Titanium-modified Austenitic Stainless Steel Research Articles

Cavity Swelling of 15-15Ti Steel at High Doses by Ion Irradiation.

The titanium-stabilized austenitic stainless steel Fe-15Cr-15Ni, which shows enhanced resistance to irradiation swelling compared with more traditional 316Ti, has been selected as a core material for fast reactors. Data on the evolution of irradiation swelling in 15-15Ti steels at very high doses, which cannot be easily achieved by neutron irradiation, are still lacking. In this paper, the swelling behavior of the titanium-modified austenitic stainless steel 15-15Ti was investigated by pre-implantation of He at room temperature followed by Ni-ion irradiation at 580 °C to peak doses of 120, 240 and 400 dpa. Relatively small cavities were observed in the zone of helium implantation, while large cavities appeared in the region near the damage peak. A correction formula for the dpa curve was proposed and applied to samples with large swelling. It was found that the steady-state swelling rate of 15-15Ti remains at ~1%/dpa even at high doses. By comparing the swelling data of the helium-implanted and helium-free regions at same doses, 70 dpa and 122 dpa, the suppression of swelling by excessive helium can be deduced at such doses.

Effect of Phosphorous Content on the Microstructure and Stress Rupture Properties of 15Cr–15Ni Titanium-Modified Austenitic Stainless Steel

The microstructure of solution-annealed and aged tensile properties and the stress rupture properties of 15Cr–15Ni titanium-modified austenitic stainless steel with different phosphorus contents were investigated using OM, SEM and TEM. The results showed that two phosphide morphologies were observed after long-term isothermal aging at 850 °C for 1000 h. One was the needle-like M2P distributed within the grain. The other was the blocky M3P distributed at the grain boundaries and twins. The tensile properties of the alloy were unaffected by the phosphorus content, but the stress rupture properties were significantly impacted. With the increase in the phosphorus content from 70 ppm to 250 ppm, the stress rupture life increased from 148 to 269.7 h. Since the strengthening effect of phosphides within the grain or at the grain boundary has been shown to improve the stress rupture properties of alloys, many nanosized granular precipitates, such as the sigma phase, carbides and phosphides, have been observed at the grain boundary, capable of alleviating the stress concentration and limit the crack propagation between two phases, improving the strength of the grain boundary. Intragranular needle-like phosphides can hinder dislocation movements effectively, which improves the intragranular strength of alloys.

Theoretical study on the correlation of swelling peaks between neutron and heavy ion irradiated 15-15Ti stainless steel

Titanium-modified austenitic stainless steels (15-15TiSS) are currently used as the fuel cladding material of fast reactors, which are subject to higher radiation damage during their application. In this work, the radiation-induced swelling of 15-15TiSS under a fast reactor neutron and heavy ion irradiation conditions was studied by the rate theory (RT). The simulated swelling properties of 15-15TiSS under fast neutron conditions were calculated initially. The swelling peak, swelling rates, and swelling-related microstructural properties are consistent with the neutron irradiation results, indicating that the selected RT model and material parameters are reasonable. Then, the swelling properties of 15-15TiSS under various damage rates were predicted by changing the radiation damage rates from 1 × 10−6 to 1 × 10−3 dpa/s. It shows that swelling peaks are strongly dependent on temperature and the swelling peaks shift ∼50 °C toward the higher temperature with each order of magnitude increase of defects generation rate. The swelling rates and swelling-related defect evolution at 1 × 10−3 dpa/s (with a swelling peak temperature of 590 °C) are consistent with that under neutron irradiation with 1 × 10−6 dpa/s (with a swelling peak temperature of 460 °C). At length, the RT-predicted heavy ions irradiation results were verified by the previous positron annihilation lifetime spectroscopy results of ions irradiated 15-15TiSS. It indicates that heavy ion irradiation can be used to study the radiation effect of materials under neutron irradiation and should be a feasible technique used in the further screening of radiation-resistant materials.

Effect of Phosphorous Content on Microstructure and the Stress Rupture Properties of 15cr–15ni Titanium Modified Austenitic Stainless Steel

Effect of Phosphorous Content on Microstructure and the Stress Rupture Properties of 15cr–15ni Titanium Modified Austenitic Stainless Steel

σ-Phase Precipitation Mechanism of 15Cr–15Ni Titanium-Modified Austenitic Stainless Steel During Long-Term Thermal Exposure

The σ-phase precipitation in 20% cold-worked 15Cr–15Ni titanium-modified austenitic stainless steel was studied during long-term aging treatment. During the isothermal aging stage, the amount of σ-phase was significantly increased at beginning and gradually became constant. The aging time only slightly affected the size and morphology of the σ-phase. Conversely, during the isochronal aging stage, the amount of σ-phase grew rapidly with the increase in the aging temperature. The σ-phase with a large amount of stacking faults was prone to nucleate around the (Ti, Mo)C particles or at the grain boundaries. The (Ti, Mo)C particles can act as effective nucleation sites, where the σ-phase directly precipitates from the austenitic matrix. From this work, the growth of σ-phase is found to be influenced by the aging temperature, and a new mechanism of σ-phase precipitation from austenite has been proposed.

Room Temperature Torsional Behavior of 15-Cr-15Ni Titanium Modified Austenitic Stainless Steel

This paper reports room temperature torsional behavior of solution annealed 15Cr-15Ni Titanium modified austenitic stainless steel (Alloy D9) up to 200% strain in the strain rate range 0.001-1/s in steps of a decade. Specimens of 10mm gauge diameter and 25mm gauge length were used in the tests. Free-end torsion tests maintaining zero axial force on the specimen were carried out till specimen rupture. Acquired torque twist data has been converted to shear stress – shear strain data using Nadai's formulation for torsion of solid bars. At lower strain rates up to 0.1/s the material exhibited periodic hardening and softening till failure. The no. of periods in hardening/softening cycles decreased with increase in strain rates. Serrated flow was observed after a certain strain level.

Dynamic Recrystallisation during Isothermal Hot Deformation in a Titanium Modified Austenitic Stainless Steel

The paper discusses the microstructural evolution during dynamic recrystallisation (DRX) of a titanium-modified austenitic stainless steel (alloy D9). Isothermal hot compression tests were conducted in a Gleeble thermo-mechanical simulator in the temperature range 1173-1373K to various strains at a constant strain rate of 0.1 and 1 s-1. The extent of DRX increased with increase in strain and temperature. Nucleation of new DRX grains was found to occur by bulging of parent grain boundary. A continuous sub-grain rotation around the original grain boundaries, which would lead to the formation of DRX nucleus in sub-grain structures, could not be confirmed from the present study. Fractions of Σ3 boundaries increased almost linearly with increase in area fraction of DRX. The generation of this Σ3 boundary was accounted for in the formation of annealing twins during DRX. The possible role of annealing twins on DRX in alloy D9 is also discussed.

Influence of State of Stress on Dynamic Recrystallization in a Titanium-Modified Austenitic Stainless Steel

The influence of the state of stress on the microstructure and dynamic recrystallization (DRX) in a titanium-modified austenitic stainless steel is assessed by performing plane-strain and uniaxial hot compression studies. Although the state of stress does not alter the mechanisms of DRX nucleation, the kinetics of DRX is hindered during plane-strain deformation vis-a-vis uniaxial deformation.

Application of Resonant Nuclear Reactions for Studying the Diffusion of Nitrogen and Silicon in Ti-Modified Stainless Steel

Resonant nuclear reaction analysis can be used for the study of diffusion behaviour of minor alloying elements in reactor structural materials. In the present paper,15N(p,αγ)12Cand30Si(p,γ)31Preactions have been used to profile the ion-implanted15Nand30Siin D9 alloy (titanium-modified austenitic stainless steel). The diffusion coefficients at various temperatures have been estimated from the broadening of the implantation profiles during thermal annealing, and the activation energies are deduced.

High temperature wear and friction behaviour of 15Cr–15Ni–2Mo titanium-modified austenitic stainless steel in liquid sodium

▶ Friction and wear studies of 20% cold-worked 15Cr-15Ni- 2Mo Titanium-modified austenitic stainless steel (alloy D9) in liquid sodium at high temperature. ▶ Low value of friction coefficients for this alloy is due to lubrication mechanism independent of oxygen content. ▶ Values well within the design criteria (as per fuel duct assembly for the Fast Flux Test Facility) for pads of wrapper material for FBRs. The sliding friction and wear behaviour of 15Cr–15Ni–2Mo titanium-modified austenitic stainless steel (alloy D9) was studied in high purity liquid sodium at high temperature viz. 473 and 823 K respectively using a reciprocating-type tribometer. Weight loss measured in specimens after the tests was negligible. Scar depth measurements on disc specimens were carried out by confocal laser scanning microscope, which revealed not much number of scratches tested at 473 and 823 K, respectively. Scanning electron microscopy on worn pin specimens revealed the presence of deformed zone in the rubbed area, and adherence of wear particles at few locations in both the cases. Damage is more in specimens tested at 823 K because of the softening of the rubbing surfaces at that temperature. Further, friction coefficient is lower at 473 K than at 823 K.

A thermo-viscoplastic constitutive model to predict elevated-temperature flow behaviour in a titanium-modified austenitic stainless steel

The experimental stress–strain data from isothermal hot compression tests over a wide range of temperatures (1073–1473K), strains (0.1–0.5) and strain rates (0.001–1s−1) were employed to formulate a suitable constitutive model to predict the elevated-temperature deformation behaviour in a Ti-modified austenitic stainless steel (alloy D9). It was observed that the Johnson–Cook (JC) model in its original form is inadequate to provide good description of flow behaviour of alloy D9 in the above hot working domain. This has been attributed to the inadequacy of the JC model to incorporate the coupled effects of strain and temperature and of strain rate and temperature. A modified constitutive model based on the Zerilli–Armstrong model has been proposed for considering the effects of thermal softening, strain rate hardening and isotropic hardening as well as the coupled effects of temperature and strain and of strain rate and temperature on flow stress. The proposed modified constitutive model could predict the elevated-temperature flow behaviour of alloy D9 over the specified hot working domain of alloy D9 with good correlation and generalization.

Grain Boundary Microstructural Control through Thermomechanical Processing in a Titanium-Modified Austenitic Stainless Steel

The present study discusses the grain boundary microstructural control in a 15Cr-15Ni-2.2Mo-Ti modified austenitic stainless steel (commonly known as alloy D9) through a one-step thermomechanical treatment. The experimental methodology adopted in this investigation was based on the strain annealing approach in which a small amount of strain (5 to 15 pct) was imparted on the solution-annealed (SA) sample. The cold-deformed samples were subsequently annealed at various temperatures (1173 to 1273 K) for different time periods (0.5 to 2 hours). It was observed that annealing after 5 pct deformation induces anomalous grain growth with a moderate increase in number fraction of coincidence site lattice (CSL) boundaries. However, a prestrain of 10 to 15 pct followed by annealing at 1273 K for 0.5 to 2 hours was found to be a suitable thermomechanical processing schedule to increase the number fraction of CSL boundaries (particularly Σ3 and its variants) significantly. Further, the well-connected network of random grain boundaries present in the SA specimen was substantially disrupted in these processing conditions due to the incorporation of Σ3 and its variants. The preceding results were discussed with reference to strain-induced grain growth vis-a-vis strain-induced boundary migration (SIBM) following deformation and annealing.

Evolution and characterization of dynamically recrystallized microstructure in a titanium-modified austenitic stainless steel using ultrasonic and EBSD techniques

A C-scan ultrasonic imaging system was used to investigate the microstructural evolution during dynamic recrystallization (DRX) of a 15Cr–15Ni–2.2Mo–Ti modified austenitic stainless steel (alloy D9). Four specimens were forged at 1273 K to different strains in the range 0.1–0.5. Specimens with true strains of 0.2 or lower did not show any variation in the amplitude of the first back-wall echo. However, a visible variation in the C-scan image was observed at and above the 0.3 strain level. This variation was attributed to the evolution of fine grains. The formation of fine grains was related to DRX, as indicated by electron backscattered diffraction. This study also revealed the characteristics of the DRX or ‘necklace grains’, as opposed to the so-called parent grains or rest of the microstructure.

Evaluation of self-welding susceptibility of an austenitic stainless steel (alloy D9) in sodium

Self-welding susceptibility of a 15Cr–15Ni–2Mo titanium-modified austenitic stainless steel (alloy D9), in both annealed and 20% cold-worked conditions, have been evaluated in flowing sodium at 823 K for 2160 and 4320 h under contact stress of 9.4 MPa. Tests were performed on flat-on-flat geometry of hollow cylindrical specimens under compression. One pair of 20% cold-worked alloy D9 vs. 20% cold-worked alloy D9 specimens tested for 4320 h was self-welded for which the breakaway shear force was measured. Scanning electron micrographs of the self-welded region showed that portions of the original interface no longer existed. The paper discusses the experimental set-up installed in the sodium loop test facility and the results of self-welding susceptibility studies on this material.

Microstructural response of titanium-modified austenitic stainless steels to neutron exposure of 70 dpa in FFTF/MOTA

JPCA, a titanium-modified austenitic stainless steel, in solution-annealed or cold-worked condition and a compositionally modified JPCA in solution-annealed condition were examined by transmission electron microscopy following irradiation in FFTF/MOTA to an exposure level of up to about 70 dpa at 390 to 600°C. At lower temperatures, all the materials developed qualitatively similar cavity-, dislocation- and precipitate-microstructures. The lower-temperature swelling peak, which appeared at near 410°C, was more efficiently suppressed by phosphorus addition than cold-working. Irradiation at or above 520°C produced substantially large swelling in solution-annealed JPCA. The cavities contributed to this higher-temperature swelling developed in association with M 6C-type precipitates. Neither cavities other than very small helium bubbles nor massive particles of M 6C-type precipitates were observed in cold-worked and phosphorus-modified materials, in which MC-type precipitates developed at very high concentration. The effect of pre-irradiation microstructure and compositional modification on the behavior of these precipitates is discussed.

Comparison of fracture behavior for low-swelling ferritic and austenitic alloys irradiated in the Fast Flux Test Facility (FFTF) to 180 DPA

Fracture toughness testing was conducted to investigate the radiation embrittlement of high-nickel superalloys, modified austenitic steels and ferritic steels. These materials have been experimentally proven to possess excellent resistance to void swelling after high neutron exposures. In addition to swelling resistance, postirradiation fracture resistance is another important criterion for reactor material selection. By means of fracture mechanics techniques the fracture behavior of those highly irradiated alloys was characterized in terms of irradiation and test conditions. Precipitation-strengthened alloys failed by channel fracture with very low postirradiation ductility. The fracture toughness of titanium-modified austenitic stainless steel D9 deteriorates with increasing fluence to about 100 displacements per atom (dpa), the fluence level at which brittle fracture appears to occur. Ferritic steels such as HT9 are promising candidate materials for fast and fusion reactor applications. The upper-shelf fracture toughness of alloy HT9 remained adequate after irradiation to 180 dpa although its ductile-brittle transition temperature (DBTT) shift by low temperature irradiation rendered the material susceptible to brittle fracture at room temperature. Understanding the fracture characteristics under various irradiation and test conditions helps reduce the potential for brittle fracture by permitting appropriate measures to be taken.

Microstructural evolution of titanium modified austenitic stainless steel after FFTF irradiation

Abstract The titanium-modified austenitic stainless steel (JPCA) and the standard type 316 steel were neutron irradiated in the FFTF/MOTA up to about 40 dpa at 370°C to 600°C. Microstructural examination proved the superior resistance to void swelling of the titanium-modified steel. Comparison with the results of HFIR irradiation experiments from the JAERI/ORNL collaboration showed the temperature dependence of void swelling to be significantly different for JPCA irradiated in FFTF. The peak swelling temperature in FFTF irradiation was below 420°C, which was about 100°C lower than that found during HFIR irradiation. Such a difference in swelling behavior would appear to be due to the effects of the significant difference in helium production rates on cavity nucleation and bubble to void conversion process.

Irradiation and thermal creep of a titanium-modified austenitic stainless steel and its dependence on cold work level

Abstract A titanium-modified austenitic stainless steel similar to the fusion PCA alloy was creep tested at three cold work levels under both thermal aging and neutron irradiation conditions. The creep behavior exhibits a complex nonmonotonic relationship with cold work level that reflects the competition between a number of stress-sensitive and temperature-dependent microstructural processes. Increasing the degree of cold work to 30% from the conventional 20% level was detrimental to its performance, especially for applications above 550°C. The 20% cold work level is preferable to the 10% level, in terms of its initial strength, in-reactor creep and swelling response.

Radiation-induced sensitization of a titanium-modified austenitic stainless steel irradiated in a spectral-tailored experiment at 60–400°C

The Radiation-induced sensitization of a titanium-modified austenitic stainless stell (PCA) irradiated at 60 to 400°C to 7 dpa in a spectral-tailored experiment was investigated by the electrochemical potentiokinetic reactivation (EPR) test technique using miniaturized specimens. The reactivation charge of PCA was increased by irradiation at 200 to 400°C, as compared to unirradiated control specimens. The occurrence of grain boundary sensitization was detected for the specimen irradiated at 400°C, but not detected for the specimens irradiated at the lower temperatures. Localized attack across grain faces was observed after EPR testing for the specimens irradiated at 200 to 400°C. The attack may be attributed to the occurrence of localized chromium depletion by RIS to matrix defect sinks. The reactivation behavior of PCA was not affected by the irradiation at 60°C.

Flow transients during strain rate jump tests in a titanium-modified austenitic stainless steel

Strain rate jump tensile tests were conducted on a titanium-modified austenitic stainless steel at room temperature with strain rate ratios of 0.5, 2, 4 and 10. Tests were also conducted at 50 K intervals in the temperature range 573–1023 K, with a strain rate ratio of 10. Flow transients, sensitive to strain as well as the magnitude and direction of strain rate jump, were observed during tensile tests on samples of a titanium-modified austenitic stainless steel. These transients were related to the difference in the waiting times associated with the rate-controlling dislocation-solute interactions. An instantaneous and a steady state strain sensitivity indices were measured and their variations with strain and temperature have been related to the occurrence of dynamic strain aging.