Cause Analysis of Massive Oxidation Scale Exfoliation from Final Reheater in 660 MW Ultra-Supercritical Boiler

doi:10.3969/j.issn.1000-7229.2014.11.023

Abstract

Abstract:

To understand the reason of massive oxidation scale exfoliation from the final reheater （RH） tubes of two 660 MW ultra-supercritical （USC） boilers after operating for 1.7×104 to 20×104 h， TP347H tubes cut from the final superheater （SH）， 3rd SH （rear plate superheater）， and final RH were investigated for chemical compositions analysis， tensile testing at ambient temperature， hardness test， metallographic examination， etc. Microstructure analysis shows that there is a layer of fine grain zone （No.9-10 grade grain size） on the inner surface of the 3rd SH and the TP347H tubes in final SH， but only the coarse grain zone can be seen in final RH. The presence of the coarse grain zone on inside surface of RH tubes causes the massive oxidation scale exfoliation from RH tubes. The grain size of TP347H tubes has great influence on the forming speed of oxide scale on the inside surface. It is found that fine grain size and high density grain boundaries can increase “short way” Cr element diffusion rates in the TP347H austenitic steel， thus increasing the resistance to steam oxidation during operating at high temperature. Finally， suggestions on material selections for 600 ℃ temperature class USC boiler tubes were presented for consideration.

Key words: ultra-supercritical unit, final reheater, oxide scale, grain size, resistance to steam oxidation

YAO Bingyin， LI Zhigang， DANG Lijun， ZHANG Zhibo， HOU Zhaotang， TANG Liying. Cause Analysis of Massive Oxidation Scale Exfoliation from Final Reheater in 660 MW Ultra-Supercritical Boiler[J]. ELECTRIC POWER CONSTRUCTION, 2014, 35(11): 132-138.

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Cause Analysis of Massive Oxidation Scale Exfoliation from Final Reheater in 660 MW Ultra-Supercritical Boiler

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