Cold-worked Austenitic Steels Research Articles

Tensile properties of modified 316 stainless steel (PNC316) after neutron irradiation over 100 dpa

ABSTRACT The effects of fast neutron irradiation on tensile properties of 20% cold-worked modified 316 austenitic stainless steel (PNC316), which was improved swelling resistance and creep strength, claddings and wrappers for fast reactors were investigated. PNC316 claddings and wrappers were irradiated in the experimental fast reactor Joyo at irradiation temperatures between 400°C and 735°C to fast neutron doses ranging from 21 to 125 dpa. The post-irradiation tensile tests were carried out at room and irradiation temperatures. Elongations of PNC316 measured by the tensile tests were maintained at an engineering level, although the material incurred significant irradiation hardening and softening depending on irradiation temperature. The maximum swelling of PNC316 wrappers was about 2.5 vol.% at irradiation temperature between 400°C and 500°C up to 110 dpa. Japanese 20% cold-worked austenitic steels, PNC316 and 15Cr-20Ni, had sufficient ductility and work-hardenability even after above 10 vol.% swelling, while they had very weak plastic instabilities.

Kinematical barriers enhanced dislocation strengthening mechanisms in cold-worked austenitic steels

We experimentally investigated microstructure evolution and strengthening behaviors of three cold-worked austenitic steels that deform via dislocations, twinning and stacking faults. The development of stacking faults and twins acting as kinematical barriers for dislocation motion was found to be influenced by stacking fault energy and accumulated dislocations simultaneously, which in turn accelerate dislocation storage rate, strengthen the alloys, and develop high strain hardening rate. We thus proposed a nonadditive strengthening equation to rationalize experimental observation by combining the dislocation-based two-internal-variable model coupled with kinematical barriers.

Deformation path effects on the internal stress development in cold worked austenitic steel deformed in tension

The effects of cold work level and strain paths on the flow stress of austenitic stainless steels, including Bauschinger effect and associated internal stresses were investigated with both mechanical testing and neutron diffraction techniques. The main objective was to assess the effects of cold rolling: to 5%, 10%, 20% and 40% reduction and uniaxial straining on the evolution of the internal strains during the re-straining to 5% tensile strain in-situ, which is relevant for stress corrosion cracking (SCC) studies. The results of mechanical testing showed that the yield strength of material increased when it was reloaded in the forward direction and decreased well below the flow stress when the loading direction was reversed, showing a strong Bauschinger effect. The magnitude of Bauschinger effect is independent on whether tensile or compressive prestraining comes first but rather on the amount of prestrain. The assessment of the effect of prestraining methods showed that the magnitude of yield asymmetry was higher in the material prestrained by uniaxial deformation than those prestrained by cold rolling. Neutron diffraction test results showed that the elastic lattice strain difference between the maximum and minimum strain values increased consistently with the applied stress during the re-straining to 5% tensile strain in-situ along the 3 orthogonal directions of the rolled plate. It also emerged that, following the in-situ loading of cold rolled materials to 5% tensile strain, the largest strain difference occurred in the material prestrained to 20% reduction. In cold rolled samples, the peak width increased with cold work levels and during re-straining to 5% along rolling, transverse to rolling and normal directions which simulated reversed condition. In contrast to the cold rolled samples, there was neither increase nor decrease in the peak width of samples prestrained by uniaxial deformation on re-straining in reverse direction. This was rationalised in terms of the development of intragranular and intergranular stresses in the two cases and implications to SCC susceptibility were discussed.

Manufacture of springs from cold-worked austenitic steel

Manufacture of springs from cold-worked austenitic steel

Temperature effect on characteristics of void population formed in the austenitic steel under neutron irradiation up to high damage dose

Radiation-induced porosity in fuel pin cladding of the BN-600 reactor fabricated of cold-worked austenitic steel 16Cr–15Ni–2Mo–2Mn irradiated to different damage dose 20–90 dpa at 410–600 °C has been examined by transmission electron microscopy. Formation and growth of various types of voids were shown to occur according to their both duration and mechanism of nucleation. Dependencies of average diameters and concentration of all void types on neutron irradiation damage dose were plotted for various temperature ranges. The change of void population with increasing dose at various temperature ranges was analyzed based on point defect kinetic. The contribution of different types of voids to swelling was examined.

Swellling of advanced austenitic stainless steels developed for the environment of heavy neutron exposure

Modified Type-316 and higher-nickel advanced austenitic steels were irradiated in the FFTF and JOYO fast reactors to a neutron dose as high as 210 dpa. At temperatures greater than 500°C, P-, Si-, and Ti-bearing cold-worked austenitic steels showed significant swelling suppression even after 150 dpa. The dominant mechanism is associated with formation of stable phosphide precipitated by addition of Ti and/or increased Si. In the lower-temperature regime, where the phosphide precipitate is not present, silicon acts to suppress swelling at contents of about 0.8 wt%. Only at this higher silicon level, increased amounts of nickel reduced the swelling. At 0.5 wt% silicon, the swelling was insensitive to the alloying elements, such as Ni, Ti and P. The effect of Si and Ni could be explained by their influence on void nucleation through effects on effective vacancy diffusion coefficient. Further improvement in swelling was expected by increasing the nickel and possibly the phosphorus contents of the advanced austenitics.

Effect of titanium content on creep strength of cold-worked 15Cr-15Ni-2.5Mo austenitic steel

The core materials, such as fuel cladding tube, in the fast breeder reactor (FBR), are required to have superior strength at elevated temperatures of around 923 K and excellent resistance to void swelling introduced by intense fast neutron irradiation to attain economic and reliable FBR performance. Cold-worked type 316 stainless steel has been used in some existing reactors. Many efforts have been made to improve the performance of candidate core materials [1, 2]. Cold working has been introduced for inhibiting the void swelling of austenitic stainless steels [3], although it accelerates structural change at elevated temperatures [4, 5]. The addition of titanium to type 316 stainless steel leads to improved swelling resistance [6]. However, data on the effect of titanium addition on the creep behaviour of cold-worked austenitic steels are lacking. Thig letter describes the effect of titanium content on the creep strength of cold-worked austenitic stainless steel similar to type 316. The base composition used was 0.07C15Cr-15Ni-2.5Mo-0.006B austenitic steel and the titanium content was varied at seven levels ranging from 0 to 1 wt %. The results of chemical analyses of the test steels are shown in Table I. They were melted in a vacuum-induction furnace and cast into 10 kg ingots. The ingots were hot-forged and hotrolled to plate of 8 mm thickness and 30 mm width. After solution-treating at 1373 K and cold rolling to produce a 60% thickness reduction, the plates were solution-treated at 1373 K and water-quenched. The grain size of the plates was approximately ASTM No.7 8. Cold working of 20% was introduced in the solution-treated plates. The plates were machined to flat specimens of 2 mm thickness and with a gauge section 6 mm wide and 25 mm long. Using the flat specimens creep-rupture tests were performed at 973 K and the microstructures were examined by transmission electron microscopy. Stress versus time to rupture curves at 973 K of the 20% cold-worked and the solution-treated specimens are shown in Figs 1 and 2, respectively. The rupture strength of both specimens increased with titanium content up to about 0.3 wt%, but decreased with increasing titanium content in the specimens containing more than 0.5 wt%Ti. The slope of stress versus time to rupture curve of the 20% cold-worked specimens containing about 0.2wt%Ti was rather gentler than that of the titanium-free specimen while, in the solution-treated specimens, the slope of the specimen with about

The strong influence of temper annealing conditions on the neutron-induced swelling of cold-worked austenitic steels

Abstract Although cold-working is known to strongly influence neutron-induced swelling of austenitic stainless steels, it also appears that the annealing temperature employed before cold-working has an equally large effect. Two irradiation studies conducted on AISI 316 stainless steel indicate that increasing the temperature of the intermediate temper anneal leads in general to a shortening of the transient regime of swelling. The evidence is consistent with the influence of both annealing temperature and irradiation temperature on carbide formation and its subsequent influence on the microchemical evolution and swelling of the steel.

Slow crack growth in 18Mn–4Cr cold-worked austenitic steel under static load in gaseous hydrogen

Tests have been performed on a cold-worked 18Mn–4Cr austenitic steel to measure rates of slow crack growth under constant load in gaseous hydrogen. The behaviour of the steel differs from that of quenched-and-tempered low-alloy steels in a number of ways: (i) slow crack growth occurs in vacuum over a wide range of stress intensity K, and hydrogen appears to accelerate this crack growth by a factor which does not depend strongly on K; (ii) the dependence of crack growth rate on K does not follow a three-stage curve, as is commonly observed in a wide range of steels; (iii) there is a strong temperature dependence of the threshold stress intensity Kth below ~30°C; (iv) a relatively large environmental addition of oxygen is only partially effective in inhibiting the effect of the hydrogen. Increased crack growth rates and scatter in the data are thought to be associated with bands of segregation and relatively high concentrations of phosphorus on some grain boundaries.MST/132

Slow crack growth in 18Mn–4Cr cold-worked austenitic steel under static load in gaseous hydrogen

AbstractTests have been performed on a cold-worked 18Mn–4Cr austenitic steel to measure rates of slow crack growth under constant load in gaseous hydrogen. The behaviour of the steel differs from that of quenched-and-tempered low-alloy steels in a number of ways: (i) slow crack growth occurs in vacuum over a wide range of stress intensity K, and hydrogen appears to accelerate this crack growth by a factor which does not depend strongly on K; (ii) the dependence of crack growth rate on K does not follow a three-stage curve, as is commonly observed in a wide range of steels; (iii) there is a strong temperature dependence of the threshold stress intensity Kth below ~30°C; (iv) a relatively large environmental addition of oxygen is only partially effective in inhibiting the effect of the hydrogen. Increased crack growth rates and scatter in the data are thought to be associated with bands of segregation and relatively high concentrations of phosphorus on some grain boundaries.MST/132

Bitter-type toroidal field magnet for ZEPHYR

Under contract to the IPP an Garching, MBB conducted a feasibility study concerning stress computations, design and material technology of a Bitter-type magnet for the ZEPHYR project. The big overall dimensions of the magnet (6,50 m diameter, 2,80 m high), access for diagnostics and neutral injection (16 ports), the possibility of remote handling of activated parts and very high forces form the main requirements for design and material technology. A design with 16 identical modules (coils) was chosen, each coil consisting of 16 Bitter-plates, plate housings and one diagnostic/neutral injection wedge. The structural parts are connected by bolts and form the bending stiff structure of the magnet. The most critical area of the magnet is the inner wedge-shaped part of the coils (throat area) with extremely high tension, compression and shear stress values, to which temperature effects contribute heavily. Steel-copper compounds are found to be the best Bitter-plate materials. Copper-plating austenitic steel can be done galvanically or by explosive techniques. Cold-worked austenitic steels (e.g. type 1.3974, 1.4406) fulfil the requirements in the throat, in the flat-plate region milder steels (e.g. ATVSMO, 1.3974) can be used. Different plate concepts are being considered. Plasma-sprayed Al 2 O 3 /TiO 2 and reinforced epoxy layers are provided as insulating materials in different magnet areas. The study has been finished in Sept. 1980 with the conclusion, that a Bitter-type magnet would be feasible, but that as the next step further activities would be absolutely necessary in the fields -stress optimization in the throat area and -material development and testing with large scale samples.

Recent Researches in Steel Metallurgy

IN a Friday evening discourse at the Royal Institution on March 17, Dr. W. H. Hatfield discussed what he described as the three principal aspects of steel research: (a) elucidation and improvement of the process of steel manufacture and manipulation; (b) the investigation of the properties of steel under variable conditions approximating to those of service; and (c) the exploration of the influence of added elements and of the effect of heat-treatment. Dr. Hatfiold showed a colour film illustrating the electric and open-hearth stool processes, the casting of ingots and forging operations. Dr. Hatfield said that one of the most complex studies is that of the changing properties of steels with varying temperature, this study not only including the properties of the steel at a particular temperature but also the degree of persistence of these properties at that temperature over long periods of time. In this connexion, details were given of a creep test carried out on a heat-resisting steel maintained under a load of 280 Ib. per sq. inch for a period of 10,000 hours at 900° C. With reference to the production of permanent strain at normal temperatures, Dr. Hatfield instanced a test now in progress at the Brown-Firth Research Laboratories. The experiment consists of a strip of cold-worked austenitic steel placed in tension under a stress of 25 tons per square inch. The tost was commenced in June 1929, and since that date the extension has been nil within the accuracy of measurement applied, that is, a sensitivity of 1/40,000 of an inch and a gauge length of 8 inches. In discussing the results achieved by the addition of alloying elements, Dr. Hatfield stated that, experimenting upon the basis of the rustless steel composition by modifying the chromium and nickel, and by adding other elements such as tungsten, molybdenum, cobalt or titanium, steels have been produced which maintain a reasonably protective film at temperatures even in excess of 1000° C., besides possessing a useful strength.