11151 Research for Creep Damage by Steam Oxidation Scale of 9Cr Boiler Tubes after Long Time Service

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Product Number: 51300-11151-SG

ISBN: 2011 11151 CP

Author: Hiroyasu Matsuda, Masahiko Takehara, Koji Kawano, Shigemitsu Kihara, Kouichi Yagi, Daisuke Horikoshi

Publication Date: 2011

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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 exfoliated (not spalling-off).

The service time of some 9Cr steel tubes have reached to 20 years and the total scale thickness has been beyond 300µm. The clarification of creep failure of 9Cr steel tube by scale exfoliation 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.

As the result, in all boilers investigated, scale exfoliation inside the tube occurred. But the reduction of the hardness is not found if there is no creep faioccur below the temperature of tube 900K.

Key words: 9Cr, boiler, tube, super heater, steam, creep, rupture, high temperature, oxidation scale, heat transfer, NDT, non destructive testing