Characterization of the subsurface properties of metastable austenitic stainless steel AISI 347 manufactured in a two-step turning process

Abstract

At high mechanical loads and below martensite deformation temperature, metastable austenitic stainless steels undergo a deformation-induced phase transformation from γ-austenite to ε- and/or α’-martensite. During cryogenic turning, this can be exploited in order to realize subsurface hardening, thus avoiding a separate hardening process. An increase of the cutting edge radius, the chamfer angle and the feed rate leads to higher passive forces, consequently a more pronounced phase transformation and ultimately to a higher microhardness. However, increasing these input variables also results in a significant increase in surface roughness.In order to eliminate this conflict of objectives between subsurface properties and surface topography, a two-step turning process is proposed. In the first process step a pronounced phase transformation and high plastic deformation of retained austenite by means of heavily chamfered tools, very high feed rates and precooling are realized in the workpiece subsurface. In the second process step, the pronounced roughness peaks are removed, while maintaining the desired subsurface properties achieved in the first step and even increasing the phase fraction of deformation-induced α’-martensite at and near the surface.In the presented study, the surface and subsurface of workpieces manufactured applying this approach were examined. The residual stresses and the phase fraction of γ-austenite, deformation-induced ε-martensite and α’-martensite after the first and second process step were measured by means of x-ray diffraction. Furthermore, the microhardness was measured in order to quantify the mechanical properties of the hardened subsurface. With the two-step turning process it was possible to generate 87 vol.-% of α’-martensite, while the workpieces manufactured in a one-step cryogenic turning process had a maximum phase fraction of 9 vol.-% α’-martensite. Consequently, the microhardness in the subsurface zone of the workpieces was higher, while also ensuring a good surface topography.