Evaluating surface hardened steels by laser-acoustics

Abstract

The use of surface acoustic waves was studied as a means of evaluating surface-hardened steels. Four different grades of steel, 42CrMo4, C45, X153CrMoV12.1 and 90MnCrV8, were laser hardened up to a depth of 1.5 mm. The construction of a laser acoustic handheld probe enabled the measurement of the propagation velocity of the surface acoustic waves on large components. The measurement yields the velocity of the surface acoustic wave depending on frequency in a range from 1 to 10 MHz. The varying frequency enables the surface acoustic waves to penetrate varying depth of the material. It was investigated, if the hardening depth and the surface hardness can be deduced from this measurement. This methodology rests on the assumption that the velocity of the acoustic wave varies, if the microstructure of the steel is hardened by a martensitic transformation. Test samples were also annealed up to 620 °C to vary the hardness of the laser hardened zone. This was performed to investigate if the acoustic wave velocity correlates with the hardness. The sensitivity of the acoustic method, which can be used to determine the hardening depth and the hardness, depends on two aspects: 1. the amount of the effect of the martensitic hardening on the wave velocity of the steel and 2. the accuracy of the velocity that can be measured. A generalized dispersion curve was calculated using the theory of the surface acoustic wave to deduce the hardening depth from the velocity. Establishing this curve requires two measurements; the measurement of the wave velocities in the hardened and non-hardened steel. Using this methodology, it was found that the hardening depth could be determined with an error not higher than 15%.