Tool wear analysis in the machining of hardened steels

Abstract

The machinability of a material can be assessed using many output parameters of the machining process, tool life being undoubtedly the most common. Tool life depends mainly on the tool wear rate, which in turn is very dependent on the prevailing wear mechanisms. It is therefore very important to study and analyze correctly the possible wear mechanisms on the rake and flank faces of the tool. When machining materials with high hardness, usually over 35 HRC, the difficulties are enormous because of the high cutting forces and heat generated, causing rapid tool wear and short tool life. When the hardness exceeds 45 HRC, the difficulties are even worse because the chips change from continuous to serrated types formed by localized shearing, increasing forces and temperatures even further. To tackle these adversities, ceramic and ultra-hard (CBN) tool materials are normally used, although other materials are also suitable. In interrupted cutting, for example, cemented carbides are frequently used. Wear mechanism analysis in hard machining is thus of particular importance. This article analyzes the tool wear mechanisms that occur in the machining of several hardened steels during continuous and interrupted cutting. All the analyses were performed after the tools have reached the stipulated end of the tool life criteria. Different types of tool material, such as cubic boron nitride, ceramics, and PVD-coated carbide inserts applied in turning and milling operations had their wear mechanisms analyzed. The main goal of this work was not to compare the tool lives of the conditions tried but to provide a greater understanding of tool wear phenomena and thus contribute to the development of tools with improved properties. Most of the worn tools had their wear region analyzed using a scanning electronic microscope (SEM) with the help of energy dispersive spectroscopy (EDS) technique. The wear analysis performed using the pictures taken in the SEM/EDS system was based on the main literature of this field of knowledge.