SAE J403 steel is a high-quality carbon structural steel with good strength, hardness, toughness and other material properties. Therefore, it has a wide range of applications. In the production process of automotive electronic parking gear, in order to improve the steel, hardness, wear resistance, fatigue strength and other material properties of the parking gear, it is necessary to conduct high-frequency induction hardening heat treatment on SAX carbon steel. The carbon steel after high-frequency induction hardening has better performance. Due to the difference in quenching process, time and material quality, the thickness of hardened layer formed after high-frequency induction hardening of materials is different, and the difference in the thickness of hardened layer has a significant impact on the properties of carbon steel materials. Therefore, strengthening the depth detection of hardened layer of carbon steel has important economic and market benefits. In the research field of hardened layer testing, the traditional testing methods are mainly metallographic method, hardness method and eddy current testing method. These testing methods have different degrees of destructiveness and unstable testing effect for the samples to be tested. The nonlinear ultrasonic testing method proposed in this paper is to use FFT to analyze the frequency spectrum of the ultrasonic signal, and use the ultrasonic nonlinear coefficient to characterize the hardened layer depth of carbon steel, so as to realize the effective detection of the hardened layer depth.
(1) Ultrasonic testing can be used to detect and characterize the depth of hardened layer of low carbon steel, as well as the quenching quality of carbon steel.
(2) The linear relationship between the ultrasonic nonlinear coefficient and the hardened layer of carbon steel was established. With the increase of the thickness of the hardened layer, the fundamental wave of the low-carbon steel sample increases continuously, and the ultrasonic linear coefficient of the material shows a growing trend. After quenching, the grain density of the material increases continuously, and the ultrasonic property of the quenched carbon steel material is better.
(3) Fitting the acquired ultrasonic signal, the relationship between the hardening layer thickness and the ultrasonic nonlinear coefficient is obtained as f (x)=a (sin (x – π))+b ((x-10) ^ 2)+c, where a is 0.00005986, b is -0.000000823, and c is 0.001596.
A large number of studies have shown that nonlinear ultrasonic testing methods can effectively characterize the properties of materials. The ultrasonic transducer with the center frequency of 5MHz is used to collect the ultrasonic signals of samples with different hardening depth, and the ultrasonic nonlinear coefficients of different samples are calculated from the amplitude of the fundamental wave and the amplitude of the second harmonic wave, so as to effectively characterize the thickness of the hardened layer of carbon steel.