Niobium (Colombium) (Nb)
Niobium is added to steel in order to stabilize 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 austenitic forming element. Yield strength is greatly improved when nitrogen is added to stainless steels.
Silicon is used as a deoxidizing (killing) agent in the melting of steel, and 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 normally a residual cobalt content in the nickel used in producing these steels.
Copper is normally present in stainless steel as a residual element. However, it is added to a few alloys to produce precipitation hardening properties or to enhance corrosion resistance particularly in sea water environments.
When added in small amounts sulphur improves machinability but does cause hot shortness. Hot shortness is reduced by the addition of manganese sulphide. Manganese sulphide has a higher melting point than iron sulphide, which would form if manganese were not present. The weak spots at the grain boudaries are greatly reduced during hot working.
Selenium is added to improve machinability.
The basic metal, iron, is alloyed with carbon to make steel and has the effect of increasing the hardness and strength of iron. Pure iron cannot be hardened or strengthened by heat treatment but the addition of carbon enables a wide range of hardness and strength.
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 AISI 200 Series of Austenitic Stainless Steels e.g. AISI 202 as a substitute for AISI 304.
Chromium is added to steel to increase resistance to oxidation. This resistance increases as more chromium is added. 'Stainless Steels' have approximately 11% chromium and a very marked degree of general corrosion resistance when compared to 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 steels to form the most important class of corrosion and heat resisting steels. These are the austenitic stainless steels, typified by 18-8, where the tendency of nickel to form Austenite is responsible for 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-nickel austenitic steels, improves resistance to pitting corrosion by chlorides and sulphur chemicals. When added to low alloy steels, Molybdenum improves high temperature strength and hot hardness. When added to chromium steels, it greatly diminishes the tendency of steels to embrittle in service or in heat treatment.
The main use of titanium as an alloying element in steel is for carbide stabilization. It combines with carbon to form titanium carbides, which are quite stable and hard to dissolve in steel. This tends to minimize the occurrence of inter-granular corrosion as with AISI 321, when adding approximately .25/.60 % titanium, the carbon combines with 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.
Phosphorus is usually added with sulphur, to improve machinability. In low alloy steels, phosphorus, in small amounts, aids strength and corrosion resistance. Experimental work indicates that phosphorus present in austenitic stainless steels increases strength. Phosphorus additions are known to increase the tendency to cracking during welding.