Non-destructive measurement of hardened surface layer thickness with velocity change of Rayleigh wave

摘要

A method to measure the layer thickness of nitrided processed steels non-destructively to assure durability is developed using Rayleigh wave velocity change. The technique uses tandem probes, a transmitter and a receiver. Since the Rayleigh wavevelocity differs depending on its frequency, the rate of velocity change is calculated by a cross-spectrum method, which calculates the difference in traveling time between the memorized wave traveling in a virgin material and that traveling in a material being tested. Velocities in some specimens of which layer thicknesses are already known are measured to obtain reference data and correlation function between the layer thickness and the rate of change in velocity is derived. The thickness of the testmaterial is then quantified from this velocity change and the correlation function. The method is applied to non-destructive measurement of steam valve rods which are nitrided to prevent them from abrasion damage. The valve rods are cylindrical and their diameters are 22mm to l50mm. Using rods with a nitrided layer of 50μm to 200μmthickness, the layer thickness obtained by the present method is confirmed to be in good agreement with those determined by destructive testing of the hardness measurement (standard error = 20μm).