Effect of elements in Alloy steels

Sulphur is not desirable in steel since it forms iron sulphide which cause brittleness and hot-shortness - a tendency to crack during hot working. Sulphur in steel forms the weak and soft sulphate "Fe"s which weakens the steel and promotes hot-shortness or brittleness at red heat. But sometimes a very small amount of sulphur is left in the steel to aid machinability. Manganese either present naturally or in small quantities added to the steel combines with small amount of sulphur that is usually present to form manganese sulphide which does not have much effect if not in large quantities and is well dispersed. If manganese sulphide is present in large quantities and in the proper form, it imparts desirable mechanical properties to steel. Therefore, certain free-machining steels which are to be machined automatically and are used for parts that will not be subjected to impact have 0.08 to 0.15% sulphur added. It is beneficial to machinability and the higher sulphur content of some steels reflect this useful property. It is detrimental to surface quality in low-carbon and low manganese steel. It decreases ductility and impact resistance. Weldability also decreases with increasing sulphur content.

Copper

• It is used in percentage of 0.1 - 0.4 to produce resistance to atmospheric corrosion.
• Acts as a strengthening agent.

Although copper is corrosion-resistant, it is not used in steel for this purpose. Copper is nowadays, used extensively in low-carbon sheets especially thin gauges and other structural steels. It has minor effect on mechanical properties. It is beneficial to corrosion resistance if amount present exceeds 0.20%. When present in appreciable amounts, it is detrimental to hot working operations. It effects forge welding and is detrimental to surface quality. It does not effect arc or acetylene welding.

Cobalt

• Improves mechanical properties such as tensile strengths, fatigue strength and hardness.
• Refines the graphite and pearlite
• It is a mild stabilizer of carbides
• Improves heat resistance
• Retards the transformation of austenite and thus increases hardenability and freedom from cracking and distortion.
• Contributes to red-hardness by hardening ferrite

Boron

• It is a  very powerful hardenability agent and increases hardenability or depth to which steel will harden when quenched.

It is being 250-750 times as effective as nickel, 75- 125 times more effective than molybdenum and about 100 times as powerful as chromium. It is expensive but used in minute quantities.

Aluminum:

• It is used to deoxidize steel
• Produces fine grain size.
• It is used to obtain non-aging characteristics alone or in the presence of other elements
• Prevents recurrence of strains in sheets and strip.