Alloy steels

Alloy steel is a general name for steels that owe their distinctive properties to elements other than carbon. They are generally classified into two major categories:
Low-alloy steel possesses similar microstructures to and requires similar heat treatments to plain carbon steels (see Section 4.3.2 on micro-alloyed steel).
High-alloy steel may be defined as a steel having enhanced properties owing to the presence of one or more special elements or a larger proportion of element than is normally present in carbon steel. This section is concerned primarily with high-alloy steels.
Alloy steels usually take the name of the element or elements,in varying percentages,having the greatest influence on the characteristics of thealloy.

Chromium Increased hardness and resistance to corrosion.
Cobalt Increased hardness,especially at high temperatures.
Manganese High tensile strength,toughness and resistance to wear.
Molybdenum Increased hardness and strength at high temperatures.
Nickel Increased tensile strength,toughness, hardness and resistance to fatigue.
Niobium Strong carbide forming effect; increases tensile strength and improves ductility.
Silicon Used as a deoxidizing agent,and has the slight effect of improving hardness.
Titanium Strong carbide forming element.
Tungsten Greater hardness,especially at high temperatures; improved tensile strength and resist- ance to wear.
Vanadium Increased toughness and resistance to fatigue.
Correct heat treatment is essential to develop the properties provided by alloying elements.

There are many alloy steels containing different combinations and percentages of alloying elements, of which some of the most popular are as follows:
High-tensile steel Used whenever there is an essential need for an exceptionally strong and tough steelcapable of withstanding high stresses. The mainalloying metals used in its manufacture are nickel,chromium and molybdenum and such steels are often referred to as nickel-chrome steels. The exact percentage of these metals used varies according to the hardening processes to be used and the properties desired. Such steels are used for gear shafts,engine parts and all other parts subject to high stress.
High-speed steels These are mostly used for cutting tools because they will withstand intense heat generated by friction and still retain their hardness at high temperatures. It has been found that by adding tungsten to carbon steel,an alloy steel is formed which will retain a hard cutting edge at high temperatures. High-speed steels are based on tungsten or molybdenum or both as the primary heat-resisting alloying element; chromium gives deep hardening and strength,and vanadium adds hardness and improves the cutting edge.

Manganese steel An addition of manganese to steel produces an alloy steel which is extremely tough and resistant to wear. It is used extensively in the manufacture of chains,couplings and hooks. Chrome-vanadium steel This contains a small amount of vanadium which has the effect of intensifying the action of the chromium and the manganese in the steel. It also aids in the formation of carbides, hardening the alloy and increasing its ductility.
These steels are valuable where a combination of strength and ductility are desired. They are often used for axle half-shafts,connecting rods,springs, torsion bars,and in some cases hand tools.
Silicon-manganese steel This is a spring steel using the two elements of manganese and silicon. These steels have a high strength and impact resistance and are used for road springs and valve springs.