Fatigue Behavior of Metastable Austenitic Stainless Steels in LCF, HCF and VHCF Regimes at Ambient and Elevated Temperatures

Abstract

Corrosion resistance has been the main scope of the development in high-alloyed low carbon austenitic stainless steels. However, the chemical composition influences not only the passivity but also significantly affects their metastability and, consequently, the transformation as well as the cyclic deformation behavior. In technical applications, the austenitic stainless steels undergo fatigue in low cycle fatigue (LCF), high cycle fatigue (HCF), and very high cycle fatigue (VHCF) regime at room and elevated temperatures. In this context, the paper focuses on fatigue and transformation behavior at ambient temperature and 300 °C of two batches of metastable austenitic stainless steel AISI 347 in the whole fatigue regime from LCF to VHCF. Fatigue tests were performed on two types of testing machines: (i) servohydraulic and (ii) ultrasonic with frequencies: at (i) 0.01 Hz (LCF), 5 and 20 Hz (HCF) and 980 Hz (VHCF); and at (ii) with 20 kHz (VHCF). The results show the significant influence of chemical composition and temperature of deformation induced α´-martensite formation and cyclic deformation behavior. Furthermore, a “true” fatigue limit of investigated metastable austenitic stainless steel AISI 347 was identified including the VHCF regime at ambient temperature and elevated temperatures.

1. Introduction

In the early stages of the development of stainless steels, the passivity of the material and, therefore, its “stainlessness” was the main scope of development [1,2]. Chemical passivity of steels in many environmental conditions is achieved by alloying at least about 11 wt% chromium to the base material. Due to an excellent combination of mechanical and technological properties, as well as corrosion resistance, austenitic stainless steels are the most prevalent group of stainless steels—widely used for components in nuclear power and chemical plants as well as a great variety of industrial, architectural and biological applications [3,4,5,6]. Since the chromium contents of typical austenitic stainless steels exceed 16 wt%, their equilibrium microstructure at room temperature would be fully ferritic, if no other austenite stabilizing alloying elements were added to the material. Elements most often used to obtain an austenitic microstructure are nickel, manganese, carbon and nitrogen. Because carbon has a very high affinity with chromium, chromium carbides are prone to develop particularly at high temperatures. This can reduce the chromium solid solution content in the austenitic matrix to less than 11 wt% and, consequently, lead to localized loss of stainlessness, especially at, or close to, the grain boundaries, resulting—under certain conditions—in intercrystalline corrosion. Therefore, in order to assure the corrosion resistivity of austenitic stainless steels, a very low carbon content and/or alloying with elements of a higher affinity with carbon, (e.g. niobium or titanium) are required [4]. In some cases, the ferromagnetic body centered cubic (bcc) δ-ferrite can be obtained in the paramagnetic face centered cubic (fcc) γ-austenitic microstructure directly after the manufacturing or melting process [7,8,9,10]. Indeed, δ-ferrite is a stable phase and its volume fraction does not change during mechanical loading. Generally, the influence of the chemical composition of Cr-Ni stainless steels on the initial microstructure at ambient temperature (AT) obtained after solution annealing heat treatment can be roughly estimated from the Cr und Ni content using the Maurer diagram [1]. Furthermore, the Schaeffler diagram provides more detailed information based on Cr and Ni equivalents taking other alloying elements besides Cr and Ni into account and is generally used for determining welding microstructures in Cr-Ni stainless steels [11].

The change in chemical composition not only influences the passivity of austenitic stainless steel but also significantly affects the metastability of austenite [12,13,14,15,16], meaning it is susceptible to undergoing phase transformations. Hence, the paramagnetic γ-austenite can transform by plastic deformation to a more stable microstructure, i.e. paramagnetic hexagonal close-packed (hcp) ε-martensite and/or body cubic centered (bcc) α´-martensite [17,18,19,20,21,22,23,24]. Scheil, one of the earliest researchers in this field, investigated the γ-austenite to α´-martensite transformation by measuring magnetic properties [25]. The change in magnetic properties by phase transformation can be used for non-destructive detection of α´-martensite as well as for monitoring fatigue processes in metastable austenites [26]. However, δ-ferrite, in the initial state of metastable austenite, has to be clearly separated from deformation-induced α´-martensite because both phases are ferromagnetic [7,8,9,10]. Deformation-induced phase transformations in metastable austenite affect significantly the fatigue process and are influenced by chemical composition [12,13,14,15,16], temperature [14,27,28,29,30] (encompassed by stacking fault energy (SFE) [31,32,33,34,35,36,37,38,39]), as well as grain size [32,40,41,42,43] strain amplitude [29,44,45,46,47] strain rate [48,49,50] and strain/stress state [51]. Additionally, SFE in fcc materials influences the slip character of dislocation, which has been summarized in the cyclic deformation map as a function of SFE and plastic strain amplitude given in [52,53,54]. For low SFE, independent of strain (stress) amplitude, the planar slip character occurs but for materials with higher SFE, a dependency of strain (stress) amplitude on dislocation slip character exists. High strain (stress) amplitude in the low cycle fatigue (LCF) regime leads to the development of the cell dislocation structure, which correlates with the wavy slip character of dislocation. The decrease of strain (stress) amplitude results in the formation of persistent slip bands in the high cycle fatigue (HCF) regime. Between the LCF and HCF regimes and lower and higher SFE, mixed dislocation structures can be observed [52,53,54]. A cyclic dislocation structure map of fcc materials with different SFE is not currently available for the very high cycle fatigue (VHCF) regime. The relationship between dislocation slip character and SFE described above was found for stable fcc materials [52,53,54]. In the case of metastable fcc austenite, SFE influences not only the dislocation slip character but also the deformation-induced phase transformation in ε and/or α´-martensite [17,18,19,20,21,22,23,24] as well as the formation of twins [55,56,57]. Figure 1 summarizes the microstructural changes in metastable austenitic Cr-Ni stainless steels due to cyclic mechanical loading, supported by TEM micrographs, which influence the cyclic deformation behavior of metastable austenitic stainless steels and consequently fatigue life. In this context, the present paper focuses on microstructural changes in metastable austenitic stainless steels, their cyclic deformation and transformation behavior at ambient and elevated temperatures in the whole fatigue regime from LCF to VHCF.

2. Materials

The investigated material was metastable austenitic stainless steel AISI 347 (X10CrNiNb1810, 1.4550) in two batches (A and B). The chemical composition of both batches is given in Table 1, which corresponds to German and international standards [60,61]. However, it is important to note that these standards do not take into consideration the metastability of the austenitic microstructure, but focus on the stainlessness of the steel. It is known that the same type of austenitic stainless steel can exist in significantly different states of metastability [16] up to a fully stable state. The material’s metastability can be characterized by experimentally estimated equations, which describe the martensitic start (M_S) or deformation-induced transformation temperature (M_d) as well as the stacking fault energy (SFE), according to the chemical composition of austenitic stainless steels. In the literature, several equations for M_S [13], M_d30 [14] and SFE [34] are given. Table 1 gives examples of these metastability parameters according to the following equations:

M_d30,Angel in °C = 413 – 462 · (C+N) – 13.7 · Cr – 9.5 · Ni – 8.1 · Mn – 18.5 · Mo – 9.2 · Si(1)

M_S,Eichelmann in °C = 1350 – 1665 · (C+N) – 42 · Cr – 61 · Ni – 33 · Mn – 28 · Si(2)

SFE in mJ/m² = 25.7 + 2 · Ni – 0.9 · Cr – 1.2 · Mn + 410 · C – 77 · N – 13 · Si(3)

The investigated austenitic stainless steel batches are in a metastable state (M_d30 is in the range of ambient temperature). M_d30 is the temperature at which 50 vol% of α´-martensite is developed by 30% of true plastic deformation [14] and was introduced for the comparison of the metastability of austenitic stainless steels. However, the deformation-induced phase transformation from γ-austenite in to α´-martensite can also take place at higher temperatures than M_d30 [46,58,59,62,63]. The amount of deformation induced α´-martensite depends: (i) on the initial conditions, given by production process, such as chemical composition and initial microstructure e.g. the grain size of austenite, dislocation arrangements/density, precipitations and (ii) on loading parameters, like deformation temperature, amount of plastic deformation, as well as the stress and strain state/rate. Therefore, determining the true M_d-temperature above which no α´-martensite formation takes place is not practically possible. In order to take into account both aspects specified above (i and ii) on the susceptibility of forming α´-martensite in metastable austenite, a method based on dynamically applied local plastic deformation and magnetic measurement was developed [16]. The parameter established by this method as I_ξ correlates very well with the grade of α´-martensite formation during cyclic loading and allows to distinguish the susceptibility of austenitic stainless steels, which have the same chemical composition and grain size, to undergo phase transformation. The I_ξ parameter of the two investigated batches of AISI 347 is given in Table 1. Table 2 summarizes the mechanical properties of the investigated material at ambient temperature (AT) and T = 300 °C as well as α’-martensite content after specimen failure.

Shown in Figure 2 are light and electron microscopy micrographs of longitudinal sections of the initial microstructure of the investigated materials in the solution-annealed state and after plastic deformation. Specimens from batch A were extracted from an original nuclear power plant surge line pipe with an outside diameter of 333 mm and a wall thickness of 36 mm. The pipe was manufactured seamless, drilled from the inside, turned from the outside and delivered in a solution-annealed state (1050 °C / 10 min / H₂0), such that in the initial state no α‘-martensite was detected. Note that the surge line pipe was investigated in the as-manufactured condition and has not been previously used in a nuclear power plant. The material of batch B was provided as rolled bars with a diameter of 25 mm in a solution-annealed state. A fully austenitic microstructure was obtained by additional annealing at 1050 °C for 35 minutes and quenching in helium atmosphere.

In Figure 2a the optical micrograph of the initial state of batch A is shown. This micrograph was taken after color etching using a Bloech & Wedl I etching agent, which is able to visualize local inhomogeneities in chemical composition [15]. A homogeneous distribution of the Cr and Ni content was detected in the microstructures of batch A. Furthermore, plastic deformation of batch A led to the homogeneous development of α´-martensite in the austenite matrix during plastic deformation (see Figure 2b,d). An electron backscatter diffraction (EBSD) micrograph of the initial state of both batches (Figure 2c,g) revealed a one-phase microstructure with annealing twins, typical for austenitic stainless steels. The same etching and observation techniques were used for characterizing the microstructure of batch B. In the case of batch B, the etching agent revealed local chemical inhomogeneities caused by slight variations of the Cr and Ni content as blue and brown bands (Figure 2e), which could not be removed during solution annealing. The blue band correlates with a lower Ni and higher Cr content, while the brown bands indicate higher Ni and lower Cr contents [15]. The band structure with a corresponding local metastability of austenitic microstructure led to a pronounced α´-martensite formation in regions with higher Cr content (Figure 2f). The local chemically induced band structure could not be observed in scanning electron micrographs using EBSD technique (Figure 2h). Instead, the EBSD image shows a homogeneous crystallographic microstructure in both cases, whereas the deformation-induced α´-martensite formation after plastic deformation can be clearly detected by EBSD images (Figure 2d,h). The comparison of EBSD grain maps (Figure 2c,g) of both batches presented a clear difference in the grain size. Quantitative evaluation yielded a mean grain size of 120 μm for batch A and of 17 μm for batch B, respectively.

3. Methods

To investigate the fatigue behavior of metastable austenitic stainless steels at ambient and elevated temperatures from the LCF to VHCF regime, servohydraulic and ultrasonic fatigue systems were used. The test temperature of T = 300 °C was achieved using an inductive heating system and control based on measurement by a type-K ribbon thermocouple in the center of the gauge length. The isothermal LCF and HCF tests at ambient temperature (AT) and T = 300 °C were performed with an MTS 100 kN servohydraulic testing system using load frequency of f = 0.01 Hz (LCF), 5 Hz and 20 Hz (HCF), see Figure 3a and b. The VHCF tests at T = 300 °C were performed with f = 980 Hz at an MTS 1 kHz servohydraulic testing system (Figure 3c). The VHCF tests at AT were realized in an ultrasonic fatigue testing system developed and built up at the authors’ institute [64,65] (Figure 3d) with an operating frequency f = 20 kHz. In order to characterize the cyclic deformation behavior in the LCF and HCF regime, respectively, an extensometer (AT and T = 300 °C) and thermocouples (only AT) were used. The area of each hysteresis loop describes the cyclic plastic strain energy dissipated per unit volume during a given loading cycle, which is mainly dissipated into heat and, hence, results in a change in specimen temperature [66,67]. Temperature was measured with one thermocouple in the middle of the gauge length (T₁) and two reference thermocouples at the elastically loaded specimen shafts (T₂, T₃). The temperature change induced by cyclic plastic deformation was calculated according to:

ΔT = T₁ − 0.5·(T₂ + T₃)(4)

The in situ detection of fatigue induced α´-martensite formation was done by magnetic Feritscope^TM (FISCHER, Windsor, CT, USA) measurements at AT (see Figure 3a). Due to the higher permeability of ferromagnetic ferrite compared to paramagnetic austenite, the response of the material to magnetic induction increases with the ferrite content. Using a non-destructive magnetic method, the Feritscope^TM measures the relative permeability of a material in the alternating magnetic field of its probe. This provides a ferrite content signal (FE%), which is also influenced by the curvature of the specimen’s surface and stress-strain state. Furthermore, to determine the ferromagnetic α´-martensite content in vol%, the Feritscope™ signal (FE%) needs to be multiplied by a factor of 1.7 [68]. Because the calibration factor was determined only for α´-martensite contents below 55 FE%, in the following diagrams the magnetically determined α´-martensite is indicated as ξ in FE% without calculating the vol% of α´-martensite. Furthermore, within one load cycle, the magnetic properties of α´-martensite are influenced by stress/strain due to the Villari effect (inverse magnetostriction), which describes a change of the magnetic susceptibility of ferromagnetic material due to mechanical stresses [26]. Therefore, an arithmetic mean value per load cycle of the measured Feritscope™ signal is given in the diagrams. For specimens loaded in LCF, HCF and VHCF tests at T = 300 °C and in VHCF tests at AT, ex situ Feritscope™ measurements were performed. The fatigue tests at AT and T = 300 °C were total strain controlled in LCF regime and stress controlled in the HCF regime. The VHCF tests at T = 300 °C were stress controlled and the ultrasonic VHCF tests at AT performed in displacement-control. All tests were performed in symmetric push-pull conditions with load ratio R = −1.

VHCF testing of metastable austenitic stainless steels at ambient temperature using an ultrasonic fatigue system is more challenging than for stable metallic materials because of transient material behavior and significant self-heating of specimens. Due to the formation of high strength α’-martensite in softer austenite, less damping of the oscillation amplitude takes place, resulting in higher displacement amplitude. Therefore, an unstable displacement amplitude occurred. A representative pulse sequence from the VHCF fatigue test on a fully austenitic microstructure in its initial state and after cyclic loading up to N~2 × 10⁶ clearly shows the challenge in performing the fatigue test with constant load amplitude (Figure 4). The first pulses were characterized by an oscillation plateau with a constant amplitude level during each pulse (Figure 4a). These results underline the necessity of a correct specimen design using FEM simulation to ensure fully reversed loading conditions with the maximum stress amplitude in the center of the gauge length and the maximum oscillation amplitude at the specimen´s end (see Figure 3d) [65]. However, due to fatigue-induced α’-martensite formation, the initial amplitude plateaus changed to pulses with an increasing displacement level (Figure 4b). This had to be leveled out by appropriate parameter adjustments during phase transformation. Further details can be found in further papers (see [65,69]).

Besides adjusting the proportional, integral and derivative (PID) parameters during VHCF tests, the deformation-induced specimen temperature increase has to be limited. Figure 5a shows the development of displacement for a pulse/pause ratio of 0.5 s/2.5 s, which is typically used for stable metallic materials [64], and which results in an effective load frequency f_eff = 3300 Hz. The progress of the specimen´s temperature is also plotted (Figure 5). It can be clearly seen that a pulse/pause ratio of 0.5 s/2.5 s cannot be used for fatigue testing of metastable austenite at ambient temperature because within two pulses the temperature increased to 200 °C (Figure 5a). To keep the specimen´s temperature below 50 °C, a change of temperature below 25 K and therefore a pulse/pause ratio of 0.06 s/2.94 s had to be used (Figure 5b). This led to an effective frequency of f_eff = 400 Hz. Theoretically, to achieve the limit of the number of cycles N_l = 2 × 10⁹ for a fatigue test with f_eff = 400 Hz, about 58 days would be required. In reality, with the development of α’-martensite, cyclic hardening of materials takes place, which reduces cyclic plasticity. Consequently, the development of the specimen’s temperature decreases, and fatigue testing could be performed with a higher pulse/pause ratio with f_eff = 1650 Hz. The adjustment of PID parameters was also less critical given that saturation of α’-martensite was achieved in the cycle regime of N~10⁸ (see Figure 10).

4. Results

Fatigue tests were performed with specimens from AISI 347 batch A taken from an original surge line pipe. Specimens from rolled bars from AISI 347 batch B represented the more metastable material and were used for selected fatigue tests to show the influence of metastability on the deformation and phase transformation behavior in the LCF regime as well as for characterizing HCF behavior at ambient temperature.

4.1. LCF Behavior

4.1.1. Ambient Temperature

Single step (constant amplitude) tests under total-strain control were performed in the LCF regime with total strain amplitudes 0.6% ≤ ε_a,t ≤ 1.6%, load frequency f = 0.01 Hz and triangular waveform. Figure 6 presents the development of stress amplitude (σ_a), the change in specimen temperature (ΔT) and α’-martensite formation (ξ) for batch A and, additionally, the results from the test with total strain amplitude ε_a,t = 1.0% for batch B. The cyclic deformation behavior of the investigated steel at ambient temperature was fundamentally determined by deformation-induced austenite-α´-martensite transformation. After a load-dependent number of cycles N, the formation of α´-martensite started and increased continuously with increasing cycle number until specimen failure (Figure 6c). The σ_a,N-curves (Figure 6a) illustrate the associated cyclic hardening processes, which led to a maximum stress amplitude for ε_a,t ≥ 1.2% in the range of the tensile strength σ_f = 569 MPa (batch A) of the solution-annealed material. Cyclic hardening was detected by temperature measurement, temperature increases in total-strain controlled fatigue tests being indicative of increases of plastic strain energy. Because of very low load frequency, the maximal change in the specimen temperature was below 3 K, which corresponds with an absolute temperature of about 28 °C and consequently bears no significant influence on the formation of α´-martensite. Figure 6c shows the development of α´-martensite content during the fatigue tests discussed above. After an incubation period, which depended on the total strain amplitude, the α´-martensite volume fraction increased continuously with the number of cycles. With increasing ε_a,t, the onset of α´-martensite formation was shifted to lower N, and a higher volume fraction of α´-martensite was measured at specimen failure. To characterize the influence of metastability of nominally the same type of austenitic stainless AISI 347 steel, specimens from batches B were cyclically loaded with total strain amplitude ε_a,t = 1%. Both batches A and B showed a continuous cyclic hardening, however, with different gradients after the first ten cycles. In the cycle range 10 < N < 100, batch A showed only a slight increase of stress amplitude, while batch B underwent a larger increase of σ_a, which correlated directly with a significant development of α’-martensite (Figure 6c) from 1 FE% to 41 FE%. As mentioned in the methods section above, ferromagnetic α’-martensite was measured in situ during fatigue testing using a Feritscope^TM sensor, for which readings above 60 FE% could be inaccurate due to lack in instrument linearity and calibration difficulties at high α’-martensite contents. For this reason, measured data in the range above 60 FE% were plotted as dashed lines in Figure 6c. After a high rate of σ_a increase between N = 10 and N = 100, batch B showed a decrease in the stress amplitude rate dσ_a/dN. However, further cyclic hardening took place which led to specimen failure at σ_a = 760 MPa, which is 120 MPa higher than the tensile strength of this material in its as-received state (Table 2). Batch A also showed evidence of a correlation between the development of α’-martensite and an increase in stress amplitude. However, the start of α’-martensite formation occurred at a higher number of cycles in comparison to batch B and the α’-martensite content was found to be, at the same number of cycles, significantly lower with the maximum value at specimen failure ξ = 30 FE%. The results of these tests clearly depicted how substantial the differences can be between the formation of deformation-induced α’-martensite in the same type of austenitic stainless steel due to differences in chemical composition and grain size and consequently the material’s metastability. It should be noted that both batches had a chemical composition within the range given in international standards for the investigated steel type.

4.1.2. Elevated Temperature T = 300 °C

Single step total-strain control fatigue tests in the LCF regime at 300 °C were performed with total strain amplitudes 0.8% ≤ ε_a,t ≤ 1.6%, a load frequency f = 0.01 Hz and triangular waveform. Figure 7 presents the development of the stress amplitude σ_a versus the number of cycles N. At 300 °C the cyclic deformation behavior of the investigated steel was characterized by initial cyclic hardening, followed by slight cyclic softening before the final stress amplitude drop associated with macro crack propagation. The microstructural changes due to cyclic plastic deformation are described elsewhere [46,63]. At these load parameters, no α´-martensite formation occurred and, compared to ambient temperature, the stress amplitude was significantly lower (compared Figure 6a with Figure 7).

4.2. HCF Behavior

For fatigue tests in the HCF regime, stress-controlled single step tests were performed. Typically, in order to obtain the limited number of cycles N_l in the range of 10⁶–10⁷ cycles, test frequencies in the range 5 Hz ≤ f ≤ 20 Hz are used for metallic materials. For N_l = 2 × 10⁶ with a load frequency of f = 5 Hz, about 5 days, for N_l = 2 × 10⁷ with a load frequency f = 20 Hz, about 12 days would be required. Usually, higher test frequencies can be used for HCF tests at ambient temperature (AT) of metallic materials but when investigating the fatigue behavior of metastable austenitic stainless steels, self-heating of the specimens has to be taken into account. Even if external cooling of the specimen is undertaken, significant increases in specimen temperature cannot be suppressed [70,71]. Hence, in this study, test frequencies for fatigue tests at nominally ambient temperature were f = 5 Hz (batch B) and for T = 300 °C f = 20 Hz (batch A).

4.2.1. At Nominally Ambient Temperature

To characterize the cyclic deformation and transformation behavior during cyclic loading in the HCF regime, specimens taken from batch B were cyclically loaded in stress-controlled single step tests with stress amplitudes in the range 225 MPa ≤ σ_a ≤ 280 MPa. In Figure 8a the resulting developments of plastic strain amplitude ε_a,p are plotted against the number of cycles N. The ε_a,p,N curves illustrate pronounced cyclic hardening after a short period of initial cyclic softening. Cyclic hardening is caused by an increase in dislocation and stacking fault density, as well as by formation of deformation-induced α´-martensite (Figure 8b). A positive influence of α´-martensite formation on fatigue life was observed, because austenite-α´-martensite transformation causes cyclic hardening, which significantly reduces plastic strain amplitude. Similarly, total-strain controlled fatigue tests in the LCF regime (Figure 6), after an incubation period dependent on stress amplitude, resulted in a continuously increasing volume fraction of α´-martensite over the number of cycles. With increasing stress amplitude, the onset of α´-martensite formation was shifted to lower N and higher α´-martensite fractions were measured at specimen failure. This development of α´-martensite in stress-controlled tests was comparable to the results given in [16] as well as for total-strain controlled fatigue tests shown in Figure 6c. However, with 1.7 FE% < ξ < 8.8 FE% the maximum values of α´-martensite at specimen failure/ultimate number of cycles were significantly lower compared to total strain-controlled LCF tests. The change in the specimen temperature during the fatigue tests is shown in Figure 8c. As in the LCF tests, changes in temperature were used to detect cyclic deformation behavior. However, in the stress-controlled fatigue tests, an increase in temperature correlated with an increase of the σ-ε hysteresis loop area, indicating cyclic softening, whereas a decrease in temperature correlated with a decrease of the σ-ε hysteresis loop area, and characterized cyclic hardening. Besides using ΔT to characterize cyclic deformation behavior, information about the general amount of self-heating is provided (Figure 8c). In consideration of Equation (4), a maximum specimen temperature of 102 °C was achieved while the value of ΔT signal shows 53 K. As is known, temperature has a significant influence on deformation-induced α´-martensite formation and consequently fatigue life. In literature [5,72,73,74,75,76,77,78,79] the fatigue life of metastable austenitic stainless steels is given as S-N curves obtained by fatigue testing with load frequencies up to 150 Hz. Obviously, specimen temperatures in HCF tests with frequency higher as 2 Hz were higher than ambient temperature; however, the information about the specimen temperatures is not at all times given. To obtain HCF results at true AT or at temperatures around 25 °C–30 °C, fatigue tests with significantly lower frequency, e.g. f = 0.2 Hz, have to be performed. This requires 115 days of test duration to achieve N_L = 2 × 10⁶, i.e. a duration that is impractical for systematic investigation of HCF behavior due to high resource expenses. In conclusion, to obtain representative results in the HCF regime in a reasonable time, it is necessary to perform fatigue tests of metastable austenite with variable test frequencies during single step fatigue tests, which effectively suppress self-heating of e.g. ΔT_max < 10 K. Therefore, it has to be noted that the presented results (and many others given in literature [5,72,73,74,75,76,77,78,79], especially regarding the development of α´-martensite and its influence on fatigue life) are influenced by specimen temperature, which needs to be taken into consideration.

4.2.2. Elevated Temperature T = 300 °C

Figure 9 shows the development of plastic strain amplitude during fatigue loading of HCF specimens from batch A at 300 °C with a frequency f = 20 Hz. As these fatigue tests were performed at elevated temperature, a higher test frequency can be used to achieve a reasonable short testing time. At the beginning of the fatigue tests, cyclic softening was detected for all stress amplitudes followed by a saturation state for stress amplitudes 165 MPa ≤ σ_a ≤ 200 MPa and finally crack growth up to specimen failure. Inversely, the stress amplitude of 160 MPa, after initial softening up to about 2 × 10⁴ cycles, led to cyclic hardening, which reduced plastic strain amplitude down to zero. Consequently, macroscopic elastic cyclic loading occurred and at this stress amplitude the limit number of cycles N_l = 2 × 10⁷ was achieved without failure. Ex situ magnetic Feritscope™ measurements quantified 0.13 FE% for this specimen and 0.00 FE% for all loadings > 160 MPa. That is, for only a small load amplitude at 300 °C, a change in the cyclic deformation behavior occurred from softening/saturation to cyclic hardening. This change in cyclic deformation character was also observed during total strain-controlled fatigue tests and detected in situ using electromagnetic induced ultrasonic signals in the specimen [46,47,63]. At 300 °C, the investigated batch A showed no α´-martensite formation during monotonic tensile loading (see Table 2) as well as in LCF [46,47,63] and HCF regimes at “higher” loading amplitudes (see Figure 7 and Figure 9). As mentioned in the introduction, the cyclic deformation behavior of metastable austenitic steels depends on load parameters such as stress amplitude, frequency, and temperature, and results in different microstructural changes from planar dislocation slip, over wavy slip, until the development of stacking faults, twins, as well as ε and α´-martensite (see. Figure 1). As the volume fraction of α´-martensite measured by Feritscope™ is relatively low (ξ = 0.13 FE%) further microstructural changes have to play a role in the cyclic hardening of metastable austenitic steel at low stress amplitude in fatigue testing at 300 °C, which are still not entirely investigated and explained.

4.3. VHCF Behavior

VHCF behavior was investigated at AT using an ultrasonic fatigue system (Figure 3d) at stress amplitudes 180 MPa ≤ σ_a ≤ 283 MPa, and a load frequency f = 20 kHz, while at T = 300 °C a servohydraulic test system was used and stress amplitudes 120 MPa ≤ σ_a ≤ 190 MPa were applied at a load frequency f = 980 Hz. All VHCF tests were performed on specimens from batch A.

4.3.1. Ambient Temperature

As σ-ε hysteresis measurements are (to date) not realizable during ultrasonic fatigue testing, and due to the macroscopically elastic behavior in the lower HCF/VHCF regime, the cyclic deformation behavior cannot be characterized using conventional data like ε_a,p, N curves. However, in situ measurement of further physical data, such as changes in the specimen temperature before and after a pulse sequence, dissipated energy and generator power, as well as “quasi in situ” measurement of magnetic properties, can be used to describe the cyclic deformation and transformation behavior of metastable austenites during fatigue testing in the VHCF regime. Figure 10 shows the aforementioned data and, in addition, the displacement amplitude as well as load frequency during an ultrasonic fatigue test with a stress amplitude of 250 MPa. After an approximately stationary phase up to N = 10⁶, cyclic hardening occurred which could be detected by a strong increase in α’-martensite content. As mentioned before, and illustrated in Figure 4, α’-martensite formation promotes transient material behavior. To keep the displacement amplitude close to a constant value of 9.5 μm, corresponding to a stress amplitude of 250 MPa, the PID parameters of the ultrasonic testing system were stepwise readjusted. The cyclic hardening of the material resulted in lower self-heating illustrated by a decreasing change in temperature. This aspect enabled longer pulse and shorter pause times which resulted in a higher effective frequency. As described in Section 3, a stepwise increase of the pulse pause ratio up to 0.72 s/0.8 s, which represents an effective frequency f_eff = 1650 Hz, led to achieving the ultimate number of cycles within 25 days. After N = 1 × 10⁸ load cycles, a lower α‘-martensite formation rate dξ/dN occurred and until the limiting number of cycles a saturation state with a stabilized α‘-martensite content of ξ = 2.2 FE% was reached. At the same time, temperature as well as displacement amplitude remained constant. In this phase, further adjustments of the displacement control were not necessary. The S-N curve resulting from single step tests is given in Figure 12c. Cyclic plastic deformation of metastable austenite at ambient temperature led to significant changes in phase distribution from single-phase austenitic to two-phase austenite/α´-martensite microstructures. At ambient temperature, and at all load amplitudes σ_a > 240 MPa, formation of α’-martensite in the range 0.3 FE% ≤ ξ ≤ 2.3 FE% took place. However, at smaller stress amplitudes σ_a < 240 MPa no deformation induced α’-martensite was measured. Fatigue failure only occurred in the HCF regime and no specimen failed in the VHCF regime beyond N = 10⁷ load cycles. Accordingly, a true fatigue limit exists for metastable austenite [80,81].

4.3.2. Elevated Temperature T = 300 °C

High precision stress-strain hysteresis measurement suitable to quantify microplastic deformations at low load amplitudes is, as with ultrasonic fatigue, impossible for tests using servohydraulic systems with a load frequency f = 980 Hz. The S-N Woehler curve resulting from constant amplitude fatigue tests at this frequency at 300 °C is given in Figure 12c. Similar to the HCF behavior at 300 °C at low stress amplitude, a very low volume fraction of α’-martensite was detected in the specimen which achieved the limit number of cycles N_l = 5 × 10⁸. Since the Ferritscope™ sensor cannot be used in situ at this testing temperature, the kinetics of α’-martensite development was analyzed by an interrupted VHCF test with a stress amplitude of 160 MPa. At defined load cycles, ex situ Feritscope™ measurements were taken at the specimen surface at ambient temperature. Figure 11 shows no α´-martensite up to N = 1 × 10⁷. The onset of α´-martensite formation occurred between N = 1 × 10⁷ and N = 1 × 10⁸. At N = 1 × 10⁸ 0.12 FE% and at N = 5 × 10⁸ 0.13 FE% were measured without specimen failure. As the measured volume fraction of α´-martensite was very low, further microstructural changes had to play a role in the cyclic hardening of metastable austenitic steel in the VHCF regime at 300 °C.

5. Summary

Figure 12 summarizes the results from fatigue tests on AISI 347 in LCF, HCF and VHCF regimes at AT and 300 °C in the form of S-N curves. The influences of metastability of two different batches of AISI 347 on cyclic deformation behavior are clearly seen in LCF tests at ambient temperature (see Figure 6). The determined total strain-controlled fatigue life in the LCF regime at AT and 300 °C is similar (Figure 12a). However, significantly different cyclic deformation behavior was observed. At AT, cyclic hardening due to α´-martensite formation took place and the transformation from a single-phase austenitic to two-phase austenite/α´-martensite microstructure developed. The results epitomize a “dynamical composite material” with changing volume fraction and distribution of “reinforcements” during cyclic loading. At 300 °C in the LCF regime, no α´-martensite was measured and cyclic deformation behavior showed slight initial cyclic hardening followed by saturation/softening. Despite significantly different cyclic deformation behavior and resulting stress amplitudes in AT and T = 300 °C, respectively, similar fatigue lifetimes were estimated. However, the specimens loaded at AT achieved stresses higher than the ultimate tensile strength. A positive influence of α´-martensite formation on fatigue life was observed in stress-controlled fatigue tests in the HCF regime (Figure 12b) where austenite-α´-martensite transformation caused hardening, which significantly reduced the plastic strain amplitude and led to increased fatigue life. The α´-martensite formation occurred in all fatigue tests at AT. At 300 °C, albeit specimens loaded with low stress amplitude showed a very small volume fraction of α´-martensite. These specimens achieved the limit number of cycles without failure. A comparison of the S-N curves from tests at AT with T = 300 °C clearly showed a decrease in fatigue strength with an increase of temperature (Figure 12b). Similar behavior was observed for VHCF tests at AT and 300 °C (Figure 12c). At AT, higher fatigue strength existed, which correlated with the development of α´-martensite at each load amplitude. At 300 °C, only specimens with the formation of a low volume fraction of α´-martensite achieved the limit of the number of cycles without failure.

6. Conclusions

The present study characterized fatigue behavior of metastable austenitic stainless steels in the LCF, HCF and VHCF regimes at two temperatures, i.e. ambient temperature and 300 °C. Based on the acquired results, the following conclusions were drawn:

• The metastability of austenitic Cr-Ni steels can, even within the specifications given in international standards, e.g. for AISI 347 stainless steel, strongly vary due to relatively small changes in chemical composition.

• Different metastability of austenitic stainless steel affects the cyclic deformation and, consequently, the fatigue lifetime behavior.

• This is caused by the fact that during fatigue, various deformation (planar or wavy slip character, formation of stacking faults, twinning) and transformation (γ -> ε, γ -> ε -> α´, γ -> α´) mechanisms with different peculiarity take place.

• The investigated metastable austenitic stainless steel AISI 347 showed a true fatigue limit in the VHCF regime at AT and 300 °C.

• Further microstructural investigations have to be performed with focus on the development of deformation/transformation mechanism maps dependent on metastability parameters, e.g. stacking fault energy and loading parameters, in the LCF, HCF and VHCF regimes in accordance with deformation maps known in the literature [52,53,54] for stable materials.

Author Contributions

M.S. planned the experimental design and wrote the majority of the present paper; A.B., T.D., R.S. and A.S. realized experimental investigations and analysis of the given results; M.S. and T.B. supervision the work.

Funding

This research was funded by Federal Ministry for Economic Affairs and Energy (BMWi), Germany and the German Research Foundation (DFG).

Acknowledgment

The authors thank BMWi as well as the DFG for financial support as part of the CRC 926 project number 172116086 ‘‘Microscale Morphology of Component Surfaces’’.

Conflicts of Interest

The authors declare no conflict of interest.

Figures and Tables

Figure 1. TEM micrographs with deformation and transformation microstructures of cyclically loaded metastable austenitic stainless steels: (a) typical dislocation density of the initial state after solution annealing, AISI 348 [58]; (b) cell dislocation structure (wavy slip character) at Nf in AISI 348 fatigued with σa = 280 MPa, R = −1 at AT with f = 5 Hz [58]; (c) dislocation accumulation in one direction (planar slip character) at Nf in AISI 348 fatigued with σa = 260 MPa, R = −1 at AT with f = 5 Hz [58]; (d) stacking faults (SF) at Nf in AISI 304 fatigued with εa,t = 0.325%, R = -1 at AT with f = 5 Hz [58]; (e) twins in AISI 347 fatigued with σa = 160 MPa, R = −1 at T = 300 °C with f = 980 Hz at N = 5 × 108, (f) ε- and α´-martensite at Nf in AISI 321 fatigued with σa = 330 MPa, R = −1 at AT with f = 5 Hz [59].

Figure 2. Optical micrographs of longitudinal sections using Bloech & Wedl I etching (a,e) initial state, (b,f) after plastic deformation as well as EBSD grain maps of (c,g) initial state and (d,h) after plastic deformation.

Figure 3. Schematic representation of the experimental setup for fatigue tests in the LCF and HCF regime at (a) AT and (b) at T = 300 °C, as well as in the VHCF regime at (c) T = 300 °C and (d) at AT.

Figure 4. Displacement amplitude of batch A during VHCF testing at (a) the beginning of the fatigue test and (b) after fatigue-induced α’-martensite formation occurred without parameter adjustments.

Figure 5. Development of the specimen’s temperature using a pulse/pause ratio of (a) 0.5 s/2.5 s and (b) 0.06 s/2.94 s for AISI 347 batch A.

Figure 6. Development of (a) stress amplitude, (b) change in temperature and (c) α’-martensite versus N during total-strain controlled LCF test of batch A and B.

Figure 7. Development of stress amplitude σa versus load cycles N during total-strain controlled LCF test at T = 300 °C of batch A.

Figure 8. Development of (a) plastic strain amplitude εa,p, (b) α’-martensite and (c) change in the specimen’s temperature ΔT versus load cycles N during HCF tests at AT with f = 5 Hz of batch B.

Figure 9. Development of plastic strain amplitude εa,p versus load cycles N in HCF tests at T = 300 °C with f = 20 Hz.

Figure 10. Displacement amplitude sa, frequency f, temperature change ΔT, power P, dissipated energy Edis and α’-martensite content ξ of batch A during VHCF test at σa = 250 MPa.

Figure 11. Development of α’-martensite during the interrupted VHCF test at σa = 160 MPa, f = 980 Hz and T = 300 °C.

Figure 12. (a) εa,t –N curve in LCF regime of batch A at AT and T = 300 °C, (b) S-N curve in HCF regime of batch A at T = 300 °C and batch B at AT, (c) S-N curve in VHCF regime of batch A at AT and T = 300 °C.

Table 1

Chemical composition in weight % and metastability parameters.

AISI 347	C	N	Nb	Cr	Ni	Mn	Mo	Si	Md30 °C	MS °C	SFE mJ/m2	Iξ FE%
Batch A	0.040	0.007	0.62	17.6	10.6	1.83	0.29	0.41	25	−189	39	0.25
Batch B	0.024	0.019	0.41	17.3	9.3	1.55	0.19	0.63	46	−81	27	0.66

Table 2

Mechanical properties and α’-martensite content after specimen failure (ξ).

AISI 347	Rp0.2 in MPa	UTS in MPa	A in %	ξ in FE%
Batch A, AT	242	569	66	4.41
Batch A, 300 °C	180	357	36	0.00
Batch B, AT	220	621	51	33.22
Batch B, 300 °C	155	428	37	0.00