Cold Work Level Research Articles

Swelling of type-316 stainless steel at high fluences in EBR-II

A test to measure swelling induced by fast neutron irradiation in unstressed specimens of type-316 stainless steel has completed irradiation in the EBR-II reactor. Results are reported and discussed which describe the swelling as a function of neutron fluence, temperature of irradiation and extent of cold work in the alloy. Density determinations showed swellings of up to 15% ΔV V f for 20% cold worked type-316 stainless steel at a neutron fluence level of 1.4 × 10 23 n/ cm 2, E > 0.1 MeV (70 dpa). The peak swelling temperature range was 550°C–600°C regardless of the extent of cold working. Increasing the cold work level reduced the swelling and tended to broaden the swelling temperature peak. Transmission electron microscopy (TEM) investigations showed that cold working had reduced the average void sizes compared to those observed in the solution annealed material.

Mechanical response of LMFBR and P1 cladding tubes under transient heating

Type 316 stainless steel tubing specimens comparable to LMFBR cladding were burst tested with relatively constant internal pressure in the 219–836 psi range and with increasing temperature. Continuous measurements of diameter change, temperature, and pressure were recorded as the samples were heated to temperatures near the melting point at rates from 10–1800°F/sec. The effects of varying initial wall thickness, cold work level, length, and thermal experience were explored. Ductile failures were observed at 10°F/sec, and stable strains at time of failure were greater than those reported by HEDL. At 200°F/sec the initially 20% cold-worked, 15-mil wall tubing produced brittle failures; while initially 40% cold worked, 10-mil wall samples displayed a mixture of ductile and brittle features. At ∼1000°F/sec the behavior of the latter material was prodominately brittle, although stable strains as large as 6% were observed. Failure temperatures were generally above 2000°F. When substantial ductility was displayed, an exponentially increasing stable strain was recorded as temperature and time progressed: from such curves temperatures corresponding to 1% strain were derived. Factors controlling the mechanical response appear to be separable by analysis based on the recorded data and the variety of materials and conditions of the tests.

Influence of Cold Work on the In-Pile Stress Rupture Strength of 16Chromium-16Nickel-Niobium Austenitic Stainless Steels

Abstract The in-pile stress rupture strength of three casts of 16Cr-16Ni-Nb stainless steel was investigated. Different pretreatments were used, namely solution annealing plus various cold-working levels up to 20%. Thermal stability under irradiation and the high-temperature embrittlement of the different material conditions were studied. The specimens were irradiated in the form of electrically heated pressurized tubes at irradiation temperatures ranging from 615 to 720°C. Irradiation time was up to 4800 h, corresponding to a maximum fast neutron fluence (E > 0.1 MeV) of 8.2 × 1025 n/m2. Neutron irradiation caused a marked reduction in stress rupture strength for the solution-annealed and the cold-worked specimens. For the solution-annealed tubes, this reduction was mainly attributed to high-temperature embrittlement; the decrease in rupture strain was less pronounced for the cold-worked tubes. The reduction in stress rupture strength came mainly from an increase in creep rate. As far as stress rupture strength and creep strength at high irradiation temperatures were concerned, the 15% cold-worked 16Cr-16Ni-Nb steel was superior to the other cold-work levels.

Void swelling in cold worked FV548 steel irradiated in a high voltage microscope after pre-injection with 10 ppm helium

Abstract A study has been made of void formation in 20% cold worked FV548 austenitic stainless steel irradiated with 1 meV electrons in the Harwell AEI EM7 high voltage microscope (HVM) after pre-injection with 10 ppm helium. In the temperature range studied, the void swelling is similar in all respects to the void swelling in 1150°C solution treated FV548 steel. This is contrary to the behaviour observed during fast neutron and heavy ion irradiation, where 20% cold work markedly reduces void swelling. Limited experiments on 60% cold worked FV548 steel show that voids form readily even at this cold work level, but the void swelling is reduced by ≈50%. Subsidiary experiments have eliminated the very high dose rate and the helium content as reasons for the lack of sensitivity to cold work: it seems likely that it is related to the production of point defects in Frenkel pairs during election irradiation, as opposed to displacement cascades.

The Effect of Varying Degrees of Cold Work on the Stress-Rupture Properties of Type 304 Stainless Steel

Cold work enhances the room temperature tensile properties of austenitic stainless steels. There are few data on the effect of varying degrees of cold work on elevated temperature properties. Such information is needed to set design limits on the use of cold-work enhanced properties in the creep range. ASTM A-193 B8 bars were given three levels of cold work. Tensile properties, creep-rupture properties, and the resulting microstructures were determined and compared with those of annealed bar. There is a temperature limit, dependent on the degree of cold work, beyond which the enhancement of creep-rupture properties is not observed.

Effect of cold work and heat-treatment on the mechanical properties of a wire drawn metastable stainless steel

Tensile strengths in excess of 600×103 psi were obtained in a Type 18-8 stainless steel using relatively simple thermomechanical treatments involving cold working after solution annealing and intermittent heat-treatments. The strength developed strongly depended on the hardness and amount of martensite formed as well as the maintenance of high work-hardening rates at large deformation strains. For a given level of cold work, martensite is shown to transform more rapidly using material annealed in the carbide precipitation temperature range as this tends to increase theMd temperature. The volume fraction martensite formed by wire drawing is shown to be related to the square of the deformation strain. Also shown are the effects of annealing temperature and cold work on the elastic modulus.

A Consideration on Limits of Cold Working in Nuclear Construction

Cold working of metals affects the mechanical properties of those metals. These effects are particularly important in the design, analysis and subsequent performance of materials used in elevated temperature nuclear construction. This paper provides an assessment of the effects of cold working on the time independent and time dependent mechanical properties of the materials most commonly encountered in liquid metal fast breeder reactors. The paper suggests specific reductions, as a function of time, temperature and cold work level, for those properties adversely affected. The final objective of this paper is to provide an assessment of the manner in which the Code now addresses fabrication effects and how it might make those assessments in the future.

High-strength Cu-Ni-Sn alloys by thermomechanical processing

The influence of prior cold work on the aging characteristics and mechanical properties response for copper-rich alloys in the Cu-Ni-Sn system has been investigated. It has been established15,16 that there exists a spinodal mode of decomposition below a critical temperatureTR, 200 to 300°C below the equilibrium phase boundary in this system. Significant age hardening response is observed in this region; however, fracture ductility is severely impaired due to a grain boundary precipitate network which develops after relatively short aging times. Cold work prior to low temperature aging is found to have relatively little influence on the incubation time for this embrittling network. It does, however, profoundly enhance the kinetics of the continuous (spinodal) transformation. It is observed that for broad variations in composition, critical combinations of prior cold work, aging time and temperature yield material with unique combinations of. yield stress and fracture ductility (for example, a Cu-9 wt pct Ni-6 wt pct Sn alloy may be processed to exhibit an 0.01 pct offset yield of 174,000 psi in conjunction with a 55 pct R.A. on fracture; significantly higher 0.01 pct offset yield values may be achieved at some reduction in fracture ductility for other NiJSn ratios). It is concluded that the resultant ductileJbrittle properties response is a consequence of a critical compctitive balance between amplitude development in the modulated structure and nucleation of the grain boundary network. The minimum level of prior cold work required to effect this balance in the Cu-9 wt pct Ni-6 wt pct Sn alloy is 75 pct R.A.

Fracture of Zircaloy cladding under simulated power ramp conditions

Behaviour of Zircaloy cladding under stresses imposed by fuel dimensional changes is discussed and results presented which suggest that the correct choice of cladding properties, in particular cold-work level and texture, is important if the probability of low ductility failures in fuel pins is to be reduced. Evidence is presented, based on laboratory tests and post-irradiation examination of cladding, which leads to the conclusion that failure of Zircaloy can take place by stress-corrosion cracking in the presence of fission product iodine and it is further demonstrated that the process is strongly stress-dependent. Consideration is given to the influence of irradiation exposure, strain rate and adverse stress systems, for instance at inter-pellet ridges or fuel cracks, on the cracking susceptibility of Zircaloy cladding. The significance of the results is discussed in relation to a proposed stress-corrosion cracking mechanism.

RELEASE OF STORED ENERGY IN OXIDE-DISPERSION-STRENGTHENED COPPER

The pattern of energy release on annealing was followed by calorimetric means for a series of oxide-dispersion-strengthened copper alloys extruded at high reduction ratio. Included among the test materials were two Cu-Al2O3 alloys prepared by salt decpmposition, several Cu-SiO2 and Cu-Al2O3 alloys prepared by internal oxidation, and a CU-CU2O alloy prepared by surface oxidation. For several of the alloys energy releases were observed that confirm the expected high levels of energy stored during high strain rate extrusion. Values of 0·41 and 0·42 cal/g were measured and correspond to levels of cold work found in copper severely deformed in torsion. The energy release was observed at temperatures high above that normal for the recrystallization of cold-worked copper.

Radiation Effects on the Mechanical Properties and Microstructure of Type-304 Stainless Steel

The mechanical properties and microstructures of Type-304 stainless steel were studied as a function of cold work, neutron irradiation, and testing temperature. True-stress, true-strain tensile tests were made on nonirradiated specimens at 70°F (21°C), 600°F (315°C), and 1300°F (700°C), and on irradiated specimens at 70°F and 600°F. Specimens were irradiated to 1.25 x 1020 n/cm2 (>1 MeV) at 110°F (43°C). Neutron irradiation increased the yield strength and ultimate tensile strength of annealed and cold-worked specimens at 70° F and at 600° F.The incremental increase in these properties decreased with increasing cold work. The elongation of nonirradiated and irradiated specimens tested at 70° F was found to increase with initial levels of cold work and then to decrease. This effect was not observed at 600° F. The most severe decreases in mechanical stability were observed in heavily deformed (greater than 20% reduction in thickness) and irradiated specimens tested at 600° F. These specimens failed in a ductile manner with total elongations as low as 1/2%. The increases in the strength and decreases in plastic stability produced by irradiation were combined by measuring the energy absorbed to plastic instability (area under the true-stress, true-strain curve up to the point of maximum load). This energy value was found to be an effective method for comparing the effects of the various variables.Cold work was found to produce large amounts of austenite-to-martensite transformation. Neutron irradiation was found to produce no measurable increase in martensite content. Transmission electron microscopy of irradiated specimens confirmed the presence of martensite and epsilon phase in Type-304 stainless steel. Irradiated specimens contained high concentrations of black dots which were not observed in nonirradiated specimens. In some instances these black dots could be resolved into loops. These black dots are presumed to be clusters of vacancies or interstitials produced by neutron radiation.