Embrittlement

There are two types of embrittlement, which could affect BWR vessel and internal components. These are (1) radiation embrittlement, which may affect core region internals, and (2) thermal embrittlement, which may affect the cast stainless steel fuel supports. 

Neutrons produce energetic primary recoil atoms, which displace large numbers of 

The actual mechanism of radiation embrittlement is not completely understood. For example, in low alloy RPV steel, radiation embrittlement is a function of both environmental and metallurgical variables. Fluence or dpa, and copper and nickel content have been identified as the primary contributors in US NRC Regulatory Guide 1.99, Revision 2 [5.1]. Other important variables include flux, temperature and phosphorus content. There is evidence that other variables such as heat treatment may also influence embrittlement. Therefore, mathematically based statistical data correlation are subject to uncertainty. 

Wrought austenitic stainless steels do not exhibit the sharp ductile to brittle transition behavior characteristic of low alloy and carbon steels. Rather, toughness losses due to irradiation tend to accumulate with increasing fluence and saturate at levels 1x10 n/m. Until recently, there was little information available to quantify the effects of radiation embrittlement on RPVIs. New information [5.2] describes the results of a fracture toughness study performed on irradiated Type 304 stainless steel reactor internal material taken from 

Susceptible to this kind of mechanisms are cast stainless steels, to a lesser extent weld metal and some Cr rich martensitic steels. Several research projects funded by the USNRC, EPRI, George Fisher Limited of Switzerland, and a consortium of Westinghouse, Framatome and EDF have evaluated mechanical property degradation which results from thermal ageing embrittlement in typical cast duplex stainless steel materials [5.3], 

Brittleness is. . the quality of a material that leads to craek propagation without appreeiable plastie deformation.  

Embrittlement is. . the reduction in the normal ductility of a metal due to a physical or chemical change.  

With these definitions in mind, a systematic classification has been made. The various types of embrittlement found in refineries and petrochemical plant equipment, susceptible steels, basic causes, and common remedies are listed in the accompanying table. 

Intrinsic Steei Quaiity refers to the metallurgical and chemical properties of steel products (plate, pipe, tubes, structurals, castings, forgings) supplied to the fabricator for conversion into process equipment. Factors related to deoxidation, controlled finishing temperatures in rolling, and cleaning up of surface defects are included. 

Fabrication and Erection. Embrittlement problems associated with forming, welding and heat treatment are included in this section, although in some instances the heat treatment is done by the steel manufacturer. 

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Gibala R and Hehemann R F (eds) 1984 Hydrogen Embrittlement and Stress Corrosion Craoking (Metals Park, OH American Soceity of Metals)... 

Thermal Oxidative Stability. ABS undergoes autoxidation and the kinetic features of the oxygen consumption reaction are consistent with an autocatalytic free-radical chain mechanism. Comparisons of the rate of oxidation of ABS with that of polybutadiene and styrene—acrylonitrile copolymer indicate that the polybutadiene component is significantly more sensitive to oxidation than the thermoplastic component (31—33). Oxidation of polybutadiene under these conditions results in embrittlement of the mbber because of cross-linking such embrittlement of the elastomer in ABS results in the loss of impact resistance. Studies have also indicated that oxidation causes detachment of the grafted styrene—acrylonitrile copolymer from the elastomer which contributes to impact deterioration (34). 

Examination of oven-aged samples has demonstrated that substantial degradation is limited to the outer surface (34), ie, the oxidation process is diffusion limited. Consistent with this conclusion is the observation that oxidation rates are dependent on sample thickness (32). Impact property measurements by high speed puncture tests have shown that the critical thickness of the degraded layer at which surface fracture changes from ductile to brittle is about 0.2 mm. Removal of the degraded layer restores ductiHty (34). Effects of embrittled surface thickness on impact have been studied using ABS coated with styrene—acrylonitrile copolymer (35). 

Deterioration. The causes of degradation phenomena in textiles (155—158, 164) are many and include pollution, bleaches, acids, alkaUes, and, of course, wear. The single most important effect, however, is that of photodegradation. Both ceUulosic and proteinaceous fibers are highly photosensitive. The natural sensitivity of the fibers are enhanced by impurities, remainders of finishing processes, and mordants for dyes. Depolymerization and oxidation lead to decreased fiber strength and to embrittlement. 

Like other perfluoropolymers. Teflon PFA is not highly resistant to radiation (30). Radiation resistance is improved in vacuum, and strength and elongation ate increased more after low dosages (up to 30 kGy or 3 Mrad) than with FEP or PTEE. Teflon PEA approaches the performance of PTEE between 30 and 100 kGy (3—10 Mrad) and embrittles above 100 kGy (10 Mtads). At 500 kGy (50 Mrad) PTFE, FEP, and PFA ate degraded. The effect of radiation on tensile strength and elongation is shown in Table 7. 

Carbon content is usually about 0.15% but may be higher in bolting steels and hot-work die steels. Molybdenum content is usually between 0.5 and 1.5% it increases creep—mpture strength and prevents temper embrittlement at the higher chromium contents. In the modified steels, siUcon is added to improve oxidation resistance, titanium and vanadium to stabilize the carbides to higher temperatures, and nickel to reduce notch sensitivity. Most of the chromium—molybdenum steels are used in the aimealed or in the normalized and tempered condition some of the modified grades have better properties in the quench and tempered condition. 

Its influence appears to result primarily from suppression of the embrittling effects of moisture in air (75). The role of chromium, on the other hand, is to reduce the embrittling effects of oxygen at temperatures above about 500°C (76). 

Louis Raymond, ed., Hjdrogen Embrittlement Prevention and Control, American Society for Testing and Materials, PhUadelphia, Pa., 1988. 

The drawbacks of cellular materials include limited temperature of appHcations, poor flammabiUty characteristics without the addition of fire retardants, possible health ha2ards, uncertain dimensional stabiUty, thermal aging and degradation, friabiUty, and embrittlement due to the effects of uv light (3,6,15). 

Polyamides, like other macromolecules, degrade as a result of mechanical stress either in the melt phase, in solution, or in the soHd state (124). Degradation in the fluid state is usually detected via a change in viscosity or molecular weight distribution (125). However, in the soHd state it is possible to observe the free radicals formed as a result of polymer chains breaking under the appHed stress. If the polymer is protected from oxygen, then alkyl radicals can be observed (126). However, if the sample is exposed to air then the radicals react with oxygen in a manner similar to thermo- and photooxidation. These reactions lead to the formation of microcracks, embrittlement, and fracture, which can eventually result in failure of the fiber, film, or plastic article. 

Film or fibers derived from low molecular weight polymer tend to embrittle on immersion ia acetone those based on higher molecular weight polymer (>0.60 dL/g) become opaque, dilated, and elastomeric. When a dilated sample is stretched and dried, it retains orientation and is crystalline, exhibiting enhanced tensile strength. The tensile heat-distortion temperature of the crystalline film is iacreased by about 20°C, and the gas permeabiUty and resistance to solvent attack is iacreased. 

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