Stainless steel is largely used in the car exhaust market and will be applied now for truck and off-road vehicles. In that field of application, designs are more and more complex with the integration of a catalytic converter and particle filter, consequence of more and more severe diesel depollution regulations. In particular, due to the necessity of reducing NOx emission established by Euro 5 standard (2009), Euro 6 (2014) and American Tier 4 (2014), new equipment were developed for diesel vehicles (truck as well as car). The most promising technology is called Selective Catalytic Reduction (SCR) and takes advantage of the reduction feature of ammonia (NH 3 ) on NOx. As NH 3 cannot be stored directly within the vehicle for safety reasons (toxicity & flammability of ammonia) urea in water solution was selected to initiate the reaction by means of a spraying nozzle. To get a better understanding of the involved hot corrosion mechanisms and afterward to improve material selection, a dedicated laboratory test was developed at Isbergues Research Center. The simulated test consists of spraying urea solution on cyclic heated stainless steel in a range from 200°C to 600°C. We evidenced a nitriding mechanism due to the urea decomposition on the surface of stainless steel at high temperature, and also the very different behaviours between austenitic and ferritic grades. The last one, in particular K41X (1.4509-441) and K33X (1.4513-molybdenum stabilized ferritic) grades show the best performance in particular when compared to the standard 304 austenitic grade. The paper will review the test set-up, the result obtained and will discuss the stainless steel grade selection for the SCR application.

No abstract

No abstract

No abstract

Weld seams often constitute critical points in the thermo-mechanical fatigue design of a stainless steel automotive exhaust manifold. Therefore, a thermal fatigue test on V-shaped specimens was developed by ArcelorMittal Isbergues Research Centre to simulate the thermo-mechanical loading of such a part. Two ferritic base metals dedicated to high temperature applications, together with various filler materials (both austenitic and ferritic grades) commonly used in the exhaust market were tested with a 250-950°C thermal cycle. The results were compared in terms of lifetime, cracking mechanisms and micro-structural evolution in order to point out the best base/filler metal combinations. For austenitic weld metals, the higher thermal expansion coefficient than ferritic weld metals (50% higher) led to a more pronounced oxidation and a higher level of stresses generated at the interface between melted zone and base metal. Consequently, the cracks were localized to this interface for austenitic filler material while they appeared in the base metal and out of the heat affected zone for ferritic filler material.