For components of an exhaust system of an automobile engine, such as an exhaust manifold and a turbine housing, spheroidal graphite cast iron and high-Si spheroidal graphite cast iron have been hitherto used. In some high-powered engines, since the exhaust gas temperature is high and even high-Si spheroidal graphite cast iron has insufficient endurance, a weld structure of stainless steel sheets, “Niresist” cast iron and ferritic stainless cast iron is adopted. Recently, as high-powered engines of automobiles have been further developed, demand for cleaning automobile exhaust gas has increased. In particular, in order to speedily clean up an exhaust gas when an engine is started, the exhaust gas has to be speedily heated to a temperature where an exhaust gas cleaning device operates. Therefore, thinning and weight reduction of the exhaust system components have become demanded because the amount of heat stripped by exhaust system components such as an exhaust manifold and a turbine housing located further to the engine side than an exhaust gas cleaning device should be reduced to the extent possible. However, in thin casts, owing to the thinning, the strength against the thermal stress becomes insufficient and the surface temperature goes up, and therefore existing spheroidal graphite cast iron is insufficient in thermal fatigue characteristics and in oxidation resistance. As the result, casts of stainless steel cast irons are partially being used (Reference 1).
[Reference 1] JP 08−225898
However, when a cast of the stainless steel cast iron of Reference 1 is used for parts such as exhaust system components, in an environment of high-temperature and high-C potential, the cast part is carburized and decreased in thermal fatigue resistance and workability. Besides, when the cast part is used in the exhaust system component of a diesel engine, a S component contained in light oil that is a fuel is burned to generate a sulfuric acid based component, and the sulfuric acid based component condenses on an inner surface of the component when the exhaust gas is cooled to tend to accelerate the corrosion (i.e., so-called sulfuric acid dew corrosion).
USPTO Backgrounds
USPTO Backgrounds