Influence of the Magnetic Field on the Behavior of Cracks in Steel after Heat Treatment to a High-Strength State

Introduction . Fatigue failure develops at stresses below the ultimate strength and is characterized by its suddenness and catastrophic consequences. Statistical data indicate that failure under cyclic loading is one of the common types of damage to materials and their performance is largely determined by their resistance to crack growth. In addition to well-known methods for achieving high-strength states, it has been proposed to use heat treatment in the magnetic field (HTMF). However, the mechanisms of crack behavior changes after such treatment and factors affecting crack resistance remain poorly understood. In this regard, this study aims to assess the effect of the structure after HTMF on the kinetic features of fatigue crack growth and the effectiveness of structural barriers formed during HTMF, preventing the destruction of steel. Materials and Methods . The kinetics of fatigue crack development was studied during cyclic testing of prismatic samples using an original setup with a special stabilizer of oscillation amplitude. The occurrence and subsequent development of a crack was recorded by the method of electric potentials. The studies were conducted on steels that had been heat-treated to achieve a high-strength state: 18Kh2N4VA steel after quenching in air with a martensite structure and 30KhGSA steel after isothermal quenching at 380℃ to a lower bainite structure. A magnetic field of 1.6 MA/m was obtained in the magnetic gap of the FL-1 electromagnet. Results . It was found that heat treatment of 30KhGSA and 18Kh2N4VA steels in a magnetic field of 1.6 MA/m led to a noticeable decrease in the rate of fatigue crack propagation. An increase in the threshold stress values for the delamination of the main crack by the tear-off mechanism was noted, which indicated an increase in durability. When analyzing the crack trajectories, an increase in their branching indicators was revealed: an increase in the standard deviation of crack inclination angles, as well as a decrease in the correlation interval of the crack bend inclination relative to the average position by 0.5 μm. These changes were due to the effect of the magnetic field on the microstructure of martensite, the formation of a greater number of effective barriers on the path of crack movement, which ultimately affected the resistance to fatigue failure of steels and their mechanical properties. Discussion . Analysis of the obtained results, based on modern theories of strength and fracture, revealed that the mechanism of viscous destruction, which was typical for the steels under study, worked by the origin, growth and coalescence of pores. Under the influence of normal stresses, vacancies settled on the surface of micropores and as a result, the pore gradually transformed into a crack. Observations of cracks in foils showed that the change in the crack trajectory did not depend on the type of heat treatment and was a random process. Conclusion . The statistical data obtained in this study allow us to conclude that after HTMF, a structure is formed that leads to an increase in the micro-tortuosity of cracks, with a steeper trajectory of bends due to frequent structural barriers. These features of crack behavior suggest that HTMF is a practical method for achieving a high-strength state in steels, which can be applied to a wide range of steel grades without requiring significant changes to their heat treatment processes. By increasing the crack resistance of steels, we can improve the safety of various devices and man-made systems, as well as reduce their costs and maintenance requirements.