Application of Stainless Steel in Automotive Exhaust Manifold

With the rapid development of automotive industry in recent years, in order to protect the environment, automotive manufacturers are striving to improve the fuel rate and purify the exhaust gas. The use of new technologies and new processes results in the exhaust gas temperature exceeding 900 C.The use of stainless steel instead of cast iron is increasing in Japan, USA, Europe and other regions. The main materials are 304, 409 and 410L stainless steel.

Exhaust manifolds made of stainless steel can be roughly divided into two categories according to their structural characteristics.One is the stamping and welding of stainless steel sheets, the other is the fabrication process of bending and then welding of stainless steel pipes, and the other is the steel pipe with dual pipe construction.There are currently two types of stainless steel used to make exhaust manifolds, one is austenitic stainless steel and the other is ferritic stainless steel.

Austenitic stainless steels have good high temperature strength, but their oxidation resistance is not as good as that of ferritic stainless steels due to their easy oxidation peeling.YUS409D (11Cr-Ti) was used to initially adapt the exhaust temperature of about 800 C. After that, titanium, vanadium and rare earth elements such as AHS steel were added to meet the increase of exhaust temperature due to the strengthening of exhaust regulations.Subsequent requirements led to the development of SUS304 (18Cr-8Ni) and SUXM15J1 (18Cr-13Ni-4Si) stainless steels.

Ferritic stainless steels have better oxidation resistance than austenitic stainless steels such as 40Cr17Si2.However, the high temperature strength is not as good as that of austenitic stainless steel.JFE Steel Group has carried out a detailed study on the high temperature strength, oxidation resistance and formability of ferritic stainless steel and developed JFE-MH1 stainless steel.[115].Ferritic stainless steels are advantageous in terms of thermal fatigue properties due to their small coefficient of thermal expansion.The main steel types used are SUH409L (11Cr-Ti-LC) and SUS 430J1L (18Cr-0.5Cu-Nb-LC, N). However, in recent years, higher requirements have been put forward for exhaust emission regulations. The exhaust temperature tends to be higher, and it is said that some of them reach above 950 C.In this case, Ti, Ni, Mo, Nb, Ta and other elements are often added to the alloy, mainly SUS444 (19Cr-2Mo), IN783LC and other stainless steels.

In addition, to reduce material costs, a series of stainless steels such as SUS429 (15Cr), SUH409L, YUS180 (19Cr-0.4Nb-0.4Cu) have been developed with the use of Cr elements.[120-123].A lot of research and practice have shown that ferritic stainless steels have better oxidation resistance and dimensional stability when used at 900 C, but their thermal deformation resistance is not ideal when used above 900 C. For austenitic stainless steels, lower Ni content stainless steels can be used at temperatures less than 1000 C, such as 40Cr22Ni10Si2, while higher Ni content austenitic stainless steels can be used.Temperature can be at 1000 C, e.g. 40Cr25Ni20Si2.

Overall, the development of exhaust manifold material is gradually from cast iron to cast steel, and finally to austenitic heat-resistant cast steel material.Considering the respective advantages of ferritic stainless steel and austenitic stainless steel, some automotive manufacturers employ dual-pipe exhaust manifolds with austenitic stainless steel for inner tube and ferritic stainless steel for outer tube.

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