Search Results

In order to meet new environmental regulations (i.e. mass of CO2 rejected in the atmosphere per km), car manufacturers are looking for new solutions to lighten chassis and structural parts in cars. High strength steels formed by hot stamping have proved to be good candidates for achieving better in-use performances together with a lighter structure. In particular, the martensitic stainless steel MaX fulfils the industrial targets for chassis parts in terms of mechanical and fatigue properties. For instance, from a cold formed baseline made of 600 MPa carbon steel, a 50 % mass reduction can be expected with a hot stamped suspension arm made of MaX and included a new clamshell design. However, those parts are often made of a complex assembly of different materials (high strength steels, aluminium and cast iron among others) which are subjected to aggressive environments in service. Therefore galvanic corrosion of those complex assemblies has to be evaluated.

The evolution of emission control standards on particulate matter and NOx has led to a significant increase of complexity of the diesel exhaust line which includes catalytic converter, particulate filter and selective catalytic reduction systems. The exhaust line is no longer a component that customers can change easily; its durability has to be studied for longer lifespan and if possible to be predicted. From a corrosion point of view, emission control systems have led to more and more severe conditions for stainless steel material used in the exhaust line. In particular, mufflers are exposed to higher temperature during the regeneration of the particle filter and also to acidification of gas condensates due to high sulphur content that can be found in diesel.

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.

Due to the evolution of emission control standards, new pollution control systems will be necessarily used for off-road vehicles and trucks exhaust systems and in the near future for passenger cars. Indeed, the will to reduce NOx emission through Euro 5 (2009) and then to Euro 6 (2014) and American EPA Tier 4 (2008-2015) imposes the implementation of a new after-treatment system within the exhaust line. One of the most promising technologies takes advantage of the reduction feature of ammonia (NH₃) on NOx. This system called Selective Catalytic Reduction (SCR) couldn't be developed by storing directly ammonia as a reduction agent on the vehicle due to its high toxicity and flammability. It is why urea is used as an ammonia generator through thermolysis reaction.