## 1 Introduction  
The cast stainless steel with a two-phase microstructure of austenite (\(\gamma\)) and \(\delta\)-ferrite have extensively been applied in nuclear industry for a long time because of their great corrosion resistance, high mechanical strength, and good weld ability [1; 2]. Some of the cast stainless steels, CF3, CF3M, CF8, CF8M, and CF20, have been used in the power plant for valves, pump casings, and the primary loop piping of pressurized water reactors (PWR). Even though these stainless steels have excellent performance, there are various mechanisms that may lead to the degradation of cast stainless steel during reactor service. One of the mechanical property degradation mechanisms is thermal embrittlement [3; 4; 5]. The high temperature is a major concern for the cast stainless steel pipe of power plants due to the microstructural changes in the ferrite phases. The spinodal decomposition is produced after a long-term isothermal aging treatment at the range 300-500 \({}^{\circ}\)C [6; 7; 8]. The reaction of decomposition can be represented as the \(\delta\)-ferrite transformed into a chromium-rich \(\alpha^{\prime}\) phase and an iron-rich \(\alpha\) phase. Even though just a phase transformation happened in the ferrite phases, the brittleness is a gradual increase in the whole cast stainless steel component. When the \(\alpha^{\prime}\) phases are precipitated, the hardness of ferrite phases would increase and become brittle. If the ferrite phases are brittle, the interface between austenite (\(\gamma\)) and ferrite has the potential to lead to cracking. Under the irradiation condition, the irradiation would cause further hardening and make the phase become more brittle. A crack may initiate a repetitive process during a long operation time, affecting cast stainless steel component integrity. In the light of this, the detection and evaluation of microstructural changes of cast stainless steel has become more important with the growing demands of power plants. Up to now, the destructive tests in laboratories have commonly been used for phase evaluation, these techniques are time-consuming and costly. It is of great interest to develop some non-destructive techniques for real time detection and evaluation of microstructural change. In the previous studies, ultrasonic-based non-destructive testing (NDT) techniques were employed for monitoring the materials [9; 10; 11; 12]. The ultrasonic method is a powerful nondestructive way of evaluating material since the characteristics of ultrasonic waves are related to the properties of the material. In this research, some ultrasonic methods were used to evaluate the microstructural change of cast stainless steel. In recent years, an inspection method known as nonlinear ultrasonic technique has proved to be sensitive enough for detection of material micro-damage and evaluation of change of microstructure. A theoretical model was proposed by [PERSON] et al. [13] to relate the nonlinear ultrasonic parameter to the local strain. The results obtained by [PERSON] et al. [14] on damage of granite samples show that the nonlinear parameter is found to have a positive correlation with the loading level. Moreover, the experimental measurements by [PERSON] et al. [15] indicate a decrease in the ultrasonic nonlinearity parameter with increasing heat treatment time. The results obtained by [PERSON] et al. [16] show that the nonlinear ultrasonic parameter has a close relationship with structural cracks. [PERSON] et al. [17] employed nonlinear ultrasonic measurements to characterize the creep damage in ASME T92 steel welded joints. The traditional ultrasonic method is based on linear theory and normally determines the properties of a material by measuring some ultrasonic parameters, such as velocity, attenuation, transmission, and reflection coefficients. However, it is difficult to detect and evaluate the evolution of microstructure by the conventional ultrasonic technique. However, the nonlinear ultrasonic technique overcomes this limitation.