Deformation heterogeneity induced coarse grain refinement of the mixed-grain structure of 316LN steel through limited deformation condition

The defect of mixed-grain structure with millimeter-grade coarse grains (MCGs) is frequently found in heavy forgings, such as nuclear main pipes manufactured from 316LN steel. Renovation of such kind forgings can only be conducted through recrystallization. However, the allowed deformation is limited because the geometric size of forgings should be maintained during the renovation. For this purpose, the deformation heterogeneity of 316LN steel with mixed-grain structures were studied experimentally, which demonstrated that the MCGs were more inclined to deform than fine grains even if MCGs are with hard orientations. Therefore, a two-stage method was studied to refine MCGs, which takes the advantage of this deformation heterogeneity. The first stage was to impose a limited deformation to the steel at room temperature, suppressing the dynamic recovery and leading the preferentially deformed MCGs to accumulate enough deformation energy storage. The second stage was rapidly heating the deformed materials to realize static recrystallization (SRX). The results showed that the MCGs were completely refined by SRX even if the reduction rate is only 8%. The SRX mechanisms and refining assessments of MCGs under different deformation conditions were discussed. This work, thus, provides an effective method to salvage the heavy structures in engineering.