Steels that contain specified amounts of alloying elements other than carbon and the commonly accepted amounts of manganese, copper, silicon, sulfur, and phosphorus are known as alloy steels. Alloying elements are added to change mechanical or physical properties. A steel is considered to be an alloy when the maximum of the range given for the content of alloying elements exceeds one or more of these limits: 1.65 wt.% Mn, 0.60 wt.% Si, or 0.60 wt.% Cu, or when a definite range or minimum amount of any of the following elements is specified or required within the limits recognized for constructional alloy steels: aluminum, chromium (up to 3.99%), cobalt, columbium, molybdenum, nickel, titanium, tungsten, vanadium, zirconium, or other element added to obtain an alloying effect. According to the previous definition, strictly speaking, tool and stainless steels are also considered alloy steels. However, the term alloy steel is reserved for those steels that contain a minute amount of alloying elements and that usually depend on thermal treatment to develop specific properties. Subdivisions for most steels in this family include through-hardening grades, which are heat treated by quenching and tempering and are used when maximum hardness and strength must extend deep within a part, while carburizing grades are used where a tough core and relatively shallow, hard surface is needed. After a surface-hardening treatment such as carburizing or nitriding for nitriding alloys, these steels are suitable for parts that must withstand wear as well as high stresses. Cast steels are generally through-hardened, not surface treated. Carbon content and alloying elements influence the overall characteristics of both types of alloy steels. Maximum attainable surface hardness depends primarily on the carbon content. Maximum hardness and strength in small sections increase as carbon content increases, up to about 0.7 wt. % C. However, carbon content greater than 0.3% can increase the possibility of cracking during quenching or welding. Alloying elements primarily influence hardenability. They also influence other mechanical and fabrication properties including toughness and machinability. Lead additions (i.e., 0.15 to 0.35 wt.% Pb) greatly improve the machinability of alloy steels by high-speed-tool steels. For machining with carbide tools, calcium-treated steels are reported to double or triple tool life in addition to improving surface finish. Alloy steels are often specified when high strength is needed in moderate to large sections. Whether tensile or yield strength is the basis of design, thermally treated alloy steels generally offer high strength-to-weight ratios. In general, the wear resistance can be improved by increasing the hardness of an alloy, by specifying an alloy with greater carbon content, or by using nitrided parts, which have better wear resistance than would be expected from the carbon content alone. Fully hardened and tempered, low-carbon (i.e., 0.10 to 0.30 wt.% C) alloy steels have a good combination of strength and toughness, at both room and low temperatures.
Carburizing alloyed steels. The properties of carburized and hardened cases depend on the carbon and alloy content, the structure of the case, and the degree and distribution of residual stresses. The carbon content of the case depends on the carburizing process and on the reactivity of iron and of the alloying elements to carburization. The original carbon content
of the steel has little or no effect on the carbon content produced in the case; hence the last two digits in the AISI-SAE specification numbers are not meaningful as far as the case is concerned. The hardenability of the case, therefore, depends on the alloy content of the steel and the final carbon content produced by carburizing. With complete carbide solution, the effect of alloying elements on the hardenability of the case is about the same as the effect of these elements on the hardenability of the core. As an exception to this statement, any element that inhibits carburizing may reduce the hardenability of the case. Some elements that raise the hardenability of the core may tend to produce more retained austenite and consequently somewhat lower hardness in the case. Alloy steels are frequently used for case hardening because the required surface hardness can be obtained by moderate quenching speeds. Slower quenching may mean less distortion than would be encountered with water quenching. It is usually desirable to select a steel that will attain a minimum surface hardness of 58 or 60 HRC after carburizing and oil quenching. Where section sizes are large, a high-hardenability alloy steel may be necessary, whereas for medium and light sections, low-hardenability steels will suffice.
The case-hardening alloy steels may be divided into two classes: high- and mediumhardenability case steels.
High-hardenability case steels. The five AISI-SAE grades 2500, 3300, 4300, 4800, and 9300 are high-alloy steels; hence both the case and the core possess a high hardenability. They are used particularly for carburized parts with thick sections, such as pinions and heavy gears. Good case properties can be obtained by oil quenching. These steels are likely to have retained austenite in the case after carburizing and quenching, and hence refrigeration may be required.
Medium-hardenability case steels. The AISI-SAE grades 1300, 2300, 4000, 4100, 4600, 5100, 8600, and 8700 have medium hardenability, which means that their hardenability is intermediate between that of plain carbon steels and the higher-alloy carburizing steels discussed previously. In general, these steels can be used for average-size case-hardened produced by oil quenching. The core properties of case-hardened steels depend on both the carbon and alloy content of the steel. Each of the general types of alloy case-hardening steel is usually made with two or more carbon contents to permit different hardenability in the core. The most desirable hardness for the core depends on the design and the type of application. Usually, where high compressive loads are encountered, relatively high core hardness is beneficial in supporting the case. Low core hardnesses may be required if great toughness is important.
The case-hardening steels may be divided into three general classes, depending on the hardenability of the core:
(i) low-hardenability core such as AISI-SAE 4017, 4023, 4024, 4027, 4028, 4608, 4615, 4617, 8615, and 8617;
(ii) medium-hardenability core such as AISI-SAE 1320, 2317, 2512, 2515, 3115, 3120, 4032, 4119, 4317, 4620, 4621, 4812, 4815, 5115, 5120, 8620, 8622, 8720, and 9420;
(iii) high-hardenability core such as AISI-SAE 2517, 3310, 3316, 4320, 4817, 4820, 9310, 9315, and 9317.
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