Creep failure of the pressure equipment operated at high temperatures in power and petrochemical industry is a frequent reason of its collapse. Ultrasonic waves' velocity is sensitive to change of material properties due to thermally induced degradation processes, especially plastic strain. The article demonstrates that measurement of ultrasonic waves' velocity ratio is able to localize plastic strain induced by various means including creep. Results are confirmed by comparison with hardness and metallographic analysis. Furthermore, statistical significance of the results is tested by t-statistics. The proposed application may help with selection of potentially critical areas during routine inspections on pressure equipment in operation.