Brando Okolo

Grinding-induced microstructural gradients and residual stresses in the surface layers of carbon steel and pure tungsten

Abstract Flat samples of AISI 1080 carbon steel and commercially pure tungsten were surface-treated using a reciprocating table-type grinding machine. Cross sections of the subsurface microstructure induced by this mechanical finishing process were prepared and imaged by focussed ion beam methods. Depth profiles of the residual normal and shear stresses were analysed using synchrotron X-ray diffraction and correlated to the observed microstructural gradients. The stress partitioning between the hard cementite and the soft ferrite phases in the ground steel samples is discussed in detail.

Grinding-Induced Residual Shear Stresses

Pearlitic steel and pure tungsten specimens were ground using a table-type grinding machine. The thin surface layers affected by the grinding process were characterized using focussed ion beam milling and microscopy. The strongly graded zone altered due to severe plastic deformation and recrystallisation was found to be less than 3m thick. The microstructure in that zone depends on the grinding parameters. Using synchrotron X-ray diffraction, the residual stresses were measured for penetration depths ranging from 0.25 m to 9 m. Based on the approach by Dölle and Hauk, the residual shear stresses were separated from the residual normal stresses. In pearlitic steel, residual shear stresses of opposite sign were observed in the two phases (ferrite and cementite) and found to be compensating each other, while shear stresses were proved to be absent in single-phase tungsten. These results underline that residual shear stresses caused by severe plastic shear deformation exist only as micro-stresses.