Degradation of mechanical strength of pyrolysis furnace tubes by high-temperature carburization in a petrochemical plant

Pyrolysis furnace tubes made of HP45 heat-resistant steel casting to handle carbonaceous gases at about 850 °C in a petrochemical plant developed longitudinal cracks after less than one-third of the expected life. Detailed microstructural analysis showed that the cracks were developed by a ductile intergranular mode. This was correlated with high-temperature carburization attack leading to massive precipitation of intergranular carbides and a mixture of carbides and sigma phase within the matrix. Experimental results suggested that the highly reducing nature of the environment precluded the tube material from developing and maintaining a surface protective oxide scale, which facilitated the rapid inward diffusion of carbon.