in the last step, just a variable decreases in the other steels. This behavior can be explained on the basis of cumulative where such a variable decreases in the other steels. This behavior can be explained on the basis of plastic strain εP and the microstructural orientation inside the steels. cumulative plastic strain εP and the microstructural orientation inside the steels.
Figure 12. Evolution of the reduction of the cross sectional area with cold drawing. Figure 12. Evolution of the reduction of the cross sectional area with cold drawing.
Mainly, two key consequences appear at the microstructural level in steels as they are drawn: (i) Mainly, two key consequences appear at the microstructural level in steels as they are the accumulation of plastic of strains and increment of material strength strength (cold work), and (ii) a drawn: (i) the accumulation plastic strains and increment of material (cold work), and microstructure orientation in the wire axial direction. In hot rolled bar (not cold drawn at all), pearlitic (ii) a microstructure orientation in the wire axial direction. In hot rolled bar (not cold drawn at all), colonies (micro‐composed of alternated ferrite ferrite and and cementite lamellae) are randomly pearlitic colonies (micro-composed of alternated cementite lamellae) are randomlyoriented. oriented. However, as the drawing degree increases, the pearlitic colonies are progressively oriented in cold the However, as the drawing degree increases, the pearlitic colonies are progressively oriented in the cold drawing direction [13] and the ferrite/cementite lamellae do the same [15], both microstructural drawing direction [13] and the ferrite/cementite lamellae do the same [15], both microstructural levels levels becoming quasi‐aligned with the wire axis or cold drawing direction in the commercial pre‐ becoming quasi-aligned with the wire axis or cold drawing direction in the commercial pre-stressing stressing wires (final stages of manufacture). The plastic cumulative strain at the end of the steel wiressteel (final stages of manufacture). The cumulative strainplastic at the end of the drawing chain, drawing chain, i.e., that necessary for the full reorientation in axial direction of both microstructural i.e., that necessary for the full reorientation in axial direction of both microstructural levels (hierarchical levels (hierarchical structures of pearlitic colonies and ferrite/cementite lamellae) ranges between 1.1 structures of pearlitic colonies and ferrite/cementite lamellae) ranges between 1.1 and 1.6 for the and 1.6 for the whole set of steel families analyzed in the present paper. This interval fully agrees whole set of steel families analyzed in the present paper. This interval fully agrees with the value with value of reported literature in the scientific [30,31], of plastic the cumulative plastic for strain of 1.5,the reported in 1.5, the scientific [30,31],literature of the cumulative strain necessary the necessary for the aforementioned completion of reorientation. aforementioned completion of reorientation. Once the microstructure (colonies and lamellae) are completely oriented in the drawing direction [13,15], the percentage of reduction of the cross sectional area Z begins to decrease,
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Once the microstructure (colonies and lamellae) are completely oriented in the drawing direction [13,15], the percentage of reduction of the cross sectional area Z begins to decrease, Metals 2016, 6, 114 15 of 18 Metals 2016, 6, 114 15 of 18 independently of the plastic strain increment (e.g., step 6 to 7 in family types B, C, D and E). The Z reduction is linked with the strong decrease of the interlamellar spacing caused during the independently of the plastic strain increment (e.g., step 6 to 7 in family types B, C, D and E). The Z independently of the plastic strain increment (e.g., step 6 to 7 in family types B, C, D and E). The Z microstructure re-orientation at the final stages of drawing [14] and the dislocation density rises in the reduction reduction is is linked linked with with the the strong strong decrease decrease of of the the interlamellar interlamellar spacing spacing caused caused during during the the ferrite lamellae, thus increasing the microstructural packing closeness and reducing the free path for microstructure re‐orientation at the final stages of drawing [14] and the dislocation density rises in microstructure re‐orientation at the final stages of drawing [14] and the dislocation density rises in dislocation movement during strain hardening of the progressively drawn steels. the ferrite lamellae, thus increasing the microstructural packing closeness and reducing the free path the ferrite lamellae, thus increasing the microstructural packing closeness and reducing the free path The FPZ (fibrous zone) was characterized by means of its size SFPZ represented in Figure 13, for dislocation movement during strain hardening of the progressively drawn steels. for dislocation movement during strain hardening of the progressively drawn steels. and by its characteristic length rFPZ shown in Figure 14. As a common trend, in all analyzed steel The FPZ (fibrous zone) was characterized by means of its size S The FPZ (fibrous zone) was characterized by means of its size SFPZ FPZ represented in Figure 13, and represented in Figure 13, and families the two parameters decrease with cold drawing. Figure 15 shows the evolution with the by its characteristic length r by its characteristic length rFPZ FPZ shown in Figure 14. As a common trend, in all analyzed steel families shown in Figure 14. As a common trend, in all analyzed steel families drawing degree of the area of the external crown SEC . It increases up to the second step of the cold the the two two parameters parameters decrease decrease with with cold cold drawing. drawing. Figure Figure 15 15 shows shows the the evolution evolution with with the the drawing drawing drawing process and then continuously decreases as εP rises (with certain exceptions in the last degree of the area of the external crown S EC . It increases up to the second step of the cold drawing degree of the area of the external crown SEC. It increases up to the second step of the cold drawing step). With regard to its average depth (Figure 16), similar results were obtained for the different process and then continuously decreases as ε process and then continuously decreases as εPP rises (with certain exceptions in the last step). With rises (with certain exceptions in the last step). With steel families. regard to its average depth (Figure 16), similar results were obtained for the different steel families. regard to its average depth (Figure 16), similar results were obtained for the different steel families. 50 50 A A B B C C D D E E
FPZ
S FPZ (%)
40 40 30 30 20 20 10 10 00 0.0 0.0
0.2 0.2
0.4 0.4
0.6 0.6
0.8 0.8 p
p
1.0 1.0
1.2 1.2
1.4 1.4
1.6 1.6
Figure 13. Evolution of the area of the FPZ with the drawing degree. Figure 13. Evolution of the area of the FPZ with the drawing degree. Figure 13. Evolution of the area of the FPZ with the drawing degree.
50 50 A A B B C C D D E E
FPZ
r (%) FPZ
40 40 30 30 20 20 10 10
00 0.0 0.0 0.2 0.2 0.4 0.4 0.6 0.6 0.8 0.8 1.0 1.0 1.2 1.2 1.4 1.4 1.6 1.6 p
p
Figure 14. Evolution of radius of the FPZ with the drawing degree. Figure 14. Evolution of radius of the FPZ with the drawing degree. Figure 14. Evolution of radius of the FPZ with the drawing degree.
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Figure 15. Evolution of the area of the external crown with the drawing degree. Figure 15. Evolution of the area of the external crown with the drawing degree. Figure 15. Evolution of the area of the external crown with the drawing degree. 100 100 AA BB CC DD EE
60 60
m
(%) xxm (%)
80 80
40 40 20 20 00 0.0 1.0 0.0 0.2 0.2 0.4 0.4 0.6 0.6 0.8 0.8 1.0 1.2 1.2 1.4 1.4 1.6 1.6 p
p
Figure 16. Evolution of the average depth of the external crown with the drawing degree. Figure 16. Evolution of the average depth of the external crown with the drawing degree. Figure 16. Evolution of the average depth of the external crown with the drawing degree.
7. Conclusions 7. Conclusions 7. Conclusions The fracture surfaces of samples corresponding to very diverse steels, with different cumulative The fracture surfaces of samples corresponding to very diverse steels, with different cumulative The fracture surfaces of samples corresponding to very diverse steels, with different cumulative plastic strain, were studied by means of scanning electronic microscope (SEM) and image analysis plastic strain, were studied by means of scanning electronic microscope (SEM) and image analysis plastic strain, were studied by means of scanning electronic microscope (SEM) and image analysis techniques. In all steels a similar fracture behavior was observed: techniques. In all steels a similar fracture behavior was observed: techniques. In all steels a similar fracture behavior was observed: The fracture initiates at zone of each wire which the fracture The fracture fracture initiates at the the central central zone each wire in in fracture microscopic microscopic ‚ The initiates at the central zone of eachof wire in which thewhich fracturethe microscopic topography topography may be classified as micro‐void coalescence (MVC) with fibrous aspect and topography may be classified as micro‐void coalescence (MVC) with fibrous and may be classified as micro-void coalescence (MVC) with fibrous aspect and propagatesaspect in a radial propagates in a radial direction through the intermediate zone up to reaching the external ring. propagates in a radial direction through the intermediate zone up to reaching the external ring. direction through the intermediate zone up to reaching the external ring. The intermediate zone shows fractography by cleavage and MVC. The fracture surface by The intermediate intermediate zone zone shows shows aa a fractography fractography by by cleavage cleavage and and MVC. MVC. The The fracture fracture surface surface by by ‚ The cleavage diminishes in favor of MVC in the last steps of the manufacturing process by cold cleavage diminishes in favor of MVC in the last steps of the manufacturing process by cold cleavage diminishes in favor of MVC in the last steps of the manufacturing process by cold drawing, so that the fracture process becomes more ductile as the drawing degree increases. drawing, so that the fracture process becomes more ductile as the drawing degree increases. drawing, so that the fracture process becomes more ductile as the drawing degree increases. Another important characteristic of such an intermediate zone is the presence of radial cracks Another important characteristic of such an intermediate zone is the presence of radial cracks ‚ Another important characteristic of such an intermediate zone is the presence of radial cracks oriented quasi‐parallel to the drawing axis or cold drawing direction, embryos of anisotropic oriented quasi‐parallel to the drawing axis or cold drawing direction, embryos of anisotropic oriented quasi-parallel to the drawing axis or cold drawing direction, embryos of anisotropic fracture behavior as a consequence of manufacture‐induced microstructural orientation. fracture behavior as a consequence of manufacture‐induced microstructural orientation. fracture behavior as a consequence of manufacture-induced microstructural orientation. With regard to the percentage values of the different parts in the fracture surface, the evolution With regard to the percentage values of the different parts in the fracture surface, the evolution With regard to the percentage values of of the different parts in the fracture surface, the evolution ‚ with with cold cold drawing drawing of of the the central central zone zone of the the fibrous fibrous aspect aspect (fracture (fracture process process zone zone or or FPZ) FPZ) is is with cold drawing of the central zone of the fibrous aspect (fracture process zone or FPZ) is similar similar in all analyzed steels and the same happens in the matter of the external ring (shear lip). similar in all analyzed steels and the same happens in the matter of the external ring (shear lip). in all analyzed steels and the same happens in the matter of the external ring (shear lip).
Acknowledgments: The authors wish to acknowledge the financial support provided by the following Spanish Acknowledgments: The authors wish to acknowledge the financial support provided by the following Spanish Institutions: Ministry for Science and Technology (MICYT; Grant MAT2002‐01831), Ministry for Education and Institutions: Ministry for Science and Technology (MICYT; Grant MAT2002‐01831), Ministry for Education and Acknowledgments: The authors wish to acknowledge the financial support provided by the following Spanish Science Grant BIA2005‐08965), Ministry Science and Grant Science (MEC; (MEC; Grant for BIA2005‐08965), Ministry for for Science and Innovation Innovation (MICINN; (MICINN; Grant BIA2008‐06810), Institutions: Ministry Science and Technology (MICYT; Grant MAT2002-01831), Ministry forBIA2008‐06810), Education and Ministry for Economy and Competitiveness (MINECO; Grant BIA2011‐27870), Junta de Castilla y León (JCyL; Ministry for Economy and Competitiveness (MINECO; Grant BIA2011‐27870), Junta de Castilla y León (JCyL;
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Science (MEC; Grant BIA2005-08965), Ministry for Science and Innovation (MICINN; Grant BIA2008-06810), Ministry for Economy and Competitiveness (MINECO; Grant BIA2011-27870), Junta de Castilla y León (JCyL; Grants SA067A05, SA111A07 and SA039A08), and the steel supplied by EMESA TREFILERÍA (La Coruña, Galicia, Spain) and TREFILERÍAS QUIJANO (Los Corrales de Buelna, Santander, Spain). Author Contributions: J.T. conceived and designed the experimental procedure; F.J.A., B.G., J.C.M., D.V. and M.L. performed the tests; all the authors analyzed the results and wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
Abbreviations The following abbreviations are used in this manuscript: FPZ SEM MFM MVC
Fracture Process Zone Scanning Electron Microscopy Micro-Fracture Map Micro-Void Coalescence
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