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% Mn, 0.60% Si, or 0.60% 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 (to 3.99%), cobalt, columbium, molybdenum, nickel, titanium, tungsten, vanadium, zirconium or other element added to obtain an alloying effect.
Technically, then, tool and stainless steels are alloy steels. In this chapter, however, the term alloy steel is reserved for those steels that contain a modest amount of alloying elements and that usually depend on thermal treatment to develop specific properties. With proper heat treatment, for example, tensile strength of certain alloy steels can be raised from about 55,000 psi to nearly 300,000 psi.
Subdivisions for most steels in this family include “through-hardenable” and “carburizing” grades (plus several specialty grades such as nitriding steels). Through-hardening grades — which are heat treated by quenching and tempering — are used when maximum hardness and strength must extend deep within a part. Carburizing grades are used where a tough core and relatively shallow, hard surface are 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 carbon content. Maximum hardness and strength in small sections increase as carbon content increases, up to about 0.7%. However, carbon contents 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 (0.15 to 0.35%) substantially improve 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.
Few exact rules exist for selecting through-hardening or surface-hardening grades of alloy steels. In most cases, critical parts are field tested to evaluate their performance. Parts with large sections — heavy forgings, for example — are often made from alloy steels that have been vacuum degassed. While in a molten state, these steels are exposed to a vacuum which removes hydrogen and, to a lesser degree, oxygen and nitrogen.
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. For applications requiring maximum ductility, alloys with low sulfur levels (<0.01%) can be supplied by producers using ladle-refining techniques.
In general, wear resistance can be improved by increasing the hardness of an alloy, by specifying an alloy with greater carbon content (without increasing hardness), or by both. The surface of a flame-hardened, medium-carbon steel, for example, is likely to have poorer wear resistance than the carbon-rich case of a carburized steel of equal hardness. Exceptions are nitrided parts, which have better wear resistance than would be expected from the carbon content alone.
For any combination of alloy steel and heat treatment, three factors tend to decrease toughness: low service temperature, high loading rates, and stress concentrations or residual stress. The general effects of these three conditions are qualitatively similar, so low-temperature impact tests (to -50°F) are useful for many applications as toughness indicators under various service conditions and temperatures.
Fully hardened-and-tempered, low-carbon (0.10 to 0.30% C) alloy steels have a good combination of strength and toughness, both at room and low temperature. Care must be taken in heat treatment of certain alloy-steel grades, however, because toughness may be decreased substantially by temper brittleness — a form of embrittlement developed by slow cooling through the range of 900 to 600°F, or by holding or tempering in this range.
When liquid quenching is impractical (because of the danger of cracking or distortion, or because of cost), various low-carbon nickel or nickel-molybdenum steels in the normalized-and-tempered condition can be used for low-temperature service.
Wrought alloy steels (and carbon steels) are classified by a series of AISI and SAE numbers that designate composition and alloy type. Letters, which are used in addition to the four-digit designations, include the suffix “H,” used for steel produced to specific hardenability limits (which allows wider composition ranges for certain alloying elements), and the prefix “E,” which indicates a steel made by the basic electric-furnace method. Other specifications, such as those issued by ASTM, specify minimum properties for critical structural, pressure-vessel, and nuclear applications.
ASTM specifications classify cast alloy steels by relating the steel to the mechanical properties and intended service condition. Chemical analysis is secondary. There are ASTM specifications for general use such as A27 or A148 when mechanical properties are critical. For low-temperature service, A352 or A757 is recommended when toughness is important. For weldability, A216 is specified when fabrication is critical, and for pressure service, A217 or A389 is recommended when a number of properties are important. Still other ASTM alloy steels are available for special applications. Other specifications such as SAE J435 are used for cast steels in automotive applications. A summary of steel-casting specifications is available from the Steel Founders’ Society of America, Des Plaines, Ill.