Work-hardening behavior and evolution of dislocation-microstructures in high-nitrogen bearing austenitic steels

Microstructural evolution during deformation at room temperature has been studied by means of transmission electron microscopy to make clear the reason for high work-hardening in high-nitrogen bearing austenitic steels. Tension tests were carried out at room temperature using three SUS316L type steels bearing 0.02 to 0.56 mass% nitrogen and 18Mn-18Cr (retaining ring) type steels bearing 0.51 to 0.84 mass%; nitrogen. It is found that both yield strength and work-hardening increase with increasing nitrogen concentration. In high-nitrogen bearing steels, planar dislocation-arrays are formed in the beginning of deformation, frequently showing multi-dipoles. Then, they overlap to make dislocation-walls and at the same time such dislocation-arrays or walls are pinned each other presumably by operation of Lomer-Cottrell reaction at their intersections. The dislocation-wall is expected to play a role similar to grain boundary for further deformation. Microstructure observed in a heavily deformed specimen looks like fine octahedral grid-structure. Thus, dislocation density in a plastically deformed specimen becomes much higher in high-nitrogen bearing steels than ina low-nitrogen bearing steel in which dislocation-cell structure is evolved.