ALLOYING METALS AND THEIR EFFECTS
| S.N | ALLOYING METAL | Effect of Alloying on Structure and Properties |
| 1 | CHROMIUM | Chromium is by far the most important alloying element in Stainless steel production. A minimum of 10.5% chromium is required for the formation of a protective layer of chromium oxide on the steel surface. The strength of this protective/passive layer increases with increasing chromium content. Chromium prompts the formation of ferrite within the alloy structure and is described as ferrite stabilizer. |
| 2 | NICKEL | Nickel improves general corrosion resistance and prompts the formation of austenite and it is an austenite stabilizer. Stainless steels with 8-9% nickel have a fully austenitic structure and exhibit superior welding and working characteristics to ferritic stainless steels. Increasing nickel content beyond 8-9% further improves both corrosion resistance especially in acid environments and workability. |
| 3 | MOLYBDENUM (AND TUNGSTEN | Molybdenum increases resistance to both pitting, crevice corrosion and general corrosion. Molybdenum and tungsten are ferrite stabilisers which, When used in austenitic alloys, must be balanced with austenite stabilisers in order to maintain the austenitic structure. Molybdenum is added to martensitic Stainless steels to improve high temperature strength. |
| 4 | NITROGEN | Nitrogen increases strength and enhances resistance to localized corrosion. It is austenite former. |
| 5 | COPPER | Copper increases general corrosion resistance to acids and reduces the rate of work- hardening (example: it is used in cold-headed products such as nails and screws). It is an austenite stabiliser. |
| 6 | CARBON | Carbon enhances strength especially, in hardenable martensitic stainless steels), but may have an adverse affect on corrosion resistance by the formation of chromium carbides. It is an austenite stabiliser. |
| 7 | TITANIUM (AND NIOBIUM & ZIRCONIUM) | Where it is not desirable or, indeed, not possible to control carbon at a low level, titanium or niobium may be used to stabilise stainless steel against intergranular corrosion. As titanium (niobium and zirconium) have greater affinity for carbon than chromium, titanium (niobium and zirconium) carbides are formed in preference to chromium carbide and thus localised depletion of chromium is prevented. These elements are ferrite stabilisers. |
| 8 | SULPHUR | Sulphur is added to improve the machinability of stainless steels. As a consequence, Sulphur-bearing stainless steels exhibit reduced corrosion resistance. |
| 9 | CERIUM | Cerium, a rare earth metal, improves the strength and adhesion of the oxide film at high temperatures. |
| 10 | MANGANESE | Manganese is an austenite former, which increases the solubility of nitrogen in the steel and may be used to replace nickel in nitrogen-bearing grades. |
| 11 | SILICON | Silicon improves resistance to oxidation and is also used in special stainless steels exposed to highly concentrated sulphuric and nitric acids. Silicon is a ferrite stabiliser. |
Crystal Structure of some metals (at room temperature)
| Metal | Structure |
| Aluminium | FCC |
| Nickel | FCC |
| Copper | FCC |
| Gold | FCC |
| Lead | FCC |
| Platinum | FCC |
| Silver | FCC |
| Iron | BCC |
| Manganese | BCC |
| Nitrogen | HCP |
| Carbon | Crystalline |
| Silicon | FCC |
| Tungsten | BCC |
| Niobium | BCC |
| Chromium | BCC |
| Vanadium | BCC |
| Zinc | HCP |
| Zirconium | HCP |
| Titanium | HCP |
| Cadmium | HCP |
| Cobalt | HCP |
| Magnesium | HCP |
| Sulphur | Orthorhombic |
| Cerium | FCC |
| Molybdenum | BCC |
