The exhaust manifold is one of the part of the exhaust system closest to the engine. The material is required to have the characteristics of perfect oxidation resistance, high temperature strength and thermal fatigue because the temperature of the exhaust gas is as high as 900° C. Additionally, it is also required that the material should have good formability so as to conduct complex shapes processing.
In the past, ductile iron was used to cast vehicle exhaust manifolds. However, with the improving performance of engine, the temperature of exhaust gas has also risen, and with the increasing need to reduce the weight of vehicle components. Therefore, the use of cast stainless steel has also increased.
There are two types of stainless steel used to investment cast exhaust manifolds: austenitic stainless steel and ferritic stainless steel.
Austenitic stainless steel has perfect high-temperature strength, but it is not as good as ferritic stainless steel in terms of oxidation resistance due to its oxide skin peeling.
Although the ferritic stainless steel has superior oxidation resistance, the high temperature strength is not as good as the austenitic stainless steel. The ferritic stainless steel has a small thermal expansion coefficient, which is advantageous in terms of thermal fatigue characteristics.
Compared with cast iron exhaust manifold, cast stainless steel exhaust manifold has the following advantages :
- Smooth inner wall and low exhaust resistance;
- The heat transfer is fast, which is conducive to the fast ignition of the tightly coupled three-way catalyst and purifies the exhaust gas;
- Light weight,which is conducive to lightweight.