Non-destructive monitoring microstructure and properties changes of 35CrMo steel cylinder using magnetic Barkhausen noise and hysteresis loop techniques

Abstract Background An important subject during the inspection and quality control of heat-treated steel components is the accurate determination of their microstructure and properties through efficient methods. Traditional laboratory-based techniques, such as optical microscope and mechanical test, are expensive and require a lot of time for samples preparation. Among the various inspection methods for ferromagnetic materials, magnetic non-destructive testing technology is very suitable for on-line detection of product quality, because it is convenient and sensitive to changes in microstructure. Objective The main task of this work is to develop a reliable magnetic method of nondestructive testing of the heat treatment quality of steel components from high strength alloy steel. More specifcally, the relationship between magnetic properties and mechanical properties is provided for rapid on-site inspection of product quality. Methods Based on hysteresis loop and Barkhausen noise method, the microstructure characterization and residual stress evaluation of 35CrMo steel cylinders under different heat treatment processes are studied. Simultaneously, microstructure observation and hardness measurement are also carried out for the experimental validation. Finally, based on the destructive mechanical test, the linear relationship between the magnetic properties of the cylinder material and its main mechanical properties is fitted. Results The quenching and tempering heat treatment processes result in significant variations in the coercive force and the RMS value of the MBN voltage of the 35CrMo steel cylinders, respectively. The surface hardness is in good agreement with coercive force, and the residual stresses levels are evaluated quantitatively via the calibration curve of stress-RMS value. In particular, the coercivity has a good fit with the hardness and strength, the corresponding correlation coefficients are 0.92 and 0.98, respectively. Conclusion To avoid destructive testing, magnetic hysteresis loop and Barkhausen noise techniques could be a better tool for the evaluation of quality of heated steel products.