The general symbol
of a chemical element is represented by:
where A is the atomic number indicating the number of protons exist in the nucleus of the atom; N is the atomic mass unit, defined as the ratio of the average mass per atom to 1/12 of the atomic mass of carbon-12 in its nuclear and electronic ground state; and Z is the chemical symbol of the element.
Manganese increases hardenability and tensile strength of steel, but to a lesser extent than carbon. It is also able to decrease the critical cooling rate during hardening, thus increasing the steels hardenability much more efficient than any other alloying elements. Manganese also tends to increase the rate of carbon penetration during carburizing and acts as a mild deoxidizing agent. However when too high carbon and too high manganese accompany each other, embrittlement sets in. Manganese is capable to form Manganese Sulphide (MnS) with sulphur, which is beneficial to machining. At the same time, it counters the brittleness from sulphur and is beneficial to the surface finish of carbon steel.
For welding purposes, the ratio of manganese to sulphur should be at least 10 to 1. Manganese content of less than 0.30% may promote internal porosity and cracking in the weld bead, cracking can also result if the content is over 0.80%. Steel with low Manganese Sulphide ratio may contain sulphur in the form of iron Sulphide (FeS), which can cause cracking (a “hot-short” condition) in the weld
Phosphorus increases strength and hardness, but at the expense of ductility and impact to toughness, especially in higher carbon steels that are quenched and tempered. As such its content in most steel is limited to a maximum of 0.05%. Phosphorus prevents the sticking of light-gage sheets when it is used as an alloy in steel. It strengthens low carbon steel to a degree, increases resistance to corrosion and improves machinability in free-cutting steels. In terms of welding, phosphorus content of over 0.04% makes weld brittle and increases the tendency to crack. The surface tension of the molten weld metal is lowered, making it difficult to control.
Sulphur improves machinability but lowers transverse ductility and notched impact toughness and has little effects on the longitudinal mechanical properties. Its content is limited to 0.05% in steels but is added to freecutting steels in amount up to 0.35% with the manganese content increased to counter any detrimental effects since sulphur is beneficial to machining. For welding, weldability decreases with increasing sulphur content. Sulphur is detrimental to surface quality in low carbon and low manganese steels and it promotes hot shortness in welding with the tendency increasing with increased sulphur.
Silicon increases strength and hardness but to a lesser extent than manganese. It is one of the principal deoxidizers used in the making of steels to improve soundness, i.e. to be free from defects, decays or damages. Silicon is present in all steels to a certain extent. Its content can be up to 4% for electric sheets that are widely used in alternating current magnetic circuits.
In welding, silicon is detrimental to surface quality, especially in the low carbon, resulphurized grades. It aggravates cracking tendencies when the carbon content is fairly high. For best welding condition, silicon content should not exceed 0.10%. However, amounts up to 0.30% are not as serious as high sulphur or phosphorus content.
For galvanizing purposes, steels containing more than 0.04% silicon can greatly affect the thickness and appearance of the galvanized coating. This will result in thick coatings consisting mainly zinc-iron alloys and the surface has a dark and dull finish. But it provides as much corrosion protection as a shiny galvanized coating where the outer layer is pure zinc.
Vanadium is used to refine grain size. Steels containing vanadium have a much finer grain structure than steels of similar compositions without vanadium. It decreases the rate of grain growth during heat treating processes and raises the temperature at which grain coarsening sets in thus improving the strength and toughness of hardened and tempered steels. Contents up to 0.05% increases hardenability while larger amounts tend to reduce hardenability due to the formation of carbide. Vanadium lessens softening on tempering and induces secondary hardness on high speed steels.
Vanadium is used in nitriding, heat resisting, tool and spring steels together with other alloying elements. It is also being utilized in ferrite/pearlite microalloy steels to increase hardness through carbonitride precipitation strengthening of the matrix.
1. Automotive Handbook, Bosch, 1st English Edition, 1978, p 154-158
2. Lawrence H. Van Vlack, Elements of Material Science and Engineering, 4th Edition, Addison-Wesley, 1980, p 31-32