Session: Research Posters

Paper Number: 119212

119212 - Influence of Long-Term Operation on Creep Deformation and Rupture Strength of mod.9cr-1mo Steel Welded Joint 

 Mod.9Cr-1Mo steels are widely used for steam pipes in ultra-super critical (USC) thermal power plants due to its superior high temperature strength and corrosion resistance properties. It has been reported that creep damage extended preferentially in the heat affected zone (HAZ) of Mod.9Cr-1Mo steel longitudinal welded joint by internal pressure. Microstructure of the welded joint gradually changes during long-term operation resulting in deterioration of creep strength. For that reason, it is necessary to clarify influence of long-term operation on creep strength of the welded joints. However, it is difficult to cut out a standard sized creep specimen with 10mm diameter directly from weld metal and narrow HAZ. Therefore, a miniature size creep specimen has to be used to obtain creep properties of constituent materials.

 In this study, a 100,000 hours operated boiler welded joint which contained coarsening microstructure (CM) within dendrite microstructure (DM) of weld metal was used as a tested material. Creep tests were performed using tensile loading type miniature specimens with 1mm diameter and 5mm parallel length of constituent materials such as base metal, HAZ and weld metal. Both standard and miniature size welded joint specimens taken from the welded joint were also tested. Creep tests for miniature specimens were performed by a special designed creep testing machine in inert gas atmosphere.

 As a result, minimum creep strain rate of the weld metal containing CM was 1000 times faster than that of new weld metal indicating significant reduction of creep deformation resistance and, was 10 times faster than a long-term used weld metal with only DM. Creep rupture time of the standard welded joint specimens, which failed at the CM in weld metal, was 4 times shorter than that failed at the DM. The miniature welded joint specimens failed at either HAZ or weld metal depending on influence of the CM containing in weld metal.

 Finite element creep analyses of both welded joint specimens were conducted to figure out stress and creep strain distribution from the base to weld metal. The standard welded joint analysis model was created by reproducing the shape of CM in the weld metal. The miniature welded joint analysis model has either CM or DM in the weld metal considering an extreme condition. Material constants in Norton’s law of the constituent materials were determined from the creep test results of the miniature specimens.

 Although the maximum value of axial stress and triaxial stress states occurred at HAZ in new standard welded specimen, those occurred at around boundary between CM and DM in the weld metal of the used welded joint. On the other hand, uniaxial stress uniformly distributed through base metal to weld metal in the used miniature welded joint specimen. Analysis results of both standard and miniature welded joint specimens with and without CM in weld metal indicated that creep strain accumulation in CM was larger than that in DM causing shorter rupture time of the welded joints with CM in weld metal.

Presenting Author: Hayato Ikegami Chiba institute of technology

Presenting Author Biography: Undergard degree in mechanical engineering Chiba institute of technology

Authors:

Hayato Ikegami Chiba institute of technology
Takashi Ogata Chiba institute of technology