Temperature embrittlement process

The main use of this type steel is for situations in which the process material may not be corrosive to mild steel, yet contamination due to rusting is not tolerable and temperatures or conditions are unsuitable for aluminum. However, prolonged use of these steels in the temperature range of 450 to 550°C causes low-temperature embrittlement of most ferritic steels with more than 12% chromium [16]. 

Low temperature. Low-temperature processes (below 0°C) contain large amounts of fluids kept in the liquid state by pressure and/or low temperature. If for any reason it is not possible to keep them under pressure or keep them cold, then the liquids will begin to vaporize. If this happens, impurities in the fluids are liable to precipitate from solution as solids, especially if equipment is allowed to boil dry. Deposited solids may not only be the cause of blockage but also in some cases the cause of explosions. It is necessary, therefore, to ensure that the fluids entering a low-temperature plant are purified. A severe materials-of-construction problem in low-temperature processes is low-temperature embrittlement. Also, in low temperature as in high-temperature operations, the equipment is subject to thermal stresses, especially during start-up and shutdown. 

One faces the problems of low-temperature embrittlement of steel claddings of SRU fuel elements, CPS absorbers as well as SRU steel structures potentially complicating the process of SNF unloading from land-based containers and subsequent shipment to the reprocessing plant ... 

If degradation processes such as high-temperature embrittlement or autorefrigeration will affect operating procedures such as pressurization during start-up, indicate such limitations as general notes to the MSD. 

The objective of FP-5 project PISA is to better understand the role of phosphorus in the embrittlement process of RPV steels and improve the predictability of the impact it can have on embrittlement. Further work is still needed to understand, in particular, the segregation mechanisms of phosphorus to internal grain boundaries in RPV steels, as a result of exposure to irradiation or elevated temperatures, and the subsequent brittle intergranular failure of the material. 

Another baisic issue is that of the frequent, high temperature embrittlement of continuous ceramic fiber composites and the extent to % hich this may be controlled by processing, for ex2mip1e the possibility that embrittlement may be less fast or as serious in composites made by chemical vapor infiltration as opposed to other processes. These latter questions heavily focus around the issue of the mechanism of toughening in these... 

The polymer is not easy to process and in injection moulding melt temperatures of 300°C are employed. In order to prevent excess embrittlement by shock cooling of the melt, mould temperatures as high as 150°C may be used. The polymer may also be compression moulded at temperatures of 250-260°C. 

If the major constituents of a solid alloy in contact with a liquid alloy are highly soluble in the latter without formation of compounds, progressive attack by solution is to be expected. If, on the other hand, a stable inter-metallic compound is formed, having a melting point above the temperature of reaction, a layer of this compound will form at the interface and reduce the rate of attack to a level controlled by diffusion processes in the solid state. By far the most serious attack, however, occurs in the presence of stresses, since in this case the liquid alloy, or a product of its reaction with the solid alloy, may penetrate along the grain boundaries, with resultant embrittlement and serious loss of strength. 

The role of the stress in embrittlement and stress-corrosion processes has been examined in some detail by employing the slow strain-rate technique . Specimens of alloy 7179-T651 tested in air or in vacuum after pre-exposure to water at 70° C or in water at various potentials at ambient temperature exhibited a reversible embrittlement in excess of that arising from testing in moist air . The embrittlement was attributed to hydrogen absorption, and recovery was thought to be due to loss of hydrogen (particularly under vacuum) or to diffusion to traps. Potentiostatic tests revealed... 

---