Influence of Elements on Steel
Steels are among the most commonly used alloys and is essentially an alloy of carbon and iron. It also contains other elements, some of which are retained from the steel marking process, other constituents are added to produce specific properties. The most common elements are listed below:
1. Aluminum (Al)
When added to molten steel, Aluminium mixes very quickly with any undissolved oxygen and is therefore considered one of the most common deoxidizers in making steel. Aluminium is also used to produce a fine grain structure and to control grain growth.
2. Carbon (C)
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.
3. Chromium (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.
4. Cobalt (Co)
Cobalt is used to increase the red hardness of a steel. It adds much life to a tool by its ability to maintain hardness and cutting ability when it’s heated to a dull red during a machining operation.
5. 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.
6. Iron (Fe)
Although it lacks strength, Iron 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.
7. Lead (Pb)
Lead is used in steel to improve machinability. In small amounts of 0.15 – 0.3% and finely divided and distributed, it has no known effect on the mechanical properties of steel.
8. Manganese (Mn)
It’s presence has three main effects; it is a mild de-oxidant acting as a cleanser taking the sulphur and oxygen out of the melt into the slag. It increases hardenability and tensile strength but decreases ductility. It combines with sulphur to form manganese sulphides, essential in free-cutting steels.
9. 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.
10. Nickel (Ni)
When added to carbon steel in amounts up to 5%, it increases the tensile strength, toughness and hardenability without loss of ductility. Often used in combination with other alloying elements, especially chromium and molybdenum. Stainless steels contain between 8% and 14% nickel.
11. Niobium (Nb/Cb )
Niobium (Columbium) increases the yield strength and, to a lesser degree, the tensile strength of carbon steel. The addition of small amounts of Niobium can significantly increase the yield strength of steels. Niobium can also have a moderate precipitation strengthening effect. Its main contributions are to form precipitates above the transformation temperature, and to delay the recrystallization of austenite, thus promoting a fine-grain microstructure having improved strength and toughness.
12. Nitrogen (N)
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 austenitic stainless steels.
13. Phosphorus (P)
Although it increases the tensile strength of steel and improves machinability, it is usually regarded as an undesirable impurity because of its embrittling effect. Most steels do not exceed 0.05% phosphorus.
14. Selenium (Se)
Selenium is added to improve machinability.
15. Silicon (Si)
In most commercial steels it is present in a range of 0.05/0.35% and acts as a powerful deoxidizing agent. It is present in higher contents in Silicon-Manganese Spring Steels and Acid and heat resisting steels.
16. Sulpher (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.
17. Tantalum (Ta)
Chemically similar to niobium and has similar effects.
18. 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- 0.6% 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.
19. Tungsten (W)
Tungsten is used as the main element in high speed tool steels. After heat treatment, the steel maintains its hardness at high temperature making it particularly suitable for cutting tools.
20. Vanadium (V)
Vanadium 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.