Magnetic investigation of strains and stresses in surface-hardened materials

The paper investigates the effect of tensile strains on the field dependence of differential magnetic permeability of double-layer composite steel 45 specimens simulative of surface hardening and steel 45 specimens subjected to laser heat hardening. It is shown that the field dependence of differential magnetic permeability is representative of the multilayer micro structure and of the changes occurring in each layer under tension. It is found that significant shifts of the magnetic permeability peaks toward stronger magnetic fields and rapid decreases in peak heights result from the involvement of different layers of the specimens in plastic deformation and proceed in a strictly defined sequence as the stress corresponding to the yield limit of each layer is reached. It is demonstrated that the lost bimodal character of the field dependence of differential magnetic permeability after unloading suggests that the material with a hardened surface layer nears fracture.