Mechanical characterization of the ASTM A335 P5 steel and reliability of radiant tubes after long operating time in a petrochemical industry furnace

This paper deals with mechanical characterization of ASTM A335 grade P5 steel specimens, taken from two radiant tubes of the Milazzo Refinery topping furnace, which were put out of service because they underwent unacceptable thickness reductions from 7 to 4 mm. Radiant tubes are designed to guarantee long operation times at the working temperature, therefore periodic inspection are scheduled to verify their reliability and dismount the damaged ones. Chemical composition resulted slightly different in the two considered tubes, even if within the limits of the Standard. Our investigations showed that the tube richer in alloying elements is characterized by a finer microstructure and better tensile and creep properties. The creep test results were compared by means of the Larson-Miller parameter; then the residual life of the two tubes was estimated numerically, by a procedure developed by us to take into account both creep data and progressive wall thickness reduction. By this way we ascertained that many years of residual life can be foreseen at temperatures up to 600°C. It is equal to some years at 700°C, but is reduced drastically to few hundreds of hours at 800°C for the less alloyed tube. We concluded that, in order to guarantee safe operating conditions at least until the subsequent scheduled plant stop, it is strongly advisable to replace the radiant tubes when their thickness is reduced to 4 mm and to monitor regularly that surface temperature is not higher than 600°C.