Ferritic Stainless Steel Grade with Improved Durability for High Temperature Exhaust Manifold 2011-01-0194

One way to respect the Euro 5 depollution norm is the downsizing of the engine, which leads to more severe in-use operating conditions especially an increase of the exhaust gas temperature. Consequently, the hot part of the exhaust system, i.e., from manifold to the catalytic converter, could be subjected to maximal temperature up to 1000°C. Moreover, an improved durability and longer life guaranties are also required for such parts. In this context, a new ferritic stainless steel grade has been developed, named K44X (AISI 444, EN 1.4521), which fulfills these new specifications and that could be applied for both fabricated manifold and turbocharger shells. The K44X, with a chromium content of 19% (weight), an addition of 2% molybdenum and 0.6% of niobium, offers excellent high temperature properties like cyclic oxidation, creep and thermal fatigue resistance, a low thermal expansion coefficient. The grade maintains a very good weldability and formability very close to the AISI 441 EN 1.4509 one. Furthermore, the molybdenum content also guaranties an additional high corrosion resistance that makes this grade an optimized solution for turbocharger application. One more advantage, the K44X ferritic stainless steel, with its higher thermo-mechanical fatigue resistance compared to the existing solutions, could bring a lightweight and thinner solution by substituting the traditional cast manifold. The paper will present all the high temperature properties in comparison with those of standard austenitic (as 304), refractory austenitic (as 309- 1.4828 or S30815 1.4835) and ferritic grades (as 441 1.4509) and some examples of automotive exhaust manifold applications.