Technically, every steel is an alloy, but not all of them have the “alloy steel” designation. To be called alloy steel, other elements must be intentionally added to the iron and carbon composition. A small percentage of alloying elements — typically, no more than 5% — is added to the mix, and these metals can improve corrosion resistance, machinability, and many other properties.
The alloying of carbon steel with other elements to obtain a wide range of desired properties is a mature science. The following summarizes the known effects of adding certain elements to steel
Carbon: In general, increasing the carbon content of steel alloys produces higher ultimate strength and hardness but may lower ductility and toughness. Carbon also increases air-hardening tendencies and weld hardness. In low-alloy steel for high-temperature applications, the carbon content is usually restricted to a maximum of about 0.15 percent in order to assure optimum ductility for welding, expanding, and bending operations. An increasing carbon content lessens the thermal and electrical conductivities of steel.
Phosphorus: High phosphorus content has an undesirable effect on the properties of carbon steel, notably on shock resistance and ductility (see the section on temper embrittlement). Phosphorus is effective, however, in improving machineability. In steels, it is normally controlled to less than 0.04 weight percent.
Silicon: Used as a deoxidizing agent, silicon increases the tensile strength ofs teel without increasing brittleness when limited to less than about 2 percent. Silicon increases resistance to oxidation, increases electrical resistivity, and decreases hysteresis losses. Thus it is used for electrical applications. Adding silicon may reduce creep rupture strength.
Manganese: Manganese is normally present in all commercial steels. The manganese combines with sulfur, thus improving hot-working characteristics. In alloy steels, manganese decreases the critical cooling rate to cause a hardened or martensitic structure and thus contributes to deep-hardening.
Nickel: As an alloying element in alloy steels, nickel is a ferrite strengthener and toughener and is soluble in all proportions. Nickel steels are easily hardened because nickel lowers the critical cooling rate necessary to produce hardening on quenching. In heat-treated steel, nickel increases the strength and toughness.In combination with chromium, nickel produces alloy steels possessing higher impact and fatigue resistance than can be obtained with straight carbon steels.
Chromium: As an alloying element in steel, chromium is miscible in iron as a solid solution, and forms a complex series of carbide compounds. Chromium is essentially a hardening element and is frequently used with a toughening element such as nickel to produce superior mechanical properties. At higher temperatures, chromium contributes increased strength and is ordinarily used in conjunction with molydenum. Additions of chromium significantly improve the elevated temperature oxidation resistance of steels.
Molybdenum: In steel, molybdenum can form a solid solution with the iron and, depending on the molybdenum and carbon content, can also form a carbide. A deeper hardening steel results. The molybdenum carbide is very stable and is responsible for matrix strengthening in long-term creep service.
Vanadium: This element is one of the strong carbide formers. It dissolves to some degree in ferrite, imparting strength and toughness. Vanadium steels show a much finer grain structure than steels of a similar composition without vanadium.
Boron: Boron is usually added to steel to improve hardenability; that is, to increase the depth of hardening during quenching.
Aluminum: Aluminum is widely used as a deoxidizer in molten steel and for controlling grain size. When added to steel in controlled amounts, it produces a fine grain size.
Sulfur: Present to some degree in all steel (less than 0.04 weight percent), sulfur forms a nonmetallic inpurity that, in large amounts, results in cracking during forming at high temperatures (hot shortness). Combining it with managanese forms a MnS compound that is relatively harmless.
Copper: Copper dissolves in steel and strengthens the iron as a substitutional element. The use of copper in certain alloys increases resistance to atmospheric corrosion and increases yield strength. However, excessive amounts of copper (usually above 0.3 percent are harmful to elevated temperature performance since the lower melting point element segregates to grain boundaries and locally melts (liquates), causing intergranular separation under applied stress.
In general, when used in combination, alloying elements may complement each other and give greater overall benefits than when used singly in much larger quantities. There are literally hundreds of wrought grades of steel that range in composition with the variation of the many major and minor alloying elements.
The simplest of these classes is known as plain carbon steel, with carbon varying between approximately 0.05 and 1.0 weight percent.Within this broad range fall three general groups according to carbon content; they are defined as follows:
1. Low carbon steels—0.05 to 0.25 percent carbon
2. Medium carbon steels—0.25 to 0.50 percent carbon
3. High carbon steels—0.50 percent and greater carbon content
Alloy steels are generally considered to be steels to which one or more alloying elements, other than carbon, have been added to give them special properties that are different than those of straight carbon steels. From the standpoint of composition, steel is considered to be an alloy steel when amounts of manganese, silicon, or copper exceed the maximum limits for the carbon steels, or when purposeful addition of minimum quantities of other alloying elements are added. These could be chromium, molybdenum, nickel, copper, cobalt, niobium, vanadium, or others.
1. Low carbon steels—0.05 to 0.25 percent carbon
2. Medium carbon steels—0.25 to 0.50 percent carbon
3. High carbon steels—0.50 percent and greater carbon content
Alloy steels are generally considered to be steels to which one or more alloying elements, other than carbon, have been added to give them special properties that are different than those of straight carbon steels. From the standpoint of composition, steel is considered to be an alloy steel when amounts of manganese, silicon, or copper exceed the maximum limits for the carbon steels, or when purposeful addition of minimum quantities of other alloying elements are added. These could be chromium, molybdenum, nickel, copper, cobalt, niobium, vanadium, or others.
The next higher class of alloyed steel useful to the piping industry is ferritic and martensitic stainless steels. These are steels alloyed with chromium contents above about 12 percent. Because of the chromium, these materials possess good corrosion resistance. They retain a ferritic (BCC) crystal structure, allowing the grades to be hardened by heat treatment.
When sufficient nickel is added to iron-chromium alloys, an austenitic (FCC) structure is retained at room temperature. Austenitic stainless steels possess an excellent combination of strength, ductility, and corrosion resistance. These steels cannot be hardened by quenching, since the austenite does not transform to martensite.
When sufficient nickel is added to iron-chromium alloys, an austenitic (FCC) structure is retained at room temperature. Austenitic stainless steels possess an excellent combination of strength, ductility, and corrosion resistance. These steels cannot be hardened by quenching, since the austenite does not transform to martensite.
A stronger type of stainless steel has been developed which takes advantage of precipitation reactions within the metal matrix made possible by addition of elements such as aluminum, titanium, copper, and nitrogen. These materials are referred to as precipitation—hardenable stainless steels. Both martensitic and austenitic stainless steels can be enhanced in this manner.As annealed, these materials are soft and readily formed. When fully hardened, through aging heat treatments, they attain their full strength potential.
What’s the difference between high and low alloy?
Most people say that high alloy is any steel with alloying elements (not including carbon or iron) that make up more than 8% of its composition. These alloys are less common, because most steel only dedicates a few percent to the additional elements. Stainless steel is the most popular high alloy, with at least 10.5% chromium by mass. This ratio gives stainless steel more corrosion resistance, with a coating of chromium oxide to slow down rusting.
Meanwhile, low alloy steel is only modified slightly with other elements, which provide subtle advantages in hardenability, strength, and free-machining. By lowering the carbon content to around 0.2%, the low alloy steel will retain its strength and boast improved formability.
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