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Microstructure-based modeling of hydrogen assisted cracking in pearlitic steels
Materials Science and Engineering A319– 321 (2001) 540– 543 www.elsevier.com/locate/msea
Microstructure-based modeling of hydrogen assisted cracking in pearlitic steels J. Toribio *, E. Ovejero Department of Materials Science, Uni6ersity of La Corun˜a, E.T.S.L Caminos, Campus de El6ina, 15192 La Coruna, Spain
Abstract This paper analyzes the process of hydrogen assisted cracking in pearlitic steels, which are increasingly cold drawn in the course of manufacturing to produce prestressing steel wires used in prestressed concrete. A microstructure-based modeling of the environmentally-assisted fracture phenomenon is proposed to rationalize the results for diferent degrees of drawing. The approach is based on two items — (i) the model by Miller and Smith of shear cracking (SC) in pearlite; (ii) the mechanism of hydrogen enhanced decohesion (HEDE) proposed by Gerberich. There is an evolution from a predominant micromechanism of SC in slightly drawn steels to a predominant micromechanism of HEDE in heavily drawn steels. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Pearlitic steels; Cold drawing; Stress corrosion cracking; Hydrogen assisted cracking; Micromechanical modeling
1. Introduction Prestressing steel is a material widely used in prestressed concrete, where cold drawn steels are needed to undergo high stress levels. These steels frequently work in harsh environments, where the risk of failure by stress corrosion cracking is greater. Whereas earlier research analyzed only the fracture and stress corrosion cracking of hot rolled steel (base material) and fully drawn prestressing steel (final commercial product) [1– 4], in this paper the process of hydrogen assisted cracking (HAC) in cold drawn pearlitic steels is elucidated by studying the intermediate stages of the cold drawing process. The environmentally-assisted fracture phenomenon is modeled on the basis of microstructure, to complete the study of fracture in air [5] and localized anodic dissolution [6].
2. Experimental The materials used in this work were high-strength steels with different degrees of cold drawing taken from * Corresponding author. Tel.: + 34-9-81167000; fax: + 34-981167170. E-mail address: [email protected] (J. Toribio).
a real manufacturing process. The relevant data concerning drawing, chemical composition, diameter evolution of the wires and mechanical properties of these steels are given in a previous paper [7]. The microstructural evolution as drawing proceeds is given in [8–11]. Drawing produces a progressive slenderizing of the pearlite colonies [8] accompanied by increasing orientation in direction quasi-parallel to the drawing axis [9], as well as reduction of the interlamellar spacing [10] and progressive orientation of the lamellae in the cold drawing direction [11]. Hydrogen embrittlement experiments were carried out in the form of slow strain rate tests on precracked steel wires. Samples were precracked by axial fatigue in the normal laboratory air environment to produce a transverse precrack, so that the maximum stress intensity factor during the last stage of fatigue precracking was Kmax = 0.30 KIC, where KIC is the fracture toughness, and the crack depth was a= 0.30 D in all cases, where D is the wire diameter. After precracking, samples were introduced in a corrosion cell containing aqueous solution of 1 g l − 1 Ca(OH)2 plus 0.1 g l − 1 NaCl (pH 12.5) to reproduce the alkaline working conditions of prestressing steel surrounded by concrete. The experimental device consisted of a potentiostat and a three-electrode assembly (metallic sample or working electrode, platinum counter-electrode and saturated
0921-5093/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 0 3 7 - 1
J. Toribio, E. O6ejero / Materials Science and Engineering A319–321 (2001) 540–543
calomel electrode as the reference one). All tests were conducted under potentiostatic control at a constant potential of −1200 mV versus SCE at which the stress corrosion cracking mechanism is HAC [2]. The applied displacement rate in axial direction was constant during each test and proportional to each wire diameter so that the smallest rate was 1.7× 10 − 3 mm per min for the fully drawn wire (steel 6 of 7 mm diameter) and the highest was 3.0× 10 − 3 mm per min for the hot rolled bar (steel 0 of 12 mm diameter).
3. Results A progressive change in the macroscopic topography as the cold drawing increases was observed in all fracture surfaces. Fig. 1 gives a 3D-view of these fracture surfaces, showing that mixed mode crack growth appears from a certain cold drawing level, with early crack deflection, which starts just at the tip of the fatigue precrack, i.e. at the very beginning of the hydrogen embrittlement test. In the first step drawing (steels 0 and 1), the crack grows in mode I in both fatigue
Fig. 1. Fracture surfaces produced by HAC in steels with different degree of cold drawing: (a) null or slight drawing (Di /D0 ] 0.90); (b) intermediate drawing (0.90 \ Di /D0 ] 0.74); (c) heavy drawing (Di / D0 B 0.74); f, fatigue crack growth; I, mode I propagation; II, mixed mode propagation; F, final fracture; Di, diameter of each drawn steel; D0, initial diameter before cold drawing.
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precracking and hydrogen-assisted cracking (Fig. 1a). In steels with an intermediate degree of drawing (2 and 3), there is a slight deflection in the hydrogen-assisted crack, and this deflection is not uniform along the crack front but has a wavy shape (Fig. 1b). For the most heavily drawn specimens (4–6), the crack deflection takes place suddenly after the fatigue precrack and the deviation angle is even higher and more or less uniform along the whole crack front (Fig. 1c). This behavior may be explained on the basis of the oriented microstructure of the material with regard to pearlite colonies [9] and lamellae [11], as the drawing becomes heavier, the cracks find easier propagation directions with lower fracture resistance. Thus, the macroscopic HAC behavior of the different steels (progressively anisotropic with cold drawing) is a direct consequence of the microstructural changes undergone during manufacture.
4. Micromechanical modeling
4.1. Hot rolled and slightly drawn steels A microstructure-based modeling of the HAC phenomenon is proposed to rationalize the experimental results as a function of the degree of cold drawing. In hot rolled and slightly drawn steels, the micromechanism of fracture by HAC (shown in Fig. 2) is the so called tearing topography surface (TTS) [12– 15]. This non-conventional microscopic fracture mode is undoubtedly associated with the hydrogen-assisted microdamage process in pearlitic steels [14,16] and it consists of a very closely spaced nucleation of voids or defects, as sketched in Fig. 2. As a matter of fact, the TTS fracture mode is a derivation of the micro-void coalescence (MVC) topography due to the presence of the hydrogen as a damaging agent, cf. [16]. Thus, the microscopic fracture mode turns from MVC to TTS, when sufficient amount of hydrogen reaches the prospective fracture nuclei so that the crack advances by TTS.
4.2. Steels with an intermediate degree of cold drawing
Fig. 2. Micromechanical model of HAC in hot rolled and slightly drawn steels: the mechanism is the so called tearing topography surface or TTS [12 – 15] with very closely spaced nucleation of voids or defects.
When the degree of drawing increases, the behavior becomes anisotropic (cf. Fig. 1b) and a micromechanical model able to account for the different degrees of cold drawing is required. It is based on two models proposed earlier — (i) the model by Miller and Smith of fracture of pearlitic microstructure by shear cracking (SC) of the cementite lamellae [17], described in a previous paper [5]; (ii) the mechanism of hydrogen enhanced decohesion (HEDE), a term coined by Ger-
542
J. Toribio, E. O6ejero / Materials Science and Engineering A319–321 (2001) 540–543
hydrogen penetration could be easier along the path opened by SC of the cementite lamellae. Fig. 3b shows the mechanism of fracture by HEDE. Both could be operative in different regions of the material near the crack tip region. As a consequence of the progressive microstructural orientation induced in the material by the cold drawing manufacturing method, the micromechanism of fracture evolves form predominant SC and hydrogen penetration in slightly drawn steels to predominant HEDE in heavily drawn steels, as described in the following section.
4.3. Hea6ily drawn steels
Fig. 3. Micromechanical model of HAC in steels with an intermediate degree of drawing: (a) hydrogen diffusion in the pearlitic microstructure and penetration along the path opened by a Miller – Smith mechanism of shear cracking (SC); (b) fracture by hydrogen enhanced delamination or debonding (HEDE).
The importance of the described micromechanism of HEDE in the fracture process by HAC of heavily drawn steels steel is even higher because of the lamellar structure of the steel (markedly oriented), which produces anisotropy regarding fracture and hydrogen diffusion, so that hydrogen diffuses mainly in the direction of the plates (D DÞ in Fig. 4a) and can weaken the bonds or interfaces between the ferrite and the cementite lamellae (which are the weakest links even before the hydrogen presence) thus contributing to the hydrogen-induced fracture by delamination (or debonding) between two similar microstructural units, i.e. at the ferrite–cementite interface or at the pearlitic colony boundaries, as sketched in Fig. 4b.
5. Conclusions
Fig. 4. Micromechanical model of HAC in heavily drawn steels: (a) hydrogen diffusion in longitudinal and transverse directions; (b) fracture by hydrogen enhanced delamination or debonding (HEDE). The microstructure is assumed to be totally oriented in the cold drawing direction.
berich to describe a kind of microscopic fracture mode promoted by hydrogen [18]. For the case of a pearlitic microstructure, it would be hydrogen enhanced delamination (or debonding) between two similar microstructural units (colonies or lamellae). Fig. 3 illustrates the two operative micromechanisms in cold drawn pearlitic steels with an intermediate degree of drawing. In Fig. 3a, it is seen that
A microstructure-based model of fracture by hydrogen assisted cracking in pearlitic steels was formulated for different degrees of cold drawing. In slightly drawn steels, the fracture micromechanism consists of tearing topography surface, a kind of closely-spaced nucleation micromechanism promoted by hydrogen. In steels with an intermediate degree of cold drawing, a mixed micromechanical model is proposed consisting of shear cracking of pearlite and hydrogen enhanced delamination. In the most heavily drawn steels, fracture takes place by hydrogen-enhanced delamination between two similar microstructural units markedly oriented in the drawing direction.
Acknowledgements The financial support of this work by the Spanish CICYT (Grant MAT97-0442) and Xunta de Galicia
J. Toribio, E. O6ejero / Materials Science and Engineering A319–321 (2001) 540–543
(Grants XUGA 11801B95 and XUGA 11802B97) is gratefully acknowledged. In addition, the authors wish to express their gratitude to EMESA TREFILERIA S.A. (La Corun˜ a, Spain) for providing the steel used in the experimental programme.
References [1] B.W. Cherry, S.M. Price, Corros. Sci. 20 (1980) 1163. [2] R.N. Parkins, M. Elices, V. Sa´ nchez-Ga´ vez, L. Caballero, Corros. Sci. 22 (1982) 379. [3] A.M. Lancha, Ph. D. Thesis, Complutense University of Madrid (1987). [4] N. Sarafianos, J. Mater. Sci. Lett. 8 (1989) 1486. [5] J. Toribio, M. Toledano, Constr. Building Mater 14 (2000) 47. [6] J. Toribio, E. Ovejero, Mater. Sci. Eng. A 319 – 321 (2001) 308.
543
[7] J. Toribio, E. Ovejero, M. Toledano, Int. J. Fracture 87 (1997) L83. [8] J. Toribio, E. Ovejero, Mater. Sci. Eng. A234-236 (1997) 579. [9] J. Toribio, E. Ovejero, J. Mater. Sci. Lett. 17 (1998) 1037. [10] J. Toribio, E. Ovejero, Scr. Mater. 39 (1998) 323. [11] J. Toribio, E. Ovejero, Mech. Time-Dep. Mater. 1 (1998) 307. [12] A.W. Thompson, J.C. Chesnutt, Metall. Trans. 10A (1979) 1193. [13] J.E. Costa, A.W. Thompson, Metall. Trans. 13A (1982) 1315. [14] J. Toribio, A.M. Lancha, M. Elices, Scr. Metall. Mater. 25 (1991) 2239. [15] J. Toribio, A.M. Lancha, M. Elices, Metall. Trans. 23A (1992) 1573. [16] J. Toribio, E. Vasseur, J. Mater. Sci. Lett. 16 (1997) 1345. [17] L.E. Miller, G.C. Smith, J. Iron Steel Inst. 208 (1970) 998. [18] W.W. Gerberich, P. Marsh, J. Hoehn, S. Venkataraman, H. Huang, in: T. Magnin, J.M. Gras (Eds.), Corrosion-Deformation Interactions (CDI’92), Les Editions de Physique, Les Ulis, 1993, p. 325.
Microstructure-based modeling of hydrogen assisted cracking in pearlitic steels J. Toribio *, E. Ovejero Department of Materials Science, Uni6ersity of La Corun˜a, E.T.S.L Caminos, Campus de El6ina, 15192 La Coruna, Spain
Abstract This paper analyzes the process of hydrogen assisted cracking in pearlitic steels, which are increasingly cold drawn in the course of manufacturing to produce prestressing steel wires used in prestressed concrete. A microstructure-based modeling of the environmentally-assisted fracture phenomenon is proposed to rationalize the results for diferent degrees of drawing. The approach is based on two items — (i) the model by Miller and Smith of shear cracking (SC) in pearlite; (ii) the mechanism of hydrogen enhanced decohesion (HEDE) proposed by Gerberich. There is an evolution from a predominant micromechanism of SC in slightly drawn steels to a predominant micromechanism of HEDE in heavily drawn steels. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Pearlitic steels; Cold drawing; Stress corrosion cracking; Hydrogen assisted cracking; Micromechanical modeling
1. Introduction Prestressing steel is a material widely used in prestressed concrete, where cold drawn steels are needed to undergo high stress levels. These steels frequently work in harsh environments, where the risk of failure by stress corrosion cracking is greater. Whereas earlier research analyzed only the fracture and stress corrosion cracking of hot rolled steel (base material) and fully drawn prestressing steel (final commercial product) [1– 4], in this paper the process of hydrogen assisted cracking (HAC) in cold drawn pearlitic steels is elucidated by studying the intermediate stages of the cold drawing process. The environmentally-assisted fracture phenomenon is modeled on the basis of microstructure, to complete the study of fracture in air [5] and localized anodic dissolution [6].
2. Experimental The materials used in this work were high-strength steels with different degrees of cold drawing taken from * Corresponding author. Tel.: + 34-9-81167000; fax: + 34-981167170. E-mail address: [email protected] (J. Toribio).
a real manufacturing process. The relevant data concerning drawing, chemical composition, diameter evolution of the wires and mechanical properties of these steels are given in a previous paper [7]. The microstructural evolution as drawing proceeds is given in [8–11]. Drawing produces a progressive slenderizing of the pearlite colonies [8] accompanied by increasing orientation in direction quasi-parallel to the drawing axis [9], as well as reduction of the interlamellar spacing [10] and progressive orientation of the lamellae in the cold drawing direction [11]. Hydrogen embrittlement experiments were carried out in the form of slow strain rate tests on precracked steel wires. Samples were precracked by axial fatigue in the normal laboratory air environment to produce a transverse precrack, so that the maximum stress intensity factor during the last stage of fatigue precracking was Kmax = 0.30 KIC, where KIC is the fracture toughness, and the crack depth was a= 0.30 D in all cases, where D is the wire diameter. After precracking, samples were introduced in a corrosion cell containing aqueous solution of 1 g l − 1 Ca(OH)2 plus 0.1 g l − 1 NaCl (pH 12.5) to reproduce the alkaline working conditions of prestressing steel surrounded by concrete. The experimental device consisted of a potentiostat and a three-electrode assembly (metallic sample or working electrode, platinum counter-electrode and saturated
0921-5093/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 0 3 7 - 1
J. Toribio, E. O6ejero / Materials Science and Engineering A319–321 (2001) 540–543
calomel electrode as the reference one). All tests were conducted under potentiostatic control at a constant potential of −1200 mV versus SCE at which the stress corrosion cracking mechanism is HAC [2]. The applied displacement rate in axial direction was constant during each test and proportional to each wire diameter so that the smallest rate was 1.7× 10 − 3 mm per min for the fully drawn wire (steel 6 of 7 mm diameter) and the highest was 3.0× 10 − 3 mm per min for the hot rolled bar (steel 0 of 12 mm diameter).
3. Results A progressive change in the macroscopic topography as the cold drawing increases was observed in all fracture surfaces. Fig. 1 gives a 3D-view of these fracture surfaces, showing that mixed mode crack growth appears from a certain cold drawing level, with early crack deflection, which starts just at the tip of the fatigue precrack, i.e. at the very beginning of the hydrogen embrittlement test. In the first step drawing (steels 0 and 1), the crack grows in mode I in both fatigue
Fig. 1. Fracture surfaces produced by HAC in steels with different degree of cold drawing: (a) null or slight drawing (Di /D0 ] 0.90); (b) intermediate drawing (0.90 \ Di /D0 ] 0.74); (c) heavy drawing (Di / D0 B 0.74); f, fatigue crack growth; I, mode I propagation; II, mixed mode propagation; F, final fracture; Di, diameter of each drawn steel; D0, initial diameter before cold drawing.
541
precracking and hydrogen-assisted cracking (Fig. 1a). In steels with an intermediate degree of drawing (2 and 3), there is a slight deflection in the hydrogen-assisted crack, and this deflection is not uniform along the crack front but has a wavy shape (Fig. 1b). For the most heavily drawn specimens (4–6), the crack deflection takes place suddenly after the fatigue precrack and the deviation angle is even higher and more or less uniform along the whole crack front (Fig. 1c). This behavior may be explained on the basis of the oriented microstructure of the material with regard to pearlite colonies [9] and lamellae [11], as the drawing becomes heavier, the cracks find easier propagation directions with lower fracture resistance. Thus, the macroscopic HAC behavior of the different steels (progressively anisotropic with cold drawing) is a direct consequence of the microstructural changes undergone during manufacture.
4. Micromechanical modeling
4.1. Hot rolled and slightly drawn steels A microstructure-based modeling of the HAC phenomenon is proposed to rationalize the experimental results as a function of the degree of cold drawing. In hot rolled and slightly drawn steels, the micromechanism of fracture by HAC (shown in Fig. 2) is the so called tearing topography surface (TTS) [12– 15]. This non-conventional microscopic fracture mode is undoubtedly associated with the hydrogen-assisted microdamage process in pearlitic steels [14,16] and it consists of a very closely spaced nucleation of voids or defects, as sketched in Fig. 2. As a matter of fact, the TTS fracture mode is a derivation of the micro-void coalescence (MVC) topography due to the presence of the hydrogen as a damaging agent, cf. [16]. Thus, the microscopic fracture mode turns from MVC to TTS, when sufficient amount of hydrogen reaches the prospective fracture nuclei so that the crack advances by TTS.
4.2. Steels with an intermediate degree of cold drawing
Fig. 2. Micromechanical model of HAC in hot rolled and slightly drawn steels: the mechanism is the so called tearing topography surface or TTS [12 – 15] with very closely spaced nucleation of voids or defects.
When the degree of drawing increases, the behavior becomes anisotropic (cf. Fig. 1b) and a micromechanical model able to account for the different degrees of cold drawing is required. It is based on two models proposed earlier — (i) the model by Miller and Smith of fracture of pearlitic microstructure by shear cracking (SC) of the cementite lamellae [17], described in a previous paper [5]; (ii) the mechanism of hydrogen enhanced decohesion (HEDE), a term coined by Ger-
542
J. Toribio, E. O6ejero / Materials Science and Engineering A319–321 (2001) 540–543
hydrogen penetration could be easier along the path opened by SC of the cementite lamellae. Fig. 3b shows the mechanism of fracture by HEDE. Both could be operative in different regions of the material near the crack tip region. As a consequence of the progressive microstructural orientation induced in the material by the cold drawing manufacturing method, the micromechanism of fracture evolves form predominant SC and hydrogen penetration in slightly drawn steels to predominant HEDE in heavily drawn steels, as described in the following section.
4.3. Hea6ily drawn steels
Fig. 3. Micromechanical model of HAC in steels with an intermediate degree of drawing: (a) hydrogen diffusion in the pearlitic microstructure and penetration along the path opened by a Miller – Smith mechanism of shear cracking (SC); (b) fracture by hydrogen enhanced delamination or debonding (HEDE).
The importance of the described micromechanism of HEDE in the fracture process by HAC of heavily drawn steels steel is even higher because of the lamellar structure of the steel (markedly oriented), which produces anisotropy regarding fracture and hydrogen diffusion, so that hydrogen diffuses mainly in the direction of the plates (D DÞ in Fig. 4a) and can weaken the bonds or interfaces between the ferrite and the cementite lamellae (which are the weakest links even before the hydrogen presence) thus contributing to the hydrogen-induced fracture by delamination (or debonding) between two similar microstructural units, i.e. at the ferrite–cementite interface or at the pearlitic colony boundaries, as sketched in Fig. 4b.
5. Conclusions
Fig. 4. Micromechanical model of HAC in heavily drawn steels: (a) hydrogen diffusion in longitudinal and transverse directions; (b) fracture by hydrogen enhanced delamination or debonding (HEDE). The microstructure is assumed to be totally oriented in the cold drawing direction.
berich to describe a kind of microscopic fracture mode promoted by hydrogen [18]. For the case of a pearlitic microstructure, it would be hydrogen enhanced delamination (or debonding) between two similar microstructural units (colonies or lamellae). Fig. 3 illustrates the two operative micromechanisms in cold drawn pearlitic steels with an intermediate degree of drawing. In Fig. 3a, it is seen that
A microstructure-based model of fracture by hydrogen assisted cracking in pearlitic steels was formulated for different degrees of cold drawing. In slightly drawn steels, the fracture micromechanism consists of tearing topography surface, a kind of closely-spaced nucleation micromechanism promoted by hydrogen. In steels with an intermediate degree of cold drawing, a mixed micromechanical model is proposed consisting of shear cracking of pearlite and hydrogen enhanced delamination. In the most heavily drawn steels, fracture takes place by hydrogen-enhanced delamination between two similar microstructural units markedly oriented in the drawing direction.
Acknowledgements The financial support of this work by the Spanish CICYT (Grant MAT97-0442) and Xunta de Galicia
J. Toribio, E. O6ejero / Materials Science and Engineering A319–321 (2001) 540–543
(Grants XUGA 11801B95 and XUGA 11802B97) is gratefully acknowledged. In addition, the authors wish to express their gratitude to EMESA TREFILERIA S.A. (La Corun˜ a, Spain) for providing the steel used in the experimental programme.
References [1] B.W. Cherry, S.M. Price, Corros. Sci. 20 (1980) 1163. [2] R.N. Parkins, M. Elices, V. Sa´ nchez-Ga´ vez, L. Caballero, Corros. Sci. 22 (1982) 379. [3] A.M. Lancha, Ph. D. Thesis, Complutense University of Madrid (1987). [4] N. Sarafianos, J. Mater. Sci. Lett. 8 (1989) 1486. [5] J. Toribio, M. Toledano, Constr. Building Mater 14 (2000) 47. [6] J. Toribio, E. Ovejero, Mater. Sci. Eng. A 319 – 321 (2001) 308.
543
[7] J. Toribio, E. Ovejero, M. Toledano, Int. J. Fracture 87 (1997) L83. [8] J. Toribio, E. Ovejero, Mater. Sci. Eng. A234-236 (1997) 579. [9] J. Toribio, E. Ovejero, J. Mater. Sci. Lett. 17 (1998) 1037. [10] J. Toribio, E. Ovejero, Scr. Mater. 39 (1998) 323. [11] J. Toribio, E. Ovejero, Mech. Time-Dep. Mater. 1 (1998) 307. [12] A.W. Thompson, J.C. Chesnutt, Metall. Trans. 10A (1979) 1193. [13] J.E. Costa, A.W. Thompson, Metall. Trans. 13A (1982) 1315. [14] J. Toribio, A.M. Lancha, M. Elices, Scr. Metall. Mater. 25 (1991) 2239. [15] J. Toribio, A.M. Lancha, M. Elices, Metall. Trans. 23A (1992) 1573. [16] J. Toribio, E. Vasseur, J. Mater. Sci. Lett. 16 (1997) 1345. [17] L.E. Miller, G.C. Smith, J. Iron Steel Inst. 208 (1970) 998. [18] W.W. Gerberich, P. Marsh, J. Hoehn, S. Venkataraman, H. Huang, in: T. Magnin, J.M. Gras (Eds.), Corrosion-Deformation Interactions (CDI’92), Les Editions de Physique, Les Ulis, 1993, p. 325.