Abstract: There are some shortcomings in application of ultrasonic probe with single crystal chip, large refraction angle and short cutting edge in welding fault detection for tubes with minor diameter and thin wall (4～8mm) such as the incorrect positioning, the difficulty to determine on the nature of the defect and the lower defect resolution. These problems were analyzed and researched, and relevant measures were formulated. So the welding fault defecting method for tubes with minor diameter and thin wall become more accurate, the escaping from detection or wrong decision were avoided, and the practical application effect was remarkable.

Key words: tube with minor diameter and thin wall, ultrasonic detection, problem, analysis