There have been trials to improve the strength of gray cast iron without losing its superior properties. Petrol engines have cylinder pressures ranging from 7 to 12 MPa, while heavy-duty diesels operate in excess of 20 MPa.
This high pressure generates much higher mechanical and thermal stresses on the cylinder block. The use of a cast iron block is widespread because of the high strength needed. However, a much stronger material is required to enable a lightweight design with decreased thickness. For these requirements, a block made of compact graphite iron has been proposed.
The graphite shape greatly influences the material characteristics of cast iron. Figure 2.17 gives a schematic representation of graphite morphology.
In the conventional casting procedure, cast iron generates a flaky shape (a)
in the iron matrix. However, when a special modification treatment is implemented on the molten iron just before pouring, graphite becomes round (b). Cast iron having this shape is called spheroidal graphite cast iron or nodular cast iron. The additive for spheroidizing is called a nodularizer. Compared to the flaky shape, this geometrical shape can avoid microstructural stress concentration to give higher mechanical strength and ductility. This is also referred to as ductile iron. However, the thermal conductivity and resistance to scuffing are not so high.
The third microstructure (c) is compact graphite iron (CGI) containing graphite of a vermicular (worm-like) form (c). This is a relatively new alloy that has improved mechanical strength without diminishing the favorable
properties of flaky graphite iron. As may be inferred from the shape, the properties of this iron are positioned between flaky and spheroidal iron. It has a higher tensile strength being 1.5 to 2 times as strong as flaky iron,
higher stiffness and approximately double the fatigue strength of flaky iron.
The thermal conductivity lies between flaky and spheroidal iron. This makes it possible to produce a cylinder block that is both thinner and stronger. The nodularizer inoculated in the molten iron gives perfect spheroidal
graphite. An imperfect spheroidizing treatment before pouring generates CGI. The nodularizer contains Mg. After the inoculation of the nodularizer, the graphite shape gradually changes to a flaky shape via a vermicular shape as shown in Fig. 2.18.6 The residual Mg content in the molten iron decreases with time due to the evaporative nature of Mg. This causes degradation of the spheroidal graphite to give CGI. An additional explanation is given in Chapter 4 and Appendix D.
The number of cast iron blocks used for petrol engines has decreased, while CGI is seen as a new cast iron for diesel engines. To meet Euro IV (2005) emission regulations, cylinder pressures of 18 MPa and higher are
being planned for car diesels. Even more stringent requirements under Euro V will come in from 2007 and 2008. Higher strength blocks with good thermal discharge properties will be required.
The science and technology of materials in automotive engines
Hiroshi Yamagata
Woodhead Publishing and Maney Publishing
on behalf of
The Institute of Materials, Minerals & Mining
CRC Press
Boca Raton Boston New York Washington, DC
WOODHEAD PUBLISHING LIMITED
Cambridge England
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