Characterisation of suitable fillers for a butt weld of creep aged X20 and virgin P91 pipes

Abstract

The need to improve thermal efficiency in coal-fired power plants and to reduce emissions has led to the development of new creep resistant steels such as P91. These developments led to partial replacement of creep aged X20 pipes with the new P91 material in the power plants. In joining the transition welds, two different filler materials (X20 and P91) were used. Microstructural, mechanical and creep test of both pipe samples cross weld joints were evaluated to identify a suitable filler material. Microstructural studies on different weldment zones(weld metal (WM), heat affected zone (HAZ) and parent material (PM)) revealed comparable shape, orientation, and quantity of the characterised creep resistant metallurgical parameters such as precipitates, sub-grains and dislocation density for both different filler material weldments irrespective of the use of either X20 or P91 WM. Carbon migration was investigated using a scanning electron microscope (SEM) and nanoindentation hardness test across the fusion line. The results revealed a localised denuded zone, with a width of ~20μm located at the fusion line region of the welded joint. The presence of the denuded zone did not affect the mechanical (tensile and hardness tests) and creep (uniaxial and small punch creep test (SPCT))results. Overall characterisation results in relation to the objectives of this study did not reveal any distinctive performance differences resulting from using either the X20 or P91 weld filler for a butt weld of creep aged X20 and virgin P91 pipes. Both weld fillers (X20 and P91) were deemed suitable since they gave comparable results based on short term uniaxial creept tests (i.e. max. 1302 hours of creep test) and from small punch creep tests (SPCT)