Creep Failure by Steam Oxidation Scale of 9Cr Boiler Tubes After Long-Time Service

  9Cr steel has been widely used for boiler tube, header and pipe because of the excellent high temperature strength.

  It is well known that the creep failure is anticipated in weld joint, like Type four cracking due to the low creep strength in softening zone of heat affected zone.

On the other hand the creep failure of 9Cr steel tube itself has been reported also. The creep failure is due to the hot spot generated by the obstruction of heat transfer in the part where the steam oxidation scale is detached (not spalling-off).

  The service time of some 9Cr steel tubes have reached to 20 years and the total scale thickness has been beyond 300 micrometer. The clarification of creep failure of 9Cr steel tube by scale detaching must be a keen issue for users.

The user group in Japan has been organized to project the research for safe and long utilization of 9Cr steel tube by the analysis of the tubes for long time service.