Hardenability Bands for Carbon and Alloy H Steels ■ J1868 Restricted Hardenability Bands for Selected Alloy Steels The place of steels in the overall UNS is documented in the standard jointly published by SAE and ASTM: SAE HS-1086/ASTM DS 56H Metals and Alloys in the Unified Numbering System. See Internet sites 4 and 5 for additional information and to acquire these standards. The new designation system permits steel manufactur- ers to modify the standard chemistry for commonly used steels to meet specific customer needs without creating an entirely new four-digit designation number. Performance requirements for press-forming, roll-forming, abrasion re- sistance, environmental corrosion resistance, cleanliness of the steel, and similar special needs can be assured more eas- ily and reliably with such a system. These situations often arise in production operations in the automotive, aerospace, appliance, construction equipment, agricultural equipment, manufacturing equipment, energy production equipment, and similar industries. A bearing steel nominally contains 1.0% carbon. Common grades are 50100, 51100, and 52100; the usual four-digit designation is replaced by five digits, indicating 100 points of carbon. Alloy Groups As indicated in Table 2–8, sulfur, phosphorus, and lead im- prove the machinability of steels and are added in signifi- cant amounts to the 11xx, 12xx, and 12Lxx grades. These grades are used for screw machine parts requiring high pro- duction rates where the resulting parts are not subjected to high stresses or wear conditions. In the other alloys, these elements are controlled to a very low level because of their adverse effects, such as increased brittleness. Nickel improves the toughness, hardenability, and cor- rosion resistance of steel and is included in most of the alloy steels. Chromium improves hardenability, wear and abra- sion resistance, and strength at elevated temperatures. In high concentrations, chromium provides significant corro- sion resistance, as discussed in the section on stainless steels. indicate the amount of carbon in the steel in hundredths of a percent. For example, when the last two digits are 20, the alloy includes approximately 0.20% carbon, and it is often said that the steel has 20 points of carbon. Small variations are allowed, say from 0.18% to 0.23%. Carbon content is part of the designation system be- cause the primary mechanical properties of strength, hard- ness, and ductility are strongly dependent on it. As carbon content increases, strength and hardness also increase dur- ing processing and heat treatment. Ductility typically de- creases so designers must balance strength and ductility when specifying a steel and its condition. It is useful to cat- egorize steels into four broad classes, low carbon, medium carbon, high carbon, and bearing steels.
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