409L stainless steel for corrosion, high temperature and fatigue resistance, and can achieve longevity and lightweight exhaust system components. 409L high temperature resistance, and can achieve high efficiency of the engine and exhaust system, and reduce the thickness of the exhaust system can reduce exhaust emissions. 409L low cost, flexibility, and finished product rate, easy to replace, is environmentally friendly new products. 409L stainless steel with excellent corrosion resistance, suitable for the extension of the life of vehicles and automotive lightweight and environmentally friendly as recyclable materials, GM, Ford, Volkswagen, DaimlerChrysler and other major automotive manufacturers in the exhaust systems made of stainless steel 100% . After the high-temperature automotive exhaust system arising from an automobile engine, the combustion exhaust gas processing speed, it is discharged to the outside automobile. The main function of the exhaust system to reduce vibration and noise, and then the exhaust gas after combustion, to convert it into clean gas. Automotive exhaust system is the longest parts automotive parts, the system must withstand extreme temperature changes (-20 ~ 850 ℃) and high-temperature, high-speed high-frequency vibrations of the exhaust gas, but the biggest change is in the car, the most complex environment, 409L can withstand temperature extremes of (-20 ~ 650 ℃). Recently, due to environmental standards increasingly stringent automobile exhaust, short drive from the city to increase and spread winter snow agents and other reasons, the use of a standard exhaust system are becoming increasingly stringent, from Europe Ⅱ- Euro Ⅳ standard, traditional steel can not meet its technical requirements, the high-end exhaust system has been used 409L material. Every vehicle using 409L stainless amount 35KGS-65KGS. [1] 2 chemical composition of 409L / S40903 stainless steel chemical composition (mass fraction,%) standard steel C≤ Cr Ti N≤ Nb≤ Ni≤ Si≤ Mn≤ P≤ S≤ American ASTM 409L / S40903 0.030 10.5 ~ 11.7 6 × C ~ 0.75 - 0.50 1.00 1.00 0.040 0.030 - 409L / S40903 stainless steel grades controls: International ISO X2CrTi12; European EN X2CrTi12 / 1.4512; Japan JIS SUH409L. 409L / S40903 stainless steel features 409L / S40903 ultra-low carbon steel is ferritic stainless steel, good oxidation resistance, good weldability. 3 mechanical properties of heat ó0.2 / MPa ób / MPa δ (%) Hardness HBW annealed steel ≤8.00 ≥275 ≥415 ≥20 ≤197 409L / S40903 stainless steel forging process specification Preheat / ℃ initial forging temperature / ℃ final forging temperature / ℃ Cooling 7601040 ~ 1120705 ~ 790 air-cooled 409L / S40903 stainless steel heat treatment annealing temperature 780 ~ 950 ℃, rapid cooling or slow cooling, hardness ≤197HBW. 409L / S40903 stainless steel uses 409L / S40903 stainless steel is mainly used in the manufacture of automotive exhaust treatment device. 4 crystalline properties (1) 409L steel crystallization of solid, liquid two-phase region is very small, less than 30 ℃;  (2) at high temperatures barely γ phase. So when 409L steel began to crystallize after pouring, due to direct contact with the mold surface, the degree of supercooling liquid is large, while also promoting the mold surface heterogeneous nucleation, therefore, a high rate of liquid phase nucleation, formation fine equiaxed surface. Surface such as the formation of fine equiaxed zone speed quickly, then titanium carbide precipitates too late because time is short. After the formation of fine equiaxed zone, generally columnar grain steels, but did not appear columnar grain steel 409L. 409L steel due to the liquid-solid two-phase region narrow temperature range, when the crystalline component supercooled small area, columnar crystals difficult to grow forward. Meanwhile, after the formation of fine equiaxed zone, after a period of time, titanium and carbon have time for diffusion to form titanium carbide particles, promote heterogeneous nucleation. These two reasons that the region generates equiaxed not appear columnar crystals. No upper section columnar crystals, all equiaxed, just different sizes. In the casting process, when the billet temperature falls to 1100 ℃, if the carbon content of more part C forms carbides with Cr, carbides chromium depleted zone appears around, there may be a small amount of δ phase. The relative performance of two different steels appear at this time, or δ phase content at 32% ~ 6O% is blank plastic zone. Steel 409L low carbon content, even if δ phase, the amount is very small. Therefore, does not have a big impact on performance.  409L steel from high to low temperature almost no α → γ phase transformation, and therefore it is easy to grow grains, there are many coarse ferrite, lower strength, in the continuous casting, the upper row, after the slab is cut prone to bend head phenomenon, affecting the normal production. The main problem is the 409L steel crystallization performance degradation due to grain coarsening [1]. 5 Knowledge stainless steel (Stainless Steel) refers to the resistance of air, steam, water and other corrosive medium and weak acid, alkali, salt and other chemical etching medium corrosion of steel, also known as acid-resistant stainless steel. Practical applications, often the weak corrosion resistance as stainless steel corrosion media, while the chemical resistance of steel known as acid corrosion of steel. As the two differences in the chemical composition, the former is not necessarily resistant to chemical corrosion, while the latter generally has a stainless steel sex. The corrosion resistance of stainless steel depends on the alloying elements contained. Types of stainless steel is often divided by organizational status: martensitic steels, ferritic steels, austenitic steel. In addition, according to the ingredients into: chrome stainless steel, chrome-nickel stainless steel and chrome manganese nitrogen stainless steel. 1, ferritic stainless steel: chromium 12% to 30%. Its corrosion resistance, toughness and weldability with the increase of chromium content increased resistance to chloride stress corrosion better than other types of stainless steel. Fall into this category have Crl7, Cr17Mo2Ti, Cr25, Cr25Mo3Ti, Cr28 and so on. Ferritic stainless steel because of high chromium content, corrosion and oxidation resistance are relatively good, but poor mechanical properties and process performance, and more for large discontinuity acid structure and antioxidant for steel. Such steel is resistant to corrosive atmosphere, nitric acid and saline solution, and has good oxidation resistance, thermal expansion coefficient, etc., for the nitric acid plant equipment and food, can also be produced parts at high temperatures, such as gas turbine parts, etc. . 2, austenitic stainless steel: more than 18% chromium, it also contains about 8% nickel and a small amount of molybdenum, titanium, nitrogen and other elements. Good overall performance, corrosion resistant variety of media. Common austenitic stainless steel grades are 1Cr18Ni9,0Cr19Ni9 like. 0Cr19Ni9 steel wC <0.08%, steel marked "0." This kind of steel contains large amounts of Ni and Cr, the steel at room temperature was austenitic state. Such steel has good ductility, toughness, weldability and corrosion resistance, corrosion resistance are good in oxidizing and reducing medium, used to produce acid and equipment, such as containers and equipment corrosion lining, pipelines, corrosion equipment parts, such as nitric acid. Austenitic stainless steels commonly used solution treatment, is about steel heated to 1050 ~ 1150 ℃, and then cooled to obtain a single-phase austenite. 3, austenitic - ferritic duplex stainless steel: the advantages of both austenitic and ferritic stainless steel, and has superplasticity. Austenitic and ferritic stainless steel accounts for about half of each organization. In the case of containing a lower C, Cr content of 18% ~ 28%, Ni content of 3% to 10%. Some steel further contains Mo, Cu, Si, Nb, Ti, N and other alloying elements. Both austenitic class steel and ferritic stainless steel features, higher compared with ferrite, ductility, toughness, brittle at room temperature, resistance to intergranular corrosion and welding performance are significantly improved, while also maintaining a ferritic stainless steel 475 ℃ brittle and high thermal conductivity, with superplastic characteristics. Compared with the austenitic stainless steel, high strength and resistance to intergranular corrosion and chloride stress corrosion has significantly improved. Duplex stainless steel has excellent resistance to pitting corrosion, but also a section of nickel stainless steel. 4, martensitic stainless steel: high intensity, but popular brands poor ductility and weldability have 1Cr13,3Cr13 martensitic stainless steel, etc., due to the higher carbon content, it has a high strength, hardness and wear resistance, but somewhat less corrosion resistance, high mechanical performance requirements for corrosion resistance on the part of some general requirements, such as springs, turbine blades, valves and other hydraulic press. Such steel is after quenching, tempering use.