Niobium-Alloyed Stainless Steel for Automotive Exhaust Systems

The stringent emission regulations associated with the ecological and economic competition in the automotive sector have forced the OEMs to adopt a continuous improvement approach in optimizing the energy efficiency of the vehicle, which demanded a faster reach of the light-off temperature of the catalytic converter (to be able to decrease the emissions), and the usage of a lighter material. The result of the increased efficiency is a higher exhaust gas temperatures up to 1000-1050°C, where the conventional cast iron exhaust system components in the hot end are not efficient anymore, and inventing a new material that is able to withstand these high temperature while keeping a good mechanical properties is a must, as well as the increase in the warranty period up to 10 years demanded a material that has superior corrosion resistance. Henceforth, cast iron has evolved to thin sheets of stainless steels, in particular, ferritic stainless steel grades are the most promising as they are found to be lighter, cheaper, and more resistant to the service conditions of the automotive exhaust systems ??Niobium alloyed stainless steels excelled in satisfying the needed requirements, as the addition of niobium to ferritic stainless steel provided the desired high temperature strength, high resistance to creep and oxidation, as well as increased the pitting corrosion resistance for a longer warranty period, all with lighter components which contributed to weight saving and consequently less CO2 emissions. Up to now, the most used grade is the DIN. 1.4509 which shows good performance, however, for temperatures exceeding 1000°C, the grades used in the hot end are 1.4521 and 1.4622 both with niobium content, and among those proposed for the cold end is the dual-stabilized 1.4512. In addition the added niobium resulted in better formability and weldability, making the niobium alloyed ferritic stainless steels suitable for the automotive exhaust application. ??The Improvements achieved upon adding Niobium could be interpreted as a result of the solid solution strengthening, pinning effect, and refining the grain size that are effective in increasing the high temperature performance, formation of precipitates and laves that increased the resistance to creep, fatigue, and increased the thermal fatigue life. Niobium stabilization effect improves the drawbaility through enhancing the γ-fibers and lowering the α-fibers and θ-fibers in the texture. As well as, its higher affinity to carbon was able to inhibit the intergranular corrosion by forming NbC instead of Cr carbides, mainly in the heat affected zone, resulting in a stainless steel that is virtually immune to sensitization. In addition, Niobium has also showed its effectiveness in increasing the oxygen storage capacity in the catalytic converter. ??Although the future of the automotive sector is running towards electrification and the elimination of the Internal Combustion Engine (ICE) and its corresponding exhaust system in order to achieve zero emissions, Niobium technology has already paved its way in the electric mobility, where niobium technology could be implemented in the energy generation of batteries as an alternative to Li-Ion batteries, fuel cells and super capacitors, in addition to other roles of Niobium in the body in white, brakes, chassis, wheels and engine parts.