PLANT MATERIAL PROBLEMSDOE-HDBK-1017/2-93Plant MaterialsFundamental requirements during design and manufacturing for avoiding fatigue failure aredifferent for different cases. For a pressurizer, the load variations are fairly low, but the cyclefrequency is high; therefore, a steel of high fatigue strength and of high ultimate tensile strengthis desirable. The reactor pressure vessel and piping, by contrast, are subjected to large loadvariations, but the cycle frequency is low; therefore, high ductility is the main requirement forthe steel. Thermal sleeves are used in some cases, such as spray nozzles and surge lines, tominimize thermal stresses. Although the primary cause of the phenomenon of fatigue failure isnot well known, it apparently arises from the initial formation of a small crack resulting from adefect or microscopic slip in the metal grains. The crack propagates slowly at first and then morerapidly when the local stress is increased due to a decrease in the load-bearing cross section. Themetal then fractures. Fatigue failure can be initiated by microscopic cracks and notches, and evenby grinding and machining marks on the surface; therefore, such defects must be avoided inmaterials subjected to cyclic stresses (or strains). These defects also favor brittle fracture, whichis discussed in detail in Module 4, Brittle Fracture.Plant operations are performed in a controlled manner to mitigate the effects of cyclic stress.Heatup and cooldown limitations, pressure limitations, and pump operating curves are all usedto minimize cyclic stress. In some cases, cycle logs may be kept on various pieces ofequipment. This allows that piece of equipment to be replaced before fatigue failure can takeplace.Work(Strain)HardeningW ork hardeningis when a metal is strained beyond the yield point. An increasing stress isrequired to produce additional plastic deformation and the metal apparently becomes strongerand more difficult to deform. Stress-strain curves are discussed in Module 2, Properties of Metals. If true stress is plottedagainst true strain, the rate of strain hardening tends to become almost uniform, that is, the curvebecomes almost a straight line, as shown in Figure 1. The gradient of the straight part of theline is known as the strain hardening coefficient or work hardening coefficient, and is closelyrelated to the shear modulus (about proportional). Therefore, a metal with a high shear moduluswill have a high strain or work hardening coefficient (for example, molybdenum). Grain sizewill also influence strain hardening. A material with small grain size will strain harden morerapidly than the same material with a larger grain size. However, the effect only applies in theearly stages of plastic deformation, and the influence disappears as the structure deforms andgrain structure breaks down.Work hardening is closely related to fatigue. In the example on fatigue given above, bendingthe thin steel rod becomes more difficult the farther the rod is bent. This is the result of workor strain hardening. Work hardening reduces ductility, which increases the chances of brittlefailure.MS-05Page 28Rev. 0
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