Abstract
An excessive concentration of boron in the 9% Cr tempered martensitic steels alloyed with nitrogen has a detrimental effect on creep performance, as relatively large boron nitride phases are formed and can act as the preferred sites for cavity nucleation. In the current research, a systematic investigation has been performed documenting details of creep damage in the heat affected zone of a weld joint constructed by using a 9% Cr Grade 92 steel. A combination of correlative characterisation techniques has been applied to monitor and quantify the evolution of creep damage within the heat affected zone through the entire creep life. The utilisation of cryo-fractography, two-dimensional electron-based microscopic characterisation and three-dimensional tomography based on focused ion beam microscopy have confirmed the close association between creep cavities and boron nitride phases from the initial stage of creep. It is revealed that the thermal transients imposed during welding result in the decohesion at the interface of boron nitrides with the surrounding matrix. This decohesion is a result of significant thermo-mechanical mismatch between boron nitrides and the tempered martensitic matrix. The voids initiated at these locations grow and eventually link-up during creep exposure, leading to cracking and fracture.
Highlights
State-of-the-art power plant steam cycles are routinely designed to operate at temperatures ≥873 K (600 °C)
Additional microstructural complexity is introduced into the Heat Affected Zones (HAZs) when the parent metal is subjected to multiple thermal cycles during conventional welding and Post Weld Heat Treatment (PWHT) processes
Some observed cavities are not associated with BN particles (Fig. 7b), whilst the others contain BN particles (Fig. 7c) with chemical composition confirmed by Energy Dispersive X-ray (EDX)
Summary
State-of-the-art power plant steam cycles are routinely designed to operate at temperatures ≥873 K (600 °C). The increased values of the design allowable stress for Grade 92 as compared to the precedented Grade 91 steel have recently resulted in the adoption of this material for the high temperature components in fossil-fired supercritical and Combined Cycle Gas Turbine (CCGT) power plants. The fabrication of the hot-section components with complex geometries typically requires numerous welds. These are usually fabricated by using a multi-pass welding process in combination with a subcritical Post Weld Heat Treatment (PWHT) [1]. In these cases, additional microstructural complexity is introduced into the Heat Affected Zones (HAZs) when the parent metal is subjected to multiple thermal cycles during conventional welding and PWHT processes. Detailed metallographic examination has confirmed the distribution of microstructure in the HAZ of a multi-pass weld that involves multiple thermal cycles [5]
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