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.
Carbon is alloyed with base metal Iron to make steel which affects the hardness and strength of material. Thus the addition of carbon enables a wide range of hardness and strength.
Manganese is added to improve hot working properties and increase strength, toughness and hardenability. Manganese, like nickel, is an austenite forming element and can been used as a substitute for nickel.
Chromium is added to the steel to increase corrosion resistance. The 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.
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, 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.
Molybdenum, when added to chromium-
Used as stabilizing elements in stainless steels. Each has a high affinity for carbon and forms carbides, which are uniformly dispersed throughout the steel. Thus, localized precipitation of carbides at grain boundaries is prevented. Compared to chromium, carbon has higher affinity to titanium to form inter-
Phosphorus is usually added with sulphur to improve machinability in low alloy steels, phosphorus, in small amounts, increases strength and corrosion resistance. Experimental work shows that phosphorus present in steel increases strength. Phosphorus additions are known to increase the tendency to cracking during welding.
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.
Selenium is added to improve machinability.
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 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.
Silicon is used as a deoxidizing (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 aluminum killed steels.
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 approximately 0.2% maximum. This problem is emphasized because there is residual cobalt content in the nickel used in producing these steels.
Chemically similar to niobium and has similar effects.
Copper is normally present in stainless steels as a residual element. However it is added to a few alloys to produce precipitation-
Increases strength, hardness, wear resistance and resistance to shock impact. It retards grain growth, permitting higher quenching temperatures. It also enhances the red-
Increases strength, wear resistance, hardness and toughness. Tungsten steels have superior hot-