Precipitation of carbides and sigma phase in AISI type 446 stainless steel under working conditions

The microstructure of tubes of AISI 446 ferritic stainless steels used in a heating furnace of a petrochemical plant was investigated using scanning electron microscopy, energy-dispersive spectrometry, X-ray diffraction, hardness and microhardness measurements. It was found that, under the working conditions the tubes were subjected, precipitation of carbides and sigma phase occurs. Heat treatment does dissolve these precipitates but the tubes cannot be regenerated due to their change in composition, which occurs preferentially at the face of the tubes closest to the flames.