The precipitation and coarsening of Laves-phase in the fine grained heat-affected zone (FGHAZ) of a 9% Cr steel P92 welded joint during thermal aging at 923 K were investigated and compared to the base metal (BM), in order to clarify their effects on the Type IV fracture. Laves-phase precipitated mostly on the prior austenite grain boundaries of the FGHAZ. In comparison with BM, FGHAZ contained more grain boundary areas and can provide more nucleation sites for Laves-phase, resulting in an accelerated precipitation and rapidly reaching to the around 1.0% of saturated volume fraction. The coarsening of Laves-phase precipitates in FGHAZ was also much faster than that in BM, enhanced by the contribution of grain boundary diffusion resulted from its finer prior austenite grains. The FGHAZ had denser and smaller Laves-phase precipitates during the precipitation period in comparison with BM, obviously improved the creep strength by precipitation hardening. However, this effect in FGHAZ reduced sharply during coarsening owing to its coarsening rate greater than that of BM. In addition to the initial coarse polygonal subgrains with low dislocation density in FGHAZ produced by the weld thermal cycle and subsequent tempering in post-weld heat treatment (PWHT), coarse Laves-phase precipitates on grain boundaries formed in the long-term thermal aging, contributing to the formation of the creep cavities, can also play a key role in Type IV fracture of welded joint in 9% Cr steels.
|Number of pages||10|
|Journal||Journal of Materials Processing Technology|
|Early online date||15 Dec 2014|
|Publication status||Published - 1 May 2015|
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- Department of Engineering and Technology - Senior Lecturer in Mechanical Engineering
- School of Computing and Engineering
- Centre for Precision Technologies - Member
- Centre for Thermofluids, Energy Systems and High-Performance Computing - Member