Role of dynamic strain aging in corrosion fatigue of low-alloy pressure vessel steel in high temperature water

Low cycle fatigue (LCF) behavior of an A533B-type low-alloy pressure vessel steel was investigated in 200 °C and 288 °C water. Major attention was paid to the role of dynamic strain aging (DSA) on corrosion fatigue behavior of the steel. It was found that DSA played a noticeable role in the present corrosion fatigue process, manifested by the strain-rate and temperature dependent cyclic stress and plastic strain amplitude response. DSA was found to occur at a low strain rate in 200 °C water or at a high strain rate in 288 °C water regardless of dissolved oxygen concentration (DO) in water. In low-DO water, DSA improved the LCF resistance of the steel to some extent, but such an effect was screened by the environmental effects in high-DO water. The interactions between DSA and surrounding environmental factors as well as their possible influence on environmentally assisted cracking and fatigue resistance of the steel in high temperature water are discussed.     

Effect of dynamic strain aging on fatigue crack growth behaviour of reactor pressure vessel steels

Abstract

Fatigue tests under constant amplitude load were conducted on compact tension specimens of SA533B3 steels with four levels of sulphur content at different temperatures. A modified capacitance type crack opening displacement (COD) gauge was shown to be suitable for fatigue crack length measurement at high temperatures. Test results obtained with different measurement techniques show good consistency. The observation that the Young's moduli measured at a strain rate of 4 × 10^?3 s^?1 for the SA533B3 steels at 150 and 300°C do not decrease with increasing temperature seems to be related to the presence of dynamic strain aging. The fatigue crack growth rates at 150 and 300°C are about two and half times slower than those tested at 400°C because dynamic strain aging prevails at 150 and 300°C. Fractographic examination results suggest that inclusions embedded in secondary cracks enhanced the fatigue crack initiation rather than the fatigue crack growth.     

Influence of dynamic strain aging pre-treatment on the low-cycle fatigue behavior of modified 9Cr–1Mo steel

The influence of dynamic strain aging (DSA) pre-treatment on the low-cycle-fatigue (LCF) behavior of modified 9Cr–1Mo steel was investigated at 550 °C. The DSA pre-treatment reduces the fatigue life, which is reflected on the fracture surface as multiple crack initiation. The samples pre-treated by DSA have higher peak tensile stress and positive mean stress effects, which is responsible for the lifetime reduction. The DSA pre-treatment does not change cross-slip mechanisms during mechanical cycling, compared without DSA process, but results in accelerating the microstructure transformation from lath to cells with low dislocation densities, which reduces the number of cycles to failure.     

Low cycle fatigue behavior of Cr–Mo–V low alloy steel used for railway brake discs

The cyclic stress–strain response and the low cycle fatigue (LCF) behavior of Cr–Mo–V low alloy steel which was used for forged railway brake discs was studied. Tensile strength and LCF properties were examined over a range from room temperature (RT) to 600 °C using specimens cut from circumferential direction of a forged disk. The fully reversed strain-controlled LCF tests were conducted at a constant total strain rate with different axial strain amplitude levels. The cyclic strain–stress relationships and the strain–life relationships were obtained through the test results, and related LCF parameters of the steel were calculated. The studied steel exhibits cyclic softening behavior and behaves Masing type, especially at higher strain amplitudes. At higher than 600 °C, carbide particles aggregated and a decarburized layer developed near the specimen surface. Micro voids distribute within the depth of 50 μm from the specimen surface could coalesce with fatigue cracks. Multiple crack initiation sites were observed on the fracture surface. The oxide film that generated at 600 °C covered the fatigue striations and accelerated the crack propagation. Final fracture area with bigger and deeper dimples showed better ductility at higher temperature. The investigated LCF behavior can provide reference for brake disc life assessment and fracture mechanisms analysis.     

Low cycle fatigue and creep–fatigue interaction behaviour of 2.25Cr1MoV steel at elevated temperature

This study reports the fatigue behaviour of 2.25Cr1MoV steel under low cycle fatigue (LCF) loading and creep-fatigue interaction (CFI) loading at 355, 455 and 555 °C. Various hold durations up to 600 s were introduced in the CFI tests at the peak/valley strain under strain or stress control. In LCF tests, the steel exhibited remarkable strengthening at 455 °C, which can be ascribed to the effect of dynamic strain aging. In CFI tests, tensile holds were found more damaging than compressive holds but considerably less harmful than the combined tensile-compressive holds. A modified plastic strain energy approach based on the damage mechanisms was proposed to predict fatigue life under LCF and CFI conditions. The predictions obtained compared very favourably with the experimental results.     

Some metallurgical aspects of Dynamic Strain Aging effect on the Low Cycle Fatigue behavior of C–Mn steels

Carbon–Manganese steels and associated welds are commonly used, and sometimes to sustain loads in the Low Cycle Fatigue domain. Nevertheless, the metallurgy of these C–Mn steels is rather complex, due to the interaction of solute atoms (carbon and nitrogen) with dislocations during deformation which leads to metallurgical instabilities: Lüders strain, Static Strain Aging (SSA) and Dynamic Strain Aging (DSA). The DSA phenomenon is an interaction during the test between solute atoms and dislocations which are submitted to an supplementary anchorage if the temperature is sufficient to allow the diffusion of solute atoms leading to a discontinuous plastic deformation localized in bands associated with serrations on the stress–strain curve. In C–Mn, the temperature domain where the phenomenon is present is from 150 °C to 300 °C. If these metallurgical instabilities induce an increase in hardness, unfortunately they produce a decrease of ductility detrimental to components safety. The results of the DSA effect on LCF behavior in C–Mn and Low Alloyed steels reported in the literature are very confused and contradictories. In this study, two C–Mn steels with a different sensitivity to DSA are investigated in the Low Cycle fatigue domain. As reported from some authors, the fatigue life seems enhance or reduce in the temperature domain where the DSA is maximum, but the decrease of the strain rate always decreases the number of cycles to failure.     

Manifestations of dynamic strain aging under low and high cycle fatigue in a type 316LN stainless steel

Influence of Dynamic strain aging (DSA) under low cycle fatigue (LCF) and high cycle fatigue (HCF) loading was investigated by conducting LCF and HCF tests on specimens over a wide range of temperature from 573 to 973 K. DSA was found to be highly pronounced in the temperature range of 823–873 K. DSA was seen to have contrasting implications under LCF and HCF deformation. The cyclic hardening owing to DSA caused an increase in the cyclic stress response under LCF, leading to decrease in cyclic life. On the other hand, the DSA-induced strengthening suppressed the crack initiation phase under HCF where the applied stress remains fixed, leading to an increase in the cyclic life.     

The effect of microstructure on the morphology of fatigue cracks in UDIMET® 720

The low cycle fatigue (LCF) behaviour of four variants of UDIMET^® 720 was investigated. The materials comprised a fine grained (approximately 10 μm), powder processed material with a fine bimodal distribution (～20 and 80 nm) of secondary γ′; the same material, but with enlarged secondary γ′ (～480 nm); a coarse grained powder processed material (grain size ～62 μm) and finally a cast and wrought material with a similar microstructural scale to the fine grained powder processed alloy, but with reduced interstitial element content. LCF testing was undertaken on corner notched square section specimens at 20, 300 and 600 °C with a frequency of 0.25 Hz, a cyclic stress range of 500 MPa and an R ratio of +0.1. At 20 and 600 °C fracture was found to be macroscopically flat for all materials. However, at 300 °C, significant shear fracture was observed in the two materials that had a fine grain size and a fine secondary γ′ size, leading to a characteristic ‘tear‐drop’ appearance. Only minor shear fracture was observed in the coarse grained and enlarged secondary γ′ materials. Tensile tests indicated that weak dynamic strain ageing occurred in all materials at 300 °C. The fine grained powder processed U720 also exhibited dynamic strain ageing at 600 °C, but this was not the case for the coarse grained or cast and wrought materials. The origin of the shear fracture are discussed and related to the microstructure.     

THE ROLE OF DYNAMIC STRAIN-AGEING IN THE ENVIRONMENT ASSISTED CRACKING OBSERVED IN PRESSURE VESSEL STEELS

Abstract— The dynamic strain ageing (DSA) behaviour of A533B Class 1 reactor pressure vessel (RPV) steel, containing 5 ppm of free nitrogen, has been investigated over a wide range of strain rates from 3 × 10⁻⁷ to 3−10⁻³s⁻¹ and temperatures from 20 to 350°C. The DSA was observed within certain temperature ranges at all strain rates tested and its hardening effect in terms of the maximum strengthening stress (̀_p) decreased linearly with the increase of log strain rate. The temperature to stimulate DSA was observed to increase with increasing strain rate. The apparent activation energies of the characteristic strain rates for the onset, and peak of DSA have been determined, and their implications have been discussed. Compared with the available data, a positive effect of free nitrogen content on ̀_p has been evaluated. It has been found that the occurrence of susceptible environment assisted cracking (EAC) of A533B steel in high temperature water is co-related to the DSA behaviour. The results suggest that DSA reduces the ductility of RPV steel and its role in enhancing the EAC of RPV steels should not be neglected, in view of the coincidence with susceptibility zones for DSA and EAC in terms of strain rate and temperature.     

Experimental study of type 316 stainless steel failure under LCF/TMF loading conditions

A detailed investigation of low cycle fatigue (LCF) and thermo-mechanical fatigue (TMF) of a 316FR type stainless steel is presented in this paper in order to identify the failure mechanism based on the experimental results and the subsequent metallography of the samples. The LCF–TMF servohydraulic testing with a temperature uniformity of less than ±5 °C within the gauge section of the specimens was employed to conduct the experimental tests. Fully-reversed, strain-controlled isothermal tests were conducted at 650 °C for the strain ranges of Δɛ = ±0.4%, ±0.8%, ±1.0% and ±1.2%. Strain-controlled in-phase (IP) thermo-mechanical fatigue tests were conducted on the same material and the temperature was cycled between 500 °C and 650 °C. Additionally, the creep–fatigue interactions were investigated with the introduction of symmetrical hold time under both LCF–TMF tests. The cyclic behaviour was further studied by performing microstructural investigations using the scanning electron microscope (SEM).     

Effect of grain size on austenite stability and room temperature low cycle fatigue behaviour of solution annealed AISI 316LN austenitic stainless steel

Abstract

The present paper investigates completely reversed room temperature low cycle fatigue (LCF) behaviour of solution annealed austenitic stainless steel AISI 316L with two different grain sizes of 90 and 139 μm developed by solution annealing treatment at 1050 and 1150°C respectively and at six strain amplitudes ranging between ± 0·375 and ± 1·00%. Complete cyclic hardening has been observed for both the grain sizes. While fine grained steel shows an improvement in cyclic life compared with that of coarse grained steel for strain amplitudes ± 0·375 and ± 0·50%, and perfectly follows the Coffin–Manson (C–M) behaviour within the experimental domain, higher cyclic life with bilinear C–M behaviour is observed in the case of coarse grained steel at ± 0·625% strain amplitude and above. Optical microscopy of fatigue fracture surfaces reveals the formation of martensite on cyclic straining predominantly at higher strain amplitudes.     

Effect of strain rate on high-temperature low-cycle fatigue of 17-4 PH stainless steels

The effect of strain rate (10⁻², 10⁻³ and 10⁻⁴ s⁻¹) on the low-cycle fatigue (LCF) behavior was investigated for 17-4 PH stainless steels in three different conditions at temperatures of 300–500 °C. The cyclic stress response (CSR) for Condition A tested at 300 and 400 °C showed cyclic hardening due to an influence of dynamic strain aging (DSA). An in situ precipitation-hardening effect was found to be partially responsible for the cyclic hardening in Condition A at 400 °C. For H900 and H1150 conditions tested at 300 and 400 °C, the CSR exhibited a stable stress level before a fast drop in load indicating no cyclic hardening or softening. At 500 °C, cyclic softening was observed for all given material conditions because of a thermal dislocation recovery mechanism. The cyclic softening behavior in Conditions A and H900 tested at 500 °C is attributed partially to coarsening of the Cu-rich precipitates. The LCF life for each material condition, tested at a given temperature, decreased with decreasing strain rate as a result of an enhanced DSA effect. At all given testing conditions, transgranular cracking was the common fatigue fracture mode.     

Fatigue crack growth rate properties of SA508 and SA533 pressure vessel steels and submerged arc weldments in room and elevated temperature air environments

The stage II fatigue crack growth rate (FCGR) properties of four pressure vessel steels (SA508 Cl 2a, SA533 Gr A Cl 2, SA508 Cl 3a and SA533 Gr B Cl 2) and two automatic submerged arc weldments (SA508 Cl 2a and SA533 Gr A Cl 2) were developed in an air environment at 24 and 288°C (75 and 550°F). Tests were conducted at load ratios of 0.20 and 0.70. The air environment FCGR properties of these four base materials and two weldments proved independent of both load ratio and temperature and conformed to the ASME Section XI air environment reference curve. Furthermore, the base and weld metal air environment FCGR properties of these six pressure vessel steels were essentially identical.     

Role of dynamic strain ageing in low cycle fatigue

Low cycle fatigue (LCF) at elevated temperatures is known to be influenced by time-dependent processes like creep, oxidation and metallurgical instabilities. Another time-dependent phenomenon namely, dynamic strain ageing (DSA) has been found to exert an influence on LCF behaviour at high temperatures. Research activities carried out in the present author’s laboratory with a view to understanding the effects of DSA on LCF are highlighted in this paper. Occurrence of DSA manifests during total strain-controlled fatigue tests in the form of serrated plastic flow in stress-strain hysteresis loops, increased cyclic work hardening and reduced plastic strain range. Further, DSA causes localization of plastic flow leading to enhanced planarity of slip and widely-spaced slip bands. Impingement of slip bands on grain boundaries causes increased grain boundary decohesion, leading to reduced fatigue life. The influence of prior microstructure such as second phase particles and grain size on the effects of DSA on LCF is also discussed.     

Low cycle fatigue behaviour of 15-15Ti at 20 and 550°C under vacuum

15-15Ti is one of the potential candidate materials for fuel rod cladding of lead-based reactors, and its fatigue properties have been investigated. Low cycle fatigue (LCF) tests of 15-15Ti have been carried out under total strain amplitudes ranging from ±0.3% to ±0.9% at 20 and 550°C in vacuum. The results show the LCF life decreases with the increase of temperature. Crack propagates more tortuously and the fatigue crack propagation (FCP) rate is lower when it is tested at 20°C compared to 550°C. In addition, the effect of twin boundaries (TBs) and stacking fault energy (SFE) on FCP rate is discussed in this paper.     

Influence of temperature on the low cycle fatigue behaviour of a modified 9Cr–1Mo ferritic steel

Low cycle fatigue (LCF) behaviour of a modified 9Cr–1Mo steel under normalized and tempered conditions is reported. The alloy was normalized at 1313K for 1 h followed by tempering at 1033K for 1 h, which resulted in a tempered martensitic structure. Total axial strain controlled LCF tests were conducted at a constant strain rate of 3×10⁻³ s⁻¹ at different strain amplitudes varying from ±0.25 to ±1.0% in the temperature range of 300–873K. The cyclic stress response behaviour, in general, showed an initial brief hardening for the first few cycles, followed by a continuous and gradual softening regime that ended in a stress plateau that continued up to the specimen failure. The fatigue life decreased as the temperature increased. The temperature effect on life was more pronounced at low strain amplitudes. The metallography of the failed samples revealed that the fatigue failure at high amplitudes of testing was marked by extensive crack branching and the formation of secondary cracks. Oxidation was found to exert major influence on LCF life reduction at 873K.     

Fatigue lifetime of AZ91 magnesium alloy subjected to cyclic thermal and mechanical loadings

In the present paper, thermo-mechanical fatigue (TMF) and low cycle fatigue (LCF) or isothermal fatigue (IF) lifetimes of a cast magnesium alloy (the AZ91 alloy) were studied. In addition to a heat treatment process (T6), several rare elements were added to the alloy to improve the material strength in the first step. Then, the cyclic behavior of the AZ91 was investigated. For this objective, strain-controlled tension–compression fatigue tests were carried out. The temperature varied between 50 and 200 °C in the out-of-phase (OP) TMF tests. The constraint factor which was defined as the ratio of the mechanical strain to the thermal strain, was set to 75%, 100% and 125%. For LCF tests, mechanical strain amplitudes of 0.20%, 0.25% and 0.30% were considered at constant temperatures of 25 and 200 °C. Experimental fatigue results showed that the cyclic hardening behavior occurred at the room temperature in the AZ91 alloy. At higher temperatures, this alloy had a brittle fracture. But also, it was not significantly clear that the cyclic hardening or the cyclic softening behavior would be occurred in the material. Then, the high temperature LCF lifetime was more than that at the room temperature. The OP-TMF lifetime was the least value in comparison to that of LCF tests. At the end of this article, two energy-based models were applied to predict the fatigue lifetime of this magnesium alloy.     

Hold-time effects on elevated-temperature low-cycle-fatigue and crack-propagation behaviors of HAYNES? 188 superalloy

Low-cycle-fatigue (LCF) and fatigue-crack-growth (FCG) behaviors of a cobalt-based HAYNES 188 superalloy were studied at temperatures ranging from 816 to 982 °C in laboratory air. Various tensile-hold times were imposed at the maximum strain and load in the LCF and FCG tests, respectively, to examine the high-temperature fatigue and creep–fatigue interactions. In this article, the effects of hold time and temperature on the cyclic-stress response, fatigue life, fracture mode, and crack-growth rate are discussed. Parameters based on the tensile-hysteresis energy are applied to correlate the LCF lives with and without hold time.     

Thermomechanical fatigue evaluation and life prediction of 316L(N) stainless steel

An attempt has been made to understand the thermomechanical fatigue (TMF) behaviour of a nitrogen-alloyed type 316L austenitic stainless steel under different temperature domains. Smooth, hollow specimens were subjected to in-phase (IP) and out-of-phase (OP) thermal–mechanical cycling in air under a mechanical strain control mode, at a strain rate of 6.4 × 10^?5 s^?1 and a strain amplitude of ±0.4%. For the sake of comparison, total strain controlled low cycle fatigue (LCF) tests were also performed at the peak temperatures of TMF cycling on similar specimens employing the same strain rate and strain amplitude. Life was found to depend on the thermal/mechanical phasing and temperature. Creep was found to contribute to life reduction in IP tests when the peak temperature of cycling was above 600 °C. A few TMF tests were performed in vacuum in order to assess environmental influence on life. Thermomechanical fatigue cycling led to the development of significant amounts of mean stresses and the stress response was generally higher compared to that of LCF tests at the peak cyclic temperatures. Also, the isothermal tests at the peak temperature of TMF cycling resulted in lower lives compared to those obtained under TMF. An attempt was made to predict the TMF life using the isothermal database and satisfactory predictions were achieved using the Ostergren’s frequency modified damage function (FMDF) approach.     

Microstructural evaluation of fatigue damage in SA533-B1 and type 316L stainless steels

Transmission electron microscopy (TEM) examinations were made on fatigued SA533-B1 low alloy steel and Type 316L stainless steel specimens with the intention to investigate the mis-orientation changes among dislocation cells and the evolution of dislocation structures. Contrary to what might be expected for the cell structures, no clear relationship between fatigue damage and the mis-orientation changes of cell walls (or subgrain boundaries) was found in the fatigued samples of SA533-B1 steel (a bcc structure); however, significant changes of dislocation structures were observed in the fatigued samples of Type 316L stainless steel (an fcc structure). This could be accounted for by their different structures as well as complicated defect structures such as subgrain boundaries, small carbides, and dislocations inhomogeneously distributed in the SA533-B1 steel. It is interesting to note that at room temperature dislocations of fatigued SS316L specimens were observed to arrange themselves on {111} slip planes, in contrast, at 300°C the dislocations tend to move from their slip planes into subgrain boundaries in the surface layers rather than in the cross sectional layers.