Cold-worked Specimens Research Articles

Investigation of irradiation defects and hardening of cold-worked vanadium alloys

Irradiation defects of V-5Cr-5Ti alloys with different pre-irradiation treatment after irradiation to 0.5 dpa at 100 °C with energetic Fe ions were investigated. For the cold-worked specimens, dislocation loops decrease in size while increasing in number density with the increase of the deformation percentage. For the annealed specimens, dislocation loops have minor difference in size whilst the number density decreases with the increasing deformation percentage. Meanwhile, dislocation loops decorating dislocations were observed. The obstacle strength factor (α) is calculated according to the dispersion barrier hardening model (DBH). For the cold-worked specimens, α increases with increasing size of dislocation loops. A linear relation, α=0.075*d−0.074 is established. At a similar size, dislocation loops in the annealed specimens have a greater α than the cold-worked specimens, possibly due to the lock of the tangled network dislocations and/or the hinder of the segmented movement of the tangled network dislocations by dislocation loops.

Optimize the corrosion behavior of AISI 204Cu stainless steel in different environments under previous cold working and welding

Enhancing corrosion resistance in stainless-ssteel alloys is a paramount objective in the petroleum industry. This study investigated the effects of the previous cold working and welding processes on the mechanical properties and corrosion rates of 204 Cu stainless steel in different aggressive environments (crude oil, freshwater, and seawater). The experimental sets were supported by microstructure analysis. The mean weight loss method was employed to determine the corrosion rates, which were optimized using the Taguchi method. The ferrite and austenite phase bands, as well as the deformed portions of austenite, are pushed to flatten out during cold working, which increases the material’s hardness. Cold-worked steels were welded, creating an annealed area around the HAZ in addition to the usual weld zones, which demonstrated partial microstructure recovery and hardness reduction. HAZ showed signs of iron overload and chromium nitride precipitation. Cold-worked specimens only showed reduced corrosion resistance to 30% of the initial rate and reduced thickness. Moreover, the Taguchi optimization technique indicated that the corrosion environment has the most effect on the corrosion rate compared to the cold work ratio for welded and non-welded stainless-steel specimens.

Effect of cold work deformationon irradiation hardening of vanadium alloys

Vanadium alloys are regarded as promising candidate structural materials for the advanced blanket concept in fusion reactors due to their low activation, good high-temperature strength and, in particular, their compatibility with liquid lithium. In the present work, six kinds of V–5Cr–5Ti alloys under heavy cold work with deformation amounts of 40%, 60% and 80%, and/or subsequent annealing were investigated. Irradiation damage of 0.1, 0.3 and 0.5 dpa was introduced in both specimens using 352.8 MeV Fe ions at 100 °C. Electron backscattered diffraction and transmission electron microscopy (TEM) were used to investigate pre-irradiation microstructures such as grains, dislocations, precipitates and bubbles. X-ray diffraction was used to evaluate the pre-existing dislocation density and TEM was used to image the irradiation defects. The change in hardness was evaluated using micro-hardness tests. Before irradiation, the hardness increased with the increasing deformation amount but decreased after subsequent annealing. Dislocation cells turning into sub-grains with low-angle boundaries were observed, while the deformation amount reached 80% in cold-worked specimens. After irradiation, hardening was observed in all specimens and at all irradiation doses, and a power-law relation was observed in dose-dependent hardening. The effect of the initial microstructure on irradiation hardening was discussed in terms of the sink strength while ignoring grains and precipitates due to their large size. Pre-existing bubbles could effectively reduce irradiation hardening compared with previous results. Meanwhile, with the increasing sink strength of dislocations, hardening decreased in a different manner in cold-worked and annealed specimens. The irradiation defects in some specimens were investigated to clarify the inherent mechanism in the relationship between the initial microstructures and irradiation hardening.

Microcrystallographical Characterization of Initiation of Intergranular Stress Corrosion Cracking on Alloy 600 in Terms of Local Stress at Grain Boundary Induced by Slip Deformation

The initiation of intergranular stress corrosion cracking (IGSCC) on Alloy 600 in a simulated pressurized water reactor primary water environment was investigated by the characterization using an electron back scattering diffraction (EBSD) on flat tensile specimens subjected to a slow strain rate testing. The IGSCC initiation was evaluated for many grain boundaries in terms of the grain boundary characteristics and the local stress generated at each grain boundary; the latter is estimated by considering the specific slip deformation determined by the Schmid factor of two grains adjacent to a grain boundary. This simplified local stress evaluation for IGSCC susceptibility based on EBSD analysis is introduced for the first time in this study. IGSCC tends to occur as a result of induced tensile stress rather than shear stress at the grain boundary, whereas no IGSCC occurred when compressive stress was applied at the grain boundary. Similar results were observed for both 10% and 20% cold worked (CW) specimens in the stress analysis. It was noted that crack initiation depends not only on the stress at the grain boundary but also on the strain concentrated around the grain boundary for the 20% CW specimen.

Cold work induced stability of retained austenite at elevated temperature in a medium carbon high silicon steel

In the present work, a potential solution has been suggested for arresting the depletion of the retained austenite during prolonged holding at elevated temperature. A medium carbon high silicon steel specimen was austenitized and then air cooled for 20s followed by austempering at 350 °C for 10 min. 20% retained austenite was measured from X-Ray diffraction studies in the specimen and 23.4 ± 1.7% uniform elongation was achieved. On holding the similar austempered specimen at 350 °C for 120 min, all the RA austenite depleted from the microstructure and uniform elongation values dropped to 5.7 ± 0.6%. In further thermomechanical treatments, austempered specimens were cold worked by 4, 7 and 10% at room temperature. The cold worked specimens were further held at elevated temperature (350 °C) for prolonged duration (120 min). Retained austenite content was observed to increase from 14 to 18% with the amount of cold work and tensile elongation value of up to 14% was observed. The decomposition of the retained austenite from the microstructure of cold worked specimens was arrested due to the increase in the dislocation density during cold work applied before holding at the elevated temperature. First principle calculations indicate migration of large number of carbon atoms towards the dislocations at elevated temperature thereby inhibiting the carbide precipitation during decomposition of retained austenite. Application of cold work to the austempered specimens could arrest the depletion of retained austenite at elevated temperature.

The Effects of Cold Work on the Incipient Pitting Morphology Evolution of 304L Stainless Steels

The purpose of this study was to investigate the effects of cold work on the incipient pitting corrosion of 304L stainless steels (SS). The as-received (0%-CW) and 20% cold-worked (20%-CW) 304L specimens were corrosion tested in a salt-spray system with a solution of 3.5% simulated seawater for the exposure times of 12 h, 24 h, 48 h, 96 h, and 192 h to characterize the pitting corrosion behavior of 304L SS. Weight gain and microstructures were measured using a microbalance and electron backscatter diffraction, respectively. The dimensions of corrosion pits—depth, length, and width—were analyzed by confocal laser scanning microscope to elucidate the pit growth process. The analysis of the rust and pit features revealed that the degree of corrosion is more severe in the 0%-CW specimen than in the 20%-CW specimen.

Synergistic Effect of Solid State Hydrogen and Cold Work Pretreatment on Oxide Films Grown on 316L Stainless Steel during Short Term Immersion in Deaerated High Temperature Water at 300 °C

The properties of the oxide films formed on solution-annealed and cold-worked 316L stainless steel (SS) specimens with and without charged hydrogen in deaerated pressurized water reactor primary water at 300 °C were investigated. The outer oxide layers of all specimens were composed of magnetite (Fe3O4) and NiFe2O4. Charged hydrogen resulted in larger outer iron-bearing oxide particles forming due to hydrogen-enhanced outward diffusion of iron cations. Prior cold-work accelerates the oxidation was observed. Charged hydrogen led to local cracks in the oxide film and enhanced the penetration oxidation beneath the metal/oxide interface. The Cr-rich inner oxide layer grown on the prior cold-worked specimen with charged hydrogen was thicker than that on the cold-work specimen or the hydrogen-charged specimen, revealing the combined effects of charged hydrogen and prior cold-work on the acceleration of the oxidation process. The working mechanism of the solid-state hydrogen effect on the oxide film was discussed.

Strain anisotropy models for refined diffraction line profile analysis in cubic metals

Abstract The present work examined the anisotropy magnitudes obtained from different elastic models of cubic metals (Cu, 5383 Al alloy, FCC austenite steel and BCC steel) to explore the origin of strain anisotropy. The results showed that stable intersections were observed from the modeled and experimental plots of the reciprocal elastic modulus (1/Ehkl) and orientation parameter (Γ). The effectiveness of quasi elasto-plastic model based method in correcting strain anisotropy was further verified in cold-worked specimens. For the important input parameters in dislocation model based diffraction line profile analysis methods, the average diffraction contrast factors of dislocations were observed to depend on elastic constants. Interesting intersections were found from linear dependence of on Γ. The conventional input values indicated distinct dependencies on given elastic constants in diffraction line profile analysis. Accordingly, a refined approach was proposed by adopting the optimized intersections as input values, by which more reliable results could be obtained in practical applications.

Characterization of wire arc additive manufactured products: A comparison between as-deposited and inter-layer cold worked specimens

The presence of large dendrites and alloy segregation in as-deposited wire arc additive manufacturing (WAAM) products results in a wide range of mechanical properties that are often inferior to wrought ones. In this study, the effect of inter-layer cold working on the microstructure and mechanical properties of AISI 347 samples manufactured by WAAM has been investigated. The average hardness of single-pass welding, as-deposited, and cold worked WAAM samples were 190 HV, 249 HV, and 323 HV, respectively. Inter-layer cold working has led to an increase in ultimate tensile strength from 576 MPa to 803 MPa and yield strength from 395 MPa to 697 MPa. Also, Inter-layer cold working and thermal energy due to the deposition of subsequent layers have resulted in microstructural evolution and grain refinement through recrystallization. Besides, a semi-quantitative analysis has shown that increasing the amount of cold working by four times resulted in 64 % decrease in average recrystallized grain size.

A Mechanistic Study of Carbon Steel Cracking in 360°C Air and Hydrogen Environments

Thirty percent cold-worked (CW) carbon steel tensile specimens were exposed to 360°C air and hydrogen environments (2 MPa H2 and 20 MPa H2) under an applied load to produce intergranular creep cracking. In this study, cutting-edge microscopy techniques were applied to characterize cracking on multiple length scales and in three dimensions. The objective was to develop a better mechanistic understanding of creep cracking in carbon steel, and the known deleterious effect of hydrogen (attack) at the micro-to-nanoscale. Amorphous carbon along the fracture path was observed in all experiments, with evidence for nanoscale cavities/methane bubbles in hydrogen exposures, particularly at cementite-ferrite boundaries. Results suggested that creep or residual stress led to breakdown of cementite to amorphous carbon, cavitation, and/or formation of methane (depending on H2 content); it is suggested that the combination of deleterious mechanisms leads to initiation and/or acceleration of creep cracking in CW carbon steel. Comparisons are made between the morphology of creep cracking in these laboratory experiments and recent results from characterization of creep cracking in ex-service carbon steel piping from a CANDU nuclear power plant. Although more subtle, similar morphology and chemistry at crack tips in laboratory and ex-service CW carbon steel suggests that the mechanism(s) of creep cracking is similar.

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.

Through-thickness microstructural evolution during grain boundary engineering type thermomechanical processing and its implication on sensitization behavior in austenitic stainless steel

Through-thickness microstructural variations and their synergistic influence on degree of sensitization in grain boundary engineering (GBE) treated 304L stainless steel was compared with solution-annealed and marginally cold-worked (~3% thickness reduction) specimens. The solution-annealed specimen has shown higher degree of sensitization due to the presence of lower fraction of Σ3 (as well as other low Σ CSL) boundaries and dominant random high-angle boundaries (RHAGBs) connectivity. However, it has shown thickness-independent sensitization behavior as grain boundary character distribution, residual strain and grain boundary network topology towards thickness direction are similar. Owing to higher residual strain, as revealed by local misorientation analysis, the cold-worked specimen showed maximum degree of sensitization (higher than solution-annealed condition) at surface. The GBE specimen has shown much lower degree of sensitization at surface due to higher-fraction of Σ3 boundaries and discontinuous RHAGBs connectivity. However, increasing fraction of RHAGBs and their connectivity, as confirmed by twinning-related domain and fractal analysis, cause more sensitization at interior of this specimen. In spite of this, the degree of sensitization at the interior of GBE specimen is lower than the solution-annealed or cold-worked specimen. Even though the residual strain in GBE specimen is higher than the solution-annealed condition, the lower degree of sensitization indicates that the grain boundary character distribution and grain boundary network topology have higher implication than residual strain in controlling sensitization.

The influence of dislocation and hydrogen on thermal helium desorption behavior in Fe9Cr alloys

Transmutation helium may causes serious embrittlement which is considered to be due to helium from clustering as a bubble in materials. Suppression of transmutation helium can be achieved by introducing trapping sites such as dislocations and impurities in materials. Here, effects of intentionally-induced dislocations and hydrogen on helium migrate and release behaviors were investigated using thermal desorption spectrometry (TDS) technique applied to well-annealed and cold-worked Fe9Cr alloys irradiated by energetic helium/hydrogen ions. Synchronous desorption of helium and hydrogen was observed, and the microstructure states during helium release at different temperatures were analyzed. High thermally stable HenD type complexes formed in cold-worked specimens, resulting in the retardation of helium migration and release. The existence of hydrogen will strongly affect the thermal helium desorption which could be reflected in the TDS spectrum. It was confirmed that hydrogen retained in the specimens can result in obvious delay of helium desorption.

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Transmutation helium may causes serious embrittlement which is considered to be due to helium from clustering as a bubble in materials. Suppression of transmutation helium can be achieved by introducing trapping sites such as dislocations and impurities in materials. Here, effects of intentionally-induced dislocations and hydrogen on helium migrate and release behaviors were investigated using thermal desorption spectrometry (TDS) technique applied to well-annealed and cold-worked Fe9Cr alloys irradiated by energetic helium/hydrogen ions. Synchronous desorption of helium and hydrogen was observed, and the microstructure states during helium release at different temperatures were analyzed. High thermally stable HenD type complexes formed in cold-worked specimens, resulting in the retardation of helium migration and release. The existence of hydrogen will strongly affect the thermal helium desorption which could be reflected in the TDS spectrum. It was confirmed that hydrogen retained in the specimens can result in obvious delay of helium desorption.

Latent heat saturation in microstructural evolution by severe plastic deformation

During plastic deformation, most of the dissipated work is released as heat, but a fraction of it, usually small, is stored in the microstructure, is called latent heat and is associated with the network of dislocations that develops. The rate of energy storage in the microstructure divided by the rate of plastic dissipation is defined as the latent heat capacity. Latent heat remains stored in the microstructure of cold worked specimens after quenching. This energy is associated with modified mechanical properties, e.g. hardness, and is released upon annealing. Saturation of this stored energy has been observed in experiments after a specific amount of plastic deformation is reached. A thermodynamically consistent model for continuous dynamic recrystallization is proposed in this paper with the aim of explaining the phenomenon of latent heat saturation and relating it to grain refinement. The proposed model has three essential features: (i) the latent heat increases in the specimen during plastic deformation as plastic work is continuously dissipated; (ii) the rate of latent heat storage per unit work, i.e. the latent heat capacity, is related to the internal architecture of the microstructure and decreases to zero as a consequence of microstructural evolution; (iii) the relationship between the latent heat and the microstructure is described through the use of two parameters: (a) the dislocation density and (b) the average grain diameter. A comparison of the proposed model with experiments is reported and a validation for the prediction of microstructural evolution, as well as the evolution of the latent heat and latent heat capacity, is provided.

Influence of delta ferrite on corrosion susceptibility of AISI 304 austenitic stainless steel

AbstractIn the current study, the influence of delta (δ) ferrite on the corrosion susceptibility of AISI 304 austenitic stainless steel was evaluated in 1Molar concentration of sulphuric acid (H2SO4) and 1Molar concentration of sodium chloride (NaCl). The study was performed at ambient temperature using electrochemical technique—Tafel plots to evaluate the corrosive tendencies of the austenitic stainless steel sample. The as-received (stainless steel) specimen and 60% cold-worked (stainless steel) specimens were isothermally annealed at 1,100°C for 2 h and 1 h, respectively, and quenched in water. The results obtained show that the heat-treated specimen and the 60% cold-worked plus heat-treated specimen exhibited higher corrosion susceptibility than the as-received specimen, which invariably contained the highest fraction of δ ferrite particles. The finding shows that the presence of δ ferrite, in which chromium (Cr), the main corrosion inhibitor segregates, does not degrade and or reduces the resistance ...

Parametric Study on the Tensile Properties of Ni-Based Alloy for a VHTR

A very high-temperature reactor (VHTR) has been studied among generation IV nuclear power plants owing to its many advantages such as high-electric efficiency and massive hydrogen production. The material used for the heat exchanger should sustain structural integrity for its life even though the material is exposed to a harsh environment at 1223 K (950 °C) in an impure helium coolant. Therefore, an enhancement of the material performance at high temperature gives a margin in determining the operating temperature and life time. This work is an effort to find an optimum combination of alloying elements and processing parameters to improve the material performance. The tensile property and microstructure for nickel-based alloys fabricated in a laboratory were evaluated as a function of the heat treatment, cold working, and grain boundary strengthener using a tension test at 1223 K (950 °C), scanning electron microscopy, and transmission electron microscopy. Elongation to rupture was increased by additional heat treatment and cold working, followed by additional heat treatment in the temperature range from 1293 K to 1383 K (1020 °C to 1110 °C) implying that the intergranular carbide contributes to grain boundary strengthening. The temperature at which the grain boundary is improved by carbide decoration was higher for a cold-worked specimen, which was described by the difference in carbide stability and carbide formation kinetics between no cold-worked and cold-worked specimens. Zr and Hf played a scavenging effect of harmful elements causing an increase in ductility.

Grain growth and static recrystallization kinetics in Co–20Cr–15W–10Ni (L-605) cobalt-base superalloy

The grain size evolution of cold-rolled L-605 cobalt-base superalloy during ultra-rapid annealing is investigated in this paper. Cold-worked specimens undergo static recrystallization, leading to grain refinement or grain coarsening depending on the annealing time and temperature. The kinetics of grain growth is found to be independent of the initial deformation. The evolution of grain size can be simply described by a grain growth model for high temperatures and long annealing times, and the mobility of interfaces is estimated by modelling. Fast annealing treatment process is a very promising technique to customize grain size and enhance mechanical strength. In particular, the reduction of annealing time is an efficient method to produce a refined microstructure through static recrystallization.

Effects of Hydrogen Water Chemistry on Stress Corrosion Cracking Behavior of Cold-Worked 304L Stainless Steel in High-Temperature Water Environments

This study measured the stress corrosion cracking (SCC) growth rates of cold-worked 304L stainless steel in oxygenated and hydrogenated coolant environments. The effects of cold work at the levels of 5, 20, and 30% on SCC were also investigated. The SCC crack growth rate increased substantially with increases in the degree of cold work in oxygenated water environment. However, after hydrogen injection, SCC propagation in all cold worked specimens was gradually inhibited. The time to crack arrest decreased with increases in the degree of cold work. The slip bands caused by cold work facilitated the initiation and growth of transgranular (TG) cracks in specimens exposed to an oxygenated coolant environment. TG cracks were easier to arrest compared with intergranular cracks after the injection of hydrogen into the coolant system.

Manufacturing Technology for Titanium Tubes from Billets Prepared by Electron-Beam Remelting

Technology is developed for manufacturing hot-pressed and cold-worked seamless tubes from titanium alloy billets prepared by electron-beam remelting, which makes it possible to reduce metal consumption and energy expended by lowering the number of production operations (excluding forging, machining, piercing, or expanding with preliminary drilling and heating before the above-mentioned operations). Billet mechanical properties and structure studied for hot-pressed and cold-worked pipe specimens show that the production parameters developed make it possible to obtain hot-pressed and cold-worked seamless tubes corresponding to specifications of domestic and overseas standard documents.