David N. French, Sc.D.
President of David N. French, Inc., Metallurgists, Northborough, MA
Summary: The following article is a part of National Board Classic Series and it was published in the National Board BULLETIN. (3 printed pages)
Metals, like everything else, are made up of atoms, and for explanations used here, are assumed to be solid spheres. 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. There is then, long-range atomic order within individual crystals. Where adjacent crystals join is a crystal boundary, a zone of short-range disorder. These crystal boundaries determine in no small way the useful properties of engineering materials when applied to steam generators.
All boiler alloys are made up of many crystals of various individual orientations. These individual crystals are called"grains." In any one grain, all atoms are arranged with one particular orientation and one particular pattern. 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.
Individual grains are viewed as being made up of the cube faces of face-centered cubic or body-centered cubic iron. Grain boundaries are usually considered to be two dimensional, but are actually a finite thickness, perhaps 2-10 atomic distances. The mismatch of the orientation of neighboring grains leads to a less efficient atomic packing within the grain boundary. Hence the atoms in the boundary have a less ordered structure and a slightly higher internal energy. The disordered atomic arrangement and higher energy can explain several features associated with material degradation found in the high-temperature operation of boilers.
Grain size can vary greatly depending on the alloy and heat treatment. For reference, a grain diameter is about 0.001" across. Thus there may be a billion (109) grains per cubic inch of alloy. Within any one grain are a large number of individual atoms. The diameter of an iron atom is about 10-8 (0.00000001) inch. So across a one-mil (0.001") grain are 100,000 (105) iron atoms. 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 ASTM grain-size number is one standard for determining the average grain size. The ASTM grain size number "N" is defined by:
where "n" is the number of grains per square inch when viewed at l00x. The usual range is from 1-9. Note that as the grains get smaller, the grain-size number gets larger.
The shape of a grain is governed by its neighbors. No grain is ever a sphere, but an irregular polyhedron that when packed together with others completely fills the space. 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. (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. Grains are said to be elongated when the characteristic dimensions are not the same, but one direction is much longer than the others.
Grain boundaries play an important role in the operational behavior of boiler steels. The next several examples will illustrate the damage done along the grain boundaries in preference to the bulk of the grains themselves.
In order to observe the microstructure, a piece of the metal is smoothly polished to a plane and mirror-like finish. 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. A small groove is left at the grain boundaries. Since a groove will not reflect light as do the flat, polished grains; the grain boundaries appear as black lines, and the structural details are visible.
Grain boundaries are regions of atomic mismatch and less dense atomic packing. Less density on an atomic scale implies bigger atomic-sized holes through which atoms can more easily move. Such atomic mobility is called "diffusion." Thus grain boundaries will oxidize or corrode more rapidly, usually referred to as "grain-boundary penetration" or "intergranular attack." Oxygen diffuses along the grain boundaries, reacts with the steel, and forms iron oxide preferentially at the grain boundaries.
In stainless steel, the diffusion of carbon along the austenite grain boundaries leads to the formation of chromium carbides. As these carbides form, they deplete the region immediately adjacent to the grain boundary of chromium. As chromium decreases, so does the corrosion resistance. Thus the grain-boundary region is more easily corroded, a condition called "sensitization."
Once these steels have been sensitized, they are subject to an intergranular corrosion attack known as (IGA). The location from which the sample was taken is the bottom of a pendant where condensate collects during shutdown. When the boiler is opened to the atmosphere, water and oxygen preferentially corrode the sensitized grain boundaries. The resultant appearance is corrosion debris, essentially metal oxides, along the grain boundaries.
Hydrogen damage occurs in waterwall tubes of high pressure utility boilers under thick, water-side corrosion deposits. Under-deposit corrosion usually occurs under acid conditions. One of the products of under-deposit corrosion is atomic hydrogen, a small atom, which easily diffuses into the steel. Hydrogen reacts with iron carbide to form ferrite and methane. The methane molecule is large, cannot diffuse through the steel, and so collects at the ferrite grain boundaries. When the methane pressure is sufficiently great, intergranular cracks form along the ferrite grain boundaries. There are two characteristic features to hydrogen damage: 1) intergranular cracks, and 2) loss of iron carbide (decarburization).
At elevated temperatures, the strength of a grain is greater than the strength of a grain boundary. At room temperature the strength of a grain boundary is greater than the strength of a grain. Where the grain-boundary strength equals the grain strength is known as the "equi-cohesive temperature." Elevated temperature failures tend to follow the grain boundaries and are referred to as "intergranular (between the grains) failures." Elevated-temperature deformation occurs by one grain moving past its neighbor, a process known as "creep." Voids form first at the juncture of several grains where deformation by grain-boundary sliding is limited. Individual voids then link up to form grain-boundary cracks, often referred to as "grain-boundary separation."
In summary, atomic arrangement at a crystal or grain boundary dictates the service behavior. Grain boundaries, being a zone of higher internal energy, may be more readily corroded or oxidized. At elevated temperatures, the grain boundaries are weaker, the grains slip past one another, and creep damage collects at grain boundaries. In under-deposit corrosion and hydrogen damage, grain boundaries are the site at which the methane collects that leads to the intergranular cracking characteristic of hydrogen damage.
Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition of the ASME Boiler and Pressure Vessel Code for current requirements.