Three-dimensional wear parameters and wear mechanisms in turning hardened steels with PCBN tools

Three-dimensional wear parameters and wear mechanisms in turning hardened steels with PCBN tools

Author’s Accepted Manuscript Three-Dimensional Wear Parameters and Wear Mechanisms in Turning Hardened Steels with PCBN Tools Denis Boing, Rolf Bertra...

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Author’s Accepted Manuscript Three-Dimensional Wear Parameters and Wear Mechanisms in Turning Hardened Steels with PCBN Tools Denis Boing, Rolf Bertrand Schroeter, Adilson José de Oliveira

PII: DOI: Reference:

S0043-1648(17)31325-X WEA102299

To appear in: Wear Received date: 6 September 2017 Revised date: 19 November 2017 Accepted date: 23 November 2017 Cite this article as: Denis Boing, Rolf Bertrand Schroeter and Adilson José de Oliveira, Three-Dimensional Wear Parameters and Wear Mechanisms in Turning Hardened Steels with PCBN Tools, Wear, This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Three-Dimensional Wear Parameters and Wear Mechanisms in Turning Hardened Steels with PCBN Tools Denis Boinga,b*, Rolf Bertrand Schroeterb and Adilson José de Oliveirac a

Department of Mechanical Engineering, Technology, Innovation and Manufacturing

Center, University Center of Brusque, Brusque, SC, Brazil b

Department of Mechanical Engineering, Laboratory of Precision Mechanics, Federal

University of Santa Catarina, Florianópolis, SC, Brazil c

Department of Mechanical Engineering, Technology Center, Federal University of Rio

Grande do Norte, Natal, RN, Brazil







[email protected] 123 Dorval Luz st, Santa Terezinha, 88352-400, Brusque-SC, Brazil.

Abstract Hard turning has been applied in a wide range of mechanical components based on ferrous materials. The application of these components is a function of mechanical properties and microstructure – that also has manufacturing process influences, i.e., cutting mechanism. This research aims to discuss the tool wear level and the correlation with the wear mechanisms in turning with PCBN tools, which deals with three steel alloys (AISI 4340, AISI 52100 and AISI D2) and considers six levels of hardness (on the interval from 35 to 60 HRC), applying the novel three-dimensional wear parameters based on Focus Variation Microscope (FVM) to wear evaluation. Considering the wear parameters that represent the amount of material removed from the tool (WRM) and tool affected area (WAA), tool wear intensity and abrasion wear mechanisms have a decreasing trend with the increase of hardness in the range of 35-50 HRC. Above 50 HRC, however, there is a tendency of increased tool wear intensity when steel hardness is increased. The fraction volume of carbides in the steel microstructures intensifies the abrasion wear mechanism. The adhesion wear mechanism showed a reduction with an increase of the steel’s hardness – identified by wear parameters WAM (adhered material volume on the tool). Based on crater wear formation, the diffusion wear mechanism had an inverse behavior when compared to adhesion. Better results concerning tool wear can be achieved when turning steels with 50 HRC. It was evidenced that the three-

dimensional wear parameters applied open new possibilities to understanding complex and specific phenomena occurring in machining processes, particularly in the machining of hardened steels.

Keywords: three-dimensional wear parameters; hard turning; PCBN tools; wear mechanisms; steel microstructure.

1. Introduction Hard turning has been widely researched and applied in the industrial environment since the 1980’s as finishing or semi-finishing operation to replace the grinding process, or as a hybrid process in association with grinding. Regarding the hard turning characteristics, it has advantages like process flexibility, material removal rate, set up time and environmental compatibility – and is mainly applied to machining multi-body and complex geometry components, i.e., gear shaft (multi-body, pulley cone, continuous and interrupted cut). The technology spread out was based on machine tools with high stiffness and dynamic stability combined with ceramic and PCBN tool materials [1–4]. Furthermore, the hard turning process feasibility is strongly dependent on the ceramic and PCBN tool behavior – the tool wear level and the tool wear mechanism [5–10] and its relationship with the cutting conditions [11–14]. The typical hard turning definition found in literature describes the process when the machined material hardness is above 45 HRC [2]. Klocke [15] supports the definition when machined material hardness is above 50 HRC. Poulachon, Moisan and Jawahir [16] suggest the hard turning definition when the ultimate steel strength is above 1700 MPa and this approach is directly related to the cutting forces behavior. According to Matsumoto, Barash and Liu [17], the cutting forces involved in cutting soft steels are relatively high and decrease as the hardness increases. When the hardness exceeds 50 HRC, the cutting forces suddenly increase. In the transition region of the cutting force, a transition in chip morphology also occurs – from continuous to saw tooth chip [8,17,18]. In addition, Luo, Liao and Tsai [19] observed lower tool wear level (flank wear) with the steel at 50 HRC, when turning of AISI 4340 steel (35-45-50-55 and 60 HRC) with ceramic and CBN tools,. The thermo-mechanical processes and phenomena described are the critical issue that make it not possible to compare hard turning to conventional turning [2,20].

Hard turning is applied in a wide range of ferrous materials. The application of the final component is a function of mechanical properties and microstructure – which also have manufacturing process influences, i.e., increasing the carbides volume fraction and size in the microstructure of the machined material is a limiting factor regarding the feasibility and process capability [21]. Considering the hard turning application, Boing [22] defined four classes of machined materials (A, B, C and D) based on tempered martensitic microstructure associated with the volume fraction of carbides. Class "A" relates to materials with less than 1 vol% volume fraction of carbides, for instance, AISI 4140, 4340 and 8640 quenched and tempered steels (hard turning application [11–13]). Class “B” encompasses materials with 1 to 5 vol% volume fraction of carbides, and a typical example is AISI 52100 steel ([23,24]). Class “C” consists of materials in the range from 5 to 15 vol% volume fraction of carbides. This class includes the cold work tool steels (AISI D2 and D6) and high-speed steels (AISI M2) ([9,25]). Finally, class “D” is composed of materials with the volume fraction of carbides of over 15 vol%, such as high-chromium white cast iron and hard-facing layer deposited on steels or cast irons [10,21,26,27]. König et al. [28] presented the first condensed discussion involving hard turning in different steel alloys, and stated that materials with high fraction volume of carbides in the microstructure promote lower tool lifetime. In addition, Poulachon et al. [25] demonstrated the material microstructure (classes A-C; 54 HRC) impact on the PCBN tools flank wear topography. Coarse carbides M7C3 (size between 10 - 15µm) generated large grooves on the flank face (class C), while in materials without carbides (Class A), smooth grooves were observed linked to martensite grain size. Steels with grade C promote shorter tool life, followed by material class B and A. The correlation between flank wear topography and carbide size in the machined material microstructure (class C – 62 HRC) was also mentioned by Arsecularatne [29]. Comparing the data of several hard turning reports that are independent of the machined material class, the main wear mechanism in PCBN cutting edges is abrasion, both in continuous and interrupted cutting. In addition to abrasion, diffusion, adhesion and attrition are secondary wear mechanisms. Diffusion is described only in continuous cutting with high CBN content tools. Moreover, chipping is common in interrupted cutting. In the different studies, the wear evaluation and the wear mechanism analysis were performed by a toolmaker’s microscope and scanning electron microscope (SEM), respectively. Despite the consolidated methods to tool wear analysis, they do not

provide three-dimensional geometric information, and with that, important information can be missed to explain machining process phenomena, mainly regarding tool wear analysis, i.e., tool wear volume and area numerical values as well as tool microgeometry modification along the cutting process – these situations can be helpful to understand the crater wear formation in PCBN cutting edges and the effective contact region in the tool-chip interface [30,31]. According to Byrne, Dornfeld and Denkena [32] high precision hard turning trends achieve ISO tolerance IT2 when combined with roughness Rz below 1µm. Besides the machine tool precision, the tool microgeometry characteristics [33], as well the level of wear is a critical issue to be evaluated. The traditional wear parameters available in ISO 3685-1993 [34], like VBB or KT, are not sensitive when the tool wear land is small and important phenomena can be ignored. The modern optical measurement methods, like the focus variation microscope (FVM), that combines the small depth of focus of an optical system with vertical scanning to measure topographical and real color information of a complex surface with intricate details, can provide resources to implement new techniques to evaluate cutting edge wear in machining processes [35,36]. In this context, this research aims to discuss the level of wear and the correlations with the wear mechanisms, applying the novel three-dimensional wear parameters based on Focus Variation Microscope (FVM) to wear evaluation in hard turning with PCBN tools in three classes of machined material (A-C) and at six levels of hardness (35-60 HRC), along the adequacy period of the tool-workpiece tribological system.

2. Experimental procedures The experiments were carried out in a horizontal CNC lathe manufacturer by Heyligenstaedt, model Heynumat 10U, with 75 kW of power in the spindle motor, the maximum speed rotation of 4500 rpm and a three-jaw chuck with a maximum chucking pressure of 45 bar. The workpiece geometry and the clamping system are shown in Figure (1).

Figure 1 – Workpiece geometry and clamping system Three steel alloys (AISI 4340 – class A, AISI 52100 – class B, and AISI D2 – Class C) were applied, and the chemical compositions are shown in Table (1). The workpieces were quenched and tempered (according to specific procedures for each alloy) to achieve six level of hardness: 35±1, 40±1, 45±1, 50±1, 55±1 and 60±1 HRC. The final mechanical properties, stress strain curves, and microstructures are shown in Table (2) and Figures (2) and (3), respectively. Table 1 – Workpiece steel alloy chemical composition Chemical composition %wt P Si Ni Cr

Steel alloy AISI 4340 AISI 52100






























Table 2 – Steel alloys mechanical properties Steel alloy

AISI 4340

AISI 52100

Hardness [HRC] 35 40 45 50 55 60 35 40 45 50 55

Mechanical properties Ultimate strength Yield strength σr [MPa] σe [MPa] 1114 1032 1245 1173 1497 1362 1977 1401 1962 1295 2028 1560 1097 997 1333 1214 1601 1465 1806 1650 2127 1955

Strain ε [%] 17.8 13.1 12.2 12.8 11.6 3.4 17.6 9.5 10 6.5 2.7


60 35 40 45 50 55 60

2334 1000 1246 1396 1530 1896 2253

1854 790 1031 1170 1339 1676 2018

2.3 11.5 8.7 5.6 5.8 2.8 1.8

Figure 2 - Stress strain curves: a) AISI 4340; b) AISI 52100 and c) AISI D2

Figure 3 - Steel microstructures for turning experiments In AISI 4340 steel the microstructure hardness 35, 40 and 45 HRC are composed of a high content of bainite and a lower level of tempered martensite. The amount of

martensite content gradually increases from 50 up to 60 HRC. AISI 52100 steel has small eutectic carbides (1-2µm), type M7C3, homogeneously distributed in the matrix. In the range of hardness from 35 up to 40 HRC the AISI 52100 steel has the microstructure composed by tempered martensite and, from 45 up to 60 HRC, refined tempered martensite. The AISI D2 steel microstructure is formed by irregular shape and distribution of M7C3 primary carbides (~15µm) and spherical secondary carbides (13µm). At a hardness of 35 HRC, the matrix is composed by cementite and ferrite. From 40 up to 60 HRC, the microstructure is constituted by tempered martensite with retained austenite. The retained austenite amount reduces when the hardness increases. The experiments were carried out with PCBN tools (Sandvik Coromant® grade 7025 or ISO H20, code SNGA 120408 S01030A). The tools have a low-content of cBN (60%) with bimodal grain distribution (1 and 3µm) in a ceramic binder Ti(C,N) and Al. Also, they have high edge resistance due to the inhibition of borides formation, such as TiB2, along with the PCBN sintering process. These properties help to avoid chipping and edge fracture, promoting a beneficial situation to machining materials with a high fraction of carbides in the microstructure [21,37]. The edge profile and microgeometry parameters are shown in Figure (4). The tools were assembled on a tool holder (code DSBNR-2020K-12) and clamped with a top clamp and a center pin combination aiming to promote high rigidity of the insert and, consequently, avoid chipping during the machining.

Figure 4 – Tool edge microgeometry

The experiments were performed along the adequacy period of the toolworkpiece tribological system – according to stage I of tool life curve (machining time ~ 54s) - to analyze the wear mechanism in PCBN cutting edges and to demonstrate the feasibility and sensitivity of the three-dimensional wear parameters. One experiment consisted of one radial turning pass on the workpiece surface from larger to smaller diameter, i.e., using a variation in the spindle rotation to maintain the cutting speed at 150 m/min. In order to avoid external influences, i.e. oxides formed on the surface in the heat treatment process, before the experiments, the workpiece surface was premachined with a dedicated tool. After this procedure, the cutting edge was replaced by a fresh PCBN edge, and the face turning started. The cutting parameters used were: cutting speed (vc) = 150 m/min, feed (f) = 0.08 mm/rev and depth of cut (ap) = 0.20 mm, in dry condition. Each experiment was carried out two times. This methodology made it possible to find the direct comparison between the three steel alloys at six levels of hardness. The methodology used to evaluate tool wear and wear mechanism consists of two steps: a) quantitative evaluation using focus variation microscope (FVM), model Infinite Focus G5 manufactured by Alicona®; b) to complement the wear evaluation and validate the FVM analysis, the worn cutting edges were evaluated with a scanning electron microscope (SEM), model JSM-6390LV manufactured by JEOL®. Before the turning tests, to create a reference geometry, each PCBN fresh cutting edge was measured by the FVM system. After the machining tests, the cutting edges were remeasured. An objective lens with 50x magnification (25nm in the vertical resolution and 1.1 µm in the lateral resolution) was used. Figure (5) shows the procedures applied. This methodology was previously reported by Danzl, Helmli and Scherer [36].

Figure 5 – Methodology of output three-dimensional wear parameters With the datasets overlapped (cutting edge before and after machining tests), it is possible to recognize three-dimensional wear parameters – which are defined in the Table (4). The parameters WRM and WMD represent the phenomenon that occurs below the reference surface and indicate the tool wear definition according to ISO 3685:1993 [34] “the change of shape of the tool from its original shape, during cutting, resulting from the gradual loss of tool material or deformation.” However, the interaction between cutting edge and the workpiece during machining can result in the adhered material on the rake or the clearance surfaces after the machining process, which is also part of the tribological system and contributes to the tool wear [35]. In this case, parameters WAM and WMH indicate the defects above of the reference surface. The W AA parameter encompasses all alteration on the used cutting edge surfaces compared with the reference (fresh) surfaces.

Table 3 – Three-dimensional wear parameters Parameter





µm µm²





Definition Volume of removed material from the tool in relation to the reference surface Maximum depth of defect in relation to the reference surface Affected tool area in relation to the reference surface Volume of adhered material on the tool in relation to the reference surface Maximum height defect in relation to the reference surface

The ISO 8785:1998 [38] defines terms and parameters related to surface imperfections. The parameters WMD, WAA and WMH are, respectively, SIMch, SIMt and SIMcd. However, the semantics adopted and listed in the Table (3) are exclusively dedicated to discussing machining process phenomena. Furthermore, considering the context of these tool wear studies, tool wear concept will not be just related to the loss of the tool material. Tool wear concept encompasses all the changes of the tool properties (geometrical and physicochemical) which modify the initial cutting conditions.

3. Results and discussion The results of the surfaces overlapped methodology are shown in Figure (6) for all tested conditions. The three-dimensional parameter values were calculated based on the middle value from the two replicates, and no significant differences were observed between identical conditions. These results promote reliability in the following trend discussions.

Figure 6 – Three-dimensional wear parameters based on datasets overlapped The fraction volume of carbides in the microstructure and the machined material hardness influenced the parameters WRM (amount of material removed from the tool) and WAA (tool affected area), according to Figures (7-8). From steel alloy class A (AISI 4340) to class C (AISI D2), independent of the level of hardness, there is a verified increase in the parameters WRM and WAA, which are directly related to machined material abrasiveness, specifically the presence of carbides in the microstructure. According to classes of machined materials defined by Boing [22], as also shown in

Figure (3), AISI 4340 steel is free of primary carbides. On the other hand, AISI 52100 bearing steel has approximately 4 vol%, and AISI D2 tool steel has approximately 10 vol% of primary carbides in the microstructure.

Figure 7 – WRM (volume of removed material from the tool) wear parameter based on surfaces overlapped.

Figure 8 – WAA (affected tool area) wear parameter based on surfaces overlapped Analyzing the parameters WRM and WAA in Figures (7-8), an interesting point to note is a tendency to decrease cutting edge wear volume and affected area with an increase of the hardness in the range from 35 to 50 HRC, mainly in steels with higher % of carbides. Above the level of hardness of 50 HRC, however, cutting edge wear volume and affected area parameters showed a tendency to increase when steel hardness rises. These behaviors are supported by the hard turning principles for the following explanations: cutting forces and the saw tooth chip formation mechanism. According to Matsumoto, Barash and Liu [17], the increase in steel hardness (range from 30 to 50 HRC) raises the chip tool interface temperature which softens the steel being cut and reduces the shear force on the tool rake face. This effect also increases the shear angle and decreases the chip thickness, which reduces the tool chip contact area on the rake face. Therefore, in this range of hardness, continuous chips are

formed. When the steel hardness exceeds 50 HRC, there is a sudden increase in cutting forces and two conflicting factors affecting the cutting mechanism: one is the increase in yield stress due to the rise in steel hardness; and the other is the reduction in yield stress due to the cutting heat generated. However, when the hardness is above 50 HRC, the steel is more brittle and the deformation energy expended in cutting is small. Consequently, the cutting heat generated from this energy is reduced, and the material softening does not play a preponderant role in high shear and friction force on the tool rake face. These phenomena are directly related to the saw tooth chips formation in steels with hardness above 50 HRC. The previous explanation can be related to the lower level of tool wear when the hardness of the material is at 50 HRC. To complement the hypothesis for the results shown in Figures (6-8), one association can be realized with the chip formation mechanism, which is the thermomechanical principle that defines hard turning. In the machining of ductile materials, the chip formation is accomplished by a severe plastic deformation in the primary shear zone, resulting in continuous chips [15]. On the other side, when hard turning is applied, a more complex chip formation mechanism can be considered. The high compressive stress created by the cutting tool in the machined material leads not to a material flow over the rake face, but the formation of a crack on the free surface. This crack releases the stored energy and thus acts as a sliding surface for the material segment, allowing the segment to be forced out between the parting surface. Simultaneously, plastic deformation and heating of the material occur at the leading edge of the cutting tool. Once the chip segment has slid away, renewed cutting pressure results in the formation of a fresh crack and chip segment. The increase of the temperature needed for plastification of a small section of the chip material is supplied by the heat created in the cutting process. The individual chip segments are linked by the small proportion of the material which is plastically deformed and heated to a high temperature [39,40]. An explanation about extended shearing stress in saw tooth chip formation in hard turning can be found in [18]. Based on the results shown in Figures (6-8) and the consolidated phenomena previously related, in addition to the influences of the cutting forces, cutting mechanisms and mechanisms of chip formation, the transition zone, from conventional to hard turning, also has influences on the level and mechanisms of the tool wear. Analyzing the cutting edge wear in Figure (6), mainly to the AISI D2 steel, until the hardness of 50 HRC, the wear of the cutting edges is mainly located on the flank face,

and there is a large amount of material adhered on the tool rake face. Above 50 HRC, the flank wear intensity rises, the amount of adhered material is reduced (Figure 10), and the formation of crater wear occurs – this is a transition of the tool wear level and strong evidence of the transition of the tool wear mechanism. Figure (9) shows the results of the parameters WMD – maximum depth of defect relative to the reference surface. Regarding the material class (A-C), the trend is similar to WRM and WAA parameters – the amount of fraction volume of carbides in the material microstructures promotes deeper defects on the cutting edge faces. In addition, the similar trend to decrease the tool wear up to 50 HRC; and above 50 HRC, the tendency to increase the tool wear with the increase in the steels hardness is evident to the alloys AISI 52100 and AISI D2 (except for AISI D2 – 45 HRC). For the AISI 4340 steel, the increase in hardness promotes lower depths of defects. The deeper defects were found on the cutting edges that turned AISI D2 (class C) steel with 60 HRC, while the lower defects, considering the parameter WMD, were found in the tools that turned AISI 4340 with 60 HRC. These results involve the change in the intensity of the tool wear mechanisms.

Figure 9 – WMD (Maximum depth of defect) wear parameter based on surfaces overlapped The wear parameters that represent the amount of material adhered on the tool (WAM) and the maximum height of defect relative to the reference surface (WMH) are shown in Figures (10-11). At lower levels of the steel hardness, the greater values of the parameters WAM and WMH were observed – except for the AISI 4340 steel which has random behavior along the hardness values. In the case of AISI D2 steel, at the lower level of hardness, the amount of adhered material is as critical as the carbides abrasiveness. On the other side, another phenomenon related to the adhered material on

the cutting edge is called TLP – tool protection layer, which can improve the tool performance and also explain the nonlinear behavior of the tool wear [6,10,41].

Figure 10 –WAM (Volume of adhered material on the tool) wear parameter based on surfaces overlapped

Figure 11 – WMH (maximum height defect) wear parameter based on surfaces overlapped In addition to assisting tool wear quantification, three-dimensional wear parameters have a purpose to help understand the tool wear mechanisms involved – mainly in function of numerical response. The trends based on parameters WRM, WAA, WMD (Figures 7-9) give clear information about a region where the wear intensity is reduced in the area where a transition from conventional to hard turning occurs – 50 HRC. Furthermore, adding the information from WAM and WMH, (Figures 10-11), the transition of the tool wear mechanism intensity needs to be considered. Based on these results, Figure (12) summarizes the results of the main wear mechanism according to Gahr [42] in the PCBN tools, considering the three steel alloys machined at six levels of hardness (35-60 HRC).

Figure 12 - Relative intensity of wear mechanisms: a) steel alloys influence and b) steel alloys microstructures (fraction volume of carbides) influence Based on the WRM (volume of removed material from the tool), WMD (maximum depth of defect) and WAA (affected tool area) parameters, a decrease in the abrasion intensity was verified with an increase in steel hardness up to 50 HRC. Above the limit of 50 HRC, the abrasion intensity has a sudden increase. The adhesion wear mechanism is correlated with the WAM (volume of adhered material on the tool) and WMH (maximum height defect) and also shows a reduction with an increase in steels hardness. Surface damage (alternating tribological stresses) and diffusion have opposite behavior: they increase with higher levels of hardness, which are correlated with higher mechanical properties of the machined material and the crater wear formation, respectively (Figure 12a). The direct comparison between SEM (scanning electron microscope) and FVM (focus variation microscope) analysis are shown in Figure (13). While the SEM image shows specific details about the wear morphology – in Figure (13a), the FVM image shows cutting edge wear quantitative information, i.e., it makes it possible to recognize the crater wear formation numerically, according to Figure (13b).

Figure 13 – SEM and FVM analysis in PCBN cutting tool in hard turning AISI D2 – 60 HRC.

Besides the information of the three-dimensional wear parameters, the overlapped datasets from fresh to worn cutting edges give a colormap response showing the affected region of the cutting edges – it makes it possible, according to the literature [15,43–45],

to associate the form of the cutting edge wear with the main wear

mechanism, i.e., flank wear: abrasion; crater wear: diffusion. The complementary analysis by the SEM microscope, to deeply investigate the main wear mechanism, is still relevant – mainly to recognize the worn cutting edge topography. However, the FVM analysis promotes faster answers about the behavior of machining processes, as well as being extremely important in industry application and process evaluation. Based on the three steel alloys (Figure 12b), the amount of carbides in the steel microstructures intensifies the abrasion and surface damage wear mechanism. Comparing the worn cutting edges morphologies in details of Figure (14), the abrasion scratches are more intense and larger in AISI D2 steel – which is directly related to the size and fraction volume of carbides in the steel’s microstructures. In the cutting process, the carbides collision against the tool edge, beyond the abrasive wear scratches, promote instability in the cutting process which can lead to cutting edge chipping and damage. According to Figure (12b), the diffusion wear mechanism is reduced when the fraction volume of carbides increases – which is related to the steel alloys thermal conductivity [31]. Finally, the adhesion mechanism is correlated to the steel’s chemical composition and metallographic constituents and it is strongly enhanced with the change of turned steels [15].

Figure 14 - Wear pattern morphologies on the cutting edges for turning AISI 4340, 52100 and D2 steels in 35 HRC – specific region cutting edge region: detail “a” from Figure (13)

Throughout the results and discussions, the three-dimensional wear parameters functionality was confirmed as a complementary method to discuss machining phenomena. It was possible to quantitatively evaluate the situations that traditionally were just evaluated based on the wear’s topographies, i.e., SEM images. Furthermore, the three-dimensional parameters are useful to evaluate the level of wear. In Figure (6), considering the AISI 52100 steel in hardness interval (35-55 HRC) the wear in the PCBN cutting edges, considering the VBB parameter, is practically the same. However, Figures (6-11) showed that the three-dimensional parameters are more sensitive to identify the wear pattern, even in the lower level of wear. This methodology can be spread out to industrial application, mainly to evaluate high precision machining aiming, for example, to evaluate the wear level of the PCBN tool in the boring process applied to manufacturing valves seats in engine blocks or bearing seats on shafts and in housing bores. The three-dimensional parameters based on focus variation technology are an appropriated equipment to evaluate wear progression and morphology on the cutting edges applied mainly in single point turning process. They also have a strong potential to be implemented in other machining processes and tools, like micromilling and microdrilling, ultraprecision components, wear in the ball nose end mill, etc. Also, the parameter WRM (amount of material removed from the tool) can be used to predict the tool life based on tool wear rate – reducing the time spent on tool life tests and streamline machining process improvement [31]. Based on these results and considering the application and the availability of the FVM technology, it is suggested to include the three-dimensional parameters (WRM, WMD, WAA, WAM and WMH) in the ISO 3685 standard or similar.

4. Conclusions Based on the results obtained in these experiments it can be concluded that in the turning of AISI 4340, 52100 and D2 steels at six levels of hardness (35-60) with a PCBN tool:


The three-dimensional wear parameters (WRM, WMD, WAA, WAM and WMH) based on FVM (focus variation microscope) are functional to identify the level of tool wear and as a complementary method to discuss machining phenomena like tool wear mechanisms;


The fraction volume of carbides in the microstructure of machined material, independent of the hardness level, had an exponential effect on the level of tool wear – especially considering the parameters WRM, WAA and WMD. These results are directly related with the carbides abrasiveness in the machined material microstructure;


The lower level of wear in the cutting edges after turning the three steel alloys was in 50 HRC. The parameters WRM and WAA showed a decreasing trend in the values of tool wear with an increase in the hardness in the range from 35 to 50 HRC. Above a hardness level of 50 HRC, however, wear presented with an increasing trend with increasing hardness. These results were associated with hard turning principles;


The parameters WAM and WMH (amount of material adhered on the tool and maximum height defect) showed a functional numerical response to recognizing the adhesion wear mechanism, which revealed a trend in reduction when the hardness of the material increased.


The authors would like to thank Sandvik Coromant® for supplying the cutting tools; Alicona® for providing resources and discussions in the measurement methods; Villares Metals® for supplying the steel alloys. Special thanks are also extended to Kelly Albrecht to the language reviews. Funding: The authors would like to acknowledge financial support from FAPESC (Foundation for Research and Innovation Support of the Santa Catarina State – Brazil) in the research project “Machining of Hardened Steels – 2015TR304”.



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Highlights 

Three-dimensional wear parameters as a methodology to evaluate tool wear and wear mechanism

Tool wear level and tool wear mechanism transition in the transition zone from continuous to hard turning – 50 HRC

Steel microstructure impact in the tool wear and wear mechanisms