INOCULATION OF METALS IN CASTING

Inoculation may be defined as addition of inoculant to molten metal in order to modify structure of metal after solidification and properties of cast metal or alloy.

Action of inoculants is not just only simple alloying effect.

Nucleation is an integral effect of inoculation.

It is partially caused by addition of nuclei to the melt.

A nucleus is a particle of matter which acts as centre to start graphitizations/crystallization.

It is applied late in melting operation/process. Some commonly applied inoculation treatments are :

(i) Grain and constituent refinement

Alloys -------------------Typical inoculants

Steel------------------------ Al, Ti

Aluminium base --------- Ti, B, Nb, Zr

Magnesium base ----------- C, Zr

Mg – Al -------------------- C

(ii) Refinement and eutectic modification

Alloys --------------------Typical inoculants

Al - Si -------------- ------ Na

Cast iron (inoculation) ------ Si, C

Cast iron (S.G. iron structure- Mg, Ce

Effects of Inoculation

Aluminium in controlled amount is added in steel to produce fine grain size and special distribution of inclusions. This improves mechanical (especially impact) properties and response of metal to heat treatment.

Titanium (0.2%) or boron (0.02%) are added at final stage of casting of light alloys (Al) reduce the cast grain size from 0.1 inch (2.5 mm) in dia to as small as 0.005 (0.125 mm) in dia.

When iron and nickel are added in small percentages to aluminium bronze, it prevents the formation of a coarse crystalline structure.

Carbon is added in small amounts to molten Mg - Al alloys for grain refinement.

Zirconium is added in small amount to magnesium alloys (without any aluminium) for grain refinement.

Eutectic structure of Al-Si alloys is modified by adding sodium in small quantity or treating with sodium salts. Sodium treatment ensures fine dispersion of Si and higher physical properties.

Inoculation of molten cast iron

– modifies structure or graphite formation

– change in graphite type obtained by inoculation of molten iron.

– improves mechanical and physical properties

– reduces inherent section sensitivity of grey cast iron

– makes material tougher, more predictable and less susceptible to changes in chemical composition.

– In graphitization inoculation of grey cast iron, inoculant favours graphite formation. prevents (undercoding) chilling and avoids formation of white iron in thin sections.

Common graphitizers are ferrosilicon, nickel silicon, Ca–Mn – Si and Si – Mn – Zr.

Quantity of graphitizer (inoculant) should be such that it can add 0.5 – 1.0% silicon to the molten iron.

Graphitizers produce more effect when added to the molten metal in laddle than when included in the furnace metal charge.

If normal grey iron is inoculated with silicon at the time of pouring, the austenite – graphite eutectic grain size and graphite flake distribution can be controlled.

In another treatment, when magnesium or cerium is added in the molten iron, the result is the spheroidal graphite structure and a ductile iron.

Near about 0.04% residual magnesium changes graphite, flakes into spheroids with huge improvement in mechanical properties of the resulting iron.