Improving the surface morphology of metastable austenitic steel AISI 347 in a two-step turning process

Abstract

The material group of metastable austenitic steels offers the possibility of surface layer hardening, integrated into the machining process. The hardening results from the superposition of strain hardening mechanisms and deformation induced phase transformation from austenite to martensite. The deformation induced formation of martensite is favored by high passive forces and low temperatures in the workpiece surface layer. High passive forces can be achieved by negative effective rake angles and large undeformed chip cross sections. However, a low depth of cut is required since otherwise large quantities of the martensite produced in the surface layer will directly be removed. An increase in the feed rate leads to a quadratic increase in the average maximum height of profile Rz due to the kinematics of the turning process. There hence is a conflict between achievable surface quality and martensite formation.This study presents a two-step turning approach to generate martensitic surface layers with low surface roughness. The aim of the first machining step is to produce as much martensite as possible, regardless of the resulting surface topography, by means of very high feed rates and cryogenic cooling. To increase the passive forces, chamfered indexable inserts with varying chamfer angles are used in this process step. In the second machining step, the surface is finished with sharp-edged PCD inserts. The aim of the second machining step is to generate the final surface topography with significantly lower roughness values, whereby as little as possible of the deformation induced martensite generated in the first machining step is to be removed. The final result of this two-step turning process is a workpiece surface layer with enhanced mechanical properties and low surface roughness.