Development of ferritic stainless steel for catalytic converter

Abstract

Hazardous emission of exhaust gases, from automobile engines causes air pollution. Increasing number of vehicles on road, types of engines used, congested traffic, outdated technology and fuel quality are some of the contributory factors in India which aggravate this problem. Effort to reduce its unjurious effects is of paramount importance. Use of catalytic converter in the exhaust system converts the noxious pollutants into harmless condition to human body and protects the atmospheric pollution. A catalytic converter is a honeycomb device coated with noble metal acting as catalyst in the oxidation of emission rendering it less harmful by forming carbon dioxide and water. There are basically two types of catalytic converters i.e. ceramic supported and metallic supported. The former one is based on cordierite converter whereas the latter one is smaller in size, durable and simpler in construction and is amenable to be easily installed right below the exhaust manifold. Some of the basic requirements for the material for metal support catalytic converter are high heat resistance to withstand engine exhaust temperature (900 C), resistance to sulphurous atmosphere, high temperature mechanical strength, good workability to produce foil (50 m), good formability to make corrugated structure. The ferritic stainless steel (Fe-Cr-Al-Y) was developed which possesses high oxidation resistance due to formation of alpha alumina film and yttrium provides adhesion of alumina film to steel and also prevents grain coarsening leading to improved ductility. The use of 409 ferritic stainless steel has replaced many critical components of the exhaust system satisfying upto 700 C. different types of materials such as ferritic stainless Fe-Cr-Al-Ti, Fe-Cr-Al-Ce and Fe3Al-base iron aluminide have been reported for catalytic converter. Use of chrome steels claded with aluminium or hot aluminised has also been reported for this purpose. The paper deals with the development of an indigenous ferritic stainless steel possessing the above requirements and aims to study their workability to achieve foil of 50 m suitable for making catalytic converter. Two experimental heats of Fe-20Cr-5Al-o.5Ti and Fe-20Cr-5Al-0.5Ce were made in air/vacuum induction furnace. The cast ingots 9100x100 mm) after homogenisation were hot forged and then hot and cold rolled with intermediate annealing to yield 50 m thick foil by pack-rolling. Annealing time and temperature were optimised by grain size measurement. Metallographic examinations by optical microscope and mechanical properties of the cast and sheet products were evaluated.