Cast iron
The turbine housing must be resistant to oxidation and possess thermal fatigue resistance at high temperatures. Diesel engines have low exhaust gas temperatures and therefore use high Si nodular cast iron or Niresist cast iron. These housings are made by sand casting using sand cores. The Si content of high-Si nodular graphite cast iron is about 14%, which raises the eutectoid transformation temperature (723 °C, the transformation
temperature from austenite to pearlite). Transformation during operation causes thermal fatigue because of the transformation strain. By raising the transformation temperature, the added Si prevents thermal fatigue in the
operating temperature range. The Si also forms a thick oxide scale on the surface which prevents oxidation corrosion at high temperatures. This alloy is also used in the exhaust manifold.
Austenitic nodular graphite cast iron is also used in the housing, and adding about 20% Ni makes the matrix of cast iron austenitic in a wide temperature range. Generally, this form is referred to as Niresist. Figure 11.7
shows the microstructure of Niresist. Round graphite particles can be seen in the austenitic matrix. The austenite structure has a high thermal expansion coefficient, and the lack of transformation under operating conditions gives superior resistance to thermal fatigue. Niresist is stronger than high-Si nodular graphite cast iron. The high strength and corrosion resistance mean that this alloy can be used in the exhaust manifold, which operates under red heat conditions. The high hardness and high thermal expansion coefficient are
also suitable for reinforcing the piston-ring groove of the aluminum piston (see Chapter 3).
The thermal inertia of the turbine housing affects start-up emissions. Low thermal inertia means that the temperature of the catalytic converter rises quickly, so that it begins to convert pollutants early. This operational design requires greater heat resistance and thin walls in the turbo housings.
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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