During the unit startup and power increase,the pressure tapping pipeline downstream of an isolation valve was broken,resulting in a large amount of steam leakage.The pipeline was analyzed by field working conditions,chemical composition,hardness,microstructure and fracture micro morphology.It was found that there were high carbide inclusions in the material structure of the pressure tapping pipeline,weld beading in the welding area,coarse microstructure and softening in the welding heat affected zone,and poor welding quality.The pipeline had significant constraints during installation,and under the action of vibration loads,it lead to fatigue cracking.The crack originated at the root of the weld toe and propagates until it fractured.
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