Researches on an Advanced Engineering Material for Automobile Industry: SG iron with better Mechanical Properties and higher friction obtained by post innoculation of tin and casting in vacuum ('depression') mould

摘要

Spheroidal (nodular or ductile) graphite iron; SG iron, has an as-cast structure containing graphite particles in the form of small rounded, "spheroidal", "globular" or "nodular" particles in a ductile metallic matrix. Industrial products of SG iron are ductile, durable, resistant to wear and fracture and their mechanical properties may further be improved, as per requirement, by post innoculation, with different elements. We have innoculated the SG iron with a minute amount of Sn, cast in vacuum ('depression') mould and have studied the behaviour of tin in SG iron. We utilized sandwich process which is very much common for production of SG iron and uses pouring of hot molten metal at about 1400-1500°C in ladle which contains the nodulant under the innoculants coverage. The reaction with the molten metal is violent. Magnesium acts as a deoxidizer (MgO) and desulphurizer (MgS). Si acts as graphitizer and promotes nucleation sites for nodules. Soon after the spheroidal treatment, when ladle is transfer to the mould line. An automobile part, Differential gear box, was cast in the specially designed mould box on which vacuum ('depression') can be imposed. Specially designed mould box was used. This type of moulding box is identical to the cope and drag commonly used in foundries except that on its one side a hole (1in. dia) is drilled which is connected on its exterior side to vacuum pump (-1 kgf/cm~2) and on the interior side it is connected with 1/2" dia perforated pipe, which is, made rectangular, giving sufficient space for pattern and encircling the mould cavity all around. Microstructure reveals well formed and evenly distributed graphite nodules. The nodule count ranges 100-125 nodules/mm~2, nodule size ranges between 35-40 microns. Microstructure reveals fine pearlitic matrix with 10-20% ferrite dominantly around the nodules i.e., bull eye structure. Effect of Tin additions upto 0.2% is SG iron were studied in the present research work. These levels of additions were sufficient to produce pearlitic structure in casting samples. Our researches have shown that tin additions promote pearlhe as cast structure, the ultimate tensile strength and elongation drop when tin is added beyond the point required to obtain a fully pearlitic matrix. However, hardness is increased. The alloy made was applied in automobile industry, no such complaint of failure of said parts, was reported.