Iron containing more than 0.15 wt.% C chemically combined is normally termed steel. This 0.15 wt.% C is a somewhat arbitrarily chosen borderline, and sometimes the nearly chemically pure “ingot iron” is referred to as mild steel. To make it even more confusing, the term mild steel is often also used as a synonym for low-carbon steels (see below), which are materials with 0.15 to 0.30 wt.% C. Steels that have been worked or wrought while hot are covered with a black scale, also called a mill scale made of magnetite (Fe3O4), and are sometimes called black iron. Cold-rolled steels have a bright surface, accurate cross section, and higher tensile and yield strengths. They are preferred for bar stock to be used for machining rods and for shafts. Carbon steels may be specified by chemical composition, mechanical properties, method of deoxidation, or thermal treatment and the resulting microstructure. However, wrought steels are most often specified by their chemical composition. No single element determines the characteristics of a steel; rather, the combined effects of several elements influence hardness, machinability, corrosion resistance, tensile strength, deoxidation of the solidifying metal, and the microstructure of the solidified metal. Standard wroughtsteel compositions for both carbon and alloy steels are designated by the SAE-AISI four-digit code, the last two digits of which indicate the nominal carbon content (Table 1).
Plain Carbon Steels
Carbon steels, also called plain carbon steels, are primarily Fe and C, with small amounts of Mn. Specific heat treatments and slight variations in composition will lead to steels with varying mechanical properties. Carbon is the principal hardening and strengthening element in steel. Actually, carbon increases hardness and strength and decreases weldability and ductility. For plain carbon steels, about 0.20 to 0.25 wt.% C provides the best machinability. Above and below this level, machinability is generally lower for hot-rolled steels. Plain carbon steels are usually divided into three groups:
(i) Low-carbon steels (e.g., AISI-SAE grades 1005 to 1030), or mild steels, contain up to 0.30 wt.% carbon. They are characterized by a low tensile strength and a high ductility. They are nonhardenable by heat treatment, except by surface hardening processes.
(ii) Medium-carbon steels (e.g., AISI-SAE grades 1030 to 1055) have between 0.31 wt.% and 0.55 wt.% C. They provide a good balance between strength and ductility. They are hardenable by heat treatment, but hardenability is limited to thin sections or to the thin outer layer on thick parts.
(iii) High-carbon steels (e.g., AISI-SAE grades 1060 to 1095) have between 0.56 wt.% and about 1.0 wt.% C. They are, of course, hardenable and are very suitable for wear-resistant and/or high-strength parts.
Low-carbon or mild steels. The lowest carbon group consists of the four AISI-SAE grades 1006, 1008, 1010, and 1015. All these grades consist of very pure iron with less than 0.30 wt.% C having a ferritic structure, and they exhibit the lowest carbon content of the plain carbon group. These steels exhibit a relatively low ultimate tensile strength and are not suitable for mechanically demanding applications. Both tensile strength and hardness rise with increases in carbon content and/or cold work, but such increases in strength are at the expense of ductility or the ability to withstand cold deformation. Hence mild steels are selected when cold formability is required. They are produced both as rimmed and killed steels. Rimmed steels are used for sheet, strip, rod, and wire where excellent surface finish or good drawing qualities are required, such as oil pans and other deep-drawn and formed products. Rimmed steels are also used for cold-heading wire for tacks, and rivets and low-carbon wire products. Aluminum-killed steels (i.e., AK steels) are used for difficult stampings or where nonaging properties are needed. Silicon-killed steels (i.e., SK steels) are preferred to rimmed steels for forging or heat-treating applications. With less than 0.15 wt.% C, the steels are susceptible to serious grain growth, causing brittleness, which may occur as the result of a combination of critical strain from cold work followed by heating to elevated temperatures. Steels in this group due to their ferritic structure do not machine freely and should be avoided for cut screws and operations requiring broaching or smooth finish on turning. The machinability of bar, rod, and wire products is improved by cold drawing. Mild steels are readily welded.
Carburizing steels. This second group consists of the 13 AISI-SAE grades 1016, 1017, 1018, 1019, 1020, 1021, 1022, 1023, 1024, 1025, 1026, 1027, and 1030. Because of their higher carbon content they exhibit enhanced tensile strength and hardness but at the expense of cold formability. For heat-treating purposes, they are known as carburizing or case-hardening steels. Killed steels are recommended for forgings, while for other uses semikilled or rimmed steel may be suitable. Rimmed steels can ordinarily be supplied with up to 0.25 wt.% C. Higher carbon content provides a greater core hardness with a given quench or permits the use of thicker sections. An increase in manganese improves the hardenability of both the core and the case along with machinability; in carbon steels this is the only change in composition that will increase case hardenability. For carburizing applications, grades AISI 1016, 1018, and 1019 are widely used for thin sections or water-quenched parts. AISI 1022 and 1024 are used for heavier sections or where oil quenching is desired, and AISI 1024 is sometimes used for such parts as transmission and rear axle gears. AISI 1027 is used for parts given a light case to obtain satisfactory core properties without drastic quenching. AISI 1025 and 1030, although not usually regarded as carburizing types, are sometimes used in this manner for larger sections or where greater core hardness is needed. For cold-formed or cold-headed parts, the lowest manganese grades (i.e., AISI 1017, 1020, and 1025) offer the best formability at their carbon level. AISI 1020 is used for fan blades and some frame members, and 1020 and 1025 are widely used for low-strength bolts. The next highest manganese types, i.e., AISI 1018, 1021, and 1026, provide increased strength. All carburizing steels can be readily welded or brazed.
Medium-carbon steels. This group consists of the 16 AISI-SAE grades 1030, 1033, 1034, 1035, 1036, 1038, 1039, 1040, 1041, 1042, 1043, 1045, 1046, 1049, 1050, and 1052 with a carbon content of between 0.31 and 0.55 wt.% C. They are usually selected for their higher mechanical properties and are frequently further hardened and strengthened by heat treatment or by cold work. They are usually produced as killed steels and are suitable for a wide variety of automotive-type applications. Increases in the mechanical properties required in section thickness or in depth of hardening ordinarily indicate either higher carbon or manganese content or both. The heat-treating practice preferred, particularly the quenching medium, has a great effect on the steel selected. In general, any of the grades over 0.30 wt.% C may be selectively hardened by induction heating or flame methods. The lower-carbon and manganese steels in this group find usage for certain types of cold-formed parts. AISI 1030 is used for shift and brake levers. AISI 1034 and 1035 are used in the form of wire and rod for cold upsetting such as bolts, and AISI 1038 for bolts and studs. The parts cold-formed from these steels are usually heat treated prior to use. Stampings are generally limited to flat parts or simple bends. The higher-carbon AISI 1038, 1040, and 1042 are frequently cold drawn to specified physical properties for use without heat treatment for some applications such as cylinder head studs. Any of this group of steels may be used for forgings, the selection being governed by the section size and the physical properties desired after heat treatment. Thus, AISI 1030 and 1035 are used for shifter forks and many small forgings where moderate properties are desired, but the deeper-hardening AISI 1036 is used for more critical parts where a higher strength level and more uniformity are essential, such as some front-suspension parts. Forgings such as connecting rods, steering arms, truck front axles, axle shafts, and tractor wheels are commonly made from the AISI 1038 to 1045 group. Larger forgings at similar strength levels need more carbon and perhaps more manganese; for instance, crankshafts are made from AISI 1046 and 1052. These steels are also used for small forgings where high hardness after oil quenching is desired. Suitable heat treatment is necessary on forgings from this group to provide machinability. It is also possible to weld these steels by most commercial methods, but precautions should be taken to avoid cracking from too rapid cooling.
High-carbon steels. These are the 14 AISI-SAE grades 1055, 1060, 1062, 1064, 1065, 1066, 1070, 1074, 1078, 1080, 1085, 1086, 1090, and 1095. These steels contain more carbon than is required to achieve maximum “as quenched” hardness. They are used for applications requiring improved wear resistance for cutting edges and to make springs. In general, cold forming cannot be used with these steels, and forming is only limited to flat stampings and springs coiled from small-diameter wire. Practically all parts from these steels are heat treated before use. Uses in the spring industry include AISI 1065 for pretempered wire and 1066 for cushion springs of hard-drawn wire; 1064 may be used for small washers and thin stamped parts, 1074 for light flat springs formed from annealed stock, and 1080 and 1085 for thicker flat springs. 1085 is also used for heavier coil springs. Finally, valve spring wire and music wire are special products.
Easily machinable carbon steels. The three AISI-SAE grades 1111, 1112, and 1113 are intended for applications where easy machining is the primary requirement. They are characterized by a higher sulfur content than comparable carbon steels, machinability improving within the group as sulfur increases but at the expense of cold-forming, weldability, and forging properties. In general, the uses are similar to those for carbon steels of similar carbon and manganese content. These steels are commonly known as Bessemer screw stock and are considered the best machining steels available. Although of excellent strength in the cold-drawn condition, they have an unfavorable property of cold shortness and are not commonly used for vital parts. These steels may be cyanided or carburized, but when uniform response to heat treating is necessary, open-hearth steels are recommended. The nine AISI-SAE grades 1109, 1114, 1115, 1116, 1117, 1118, 1119, 1120, and 1126 are used where a combination of good machinability and more uniform response to heat treatment is required. The lower-carbon varieties are used for small parts that are to be cyanided or carbonitrided. AISI 1116, 1117, 1118, and 1119 contain more manganese for better hardenability, permitting oil quenching after case-hardening heat treatments. The higher-carbon 1120 and 1126 provide more core hardness when this is needed. Finally, grades AISI-SAE 1132, 1137, 1138, 1140, 1141, 1144, 1145, 1146, and 1151 exhibit a composition similar to that of carbon steels of the same carbon level, except they have a higher sulfur content. They are widely used for parts where large amounts of machining are necessary or where threads, splines, or other contours present special problems with tooling. AISI 1137 is widely used for nuts and bolts. The higher-manganese grades 1132, 1137, 1141, and 1144 offer greater hardenability, the higher-carbon types being suitable for oil quenching for many parts. All these steels may be selectively hardened by induction or flame heating.
No comments:
Post a Comment