Non-Destructive Testing Of The Thickness Of Nitriding Layer On Surface Of 38CrMoAl Steel By Using Laser Ultrasound Surface Wave

With the rapid development of modern industrial technology,metal materials are often in a harsh working environment in industrial production,and the structural surface is inevitably damaged to varying degrees,and the life of the workpiece depends largely on the surface properties of the material.The surface strengthening layer plays an important role in improving the fatigue resistance of metal parts,and the thickness of the strengthening layer is one of the important indexes for evaluating the performance of the strengthening layer.Therefore,it is necessary to perform nondestructive testing on the thickness of the strengthening layer.In order to realize the rapid non-destructive detection of the thickness of the surface strengthening layer of metal materials,a new type of detection method-laser ultrasonic surface wave technology came into being.Laser ultrasonic surface waves have the advantages of long propagation distance,wide detection range and easy scanning.They overcome the application constraints of traditional ultrasonic in harsh environments(high temperature,heavy load,corrosion,etc.)and have become an important technical means in the field of nondestructive testing.one.The paper studied the method of nondestructive testing of the thickness of nitriding layer on the surface of 38 CrMoAl steel by laser ultrasonic surface wave,which mainly includes the following aspects.First of all,this paper introduces the principle of laser excitation of surface acoustic waves under the thermoelastic mechanism and cauterization mechanism.The thermoelastic excitation mechanism for the surface of the material is discussed.The temperature field,displacement field and stress field of the laser ultrasonic are analyzed according to the laws of thermodynamics in solid media.the basic characteristics of laser ultrasonic surface waves are introduced,and the sensor method and optical method commonly used in laser ultrasonic technology are summarized.Secondly,the laser ultrasonic visualization system with automatic scanning function is introduced.The working principle is that the excitation source generates the pulsed laser.At the same time,the pulsed laser is used to scan the detected workpiece through the electric small mirror.The laser interferometer is used to vertically illuminate the fixed point to collect the ultrasonic wave.The surface displacement signal is processed by the algorithm to generate a dynamic image of the ultrasonic wave propagation.Subsequently,the crack depth was detected by laser ultrasonic surface wave.The interaction process between the surface acoustic wave excited by the laser point source and the surface defects at different depths was observed experimentally.The results show that the reflection and transmission coefficient of the surface acoustic wave change with the depth of the surface defect;when the depth is different,the dynamic image of the surface acoustic wave propagates with time and the change of the B-scan signal.According to the relationship between the depth of the defect and the reflection and transmission coefficient,the laser ultrasonic visualization device is used to evaluate the crack depth by non-destructive measurement.In addition,it is also verified that surface waves of different frequencies have different propagation depths.Finally,the thickness of the nitriding layer on the surface of 38 CrMoAl steel was detected by laser ultrasonic surface wave.Since the surface wave propagation velocity is related to the properties of the material,and the surface wave propagation depth is about one wavelength ?(?=v/f),the surface ultrasonic wave is obtained by extracting the laser ultrasonic surface wave velocity at different frequencies 1).The dispersion curve.The experimental results show that there is a special point on the dispersion curve of the samples with different nitriding layer thicknesses.The center frequency corresponding to this point has a monotonous decreasing relationship with the thickness of the nitriding layer.With this relationship,the measurement of the thickness of the material nitrided layer can be achieved.