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in vacuo) on low cycle fatigue characteristics of a duplex stainless steel
International Journal of Fatigue 21 (1999) S119–S125 www.elsevier.com/locate/ijfatigue
Effect of testing atmosphere (air/in vacuo) on low cycle fatigue characteristics of a duplex stainless steel L. Llanes a
a,*
, A. Mateo a, P. Villechaise b, J. Me´ndez b, M. Anglada
a
Departament de Cie`ncia dels Materials i Enginyeria Metal•lu´rgica, ETSEIB, Universitat Polite`cnica de Catalunya, Barcelona E-08028, Spain b Laboratoire de Me´canique et de Physique des Mate´riaux (URA Nº. 863/CNRS), ENSMA, 86960 Futuroscope Cedex, France
Abstract The effect of testing atmosphere on low cycle fatigue characteristics of a duplex stainless steel has been studied at room temperature. Fatigue tests have been conducted under fully reversed plastic strain control and constant plastic strain rate in two different environments: air and in vacuo. The material has been investigated under two distinct conditions: as annealed or unaged and as thermally aged, corresponding to different dominant cyclic deformation mechanisms at the plastic strain amplitudes chosen for the study. In vacuo testing resulted in longer fatigue lives, and consequently, higher cumulative plastic strain than in air experiences for both material conditions. Although prominent fatigue micromechanisms for a given plastic strain amplitude did not seem to be affected by testing atmosphere, for both unaged and aged conditions, strain localization and cracking phenomena were enhanced in air as compared to vacuum. The experimental results were finally discussed in terms of fatigue micromechanisms-environment interactions. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Low cycle fatigue; Environment; Duplex stainless steels; Strain localization
1. Introduction Duplex ferrite-austenite stainless steels (DSSs) are being extensively used for applications in the power, chemical, pulp and paper, and oil industries [1]. The main reasons for their increasing usage are the excellent combination of properties (mechanical strength, toughness, stress corrosion resistance and weldability, among others) as well as the attractive performance parameters (properties to cost ratio, life cycle cost, maintenance expenditures, etc.) that DSSs offer as compared to other alternatives, particularly single-phase ferritic and austenitic stainless steels (α-SS and γ-SS respectively). The fact that many of these applications involve cyclic loading, under normal operating conditions, implies a mandatory need for a better knowledge of the fatigue behavior of DSSs as an important parameter for structural design criteria. Following the above idea, in the last ten years there has been sustained activity towards improved under-
* Corresponding author. Tel.: +34-93-401-6713/06; fax: +34-93401-6706/6600. E-mail address: [email protected] (L. Llanes)
standing of the influence of some intrinsic and extrinsic factors on the fatigue crack initiation and propagation phenomena in DSSs. Such extensive research efforts have demonstrated that material parameters (chemical composition and microstructure), thermal aging and environment greatly affect the fatigue characteristics of these materials [2–15]. This paper will be limited to a quite particular segment of the variety of combinations among environment, loading conditions and material, i.e. atmospheric (air/in vacuo) effects on the high strain fatigue regime of a given DSS. Studies on the influence of environment on the fatigue characteristics of DSSs have concentrated on evaluating corrosion fatigue resistance of these materials in severe aqueous media, e.g. chloride-containing or diluted sulfuric acid solutions, the type of aggressive environments for which DSSs have been conventionally designed and applied (see Refs. [4,11] for an extended literature review on this field). On the other hand, investigations dealing with atmospheric effects on fatigue characteristics of DSSs are only a few [3,4] and they exclusively refer to influence on fatigue propagation of long cracks using air as reference environment. The results of these studies not only point out the complexity of understanding the coupling mechanisms of environment sensitive
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L. Llanes et al. / International Journal of Fatigue 21 (1999) S119–S125
cracking under cyclic loading for each phase but also raise the question on the suitability of damp air as reference media for a complete comprehension of the interaction between microstructure and the referred mechanisms that could lead to improved alloy design. The purpose of this work is therefore to document and discuss the influence of testing atmosphere (air/in vacuo) on the low cycle fatigue (LCF) characteristics of a DSS. In doing so, from the cyclic stress-strain curve of a widely used DSS, three well-defined plastic strain amplitudes (⌬⑀pl/2) were selected to perform constant plastic strain control tests and compare the number of cycles to reach failure in both environments. Emphasis was placed on evaluating fatigue surface damage and fractographic features at given testing atmosphere. The information gathered was then used for analyzing the interaction between environment and fatigue micromechanisms, the latter being dependent upon the imposed ⌬⑀pl/2 and the embrittlement degree of the ferritic constituent (artificially modified by thermal aging) [12,15].
2. Experimental procedure The material studied in this investigation is a mediumalloy DSS of type UNS S31803 (SAF 2205) whose detailed chemical composition has been indicated elsewhere [16]. The material was received as cylindrical rods of 20 mm diameter. It was subsequently annealed at 1050°C for 1 hour followed by water quenching. The resulting microstructure consisted of 45% austenite embedded in a ferritic matrix. The heat treated rods were then machined into cylindrical threaded specimens with a diameter of 6 mm and gage length of 10 mm. All samples were first hand-polished and then electropolished, before testing. Cyclic deformation tests were performed in a symmetrical uniaxial push-pull mode on an electromechanical machine at 25°C in laboratory air or in vacuo (pressure ⬍10⫺3 Pa). Tests were performed in plastic strain controlled mode. From the cyclic stress-strain response for the material studied, determined and documented in a previous work [13,16], three ⌬⑀pl/2s: 6×10⫺4, 2×10⫺3, and 6×10⫺3, were specifically chosen in order to evaluate environmental effects as related to different cyclic deformation mechanisms. The plastic strain rate for all tests was held constant at 2×10⫺3 s⫺1. Attempting to speculate about the intrinsic deformation-environment relationship for each phase at given ⌬⑀pl/2, additional experimental information may be gathered through fatigue testing of thermally aged material. Previous experience by the authors [12] indicates that, as a consequence of such thermal treatment, the ferritic matrix gets embrittled and therefore the cyclic response of DSSs is strongly changed, as compared to that of unaged material, at ⌬⑀pl/2s for which ferrite was plas-
Table 1 Tensile properties of unaged (UA) and aged, at 475°C for 200 hours (A200), UNS S31803 DSS studied in this investigation Material
0.2% offset Yield strength, sY (MPa)
Ultimate tensile Area reduction, AR (%) strength, sUTS (MPa)
UA A200
468 780
686 955
46 21
tically active in the annealed condition, i.e. above 2– 3×10⫺4 for the DSS studied here [13,16]. Hence, a set of thermally aged material (at 475°C for 200 hours, A200) was tested under similar conditions as those described above for the annealed or unaged (UA) one to investigate possible changes of environmental effects on fatigue life characteristics as related to alterations on the dominant cyclic deformation mechanisms. The basic tensile parameters of the studied DSS under both conditions are summarized in Table 1. Finally, external and fracture surfaces of all the tested specimens were carefully analyzed by scanning electron microscopy (SEM). Special attention was directed towards examining fatigue damage features in each phase of the DSS.
3. Results and discussion 3.1. Cyclic response and fatigue life characteristics The relationship between the stress amplitude, ⌬s/2, and the number of cycles for the tests performed in vacuo is plotted in Fig. 1 for the different material conditions and ⌬⑀pl/2s investigated. The ⌬s/2 evolution observed during fatigue in vacuo was similar to that in air; thus, results obtained under the latter condition are
Fig. 1. Cyclic hardening-softening response of unaged (UA) and aged (A200) DSS under plastic strain control testing in vacuo and in air.
L. Llanes et al. / International Journal of Fatigue 21 (1999) S119–S125
only indicated by the time at which ⌬s/2 dropped off suddenly, corresponding to rapid propagation of the fatal crack. As previously found for the cyclic response of another DSS within the strain range here studied [12,17], hardening is always observed during the first few cycles, followed by a gradual softening which is more pronounced for the aged material. For the annealed material a saturation stage is reached for all the imposed strain amplitudes. On the other hand, the achievement of such steady state for the aged condition is only observed for the lowest amplitude and under inert environment. All the other testing conditions yield, after the primary hardening, a continuous ⌬s/2 decrease with increasing number of cycles up to failure. The peak ⌬s/2 for a given ⌬⑀pl/2 is about 250–400 MPa higher for the aged material than for the annealed one; however, the ⌬s/2 difference diminishes with increasing number of cycles up to values between 50 and 200 MPa under final conditions. This behavior should be associated initially with the overall yield strength increment observed for the aged material and later with the synergetic interactions among dislocation activity, deformation twinning, demodulation of spinodal microstructure and activation of other fracture micromechanisms (e.g. cleavage) in ferrite at higher cumulative plastic strain, i.e. with increasing number of cycles and/or imposed ⌬⑀pl/2 [12]. Effects of testing atmosphere on LCF life of the DSS studied under both annealed and thermally aged conditions are given in Fig. 2. The fatigue lives in vacuo were more than ten times longer than those in air at the lowest ⌬⑀pl/2 studied. This value is similar to that observed for copper [18,19] or commercial iron [20] but much higher than the one evaluated for a single-phase γ-SS [21] under similar testing conditions. At applied ⌬⑀pl/2s of 2×10⫺3 and 6×10⫺3, the fatigue life ratio related to both environments decreases to values between 2 and 4, a range close to that typically found
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for other metallic materials within such high strain regime [18–22]. Independently of the applied ⌬⑀pl/2, thermal aging does not seem to affect the relative influence of environment on the fatigue life characteristics of the DSS studied. Taking into consideration that distinct cyclic deformation mechanisms are active for annealed and aged materials under the testing conditions here studied [12,17], such finding was rather unexpected. Concomitantly, testing atmosphere does not influence the existence of a transition behavior associated with aging effects on the fatigue life of DSSs at ⌬⑀pl/2 about 2– 3×10⫺3. This relationship between thermal aging and fatigue life characteristics of DSS, as a function of imposed ⌬⑀pl/2, has been documented and analyzed previously [14,16,23] and will not be discussed further in this work. Nevertheless, it is worthwhile to point out that microstructurally different DSSs, as those studied by Iturgoyen and Anglada (38%g/62%a; 0.07%N) [14], Nystro¨m and Karlsson (75%g/25%a; 0.20%N) [23] and in this investigation (45%g/55%a; 0.13%N), exhibit such a transition at quite close applied ⌬⑀pl/2 values, yielding another example of the complex deformation interaction existing in these two-phase materials. Finally, the experimental points followed a Manson-Coffin correlation, i.e. ⌬⑀pl/2= ⑀f⬘(2Nf)c. The corresponding ⑀f⬘ and c parameters are given in Table 2. Here, as a consequence of the more pronounced fatigue life enhancement found with decreasing applied ⌬⑀pl/2, the c coefficient increases when comparing results from tests performed in vacuo with those conducted in air. 3.2. Fatigue damage and fractographic features Fatigue damage on the external surfaces of all the tested samples was evaluated by SEM. Typical features induced by fatigue in annealed and aged material are presented in Figs. 3–5. As expected, from previous experience on cyclic deformation of DSSs [12,13,15– 17], plastic activity is found in both phases for the unaged condition, whereas it gets concentrated in austenite for the aged one. For the annealed material, although strain localization is enhanced in ferrite with increasing ⌬⑀pl/2, crack nucleation sites are evenly distributed Table 2 LCF properties (Manson-Coffin parameters) of the DSS investigated, under UA and A200 conditions, in air and in vacuo Material-Testing atmosphere
Fig. 2. Effect of testing atmosphere on the LCF life of DSS under annealed (UA) and aged (A200) conditions.
UA-air UA-in vacuo A200-air A200-in vacuo
Fatigue ductility coefficient, ⑀f⬘ 0.708 0.552 0.094 0.077
Fatigue ductility exponent, c ⫺0.668 ⫺0.547 ⫺0.429 ⫺0.356
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Fig. 3. Slip features on the external surface of UA-DSS tested at ⌬⑀pl/2 of 2×10⫺3 in air.
Fig. 5. Fatigue surface damage features in A200-DSS tested at ⌬⑀pl/2 of 2×10⫺3: (a) in air, and (b) in vacuo. Fig. 4. Nucleation and early growth of microcracks in A200-DSS tested at ⌬⑀pl/2 of 6×10⫺3 in vacuo.
among ferrite, austenite, and at the interfaces between them for all the ⌬⑀pl/2s studied. Under aged conditions, microcracks nucleate at either marked extrusions in austenite (dominant) or through local cleavage in the embrittled ferritic matrix. The higher the imposed ⌬⑀pl/2, the more widespread the occurrence of the latter fracture micromechanism. Although the above fatigue micromechanisms were independent of the testing atmosphere, the density of microcracks at failure was higher in the samples tested in air than in those evaluated in vacuo for a given applied ⌬⑀pl/2s. On the other hand, the opposite was observed in terms of density of slip bands in the plastically active phases. An example of the above remarks is given in Fig. 5. The facts that: 1) higher cumulative plastic strain at failure, for a given ⌬⑀pl/2, was always attained in the experiences conducted in vacuo than in those carried out in air; and 2) most of the observed microcracks were associated with slip-related phenomena, allow to translate the found higher microcrack density in similarly higher strain localization levels within the corresponding
slip features for the specimens tested in air. Hence, the detrimental environment (air) effect on fatigue life should be described in terms of enhanced strain localization, and thus, easier microcrack initiation and early growth, the latter being an alternative mechanism of stress relaxation and further deformation. Under vacuum, such cracking phenomena are delayed as a consequence of less localized strain, possibly associated with a more reversible deformation at slip bands. From a fractographic viewpoint, there were not discernible differences on the fracture surface appearance of materials tested under distinct environmental conditions. They were characterized, as it has been reported before [3,4,14,23], by ductile striations in both phases for the annealed material (Fig. 6a). On the other hand, for the aged condition similar striations were exclusively found in austenite, whereas cleavage was observed to be the dominant fractographic characteristic in the ferritic matrix (Fig. 6b). With increasing applied ⌬⑀pl/2 ‘tire tracks’ were widely observed, as clearly shown in Fig. 6c. The occurrence of these periodic markings under low-cycle tension-compression fatigue in the DSS studied here is in total agreement, in terms of testing con-
L. Llanes et al. / International Journal of Fatigue 21 (1999) S119–S125
Fig. 6. Fracture surfaces of studied specimens: (a) ductile striations in UA-DSS tested at ⌬⑀pl/2 of 2×10⫺3 in air; (b) ductile striations and cleavage features in A200-DSS tested at ⌬⑀pl/2 of 2×10⫺3 in air; and (c) ‘tire-tracks’ in UA-DSS tested at ⌬⑀pl/2 of 6×10⫺3 in vacuo.
ditions, with previous findings of tire tracks in other structural materials [24]. 3.3. Fatigue micromechanisms-environment interactions in DSS The detrimental ambient air influence on the fatigue characteristics at room temperature of metals, as compared to those observed in an inert atmosphere, is well established to be related to the presence of moisture in
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the surrounding environment. However, similar concordance does not exist about the particular mechanism governing it. Thus, depending upon base metal, additional elements, microstructures, load ratio, testing frequency and crack growth rate, the sensitivity to air environment is commonly rationalized in terms of mechanisms related to either active species adsorption (or chemisorption) or hydrogen embrittlement [25]. A large number of investigations in different metallic systems allow to point out a quite strong and mutually interactive relationship between strain localization (and cracking) and each of these mechanisms. From a simple comparison of atmospheric effects on fatigue life of unaged DSS and a single-phase g-SS previously studied [21], i.e. without regarding the influence of microstructural as well as morphological and crystallographic texture differences in austenite (often relevant but not possible to evaluate with the existing data) for both materials, it could be conjectured that the intrinsic environmental sensitivity of ferrite is higher than that of austenite. This is particularly true at applied ⌬⑀pl/2 of 6×10⫺4, condition for which not only the differences are more marked but also plastic deformation is distributed more evenly between both phases; and therefore, the results could be more amenable of comparison in terms of similar local plastic strain within austenite. Speculating that lower fatigue lives in air are associated with some type of hydrogen involvement (either as adsorbed specie or as absorbed and embrittling element), the higher sensitivity of ferrite with respect to that of austenite should be a direct consequence of the widely different hydrogen solubility and diffusivity in both phases, lower and higher respectively in the former than in the latter. For the annealed material, it is envisioned that these intrinsic differences are enlarged, with respect to the single-phase g-SS, by the extrinsic effect associated with the plastic activity of ferrite, because the possible role of slip bands as both hydrogen trapping sites at the ferrite surface and hydrogen diffusion channels within the plastic zone at the austenite-ferrite interfaces, the main barriers for the growth of crack embryos. Thermal aging of DSS allowed to explore possible changes of environmental effects on their fatigue life characteristics as related to alteration on the dominant cyclic deformation mechanisms. Under these conditions, a significant increase in the local cyclic yield stress of the ferritic matrix is induced; and consequently, plastic strain gets concentrated, and microcracks are mainly generated, within austenite. This is true even at the highest imposed ⌬⑀pl/2s studied here, testing conditions for which microcracks were also observed to nucleate in ferrite, although less often than in austenite. Hence, finite fatigue life for aged DSS is mostly given for the possibility of microcracks initiated in austenite, acting as stress concentrators, to localize significant strain in its immediate ferrite neighborhood to induce irreversible
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deformation processes. From this viewpoint, the damage phenomena predominantly observed were local cleavage (lattice decohesion) and highly localized slip; for instance, A and B in Fig. 4 respectively, both well known to be enhanced by either hydrogen adsorption or hydrogen embrittlement. Based upon these ideas, as described above for the annealed material, environmentstrain localization-cracking interactions should also be recalled here as responsible for the pronounced atmospheric effects determined. However, an attempt to rationalize environmental effects on the fatigue life of aged material from those attained for the unaged one seems to be physically meaningless. This is mainly because the particular processes developing and the degree to which they affect such behavior are quite distinct under both conditions. First, slip feasibility is much more limited in the embrittled ferrite, a characteristic well established to enhance environmental sensitivity, than in the plastically operative one in the annealed material. Second, the local plastic strain sustained by austenite at a given imposed ⌬⑀pl/2 is higher for the aged material than for the unaged one; thus, from a simple austenitic viewpoint, environmental effects should be expected to be less on the former than on the latter. Third, the influence of the microstructural changes resulting from the thermal aging, particularly in the ferritic matrix, yields a complete different scenario in terms of the intrinsic sensitivity of this phase to hydrogen adsorption and embrittlement, as recently shown by Iacoviello et al. [26] under static loading conditions. Clearly, the interplay among all these processes is not only very difficult to elaborate but also completely different from that existing for the unaged material. Finally, taking the above issues into consideration, it is expected that DSSs with distinct microstructural (volume fraction, grain size) and crystallographic (texture) characteristics will exhibit different thermal aging effects on the relative environmental influence on their fatigue characteristics.
4. Conclusions A study of the influence of testing atmosphere (air/in vacuo) on LCF characteristics of a DSS at room temperature provides the following conclusions: 1. In air testing resulted in a marked fatigue life lessening as compared to that in vacuo, with the fatigue life ratio between both environments decreasing with increasing imposed plastic strain amplitude. 2. From an extensive and detailed SEM study of the external and fracture surfaces of all the samples studied, ambient air effects on fatigue surface damage should be analyzed in terms of rather enhancing strain localization and cracking phenomena (nucleation and
early growth) than promoting distinct fatigue micromechanisms. 3. The pronounced atmospheric effects determined in the DSS studied here are found to be independent of aging conditions. However, such independence does not seem to be the result of a physically-based transition behavior from one condition to the other; thus, it should not be simply generalized to other DSS with different chemical and microstructural characteristics.
Acknowledgements This research was supported by the Spanish CICYT (Grant MAT96-1009) and by the Spanish MEC and the French MAE (Picasso Programme HF1997-0204). The authors wish to express their sincere appreciation to M. Marsal for her assistance on the SEM studies. They also thank A. Girone`s who assisted in the sample preparation for testing and SEM examination.
References [1] Charles J, Verneau M, Bonnefois B. In: VDEh, editors. Stainless Steels ’96. Du¨sseldorf/Neus: VDEh, 1996:97–103. [2] Magnin T, Lardon JM. Mater Sci Eng 1988;A104:21. [3] Mayaki MC. Ph.D. Thesis, University of Nottingham, Nottingham, 1989. [4] Marrow TJ. Ph.D. Thesis, Cambridge University, Cambridge, 1991. [5] Iturgoyen L. Ph.D. Thesis, Universitat Polite`cnica de Catalunya, Barcelona, 1994. [6] Vogt J-B, Messai A, Foct J. In: Gooch TG, editor. Duplex Stainless Steels ’94, vol. 1, paper 33. Cambridge: TWI, 1994. [7] Perdriset F, Magnin T, Cassange T, Hoch P, Dupoiron F. In: Gooch TG, editor. Duplex Stainless Steels ’94, vol. 3, paper 13. Cambridge: TWI, 1994. [8] Degallaix S, Seddouki A, Degallaix G, Kruml T, Pola´k J. Fatigue Fract Engng Mater Struct 1995;18:65. [9] Bergengren Y, Larsson M, Melander A. Mater Sci Tech 1995;11:1275. [10] Nystrom M. Ph.D. Thesis, Chalmers University, Go¨teborg, 1995. [11] Johansson R. In: Fatigue and Fracture, ASM Handbook, vol. 19. Materials Park (OH): ASM International, 1996:757–68. [12] Llanes L, Mateo A, Iturgoyen L, Anglada M. Acta Mater 1996;44:3967. [13] Mateo A. Ph.D. Thesis, Universitat Polite`cnica de Catalunya, Barcelona, 1997. [14] Iturgoyen L, Anglada M. Fatigue Fract Engng Mater Struct 1997;20:917. [15] Llanes L, Mateo A, Violan P, Me´ndez J, Anglada M. Mater Sci Eng 1997;A234-236:850. [16] Mateo A, Violan P, Llanes L, Me´ndez J, Anglada M. In: Lu¨tjering G, Nowack H, editors. Fatigue ’96. Oxford: Pergamon, 1996:209–14. [17] Mateo A, Llanes L, Iturgoyen L, Anglada M. Acta Mater 1996;44:1143. [18] Christ H-J, Mughrabi H, Wittig-Link C. In: Luka´s P, Pola´k J, editors. Basic Mechanisms in Fatigue of Metals. Brno: Elsevier, 1988:83-92.
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[19] Gotte L, Me´ndez J, Villechaise P, Violan P. In: Lu¨tjering G, Nowack H, editors. Fatigue ’96. Oxford: Pergamon, 1996:167– 72. [20] Verkin BI, Grinberg NM. Mater Sci Eng 1979;41:149. [21] Gerland M, Me´ndez J, Violan P, Ait Saadi B. Mater Sci Eng 1989;A118:83. [22] Beloucif A. Ph.D. Thesis, Ecole Nationale Superieure des Mines de Saint-Etienne, Saint-Etienne, 1995.
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[23] Nystro¨m M, Karlsson B. Mater Sci Eng 1996;A215:26. [24] Gross TS. In: Fatigue and Fracture, ASM Handbook, vol. 19. Materials Park (OH): ASM International, 1996:42–60. [25] Petit J, De Fouquet J, He´naff G. In: Carpinteri A, editor. Handbook of Fatigue Crack Propagation in Metallic Structures. Amsterdam: Elsevier, 1994:1159–204. [26] Iacoviello F, Habashi M, Cavallini M. Mater Sci Eng 1997;A224:116.
Effect of testing atmosphere (air/in vacuo) on low cycle fatigue characteristics of a duplex stainless steel L. Llanes a
a,*
, A. Mateo a, P. Villechaise b, J. Me´ndez b, M. Anglada
a
Departament de Cie`ncia dels Materials i Enginyeria Metal•lu´rgica, ETSEIB, Universitat Polite`cnica de Catalunya, Barcelona E-08028, Spain b Laboratoire de Me´canique et de Physique des Mate´riaux (URA Nº. 863/CNRS), ENSMA, 86960 Futuroscope Cedex, France
Abstract The effect of testing atmosphere on low cycle fatigue characteristics of a duplex stainless steel has been studied at room temperature. Fatigue tests have been conducted under fully reversed plastic strain control and constant plastic strain rate in two different environments: air and in vacuo. The material has been investigated under two distinct conditions: as annealed or unaged and as thermally aged, corresponding to different dominant cyclic deformation mechanisms at the plastic strain amplitudes chosen for the study. In vacuo testing resulted in longer fatigue lives, and consequently, higher cumulative plastic strain than in air experiences for both material conditions. Although prominent fatigue micromechanisms for a given plastic strain amplitude did not seem to be affected by testing atmosphere, for both unaged and aged conditions, strain localization and cracking phenomena were enhanced in air as compared to vacuum. The experimental results were finally discussed in terms of fatigue micromechanisms-environment interactions. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Low cycle fatigue; Environment; Duplex stainless steels; Strain localization
1. Introduction Duplex ferrite-austenite stainless steels (DSSs) are being extensively used for applications in the power, chemical, pulp and paper, and oil industries [1]. The main reasons for their increasing usage are the excellent combination of properties (mechanical strength, toughness, stress corrosion resistance and weldability, among others) as well as the attractive performance parameters (properties to cost ratio, life cycle cost, maintenance expenditures, etc.) that DSSs offer as compared to other alternatives, particularly single-phase ferritic and austenitic stainless steels (α-SS and γ-SS respectively). The fact that many of these applications involve cyclic loading, under normal operating conditions, implies a mandatory need for a better knowledge of the fatigue behavior of DSSs as an important parameter for structural design criteria. Following the above idea, in the last ten years there has been sustained activity towards improved under-
* Corresponding author. Tel.: +34-93-401-6713/06; fax: +34-93401-6706/6600. E-mail address: [email protected] (L. Llanes)
standing of the influence of some intrinsic and extrinsic factors on the fatigue crack initiation and propagation phenomena in DSSs. Such extensive research efforts have demonstrated that material parameters (chemical composition and microstructure), thermal aging and environment greatly affect the fatigue characteristics of these materials [2–15]. This paper will be limited to a quite particular segment of the variety of combinations among environment, loading conditions and material, i.e. atmospheric (air/in vacuo) effects on the high strain fatigue regime of a given DSS. Studies on the influence of environment on the fatigue characteristics of DSSs have concentrated on evaluating corrosion fatigue resistance of these materials in severe aqueous media, e.g. chloride-containing or diluted sulfuric acid solutions, the type of aggressive environments for which DSSs have been conventionally designed and applied (see Refs. [4,11] for an extended literature review on this field). On the other hand, investigations dealing with atmospheric effects on fatigue characteristics of DSSs are only a few [3,4] and they exclusively refer to influence on fatigue propagation of long cracks using air as reference environment. The results of these studies not only point out the complexity of understanding the coupling mechanisms of environment sensitive
0142-1123/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 1 1 2 3 ( 9 9 ) 0 0 0 6 3 - 8
S120
L. Llanes et al. / International Journal of Fatigue 21 (1999) S119–S125
cracking under cyclic loading for each phase but also raise the question on the suitability of damp air as reference media for a complete comprehension of the interaction between microstructure and the referred mechanisms that could lead to improved alloy design. The purpose of this work is therefore to document and discuss the influence of testing atmosphere (air/in vacuo) on the low cycle fatigue (LCF) characteristics of a DSS. In doing so, from the cyclic stress-strain curve of a widely used DSS, three well-defined plastic strain amplitudes (⌬⑀pl/2) were selected to perform constant plastic strain control tests and compare the number of cycles to reach failure in both environments. Emphasis was placed on evaluating fatigue surface damage and fractographic features at given testing atmosphere. The information gathered was then used for analyzing the interaction between environment and fatigue micromechanisms, the latter being dependent upon the imposed ⌬⑀pl/2 and the embrittlement degree of the ferritic constituent (artificially modified by thermal aging) [12,15].
2. Experimental procedure The material studied in this investigation is a mediumalloy DSS of type UNS S31803 (SAF 2205) whose detailed chemical composition has been indicated elsewhere [16]. The material was received as cylindrical rods of 20 mm diameter. It was subsequently annealed at 1050°C for 1 hour followed by water quenching. The resulting microstructure consisted of 45% austenite embedded in a ferritic matrix. The heat treated rods were then machined into cylindrical threaded specimens with a diameter of 6 mm and gage length of 10 mm. All samples were first hand-polished and then electropolished, before testing. Cyclic deformation tests were performed in a symmetrical uniaxial push-pull mode on an electromechanical machine at 25°C in laboratory air or in vacuo (pressure ⬍10⫺3 Pa). Tests were performed in plastic strain controlled mode. From the cyclic stress-strain response for the material studied, determined and documented in a previous work [13,16], three ⌬⑀pl/2s: 6×10⫺4, 2×10⫺3, and 6×10⫺3, were specifically chosen in order to evaluate environmental effects as related to different cyclic deformation mechanisms. The plastic strain rate for all tests was held constant at 2×10⫺3 s⫺1. Attempting to speculate about the intrinsic deformation-environment relationship for each phase at given ⌬⑀pl/2, additional experimental information may be gathered through fatigue testing of thermally aged material. Previous experience by the authors [12] indicates that, as a consequence of such thermal treatment, the ferritic matrix gets embrittled and therefore the cyclic response of DSSs is strongly changed, as compared to that of unaged material, at ⌬⑀pl/2s for which ferrite was plas-
Table 1 Tensile properties of unaged (UA) and aged, at 475°C for 200 hours (A200), UNS S31803 DSS studied in this investigation Material
0.2% offset Yield strength, sY (MPa)
Ultimate tensile Area reduction, AR (%) strength, sUTS (MPa)
UA A200
468 780
686 955
46 21
tically active in the annealed condition, i.e. above 2– 3×10⫺4 for the DSS studied here [13,16]. Hence, a set of thermally aged material (at 475°C for 200 hours, A200) was tested under similar conditions as those described above for the annealed or unaged (UA) one to investigate possible changes of environmental effects on fatigue life characteristics as related to alterations on the dominant cyclic deformation mechanisms. The basic tensile parameters of the studied DSS under both conditions are summarized in Table 1. Finally, external and fracture surfaces of all the tested specimens were carefully analyzed by scanning electron microscopy (SEM). Special attention was directed towards examining fatigue damage features in each phase of the DSS.
3. Results and discussion 3.1. Cyclic response and fatigue life characteristics The relationship between the stress amplitude, ⌬s/2, and the number of cycles for the tests performed in vacuo is plotted in Fig. 1 for the different material conditions and ⌬⑀pl/2s investigated. The ⌬s/2 evolution observed during fatigue in vacuo was similar to that in air; thus, results obtained under the latter condition are
Fig. 1. Cyclic hardening-softening response of unaged (UA) and aged (A200) DSS under plastic strain control testing in vacuo and in air.
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only indicated by the time at which ⌬s/2 dropped off suddenly, corresponding to rapid propagation of the fatal crack. As previously found for the cyclic response of another DSS within the strain range here studied [12,17], hardening is always observed during the first few cycles, followed by a gradual softening which is more pronounced for the aged material. For the annealed material a saturation stage is reached for all the imposed strain amplitudes. On the other hand, the achievement of such steady state for the aged condition is only observed for the lowest amplitude and under inert environment. All the other testing conditions yield, after the primary hardening, a continuous ⌬s/2 decrease with increasing number of cycles up to failure. The peak ⌬s/2 for a given ⌬⑀pl/2 is about 250–400 MPa higher for the aged material than for the annealed one; however, the ⌬s/2 difference diminishes with increasing number of cycles up to values between 50 and 200 MPa under final conditions. This behavior should be associated initially with the overall yield strength increment observed for the aged material and later with the synergetic interactions among dislocation activity, deformation twinning, demodulation of spinodal microstructure and activation of other fracture micromechanisms (e.g. cleavage) in ferrite at higher cumulative plastic strain, i.e. with increasing number of cycles and/or imposed ⌬⑀pl/2 [12]. Effects of testing atmosphere on LCF life of the DSS studied under both annealed and thermally aged conditions are given in Fig. 2. The fatigue lives in vacuo were more than ten times longer than those in air at the lowest ⌬⑀pl/2 studied. This value is similar to that observed for copper [18,19] or commercial iron [20] but much higher than the one evaluated for a single-phase γ-SS [21] under similar testing conditions. At applied ⌬⑀pl/2s of 2×10⫺3 and 6×10⫺3, the fatigue life ratio related to both environments decreases to values between 2 and 4, a range close to that typically found
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for other metallic materials within such high strain regime [18–22]. Independently of the applied ⌬⑀pl/2, thermal aging does not seem to affect the relative influence of environment on the fatigue life characteristics of the DSS studied. Taking into consideration that distinct cyclic deformation mechanisms are active for annealed and aged materials under the testing conditions here studied [12,17], such finding was rather unexpected. Concomitantly, testing atmosphere does not influence the existence of a transition behavior associated with aging effects on the fatigue life of DSSs at ⌬⑀pl/2 about 2– 3×10⫺3. This relationship between thermal aging and fatigue life characteristics of DSS, as a function of imposed ⌬⑀pl/2, has been documented and analyzed previously [14,16,23] and will not be discussed further in this work. Nevertheless, it is worthwhile to point out that microstructurally different DSSs, as those studied by Iturgoyen and Anglada (38%g/62%a; 0.07%N) [14], Nystro¨m and Karlsson (75%g/25%a; 0.20%N) [23] and in this investigation (45%g/55%a; 0.13%N), exhibit such a transition at quite close applied ⌬⑀pl/2 values, yielding another example of the complex deformation interaction existing in these two-phase materials. Finally, the experimental points followed a Manson-Coffin correlation, i.e. ⌬⑀pl/2= ⑀f⬘(2Nf)c. The corresponding ⑀f⬘ and c parameters are given in Table 2. Here, as a consequence of the more pronounced fatigue life enhancement found with decreasing applied ⌬⑀pl/2, the c coefficient increases when comparing results from tests performed in vacuo with those conducted in air. 3.2. Fatigue damage and fractographic features Fatigue damage on the external surfaces of all the tested samples was evaluated by SEM. Typical features induced by fatigue in annealed and aged material are presented in Figs. 3–5. As expected, from previous experience on cyclic deformation of DSSs [12,13,15– 17], plastic activity is found in both phases for the unaged condition, whereas it gets concentrated in austenite for the aged one. For the annealed material, although strain localization is enhanced in ferrite with increasing ⌬⑀pl/2, crack nucleation sites are evenly distributed Table 2 LCF properties (Manson-Coffin parameters) of the DSS investigated, under UA and A200 conditions, in air and in vacuo Material-Testing atmosphere
Fig. 2. Effect of testing atmosphere on the LCF life of DSS under annealed (UA) and aged (A200) conditions.
UA-air UA-in vacuo A200-air A200-in vacuo
Fatigue ductility coefficient, ⑀f⬘ 0.708 0.552 0.094 0.077
Fatigue ductility exponent, c ⫺0.668 ⫺0.547 ⫺0.429 ⫺0.356
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Fig. 3. Slip features on the external surface of UA-DSS tested at ⌬⑀pl/2 of 2×10⫺3 in air.
Fig. 5. Fatigue surface damage features in A200-DSS tested at ⌬⑀pl/2 of 2×10⫺3: (a) in air, and (b) in vacuo. Fig. 4. Nucleation and early growth of microcracks in A200-DSS tested at ⌬⑀pl/2 of 6×10⫺3 in vacuo.
among ferrite, austenite, and at the interfaces between them for all the ⌬⑀pl/2s studied. Under aged conditions, microcracks nucleate at either marked extrusions in austenite (dominant) or through local cleavage in the embrittled ferritic matrix. The higher the imposed ⌬⑀pl/2, the more widespread the occurrence of the latter fracture micromechanism. Although the above fatigue micromechanisms were independent of the testing atmosphere, the density of microcracks at failure was higher in the samples tested in air than in those evaluated in vacuo for a given applied ⌬⑀pl/2s. On the other hand, the opposite was observed in terms of density of slip bands in the plastically active phases. An example of the above remarks is given in Fig. 5. The facts that: 1) higher cumulative plastic strain at failure, for a given ⌬⑀pl/2, was always attained in the experiences conducted in vacuo than in those carried out in air; and 2) most of the observed microcracks were associated with slip-related phenomena, allow to translate the found higher microcrack density in similarly higher strain localization levels within the corresponding
slip features for the specimens tested in air. Hence, the detrimental environment (air) effect on fatigue life should be described in terms of enhanced strain localization, and thus, easier microcrack initiation and early growth, the latter being an alternative mechanism of stress relaxation and further deformation. Under vacuum, such cracking phenomena are delayed as a consequence of less localized strain, possibly associated with a more reversible deformation at slip bands. From a fractographic viewpoint, there were not discernible differences on the fracture surface appearance of materials tested under distinct environmental conditions. They were characterized, as it has been reported before [3,4,14,23], by ductile striations in both phases for the annealed material (Fig. 6a). On the other hand, for the aged condition similar striations were exclusively found in austenite, whereas cleavage was observed to be the dominant fractographic characteristic in the ferritic matrix (Fig. 6b). With increasing applied ⌬⑀pl/2 ‘tire tracks’ were widely observed, as clearly shown in Fig. 6c. The occurrence of these periodic markings under low-cycle tension-compression fatigue in the DSS studied here is in total agreement, in terms of testing con-
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Fig. 6. Fracture surfaces of studied specimens: (a) ductile striations in UA-DSS tested at ⌬⑀pl/2 of 2×10⫺3 in air; (b) ductile striations and cleavage features in A200-DSS tested at ⌬⑀pl/2 of 2×10⫺3 in air; and (c) ‘tire-tracks’ in UA-DSS tested at ⌬⑀pl/2 of 6×10⫺3 in vacuo.
ditions, with previous findings of tire tracks in other structural materials [24]. 3.3. Fatigue micromechanisms-environment interactions in DSS The detrimental ambient air influence on the fatigue characteristics at room temperature of metals, as compared to those observed in an inert atmosphere, is well established to be related to the presence of moisture in
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the surrounding environment. However, similar concordance does not exist about the particular mechanism governing it. Thus, depending upon base metal, additional elements, microstructures, load ratio, testing frequency and crack growth rate, the sensitivity to air environment is commonly rationalized in terms of mechanisms related to either active species adsorption (or chemisorption) or hydrogen embrittlement [25]. A large number of investigations in different metallic systems allow to point out a quite strong and mutually interactive relationship between strain localization (and cracking) and each of these mechanisms. From a simple comparison of atmospheric effects on fatigue life of unaged DSS and a single-phase g-SS previously studied [21], i.e. without regarding the influence of microstructural as well as morphological and crystallographic texture differences in austenite (often relevant but not possible to evaluate with the existing data) for both materials, it could be conjectured that the intrinsic environmental sensitivity of ferrite is higher than that of austenite. This is particularly true at applied ⌬⑀pl/2 of 6×10⫺4, condition for which not only the differences are more marked but also plastic deformation is distributed more evenly between both phases; and therefore, the results could be more amenable of comparison in terms of similar local plastic strain within austenite. Speculating that lower fatigue lives in air are associated with some type of hydrogen involvement (either as adsorbed specie or as absorbed and embrittling element), the higher sensitivity of ferrite with respect to that of austenite should be a direct consequence of the widely different hydrogen solubility and diffusivity in both phases, lower and higher respectively in the former than in the latter. For the annealed material, it is envisioned that these intrinsic differences are enlarged, with respect to the single-phase g-SS, by the extrinsic effect associated with the plastic activity of ferrite, because the possible role of slip bands as both hydrogen trapping sites at the ferrite surface and hydrogen diffusion channels within the plastic zone at the austenite-ferrite interfaces, the main barriers for the growth of crack embryos. Thermal aging of DSS allowed to explore possible changes of environmental effects on their fatigue life characteristics as related to alteration on the dominant cyclic deformation mechanisms. Under these conditions, a significant increase in the local cyclic yield stress of the ferritic matrix is induced; and consequently, plastic strain gets concentrated, and microcracks are mainly generated, within austenite. This is true even at the highest imposed ⌬⑀pl/2s studied here, testing conditions for which microcracks were also observed to nucleate in ferrite, although less often than in austenite. Hence, finite fatigue life for aged DSS is mostly given for the possibility of microcracks initiated in austenite, acting as stress concentrators, to localize significant strain in its immediate ferrite neighborhood to induce irreversible
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deformation processes. From this viewpoint, the damage phenomena predominantly observed were local cleavage (lattice decohesion) and highly localized slip; for instance, A and B in Fig. 4 respectively, both well known to be enhanced by either hydrogen adsorption or hydrogen embrittlement. Based upon these ideas, as described above for the annealed material, environmentstrain localization-cracking interactions should also be recalled here as responsible for the pronounced atmospheric effects determined. However, an attempt to rationalize environmental effects on the fatigue life of aged material from those attained for the unaged one seems to be physically meaningless. This is mainly because the particular processes developing and the degree to which they affect such behavior are quite distinct under both conditions. First, slip feasibility is much more limited in the embrittled ferrite, a characteristic well established to enhance environmental sensitivity, than in the plastically operative one in the annealed material. Second, the local plastic strain sustained by austenite at a given imposed ⌬⑀pl/2 is higher for the aged material than for the unaged one; thus, from a simple austenitic viewpoint, environmental effects should be expected to be less on the former than on the latter. Third, the influence of the microstructural changes resulting from the thermal aging, particularly in the ferritic matrix, yields a complete different scenario in terms of the intrinsic sensitivity of this phase to hydrogen adsorption and embrittlement, as recently shown by Iacoviello et al. [26] under static loading conditions. Clearly, the interplay among all these processes is not only very difficult to elaborate but also completely different from that existing for the unaged material. Finally, taking the above issues into consideration, it is expected that DSSs with distinct microstructural (volume fraction, grain size) and crystallographic (texture) characteristics will exhibit different thermal aging effects on the relative environmental influence on their fatigue characteristics.
4. Conclusions A study of the influence of testing atmosphere (air/in vacuo) on LCF characteristics of a DSS at room temperature provides the following conclusions: 1. In air testing resulted in a marked fatigue life lessening as compared to that in vacuo, with the fatigue life ratio between both environments decreasing with increasing imposed plastic strain amplitude. 2. From an extensive and detailed SEM study of the external and fracture surfaces of all the samples studied, ambient air effects on fatigue surface damage should be analyzed in terms of rather enhancing strain localization and cracking phenomena (nucleation and
early growth) than promoting distinct fatigue micromechanisms. 3. The pronounced atmospheric effects determined in the DSS studied here are found to be independent of aging conditions. However, such independence does not seem to be the result of a physically-based transition behavior from one condition to the other; thus, it should not be simply generalized to other DSS with different chemical and microstructural characteristics.
Acknowledgements This research was supported by the Spanish CICYT (Grant MAT96-1009) and by the Spanish MEC and the French MAE (Picasso Programme HF1997-0204). The authors wish to express their sincere appreciation to M. Marsal for her assistance on the SEM studies. They also thank A. Girone`s who assisted in the sample preparation for testing and SEM examination.
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