Application of Stainless Steel in Automobile Exhaust System
Stainless steel has been used in automobile manufacturing for a long time, and the most commonly used are mainly decorative materials, one is decorative materials, mainly stainless steel handles, decorative strips, pedals, etc.; the other is exhaust systems, which are mainly applied to stainless steel corrosion resistance, High-temperature resistance performance. Among them, the stainless steel consumption of the exhaust system accounts for the vast majority.
According to the latest statistics released by the China Association of Automobile Manufacturers on January 11, my country’s automobile production and sales in 2009 have completed 13.791 million and 13.644 million respectively, an increase of 48% and 46% year-on-year. If a car uses an average of 30Kg of stainless steel to calculate, the automobile industry in 2009 consumed 413,700 tons of stainless steel. This article mainly introduces the application and development of stainless steel in automobile exhaust systems.
The automobile exhaust system is composed of 7 components, which are the exhaust manifold, front pipe, flexible pipe, catalytic converter, center pipe, main muffler, and tailpipe in order of distance from the engine. Some models are also equipped with several catalytic converters and auxiliary silencers.
The service temperature and use environment of each component of the exhaust system is different. Considering the manufacturing cost, different materials of stainless steel and other materials will be used. For example, in the high-temperature part (hot end) close to the engine, the main characteristics required for the component materials are high-temperature strength, thermal fatigue characteristics, high-temperature oxidation resistance, and salt corrosion resistance; while in the part far away from the engine (cold end),
The material is required to be resistant to salt and exhaust gas condensation corrosion. In addition, the tailpipe exposed at the rear of the car body should be beautiful in appearance in addition to corrosion resistance. The function required performance, and materials used for each component are described in detail below.
Exhaust Manifold
The exhaust manifold is a component that collects exhaust gas discharged from each cylinder of the engine and discharges it to the exhaust pipe. Because the exhaust manifold is directly installed on the engine, and the exhaust gas temperature can be as high as about 900°C, the materials used must have excellent oxidation resistance, high-temperature strength, and thermal fatigue properties; in addition, due to the complex shape of the exhaust manifold, The material used also needs to have excellent formability.
According to the manufacturing method, stainless steel exhaust manifolds can be divided into two categories: one is formed by stamping and welding of steel plates; the other is formed by bending and welding steel pipes. The latter also includes the casing structure.
There are two types of stainless steel used as exhaust manifolds, austenitic stainless steel, and ferritic stainless steel. Austenitic stainless steel has excellent high-temperature strength, but because the oxide scale is easy to peel off, its oxidation resistance is not as good as ferritic stainless steel. The austenitic stainless steel used for exhaust manifolds includes SUS304 (18Cr-8Ni), SUSXM15J1 (18Cr-13Ni-4Si), and so on. On the other hand, although ferritic stainless steel has good oxidation resistance, its high-temperature strength is not as good as austenitic stainless steel.
However, due to the small thermal expansion coefficient of ferritic stainless steel, it is superior to austenitic stainless steel in terms of thermal fatigue characteristics. The ferritic stainless steel used in exhaust manifolds includes SUH409L (11Cr-Ti-LC), SUS430J1L (18Cr-0.5Cu-Nb-LC, N), etc.
In recent years, as exhaust gas emission regulations have become increasingly strict, the exhaust gas temperature has also risen to about 950°C. At this time, SUS444 (19Cr-2Mo) series stainless steel can be used. In addition, in order to reduce costs, SUS429 (15Cr) stainless steel with a lower cr content is also used. In an exhaust manifold with a sleeve structure, the inner tube is usually made of austenitic stainless steel, and the outer tube is made of ferritic stainless steel.
Front Tube
The front pipe is located between the exhaust manifold and the flexible pipe. In order to ensure the catalytic activity of the downstream catalytic converter, the heat radiation of the front pipe must be minimized to prevent the exhaust gas temperature from decreasing. In addition, the front pipe must also have the effect of reducing noise. For these reasons, the front tube is shifting from the traditional single-layer tube structure to the thin-walled casing structure. The materials used for the front tube are ferritic stainless steel such as SHU409L, SUS436L (17Cr-1Mo-LC, N), and SUS430J1L.
When the casing structure is adopted, austenitic stainless steel is sometimes used for the inner tube. In the future, low-cost ferritic stainless steel with excellent oxidation resistance and thermal fatigue properties will continue to maintain its status as the main material of the front tube. In addition, in order to cope with the trend of rising exhaust gas temperature and tube wall thinning, steels with better high-temperature performance, such as SUS429 grade stainless steel, will be used.
Flexible Tube
The flexible pipe is located between the exhaust manifold and the catalyst converter, mainly to prevent the vibration of the engine from being transmitted to other parts of the exhaust system. The flexible tube is composed of a corrugated sleeve and a stainless steel wire mesh braid that wraps the sleeve. The material of the flexible pipe needs to have good high-temperature fatigue characteristics, and at the same time, because it is made into a corrugated shape, the material must have good workability. In addition, snow melting salt is used to prevent road freezing in cold areas, so the outside of the flexible pipe is required to have high-temperature salt corrosion resistance.
At present, the material used for flexible pipes is mainly SUS304. However, considering that the outside of the pipe needs to be resistant to high-temperature salt corrosion in cold areas, SUSXM15J1 with higher performance is also used. This steel grade is added with Si and increased the content of Ni. . Recently, SUS316L (18Cr-12Ni-2.5Mo-LC), which is more economical than SUSXM15J1 and has better resistance to intergranular corrosion than SUS304, has also begun to be used.
Catalytic converter (catalyst carrier and shell)
In order to reduce the pollution of car exhaust to the air, it is necessary to install an exhaust purification system called a “catalytic converter”. It can convert carbon monoxide and hydrocarbon pollutants in exhaust gas into carbon dioxide and water, and convert nitrogen oxide (NO) pollutants into nitrogen and oxygen. The converter is composed of a catalyst, auxiliary catalyst, sealing coating, and catalyst carrier.
Since the catalytic converter is located close to the exhaust manifold and under the chassis, it must be able to withstand the harsh service environment such as high temperature and vibration. For the catalyst carrier, in addition to the cordierite ceramic carrier, a metal carrier made of ferritic stainless steel foil was also used. The reason for using ferritic stainless steel foil is its good thermal shock performance and small heat capacity.
The metal catalyst carrier is composed of a honeycomb mandrel made of stainless steel foil and a shell made of stainless steel plate. The honeycomb mandrel is made by winding flat foil and corrugated foil and joining them by brazing, welding, or the like. The shell must have good high-temperature strength, high-temperature salt corrosion resistance, and processability.
Because the foil is required to have excellent oxidation resistance, Fe-Cr-Al ferritic stainless steel, such as Fe-20Cr-5A1 alloy, is used. In order to enhance the adhesion of the oxide film, some steel grades have also added a small number of rare earth elements (REM) such as Hf, Sc, Y, and Ce. Its development trend is to further improve the oxidation resistance of stainless steel foil.
The housing of the catalytic converter mainly uses SUS430 (17Cr) stainless steel, but in order to cope with the increase in exhaust gas temperature, ferritic stainless steel with better high-temperature performance has been used, such as SUS430J1L and SUS429 series.
Central Tube
The central pipe connects the catalyst converter and the main muffler. Some models also have a secondary muffler installed in the center tube. The exhaust gas temperature at this location is relatively low, so the high-temperature performance of the center tube is not very high, but the requirements for its corrosion resistance are very high. The corrosion resistance of the inner surface is mainly for the corrosion caused by the condensed water in the exhaust gas, while the outer surface has to resist the salt corrosion caused by the snow melting salt.
Traditionally, the center tube is made of aluminized carbon steel plate, but as a measure to resist corrosion on the inner and outer surfaces, the application of stainless steel is increasing rapidly. The stainless steel used for the center tube is SUS410 (13Cr), SUH409L, and SUS430.
Main Silencer
The function of the main muffler is to absorb and reduce the noise when exhausting exhaust gas. It is composed of a shell, inner panel, inner tube, end panel, and other parts. When the car is driving for a long distance, due to the heating effect of the exhaust gas, the maximum temperature of the main muffler can reach about 400 ℃, so moisture will not condense in it at this time.
But when the car is driving a short distance, the temperature of the main muffler does not rise, so moisture will condense in it. Condensed water contains NH+4, CO23, SO24, Cl-, and organic acids. When the engine is started and stopped repeatedly, these substances are gradually concentrated. The appearance of condensate creates a very corrosive and humid environment inside the muffler. On the other hand, the outer surface of the muffler is exposed to the corrosive environment of snow-melting salt.
At present, the main muffler material is mainly SUS436L, but the use of SUS436J1L (17Cr-0.5Mo) is increasing because the steel grade reduces the amount of Mo added and thus reduces the cost. In view of the beautiful appearance and good corrosion resistance of aluminized stainless steel plate, it is sometimes used for housings and end panels.
Tailpipe
The tailpipe is located behind the main muffler and is the final outlet of the exhaust gas. It is also a visible part. Therefore, the tailpipe must not only be corrosion-resistant but also beautiful. Since the exhaust gas temperature at the tailpipe is already very low, it is sufficient to use an aluminized steel plate. But sometimes for aesthetics, high-purity ferritic stainless steels such as SUS409L, SUS430, and SUS436 and austenitic stainless steel SUS304 are also used. When there are special requirements for appearance, the main steel grades other than chrome plating or hot-dip aluminum plating are also used, and YUS731 can withstand high-temperature salt corrosion.
The automotive industry is a major consumer of stainless steel. Its application parts mainly exhaust systems and various decorative parts. The 400 series stainless steel is mainly used, and the representative steel types are 409L, 436L, 430, etc.
The automobile industry has become a pillar industry for stainless steel consumption. In view of the fact that China’s automobile industry is still in a rapid development stage, the role of stainless steel in automobile manufacturing still has great development. Stainless steel has excellent performance, environmental protection, and recyclable characteristics. I believe that in the next few years, the automotive industry, especially the application of stainless steel in automotive exhaust systems, will have greater growth in terms of quantity and steel varieties.