Characterized by a high hardness, strength, and fragility, but a poor thermal conductivity, hardened steel is typically regarded as a difficult-to-machine material [
1], and its finish cutting is usually conducted using PCBN tools. During the machining hardening of steel, heavier mechanical and thermal loads occur in the cutting area [
2,
3], and thus at high temperature and high pressure, the tool-workpiece and tool-chip friction become severe, resulting in quick rake and flank wear. In particular, when the tool wear develops beyond a certain extent, the cutting force and cutting temperature clearly increase and vibrations will even occur [
4‐
7], which will cause further breakdowns such as chipping, fragmenting, rupturing, and peeling. This has a serious impact on the machining surface quality, and improvements of the hard machining performance plateau.
Scholars both at home and abroad have carried out numerous related studies on PCBN tool wear. Chou et al. [
8] researched the effects of the CBN content on the tool wear during hard machining, and concluded that a tool with a low CBN content has a relatively smaller wear rate than a tool with a high CBN content, and its machining surface quality is more satisfactory. Zhou et al. [
9] conducted an experimental study on the machining of hardened steel 100Cr6 using a PCBN tool, and determined the effects of the chamfer angles on the tool wear. Based on an experiment on the cutting of hardened steel AISI4340, Coelho et al. [
10] studied the influence of the coating types on the PCBN tool wear, and found that a TiAlN nano coating cutting tool is the least worn, a tool coated with TiAlN is less worn, a tool coated with AlCrN is more worn, and an uncoated tool is the most worn. Arsecularatne et al. [
11] studied the effects of the cutting parameters on the tool wear in the machining of hardened steel AISI D2 using a PCBN tool, and proposed optimum cutting parameters aiming at the tool life and volume of the material removal. Poulachon et al. [
12] investigated the influence laws of the PCBN tool wear morphology, based on different hardened steels, and revealed that the tool flank grooves have a correlation with the microstructure of these steels. Zhao et al. [
13] focused on the surface roughness evaluation during hard turning and found that the cutting-edge radius of the PCBN tool has a significant influence on the surface roughness and tool wear. Das et al. [
14] carried out an experiment on the hard turning of AISI 4140 steel using ceramic tools coated with PVD-TiN and revealed that the surface roughness is mainly influenced by the feed and cutting speed. Anmark et al. [
15] researched the fine machining of carburizing steel using PCBN tools, and determined the correlation between the tool wear mechanism and the inclusion components in the workpiece materials. Ramanuj et al. [
16] compared the wear characteristics in the hard turning of AISI D2 steel using a coated carbide cutter and a ceramic cutter individually, and found that the wear mechanisms of the coated carbide tool are mainly abrasion, diffusion, and notching wear, whereas the ceramic tool showed a good stability without a catastrophic failure. Mahfoudi et al. [
17] investigated the tool wear that occurs when the PCBN tool is used to machine harden steel AISI 52100 at a high speed, and established a crater wear prediction model based on the temperature distribution in the tool-chip contact area. In addition, Huang et al. [
18] built a model for predicting the rake crater wear depth of a PCBN tool, and verified its accuracy experimentally, during which the PCBN tool was used to cut hardened steel AISI 52100. Combining FEA with the cutting experiments, Özel et al. [
19,
20] conducted comparative research into the wear morphology and wear process of two different PCBN tools with a variable rounded edge and an invariable chamfered edge. In addition, other researchers have focused on the tool wear mechanisms, such as abrasive wear, bond wear, diffusion wear, and chemical wear, during hard turning [
21‐
24].
In conclusion, the current study on tool wear during hard turning focuses on PCBN tools with an invariable chamfered edge. In comparison, a tool with a variable cutting edge can form a spatially curved edge and therefore improve the thermolysis of the chip removal and tool life, reducing the cutting resistance. Such merits have drawn the attention from concerned scholars, and have been preliminary explored regarding tool fabrication techniques [
25‐
27] and the cutting performance [
28‐
30]. A PCBN tool with a variable chamfered edge was studied, and research into its wear morphology and mechanism was carried out based on a high-speed hard turning experiment.