Because stainless steel has good properties such as corrosion resistance, heat resistance, and workability, among others, it is utilized flexibly and effectively in the manufacture of vehicle components. Each automobile produced now consumes approximately 20 to 30 kg of stainless steel.

Because of its great machinability, stainless steel was initially utilized as a decorative covering material in the automotive industry, but it is now widely used in many other parts. Among these, the number of components used in vehicle exhaust systems has grown dramatically, now accounting for more than half of all stainless steel used in cars.

Automobile manufacturers mandate that materials with exceptional heat resistance and corrosion resistance be used in the exhaust system components to meet society’s expectations for ever-stricter automotive exhaust pollution requirements and lower fuel costs by lightening the weight of the vehicle.

In light of this, stainless steel tubing has largely replaced the castings and aluminized sheets previously utilized for the exhaust system’s components. Additionally, higher-performance stainless steel rather than general-purpose stainless steel has been used as the material.

The exhaust manifold, the front pipe, the coiled tubing, the catalytic converter, the middle pipe, the primary muffler, and the tail pipe are the seven parts that make up the vehicle exhaust system close to the engine. Depending on the type of vehicle, it may be possible to install numerous catalytic converters or an additional muffler. The exhaust manifold, which is the one that is nearest to the engine, collects exhaust gases and directs them to the exhaust pipe.

The material utilized must have great oxidation resistance, high-temperature strength, and thermal fatigue properties due to the exhaust gas temperature, which can reach 900 °C. Additionally, the material needs to have superior forming capabilities due to the requirement for processing into complex shapes. The structure of a stainless-steel exhaust manifold may be split into two categories: type one is formed by ramming, welding, and assembling steel plates, and type two is formed by processing steel pipe bending and welding assembly. There are various types of steel pipes, including composite pipe structures.

The exhaust manifold is made of austenitic stainless steel and ferritic stainless steel, both of which are forms of stainless steel. Despite having advantages in high-temperature strength, austenitic stainless steels are susceptible to oxide skin peeling. Its oxidation resistance is therefore lower than that of ferritic stainless steels. Ferritic stainless steels are superior to austenite stainless steels in terms of oxidation resistance, but they are less strong at high temperatures.

Ferritic stainless steels perform better in terms of thermal fatigue due to their low coefficient of thermal expansion. Presently utilized ferritic grades include SUS 430J1L and SUH 409L. However, with more rigorous emission standards in place recently, there has been a tendency for exhaust gas temperatures to rise, occasionally as high as 950 °C. The grade SUS 444 series is utilized in this situation.

Additionally, SUS 429 grade with decreased Cr is an option as a cheap material. The majority of the inner tube of an exhaust manifold with a composite pipe structure is made of austenitic stainless steel, while the external tube is made of ferritic stainless steel.