Most equipment made of 0.5Mo steel has been operating successfully for a long term in the petroleum and petrochemical industries. However, unusual incidents of hydrogen attack below the Nelson chart imply that metallurgical variations such as microstructure and concentration of impurity elements have an important role in hydrogen attack resistance. The deletion of the Nelson curve is a major concern to the oil refining industry, which now has to assess the integrity of aged, existing equipment made of 0.5Mo steel.
The sample of 156 heat was investigated by ultrasonic inspection and metallurgical examination. All damaged samples taken from retired vessels and piping after long term service possessed an unusual microstructure of pro-eutectoid ferrite and pearlite with massive M23C6 carbides. On the other hand, all components with standard pro-eutectoid ferrite and acicular bainitic microstructures with Fe3C were not damaged below the critical curve of Nelson chart 3rd edition.
Based on these surveys of the hydrogen attack incidents, a critical operating limit of temperature and hydrogen partial pressure was determined which accounts for variations in microstructure and the type of precipitated carbide.
