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Sustainable hard turning of gas carburized SAE 8620 steel: comparative analysis of PCBN tool performance and surface integrity under dry and MQL conditions
Dieser Artikel geht auf das nachhaltige Hartdrehen von gasverkohlten SAE 8620-Stählen ein und vergleicht die Leistung von PCBN-Werkzeugen unter Trocken- und Minimum Quantity Lubrication (MMS) -Bedingungen. Die Studie untersucht die Mechanismen des Werkzeugverschleißes, einschließlich des Flanken- und Kraterverschleißes, und ihren Verlauf unter verschiedenen Schmierstrategien. Sie untersucht auch die Integrität der Oberfläche, wobei sie sich auf die Bildung weißer und dunkler Schichten konzentriert, und den Einfluss von MMS auf die Verringerung dieser Phänomene. Die Analyse erstreckt sich auf Bearbeitungskräfte und Spanmorphologie und bietet ein umfassendes Verständnis des Schneidprozesses. Die Forschungsergebnisse kommen zu dem Schluss, dass MMS die Standzeit und Oberflächenintegrität der Werkzeuge signifikant erhöht und damit eine praktikable Alternative zu herkömmlichen Kühlmethoden für Überschwemmungen darstellt. Darüber hinaus vergleicht der Artikel das Verhalten von SAE 8620 mit SAE 1045 Stahl, wobei die einzigartigen Herausforderungen durch die kohlensäurehaltige Schicht von SAE 8620 hervorgehoben werden. Die detaillierte metallographische Analyse und die Diskussion über die Schneidkräfte bieten wertvolle Erkenntnisse für Fachleute, die darauf abzielen, Hartdrehprozesse für mehr Effizienz und Nachhaltigkeit zu optimieren.
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Diese Zusammenfassung des Fachinhalts wurde mit Hilfe von KI generiert.
Abstract
The manufacturing industry is increasingly seeking sustainable alternatives to traditional grinding processes for finishing hardened components. This research presents an in-depth evaluation of the hard turning process applied to carburized, quenched, and tempered SAE 8620 steel employing polycrystalline cubic boron nitride (PCBN) inserts. Induction-hardened SAE 1045 steel served as a benchmark for comparative purposes, owing to its widespread use in similar industrial contexts of SAE 8620. The research compares dry machining and Minimum Quantity Lubrication (MQL) conditions, analyzing their respective effects on tool performance and surface integrity. The experimental methodology comprised the systematic monitoring of machining forces, flank and crater wear, surface roughness, chip morphology, and white layer formation. The results indicate that the specific microstructure of SAE 8620, resulting from the carburizing process, induces wear patterns distinct from those observed in other steels of equivalent hardness. The application of MQL effectively mitigated both flank and crater wear compared to dry cutting, although it significantly altered the surface topography and the metallurgical characteristics of the white layer. It is concluded that the replacement of grinding by PCBN hard turning is a viable and efficient strategy for gas carburized SAE 8620 steel. Furthermore, MQL proved superior to dry machining in extending tool life by suppressing thermal and tribological mechanisms, though it necessitates rigorous control over roughness parameters and surface integrity.
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1 Introduction
The implementation of high-hot-hardness tools with geometrically defined cutting edges for machining hardened steels has become a consolidated practice in the manufacturing industry, effectively substituting traditional grinding processes. This transition is driven by the ability to machine complex geometries, lower operational costs, and a significantly reduced environmental footprint. Such advancements are underpinned by the widespread adoption of Polycrystalline Cubic Boron Nitride (PCBN) and ceramic tools, integrated with high-stiffness, high-precision machine tools capable of meeting stringent dimensional tolerances and surface finish requirements.
In alignment with global sustainability and cost-reduction trends, Minimum Quantity Lubrication (MQL) has emerged as a critical technology in industrial plants. MQL not only lubricates but also alters the cooling rate. MQL can act as a localized quenching medium during cutting, which directly influences the thickness of the white layer. This shift has prompted the scientific community to seek a deeper phenomenological understanding of its mechanisms and potential. The economic and environmental scale of traditional lubrication is substantial; for instance, the annual consumption of water-soluble oil by the Ford Motor Company exceeds 20 million liters - in 2025, these numbers are even more critical due to automakers’ Net Zero targets [1]. The financial burden encompasses not only the procurement of the fluid, but also significant expenditures related to maintenance, monitoring, and disposal. Consequently, dry or “near-dry” machining has gained prominence. To enhance the efficiency of dry systems and support operations that traditionally rely on flood cooling, MQL technology is being continuously investigated and optimized.
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The production of hardened components also relies on advanced heat treatment practices to optimize costs and minimize environmental impact. Heat treatments can be categorized into thermal processes—such as annealing, normalizing, and quenching—and thermochemical processes, which include diffusion-based treatments like carburizing and nitriding. For the SAE 8620 steel, the standard procedure involves carburizing followed by quenching and tempering to achieve the required hardness. This process aims to tailor the material for specific loading and tribological conditions, imparting properties distinct from its normalized state. While induction hardening is often cited as a more flexible and cost-effective alternative for certain applications, carburizing remains the benchmark for components requiring high surface hardness and a tough core, such as those made from SAE 8620.
Modern manufacturing processes for hardened materials must prioritize flexibility, ecological compatibility, cost-effectiveness, and production agility. The state-of-the-art converges on three key pillars: the substitution of grinding by hard turning, the optimization of heat treatments, and the replacement of flood cooling with MQL. In the specific context of SAE 8620 steel, the reduction or elimination of cutting fluids is highly desirable due to the positive impact on environmental management.
Hard turning is particularly suitable for dry conditions; however, the loss of lubrication and cooling effects - specifically friction reduction and heat dissipation - can be detrimental. The challenge lies in mitigating these effects without compromising the beneficial thermal softening phenomenon occurring in the cutting zone. MQL systems address this by providing lubrication and cooling without the aggressive heat removal associated with flood cooling. Furthermore, MQL facilitates chip evacuation and protects both the workpiece and the machine tool from oxidation.
The primary objective of this research is to evaluate the wear mechanisms of PCBN tools, machining forces, chip morphology, and the resulting surface and subsurface integrity (specifically white and dark layer formation) during the hard turning of carburized and quenched SAE 8620 steel under dry and MQL conditions. In the present work, induction-hardened SAE 1045 steel served as a benchmark for comparative purposes, owing to its widespread use in similar industrial contexts to SAE8620. The specific objectives are the following: to evaluate the progression of flank and crater wear, as well as notch occurrence in PCBN tools, as a function of the lubrication-cooling medium; to assess the surface integrity of the workpiece through high-precision roughness measurements; to analyze the subsurface integrity of the machined region, focusing on the formation and depth of white and dark layers; to correlate cutting forces with the specific microstructural state of the carburized SAE 8620 and the lubrication conditions; to characterize the chip morphology and microstructure to understand the phenomenological impact of MQL on the cutting process.
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2 Literature review
2.1 Industrial context of hard turning
The modern manufacturing industry is characterized by a relentless pursuit of increased productivity, cost reduction, and minimized environmental impact. In this context, the machining of hardened parts, commonly referred to as hard turning, has gained significant traction as a robust alternative to traditional grinding processes [1]. Hard turning involves the machining of materials with hardness levels typically exceeding 45 HRC, often reaching up to 65 HRC. The shift from grinding to hard turning is driven by several factors: the ability to machine complex geometries in a single setup, higher material removal rates, and the potential for dry machining or the use of sustainable cooling strategies [2].
For components such as gears, shafts, and bearings, SAE 8620 steel is a primary choice due to its excellent response to carburizing treatments, which result in a hard, wear-resistant surface and a tough, ductile core [3]. However, the high hardness of the carburized layer poses significant challenges to tool life and surface integrity, necessitating the use of advanced cutting tool materials and optimized lubrication-cooling techniques [4].
2.2 PCBN cutting tool technology in hard machining
The success of hard turning is intrinsically linked to the development of ultra-hard tool materials. Polycrystalline Cubic Boron Nitride (PCBN) is the second hardest material known, surpassed only by diamond. Unlike diamond, PCBN exhibits high chemical stability when machining ferrous alloys at high temperatures, as it does not react with the iron in the workpiece [5].
PCBN tools are categorized based on their CBN content and the type of binder used (typically ceramic or metallic). High-CBN content tools (approx. 80–90%) are generally preferred for heavy-duty applications or interrupted cuts due to their higher toughness, while low-CBN content tools (approx. 40–60%) with ceramic binders (such as TiN or TiC) are optimized for continuous finishing operations on hardened steels [6]. The ceramic binder enhances the chemical wear resistance, which is crucial when machining SAE 8620, where interface temperatures can exceed 1000 °C [7]. The high hot hardness of PCBN allows it to maintain a sharp cutting edge even when the workpiece material undergoes thermal softening in the primary shear zone [8].
2.3 Metallurgical characteristics of case-hardened SAE 8620 steel
SAE 8620 is a low-alloy nickel-chromium-molybdenum steel. Its performance in hard turning is dictated by the microstructural changes induced during gas carburizing and subsequent quenching. The process increases the carbon content in the surface layer (typically to 0.8% − 1.2%), which, upon quenching, transforms into martensite [9].
A critical distinction in the machinability of hardened steels lies in the morphology of the martensite. According to Krauss [10], the carbon content determines whether the martensite forms as “lath” or “plate/twinned” structures. In SAE 8620, the high carbon concentration in the case leads to the formation of plate martensite, which is significantly brittle and has a hardness [11]. This brittleness affects the chip formation mechanism, often leading to the generation of segmented or “sawtooth” chips, which exert oscillatory forces on the PCBN tool [12].
2.4 Mechanisms of heat generation and tool wear
In hard turning, nearly all the mechanical energy consumed is converted into heat. There are three primary zones of heat generation: the primary shear zone (plastic deformation), the secondary shear zone (tool-chip interface friction), and the tertiary zone (tool-workpiece interface friction) [13]. Due to the high hardness of SAE 8620, the energy required for material removal is substantial, leading to extreme temperatures at the tool tip.
Tool failure in PCBN inserts typically occurs through gradual wear rather than catastrophic fracture, provided the cutting parameters are optimized. The two dominant wear modes are:
1.
Flank Wear (VB): Caused primarily by abrasive action between the tool and the hardened workpiece. It directly impacts dimensional accuracy and surface roughness [14].
2.
Crater Wear (KT): Occurs on the rake face due to chemical diffusion and abrasion at high speeds. The ceramic binder in low-CBN tools helps mitigate this by providing a chemical barrier [15].
The high temperatures also facilitate the “thermal softening” of the workpiece, which, paradoxically, can reduce cutting forces. However, if the temperature is not controlled, it accelerates the diffusion of boron and nitrogen into the chip, leading to rapid tool degradation [16].
2.5 Minimum quantity lubrication (MQL) in hard turning
Traditional flood cooling is often ineffective in hard turning because the high contact pressures at the tool-chip interface prevent the fluid from reaching the cutting edge. Furthermore, the thermal shock induced by flood cooling can lead to micro-cracking in PCBN tools [17]. Minimum Quantity Lubrication (MQL) has emerged as a superior alternative.
MQL involves atomizing a very small amount of biodegradable oil (typically 10 to 100 ml/h) in a stream of compressed air. The high-velocity air-oil mist can penetrate the capillary spaces at the interface more effectively than liquid coolants [18]. In the machining of SAE 8620, MQL provides two main benefits:
Lubrication: Reducing the friction coefficient at the rake and flank faces, which lowers the rate of crater and flank wear [19].
Cooling: Although the cooling capacity of MQL is lower than flood cooling, the compressed air stream provides sufficient convective cooling to stabilize the tool temperature and prevent excessive thermal expansion [20].
Research by Dhar et al. [21] indicates that MQL can reduce cutting temperatures by 10–15% compared to dry machining, which is critical for preserving the tool’s hardness and the workpiece’s surface integrity.
2.6 Surface integrity: roughness and the “White layer”
Surface integrity is the most critical factor when replacing grinding with hard turning. It encompasses surface roughness, microstructural changes, and residual stresses.
2.6.1 Surface roughness
Hard turning is capable of producing finishes comparable to grinding (Ra < 0.4 μm). The roughness in hard turning is influenced by the tool’s nose radius (rƐ), feed rate (f), and tool wear. For SAE 8620, the use of MQL has been shown to produce a more consistent surface finish over time by delaying the progression of flank wear [22].
2.6.2 The white layer phenomenon
A major concern in hard turning is the formation of the “white layer” - a very hard (up to 1200 HV), brittle, and featureless layer observed under optical microscopy after etching with Nital [23]. This layer results from:
Rapid Heating and Quenching: The surface temperature exceeds the transformation temperature, followed by rapid cooling by the bulk of the material.
Severe Plastic Deformation: Inducing grain refinement and phase transformations [24].
In SAE 8620, the white layer can be detrimental as it may contain micro-cracks that serve as initiation sites for fatigue failure. Studies suggest that MQL can mitigate white layer thickness by reducing the peak temperatures during the cut [25].
2.6.3 Residual stresses
Unlike grinding, which often induces tensile residual stresses (detrimental to fatigue life), hard turning typically generates compressive residual stresses at the surface [26]. This is due to the dominant effect of mechanical deformation over thermal expansion. Compressive stresses inhibit crack propagation, thereby enhancing the service life of components like gears and bearings [27].
2.7 Chip formation and cutting forces
The chip formation in hard turning of SAE 8620 is characterized by the “sawtooth” or segmented morphology. This occurs due to a cyclic process of plastic deformation followed by adiabatic shear localization or crack initiation [28]. The frequency of segment formation is high, leading to high-frequency force oscillations.
Cutting forces in hard turning are generally higher than in conventional turning. The passive force (Fp or radial force) is often the largest component due to the small depths of cut (ap) and the large nose radius of the tool relative to the feed rate [29]. This high passive force can lead to vibrations and dimensional inaccuracies if the machine-tool system lacks sufficient rigidity. MQL helps stabilize these forces by maintaining a more constant friction condition at the interface [30].
2.8 Comparative analysis: SAE 8620 vs. SAE 1045
While both steels are used in hardened conditions, their behavior in hard turning differs significantly. SAE 1045, when induction hardened, typically features a lath martensite structure with lower carbon content in the matrix. In contrast, the carburized SAE 8620 features a high-carbon plate martensite [31]. The higher brittleness of the SAE 8620 case leads to more aggressive abrasive wear on the PCBN tool, whereas the SAE 1045 tends to exhibit more adhesive wear components. Consequently, the tool life when machining SAE 8620 is generally shorter, necessitating more precise control of the MQL parameters [32].
2.9 Final remarks on literature review
This literature review confirms that hard turning of case-hardened SAE 8620 steel using PCBN tools is a technically viable and environmentally sustainable process. The application of MQL technology plays a pivotal role in enhancing tool life and ensuring surface integrity by providing effective lubrication and cooling. The transition from plate martensite in the carburized layer dictates a unique wear behavior that requires specialized tool grades and optimized cutting conditions. Future research should focus on the long-term fatigue performance of hard-turned SAE 8620 components to fully validate the replacement of grinding in critical automotive applications.
3 Materials and methods
3.1 Workpiece material and heat treatment
The material investigated was the SAE 8620 low-alloy steel, selected for its suitability in applications requiring a hard, wear-resistant surface layer supported by a ductile core. Cylindrical specimens (diameter: 50 mm; length: 200 mm) were prepared from normalized bar stock. The chemical composition of the samples was verified via optical emission spectrometry (BRUKER model Q8 MAGELLAN), as detailed in Table 1.
Table 1
Chemical Analysis of SAE 8620 workpiece
Fe
C
Si
Mn
P
S
Cr
Ni
97,5
0,162
0,233
0,775
0,0127
0,0171
0,584
0,459
Mo
Cu
Al
Ti
V
Pb
Co
Sn
0,160
0,147
0,0265
0,00077
0,0022
< 0,001
0,0073
0,0157
The specimens were subjected to a thermochemical case-hardening process consisting of gas carburizing followed by quenching and tempering. The process was conducted in an IPSEN batch-type furnace (Fig. 1).
The heat treatment sequence - detailed in Table 2 - involved carbon diffusion at elevated temperatures, followed by austenitizing, oil quenching, and tempering to achieve a surface hardness of 58–62 HRC and an effective case depth of 0.8–1.2 mm. These parameters directly influence the microstructural gradient (high-carbon martensite at the surface transitioning to tempered martensite in the core), which is critical for evaluating tool wear and surface integrity in your PCBN hard turning experiments. Key Technical Notes:
Carbon Potential (Cp): Maintained at 1.1% during both diffusion and austenitizing stages to ensure uniform carbon enrichment without excessive carbide formation.
Temperature Sequence: The step-down from 970 °C (diffusion) to 880 °C (austenitizing) minimizes grain growth while promoting martensitic transformation during quenching.
Quenching Agitation: The 15-minute oil agitation at 60 °C optimizes heat transfer, reducing distortion risks in cylindrical specimens.
Tempering at 300 °C: This elevated temperature targets stress relief while preserving hardness, potentially influencing white layer formation during hard turning.
Table 2
Heat treatment process parameters for carburizing, quenching, and tempering of SAE 8620 steel
Post-treatment characterization included Vickers microhardness profiling (load: 0.3 kgf; ISO 6507-1) across the cross-section, revealing a gradual transition from the carburized martensitic case to the tempered martensite core. The carbon-enriched layer was confirmed via optical emission spectrometry, ensuring compliance with SAE 8620 specifications (e.g., carbon content > 0.8 wt% at the surface). The effective case depth and microhardness profile were characterized using a NEWAGE MT61 microhardness tester (Fig. 2), following ISO 2639 standards. In addition to microhardness measurement, the metallographic structure was also analyzed, and the results are illustrated in Fig. 3.
SAE 8620: Tempered martensite (90%), martensite in medium “lace” thickness 10 μm, retained austenite (10%). Presence of carbides distributed in the matrix
3.2 Machining setup, cutting tools and cutting parameters
Hard turning experiments were performed on a CINCINNATI MILACRON HAWK 150 CNC lathe (Fig. 4), characterized by high structural rigidity suitable for hardened material processing.
Hard turning operations were conducted using Kyocera TNGA160412 (grade KBN525) indexable inserts. This specific low-content Polycrystalline Cubic Boron Nitride (PCBN) composition (55% CBN) features a sub-micron grain architecture (1 μm) integrated with a Titanium Nitride (TiN) ceramic binder. Such a micro-grain structure is engineered to provide an optimal balance between chemical wear resistance and fracture toughness, making it highly effective for the continuous and semi-interrupted finishing of carburized steels where resistance to mechanical shock is critical. To withstand the substantial passive forces characteristic of the process, the tools were secured in an MTJNR2525 holder (Fig. 5), resulting in a negative rake angle for cutting edge reinforcement. The comprehensive mechanical properties of the KBN525 grade are summarized in Table 3.
Specifications of the indexable insert used for the experiments
The cutting parameters for the hard turning of SAE 8620 steel were structured into two distinct experimental phases to evaluate the transition from initial phenomenological understanding to a comprehensive analysis of tool wear and surface integrity (Table 4). The technical context of parameters selection is the following:
Cutting Speed (Vc): The selection of 150 m/min for the second phase was motivated by the more significant results obtained during preliminary trials. This value aligns with industrial standards for finishing operations on hardened steels using low-content PCBN tools.
Feed Rate (f): A constant feed rate of 0.08 mm/rev was maintained across all tests to ensure a consistent theoretical surface finish, allowing for a direct comparison of the effects of lubrication (Dry vs. MQL) and depth of cut.
Depth of Cut (ap): The increase from 0.1 mm to 0.2 mm in Phase 2 was designed to intensify the thermomechanical load on the tool-chip interface, thereby facilitating a clearer observation of wear mechanisms and the formation of the white layer.
Tool Geometry: As noted in the methodology, the depth of cut (ap) utilized in both phases is smaller than the tool nose radius (rƐ= 1.2 mm), which is a critical condition for high precision hard turning where the effective cutting edge is restricted to the nose radius arc.
Table 4
Cutting parameters for SAE 8620
3.3 Lubrication and cooling conditions
Two distinct environmental conditions were evaluated: dry cutting and Minimum Quantity Lubrication (MQL). The MQL system utilized an ACCOLUBE applicator (Fig. 6) delivering a vegetable-based lubricant (LB2000).
The MQL parameters were strictly controlled at a flow rate of approximately 40 ml/h and a constant air pressure of 6 bar. The lubricant was delivered through a dual-nozzle system positioned to target both the flank and rake faces of the tool (Fig. 7), ensuring optimal penetration into the tool-chip interface.
Fig. 7
(a) Force measurement platform installed in the Turning Center application, (b) nozzles with magnetic support and (c) detail of the nozzle position
Tool wear progression was monitored systematically according to ISO 3685 standards. Flank wear (VBB) crater wear (KT), and notch wear (VBn) were measured using an OLYMPUS BX51RF metallurgical microscope and a ZEISS SUPRA 55 Scanning Electron Microscope (SEM) for high-resolution fractographic analysis.
Surface integrity was assessed through roughness measurements (Ra and Rz) using a TAYLOR HOBSON SURTRONIC S116 profilometer. Machining forces (cutting, feed, and passive forces) were acquired in real-time using a KISTLER 9257BA piezoelectric dynamometer integrated with a multi-channel amplifier and data acquisition system. Finally, chip morphology and subsurface metallurgical alterations (white layer formation) were analyzed via metallographic preparation and etching with Nital 2%, followed by optical and electron microscopy.
3.5 Microstructural characterization and hardness profiling
To evaluate the thermochemical treatment’s effectiveness and the subsequent impact of hard turning on surface integrity, a comprehensive microstructural characterization was performed.
3.5.1 Metallographic preparation and etching
Cross-sectional samples were extracted from the machined specimens using a precision diamond saw under constant cooling to prevent additional thermal alterations. The samples were then hot-mounted in resin and subjected to a standardized grinding and polishing sequence. Grinding was performed using silicon carbide (SiC) abrasive papers (from 220 to 1200 grit), followed by polishing with diamond suspensions on synthetic cloths to achieve a mirror-like finish.
The microstructural features, including the martensitic matrix and the resulting surface alterations (white and dark layers), were revealed by chemical etching with Nital 2% (a solution of 2% nitric acid in ethanol). The etching duration was strictly controlled (between 5 and 10 s) to ensure optimal contrast of the carburized case and the heat-affected zones.
3.5.2 Microscopy and surface integrity analysis
Microstructural analysis was conducted using an Olympus BX51RF optical microscope (OM) equipped with a high-resolution digital camera and image analysis software. For higher magnification and detailed fractographic analysis of the tool-chip interface and subsurface alterations, a ZEISS SUPRA 55 Scanning Electron Microscope (SEM) was utilized. This allowed for the precise measurement of the white layer thickness and the characterization of the transition zone between the carburized surface and the base material.
3.5.3 Vickers microhardness profiling
The hardness gradient from the treated surface toward the core was determined using a NEWAGE MT61 microhardness tester in accordance with ISO 6507 standards. Vickers microhardness (HV) measurements were performed using a load of 0.5 kgf (HV 0.5) and a dwell time of 15 s.
The microhardness profile was mapped starting at 50 μm from the surface, with subsequent indentations spaced at 100 μm intervals until reaching the core hardness. This procedure was essential to define the effective case depth (ECD), typically defined as the distance from the surface where the hardness remains above 550 HV (or approximately 50 HRC). Additionally, microhardness measurements were taken in the immediate subsurface region (within the first 50 μm) using a lower load of 0.1 kgf (HV 0.1) to detect localized softening or hardening induced by the thermomechanical loads of the hard turning process under dry and MQL conditions.
4 Results and discussions
This section presents a comprehensive analysis of the experimental results obtained from carburized, quenched, and tempered SAE 8620 steel employing polycrystalline cubic boron nitride (PCBN) inserts and Induction-hardened SAE 1045 steel - served as a benchmark for comparative purposes, owing to its widespread use in similar industrial contexts - under both Minimal Quantity Lubrication (MQL) and dry cutting conditions.
Phase 1
4.1 Tests with cutting depth of 0.1 mm
Flank wear (VBB) is a critical indicator of tool life and machining performance. Its evolution provides insights into the dominant wear mechanisms at play. The initial series of tests employed a shallow cutting depth of 0.1 mm, designed to investigate the fundamental wear mechanisms under less aggressive conditions, allowing for a clearer observation of gradual wear progression.
4.1.1 Gradual flank wear for 0.1 mm cutting depth
4.1.1.1 Wear evolution curves for 100 m/min and 150 m/min – SAE 1045 and SAE 8620
Figure 8(a) illustrates the flank wear evolution for SAE 1045 at a cutting speed of 100 m/min under both dry and MQL conditions. It is evident that MQL reduces the rate of flank wear, extending tool life by mitigating thermal effects and friction. The lubricating film provided by MQL minimizes direct tool-workpiece contact, thereby reducing adhesive and abrasive wear components [31]. Similarly, Fig. 8(b) presents the flank wear evolution for SAE 1045 at an elevated cutting speed of 150 m/min. At this higher speed, the thermal load on the cutting edge is substantially increased. While MQL still offers considerable benefits, the wear rate is generally higher than at 100 m/min, indicating a more aggressive wear regime where thermal softening of the tool binder and diffusion wear become more prominent [32]. However, the evaporative cooling effect of MQL becomes particularly crucial here, helping to stabilize the cutting edge temperature and delay catastrophic failure.
Fig. 8
Flank wear (VB) evolution as a function of cutting time for SAE 1045 and SAE 8620 steel at (a) cutting speed of 100 m/min and (b) cutting speed of 150 m/min
For the more challenging SAE 8620 steel, Fig. 8(a) displays the flank wear progression at 100 m/min. The presence of hard carbides within the SAE 8620 microstructure, resulting from its alloy composition and heat treatment, leads to a more pronounced abrasive wear component compared to SAE 1045 [33]. This is reflected in generally higher wear rates for SAE 8620 under similar conditions. MQL still provides a reduction in wear, but its effectiveness in combating the abrasive action of carbides is somewhat diminished. Figure 8(b) details the flank wear evolution for SAE 8620 at 150 m/min. At this speed, the combined effects of high temperature and severe abrasion from the carbides in SAE 8620 accelerate tool degradation. The wear curves show a steeper slope, indicating a rapid progression towards the end of tool life. The MQL strategy, while beneficial, struggles to fully counteract the aggressive wear mechanisms, including micro-chipping and fracture of the PCBN grains due to repeated impact with hard phases [34].
4.1.1.2 Wear analysis for 100 m/min and 150 m/min
An analysis of the wear patterns reveals distinct differences between the two steel types and lubrication conditions. For SAE 1045, under dry conditions, the primary wear mechanism observed was abrasive wear, exacerbated by high temperatures leading to localized softening of the PCBN binder [35]. MQL application effectively reduced both abrasive and adhesive wear by providing a lubricating film that lowered the coefficient of friction and by offering a cooling effect that mitigated thermal softening.
Figures 9, showing optical images of worn tool inserts after machining SAE 8620 at 100 m/min and 150 m/min respectively, clearly illustrate the severe abrasive wear and localized chipping on the flank.
Fig. 9
Flank wear on PCBN tool with ap = 0.1 for SAE 8620 at (a) 100 m/min and (b) 150 m/min under dry and MQL conditions
In contrast, SAE 8620 exhibited more complex wear behavior than SAE1045. The presence of hard alloy carbides, such as chromium carbides, within the tempered martensitic matrix of SAE 8620 significantly increased the abrasive component of wear [36]. These carbides act as highly abrasive particles, leading to micro-fracture and pull-out of PCBN grains from the tool matrix. Diffusion wear, where elements from the workpiece diffuse into the tool material at high temperatures, was also observed, particularly at 150 m/min, contributing to the degradation of the PCBN cutting edge [37]. MQL helped to reduce the overall temperature, thereby slowing down diffusion processes, but its ability to prevent mechanical abrasion from the carbides was limited.
4.1.2 Notch measurement
Notch wear (VN) occurs at the intersection of the tool cutting edge and the machined surface, corresponding to the depth of cut line. This region experiences high stress concentrations, severe rubbing, and often localized work hardening of the workpiece material [38]. Figure 10 presents a comparative analysis of notch wear for SAE 1045 and SAE 8620 at 100 m/min under dry and MQL conditions. For SAE 1045, notch wear was generally less severe under MQL conditions, as the lubricant reduced friction and heat generation at this critical interface. The more homogeneous microstructure of SAE 1045, primarily tempered martensite, tends to work harden less aggressively than SAE 8620, contributing to lower notch wear.
Fig. 10
Notch wear (VN) comparison with ap = 0.1 for (a) SAE 1045 and (b) SAE 8620 at 100 m/min under dry and MQL conditions
In contrast, SAE 8620 consistently exhibited higher notch wear, as depicted in Fig. 11 for 150 m/min. The presence of hard carbides in SAE 8620, combined with its propensity for work hardening, creates a highly abrasive environment at the depth of cut line. These carbides can act as micro-indenters, leading to accelerated wear in this region. MQL provided some mitigation by reducing friction, but the mechanical interaction with the hard microstructure of SAE 8620 remained a dominant factor.
Fig. 11
Notch wear (VN) comparison for (a) SAE 1045 and (b) SAE 8620 at 150 m/min under dry and MQL conditions for ap = 0.1 mm
Surface roughness (Ra) is a critical quality parameter for machined components. Notch wear, particularly primary notch wear (at main cutting tool edge), has a direct and significant impact on the resulting surface finish. Figure 12 presents the average surface roughness (Ra) values obtained for SAE 1045 and SAE 8620 under Vc = 100 m/min and Vc = 150 m/min.
Fig. 12
Average surface roughness (Ra) values for (a) SAE 1045 and SAE8620 at Vc = 100 m/min and (b) for SAE 1045 and SAE8620 at Vc = 150 m/min
Paulo & Ferreira [39] observed in their research that as notch wear increased, the surface roughness also deteriorated. The irregular geometry created by the notch on the cutting edge directly translates into grooves and irregularities on the machined surface. In this research (Fig. 12) MQL generally led to higher Ra values for SAE 1045, attributed to higher notch wear and a less stable cutting process. For SAE 8620, as shown in Fig. 12, the surface roughness values were consistently lower than for SAE 1045, especially under dry conditions. The MQL strategy, while improving Ra compared to dry cutting, could not fully overcome the inherent challenges posed by the microstructure of SAE 8620.
4.1.4 Crater wear: final measurement and image analysis
Crater wear (KT) occurs on the rake face of the cutting tool due to a combination of high temperatures, chemical affinity between the tool and workpiece materials, and diffusion processes [40]. Figure 13(a) shows typical craters wear profiles on the PCBN tool for SAE 1045 at 100 m/min. For the 0.1 mm cutting depth tests, crater wear was generally less dominant than flank wear, primarily due to the relatively small contact area between the chip and the rake face at this shallow depth of cut. However, at higher cutting speeds (150 m/min), especially under dry conditions, the elevated temperatures promoted diffusion wear, leading to the formation of small craters.
Fig. 13
Optical images of crater wear on the rake face for PCBN tool after machining (a) SAE 1045 and (b) SAE 8620 at 150 m/min under MQL and dry machining
For SAE 8620 at Fig. 13(b), crater wear was also observed, but its progression was often overshadowed by the more rapid development of flank and notch wear, which typically led to tool failure first. MQL application significantly reduced crater wear by lowering the interface temperature, thereby inhibiting diffusion and chemical reactions between the tool and chip materials [40].
Phase 2
4.2 Tests with cutting depth of 0.2 mm
Increasing the cutting depth to 0.2 mm introduced more aggressive machining conditions, leading to higher cutting forces, increased heat generation, and accelerated tool wear. This section details the observations under these more demanding parameters.
4.2.1 Flank and notch wear measurement
With a cutting depth of 0.2 mm, the wear rates for both flank and notch wear increased significantly compared to the 0.1 mm depth. Figure 14(A) illustrates the accelerated flank wear progression for SAE 1045 at 150 m/min. The larger chip load and increased contact area intensified abrasive and adhesive wear. Figure 14 (B) shows that MQL continued to demonstrate its effectiveness, significantly extending tool life by maintaining lower cutting temperatures and reducing friction. The evaporative cooling of the MQL fluid played a crucial role in preventing thermal softening of the PCBN binder and the workpiece material near the cutting zone.
Fig. 14
(A) Flank wear evolution for SAE 1045 steel at a cutting depth of 0.2 mm and 150 m/min under dry condition, presenting (a) chipping, (b) adhesion, (c) abrasion, (d) plastic deformation, (e) notch wear, and (f) micro-chipping; and (B) Flank wear evolution with MQL showing notch wear at left and right, and flank wear at the center of the image
For SAE 8620 under dry machining with cutting speed of 150 m/min, as shown in Fig. 15(A), the flank wear at 0.2 mm depth of cut was even more pronounced. The increased chip volume and interaction with the hard carbides in SAE 8620 led to rapid tool degradation. Micro-chipping and macroscopic fracture of the cutting edge were more frequently observed, indicating a transition towards brittle failure mechanisms in addition to gradual wear [41]. In Fig. 15(B) MQL provided some benefit, but the inherent abrasiveness of SAE 8620 at this higher cutting depth presented a remarkable challenge to tool integrity.
Fig. 15
(A) Flank wear evolution for SAE 8620 steel at a cutting depth of 0.2 mm and 150 m/min under dry and (B) machining under MQL conditions
The increased cutting depth and accelerated wear mechanisms had a direct impact on surface roughness. Figure 16(a) presents for Vc =150 m/min the surface roughness (Ra) values for SAE 1045 at 0.2 mm depth. While MQL still yielded better surface finishes compared to dry cutting, the overall Ra values were higher than those observed at 0.1 mm depth. This is attributed to the larger chip load, increased vibration, and more pronounced notch wear.
Fig. 16
Average surface roughness (Ra) values for (a) SAE 1045 steel and (b) SAE 8620 at a cutting depth of 0.2 mm under dry and MQL conditions
Figure 12(b) illustrates the roughness measurement results for SAE8620 material, under dry and MQL conditions, at a cutting depth of 0.2 mm. The measurements confirm that, with normal flank wear progressing and minimal notch wear incidence, its evolution does not necessarily imply increasing roughness. A cyclic alteration of the tool flank profile was observed, evidenced by changes in roughness. At a machined length of 17,274 mm, when the roughness value was 0.78 μm, a decrease began, extending until the end of the tests, reaching a value of 0.34 μm. The same trend was observed for dry machining: the reduction in roughness initiated at a machined length of 16,769; when the roughness was 0.7 μm. Subsequently, the value decreased to 0.6 μm and concluded the tests with a roughness of 0.38 μm.
4.2.3 Crater wear
At a cutting depth of 0.2 mm, crater wear became a more significant factor, particularly at higher cutting speeds (Vc = 150 m/min) and under dry conditions. The increased chip-tool contact area and higher interface temperatures promoted diffusion and chemical wear mechanisms. Figure 17(a) shows a more developed crater on a PCBN tool after machining SAE 1045 dry at 150 m/min with 0.2 mm depth. The crater depth (KT) weas notably larger than those observed at 0.1 mm depth. For SAE 8620, in Fig. 17(b), while flank and notch wear remained dominant, crater wear also progressed more rapidly. The higher thermal conductivity of SAE 8620, combined with the intense friction, led to significant heat generation at the rake face. MQL proved highly effective in mitigating crater wear by reducing the interface temperature and providing a lubricating barrier, thereby slowing down the diffusion of elements between the tool and the hot chip [42].
Fig. 17
Optical image of a well-developed crater wear on the rake face of PCBN tool (a) for SAE 1045 and (b) after SAE 8620 machining at 150 m/min and 0.2 mm depth under dry conditions
Figure 18 shows the machining forces For SAE 8620. The forces were generally higher than for SAE 1045, reflecting the increased resistance to deformation due to its higher hardness and the presence of hard carbides. However, MQL still provided a noticeable reduction in forces, albeit to a lesser extent than for SAE 1045. The reduction in forces by MQL is beneficial not only for energy efficiency but also for reducing tool wear, as lower forces translate to reduced mechanical stresses on the cutting edge [43, 44]. The passive force (Fp) was particularly sensitive to the condition of the flank wear land and the presence of notch wear, often increasing significantly as wear progressed.
Fig. 18
Machining forces for SAE 8620 at 0.2 mm depth of cut under dry and MQL conditions
Chip morphology provides valuable information about the deformation mechanisms, thermal conditions, and friction at the cutting zone. For SAE 8620, the chips were consistently more segmented and often appeared as short, broken fragments, as illustrated in Fig. 19 (1–3) for dry cutting. This is attributed to the higher hardness and lower ductility of SAE 8620, particularly due to the presence of hard carbides, which promote brittle fracture during chip formation [45]. The chips often exhibited a serrated or saw-tooth appearance, indicative of adiabatic shear banding. MQL (Fig. 19 4–6) had a less pronounced effect on the overall segmentation of SAE 8620 chips, but it did reduce the tendency for chip adhesion and improved chip evacuation.
Fig. 19
Chip morfology for SAE 8620 at 0.2 mm depth of cut under dry and MQL conditions
For SAE 1045 under dry conditions the chips were generally continuous or segmented, with evidence of significant plastic deformation. The high temperatures associated with dry cutting often led to thermal softening of the chip, promoting continuous chip formation. Under MQL conditions the chips for SAE 1045 tended to be more tightly curled and often more segmented. The cooling effect of MQL reduced the temperature in the shear zone, leading to less thermal softening and a more brittle fracture mechanism during chip formation [46]. The lubrication also facilitated smoother chip flow, reducing friction and preventing chip adhesion to the tool.
4.2.6 Chip metallography
Metallographic analysis of the chips provides detailed insights into the microstructural changes induced by the severe plastic deformation and high temperatures during machining. For SAE 8620 chips shown in Fig. 20, the white layer was generally more prominent and thicker than in SAE 1045 chips under similar dry conditions. This is attributed to the higher thermal conductivity of SAE 8620 and the more intense friction generated by its harder microstructure, leading to higher interface temperatures. The white layer in SAE 8620 chips often exhibited a finer grain structure and higher microhardness compared to SAE 1045. MQL again proved effective in reducing the thickness and prevalence of the white layer in SAE 8620 chips, confirming its role in thermal management. For SAE 1045 chips machined with MQL the white layer was significantly thinner or, in some cases, completely absent. The cooling effect of MQL reduced the peak temperatures at the chip-tool interface, preventing the material from reaching the austenitizing temperature or reducing the duration of exposure to high temperatures, thus inhibiting the formation of this thermally induced layer.
Fig. 20
Metallographic cross-section of chips from SAE 8620 machined under (a) MQL and (b) dry conditions, illustrating a prominent white layer
For SAE 8620, the as-received material exhibited a tempered martensitic structure with dispersed carbides, typical for a carburizing steel after hardening and tempering. Its initial microhardness was approximately 600–650 HV. After machining, significant changes were observed in the subsurface. At Fig. 21(a) at dry machining a distinct “white layer” was observed on the machined surface, characterized by an extremely high microhardness, often exceeding 800–900 HV. This layer, typically 5–20 μm thick, is an untempered martensitic structure formed due to rapid heating above the austenitizing temperature followed by severe quenching by the bulk material [47]. Below the white layer, a “dark layer” or softened zone was sometimes observed, where the original tempered martensite had been over-tempered due to the heat input, resulting in a slight reduction in hardness. Further below, the hardness returned to the bulk material’s value.
Fig. 21
Metallographic cross-section of SAE 8620 steel machined (a) dry, illustrating the white layer on the surface and (b) machined with MQL, showing a thinner or absent white layer
Under MQL conditions, Fig. 21(b), the white layer was significantly thinner or, in some cases, completely absent. The peak hardness on the surface was also reduced, indicating less severe thermal transformation. The cooling effect of MQL effectively mitigated the extreme thermal gradients responsible for white layer formation. At Fig. 21(b) a metallographic image of SAE 8620 machined with MQL, confirms the reduction in white layer thickness and a more uniform subsurface microstructure.
5 Conclusions
This research work evaluated the wear mechanisms of PCBN tools, machining forces, chip morphology, and the resulting surface and subsurface integrity (specifically white and dark layer formation) during the hard turning of carburized and quenched SAE 8620 steel under dry and MQL conditions, and leads to the following specific conclusions:
3.
Tool Wear and Machinability: SAE 8620 exhibited high flank and crater wear rates. This behavior is attributed to the presence of alloying elements and the formation of hard carbides during the carburizing process, confirming that microstructural composition, rather than surface hardness alone, governs tool degradation in hard machining.
4.
Effectiveness of MQL: The application of MQL significantly reduced both flank and crater wear compared to dry machining. The effectiveness of the MQL system was enhanced by the capillarity effect within the abrasive grooves of the tool flank, facilitated by the high-pressure atomization of the lubricant, which effectively cooled and lubricated the tool-chip interface.
5.
Surface Integrity and Roughness: SAE 8620 demonstrated excellent surface finish results, maintaining Ra values below 1.5 μm. These values are compatible with traditional grinding standards, validating the feasibility of replacing abrasive processes with hard turning for this specific alloy. Notably, the surface roughness remained stable and similar under both dry and MQL conditions.
6.
Notch Wear Stability: SAE 8620 maintained a consistent notch wear progression regardless of the depth of cut (ap = 0.1 mm to 0.2 mm). This stability further underscores the decisive role of the carbide-rich carburized layer in defining the tool-workpiece interaction.
7.
Thermal and Metallurgical Effects: The use of MQL effectively reduced the incidence of “white layer” (re-tempering/recrystallization) and “dark layer” (double tempering) on the machined surface. This reduction in metallurgical damage is a direct result of lower cutting zone temperatures and decreased friction provided by the lubricant.
8.
Cutting Forces and Chip Morphology: Passive force (Fp) was the predominant component during the trials and was high for SAE 8620. MQL application promoted chip embrittlement, leading to the formation of “finely segmented” chips, which, combined with reduced crater wear, prevented excessive chip elongation and improved process stability.
In summary, hard turning with PCBN tools under MQL conditions is a robust and technically viable alternative for finishing gas carburized SAE 8620 steel components, offering grinding-quality finishes while enhancing tool life and surface metallurgical integrity.
Declarations
Competing interests
The authors have no relevant financial or non-financial interests to disclose.
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Sustainable hard turning of gas carburized SAE 8620 steel: comparative analysis of PCBN tool performance and surface integrity under dry and MQL conditions
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Marco Aurélio Sampaio
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