Effect of hardening on the magnetic behavior of AISI 1045 steel

Hardening is an effective method to improve the hardness of the steel construction elements of an electric machine, such as rotor shaft. Surface hardening is typically employed for shafts as it provides a material with hard wear-resistant surface (martensite) and tough and ductile core (ferrite and pearlite), with the transitional zone between them (martensite, ferrite, and pearlite). The investigations were performed on the specimens of AISI 1045 steel, which is commonly used in electric machines. There were three categories of specimens, each of them representing a region in a surface hardened shaft – normalized specimens found in as-delivered steel (core), partially hardened specimens obtained by partial hardening of normalized specimens (transitional zone), and fully hardened specimens (hardened surface), also obtained from the normalized steel. Comparison of the magnetization curves showed that partially and fully hardened specimens exhibited a decrease of saturation magnetic flux density for about 11% and 7%, respectively, compared to the normalized specimens. The magnetizing curves from this study can be directly used in the magnetic simulations of a surface-hardened shaft. This allows more accurate electromagnetic design of electric machines, in which the shaft represents an important part of the magnetic circuit, e.g. two-pole wound-rotor synchronous machine.