[en] Creep behaviour in vacuum of Types 304 and 316 stainless steels of carbon content 0.03 wt-% and intercept grain size 12 +- 3 μm has been studied for the temperature range 550-750₀C (0.49-0.61 Tsub(m)) and stresses up to 13 MPa. Creep was characterized by Bingham behaviour with, at lower temperatures, an observed transition to power-law behaviour at stresses above 7-10 MPa. Strain rate per unit stress for the Bingham behaviour was typically within a factor of 2 of predictions for Coble creep at higher temperatures for Type 304 while observed values were lower than predicted by a factor as large as 5 at 550₀C. Values for Type 316 were a factor of 5 below predictions for a Mo-free steel even at higher temperatures, the discrepancy increasing by a further factor of 3 at 600₀C. Precipitation of M²³C⁶ during (rather than prior to) testing increased the duration of primary creep and prevented the decrease of σ⁰ from 1 to 0.1 MPa otherwise occurring with increase in temperature, but it had no effect on final strain rate per unit stress. Transition to power-law creep was observed at stresses a factor of 10 below levels expected from single power-law extrapolation creep data for high stresses. (author)

[en] The design of high-temperature components has become much more sophisticated in the last decade, demanding considerably more materials data. Data requirements for Type 316 steel are discussed in this paper in relation to the Commercial Demonstration Fast Reactor. Although guidance can be obtained from ASME Code Case N47, there are many areas where additional information is considered necessary. The present data position is given and areas for further work identified. Particular emphasis needs to be given to obtaining additional data on creep/fatigue, strain-controlled high-cycle fatigue, fracture toughness and low temperature deformation. Further information is also required on long-term stress rupture, creep deformation, stress relaxation and crack growth. Although the paper is centred on the properties of solution-annealed Type 316 steel, for a full design endorsement it will also be necessary to establish the effects of thermal aging, sodium and irradiation environment and the properties of weld metal. (author)

[en] Some of the advantages to be gained for nuclear fuel cladding applications by dispersion strengthening a 20Cr-25Ni austenitic steel with TiN are lost due to the large extent of the primary creep region. Experimental creep data are presented which show that the primary creep can be suppressed by comparatively small amounts of cold work (approximately 5%) and if the level of cold work (or prestrain) is less than approximately 10% recovery is limited and recrystallization does not occur under the conditions of the tests. However, creep strain induced softening has been observed which may be explained in terms of the degeneration of the stabilized dislocation network to that of the non-cold-worked state. A simple power law expression has been fitted to the initial stage of creep of the 5% cold-worked alloy. This is more amenable to encoding in computer models and gives a better fit at low strains than the bimodal Garofalo equation. (author)

[en] The dynamic mechanical properties of some austenitic stainless steels, such as AISI 321, AISI316L, AISI 316H, AISI 304L, and DIN 4981 in virgin, welded, thermally aged and irradiated conditions, have been determined at strain rates up to 600 s^-₁ and temperatures up to 950₀C. The dynamic response of virgin austenitic steels at temperatures of 400 and 550₀C is widely spread, passing from a substantial dynamic hardening behaviour to dynamic softening behaviour. More homogeneous is the dynamic response of the same austenitic stainless steels damaged by welding, thermal aging and irradiation. In fact, all these damaged materials, tested at 400 and 550₀C, show a grade of dynamic softening and reduction of ductility which will influence the safety coefficients and the maximum allowable strain for structures submitted to extreme dynamic loading conditions. (author)

[en] Recent advances in the modelling of grain boundary precipitation and segregation in austenitic steels are described, and the results of applying these models to experimental data in a range of austenitic steels are presented. The case for M²³C⁶ in Alloy 800 is selected for detailed discussion of grain boundary precipitation. Earlier precipitation models are adapted to account for: (1) nucleation kinetics; (2) variations in precipitate morphology; and (3) changes in solute supply rate after longer aging times. The model caters for cap-shaped and faceted grain boundary precipitates. The fit between experimentally determined isothermal precipitation curves and those predicted by the model is shown to be reasonably good, especially in the aging and service temperature range for austenitic steels (873-1100 K). The nonequilibrium grain boundary segregation process is described and a model which quantifies the process is developed. The predictions of the model indicate that the solution treatment temperature and the cooling rate from the solution treatment temperature have a profound effect on the amount of impurity segregation occurring to grain boundaries. Examples of reasonable agreement between experiment and theory for segregation of B in Type 316 steel, Ti in 12R72 steel and Al in Inconel 600 are presented. The improved precipitation and segregation models show that a better understanding of creep behaviour, carbide aging treatments and intergranular embrittlement in austenitic steels is becoming possible. (author)

[en] The authors draw on their experience of steels for nuclear purposes to discuss the production in recent years of a new type of austenitic stainless steel, SIAS 316LB (corresponding to AFNOR Z2CND 18-12, AUTAAS 722 with low C content and additions of B and N). This steel grade is employed in nuclear energy, particularly on fast reactors where thermal, mechanical and chemical stresses are considerably more severe than those which effect conventional thermal reactors. The simultaneous addition of interstitial alloying elements such as B and N substantially improves the hot mechanical resistance for protracted use; in particular boron affects creep properties and changes the sensitive area to higher temperatures and to longer time periods. The authors' purpose is to point out the metallurgical properties obtained in conditions of industrial-scale production of the material developed at the Cogne factory in Aosta. These properties are compared to those of the conventional AISI 316 grade of steel. The influence of the hot working processes on the material properties (heating condition, reduction ratios, etc.) is underlined taking into consideration forgings of large dimensions. The relationship between microstructure (precipitation of second phases containing B and N at the grain boundaries) and properties is also pointed out by TEM analysis. (author)

[en] The effect that prior cold working with 7% and 14% deformation has upon the creep behaviour of AISI alloy 314 has been investigated at 900₀C. For short-term testing the pretreated condition exhibits a beneficial effect in terms of creep and rupture strength, whereas the ductility is reduced. With increasing test times the strength and ductility of the material gradually approach those of the recrystallized condition. From structural analyses it is concluded that as long as the high dislocation density associated with the preformed condition remains, the strength is increased and the grain boundaries are sensitive to crack nucleation and growth. On the basis of the observation that the creep properties recover fully it is concluded that prior cold work at the applied level does not damage the material. (author)

[en] The thermal fatigue crack initiation and propagation promoted by thermal quenches of AISI 316 and 304 stainless steels have been studied to correlate with their known behaviour in isothermal strain cycling fatigue at elevated temperatures. Axially unconstrained specimens of rectangular section were held at bulk temperatures of 250-500₀C and symmetrically water-quenched on the narrow faces to give equivalent surface strain ranges from 2.8 x 10^-₃ to 5.4 x 10^-₃. Crack initiation in smooth samples showed an apparent threshold at a surface strain range of 2.8 x 10^-₃ equivalent to a thermal amplitude of 150₀C, no cracking being produced in 500000 cycles. The crack growth in prenotched samples was evaluated by direct observation and by subsequent fractography and showed two modes of growth. The crack growth was strain-controlled during the early stages of propagation where the crack tip was within the surface zone under conditions of fully plastic cyclic yield. At greater depths the propagation rates in the remaining elastically cycled material were found to correlate with calculated stress intensity values. In the chosen symmetrically quenched axially unconstrained configuration the crack growth rates decreased towards the centre of the specimen, indicating a crack arrest condition as expected from analysis. The results indicated a good correlation with the fracture behaviour observed from isothermal strain cycling fatigue behaviour in an air environment. (author)

[en] Two consumables suitable for welding AISI 316 have been studied. Both contained 3-8% delta-ferrite in the as-welded state but were of different nominal compositions, namely 19Cr-12Ni-3Mo and 17Cr-8Ni-2Mo. The results of a series of uniaxial stress relaxation tests carried out over a range of temperatures from 650 to 850₀C indicate that it was practicable to achieve acceptable residual stress levels even at the lower end of this temperature range; the inference was, therefore, that the final choice of stress relief treatment will be determined by the metallurgical changes that simultaneously occur. The isothermal transformation kinetics of the delta-ferrite were determined over the temperature range 600-800₀C for both weld metals and the nature and morphology of the phases formed was studied. Transformation proceeded more rapidly in the 19Cr-12Ni-3Mo deposit than in the 17Cr-8Ni-2Mo deposit. Electron microscopy revealed that in both weld metals the delta-ferrite acted as a focus for a series of complex precipitation reactions involving initially the simultaneous formation of M²³C⁶ and reversion of delta to ß followed by precipitation of intermetallic phases chi and σ. Tensile impact and creep properties of aged material have been determined and are discussed in terms of the structural features observed. (author)

[en] The reasons for the application of higher process temperatures are explained. The properties of stainless steel are compared with those of other materials such as molybdenum. Factors influencing the choice of the material such as availability of material data at high temperature, controllability, and strength of heat-affected zone are discussed. The process of designing a structure for safe and economic high-temperature application is outlined: design-by-analysis in contrast to the design-by-rule which is general practice for low-temperature applications. The rules laid down in the ASME Pressure Vessel Code Case N47 are explained as well as the procedure for inelastic stress calculations. (author)