Investigation of microstructural changes in CR9MO1 due to thermal aging using SHG and NRUS methods
Fahse, Daniel Niklas
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This research improves the experimental setup for non-contact nonlinear resonance ultrasound spectroscopy (NRUS) developed by Maier et al. and enhances the post processing to increase the operator-independence and lower the influence of environmental changes on the measurement by modifications in the measurement procedure and the post processing. Furthermore, the influence of ambient temperature is theoretically described. This theoretical approach is valuated on data collected with a temperature measurement added to the NRUS measurement setup. This approach could simplify the measurement procedure in the future by using the temperature measurement. The improved measurement setup is then used to study thermal aging by measuring the non-classical hysteretic nonlinearity parameter alpha of the ferritic martensitic steel Cr9Mo1 at increasing thermal aging time. The results are compared to other monitoring techniques such as hardness, Young’s modulus and quadratic nonlinearity parameter beta. alpha and beta show surprisingly analogous functional dependency on thermal aging. To gain more insight about the microstructural effects and how they effect both nonlinearity parameters, the results are also compared to an in the same way monitored thermal aging process of a 17-4PH precipitate hardening stainless steel. The study is concluded with a hypothesis which can explain the observed functional dependency on aging time.