Surface integrity and modification of SA508Gr.3Cl.2 RPV steel induced by high-speed face milling process

SA508Gr.3Cl.2 steel is extensively employed in fabricating nuclear reactor facilities consisting of reactor pressure vessels and steam generators. The functional performance of components is affected by the properties of superficial layers, and the machining process is indispensable for the final forming. Hence, it is essential to modify and improve surface quality via the regulation of machining parameters. This paper presents a comprehensive investigation of the effects of high-speed face milling process on the surface modification of SA508Gr.3Cl.2 steel, with particular attention to surface topography, microstructure alteration, micro- and nano-hardness, and residual stresses under semi-finishing conditions. To control the machined surface integrity via regulating milling parameters, surface characteristics are compared under various milling speeds and feeds. Results indicate that the unwanted geometrical defects in the form of feed marks, plowing grooves, surface tearing, and side flow are inherited on the machined surface, and the surface roughness is maintained at a relatively lower level, up to Sa 0.822 μm and Sz 5.824 μm. In addition, the material dragging layers with different thicknesses between 4.1 and 8.5 μm are introduced into the regions near the machined surface, which are subjected to intense plastic activities, including dislocation proliferation, grain boundary deflection, grain refinement, and elongation. There is no evidence of white layer and phase transformation at the superficial layer of machined SA508Gr.3Cl.2 steel. Besides, the increased milling speed and feed are responsible for thicker plastic deformation layers and greater hardening depths and promote surface hardening. A cutting configuration with a higher milling speed and lower feed is recommended to obtain attractive component surfaces with fewer geometrical defects, lower roughness, lower tensile residual stresses, proper hardening, and no white layer and phase transformation.