Application of Microfocus Rod Anode in Qualification of Tube-to-Tube-Sheet Welds in Steam Generators

Abstract

Tube-to-tube-sheet (TTS) welds, occurring in heat exchangers, condensers, and steam generators, are exempted from volumetric nondestructive examination, such as radiography or ultrasonic tests, due to their geometry. An approach of stricter quality control at qualification stage is adopted, apart from tests like liquid penetrant test and leak test applied to production welds. For qualification purpose, a mock up block is destructively examined for weld quality by microscopy and/or macroscopic examination. This paper shows that it is possible to carry out radiography of TTS welds using microfocal x-ray unit with back-throw type rod anode attachment, which sends radiation in nearly radial direction. It demonstrates the use of the same technique for the qualification weld block also. It further shows that the qualification weld slices can be radiographed in axial direction to enable 100% volumetric examination. Besides, it will make metallography more effective and meaningful; instead of a random section, it can then address the plane with a flaw using the help from radiography test. The paper suggests that the code adopt radiography in addition to metallography during qualification for better quality assurance.