Cold-worked Austenitic Stainless Steels Research Articles

Experimental and numerical analysis of mechanical fields on cross-shaped specimens for stress corrosion cracking of cold-worked austenitic stainless steels exposed to primary environment

A stress corrosion cracking (SCC) test was performed on a cold-worked austenitic stainless steel in simulated primary water, using a cross-shaped specimen permitting sequential loading. Crack density and location were investigated by scanning electron microscopy after the SCC test. To analyse the mechanical fields in the cracking areas, finite element simulations of the whole mechanical loading were conducted, involving both strain-path and temperature changes. Combined isotropic-kinematic hardening was used as constitutive equation and identified with tensile tests performed at room temperature and at 340 °C. Partial validation of the model was obtained by comparison of numerical strain fields with experimental measurements obtained by digital image correlation performed on a representative “ex-situ” test in air. Variations of the strain and stress fields during this test were discussed in relation with the cracking network observed at the end of the SCC test.

Influence of Cold Working on Hydrogen Diffusion Behaviour in an Austenitic Steel with Transformation-Induced Plasticity

The main objective of this work was to obtain information about the hydrogen diffusion behaviour in a cold-worked austenitic stainless steel (X3CrMnNiMoN17-8-4) in which deformation-induced martensite formation occurs during mechanical deformation. Three different states of pre-deformation (31 %, 39 % and 49 %) that showed induced phase transformation from austenite to α’martensite as well as the solution-annealed material were part of this study. All samples were charged with hydrogen in a 0.1 M NaOH solution. This charging took place electrolytic with 10 mA cm-2 at three different temperatures (50 °C; 65 °C and 80 °C) in the double cell according to Devanathan and Stachurski. Due to the very slow diffusion of hydrogen through austenite, the samples were not charged until the equilibrium state was reached. To find out the necessary diffusion parameters, the data were fitted with numerical optimisation. Using this method, the effective diffusion coefficients of charging could be determined for all material states. The study also contains microscopic analyses to visualize the effect of cold working on the microstructure of the material. The appearance of α’-martensite significantly contributes to the susceptibility to hydrogen uptake leading to increasing diffusion coefficients in relation to higher pre-deformation.

Experimental Study of the Post-Fire Mechanical and Material Response of Cold-Worked Austenitic Stainless Steel Reinforcing Bar.

This paper is concerned with the behaviour of stainless steel reinforcing bar following exposure to elevated temperatures from a fire, followed by subsequent cooling. Stainless steel-reinforced concrete is an increasingly popular solution for structural applications which require corrosion resistance, excellent mechanical properties, and long life cycles with little maintenance. In addition, although stainless steel reinforcement has a higher initial cost compared with traditional carbon steel bars, the overall life cycle costs are likely to be quite similar, owing to the lack of maintenance required for stainless steel materials. There is no information available in the literature on the post-fire properties of austenitic stainless steel reinforcement, although these data are essential for any engineer who wishes to study the structural integrity of a reinforced concrete component or system following a fire. Accordingly, this paper presents a detailed discussion and analysis from the results of a series of laboratory experiments on three grades of austenitic stainless steel reinforcement following various levels of temperature exposure and also different cooling rates. Both the mechanical and metallurgical properties are examined, and the behaviour is compared to that of B500B carbon steel reinforcement. It is shown that the stainless steel bars retained their mechanical properties under the majority of the scenarios examined and to a greater degree than traditional materials. This is important for the rehabilitation and salvage of existing reinforced concrete structures following a fire and also to avoid unnecessary demolition and replacement.

Interaction of Liquid Lead Bismuth and Materials in Spallation Target

Material choices for liquid lead bismuth spallation target are some of austenitic stainless steel, ferrite martensitic steel and cold-worked austenitic stainless steel. In order to ensure materials resistance to irradiation and corrosion as well as compatibility with lead bismuth, it is appropriate to lower the incident proton current density and the process temperature, in which temperature range engineering design can control to work, especially in ADS (Accelerator-Driven nuclear transmutation System) concept. The lower limit temperature is determined from the physical melting temperature and the engineering efficiency of the steam generator involved in process control. The material related issues for liquid lead bismuth are mass loss by impinging secondary flow, wettability at the device interface for ultrasonic waves application, detachable control of the slag in the flowing system, stabilized electrical resistance between the material and the liquid lead bismuth interface. Electromagnetic fluid analyses show how flow rate relates electrical resistivity of flow channel material.

Microstructural Evolution of Laser-Welded Dissimilar Lap Joints of Martensitic Abrasion Resistant Steel and Cold-Worked Austenitic Stainless Steel

This paper investigates the effect of the energy input on the microstructure evolution and mechanical properties of laser-welded dissimilar lap joints of cold work-hardened austenitic stainless steel (CW-ASS) and martensitic abrasion resistant steel (AR600). Microstructure characterization of the welds was conducted using optical microscopy and electron backscatter diffraction in a scanning electron microscope. Subsequently, the mechanical properties of the dissimilar lap joints were determined using microhardness measurements and tensile tests. The microstructure observations show that the phase structure in the fusion zone (FZ) is predominantly ferritic at both energy inputs. Besides, the solidification microstructure in the FZ resembles the cast structure composed of cellular and columnar dendrites with exhibiting elemental segregations. The hardness reaches its peak in the FZ. However, the FZ near AR600 steel exhibited higher hardness values than that near CW-ASS. The dissimilar lap joints welded at low energy input 160 J/mm achieved a higher shear strength than those welded at high energy input 320 J/mm due to the softening of the weld in the former lap joint.

Anodic Polarization Behavior of Cold-Worked Austenitic Stainless Steel: A Newer Approach

The anodic potentiodynamic polarization behavior of various grades of cold-worked (rolling and machining) austenitic stainless steels were studied. Deformed specimens were characterized by Electron backscattered diffraction (EBSD) and Fourier transform infrared spectroscopy (FTIR)-imaging. The FTIR-imaging was used to quantify chromium oxide (Cr2O3) spectra. It was observed, average area under Cr2O3 spectra was decreased with increase in cold -working. The certain regions in deformed microstructure showed higher value of area under Cr2O3 spectra, indicated stability of passivation film that needs to be explored. There was no microstructurally different between machined specimens.

Mechanical Fields on Cross-Shaped Specimens for Stress Corrosion Cracking of Cold-Worked Austenitic Stainless Steels

The aim of this work is to better understand the evolution of the mechanical fields in cross-shaped specimens used to study the stress corrosion cracking (SCC) susceptibility of cold-worked stainless steels exposed to the primary water of nuclear power plants. Cross-shaped specimens are used since a loading path change leads to more severe cracking than a monotonous loading. During SCC tests, only the applied load and displacement can be measured as a result of experimental equipment limitations. Therefore, a mechanical test is performed ex-situ under similar condition as the ones applied during the SCC tests. The experimental deformation fields are measured by digital image correlation (DIC) and a numerical model is developed with ABAQUS to calculate the mechanical fields. The obtained results will allow us to calibrate SCC tests already performed but also to know the experimental boundary conditions to apply to reach a given strain value during forthcoming SCC tests.

Improvement of Strength and Impact Toughness for Cold-Worked Austenitic Stainless Steels Using a Surface-Cracking Technique

For cryogenic applications, materials must be cautiously selected because of a drastic degradation in the mechanical properties of materials when they are exposed to very low temperatures. We have developed a new technique using a cold-working and surface-cracking process to overcome such degradation of mechanical properties at low temperatures. This technique intentionally induced surface-cracks in cold-worked austenitic stainless steels and resulted in a significant increase in both strength and fracture at low temperatures. According to the microstructure observations, dissipation of the crack propagation energy with surface-cracks enhanced the impact toughness, showing a ductile fracture mode in even the cryogenic temperature region. In particular, we obtained the high strength and toughness materials by a surface-cracking technique at 5% cold-worked specimen with surface-cracks.

Twin boundary-accelerated ferritization of austenitic stainless steels in liquid lead–bismuth eutectic

Exposure of austenitic stainless steels to liquid lead–bismuth eutectic with low concentration of dissolved oxygen typically results in selective leaching of highly-soluble alloying elements and ferritization of the dissolution-affected zone. In this work, focused ion beam, transmission electron backscatter diffraction and scanning transmission electron microscopy were utilized to elucidate early-stage aspects of the dissolution corrosion process of cold-worked austenitic stainless steels exposed to static, oxygen-poor liquid lead–bismuth eutectic at 450°C for 1000h. It was found that deformation-induced twin boundaries in the cold-worked steel bulk provide paths of accelerated penetration of the liquid metal into the steel bulk.

Mechanical factors in primary water stress corrosion cracking of cold-worked stainless steel

Finite element analysis was performed on a compact tension specimen to determine the stress and strain distributions near a crack tip. Based on the results, the crack tip stain rates by crack advance and creep rates near crack tip were estimated. By comparing the dependence of the mechanical factors on the stress intensity factor and yield stress with that of the SCC crack growth rates, it was tried to identify the main mechanical factor for the primary water stress corrosion cracking (PWSCC) of cold-worked austenitic stainless steels. The analysis results showed that the crack tip normal stress could be the main mechanical factor controlling the PWSCC, suggesting that the internal oxidation mechanism might be the most probable PWSCC mechanism of cold-worked austenitic stainless steels.

Intergranular Stress Corrosion Cracking Growth Behavior of Ni-Cr-Fe Alloys in Pressurized Water Reactor Primary Water

The purpose of this research was to compare the stress corrosion cracking (SCC) resistance of materials used at the present time for steam generator (SG) tubing in pressurized water reactor (PWR) primary water. Our results in PWR primary water for 20% cold-worked (CW) Alloy 800 (UNS N08800) are compared with already published data for SCC growth from 20%CW Alloy thermally treated (TT)690 (UNS N06690), 20%CW Alloy mill-annealed (MA)600 (UNS N06600), and 20%CW austenitic stainless steels. The second purpose was to examine the dependence of SCC growth on nickel and chromium in PWR primary water; the objective was to obtain the basic knowledge to understand SCC behavior of SG tubing materials. The third objective was to understand whether accelerated testing at higher temperatures is appropriate for predicting SCC initiation and growth at lower temperatures. For these objectives, SCC growth was measured in PWR primary water at 290, 320, 330, 340, and 360°C under static load conditions. Tests were performed us...

Investigations of twin boundary fatigue cracking in nickel and nitrogen-stabilized cold-worked austenitic stainless steels

Implant retrieval studies have indicated that the primary cause of failure in stainless steel devices is fatigue, and time or cycles required for fatigue crack initiation often consumes the majority of implant lifetime. Stainless steels with significant nitrogen additions have shown an improved fatigue response, but have also shown a peculiar preference for fatigue crack initiations at or along annealing twin boundaries in the face-centered cubic (FCC) materials. In a recent comparison study on cold-worked implant grade stainless steels, a number of fatigue crack initiations were found along former annealing twin boundaries on both nitrogen-stabilized austentitic (HNASS) and nickel-stabilized austenitic steels. Further investigations were warranted to determine the crystallographic conditions present around these annealing twin boundary cracks, since not every twin boundary showed crack initiation. The present study examined the crystallographic conditions present around each of the former annealing twin boundary cracks relative to the applied loading direction. It was determined that the former annealing twin boundary cracks showed the complete range of misorientation deviations allowed by the Brandon criterion. The textures of the cracked twin boundaries were found to be random relative to the overall global textures of the materials. Most of the cracked twin planes in the HNASS steel were shown to be high angles, and in many cases were nearly perpendicular to the material surface. The nickel-stabilized steel showed a preference for lower twin plane inclination angles relative to the material surface. High Schmid factors were shown for all grains surrounding the cracked twin boundaries indicating each grain was oriented favorably for slip relative to the applied loading direction. A high Taylor factor mismatch was also shown across most of the cracked twin boundaries in both steels indicating strong difference in expected yield response for each of the grains which suggest localized strain incompatibility was another important factor in twin boundary cracking.

Fatigue Life of the Strain Hardened Austenitic Stainless Steel in Simulated Pressurized Water Reactor Primary Water

Over the last 20 years or so, many studies have revealed the deleterious effect of the environment on fatigue life of austenitic stainless steels in primary water reactor (PWR) primary water. The fatigue life correlation factor, so-called Fen, which corresponds to the ratio of fatigue life in air at room temperature to that in water under reactor operating conditions, has been standardized to consider the effect on fatigue life evaluation, and the formulations are function of strain rate and temperature due to their noticeable negative effect compared with other factors (Chopra and Shack, 2007, “Effect of LWR Coolant Environments on the Fatigue Life of Reactor Materials,” Final Report, Report No. NUREG/CR-6909, ANL-06/08; Codes for Nuclear Power Generation Facilities, 2009, "Environmental Fatigue Evaluation Method for Nuclear Power Plants," JSME S NF1-2009, The Japan Society of Mechanical Engineers, Tokyo, Japan). However, mechanism causing fatigue life reduction remains to be cleared. As one of the possible approaches to examine the underlying mechanism of environmental effect, the authors focused on the effect of plastic strain, because it could lead microstructural evolution on the material. In addition, in the case of stress corrosion cracking (SCC), it is well known that the strain-hardening prior to exposure to the primary water can lead to remarkable increase of the susceptibility to cracking (Vaillant et al., 2009, “Stress Corrosion Cracking Propagation of Cold-Worked Austenitic Stainless Steels in PWR Environment,” 14th International Conference on Environmental Degradation of Materials in Nuclear Power Systems; Couvant et al., 2009, "Development of Understanding of the Interaction Between Localized Deformation and SCC of Austenitic Stainless Steels Exposed to Primary PWR Environment," 14th International Conference on Environmental Degradation of Materials in Nuclear Power Systems). However, its effect on fatigue life has not necessarily been cleared yet. The main effort in this study addressed the effect of the prior strain-hardening on low cycle fatigue life in the primary water. A plate of 304LSS was strain hardened by cold rolling or tension prior to fatigue testing. The tests were performed under axial strain control at 300 °C in primary water including B/Li and hydrogen, and in air. The effect on environmental fatigue life was investigated through a comparison of Fen in experiments and in regulations, and also the effect on the fatigue limit defined at 106 cycles was discussed.

Cold-worked austenitic stainless steels in passenger railcars and in other applications

The structural applications of cold-worked austenitic stainless steels are reviewed in a historical perspective, with an emphasis on passenger railcars. The base materials are described, including their mechanical and technological properties. Some notions of car body design are presented, together with the challenges related to design practices involving the discussed materials. The fabrication processes are described. Information on resistance and arc welding processes is provided, including their specific aspects concerning the discussed materials, a description of the equipment, procedures qualification, reference standards, and production control. Challenges relative to the wider application of cold-worked austenitic stainless steels are discussed.

Corrosion behavior of cold-worked austenitic stainless steels in liquid lead–bismuth eutectic

The effect of cold working on the corrosion behavior of austenitic stainless steels in liquid lead–bismuth eutectic (LBE) was studied to develop accelerator-driven systems for the transmutation of long-lived radioactive wastes and lead–bismuth cooled fast reactors. Corrosion tests on solution-treated, 20% cold-worked and 50% cold-worked 316SS and JPCA (15Cr–15Ni–Ti) were conducted in oxygen-controlled LBE. Slight ferritization caused by Ni dissolution and Pb–Bi penetration were observed for all specimens in the corrosion test conducted at 500°C for 1000h in liquid LBE with an intermediate oxygen concentration (1.4×10−7wt.%). In the corrosion test performed at 550°C for 1000h in liquid LBE with a low oxygen concentration (4.2×10−9wt.%), the depth of the ferritization of 316SS and JPCA increased with the extent of cold working. Only oxidation was observed in the corrosion test that was performed at 550°C for 1000h in liquid LBE with a high oxygen concentration (approximately 10−5wt.%). Cold working accelerated the formation of the double layer oxide and increased the thickness of the oxide layer slightly. In contrast, the ferritization accompanied by Pb–Bi penetration was widely observed with oxidation for all specimens corrosion tested at 550°C for 3000h under the high-oxygen condition. Cold working increased the depth of the ferritization of 316SS and JPCA. It is considered that cold working accelerated the ferritization and Pb–Bi penetration through the enhanced dissolution of Ni into LBE due to an increase in the dislocation density under conditions in which the protective oxide layer was not formed in liquid LBE.

An EBSD based comparison of the fatigue crack initiation mechanisms of nickel and nitrogen-stabilized cold-worked austenitic stainless steels

Nitrogen additions are known to improve both the pitting potentials and fatigue response of high-nitrogen austenitic stainless steels (HNASS). HNASS alloys are of particular interest to the biomedical industry due to patient allergy concerns. Available HNASS fatigue studies in the literature were all performed on materials in the solution annealed condition, and not the 20–30% cold-worked condition often ordered for implant manufacturing. Previous studies in our laboratories evaluated the corrosion fatigue response of commercially available cold-worked 316L and HNASS implant grade steels, and found evidence of unusual cleavage-like facets in the crack initiation and early propagation regions of the fracture surfaces in the HNASS. The 316L steel showed a typical ductile fatigue striated fracture surface with no evidence of cleavage-like facets. These findings suggested the fatigue crack initiation mechanisms may be different between the two austenitic steels. The purpose of the present study was to compare and contrast fatigue crack initiation mechanisms in the cold-worked 316L and HNASS steels, using an SEM and EBSD based examination technique, under both low-cycle and high-cycle conditions. Under both fatigue conditions, cracks were shown to preferentially initiate along former annealing twin boundaries in the HNASS steel. In contrast, fatigue crack initiation was shown to occur primarily along extrusions or intrusions associated with trans-granular slip markings in the 316L steel.

Thermal Stability of Microstructure in Grain Boundary Character Distribution-Optimized and Cold-Worked Austenitic Stainless Steel Developed for Nuclear Reactor Application

AbstractGrain boundary character distribution-optimized (GBCD) Type 316 corresponding austenitic stainless steel and its cold-worked ones (GBCD+CW) are one of prospective nuclear materials to be considered for next generation energy systems. These steels were thermally-aged at 973 K for 1 and 100 h and were examined by transmission electron microscopy (TEM) to evaluate microstructural stability during high temperature exposure. TEM results revealed that microstructures of both specimens prior to ageing contained step-wise boundaries which is composed of coincidence site lattice (CSL) and random grain boundaries and also that the GBCD+CW specimens had dislocation cells and networks as well as deformation twins whereas as the GBCD one possessed few dislocations. After thermal ageing, the precipitates formed on not only random grain boundaries but also dislocations, contributing to prevent significant microstructural change occurring such as recrystallization and dislocation recovery.

Annealing of Cold-worked Austenitic Stainless Steels.

This article reviews the phenomena involved during the annealing of cold worked austenitic stainless steels. Initially the cold worked state is discussed, with special emphasis on the formation of deformation induced martensites. Following, the phenomena of martensite reversion, recovery, recrystallization and grain growth are discussed. The interactions between primary recrystallization and precipitation and between precipitate dissolution and secondary recrystallization are dealt with in detail. Finally, the textures resulting from hot and cold working, and from primary and secondary recrystallization, are presented.

Swelling of cold-worked austenitic stainless steels irradiated in HFIR under spectrally tailored conditions

The effects of cold working and impurities on swelling behavior in austenitic stainless steels irradiated at 400 °C to 17.3 dpa under spectrally tailored conditions in the Oak Ridge research reactor and high flux isotope reactor were investigated. The specimens were 20% cold-worked JPCA, 316R, K (low carbon (0.02%)) and C (low carbon (0.02%) and doped with 0.08% niobium). The helium generation rate was about 15 appm He/dpa. Cavities, dislocation loops and carbides were formed by irradiation in these steels. The swelling in the JPCA-CW and 316-CW was 0.003% and 0.004%, respectively and in the C-CW and K-CW was 0.02% and 0.01%, respectively. Swelling in K and C steels was strongly reduced by 20% cold-work, and the swelling in JPCA-CW and 316R-CW steels was comparable to JPCA-SA and 316R-SA steels. The synergistic treatments of addition of some impurities and cold working are very effective for the suppression of swelling at 400 °C in austenitic stainless steels.

Stainless steel thread forming screws and their susceptibility to stress corrosion cracking

Thread forming fasteners incorporate both drilling and hole tapping features and are commonly used in the construction industry to fix together steel sheets of different material types. With this practical application in mind, fasteners manufactured from martensitic and austenitic stainless steels have been subjected to alternating corrosion conditions in accordance with test standards DIN 50021-SS and DIN 50018-K WF 2·0. The torque applied to the screws during these tests was controlled to place the fasteners under equal tensile loads, independent of their tensile strength. Thus, the results provided information on their relative susceptibilities to stress corrosion cracking. At the end of the tests, up to 80% of the martensitic stainless steel drilling and tapping screws had failed due to hydrogen induced stress corrosion cracking. The fasteners manufactured from austenitic materials withstood identical test conditions without any evidence of cracking or crack initiation. It is concluded that fasteners manufactured from modified martensitic stainless steel are more susceptible to stress corrosion cracking under the conditions of test than those made from cold worked austenitic stainless steels. This suggests that in practical applications the potential for catastrophic failure due to stress corrosion cracking could be considerably higher in modified martensitic fasteners in comparison with austenitic stainless steel fasteners, including those with hardened carbon steel drill points.