Non Contact Surface Defect Detection Technology Of Laser Ultrasonic On Hot Metal

The chemical industry pressure vessel majority runs under the harsh operating modes such as high temperature, high pressure and so on. Along with the lengthening of enlistment period, the damage problems are also constantly increasing, which may lead to security incidents. In order to solve these problems, some methods are proposed. Among these solutions, the non contact laser ultrasonic technology has significant advantages.It can detect the defects on hot metals over a long distance, and can not be affected by heat tolerance ability, sound energy attenuation and so on. Therefore, it is widely used to realize the nondestructive testing under complex environments.In this paper, the theoretical and experimental studies on surface defect detection of hot metals are carried out. Theories about laser inducing surface acoustic waves(SAW) and detection methods are introduced in detail. The process of the interaction between SAW and surface defects is discussed by using the finite element analysis methods. Simulation is carried out at different temperatures, with the rising of the temperature, the velocity reduces and the amplitude attenuation of the SAW increases. The numerical simulation about the scanning laser point source method at 500 degrees Celsius is done to compare with the simulation at room temperature, the results proved that the scanning laser point source method can be applied at hot metals. The effects of defects in different depth on SAW propagation are analyzed.With the rising of the defect depth, the amplitude of the reflected wave increases, and the amplitude of the transmitted wave decreases.Non contact defect detection system based on high temperature is set up. The effect of the temperature on defect detection is verified by experiments. During the experiments, the relationship between them is obtained, and the scanning laser point source method is used to analysis the location and depth of the defect on the surface of the hot metal successfully.Through experiments, the relationship between the depth and amplitude can be expressed in relational expressions.