Handbook-Steels Classified

Continued on next page...

In one group of alloy steels, usually termed ”high-strength” or "high- strength, low-alloy”, the addition of small

amounts of nickel, chromium, or molybdenum (less than 1% ) raises tensile strength appreciably. Although these steels cost more (per pound or per kilogram) than plain carbon steels, steel costs for structures specifically designed to utilize their higher strength are less than the steel costs for equally strong structures designed in terms of plain carbon steels. (In fact, several of the ”high-strength” steels are not truly ”alloy” steels at all; their extra strength is achieved at least partly by holding the phosphorus content below the normal limits for carbon steels.) These steels are readily welded by almost any process. Even stronger than the ”high-strength, low-alloy” steels are the structural steels generally termed ”high-strength, heat-treated alloy”. Such a steel may contain somewhat more than 1% nickel, about 1% copper, and 0.5% or less molybdenum. These steels, which must be heat-treated after rolling, have yield strengths 50-100% higher than the yield strengths of straight-carbon steels of comparable carbon content. They usually cannot be welded without some sacrifice in mechanical properties. Beyond these structural steels there lies a very large group of steels generally termed simply ”alloy .steels”. In such steels, the nickel or chromium content may reach 3.75%, with up to 0.30% molybdenum specified for many grades. In very few of these steels does the total alloy-metal content exceed 5% . In this group, chromium is the most frequently encountered alloying metal. It generally adds both strength and ductility, and is especially useful in alloys designed for heat treatment. Nickel contributes to both greater strength and greater shock resistance. Chromium and molybdenum are used in combination (in ”chrome-moly” steels) for their ability to retain strength and resist creep under stress at elevated temperatures. High-Alloy Steels. Metallurgists do not often use the term ”high-alloy” but we shall use it here to mean steels which contain more than 5% alloying metals. Most ”high-alloy” steels are formulated for corrosion resistance or heat resistance or a combination of both, and use chromium or nickel or a combination of the two as the principal alloying ingredients. Many of these steels are classified as ”stainless steels” and virtually all are produced in electric furnaces, The grades most widely used for corrosion resistance are the ”18-8” stainless steels, which contain approximately 18% chromium and 8% nickel. In addition to being totally resistant to atmospheric corrosion,