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Fracture of progressively drawn pearlite steels under triaxial stress states

J. Toribio Department of Materials Engineering, University of Salamanca, Spain

Abstract

In this paper the fracture behaviour of progressively drawn pearlite steels is studied under triaxial stress states produced by surface defects like cracks. To this end, samples from different stages of an industrial manufacturing process were analyzed. The real manufacture chain was stopped in the course of the process, and samples of five intermediate stages were extracted, apart from the original material or base product (hot rolled bar: not cold drawn at all) and the final commercial product (prestressing steel wire: heavily cold drawn). Results demonstrated that progressive cold drawing affects clearly the fracture performance of the materials, so that the most heavily drawn steels exhibit anisotropic fracture behaviour with crack deflection, i.e., a change in crack propagation direction which deviates from the original mode I propagation and approaches the wire axis or cold drawing direction, thereby producing a mixed mode stress state. At the microscopical level, clear changes are observed in the micrographs with appearances from cleavage-like in the slightly drawn steels to predominant micro-void coalescence in the heavily drawn steels. Keywords: Pearlitic steel, steelmaking, manufacturing, cold drawing, anisotropic fracture behaviour, fracture micromechanisms.

1 Introduction

High-strength prestressing steel wires for civil engineering use in prestressed concrete structures are manufactured by cold drawing a previously hot rolled pearlitic steel bar in several passes to increase the yield strength. The steelmaking process in the form of progressive cold drawing produces important plastic deformations in the material and activates a strain hardening mechanism which is responsible for the extremely high yield strength useful for structural

Damage and Fracture Mechanics VIII, C. A. Brebbia & A. Varvani-Farahani (Editors)© 2004 WIT Press, www.witpress.com, ISBN 1-85312-707-8

engineering. This heavy drawing also produces microstructural effects in the steel with potential consequences in the matter of fatigue and fracture behaviour. Therefore, although the classical mechanical properties (yield strength, ultimate tensile stress) of high-strength cold drawn pearlitic steels are known to be improved during the steelmaking procedure, further research is needed to provide more insight into the effects of this manufacture method on fracture. Previous research on this topic dealt only with the first and last steps of the cold drawing manufacturing process, i.e., the hot rolled bar (base material) and the commercial cold drawn prestressing steel wire (high-strength material to be used in prestressed concrete). Ref [1] presents a general overview, while [2] analyzes the anisotropic fracture behavior of the fully drawn wire. In this paper the fracture performance of steels with intermediate levels of cold drawing is studied under triaxial stress states produced by crack-like defects. Thus the drawing intensity (or straining level, or degree of strain hardening, given by the number of cold drawing steps undergone by the steels) is treated as the fundamental variable to elucidate the effects of the manufacturing route on the posterior fracture performance of the material.

2 Materials and effect of cold drawing

A group of high strength steels with the same chemical composition (Table 1) was used in this work. All of them were obtained from the same coil of a previously hot rolled steel which is used to manufacture prestressing steel wires after cold drawing. In this paper different degrees of cold drawing associated with intermediate steps of a manufacturing process are analyzed. The number of drawing steps applied to each steel is indicated by its own name. The range goes from steel number 0 which is not cold drawn at all up to the prestressing steel wire, number 6, which has undergone six passes of cold drawing and represents the final commercial product. Thus the drawing intensity (or straining level) is treated as the fundamental variable to elucidate the consequences of the manufacturing route on the posterior fracture behaviour of the material. Table 2 gives the diameter (Di) of all the steels (the number of the drawing steps applied to each one is indicated by its own name), the cold drawing degree Di/D0, the Young's modulus (E), the 0.2 proof yield strength (σY), the ultimate tensile strength (σR) and the Ramberg-Osgood parameters (P and n). A clear improvement of mechanical properties with cold drawing is observed, as shown in Figure 1 by means of the stress-strain curves of the different steels.

Table 1: Chemical composition (wt %) of the steel. _________________________________________________________________ C Mn Si P S Cr V Al _________________________________________________________________ 0.80 0.69 0.23 0.012 0.009 0.265 0.060 0.004 _________________________________________________________________


4 Damage and Fracture Mechanics VIII

Table 2: Nomenclature, diameter reduction and mechanical properties of the steels.

_________________________________________________________________ Steel 0 1 2 3 4 5 6 _________________________________________________________________ Di (mm) 12.00 10.80 9.75 8.90 8.15 7.50 7.00 Di/D0 1 0.90 0.81 0.74 0.68 0.62 0.58 E (GPa) 197.4 201.4 203.5 197.3 196.7 202.4 198.8 σY (GPa) 0.686 1.100 1.157 1.212 1.239 1.271 1.506 σR (GPa) 1.175 1.294 1.347 1.509 1.521 1.526 1.762 P (GPa) 1.98 2.26 2.33 2.49 2.50 2.74 2.34 n 5.89 8.61 8.70 8.45 8.69 7.98 11.49 _________________________________________________________________

E: Young's modulus, σY: yield strength, σR: ultimate tensile stress (UTS) P, n: Ramberg-Osgood parameters: ε= σ/E +(σ/P)n

0

0.4

0.8

1.2

1.6

2

0 1 2 3 4 5 6

0123456

ε

(GPa

)

(%)

Figure 1: Stress-strain curves of the different steels.

Metallographic techniques were used to reveal the microstructure of the steels. Longitudinal and transverse sections of the steel wires were cut and mounted to undergo four grinding stages, from 320 to 1200 grit, and three polishing passes followed by etching in Nital 2%. Scanning electron microscopy was used to resolve the lamellae structure of the pearlite, as described elsewhere [3]. In the pearlitic microstructure of eutectoid steels the basic unit is the pearlite colony formed by ferrite and cementite lamellae parallel to each other and with an orientation different from that of the lamellae of neighbouring colonies. Cold drawing produces changes in the pearlite colony (first microstructural unit) in the form of slenderising [4] and progressive orientation parallel to the wire axis or cold drawing direction [5]. In a finer microstructural scale, the lamellar pearlitic structure itself made up of ferrite and cementite is the following level of analysis


Damage and Fracture Mechanics VIII 5

(second microstructural unit), the key parameter being the pearlite interlamellar spacing. Cold drawing also produces changes in the lamellae in the form of decrease of interlamellar spacing [6] and orientation parallel to the main axis or cold drawing direction [7]. Figure 2 shows a longitudinal metallographic section (i.e., axial cut) of the steel 6 (the most heavily drawn wire) taken at an intermediate point between the centre and the surface of the wire, i.e., at a point located at an approximate distance of D/4 from the wire axis, D being the wire diameter. Both microstructural levels (colonies and lamellae) are shown to align in a direction quasi-parallel to the wire axis or cold drawing direction. The features explained above are the general trends of microstructural evolution produced by the wire drawing. In addition, there are some exceptional pearlitic pseudocolonies (cf. [3]) which are extremely slender, aligned quasi-parallel to the drawing direction (wire axis) and whose local interlamellar spacing is clearly anomalous —specially high— in comparison with the average (or global) spacing in the specific steel due to the fact that the cementite plates are not oriented along the wire axis direction and in some cases are pre-fractured by shear during the manufacturing process. These characteristics make them local precursors of micro-cracking, i.e., preferential fracture paths with minimum local resistance to damage, fracture or cracking.

Figure 2: Metallographic analysis of steel 6 (the most heavily drawn), showing the two basic microstructural levels of pearlitic colonies and ferrite/cementite lamellae. The vertical side of the micrograph corresponds to the wire axis or cold drawing direction. Samples were taken at D/4 from the axis, D being the wire diameter.

3 Experimental programme

To analyze the fracture mechanisms in the steels with different degrees of cold drawing, cylindrical precracked samples (cf. Figure 3) were taken from the steel wires, with different fatigue precrack depths from 0.25D to 0.50 D, where D is the wire diameter given in Table 2 for each steel. Samples were subjected to axial fatigue (tensile loading/unloading in the direction of the wire axis) with a minimum load near to zero, i.e., R = σmin/σmax = 0, load frequency of 10 Hz and



load control. The crack depth was continuously monitored by a method based on the compliance of the sample. Later, the fatigue precracked rods were subjected to monotonic tensile loading up to final fracture of the whole specimen at a crosshead speed of 3 mm/min [8]. A total number of 37 fracture tests were performed, more or less uniformly distributed among the wire diameters (i.e., among all degrees of cold drawing), although fewer tests were conducted on the intermediate levels of cold drawing due to the scarcity of material from these stages of the manufacturing route. Nevertheless, at least three replicate tests were made for each material condition (degree of drawing).

Figure 3: Precracked specimens used in the fracture tests.

4 Experimental results

A key experimental difference in fracture behaviour between slightly drawn and heavily drawn steels was observed, as sketched in Figure 4. While the slightly drawn steels (0-3) exhibit an isotropic fracture behaviour, the heavily drawn steels (4-6) exhibit a clearly anisotropic fracture behaviour in the form of crack deflection after the fatigue precrack with a deviation angle of almost 90º from the initial crack plane and further propagation in a direction close to the initial one. This anisotropic fracture behaviour of the most heavily drawn steels may be explained on the basis of the metallographic analysis (cf. Figure 2). Figure 5 shows the fracture surfaces of the progressively drawn steels. A general evolution may be observed from a typically brittle behaviour with a flat fracture surface in steel 0 (and slightly drawn steels) to a more ductile behaviour with a more or less irregular and stepped fracture surface in steel 6 (and heavily drawn steels). Accordingly, the fracture behaviour evolves from isotropic to anisotropic as the degree of cold drawing increases. The stepped appearance of



the fracture surfaces in heavily drawn steels is consistent with their strength anisotropy because there are a lot of step embryos (or meso-steps) and finally one of them becomes the final 90º-step which produces the macroscopic crack deflection. Thus the meso-roughness increases with the degree of cold drawing.

(a) (b)

Figure 4: Fracture modes: (a) slightly drawn steels 0-3; (b) heavily drawn steels 4-6; f: fatigue precracking; I: mode I subcritical cracking, II: 90º propagation step; F: final fast fracture.

To analyze the microscopic modes of fracture, a fractographic analysis by scanning electron microscopy (SEM) was performed on the fracture surfaces of all the broken samples, and results are given in Figure 6. In the first stages of cold drawing (steel 0 which is not cold drawn at all and steel 1) the microscopic fracture mode is cleavage-like. In steels 2 and 3 the fracture process initially develops by micro-void coalescence (MVC) and continues by cleavage. Thus a first MVC region is found before the cleavage-like (brittle) area, the depth of this MVC region being an increasing function of the degree of cold drawing. The most heavily drawn steels (4 to 6) exhibit anisotropic fracture behaviour with a 90º-step, as described above, although certain mode I crack growth appears before the step over a distance in which the MVC micro-fracture mode is predominant although the meso-roughness is higher, as explained above. Figure 7 offers a sketch of the microscopic fracture modes in heavily drawn steels, showing that fracture surfaces are MVC before the step (when the mode I crack growth distance is different from zero) and MVC with some cleavage (C) facets after. Thus the 90º-step appears at some distance from the fatigue precrack border, and this distance decreases as the degree of cold drawing degree increases, i.e., the step gets closer to the fatigue precrack border as the drawing becomes heavier, and in the fully drawn steel (steel 6) the step is located just at the fatigue precrack border. The microscopic appearance of the step is shown in the scanning electron micrographs of Figure 8, and it resembles shear fracture. It is consistent with a mechanism of cracked by debonding or delamination of two microstructural fracture units: the perlitic colonies or the ferrite/cementite lamellae.



Figure 5: Fracture surfaces in all the steels used in the experimental programme.

STEEL 1

STEEL 3 STEEL 2

STEEL 5 STEEL 4

STEEL 6

STEEL 0



Figure 6: Microscopic fracture modes in all the steels used in the experimental

programme. The micrographs correspond to the whole surface in steels 0 and 1, the main fracture surface after the initial MVC band in steels 2 and 3, and the main fracture surface after the propagation step in steels 4 to 6. Fracture propagates from the bottom to the top.

STEEL 2 STEEL 3

STEEL 0

STEEL 4

STEEL 6

STEEL 1

STEEL 5



Figure 7: Sketch of cracking profile and microscopic fracture modes in heavily drawn steels (steels 4 to 6). It is drawn in a plane perpendicular to the crack front at its deepest point.

Figure 8: Scanning electron micrograph of the 90º-propagation step in steel 6 (propagation from left to right).

5 Conclusions

The results of this study show that progressive cold drawing affects clearly the fracture performance of pearlitic steels for use in prestressed concrete. While the fracture behaviour of slightly drawn steels is isotropic, the most heavily drawn steels exhibit strength anisotropy and crack deflection. At the microscopic level, clear changes are observed in the micrographs with appearances from cleavage-like (brittle) in the slightly drawn steels to predominant micro-void coalescence (ductile) in the heavily drawn steels.



Acknowledgements

The financial support (Grant MAT2002-01831) of this work by the Spanish Ministry of Science and Technology (MCYT) and FEDER is gratefully acknowledged. In addition, the author wishes to express his gratitude to EMESA TREFILERIA S.A. (La Coruña, Spain) for providing the steel used in the experimental programme.

References

[1] Elices, M. Fracture of steels for reinforcing and prestressing concrete. Fracture Mechanics of Concrete: Structural Application and Numerical Calculation, eds. G.C. Sih & A. DiTommaso, Martinus Nijhoff Publishers, Dordrecht, pp. 226-271, 1985.

[2] Astiz, M.A, Valiente, A., Elices, M. & Bui, H.D. Anisotropic fracture behaviour of prestressing steel. Life Assessment of Dynamically Loaded Materials and Structures-ECF5, ed. L.O. Faria, EMAS, West Midlands, pp. 385-393, 1984.

[3] Ovejero, E. Fractura en ambiente agresivo de aceros perlíticos con distinto grado de trefilado, Ph. D Thesis, University of La Coruña, 1998.

[4] Toribio, J. & Ovejero, E., Microstructure evolution in a pearlitic steel subjected to progressive plastic deformation. Materials Science and Engineering, A234-236, pp. 579-582, 1997.

[5] Toribio, J. & Ovejero, E., Microstructure orientation in a pearlitic steel subjected to progressive plastic deformation. Journal of Materials Science Letters, 17, pp. 1037-1040, 1998.

[6] Toribio, J. & Ovejero, E., Effect of cumulative cold drawing on the pearlite interlamellar spacing in eutectoid steel. Scripta Materialia, 39, pp.323-328, 1998.

[7] Toribio, J. & Ovejero, E., Effect of cold drawing on microstructure and corrosion performance of high-strength steel. Mechanics of Time-Dependent Materials, 1, pp. 307-319, 1998.

[8] Toledano, M. Fatiga y fractura de aceros perlíticos con distinto grado de trefilado, Ph. D. Thesis, University of La Coruña, 1998.

[9] Miller, L.E. & Smith, G.C., Tensile fractures in carbon steels. Journal of the Iron and Steel Institute, 208, pp. 998-1005, 1970.

[10] Porter, D.A., Easterling, K.E. & Smith G.D.W., Dynamic studies of the tensile deformation and fracture of pearlite. Acta Metallurgica, 26, pp. 1405-1422, 1978.

[11] Park, Y.J. & Bernstein, I.M., The process of crack initiation and effective grain size for cleavage fracture in pearlitic eutectoid steel. Metallurgical Transactions, 10A, pp. 1653-1664, 1979.

[12] Lewandowski, J.J. & Thompson, A.W., Microstructural effects on the cleavage fracture stress of fully pearlitic eutectoid steel. Metallurgical Transactions, 17A, pp. 1769-1786, 1986.



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