102 - Sorelmetal

1 1024 6 % SILICON DUCTILE IRONSFOR HIGH TEMPERATURE SERVICEbyHans Roedter, Sorelmetal Technical ServicesThe 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.

2 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.

3 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 Ductile Iron has a silicon content of 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.

4 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 are several grades of this material which are used; they are described in the table % Si240 BHN 18 %Heat resistance up to 1400oF (750oC).4 - 5 % Si, 2 % Mo200 260 BHN5 15 %Heat resistance up to 1600oF (880oC).

5 High temperature - 6 % Siup to 2 % Mo240 320 BHNUp to 5 %Heat resistance up to 1740oF (950oC).Very high temperature - 6 % Si2 % Mo, V, NiUp to 2 % Cr> 300 BHNVery brittleGood heat temperature wear ProductionChemistryDesired chemistry is generally obtained in the same manner as regular Ductile Iron. Only the carbon, silicon and molybdenum contents need to be adjusted to get the right composition. In addition all returns should be kept separate to reduce alloying costs and, especially when using Mo, to keep it out of regular ferritic castings. The carbon content is lower than in regular Ductile Iron, to - Usually high carbon content contributes to reduce the shrinkage tendency; however, if the carbon equivalent is too high, the iron becomes less fluid and defects looking like cold shuts may be seen.

6 The main cause of these defects is not directly related to pouring temperature or pouring and filling time, but rather to the high carbon content and the tendency of high silicon materials to form oxides at melting temperatures. It is thus necessary to fill the mould quickly, but with minimum turbulence. The benefit of a reduced carbon content is the increase in mechanical properties. High carbon along with higher silicon and cerium contents may promote chunk graphite and/or graphite flotation in these castings. Sometimes gas defects are related also to a too high carbon content.

7 For all these reasons it is important to watch and set the carbon content carefully. Many of the defects found in SiMo castings are related to the carbon content must be kept below and preferably at about Using Sorelmetal to control manganese and keep the other residual elements in check is always advisable. This helps to promote a ferritic structure and reduce the brittleness. Phosphorus and sulphur should be maintained as low as possible. Molybdenum should also be maintained at the low end of the specification, since it is a strong carbide-promoting element and has a high tendency to segregate into intercellular regions.

8 It therefore also increases the possibility for shrinkage porosity and reduces is normally added to the melt in the furnace, at the end of the melting operation, but before taking a chemistry sample. Such a procedure ensures a homogeneous distribution of the element in the melt and a recovery of Mo of about 95%.Pouring TemperaturePouring temperature should be about 100 F (40 C) higher than the normal pouring temperature used forcastings with similar section sizes. Shake-outSince SiMo is somewhat brittle as-cast, it is necessary to take care at the shake-out and during fettling.

9 A very common defect of SiMo is cracked castings due to rough treatmentMost SiMo is made without any heat treatment. However, sometimes it is advisable to heat treat it to dissolve the carbides, improve machinability and eliminate stresses induced during solidification. If the amount of Mo carbides is high it is necessary to heat treat at or above 1650 F (900 C) to dissolve : The reader is also referred to Suggestion Sheet 71 for further information on this same SiMo March 2006