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MMPDS-01 31 January 2003 2.3 LOW-ALLOY STEELS (AISI GRADES AND PROPRIETARY GRADES) 2.3.0 COMMENTS ON LOW-ALLOY STEELS (AISI AND PROPRIETARY GRADES) 2.3.0.1 Metallurgical Considerations — The AISI or SAE alloy steels contain, in addition to carbon, up to about 1 percent (up to 0.5 percent for most airframe applications) additions of various alloying elements to improve their strength, depth of hardening, toughness, or other properties of interest. Generally, alloy steels have better strength-to-weight ratios than carbon steels and are somewhat higher in cost on a weight, but not necessarily strength, basis. Their applications in airframes include landing-gear components, shafts, gears, and other parts requiring high strength, through hardening, or toughness. Some alloy steels are identified by the AISI four-digit system of numbers. The first two digits indicate the alloy group and the last two the approximate carbon content in hundredths of a percent. The alloying elements used in these steels include manganese, silicon, nickel, chromium, molybdenum, vanadium, and boron. Other steels in this section are proprietary steels which may be modifications of the AISI grades. The alloying additions in these steels may provide deeper hardening, higher strength and toughness. These steels are available in a variety of finish conditions, ranging from hot- or cold-rolled to quenched-and-tempered. They are generally heat treated before use to develop the desired properties. Some steels in this group are carburized, then heat treated to produce a combination of high surface hardness and good core toughness. 2.3.0.2 Manufacturing Conditions — Forging — The alloy steels are only slightly more difficult to forge than carbon steels. However, maximum recommended forging temperatures are generally about 50EF lower than for carbon steels of the same carbon content. Slower heating rates, shorter soaking period, and slower cooling rates are also required for alloy steels. Cold Forming — The alloy steels are usually formed in the annealed condition. Their formability depends mainly on the carbon content and is generally slightly poorer than for unalloyed steels of the same carbon content. Little cold forming is done on these steels in the heat-treated condition because of their high strength and limited ductility. Machining — The alloy steels are generally harder than unalloyed steels of the same carbon content. As a consequence, the low-carbon alloy steels are somewhat easier to finish machine than their counterparts in the carbon steels. It is usually desirable to finish machine the carburizing and through-hardening grades in the final heat-treated condition for better dimensional accuracy. This often leads to two steps in machining: rough machining in the annealed or hot-finished condition, then finish machining after heat treating. The latter operation, because of the relatively high hardness of the material, necessitates the use of sharp, welldesigned, high-speed steel cutting tools, proper feeds, speeds, and a generous supply of coolant. Mediumand high-carbon grades are usually spheroidized for optimum machinability and, after heat treatment, may be finished by grinding. Many of the alloy steels are available with added sulfur or lead for improved machinability. However, resulfurized and leaded steels are not recommended for highly stressed aircraft and missile parts, because of drastic reductions in transverse properties. Welding — The low-carbon grades are readily welded or brazed by all techniques. Alloy welding rods comparable in strength to the base metal are used, and moderate preheating (200 to 600EF) is usually necessary. At higher carbon levels, higher preheating temperatures, and often postwelding stress relieving, are required. Certain alloy steels can be welded without loss of strength in the heat-affected zone provided that the welding heat input is carefully controlled. If the composition and strength level are such that the strength of the welded joint is reduced, the strength of the joint may be restored by heat treatment after welding. 2-10
MMPDS-01 31 January 2003 Heat Treatment — For the low alloy steels, there are various heat treatment procedures that can be applied to a particular alloy to achieve any one of a number of specific mechanical (for example tensile) properties. Within this chapter, there are mechanical properties for three thermal processing conditions: annealed, normalized, and quenched and tempered. The specific details of these three thermal processing conditions are reviewed in Reference 2.3.0.2.5. In general, the annealed condition is achieved by heating to a suitable temperature and holding for a specified period of time. Annealing generally softens the material, producing the lowest mechanical properties. The normalized condition is achieved by holding to a slightly higher temperature than annealing, but for a shorter period of time. The purpose of normalizing varies depending on the desired properties; it can be used to increase or decrease mechanical properties. The quenched and tempered condition, discussed in more detail below, is used to produce the highest mechanical properties while providing relatively high toughness. The mechanical properties for these three processing conditions for specific steels are as shown in Tables 2.3.1.0(c), (f), and (g). Maximum hardness in these steels is obtained in the as-quenched condition, but toughness and ductility in this condition are comparatively low. By means of tempering, their toughness is improved, usually accompanied by a decrease in strength and hardness. In general, tempering temperatures to achieve very high strength should be avoided when toughness is an important consideration. In addition, these steels may be embrittled by tempering or by prolonged exposure under stress within the “blue brittle” range (approximately 500 to 700EF). Strength levels that necessitate tempering within this range should be avoided. The mechanical properties presented in this chapter represent steels heat treated to produce a quenched structure containing 90 percent martensite at the center and tempered to the desired F tu level. This degree of through hardening is necessary (regardless of strength level) to insure the attainment of reasonably uniform mechanical properties throughout the cross section of the heat-treated part. The maximum diameter of round bars of various alloy steels capable of being through hardened consistently are given in Table 2.3.0.2. Limiting dimensions for common shapes other than round are determined by means of the “equivalent round” concept in Figure 2.3.0.2. This concept is essentially a correlation between the significant dimensions of a particular shape and the diameter of a round bar, assuming in each instance that the material, heat treatment, and the mechanical properties at the centers of both the respective shape and the equivalent round are substantially the same. For the quenched and tempered condition, a large range of mechanical property values can be achieved as indicated in Table 2.3.0.2. Various quench media (rates), tempering temperatures, and times can be employed allowing any number of processing routes to achieve these values. As a result of these processing routes, there are a large range of mechanical properties that can be obtained for a specific alloy. Therefore, the properties of a steel can be tailored to meet the needs for a specific component/application. Because of the potential for several different processing methods for these three conditions, the MIL, Federal, and AMS specifications do not always contain minimum mechanical property values (S-basis). They may contain minimum mechanical property values for one specific quenched and tempered condition. Those specifications cited in this Handbook that do not contain mechanical properties are identified with a footnote in Tables 2.3.1.0(a) and (b). The possible mechanical properties for these alloys covered in the specifications for the normalized, and quenched and tempered conditions in Table 2.3.0.2 are presented in Tables 2.3.1.0 (g 1 ) and (g 2 ). Users must rely on their own in-house specifications or appropriate industry specifications to validate that the required strength was achieved. Therefore, no statistical basis (A, B, S) for these values are indicated in Tables 2.3.1.0 (g 1 ) and (g 2 ). 2-11
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MMPDS-01 31 January 2003 100 Streng
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. MMPDS-01 31 January 2003 100 80 P
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MMPDS-01 31 January 2003 2.4.3 9Ni-
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