Keywords

1 Introduction

316LN stainless steel is selected for its high-temperature (HT) service conditions as it shows good weldability, intergranular stress corrosion and chloride-induced crack, adaptability with sodium coolants, and corrosive environments [2, 3, 67, 75]. It has been extensively used in nuclear power plants, thermal power plants, and aircraft components that operate at elevated temperature [33, 43, 73]. High nitrogen content (0.22%) enhances elevated temperature creep resistance, fatigue, and fracture strength [13, 31, 60]. During thermomechanical processes, 316LN SS experiences several metallurgical changes including flow instabilities, work hardening, dynamic recovery, and dynamic recrystallization [51]. Dynamic recrystallization nucleation produces in subgrain growth and grain boundary projecting mechanism [71]. Mechanical strength of 316LN SS decreases with increasing temperature. The strain-induced martensite increases the fracture stress during the impact test [72]. Under high-temperature start–stop conditions, temperature and load vary with time, resulting in a combination of creep and fatigue deformation [29, 32, 37, 61]. Due to cavitation damage, creep damage can occur during the holding period and is usually manifested as a creep cavity at the grain inner edge [74]. While in creep-fatigue interactions, strong cavitation damage is observed in this material, and fatigue crack damage develops on the surface. When this interaction occurs, the failure mode is mixed (transgranular as well as intergranular), as shown in Fig. 1 [16, 48]. Elastic–plastic fracture mechanics (EPFM) is a tool for determining the leak before break (LBB) of pressurized piping systems and estimating the service life of reactor vessels [30].

Fig. 1
A schematic diagram of the initiations, failures, and interaction of creep and fatigue on a graph representing total strain range in delta epsilon subscript t versus number of cycles in N.

Schematic diagram of CFI and failure modes due to fatigue and CFI [73]

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2 Research Background

Structural components which are subjected to cyclic loading such as elements of nuclear power plants, aircraft, and rotating machinery, fatigue life assessment study were presented by [23]. There are three stages: initiation of cracks, development of short crack to long crack, and ultimately fracture of the component [68]. The Paris rule is generally accepted as a useful method for measuring fatigue crack extension rate with a stress intensity factor [20, 63]. Modeling and simulating the fatigue crack extension mechanism of various structural components is of utmost importance to ensure safety and reliability under different loading conditions [17, 21]. An enormous number of numerical techniques have evolved to simulate the crack extension problem [26, 27] and fatigue [24] like as the finite element method (FEM), boundary element method (BEM), and an extended finite element method (XFEM) [13]. XFEM is an effective numerical technique for simulating various discontinuities, such as cracks, contact [34, 36] inclusions [35], and cavity without the need for a finite element mesh to correspond to these discontinuities [13, 25].

Samuel et al. [52], studied the tensile work hardening behavior of 316LN SS at different temperatures and strain rates. It has been observed that the tensile properties of the material are changed at intermediate temperatures due to dynamic stress aging. Experimental results show that the load-deformation curves were smooth at ambient and HT, whereas at a medium temperature, the irregular flow was obtained. Srinivasan et al. [66], investigated the LCF and CFI behavior of 316LN SS under different experimental condition such as various deformation behavior on this material reported based on several mechanisms including DSA under stress, creep, oxidation, and sub-structural recovery. Using the data generated in that study, an artificial neural network (ANN) model enabled life prediction model under LCF and CFI conditions.

Kim et al. [28], conducted LCF and CF tests for two materials (316L and 316LN SS) with continuous cycling and 10-min hold period at temperature 600 K in air. It was noticed that the addition of nitrogen increased FL, CF life, and saturation stress. It is reported that the fracture mode in the fatigue test was transgranular and for the CF test was intergranular. In addition, the dislocation structure for 316L was cellular and planar for 316LN in the fatigue, creep-fatigue test. Precipitation of carbides at grain boundaries has been reported in CF tests, and it has been suggested that these precipitation can be avoided with addition of nitrogen. Roy et al. [49] examined the LCF tests of 316LN SS at room temperature with the range of strain amplitude (± 0.3 to ± 1.0%) and strain rate (3 × 10−3s−1). The material exhibited non-masing behavior at higher strain rates and masing behavior at lower strain rates. It has been reported that FL decreases with increasing stress amplitude, and the hysteresis loop behavior was in good agreement with the test results. Babu et al. [3], conducted FCG test on 316LN SS at room temperature; the nitrogen percentage was varied through 0.8, 0.14, and 0.22 wt%. In this study, the direct current potential drop (DCPD) technique was used to estimate FCG, and the compliance technique was used to estimate the crack closure. It was observed that the threshold stress intensity factor \(\left( {\Delta K{\text{th}}} \right)\) and effective threshold stress intensity factor \(\Delta K_{{{\text{eff}}.}} {\text{th}}\) were different for various nitrogen contents. Also, the fracture surface roughness parameter was used to quantify the slip irreversibility model. The cracks are often formed by the microcavity nucleating, growing, and converging, as shown in Fig. 2 [75].

Fig. 2
A diagram exhibits 4 illustrations of the progression of cavity related cracks. It starts with six separate particles and ends with 2 cracked areas.

Crack formation involves a cavity nucleation, b cavity growth, c cavity polymerization, and d fracture [75]

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3 Factors Affecting Mechanical Properties of 316LN SS

The effect of temperature and strain rate, hold period, nitrogen content, and dynamic strain aging on deformation behavior of 316LN SS has been discussed in this study.

3.1 Effect of Temperature and Strain Rate

The load-deformation curve is smooth at room and elevated temperature although irregular flow curves were obtained at intermediate temperatures. The sufficient plastic stress–strain data for 316LN SS are best expressed in terms of Ludwigson relation at ambient temperature [8, 72]. Several flow parameters observed in the Ludwigson–Voce equation varies with deformation rate and temperature [8]. The decrease in FL with an increase in the temperature range (673–873 K) is ascribed to detrimental effects of DSA, and beyond 873 K, the FL was affected by oxidation [56, 64]. This material exhibited a higher ratio of normalized stress with increasing temperature (573–873 K) and also observed irregular fluctuation in tensile properties due to DSA at a medium temperature and a normal behavior at elevated temperatures [66]. Isothermal cycling at the HT of thermomechanical fatigue yielded lower endurance than in-phase and out-of-phase cycling [44]. The ratcheting strain increases with an increase in temperature, and fatigue life decreased because of higher plastic deformation [58].

In the load-deformation curve, irregular flow/twitching at the mean temperature is observed, related to the attractive interaction between the solute and the mobile dislocation throughout deformation, known as DSA [45, 52, 56]. DSA temperature regime is sensitive to nitrogen percentage, with temperatures ranging from 673 to 873 K (Fig. 3a) for nitrogen percentage less than 0.11 wt% and from 773 to 873 K (Fig. 3b) for nitrogen percentage greater than 0.11 wt% [45]. The direction of generating a set of non-cellular displacements increased with increasing DSA. 316LN SS is available in two irregular flow temperature regimes, such as the relatively low temperature (523–598 K) and the high temperature (673–923 K) [8].

Fig. 3
2 curve graphs represent tensile stress amplitude in megapascal versus number of cycles. The 6 plotted data, including those under D S A regime, in the 0.11% nitrogen graph exhibit relatively lower peaks compared to those in the 0.22% Nitrogen graph.

Cyclic stress response (CSR) curves of 316LN SS alloyed with different nitrogen contents [45]

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A low-cycle fatigue test was conducted for short cycles, and no evidence of dynamic precipitation in the temperature range associated with rapid cycle hardening. That study explicitly ruled out a precipitation effect in the rapid hardening of 316LN SS. The fatigue strength decreased by increasing the temperature and decreasing the strain rate from 3 × 10−2 to 3 × 10−5s−1 [66]. At all strain rates of 773 and 823 K, DSA played a vital role in fatigue deformation and fracture. In tensile tests at 873 K, DSA has been reported to result in a decrease in service life and increased fatigue strength. DSA has been described to cause hardening at an earlier fatigue life stage, and nitrogen retarded influence of DSA [28]. Samantaray et al. [50] This article has reported the influence of strain rate, deformation, working environment (temperature), and method of loading on tensile behavior of 316LN, and a correlation between deformation and microstructural features has been established. The higher temperature regime of 316LN SS serrated flow conflicts with the temperature range (773–973 K) in which precipitation of chromium carbide has been observed. In the low-temperature range (523–623 K), the diffusion of interstitial solutes to displacements is responsible for serrated flow. The mechanism accountable for the serrated yielding in the HT range (973–923 K) is an alternative solute like Cr [54].

3.2 Effect of Hold Period

The SR behavior in the cyclic regime at half-life was presented by the CFI, which introduced hold time at the peak tensile strain or peak compression strain as shown in Fig. 4 [73].

Fig. 4
A line graph represents stress, with peaks of tension and compression, versus time. The plotted data exhibits parallel peaks in both regions where delta and sigma measurements are depicted.

In hold-time tests, different stress values were found [73]

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CFI tests have been conducted at 873 K to estimate the impact of the hold period (1–90 min). It has been reported that there was a reduction in fatigue resistance with an increase in hold time. CFI is caused by a flaw in the grain boundary that leads to creep in the intergranular cavity, which can lead to intergranular gaps and cracks that coincide with fatigue cracking and constitute the most common cause of premature failure [66]. It has been found that SR varies with nitrogen content and saturating stress during the hold time [6, 40]. In CF experiments, the creep deformation occurring during the hold period has been evaluated to play a vital role in nucleation and growth of voids indicating the occurrence of an intergranular failure mode attributed to gap nucleation at the grain boundary [28].

The occurrence of creep during hold time results in the reduction of cyclic stress to attain a given strain in creep-fatigue tests [61]. Declaring that 316LN stainless steel has a high DSA in the temperature range (573–923 K), cyclic hardening behavior develops. However, the planar dislocation structure deteriorates, and the dislocation density decreases during the hold time; the resistance to dislocation motion decreases [10, 28]. In CF tests, at a constant temperature and plastic strain range, FL is found to be reduced, while the tensile hold time is increased [39]. Sarkar et al. [57], Conducted creep-fatigue interaction experiment at elevated temperature (650 ˚C) for 316LN SS where selected loading parameter is below endurance strength with high load ratio. They reported that small load amplitude shows substantial CFI which indicates the influence of stress amplitude on the failure mechanism of material at higher temperatures.

3.3 Effect of Nitrogen Content and Dynamic Strain Aging

Nitrogen has been reported to affect fatigue and creep properties at HT by modifying metallurgical factors like dislocation, precipitation, and DSA. With the addition of nitrogen percentage, the dislocation structure is changed from unitary to planar in the fatigue and CF tests [40]. The temperature and time for carbide precipitation have been reported to increase by the addition of nitrogen [41]. The integrated effect of precipitation strengthening by excellent carbides and substantial solution strengthening by nitrogen in 316LN SS resulted in increased creep rupture life and decreased creep life [38]. High-temperature LCF life increased with the adding nitrogen because it produced planar slip and restrained DSA [28, 29, 43, 65].

The fatigue strength at 300 and 77 K increases with the addition of nitrogen [70]. The creep and tensile endurance of 316LN SS was reported to improve significantly by enhancing the nitrogen percentage [37, 44] and also improving fracture toughness at cryogenic temperatures [31]. Surface and internal creep damage decreased with an increase in nitrogen percentage [13]. The steady-state creep rate, the region of intragranular cracks, and surface cracks reduced with an increase in nitrogen percentage [62]. The addition of nitrogen to 316LN SS improved corrosion resistance by preventing pitting, intergranular corrosion, sensitization, corrosion fatigue, and stress corrosion cracking [18]. Two typical manifestations of dynamic strain aging have elastic shakedown and strain burst [55]. The cyclic stress response of 316LN steel is observed to show initial hardening, softening, saturation, and eventual fracture over a long period [45, 49]. Addition nitrogen in austenitic stainless steels (304LN and 316LN SS) shows a beneficial effect on flow strength via strain hardening characteristics which can be attributed to bulk material strengthening and inclination toward planar slip hence, trim downs the susceptibility to dynamic recovery [42].

The effect of nitrogen on the CSR was observed in the LCF test with a strain amplitude of 0.6% and an ambient temperature of 300 K. The degree of softening increased by increasing the nitrogen percentage, especially for 0.14 and 0.22% at 300 K [47]. The CSR with a strain amplitude of ± 0.6% develops into a plastic deformation greater than the strain amplitude of ± 0.4% [59]. The CSR increased with decreasing strain rate as depicted in Fig. 5 which shows variation of half-life tensile stress amplitude (HLSA) with respect to nitrogen percentage at different strain rates of \(3\times {10}^{-5}{\mathrm{s}}^{-1} \mathrm{and} 3\times {10}^{-4}{\mathrm{s}}^{-1}\) and temperatures [43]. The CSR of 316LN SS under high-temperature fatigue loading is influenced by multiple factors such as dislocation–dislocation and interaction, DSA, recovery process, and creep [12, 32]. Dynamic strain aging (DSA) is based on time and temperature phenomena. When DSA reduces ductility, it may affect crack propagation at high temperatures, as seen in Fig. 6 [29]. The activation energy of 200 kJ \({\mathrm{mol}}^{-1}\) was attained in temperature range (673–873 K), which encourages the opinion that the diffusion of exchange elements such as Cr to dislocation was produced by the DSA [65]. In LCF experiments, it is also significant that nitrogen percentage greater than 0.07 wt% results in substantially larger cyclic tensile stresses [29]. Ganesan et al. [11], studied the deformation behavior of 316LN SS at high temperature and constant load (creep) for varying nitrogen percentage at 650 ˚C and reported that material creep resistance increases with increasing nitrogen percentage (0.07–0.22%) while creep ductility decreases. Increasing nitrogen percentage increases the tendency of intergranular deformation, while in the case of low nitrogen it shows both transgranular as well as intergranular cracking. Fatigue crack propagation at elevated temperature is governed by intergranular mode, due to the synergetic effect of DSA and oxidation, while in absence of oxidation, fatigue damage will transgranular [76].

Fig. 5
2 curve graphs represent the half life tensile stress amplitude in megapascal versus nitrogen content in 316 L N S S weight %. The plotted 823 K, 773 K, and 873 K data in the first graph exhibit rising trajectories, while 823 K declines in the second graph.

Effect of temperature on 316 LN with two strain rates and variation in nitrogen content [43]

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Fig. 6
2 line graphs represent the yield stress in megapascal and elongation in % versus temperature in degrees Celsius. N 04 and N 10 data in the first graph exhibit decline, while the data in the second graph exhibit post-fluctuation rising trends.

Effect of temperatures on yield stress and elongation for 316LN SS [29]

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The influence of nitrogen percentage in 316LN on LCF life can be obtained only when the time-dependent effects are minimum and the failure mode is typically transgranular, as shown in Fig. 7, [43].

Fig. 7
A block diagram exhibits the creation of optimum nitrogen content from the minimum and maximum D S A, precipitation, creep, and oxidation effects via transgranular and intergranular fractures.

Increased LCF life for 316LN SS with optimum nitrogen percentage is due to time-dependent processes

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The flow stress rises with reducing deformation rate but increases with increasing temperature [46, 65]. It was observed that for 0.14% N steel, the strain increased by 0.4% with increasing temperature (773–873 K), and the strain amplitude was 0.6%, showing DSA. The initial cyclic hardening of 773 and 300 K, with a deformation magnitude of 0.6%, occurred during DSA. In the stress–strain hysteresis loop, 0.07 and 0.22% N of the steel at all stress amplitudes of 873 and 823 K were irregular flow, independent of the nitrogen. A continuous cyclic decrease in the degree of stress drop associated with tensile results was observed over the half-life period [46]. The influence of nitrogen on FL was observed at 773 and 873 K with strain amplitudes between 0.25 and 1.0%. It was also observed that the fatigue life increases or decreases with changing nitrogen content or becomes saturated with temperature and amplitude of applied strain at 0.14% N [47]. Reddy et al. [47], conducted LCF tests with a strain amplitude of 0.25% and at temperatures 773 and 873 K. It has been reported that the cyclic strain is influenced by DSA and secondary cyclic hardening (SCH) for 0.14% N steel. DSA is exhibited as a general increase in stress response with increasing temperature except at 873 K, whereas SCH is observed throughout the full temperature range from 773 K to 873 K with different amounts of hardening (during SCH) at each temperature. During SCH, increasing the nitrogen content and decreasing the temperature can increase the degree of hardening. At 773 K, DSA and SCH develop matrix hardening, and the life of LCF falls with increasing nitrogen content due to the rapid propagation of cracks during matrix.

Hardening [45, 47]. In LCF tests with a strain amplitude of ± 0.4% and a temperature of 873 K, 316LN SS displayed constant CSR from the initial cycle until it reached a near-saturated state before a rapid decrease in stress (GV and GA 2018). In the high-temperature regime, deformation rates of the load-deformation curves and temperature-dependent fragmentation flow dissipation occur. [69] In a research paper, the effect of sub-creep temperature on dynamic strain aging which influence the hardening behavior was analyzed. 316LN SS, DSA exists between 523 and 923 K; the material tends to have minimal ductility, and it has been observed that fracture resistance diminishes with rising temperature [9].

3.4 Effect of Temperature on Fatigue Crack Growth

The FCG experiments were conducted at temperatures from 300, 573, and 823 K in the air using the \(\Delta K\) decreasing mode under a constant R of 0.1 at a loading frequency of 15 Hz as shown in Fig. 8. The online crack length was estimated with the help of the direct current potential drop (DCPD) method [4]. The 0.22% N steel exhibited a larger FCG rate and lower threshold stress intensity factor \(\left( {\Delta K{\text{th}}} \right)\) in the Paris regime than the 0.14% N steel. The FCG results with crack closure correction, FCG resistance of 0.14% N steel is better than the other two 0.08 and 0.22% N steel in Paris and threshold regimes [3, 60]. Nitrogen is useful to the LCF resistance while the high slip planarity and decrease in stacking fault energy (SFE), which produces a high tendency to slip reversibility and reduces the cyclic strain localization [32, 44]. The 316LN SS will damage by creating stacking fault rings during the comparable stress that leads to a significant level of nearly 600 MPa due to several encouraging measures including radiance, raising strain, and diminishing test temperature [7]. A complex microstructural phenomenon governs the initiation of intragranular cracks during the LCF test [62]. Plastic deformation induces an effect on fatigue crack propagation was reported in [19].

Fig. 8
A line graph represents d a per d N in millimeter per cycle versus delta K in megapascal meter raised to 0.5, where the S S 316 L N material has a load ratio of 0.1 and a frequency of 15 hertz. 300, 623,673, 723, 773, and 823 K values rise with inflection points marked at 823 and 673.

Effect of various temperatures on the FCG behavior of SS316LN [4]

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While in the case of various loading patterns, loading patterns are associated with crack tip stress–strain behavior which controls the fatigue crack propagation rate [22]. The anisotropic behavior of SS 316LN at this microscopic level has been shown to perform an essential role in the development of fatigue cracks. Fatigue cracking depends on 316LN stainless steel’s graininess for surface cracks without damages and existing cracks [9, 63]. (Zhang et al. [76]) found that the environmental-aided effect on FCG rate was equivalent to the K value and that under the same K and rise time situations, the increase in FCG rate in HT water became more evident as the load ratio (R) value raised. The influence of grain boundary atmosphere distribution and grain size on corrosion fatigue analysis of 316LN SS in borated and lithiated HT water was examined by [14]. The grain boundary engineering method, rather than increasing the fraction of low-coincidence site lattice boundaries, was found to improve FL due to grain refining. The 316LN SS undergoes significant hardening under both cold and hot conditions. The plastic deformation of 316LN SS was formulated and depends on various processing parameters like deformation amplitude, deformation rate, temperature, and load type [50]. Samuel et al. [53], In this article, effect of varying load ratios on fatigue crack threshold has been reported and found that threshold stress intensity factor has an inverse relation with load ratio. The hardness value of 316LN stainless steel increases with increasing deformation rate, reaching a maximum amount of 350 HV at a maximum applied deformation rate of 3 \(\times {10}^{-3}{\mathrm{s}}^{-1}\) [1]. The growth rate of fatigue cracking decreases with an increasing load angle. Using different ranges of stress intensity factors, FCG rate da/dn of mode-I is lower than that of the mixed mode [5].

4 Conclusion

The interaction between creep-fatigue and fatigue has been studied in this review. In the secondary cyclic hardening process, hardening is increased by increasing the nitrogen content and with the decrease in temperature. Dynamic strain aging is a time and temperature-dependent phenomenon, so yield strength increases with decreasing deformation rate or increasing temperature. Adding nitrogen increases the life of the high-temperature low-cycle fatigue as it creates glide and preserves dynamic strain aging. With increased hold duration, fatigue life decreases.