Chemical properties are herein defined as those material characteristics that are dictated by the elemental constituency of the solid. The explanation will continue with Piping & Fabrication and let's continue. This is usually measured by the relative atomic weight percent of the various elements (metals or nonmetals) or compounds within the material. Metals 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., machine ability) 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 weld ability. 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.
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., machine ability) 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 weld ability. 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.
No comments:
Post a Comment