EFFECTS OF ALLOYING ELEMENTS IN STEEL Steel, by definition, is a combination of iron and carbon. Various other elements are added to steel to improve physical properties and to produce special properties, such as resistance to corrosion or heat. The specific effects of the addition of such elements are outlined below: ALUMINUM (AI). Aluminum is a deoxidizer and degasifier. It retards grain growth and is used to control austenitic grain size. In nitriding steels, aluminum aids in producing a uniformly hard and strong nitrided case when used in amounts of 1.00% - 1.25%. BORON (B). A potent and economical addition to a fully deoxidized steel; normally used in alloy steels. Added to the melt in extraordinarily small amounts (on the order of 0.001%), it has a powerful effect on hardenability. During times of shortages of nickel, chromium and molybdenum, boron is used to replace a portion of these elements which are used to increase hardenability. Boron cannot be added in large amounts as it caused hot-shortness (brittleness in steel in the hot forming range). BISMUTH (Bi). Used in the same manner as lead as an additive in small amounts to improve machinability in the faster machining grades of certain proprietary screw machine steels. Carbon (C). While carbon is not usually considered an alloying element, it is the most important constituent of steel. It increases the tensile strength, hardness and resistance to wear and abrasion. However, carbon lowers the ductility, machinability and toughness. CHROMIUM (Cr). Chromium increases tensile strength, toughness, hardness and hardenability, as well as resistance to wear and abrasion. It also increases resistance to corrosion and scaling at elevated temperatures. COBALT (Co). Cobalt increases strength and hardness in addition to permitting higher quenching temperatures. Also, it intensifies the effects of the other major elements in more complex steels. COLUMBIUM (Cb). Columbium in stainless steel has an effect similar to titanium and tantalum in making the steel more resistant to carbide precipitation and the resulting inter-granular corrosion. COPPER (Cu). Copper improves resistance to atmospheric corrosion and increases the tensile and yield strength with very little loss in ductility. IRON (Fe). Iron is the chief element from which the various steels are made. Pure iron lacks strength, is very soft and ductile and does not respond satisfactorily to heat treatment. Commercial iron normally contains other elements which produce the required physical properties. LEAD (Pb). Lead, while not strictly an alloying element, is added to improve machinability. It is almost completely insoluble in steel, and minute lead particles, dispersed throughout the steel, reduce friction where the cutting edge contacts the work. Also, the addition of lead improves chip-breaking formations. 56 MANGANESE (Mn). <strong>Man</strong>ganese is a deoxidizer and degasifier. It also reacts with sulphur to improve forgeability. <strong>Man</strong>ganese increase tensile strength, hardness, hardenability, resistance to wear and the rate of carbon penetration in carburizing. It also decreases the tendency toward scaling and distortion. MOLYBDENUM (Mo). Molybdenum increase strength, toughness, hardness, and hardenability as well as creep resistance and strength at elevated temperatures. It improves machinability, corrosion resistance and intensifies the effects of the other alloying elements. In hot-work steels, molybdenum increases red-hardness properties. NICKEL (Ni). Nickel increases strength and hardness with no loss of ductility and toughness. It also increases resistance to corrosion and scaling at elevated temperatures when introduced suitable quantities in high chromium stainless steels. NITROGEN (N). Important in several respects; 1) as a strong austenitizer which can substitute for a portion of the nickel in stainless steels; 2) as an element in nitriding and carbonitriding certain alloy steels containing aluminum or chromium to produce an extremely hard case; 3) added to the melt of some of the freemachining steels to enhance machinability by producing a vary fine chip. PHOSPHORUS (P). Phosphorus increases strength and hardness and improves machinability. However, it adds brittleness or cold-shortness to steel. SELENIUM (Se). Related to sulphur and tellurium in the chemical classification of elements, it has the similar effect of improving machinability when added in small amounts to some freemachining steels. SILICON (Si). Silicon is a deoxidizer and degasifier. Also, it increases the tensile and yield strength, forgeability, hardness and magnetic permeability. SULPHUR (S). Sulphur improves machinability in free-cutting steels. It decreases weldability, ductility and impact strength. Also, the addition of sulphur without sufficient manganese produces brittleness at red heat. TANTALUM (Ta). Tantalum is used as a stabilizing element in stainless steels. It has a high affinity for carbon and forms carbides which are uniformly dispersed throughout the steel, thus preventing localized depletion of carbon at grain boundaries. TITANIUM (Ti). Titanium, like tantalum and columbium, is added to stainless steels to make them resistant to harmful carbide precipitation. TUNGSTEN (W). Tungsten increases strength, toughness and hardness. At elevated temperatures tungsten steels have superior hotworking characteristics and greater cutting efficiency. VANADIUM (V). Vanadium increases strength, hardness, and impact resistance. By retarding grain growth vanadium permits higher quenching temperatures. It also improves the red-harness properties of high-speed mental cutting tools and intensifies the individual effects of other major elements.
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