Creep detection of Hastelloy X material for gas turbine components with nonlinear ultrasonic frequency modulation

Abstract

Creep damage is one of the main failure modes in hot-gas‑leading components in gas turbines, which results from high temperatures along with mechanical loads. The aim of this study is to clarify the metallurgical creep behaviour of the Hastelloy X material and detect and evaluate creep damage at an early stage with a nonlinear ultrasonic modulation technique. For this purpose, multiple samples were examined to demonstrate that pores and microcracks in grain boundaries spread from the outside to the inside. Inside the specimen, molybdenum was identified as the main precipitation element. In addition, the chromium diffusion in the outer areas led to the depletion of this element and favoured the formation of pores and microcracks. Failures were proven with nonlinear dual-frequency ultrasound technology. Moreover, two different longitudinal waves were sent into the samples to use harmonic and modulated response frequencies for evaluation. As a result, harmonic frequencies offered a favourable prediction of pore sizes, whereas defined sideband frequencies reacted very sensitively to the damage density and area distribution of the failures. This study offers a method for detecting creep damage with nonlinear ultrasound techniques at an early stage as well as for differentiating between pores, microcracks, dislocations and precipitation. Therefore, the design of future gas turbine components made of Hastelloy X can be adapted with regard to the shown metallurgical behaviour and damage signatures.