A new ferritic stainless steel with improved thermo-mechanical fatigue resistance for exhaust parts

Over the past few years, car manufacturers have been considering ever higher service temperatures for the engine in order to comply with the constraints of depollution standards. The requirements in terms of exhaust gas temperature could easily reach and overtake the limits of common stainless steel grades used for such applications in the coming years. A new ferritic stainless steel - named K44X - with increased high temperature resistance has therefore been developed to withstand service temperature up to 1000 degrees C. K44X belongs to EN 1.4521 and AISI 444 classifications and is composed of approximately 19% Cr, 2% Mo and 0.6% Nb. This specific composition leads to better mechanical properties, higher creep and fatigue resistance than EN 1.4509, while keeping comparable weldability and formability. Its coefficient of thermal expansion is lower in comparison to austenitic stainless steel grades and its resistance to cyclic oxidation is improved significantly. High-temperature properties (mechanical properties, creep, cyclic oxidation resistance, and high cycle fatigue) of K44X are presented in this paper and compared with common ferritic and austenitic stainless steels used in the hot end of exhaust lines. A thermal fatigue test - designed to reproduce exhaust manifold service conditions - has also been carried out with the highest temperatures of the cycle in the range of 850-1000 degrees C. The results of these thermal fatigue tests were compared with the above-mentioned stainless steels. A thermal fatigue damage criterion was then identified based on these experimental results and using a cyclic behaviour law obtained from isothermal low cycle fatigue tests.