BS in Materials Engineering
Materials Engineering Department
Corrugated 0.05 in thick strips of Fe-Cr-Al ferritic stainless steel samples with 4% and 6% aluminum were heat treated, and the resulting Al2O3 oxide was analyzed using SEM imaging. Three heat treatment cycles were carried out for each sample. The heat treatments were based on future metal-substrate catalytic converter processing prior to applying the washcoat Each heat treatment started with a 90 minute ramp from room temperature to 538°C and a 32.5 minute ramp from 538°C to 899°C. The three different dwell periods at 899°C were 4 hours, 6 hours, and taking a sample out every hour for 4 hours and letting it air cool. The full 4 and 6 hour cycles were finished with an in-furnace cool recorded by an external thermocouple and datalogger. Additional samples were included in the 6 hour cycle that were flattened by pressing a metal cylinder across the top of the corrugated sheet. All samples were sputtered with a 300-Å-thick layer of gold to improve resolution in the SEM. Cold rolling to form the 0.05 in sheet and possibly the corrugation deformed the microstructure and altered the surface, which produced different oxide structures during heat treatment in linear patterns. Also, the different regions of the corrugation (ridge, valley and slope) produced different oxide formations. The ridges showed a flatter oxide, the valleys showed a linear pattern with alternating needle-like and rough granular oxides, and the slopes showed needle-like oxide with patches of smooth rounded oxide. The desired oxide is a needle-like structure which provides a large surface area for the catalyst layer (washcoat) during the production of catalytic converters.