High Silicon Molybdenum Ductile Iron
High Silicon Molybdenum Ductile Iron
The demand for more environmentally friendly vehicles with increased fuel efficiency has expanded the need for specialised materials like SiMo (high silicon and high silicon-molybdenum) Ductile Iron. These vehicles usually have smaller engines running at higher temperature and at higher compression rate. This requires, for exhaust manifolds and turbo charger housings, special materials designed to operate at high temperature: SiMo Ductile Iron meets the requirements. Moreover, these castings are able to perform and maintain dimensions for many thousands of cycles that can range from below freezing temperatures to those greater than 1400oF (750oC). These properties also make SiMo materials suitable for large turbine castings and many types of furnace applications.
Ductile Iron containing higher silicon and molybdenum has been developed to meet these desired characteristics. Increasing silicon content promotes the stability of the microstructure and properties at high temperature by forming a highly ferritic matrix structure and by raising the austenite transformation temperature. SiMo Ductile Iron also has a high resistance to growth and oxidation, which is much improved, when compared to gray iron, because of the nodular form of the graphite. It is also more resistant to creep (plastic deformation under load), which increases at higher stress levels and higher temperatures.
Regular Ductile Iron has a silicon content of 2.0 Ð 3.0%. Increasing the silicon concentration above these levels improves resistance to scaling and increases yield strength, but lowers toughness and elongation. Therefore, the material can be very brittle at room temperature. It usually has a ferritic structure, but may also contain pearlite and carbides. Oftentimes a heat treatment is necessary to transform pearlite and carbides, if present, in order to reduce this brittleness. The addition of molybdenum and other alloying elements may be necessary to improve the serviceability and properties of the material at high temperature.
There are several grades of this material which are used; they are described in the table below:
| Composition | Hardness | Elongation | Applications |
| 4-5 % Si | 240 BHN max. | 5 – 18 % | Heat resistance up to 1400oF (750oC) |
| 4 - 5 % Si, 0.5 – 2 % Mo |
200 – 260 BHN | 5 – 15 % | Heat resistance up to 1600oF (880oC). High temperature resistance. |
| 4 - 6 % Si up to 2 % Mo |
240 – 320 BHN | Up to 5 % | Heat resistance up to 1740oF (950oC). Very high temperature resistance. |
| 4 - 6 % Si 2 % Mo, V, Ni Up to 2 % Cr |
> 300 BHN | Very brittle | Good heat resistance. |
