Study On Characterization Of Laser Hardened Layer By Laser Ultrasonic Surface Wave

Laser hardening is a new surface treatment technology,which has shown the potential to replace the traditional surface quenching technology.Two key characteristics can be used to describe the properties of the hardened layer constructed by laser hardening on the surface of the material,namely,the thickness and the microstructure of the hardened layer.However,the detection of laser hardened layer is still in the traditional damage detection stage,namely,metallography and hardness method.Destructive detection method is inefficient and destructive.It is not suitable for complex,large and costly parts,and cannot achieve full coverage detection.At present,the nondestructive testing and characterization techniques of hardened layer properties are still lacking.Laser ultrasonic(LU)surface wave technology has the advantages of wide band,non-contact and non-destructive,and can be used to detect and characterize laser hardened layers.Therefore,this paper aims to establish a nondestructive characterization method of laser hardened layer based on LU surface wave technology.The specific research content includes the following aspects:Firstly,two kinds of LU excitation mechanisms based on ablative effect and thermoelastic effect are introduced.The wave equation and dispersion equation of surface wave in a bilayer model are derived,and the inversion method of dispersion curve of surface wave is introduced.The finite element method(FEM)is used to solve the ultrasonic field excited by laser in block material,and the velocity of surface wave is obtained and its propagation characteristics are analyzed.Secondly,a two-layer model of laser hardened layer and matrix based on elastic modulus change is presented,and a theoretical method to solve the depth of hardening is presented.The positive and abnormal dispersion effects of the two-layer model are simulated by FEM,and the validity of the calculation method for the depth of hardening layer is verified.Thirdly,45 steel and Cr12 Mo V were taken as experimental research objects,and the samples of the two materials were pretreated and then processed by laser hardened.The microstructure and depth of hardening were studied by destructive metallography and hardness methods.The results show that the laser hardening results in martensitic transformation in quenching center,grain refinement,tempering and precipitation of granular carbides in lap zone.The effective depth and total depth of hardening for the two materials were obtained by metallographic method and hardness method respectively.However,the results of the two methods have obvious differences in the overall variation trend and specific values,indicating that the two methods have limitations and can be used as reference,but not as an absolute standard.Finally,a nondestructive testing method based on dual-source laser ultrasonic scanning is proposed to characterize the laser hardened layer.Laser hardened 45 steel and Cr12 Mo V were scanned under all-optical conditions.The distribution information of hardening depth was measured by inverse solution of surface waves dispersion curve.Compared with the results of metallography and hardness method,the validity of the measurement results of laser ultrasonic method was proved.According to the attenuation of surface waves and metallographic results,it is found that the attenuation of surface waves decreases with the martensitic transformation in the quenching center,while the attenuation of surface waves increase with the precipitation of tempered carbides in the lap zone.The inverse dispersive curve method solves the problem of laser hardening depth,while the acoustic attenuation method solves the problem of surface microstructure change.Combined with these two methods and based on the scanning data of one experiment,a relatively comprehensive nondestructive evaluation of laser multi-pass hardened steel was carried out.Through the above research,the feasibility of using the laser ultrasonic surface wave nondestructive testing and evaluation of laser hardened layer proposed in this paper is verified,which provides a fast and effective nondestructive testing method for laser hardened layer,and has good industrial application value.