Material Properties of Piping Materials
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. 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 carbon- hydrogen 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 unit weight 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 longer range effects of atomic structure. These properties fall into three categories: chemical, mechanical, and physical.
Chemical Properties of Metals
Chemical properties are herein defined as those material characteristics that are dictated by the elemental constituency of the solid. This is usually measured by the relative atomic weight percent of the various elements (metals or nonmetals) or compounds within the material.
Metals are are not usually used in their pure form. Rather, secondary elements are purposely added to improve or modify their behavior. This addition of secondary elements is called alloying, and the elements added fall into two categories, based on the relative size of the atoms. Atoms significantly smaller than those of the parent metal matrix fit into spaces between the atoms in the lattices’ interstices and are called interstitial alloying elements. Carbon added to iron, creating steel, is the most common example. Larger-sized atoms will substitute for parent metal atoms in their matrix locations, thus the name substitutional alloying elements. Examples of this include zinc substituting for copper atoms in copper, creating brass; and tin substituting for copper atoms, creating bronze alloys.
Pure metals possess relatively low strength. Adding an alloying element will increase the strength of a metal’s atomic matrix because the atomic lattice is strained locally by the foreign atom, creating a larger impediment for the sliding of planes of atoms across one another during plastic flow. This is true whether the alloying element is interstitial or substitutional; however, the former generally serve as better lattice strengtheners. Strength properties are often improved to the detriment of ductility. Proper alloying, combined with appropriate metal processing and heat treatment, results in optimization of material properties.
Elements are also added to metals to improve or modify their corrosion or oxidation characteristics, or to improve manufacturability (e.g., machineability) and/or electrical properties, among other effects. However, it is important to note that alloying done to optimize one material property may act to the detriment of others.
Carbon steels, the most common of the construction materials, always contain the elements carbon, manganese, phosphorous, sulfur, and silicon in varying amounts. Small amounts of other elements may be found either entering as gases during the steel-making process (hydrogen, oxygen, nitrogen), or introduced through the ores or metal scrap used to make the steel (nickel, copper, molybdenum, chromium, tin, antimony, etc.). The specific effect of each of these elements on steel properties will be addressed later in the chapter. Addition of significant quantities of the interstitial element carbon will result in high strength and hardness—but to the detriment of formability and weldability. A great amount of research has gone into the development of the principal metals used in piping design and construction; thus the specification limits must be vigorously adhered to in order to assure reliability, predictability, and repeatability of material behavior.
The number of elements alloyed with a parent metal, and the acceptable range of content of each, are identified in the material specification (e.g., ASTM, API, ASME). Tests appropriate for determining the elemental constituency of an alloy have been standardized and are also described in ASTM specifications. The material specifications also stipulate whether the chemical analysis of an alloy may be reported by analyzing a sample of the molten metal, or taken from a specimen removed from the final product. The former is commonly referred to as a ladle analysis, and the latter as a product or check analysis. This ‘‘chemistry’’ of a construction material is reported on a material test report which may be supplied by the material manufacturer upon request.
Mechanical Properties of Metals
Mechanical properties are critically important to the design process. They are defined as the characteristic response of a material to applied force. The standardized test methods for measuring these properties are described in ASTM specifications. Properties fall into two general categories, strength and ductility. Some proper- ties, such as material toughness, are dependent on both strength and ductility. The most widely known and used material properties, as defined by ASTM, are described
in the following paragraphs.
Modulus of Elasticity (Young’s Modulus). The modulus of elasticity is the ratio of normal stress to corresponding strain for tensile or compressive stresses. This ratio is linear through a range of stress, known as Hooke’s law. The material behavior in this range is elastic (i.e., if the applied load is released the material will return to its original, unstressed shape). The value of the slope in the elastic range is defined as Young’s Modulus.
The modulus of elasticity is measured using the tension test, the most widely used test applied to engineering materials. The test consists of applying a gradually increasing load in either tension or compression, in a testing machine, to a standardized test specimen (Fig. A3.2).
The applied load is continuously monitored, as is test specimen elongation or contraction under load. These measured quantities are generally represented on a coordinate axis, called a stress-strain curve (Fig. A3.3).
The modulus of elasticity and other strength properties are established from this Curve.
Yield Strength. When a specimen is loaded beyond the point where elastic behavior can be maintained the speci- men will begin to deform in a plastic manner. Most materials do not abruptly transform from purely elastic to purely plastic behavior. Rather, a gradual transition occurs as represented by a curve, or knee, in the stress-strain curve. Lacking an abrupt and easily definable point representing transition from elastic to plastic behavior, several standardized methods have been defined by ASTM to determine the yield strength used as the engineering property. The most common is termed the 0.2 percent offset method. In this approach a line is drawn parallel to the elastic portion of the curve anchored to a point displaced 0.2 percent along the strain axis. (Fig. A3.4).
The yield strength corresponds to the calculated value of the load indicated at the intersection point of the drawn line, divided by the original cross-sectional area in the gauge length of the tensile specimen. By convention, this test is per- formed at a constant rate of strain, and is reported as newtons per square meter, or as pounds per square inch of cross section in English units.
Ultimate Tensile Strength. Upon further increase of applied load under constant strain rate, the specimen will continue to stretch until the loss of load-carrying cross section caused by specimen thinning during the test (due to Poisson’s ratio) cannot withstand further load increase. The ultimate tensile strength constitutes the maximum applied load divided by the original specimen cross-sectional area.
Elongation and Reduction of Area. The ductility of the test specimen can be established by measuring its length and minimum diameter before and after testing. Stretch of the specimen is represented as a percent elongation in a given length (usually 2 or 8 in) and is calculated in the following manner:
The diameter of the test specimen will decrease, or neck down, in ductile materials. Another standard measure of ductility is the reduction of area of the specimen, defined as follows:
Hardness. This is a measure of the material’s ability to resist deformation, usually determined by a standardized test where the surface resistance to indentation is measured. The most common hardness tests are defined by the indentor type and size, and the amount of load applied. The hardness numbers constitute a non- dimensioned, arbitrary scale, with in- creasing numbers representing harder surfaces. The two most common hardness test methods are Brinell Hardness and Rockwell Hardness, with each rep- resenting a standardized test machine with its own unique hardness scales. Hardness loosely correlates with ulti- mate tensile strength in metals (Fig. A3.5).
Approximate hardness conversion numbers for a variety of material types, including steels, can be found in ASTM Specification E140.
Toughness. Sudden fracture, exhibiting little ductility in the vicinity of the break, occurs in certain metals when load is rapidly applied. The capability of a material to resist such a brittle fracture is a measure of its toughness.
Highly ductile materials (those possessing an FCC lattice, for example) exhibit considerable toughness across a full range of temperatures. Other materials, such as BCC-based carbon steels, possess a level of toughness that is dependent on the metal temperature when the load is applied. In these metals, a transition from brittle to ductile behavior occurs over a narrow range of temperatures.
The two most common methods used to measure metal toughness are the Charpy Impact test, defined in ASTM specification E 23, and the Drop-Weight test, defined in ASTM E 208. The Charpy test employs a small machined specimen with a machined notch that is struck by a pendulum weight. #Little_PEng
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