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Effects Of Alloy Steel Elements


Steel is basically iron alloyed to carbon with certain additional elements to give the required properties to the finished melt. Listed below is a summary of the effects various alloying elements in steel.



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Iron (Fe)

Although it lacks strength, is very soft and ductile, and does not respond to heat treatment to any degree. Iron is the primary element in steel. With the addition of other alloying elements required mechanical properties can be achieved.

Aluminum(Al) Hardens considerably by solid solution.
Boron (B)

As boron possesses a high cross section for neutron absorption, it is used to alloys steels for controllers and shields of atomic energy plants. Austenitic 18/8 CrNi steels can be raised to increased yield point and strength with boron by means of precipitation hardening, but corrosion resistance is reduced in the process. Precipitation induced by room temperature increases the strength properties of high- temperature austenitic steel types in the high temperature range. In structural steels, this element improves through hardening and thus causes an increase in core strength in casehardening steels. A reduction in weldability must be expected in boron-alloyed steels. It improves corrosion resistance.

Carbon(C)

The basic metal, iron, is alloyed with carbon to make steel and has the effect of increasing the hardness and strength by heat treatment but the addition of carbon enables a wide range of hardness and strength. Directly influences the hardness and strength of the steel, decreases toughness.

Chromiun(Cr)

Chromium is added to the steel to increase resistance to oxidation. This resistance increases as more chromium is added. 'Stainless Steel' has approximately 11% chromium and a very marked degree of general corrosion resistance when compared with steels with a lower percentage of chromium. When added to low alloy steels, chromium can increase the response to heat treatment, thus improving hardenability and strength. Improves Hardenability, high temperature strength, wear resisitance and even corrosion resistance(Cr>14%).

Cobalt(Co)

Cobalt becomes highly radioactive when exposed to the intense radiation of nuclear reactors, and as a result, any stainless steel that is in nuclear service will have a cobalt restriction, usually aproximately 0.2% maximum. This problem is emphasised because there is residual cobalt content in the nickel used in producing these steels. Improves retention of hardness at high temperatures and magnetic permeability.

Copper (Cu)

Copper is normally present in stainless steels as a residual element. However it is added to a few alloys to produce precipitation hardening properties.

Lead (Po)

Is used in steel to improve machinability. In small amounts of .15 - .30% and finely divided and distributed, it has no known effect on the mechanical properties of steel.

Manganese (Mn)

Manganese is added to steel to improve hot working properties and increase strength, toughness and hardenability. Manganese, like nickel, is an austenite forming element and has been used as a substitute for nickel in the A.I.S.I 200 Series of Austenitic stainless steels (e.g. A.I.S.I 202 as a substitute for A.I.S.I 304) It improves hardenability , ductility  and wear resistance. Mn eliminates formation ofharmful iron sulfides, increasing strength at high temperatures.

Molybdenum(Mo)

Molybdenum, when added to chromium-nickel austenitic steels, improves resistance to pitting corrosion especially by chlorides and sulphur chemicals. When added to low alloy steels, molybdenum improves high temperature strengths and hardness. When added to chromium steels it greatly diminishes the tendency of steels to decay in service or in heat treatment. Increases yield point and strength. Improves Hardenability and wear resistance. Manganese Lowers effects of Iron sulphides, increases hardenability, wear resistance.

Nickel(Ni)

Nickel is added in large amounts, over about 8%, to high chromium stainless steel to form the most important class of corrosion and heat resistant steels. These are the austenitic  stainless steels, typified by 18-8, where the tendency of nickel to form austenite is responsible for a great toughness and high strength at both high and low temperatures. Nickel also improves resistance to oxidation and corrosion. It increases toughness at low temperatures when added in smaller amounts to alloy steels. Increases toughness at lower temperatures, slows the corrosion process.

Niobium (Columbium)

Niobium is added to steel in order to stabilise carbon, and as such performs in the same way as described for titanium. Niobium also has the effect of strengthening steels and alloys for high temperature service.

Nitrogen

Nitrogen has the effect of increasing the austenitic stability of stainless steels and is, as in the case of nickel, an austenite forming element. Yield strength is greatly improved when nitrogen is added to austenitic stainless steels.

Phosphorus (P)

Phosphorus is usually added with sulphur to improve machinability in low alloy steels, phosphorus, in small amounts, aids strength and corrosion resistance. Experimental work shows that phosphorus present in austenitic stainless steels increases strength. Phosphorus additions are known to increase the tendency to cracking during welding.

Selenium Selenium is added to improve machinability.
Silicon(Si)

Silicon is used as a deoxidising (killing) agent in the melting of steel, as a result, most steels contain a small percentage of silicon. Silicon contributes to hardening of the ferritic phase in steels and for this reason silicon killed steels are somewhat harder and stiffer than aluminium killed steels. It is a deoxidizing agent. Increases strength and wear resistance.It is of importance that certain elements are as low as possible. Elements like Hydrogen, Oxygen, Nitrogen, Phosphorous, Sulphur create defects inside & also reduce the mechanical properties of steels.

Sulphur (S)

When added in small amounts sulphur improves machinability but does not cause hot shortness. Hot shortness is reduced by the addition of manganese, which combines with the sulphur to form manganese sulphide. As manganese sulphide has a higher melting point than iron sulphide, which would form if manganese were not present, the weak spots at the grain boundaries are greatly reduced during hot working.

Titanium (Ti) The main use of titanium as an alloying element in steel is for carbide stabilisation. It combines with carbon to for titanium carbides, which are quite stable and hard to dissolve in steel, this tends to minimise the occurrence of inter-granular corrosion, as with A.I.S.I 321, when adding approximately 0.25%/0.60% titanium, the carbon combines with the titanium in preference to chromium, preventing a tie-up of corrosion resisting chromium as inter-granular carbides and the accompanying loss of corrosion resistance at the grain boundaries. It improves strength and corrosion resistance, limits austenite grain size. Also increases the melting point. Chemically similar to niobium and has similar effects.
Tungston(W)

Promotes red hardness and hot strength in addition to producing dense grain and a keen cutting edge. These properties make Tungsten Steels very useful for hot working applications such as cutting tools when the steel is hot enough to be low red in color. Improves toughness, grainstructure, high temperature strength, wear resistance, etc.

Vanadium(V)

Is a strong deoxidizer and promotes fine grain structure. It helps steel resist softening at elevated temperatures and seems to resist shock better than steels without it. Increases wear resistance, high temperature strength, retention of hardness etc.

Zirconium (Zr)

Increases strength and limits grain sizes. Highly effective hardenability agent, improves deformability and machinability.

It is of importance that certain elements are as low as possible.Elements like Hydrogen, Oxygen, Nitrogen, Phosphorous, sulphur. create defects inside also reduce the mechanical properties of steels. An alloy is a substance consisting of two or more chemical elements with one of them being a metal thus resulting in desired metallic properties.Steel is an alloy of Iron and carbon.