SURFACE MODIFICATION OF CONTINUOUSLY COOLED BAINITIC STEELS: INTERACTIONS BETWEEN SHOT PEENING, PLASMA NITRIDING AND SURFACE INTEGRITY

Abstract

Continuously cooled bainitic steels are an alternative to quenched-and-tempered steels for forged components because high mechanical strength can be achieved directly after hot deformation and controlled cooling. However, applications subjected to wear and fatigue require surface properties that are not always provided by the bulk material. This review analyzes sequential shot peening and plasma nitriding, emphasizing the interactions among topography, plastic deformation, residual stresses, microstructure, and nitrided-layer formation. The literature shows that shot peening can increase dislocation density, refine the near-surface microstructure, and introduce compressive residual stresses, thereby creating conditions that may favor nitrogen diffusion. Nevertheless, this effect is not universal: high compressive stress levels and excessive topographical changes may restrict nitrogen uptake and transport. Studies on sequential treatments indicate that the final tribological response depends on surface integrity as a system rather than on isolated hardness or roughness parameters. In this context, functional parameters derived from the Abbott–Firestone material-ratio curve provide complementary information on the portion of the surface effectively involved in contact. Shot peening followed by plasma nitriding can therefore improve load-bearing capacity and wear resistance, provided that the parameters of both processes are matched to the initial substrate condition.