The Spalling of Steam-Grown Oxide Scale from SUS 321 HTB Tube Steel

摘要

It is well known that oxide scales formed on the steam side of the superheater and reheater tubes in power generation boilers sometimes spall after a long time operation. Spalling of oxide scales may cause blockage of the tubes as well as erosion of the first stage nozzles and buckets of the turbines. This study was carried out to obtain the knowledge about the spalling behaviour of the steam oxide scale in stainless-steel tubes. SUS 321 HTB tube specimens were heated in steam atmosphere at 600-750 deg C for more than 300 h, and then cooled with 50-170 deg Ch~(-1). Mass change and cross section of specimens were examined to evaluate the scale spalling. Obtained results are summarized as follows:Micro-cracks were observed along the interface between the outer and inner oxide scales for all specimens but macroscopic spalling of the outer scale occurred only for the specimens heated at >700 deg C.Extension of the interface separation was independent of heating temperature and cycles, and increased with inner scale thickness.Spalling of outer oxide scales was strongly affected by heating temperature and heating cycles.