Abstract
Premature creep failures at the intercritical heat affected zone (ICHAZ) of creep-resistant steel weldments have been frequently reported. However, the creep degradation mechanism of different microstructure constituents in ICHAZ is complicated and needs further clarification. In this work, Grade 91 steel was intercritically heat-treated at a temperature (860 °C) between the critical temperatures AC1 and AC3, and a correlation between microstructure and mechanical properties of the heat-treated specimen was built. The effects of austenitization and tempering resulting from the intercritical treatment (IT) differentiated the local strain energies between the two microstructure constituents: newly transformed martensite (NTM) and over-tempered martensite (OTM). The formation of NTM grains led to a hardness increase from 247 HV0.5 in the base metal to 332 HV0.5 in the IT specimen. The ultimate tensile strength (UTS) increased from 739 MPa in the base metal to 1054 MPa in the IT specimen. Extensive growth of the OTM grains and rapid recovery of NTM grains took place simultaneously in the IT specimen during a typical tempering at 760 °C. These microstructure degradations led to a lowered hardness of 178 HV0.5, a reduced UTS of 596 MPa, and a poor creep resistance with a minimum creep strain rate of 0.49 %/h at 650 °C in an IT + tempering (ITT) specimen.
Highlights
Nine to twelve percent chromium creep strength-enhanced ferritic (CSEF) steels are widely used to fabricate high-temperature steam components, for example, headers and mainstream pipes, in thermal power plants [1,2,3]
The intercritical thermal cycle in the intercritical heat-affected zone (HAZ) (ICHAZ) has promoted multiple microstructural evolutions occurring simultaneously, including partial austenitization, precipitate dissolution or coarsening, and matrix tempering [10,11,12], which have led to very complicated microstructures [13]
Shows a typical tempered martensite microstructure with fine precipitates distributed along the prior austenite grain boundaries and martensitic packets/blocks after tempering
Summary
Nine to twelve percent chromium creep strength-enhanced ferritic (CSEF) steels are widely used to fabricate high-temperature steam components, for example, headers and mainstream pipes, in thermal power plants [1,2,3]. The structural integrity issues of these welded components exposed to severe service conditions (higher steam temperatures and pressures) in advanced ultra-supercritical power plants are drawing increasing attention from safety and maintenance management perspectives Among these issues, premature failures—especially Type IV cracking at the heat-affected zone (HAZ) of welded joints—significantly shorten the service lifetimes of those components [4,5,6,7]. A typical intercritical microstructure features a mixture of untempered martensite, transformed from austenite formed during heating, and overtempered martensite (OTM) originating from the base metal (BM) [14,15] This weak microstructure in the ICHAZ, which results in faster creep strength degradation, acts as a metallurgical notch across welds.
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