MATERIAL PROPERTIES OF PIPING MATERIALS

FIGURE A3.1 The three most common crystal structures in metals
and alloys. (a) Face-centered cubic (FCC); (b) body-centered
cubic (BCC); (c) hexagonal close-packed (HCP).

The behavior of piping material can be understood and predicted by studying a number of properties of the material. Appreciation of how a material will perform must extend all the way down to the atomic components of the material. Metals are crystalline in structure, composed of atoms in precise locations within a space lattice, and Piping & Fabrication will explain more and more about Material Properties of Piping Materials.

The smallest component of the crystalline structure is called a unit cell, the smallest repeating building block of the material. For example, iron and iron-based alloys exist in two unit cell forms, the body-centered cubic (BCC) and the face centered cubic (FCC) structure, shown in Fig. A3.1. They are differentiated in the way the atoms are arranged in repeating patterns. The body-centered cubic structure is represented by a cube with atoms at all eight corners, and one atom in the center of the cube. The face-centered lattice is represented by atoms at the eight corners of the cube, plus one atom located at the center of each of the cube’s six faces. The crystal structure naturally assumed by a material dictates some of the fundamental properties of the material. For example, FCC materials are generally more ductile than BCC materials. This is basically because FCC crystals are the most tightly packed of metallic structures and, as such, allow for more planes of atoms to slide across one another with the least amount of resistance (the fundamental atomic motion involved in what is called plasticity).

Metallic materials consist of these and other ordered crystal structures. Some metals, most notably iron, change their crystal structure as temperature varies. Structure may also change as certain other elements are added in the form of alloying additions. These changes are used to advantage by metallurgists and are the basis for developing and manipulating important material behavior, such as the heat treatability of carbon and low alloy steels.

Plastics may be defined as synthetic materials whose chief component is a resin or resin equivalent. The term plastic covers a very broad range of materials that contain, as an essential ingredient, one or more organic polymetic substances. They possess large molecular weight, formed by the chemical combination of carbonhydrogen atom chains (monomers to polymers). The atomic structure is thus ordered and predictable, but dissimilar from that of metals. Many plastics have greater strength per unitweight than metal, but suffer due to lower impact strength, chemical stability, and thermal and aging stability. However, plastics fill an important niche in the piping engineer’s repertoire.

Ceramic materials are composed of the oxides of metal arranged in ordered atomic structures similar to that of metals. The atomic constituents are electronically different, resulting in rigid, predictable behavior, but with an inherent lack of plasticity compared to metals.

Glasses form the other extreme of the atomic structure spectrum. Their atomic makeup is essentially that of a liquid; the structure is actually a solid with no ordered arrangement of atoms. These atomic characteristics (i.e., the natural arrangement of the atoms, as well as the specific elements involved and their electronic characteristics) establish the fundamental properties of engineering materials. The properties that a engineer requires to design and construct a piping system are a manifestation of the longerrange effects of atomic structure. These properties fall into three categories: chemical, mechanical, and physical.