THE EFFECT OF TEMPERATURE AND MEAN STRESS ON THE FATIGUE BEHAVIOUR OF TYPE 304L STAINLESS STEEL INTRODUCTION

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1 THE EFFECT OF TEMPERATURE AND MEAN STRESS ON THE FATIGUE BEHAVIOUR OF TYPE 34L STAINLESS STEEL H.-J. Christ, C. K. Wamukwamba and H. Mughrabi The fatigue behaviour of the austenitic stainless steel AISI34L was studied in stress-controlled tests as a function of temperature and superimposed mean stress. The cyclic deformation behaviour is characterized by a relatively large plastic strain amplitude at the beginning of the test, a pronounced cyclic hardening during the first cycles and a subsequent stabilized condition. Dynamic strain ageing leads to a maximum in cyclic strength which is reached at about 45 C. Consequently, a maximum of the number of cycles to failure was found close to this temperature. Both compressive and tensile mean stresses reduce the plastic strain amplitude of the cyclic saturation state and give rise to cyclic creep which was found to be much higher at tensile mean stresses as compared to compressive mean stresses. However, the effect of cylic creep is less important at temperatures at which the material exhibits relatively high strength. Then, at low stress amplitudes, positive mean stresses were found to even extend cyclic life, since the beneficial effect of a reduced plastic strain amplitude outweighs the detrimental influence of cyclic creep. INTRODUCTION Austenitic stainless steels such as AISI 34L are often used for high-temperature applications. In many cases corresponding components are subjected to a combination of static and cyclic load. Hence, a detailed knowledge of the high-temperature fatigue behaviour of these materials is necessary, taking the effect of a superimposed mean stress into account. Earlier studies on the isothermal and thermomechanical cyclic stress-strain response of AISI 34L (1,2) clearly documented the important effect of dynamic strain ageing (DSA) on cyclic deformation behaviour and fatigue life. In an intermediate temperature interval the occurrence of DSA processes leads to a change in the dislocation slip character from a wavy type to a more planar type. Various mechanisms are proposed to be responsible for the impeded dislocation motion. Besides the interaction of dislocations with carbon-vacancy pairs, clusters of carbon atoms (3) and substitutional Cr atoms (4), also the formation of small shearable coherent carbide precipitates during cyclic loading is of significance. These processes give rise to an enhanced cyclic strength. Consequently, one would expect that at constant plastic strain amplitude the increase of the saturation stress amplitude in the temperature range of DSA would lead to a reduction of cyclic life. Institut für Werkstofftechnik, Universität-GH-Siegen. School of Engineering, University of Zambia.

2 Institut für Werkstoffwissenschaften, Lehrstuhl 1, Universität Erlangen-Nürnberg.

3 However, a continuous decrease of fatigue life with increasing temperature was observed, since environmental effects as well as creep-fatigue interaction gain importance. Moreover, if an extremely planar slip mode is established, as found in a related study on AISI 316L (5), the negative effect of the enhanced stress amplitude can be outweighed by the beneficial influence of a more homogeneous slip distribution, if testing is done in vacuum. In most of the previous studies on the fatigue behaviour of type AISI 34L stainless steel at elevated temperature, stress control was applied (e.g. (6,7)). In this testing mode, the enhanced cyclic strength in the temperature range of DSA should lead to a reduced plastic strain amplitude, less cyclic creep (if mean stresses are applied) and prolonged cyclic life. Unfortunately, no particular attention was paid to dynamic strain ageing and its consequences in these studies. This paper attempts to deal with these aspects. The results of a study of the cyclic stress-strain behaviour, the cyclic life and the corresponding microstructural changes of AISI 34L as a function of stress amplitude, temperature and mean stress are reported. Special emphasis is put on the temperature range of DSA. The results presented were obtained as part of a comprehensive comparative study on a ferritic and an austenitic steel (8,9). The work (8) was performed at the University of Erlangen- Nürnberg, where all authors were present at that time. The behaviour of the ferritic steel used in this study has been reported elsewhere (1). EXPERIMENTAL PROCEDURE Prior to machining of the specimens for fatigue testing, the austenitic stainless steel AISI 34L (composition in wt%:.2 C,.4 Si, 1.68 Mn, 18.1 Cr, 9.9 Ni,.26 Cu,.34 Mo, and.1 V) was subjected to a two-step heat treatment. In a first step, the alloy was solution annealed at 11 C for 3 hours and water quenched. In order to obtain a microstructure that would be stable at the test temperatures, an annealing at 6 C for 1 hours was carried out subsequently. The resulting microstructure is characterized by a mean grain size of 114 m, a high portion of twins and carbide precipitates of type M 23 C 6 located at the grain boundaries. Specimens with cylindrical gauge length were machined from bars of 25 mm in diameter. Prior to testing, the gauge length of the samples was mechanically ground and polished and finally electropolished in a last preparation step to avoid any influence of the surface condition on fatigue life. An electro-mechanical test system was used for the fatigue tests. This system is equipped with a three-zone resistance furnace and hydraulic grips suitable for testing at elevated temperatures. While most of the tests were carried out in laboratory air, during some of the experiments the furnace was continuously flushed with a gas mixture consisting of Ar with 1% H 2 in order to reduce oxidation effects. Cyclic loading was carried out at a frequency of.25 Hz using stress control. Each test was performed at a constant temperature T ranging from room temperature up to 65 C and at a constant stress amplitude /2 between 2 MPa and 365 MPa. In order to study the effects arising from mean stresses m, values of m between 6 MPa and 6 MPa were superimposed in these stress-controlled tests. In all cases, the strain response was measured directly within the gauge length by means of a capacitive high-temperature extensometer. In order to characterize the dislocation arrangements and to reveal differences in the microstructure due to the influence of temperature and mean stress, transmission electron microscopy (TEM) was applied to samples cut both parallel and perpendicular to the stress

4 axis and thinned electrolytically. The crack initiation site and the type of crack propagation were determined mainly by using scanning electron microscopy (SEM) to study the surface of the samples within the gauge length as well as the fracture surface.

5 RESULTS AND DISCUSSION Cyclic Deformation Behaviour and Fatigue Life under Symmetric Loading Conditions The general cyclic deformation behaviour of the material is characterized by a large plastic strain amplitude at the beginning of the test, a pronounced cyclic hardening during the first few cycles and a subsequent stabilized behaviour. This cyclic stress-strain response can be attributed to the relatively soft condition prior to cyclic loading, the fast increase of the dislocation density at the beginning of cyclic plastic deformation and the subsequent formation of a characteristic steady-state dislocation arrangement. Only at room temperature and high stress amplitudes, a secondary cyclic hardening was observed which is a result of deformation-induced martensite formation (11). As a consequence of dynamic strain ageing processes, enhanced cyclic strength is reached in an intermediate temperature range around 45 C. DSA manifests itself in a minimum of the saturation plastic strain amplitude ( pl /2) S, since the stress amplitudes were held constant in the tests of this study. Figure 1 shows in an exemplary fashion for a /2 value of 25 MPa the dependence of ( pl /2) S on the temperature. Data points obtained in laboratory air and in Ar gas containing 1% H 2 are plotted. As expected, the cyclic deformation response is independent of the oxidation event on the surface. This does not hold true for the number of cycles to failure N f (Fig. 1). Obviously, the effect of the surrounding environment on N f is most pronounced in the intermediate temperature range. This can be understood, since on the one hand at low temperatures the interaction with the gas phase is not of significance for the corrosion-resistant material studied. On the other hand, SEM investigation documented that at high temperatures the material suffers damage in the interior due to fatigue-creep interaction and that the Ar-H 2 gas mixture does not prevent the alloy from oxidation. More important than the change of N f with environment is the observation that cyclic life is enhanced in the temperature range of DSA under the test conditions used (i.e., constant stress amplitude). The location of the relative maximum in the course of N f versus temperature corresponds to that of the minimum of ( pl /2) S. The impeding effect of DSA processes to dislocation motion leads to a change in the characteristic dislocation arrangement of the cyclic saturation state with temperature. Figure 2 shows three TEM micrographs of typical stabilized structures formed during cyclic loading at a stress amplitude of 25 MPa at temperatures below (Fig. 2a), within (Fig. 2b) and above (Fig. 2c) the temperature range of DSA. At low temperatures, the dislocations form cells that exhibit relatively broad and disorderly cell walls. The dislocation arrangement found at 525 C is very similar, but appears to be slightly more orderly (Fig. 2c). In the temperature range of DSA (Fig. 2b) the dislocations form broad walls and bundles. The Effect of Mean Stress on Cyclic Deformation Behaviour In order to understand the influence of a mean stress on cyclic life, two aspects must be taken into account. Firstly, a mean stress may change the cyclic transient (softening/hardening) behaviour and hence may affect the extent of cyclic plasticity. Secondly, cyclic creep may take place so that a monotonic plastic strain is superimposed on the cyclic one.

6 Figure 3 deals with the first aspect by showing the influence of mean stress on the value of ( pl /2) S. The data points are plotted as a function of temperature. The course of the curves is not fundamentally affected by the mean stress value applied, and shows the cyclic strength increase in the temperature range of DSA. Interestingly enough, both compressive and tensile mean stresses reduce the plastic strain amplitude of the cyclic saturation state, though it should be noted that this effect is more marked, if the superimposed mean stress is negative, i.e. compressive. An explanation of this behaviour can be given on the basis of the strong cyclic hardening shown by the alloy at the beginning of cyclic loading. Obviously the maximum absolute value of stress is more important for the extent of cyclic hardening than the stress amplitude. Information on the second aspect is provided in Fig. 4 for room temperature in the form of a representation of the plastic mean strain versus the number of loading cycles for various values of m. Generally, the extent of cyclic creep occurring during fatigue testing as a consequence of a superimposed mean stress was found to be much higher at tensile mean stresses as compared to negative values. However, the effect of cyclic creep is less important, if the material exhibits a relatively high strength (i.e., at temperatures of DSA). TEM observations revealed that mean stresses did not cause a marked change in the microstructure. However, a tendency for positive mean stresses to promote dislocation cells and for negative mean stresses to form more planar arrangements was found. These observations are in accord with the total plastic deformation taking place under these conditions. Effect of Mean Stress on Cyclic Life Tensile mean stresses are usually considered as detrimental to cyclic life while compressive mean stresses may be beneficial. However, under certain conditions, a positive mean stress can lead to an increase of N f. This is shown in Fig. 5 for room temperature (a) and 525 C (b). The basic requirement for a cyclic life extension as a result of a tensile mean stress is that the positive effect of a stronger initial cyclic hardening (i.e., the reduction of ( pl /2) S ) outweighs the negative influence of cyclic creep. For the material studied this is fulfilled, if the temperature provides high cyclic strength (e.g. by means of DSA) and the material is subjected to a low stress amplitude and to a moderate mean stress. REFERENCES (1) Petry, F., Christ, H.-J. and Mughrabi, H., "Microstructure and Mechanical Properties of Materials", Tenckhoff, E. and Vöhringer O. (eds), DGM Informationsgesellschaft Verlag, Oberursel, 1991, (2) Zauter, R., Petry, F., Christ, H.-J. and Mughrabi, H., "Thermomechanical Fatigue Behavior of Materials", ASTM-STP 1186, Sehitoglu, H. (ed.), Philadelphia, 1993, 7-9. (3) Rose, K. S. B. and Glover, S. G., Acta Metallurgical, Vol. 14, 1966, (4) Jenkins, C. F. and Smith, G. V., TMS-AIME, Vol. 245, 1969, (5) Gerland, M., Mendez, J., Lépinoux, J. and Violan, P., Mater. Sci. Engng, Vol. A164, 1993, (6) Turner, A. P. L., Met. Trans. A, Vol. 1A, 1979, (7) Turner, A. P. L. and Martin, T. J., Metall. Trans. A, Vol. 11A, 198,

7 (8) Wamukwamba, C. K., Doctorate Thesis, Universität Erlangen-Nürnberg, (9) Mughrabi, H. and Christ, H.-J., ISIJ Intern., Vol. 37, 1997, (1) Christ, H.-J., Wamukwamba, C. K. and Mughrabi H., Mater. Sci. Engng, Vol. A234- A236, (11) Bayerlein, M., Christ, H.-J. and Mughrabi H., Mater. Sci. Engng, Vol. A114, 1989, L11-L16.

8 plastic strain amplitude [%] number of cycles to failure plastic strain amplitude [%] 1 1 air Ar + 1% H temperature [ C] Figure 1. Plastic strain amplitude of saturation and cyclic life at a stress amplitude of 25 MPa a) b) c) Figure 2. TEM micrographs of the stabilized dislocation arrangement formed at a stress amplitude of 25 MPa at 25 C (a), 4 C (b), and 525 C (c) 1 /2 = 25 MPa m = MPa 2 MPa -2 MPa -4 MPa temperature [ C] Figure 3. Effect of mean stress on plastic strain amplitude of saturation

9 number of cycles to failure number of cycles to failure plastic mean strain [%] m = 2 MPa 6 MPa MPa T = 2 C /2 = 3MPa -2 MPa -6 MPa number of cycles Figure 4. Plastic mean strain as a function of cycle number for tests with mean stress 5 4 b T = 2 C /2 = 25 MPa MPa mean stress [MPa] 5 4 a T = 525 C /2 = MPa 1 25 MPa mean stress [MPa] Figure 5. Effect of mean stress on fatigue life for 25 C (a) and 525 C (b)