Machinery's Handbook, 31st Edition
394 Standard Steels a final heat treatment; however, they may be annealed, normalized, case hardened, or quenched and tempered to enhance fabrication or mechanical properties. Carbon steels may be killed, semikilled, capped, or rimmed, and, when necessary, the method of deoxi dation may be specified. Low-Alloy Steels: Low-alloy steels contain up to 8 percent of one or more alloying ele- ments. Various alloy compositions are designed to enhance mechanical properties, fab- ricating characteristics, weathering performance, hardenability, or any other attribute of the steel. High-Alloy Steels: These steels contain more than 8 percent of alloying elements, in combinations designed to achieve enhanced performance characteristics. Types of high-alloy steels include corrosion-resistant steels, heat-resistant steels, and wear-resis- tant steels. Stainless steels and some tool steels are important members of the high-alloy steels category. Stainless Steels: These high-alloy steels owe much of their corrosion resistance to rela- tively large amounts of chromium. Other elements also may be added to increase corrosion resistance, but their usefulness in this respect is limited. Stainless steels generally contain at least 10 percent chromium, with or without other elements; however, it has been custom - ary in the United States to include in this classification those steels that contain as little as 4 percent chromium. Together, these steels form a family known as stainless and heat- resisting steels , in which some members possess very high strength and oxidation resis - tance. Few, however, contain more than 30 percent chromium or less than 50 percent iron. The most common stainless steels can be divided into groups based on their metallur - gical structures: austenitic , ferritic , and martensitic . Precipitation-hardening stainless steels are classified as either semi-austenitic or martensitic. Duplex stainless steels con- tain approximately equal amounts of austenite and ferrite. In each group, the standard grades are defined by their compositional ranges, within which stainless steel will exhibit certain desired attributes. Variations in composition are made to obtain specific proper - ties. For instance, added chromium, molybdenum, and nitrogen give super duplex grades enhanced corrosion resistance. The austenitic grades are nonmagnetic in the annealed condition, although some may become slightly magnetic after cold-working. They can be hardened only by cold-working, and not by heat treatment, and combine outstanding corrosion and heat resistance with good mechanical properties over a wide temperature range. The austenitic grades are fur ther classified into two subgroups: the chromium-nickel types and the less frequently used chromium-manganese-low-nickel types. The basic composition in the chromium-nickel group is widely known as 18-8 (Cr-Ni) and is the general-purpose austenitic grade. This grade is the basis for over 20 modifications that can be characterized as follows: the chro mium-nickel ratio has been modified to change the forming characteristics; the carbon content has been decreased to prevent intergranular corrosion; the elements niobium or titanium have been added to stabilize the structure; or molybdenum has been added or the chromium and nickel contents have been increased to improve corrosion or oxidation resistance. The standard ferritic grades are always magnetic and contain chromium but no nickel. They can be hardened to some extent by cold-working, but not by heat treatment, and they combine corrosion and heat resistance with moderate mechanical properties and decora tive appeal. The ferritic grades generally are restricted to a narrower range of corrosive conditions than the austenitic grades. The basic ferritic grade contains 17 percent chro mium. In this series, there are free-machining modifications and grades with increased chromium content to improve scaling resistance. Also in this ferritic group is a 12 percent chromium steel (the basic composition of the martensitic group) with other elements, such as aluminum or titanium, added to prevent hardening. The standard martensitic grades are magnetic and can be hardened by quenching and tempering. They contain chromium and, with two exceptions, no nickel. The basic marten sitic grade normally contains 12 percent chromium. There are more than 10 standard
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