Evaluation of Work Hardening and Tensile Strength for Press Worked Products Plastic Strained from Multiaxial Direction

Abstract

Tensile strength and work hardening characteristics in the case that strain was applied from multi-axial directions by press processing and in the case that uniaxial strain was applied by tensile test were compared. Furthermore, in a unified manner with the results of the tensile tests, fatigue tests on plate material subjected to multiaxial strain were conducted, and the strength reliability of the pressed product was evaluated. Tensile strength of mild-steel plate subjected to uniaxial or multiaxial plastic strain was increased. If a tensile test is carried out again after a certain period of time has elapsed by applying strain, not only yield stress but also tensile strength are increased. This trend is due to strain aging inside the material. Degree of work hardening is higher when multiaxial strain is applied compared to when uniaxial strain is applied. For materials work hardened by applying strain, their hardness and the tensile strength are proportional regardless of the load status of strain. Accordingly, tensile strength of a product subjected to different strains can be evaluated uniformly by measuring the product's hardness. When strain is applied from multiple axes in press working, as well as tensile strength, fatigue strength is improved by work hardening. According to the results the microscopic analysis by X-ray diffraction combined with the full-width-at-half-maximum method, dislocation density is increased when multiaxial strain is applied compared to when single-axis strain is applied, and the degree of work hardening is greater. The full width at half maximum is correlated with tensile strength regardless of the state of strain applied to the material. TEM observation of dislocations revealed that the third-step-pressed product subjected to multiaxial strain has a coaxial, fine-grain dislocation-cell structure. On the other hand, in the case of pre-strained (plastic strain of 20%) material subjected to a uniaxial tensile load, the cell structure was unclear, and even the cells that were produced had coarse grains stretched in the tensile direction.