Identification and Characterization of Intercritical Heat-Affected Zone in As-Welded Grade 91 Weldment
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row intercritical heat-affected zone (ICHAZ) forms at the outer edge of the heat-affected zone in a steel weldment due to exposing a peak temperature between AC1 and AC3 critical temperatures during welding thermal cycles.[1] The Type IV cracking, a premature and insidious creep failure mode in creep-resistant steel weldments, is always observed in the ICHAZ.[2–5] In failure analysis of Grade 91 steel (9Cr-1Mo-V-Nb, ASTM A335 P91) weldments, the ICHAZ is found to have the highest recovery/recrystallization rate during post-weld heat-treatment (PWHT) and during high-temperature creep.[6–9] To clarify the Type IV cracking mechanism(s), phase transformation and characteristic structure in the ICHAZ need to be identified carefully. It is reported that preferential nucleation and limited growth of austenitic grains due to insufficient peak temperature and short soaking time in ICHAZ lead to the unique mixture of ‘‘new’’ and ‘‘old’’ grains on-cooling.[1,2] Preferential nucleation sites for austenitization are reported to shift from lath boundaries to prior austenite grain boundaries at increased temperatures in ultralow-carbon 13 pct Cr-6
YIYU WANG and RANGASAYEE KANNAN, Graduate Students, and L. LI, Professor, are with the Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2V4 Canada. Contact e-mail: [email protected] Manuscript submitted February 17, 2016. Article published online September 22, 2016 5680—VOLUME 47A, DECEMBER 2016
pct Ni martensitic stainless steel.[10] But these preferential nucleation sites of partial austenite transformation have not been verified in the ICHAZ. Furthermore, the crystallography and morphology of lath martensite, on-cooling within the ICHAZ, have not been studied. Park[4] proposed a local band, free of martensitic transformation within HAZ after the welding cycle, which would have caused the local high residual stress and limited dislocation density. The exact location of this martensite-free band has remained unknown. Additionally, a soft region was reported to be associated with the ICHAZ where the Type IV cracking occurs.[4,11] But the exact location of this soft region is still uncertain. To answer the questions above, the first priority is to pinpoint the precise location of the ICHAZ. Metallurgical identification of the ICHAZ is challenging due to its narrow width (less than 200 lm) in the entire range of HAZ. The resolution of conventional optical microscopy is not high enough to observe the heterogeneous structures, such as martensitic blocks/laths and fine precipitates. Under the scanning electron microscope (SEM), it is difficult to locate ICHAZ without etching the specimen first. Even with an etched specimen, any soft region still cannot be directly identified based on morphology alone. On the other hand, for phase identification and orientation analyses by using EBSD, the etched specimen surface may lose some critical structural information. A repeatable and convenient method is needed to positively identify and study the ICHAZ.[12] In this wo
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