A small groove is left at the grain boundaries. The prepared surface is chemically attacked with dilute acid for a short period, a process called "etching." The grain-boundary atoms are more easily and rapidly dissolved or "corroded" than the atoms within the grains. In order to observe the microstructure, a piece of the metal is smoothly polished to a plane and mirror-like finish. The next several examples will illustrate the damage done along the grain boundaries in preference to the bulk of the grains themselves. Grain boundaries play an important role in the operational behavior of boiler steels. Grains are said to be elongated when the characteristic dimensions are not the same, but one direction is much longer than the others. (For example, this attempt to minimize the surface-to- volume ratio, or surface energy, is the driving force to the spheroidization of iron carbide in ferritic steels.) Grains are said to be equiaxed when the characteristic dimensions are the same in all directions. Its characteristic dimension is referred to as a "diameter." At equilibrium the shape tends to minimize the grain-boundary surface area for a given volume of metal within a grain. No grain is ever a sphere, but an irregular polyhedron that when packed together with others completely fills the space. The shape of a grain is governed by its neighbors. Note that as the grains get smaller, the grain-size number gets larger. Where "n" is the number of grains per square inch when viewed at l00x. The ASTM grain size number "N" is defined by: The ASTM grain-size number is one standard for determining the average grain size. With a grain boundary perhaps 2-10 atomic dimensions thick, the grains are clearly seen to be regions of long-range atomic order, and the grain boundaries, regions of short-range atomic disorder. The diameter of an iron atom is about 10-8 (0.00000001) inch. Within any one grain are a large number of individual atoms. Thus there may be a billion (109) grains per cubic inch of alloy. For reference, a grain diameter is about 0.001" across. Grain size can vary greatly depending on the alloy and heat treatment. The disordered atomic arrangement and higher energy can explain several features associated with material degradation found in the high-temperature operation of boilers. Hence the atoms in the boundary have a less ordered structure and a slightly higher internal energy. The mismatch of the orientation of neighboring grains leads to a less efficient atomic packing within the grain boundary. Grain boundaries are usually considered to be two dimensional, but are actually a finite thickness, perhaps 2-10 atomic distances. Individual grains are viewed as being made up of the cube faces of face-centered cubic or body-centered cubic iron. The juncture between adjacent grains is called a "grain boundary." The grain boundary is a transition region in which some atoms are not exactly aligned with either grain. These individual crystals are called"grains." In any one grain, all atoms are arranged with one particular orientation and one particular pattern. These crystal boundaries determine in no small way the useful properties of engineering materials when applied to steam generators.Īll boiler alloys are made up of many crystals of various individual orientations. Where adjacent crystals join is a crystal boundary, a zone of short-range disorder. There is then, long-range atomic order within individual crystals. For our general purposes, atoms within a metallic crystal or grain are regularly arranged over great distances, distances that are huge when compared with atomic dimensions. Metals, like everything else, are made up of atoms, and for explanations used here, are assumed to be solid spheres.
#Grain boundary series#
Summary: The following article is a part of National Board Classic Series and it was published in the National Board BULLETIN. French, Inc., Metallurgists, Northborough, MA
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