Steel Surface Modification by Cathodic Carburizing and Anodic Polishing under Conditions of Electrolytic Plasma
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l Surface Modification by Cathodic Carburizing and Anodic Polishing under Conditions of Electrolytic Plasma S. A. Kusmanova, *, I. V. Tambovskiia, S. S. Korablevaa, and P. N. Belkina aKostroma
State University, Kostroma, 156005 Russia *e-mail: [email protected]
Received November 7, 2019; revised November 22, 2019; accepted November 24, 2019
Abstract—Low-carbon steel (Type 20) is subjected to cathodic carburizing followed by quenching and anodic plasma electrolytic polishing. The structure of the surface layer and its elemental composition are established, and the microhardness distribution within the surface layer, as well as its surface roughness, are measured. The microhardness of the surface layer exceeds 900 HV after carburizing at 900°C for 20 min in a solution containing ammonium chloride and glycerol. It is established that an increase in the surface roughness Ra to 9 μm after cathodic carburizing is related to the formation and destruction of an outer oxide layer. It is shown that the surface roughness can be reduced to 3.5 μm by anodic polishing in a 3% solution of ammonium chloride for 180 s. Keywords: plasma electrolytic carburizing, plasma electrolytic polishing, steel, microstructure, surface roughness, surface morphology DOI: 10.3103/S1068375520050099
INTRODUCTION Plasma electrolytic carburizing (PEC) has been studied for almost 60 years [1]. Different research groups have accumulated considerable information on the subject. Different electrolyte compositions for saturating steels and alloys with carbon and modes of processing were developed; the structure and phase composition of carburized layers were investigated along with their tribologic, corrosion, and other properties [2]. Some attention was paid to problems of the physicochemical mechanism of PEC, which, unlike conventional carburization processes, involves the effects of a large number of factors. The conditions for diffusional saturation with carbon in pack or gas carburizing processes are determined by the composition, temperature, and processing duration [3]. The key processes are precipitation of the saturating component, its adsorption at the surface being processed, and subsequent diffusion of atomic carbon. The oxidation of steel by oxygen is possible but is of secondary importance. However, PEC conditions are determined not only by the composition of electrolyte, temperature, and processing duration but also by the polarity of the part being processed, hydrodynamic conditions in the electrolyzer, and unavoidable side processes on the surface of the part. These include anodic dissolution and high-temperature oxidation during anodic PEC, the action of electrical discharges during cathodic processing, and a complex, stressed state of the upper layer after the part was cooled in the
electrolyte. The features of saturation with carbon occurring in the gas-vapor envelope around the part determine the advantages of the technique to a great extent (e.g., high processing rate and the possibility of quenching without reheating), but this
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