Work (Strain) Hardening

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