# Research on Residual Life Estimation Method for KMN Steel Based on Nonlinear Ultrasonic Testing
## 2 Materials and Methods  
KMN steel is a low-alloy high-strength steel with a grade of 15 Cr2 Mo1. Its mechanical properties and chemical composition are shown in Tables 1 and 2. The heat treatment process is shown in Table 3. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Mechanical Properties** & \(\mathbf{R_{\text{m}}}\) **(MPa)** & \(\mathbf{R_{\text{p0.2}}}\) **(MPa)** & \(\mathbf{E}\) **(GPa)** & \(\mathbf{\rho}\) **(kg/m\({}^{3}\))** & \(\mathbf{HV}\) **(kgf\(\cdot\)mm\({}^{-2}\))** \\ \hline KMN & 1193 & 1072 & 205 & 7840 & 335 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mechanical properties of KMN. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Heat Treatment** & **Temperature (°C)** & **Holding Time (h)** & **Cooling Method** \\ \hline Quenching & 970 \(\pm\) 10 & 2.5–3 & Oil cooling \\ Tempering & 570 \(\pm\) 10 & 4–5 & Air cooling \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Chemical composition of KMN (wt.%). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Mechanical Properties** & \(\mathbf{R_{\text{m}}}\) **(MPa)** & \(\mathbf{R_{\text{p0.2}}}\) **(MPa)** & \(\mathbf{E}\) **(GPa)** & \(\mathbf{\rho}\) **(kg/m\({}^{3}\))** & \(\mathbf{HV}\) **(kgf\(\cdot\)mm\({}^{-2}\))** \\ \hline KMN & 1193 & 1072 & 205 & 7840 & 335 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mechanical properties of KMN. The heat treatment process of KMN steel involves quenching at 980 \(\pm\) 10 \({}^{\circ}\)C for 1-1.5 h followed by oil cooling then tempering at 700-750 \({}^{\circ}\)C for 1.5-2 h before air cooling. The metallographic specimen of KMN is corroded by nitric acid alcohol. The microstructure of KMN is shown in Figure 1. The metallographic structure is mainly needle-like and lath-like martensite with some tiny inclusions. First, an ET-10d-240 vibration test bench was used to carry out fatigue tests on KMN specimens with different cycles. Fatigue specimens with different degrees of damage were obtained for non-linear ultrasonic testing, as shown in Figure 2. Before the fatigue test, the surface of the specimen was mechanically polished until no obvious scratches were visible to the naked eye. The surface condition of each specimen was kept consistent. Figure 1: Microstructure of the KMN steel. Figure 2: Fatigue testing machine and fixtures. (**a**) Tension and bending fatigue test. (**b**) Bending fatigue test. Then, the nonlinear ultrasonic testing of the KMN fatigue specimen was studied. In the experiment, the longitudinal wave oblique incidence method was used to excite and receive an ultrasonic Lamb wave in S\({}_{1}\)-S\({}_{2}\) mode. The experimental equipment for the ultrasonic Lamb wave excitation and the measurement is shown in Figure 3[22]. The RiceSNAP system can generate a pulse signal. The signal is transmitted to the ultrasonic piezoelectric transducer after passing through the attenuator. The transducer generates an ultrasonic vibration and is incident on the KMN specimen at a specific angle. After propagation, it reaches the receiving transducer, and the received signal is amplified and input into the oscilloscope. Figure 3: Nonlinear ultrasonic testing system. (**a**) RitecSNAP ultrasonic system. (**b**) Specim and probes. (**c**) Fracture. The RitecSNAP ultrasonic system was used to perform ultrasonic Lamb wave nonlinear measurements on KMN specimens with different bending fatigue damage. During the experiment, a narrowband piezoelectric transducer (2.25 MHz, Olympus NDT Panametrics, A543S) with a center frequency of 2.25 MHz was used as the transmitting probe and a broadband piezoelectric transducer (5 MHz, Olympus NDT, V544) with a center frequency of 5 MHz was used as the receiving probe. The incident angle is 27\({}^{\circ}\) and the Lamb wave signal excited at such a frequency and angle meets both the phase velocity and group velocity matching conditions. The corresponding low-pass filter is 2 MHz, and the high-pass filter is 4 MHz. In the experiment, a gain of 20 dB and an attenuation of 3 dB were used.