Surface ultrasonic wave detection technology was studied on the longitudinal defect on the outer surface of T91 steel tube.The change rule of acoustic beam's incidence angle on the surface of tube was analyzed.According to the tube's size and the acoustic velocity in T91 steel tube

appropriate incidence angle and point of the probe were calculated.Artificial defect reference block was designed

compared with the natural defect test block

a reasonable detection technology was formulated.The test results show that the designs of the probe and the process have advantages of high sensitivity and signal-to-noise ratio.This technique solves the problem of no current reference standard for the detection of surface wave of small diameter tube.It also provides reference for other steel tube.