Effect of a continuous and gas-cyclic plasma nitriding on the quality of nanostructured austenitic stainless steel

Plasma nitriding is an effective method of the austenitic stainless chromium-nickel steels hardening. Usage of the low-energy electron beams (to 1 keV), that is typical for plasma generator, provides smaller loss of energy then usage of gas discharges. Low-energy electron beams allow change current density and ion energy independently of one another without using additional external heating devices. The lack of ion-plasma nitriding is deterioration of nitrided surface quality. Nanostructured deformation processing could be applied before nitriding with the aim of increasing growth rate of nitrided layer and reduction of nitrided surface roughness. In the paper, the influence of combined processing including nanostructuring frictional treatment by sliding indentor and following continuous and gas-cyclic plasma nitriding in electron-beam plasma at a temperature of 450 °С and 500 °С on hardening and surface quality of austenitic steel AISI 321 (0.04 wt.% С; 16.77 wt.% Cr; 8.44 wt.% Ni; 1.15 wt.% Mn; 0.67 wt.% Si; 0.32 wt.% Ti; 0.31 wt.% Cu; 0.26 wt.% Mo; 0.12 wt.% Co; 0.12 wt.% V; 0.04 wt.% P; 0.03 wt.% Nb; 0.005 wt.% S; and Fe for balance) is studied. It is established that friction treatment leads to occurrence of 95 vol. % α´ strain-induced martensite and increasing of microhardness to 780 HV0.025 on nitrided surface. On the nanostructured by frictional treatment surface of metastable austenitic steel AISI 321 (in contrast to nitrided coarse-grained steel) after continuous plasma nitriding in electron-beam plasma pore formation and intensive blistering is observed. Blistering is characterized by forming of steam blows and surface blowout. Obvious blistering appears due to advanced nitrogen diffusion into nanostructured surface with α´ stain-induced martensitic structure. Improvement in quality of the nitrided steel surface, hardened by frictional treatment (decreasing of blistering, pore formation and roughness), is achieved by means of: 1) gas-cyclic plasma nitriding is carried out at the conditions of periodic alternation nitriding semicycles in mixture of argon and nitrogen and denitriding (without nitrogen supply); 2) nitriding temperature decreasing from 500 °С to 450 °С. However after gas-cyclic plasma nitriding of nanostructured surface lower micro-hardness level (1 160…1 200 HV0.025) then after continuous nitriding (1 370…1 450 HV0.025) is observed.

austenitic stainless steel, frictional treatment, electron beam, plasma, gas-cyclic nitriding, strain-induced martensite, microhardness, roughness, pores