ALLOYING ELEMENTS IN STEEL

Alloying elements are classified according to their faculty in forming carbides, austenite or ferrite, and with a view to the purpose for which they are added to ordinary steels. According to the alloying percentage, every element can impart unique and specific characteristics to the steel. The combination of various elements, as utilized in modern metallurgy, can enhance this effect. However, certain combinations of alloying elements may result in constituents which, far from producing a favorable cumulative effect with regard to a certain property, may counteract each other. The mere presence of alloying elements in steel is but a basic condition for the desired characteristic which can be obtained only by proper processing and heat treatment. The principal effect and influences of alloying and accompanying elements are outlined below.

CARBON ( C)
ALUMINUM ( AL )
ANTIMONY ( Sb )
ARSENIC ( As )
BERYLLIUM ( Be )
BORON ( B )
CALCIUM ( Ca )
CHROMIUM ( Cr )
COBALT ( Co )
COPPER ( Cu )
HYDROGEN ( H )
LEAD ( Pb )
MANGANESE ( Mn )
MOLYBDENUM ( Mo )
NICKEL ( Ni )
NITROGEN ( N )
OXYGEN ( O )
PHOSPHORUS ( P )
SILICON ( Si )
SULFUR ( S )
TIN ( Sn )
VANADIUM ( V )
WOLFRAM ( W = TUNGSTEN Tu )

CHROMIUM ( Cr )
Of all the common alloying elements, chromium ranks near the top in promoting harden ability. It makes the steel apt for oil or air hardening. It reduces the critical cooling rate required for martensite formation, increases harden ability and thus improves the aptitude for heat treatment. On the other hand, impact strength is weakened. Chromium forms carbides that improve edge-holding capacity and wear resistance. High temperature strength and resistance to high pressure hydrogenation are also enhanced. Non-scaling properties are boosted by increasing chromium contents. A chromium content of 3.99% has been established as the maximum limit applicable to constructional alloy steels. Contents above this level place steels in the category of heat resisting or stainless steels.

COBALT ( Co )
Does not create carbides, it inhibits grain growth at elevated temperatures and considerably improves the retention of hardness and hot strength. Therefore it is a frequent alloy constituent in high speed steels, hot work steels and high-temperature steels. It encourages the formation of graphite. It also intensifies the individual effects of other major elements in more complex steels.

COPPER ( Cu )
Is added to steel primarily to improve the steel's resistance to atmospheric corrosion. Amounts added to steels for this purpose typically range from 0.20% to 0.50%. Copper is scarcely used for steel alloys because it concentrates under the oxide layer and, by penetrating the grain boundary, imparts the steel a surface liable to suffer in hot working operations. It is therefore regarded as being harmful to steel.

HYDROGEN ( H )
Harmful to steel, it causes embrittlement by decreasing of elongation and reduction of area without any increase of yield point and tensile strength. It is the source of the redoubtable snow-flake formation and favors the formation of ghost lines. Atomic hydrogen engendered by pickling penetrates into the steel and forms blowholes. At elevated temperatures moist hydrogen acts as a decarburizing agent.

LEAD ( Pb )
Used in quantities of 0.15% to 0.35% for free-machining steel as its very fine, suspension-like distribution ( lead is insoluble in steel ) permits to obtain short chips and clean surfaces, hence an improved machinability. Lead amounts as mentioned above will in no way affect the mechanical properties of steel.

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