Effect of preliminary nanostructuring frictional treatment on the efficiency of nitriding of metastable austenitic steel in electron beam plasma

Improving the scratch test properties of plasma-nitrided stainless austenitic steel by preliminary nanostructuring frictional treatment

“…The steel nitrided after frictional treatment has more stable tribological properties at the microscopic level and higher resistance to elastic-plastic deformation and fracture (Figs. 1 to [4]. This is due to a relatively small penetration depth of a Vickers indenter compared to the thickness of the analyzed nitrided layers.…”

“…This is due to a relatively small penetration depth of a Vickers indenter compared to the thickness of the analyzed nitrided layers. It has been found by the nuclear reaction method [4] that, after nitriding of the AISI 321 steel in electron beam plasma at 400 °С, both non-deformed and frictional treated specimens showed nitrogen concentration of 31-32 at.% at a depth of 2.25 µ from the surface. In what follows, we discuss possible reasons for the differences detected in the scratch tests.…”

“…4а). The absence of a pronounced relief results from the high dispersion of the structure under frictional treatment [5,6] and, possibly, the features of the formation of the stress state under subsequent nitriding of a deeper layer [4]. Nanostructuring frictional treatment not only favors nitrogen diffusion to a greater depth owing to a great length of the nanograin boundaries and their high diffusion activity, but also supplies the smoother transition from the high-strength nitrided layer to the soft bulk metal due to strain-hardening, which can reach ~450 µm in depth for metastable austenitic steel [10].…”

“…After electropolishing and nitriding, nitrogen is concentrated at a smaller depth near the surface, with a sharper transition from the high-strength layer to the softer matrix, thus creating a high level of residual stresses. Besides, when the preliminarily deformed state is nitrided, 95 vol% of strain-induced α´-martensite is formed on the surface of the AISI 321 steel as early as under frictional treatment [4,8]. This prevents the development of the γ→α transformation under nitriding (as distinct from nitriding of the undeformed steel [9]) and, accordingly, the appearance of additional stresses induced by the bcc-phase with a greater unit volume than that of the fcc-phase.…”

“…Since the effectiveness of nitriding is controlled by diffusion processes in a material being modified, it is important and urgent to increase the rate of nitrogen diffusion into the material bulk at low nitriding temperatures. Nanostructuring frictional treatment preceding the operation of nitriding in electron beam plasma reduces the temperature of effective nitriding to 350 °С [4]. Besides, in the frictional treatment of austenitic Cr-Ni steel with an artificial diamond indenter in argon, simultaneously with effective hardening, low surface roughness (Ra ~0.1 µm) is achieved [5,6], this also being an important aspect in the implementation of finishing technologies.…”