Specific Features of the Behavior of the Coercive Force in Low-Carbon Plastically Deformed Steels

Abstract

The relationship between the coercive force in low-carbon steels under plastic extension and compression and the values of deformation and actual and residual stresses are studied. This relationship is investigated for both "slow" loading (when an equilibrium deformation is attained for each load value) and "fast" loading (when such equilibrium is not attained). It is shown that (i) a comparatively small increase in the coercive force in a loaded condition is due only to an increase in the density of dislocations in the process of plastic extension; (ii) a significant steep increase in the coercive force accompanying removal of the load from a plastically stretched specimen is fully due to residual compression stresses; (iii) the values of the coercive force under "slow" and "fast" loading are significantly different in the region of small deformations less than 2.5%; (iv) these values are close to each other in the loaded state for all deformations up to 10%; (v) a relief of the compression stress that creates plastic deformations causes a steep decrease in the coercive force that is as large as its increase following relief of plastic extension; this is explained by the emergence of a significant residual tension stress. The obtained results are of importance for the use of the method based on measuring the coercive force to test steel structures under the conditions when plastic deformations develop. INTRODUCTIONElastic and plastic stresses (exceeding the yield strength of steel σ fl ) are known [1-6] to affect such magnetic parameters of ferromagnetic steels as coercive force H c , residual magnetization, and initial magnetic permittivity (i.e., the quantities widely used in the magnetic method of nondestructive testing (NT) [7]) in a different way. These differences are obviously related to the differences between the mechanisms of elastic and plastic deformations [8]. Namely, while a growth of stress σ during elastic deformation results in changes in interatomic distances and in the volume of the metal (Poisson's ratio ν has approximately the same value of 0.3 for all steels, and density of dislocations ρ increases insignificantly), a plastic deformation develops owing to the move of dislocations along slip planes in each grain. (The metal volume does not change, thanks to which ν = 0.5, and the density of dislocations, contrastingly, steeply increases [8].)Note that an increase in ρ per se results in a change in the magnetic structure of steel. This phenomenon is due to the appearance around dislocation clusters of nonuniform fields belonging to the second and third kind [9]. This causes a growth of critical remagnetization fields and of both 180 ° and 90 ° domain borders (DBs) in steel. The 180 ° DBs increase owing to the growth of the gradients of internal stresses [10]. In regard to the 90 ° DBs, the stresses per se play the role of potential barriers for remagnetization [11]. The mechanisms that control these effects are considered in detail in [11]. As a result, an increase in the dislocation density ...

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