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Work  (Strain)  Hardening

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PLANT MATERIAL PROBLEMS DOE-HDBK-1017/2-93 Plant Materials Fundamental  requirements  during  design  and  manufacturing  for  avoiding  fatigue  failure  are different for different cases.   For a pressurizer, the load variations  are fairly low, but the cycle frequency is high; therefore, a steel of high fatigue strength and of high ultimate tensile strength is  desirable.    The  reactor  pressure  vessel  and  piping,  by  contrast,  are  subjected  to  large  load variations, but the cycle frequency is low; therefore, high ductility is the main requirement for the  steel.    Thermal  sleeves  are  used  in  some  cases,  such  as  spray  nozzles  and  surge  lines,  to minimize thermal stresses.   Although the primary cause of the phenomenon of fatigue failure is not well known, it apparently arises from the initial formation of a small crack resulting from a defect or microscopic slip in the metal grains.  The crack propagates slowly at first and then more rapidly when the local stress is increased due to a decrease in the load-bearing cross section.  The metal then fractures.  Fatigue failure can be initiated by microscopic cracks and notches, and even by  grinding  and  machining  marks  on  the  surface;  therefore,  such  defects  must  be  avoided  in materials subjected to cyclic stresses (or strains).  These defects also favor brittle fracture, which is 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 used to  minimize  cyclic  stress.     In  some  cases,  cycle  logs  may  be  kept  on  various  pieces  of equipment.   This allows that piece of equipment to be replaced before fatigue failure can take place. Work  (Strain)  Hardening W ork  hardening  is  when  a  metal  is  strained  beyond  the  yield  point.    An  increasing  stress  is required to produce additional plastic deformation and the metal apparently becomes stronger and more difficult to deform. Stress-strain  curves  are  discussed  in  Module  2,  Properties  of  Metals.   If  true  stress  is  plotted against true strain, the rate of strain hardening tends to become almost uniform, that is, the curve becomes almost a straight line,  as shown in Figure 1.   The gradient of the straight part of the line is known as the strain hardening coefficient or work hardening coefficient,  and is closely related to the shear modulus (about proportional).  Therefore, a metal with a high shear modulus will have a high strain or work hardening coefficient (for example, molybdenum).   Grain size will also influence strain hardening.   A material with small grain size will strain harden more rapidly than the same material with a larger grain size.   However, the effect only applies in the early stages  of plastic  deformation, and the  influence disappears as  the structure  deforms and grain structure breaks down. Work hardening is closely related to fatigue.   In the example on fatigue given above, bending the thin steel rod becomes more difficult the farther the rod is bent.   This is the result of work or strain hardening.   Work  hardening reduces ductility, which  increases the chances of  brittle failure. MS-05 Page 28 Rev. 0



   


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