Effect of deep cold rolling on mechanical properties and microstructure of nickel-based superalloys

Mechanical surface treatments are performed on aerospace components for fatigue life enhancement by introducing beneficial compressive stress profiles and material strengthening. This paper studies the influence of Deep Cold Rolling (DCR) on the residual stress distribution, work hardening and the microstructure modification of two nickel-based superalloys, IN100 and RR1000. Two different diameters (6.3 mm and 12.6 mm) of the rolling ball inserts were investigated in this analysis. The hardness and residual stresses at the subsurface after DCR were analyzed along the rolling and transverse directions. Electron BackScatter Diffraction (EBSD) technique was used to characterize the microstructure of the samples both qualitatively and quantitatively. The degree of work hardening of fine grain RR1000 after DCR was characterized using full width at half-maximum (FWHM) of the X-ray diffraction peaks and Grain Orientation Spread (GOS) profiles acquired by EBSD characterization. The results clearly indicate that deep cold rolling introduces compressive residual stresses as deep as 1 mm, with significant work hardening at the subsurface in the coarse grain IN100. DCR resulted in the grain refinement, increase in low angle grain boundaries and clustering of dislocation density around the carbides in IN100. Depending on the ball diameter, DCR of RR1000 induced compressive residual stresses up to 700 gm and work hardening till the depths of 400 mu m. Additionally, severe deformation of grains occurred near the rolling surface. The larger diameter of the rolling ball resulted in high degree of work hardening and better residual stress distribution deeper into the materials.