Severe internal attack was experienced in the hottest part of 2.25Cr 1Mo steel charge heater tubes in a Continuous Catalyst Regeneration (CCR) Platformer unit. The metal loss was found to be caused by so-called metal dusting. Metal dusting can occur due to the high carbon activity in the CCR pnxess gas. If active metal surfaces are exposed to this gas at relatively high temperature levels of about 625 to 675 °C, metal dusting might occur whilst forming whisker-type graphite. The temperature range for metal dusting is very narrow, in CCR process gas it can occur over a range of about 25 to 50 °C depending on the metallurgy. At slightly higher temperatures carburization occurs, and this caused in a few cases rather thick carburized layers in 9Cr 1Mo steel tubes which also led to failure.
Control of the skin temperatures of the heater tubes to safe levels (in combination with control of sulphur and chloriding agent levels in the process gas) can prevent this type of failures.
Kevwords: 2.25Cr 1Mo, 9Cr 1Mo, metal dusting, carburization, CCR Platformer units