Analysis of white layers formed in hard turning of AISI 52100 steel

Analysis of white layers formed in hard turning of AISI 52100 steel

Materials Science and Engineering A 390 (2005) 88–97 Analysis of white layers formed in hard turning of AISI 52100 steel A. Ramesha , S.N. Melkotea,∗...

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Materials Science and Engineering A 390 (2005) 88–97

Analysis of white layers formed in hard turning of AISI 52100 steel A. Ramesha , S.N. Melkotea,∗ , L.F. Allardb , L. Riesterb , T.R. Watkinsb a

The George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA b High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Received 4 December 2003; received in revised form 15 July 2004; accepted 3 August 2004

Abstract The formation mechanisms and properties of white layers produced in machining of hardened steels are not clearly understood to date. In particular, detailed analysis of their structure and mechanical properties is lacking. This paper investigates the differences in structure and properties of white layers formed during machining of hardened AISI 52100 steel (62 HRC) at different cutting speeds. A combination of experimental techniques including transmission electron microscopy (TEM), X-ray diffraction (XRD), and nano-indentation are used to analyze the white layers formed. TEM results suggest that white layers produced at low-to-moderate cutting speeds are in large part due to grain refinement induced by severe plastic deformation, whereas white layer formation at high cutting speeds is mainly due to thermally-driven phase transformation. The white layers at all speeds are found to be comprised of very fine (nano-scale) grains compared to the bulk material. XRD-based residual stress and retained austenite measurements, and hardness data support these findings. © 2004 Elsevier B.V. All rights reserved. Keywords: White layers; Hard turning; Plastic deformation; Phase transformation

1. Introduction Machining of hardened steels (>45 HRC) using single point turning, commonly referred to as hard turning, is of considerable interest to manufacturers of ball bearings, and other mechanical power transmission components. Bearing steels (e.g. AISI 52100) are the most common examples of these materials, which are conventionally finished by grinding. Hard turning has several potential advantages over grinding including the ability to be carried out dry, greater flexibility in producing complex geometric forms, lower machine tool cost, and comparable surface finish [1,2]. Unfortunately, hard turning is yet to find widespread industry acceptance as a viable finishing operation primarily due to the formation of undesirable microstructural artifacts in the near-surface layers after machining. These artifacts are often referred to as “white layers” because they appear featureless and white when viewed in an optical microscope after polishing and etching. The white layer is known to ∗

Corresponding author. Tel.: +1 404 894 8499; fax: +1 404 894 9342. E-mail address: [email protected] (S.N. Melkote).

0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.08.052

be a function of the cutting conditions and its thickness ranges from a few microns to a few tens of microns [3–5]. It is commonly thought to be composed of un-tempered martensite [6–7]. Furthermore, based mainly on knowledge of thermal damage or “burn” in grinding processes, white layers are generally considered to be detrimental to fatigue life since they are known to be hard and brittle and can be associated with tensile surface residual stresses [7,8]. Besides machining and grinding [7–10], white layers have been reported in other operations such as rubbing [11], railwheel contact [12–14] and under a wide range of process conditions. Griffiths [8] attributes white layer formation to one or more of the following possible mechanisms: (i) rapid heating and quenching, which results in transformation products, (ii) severe plastic deformation, which produces a homogenous structure or one with a very fine grain size, and (iii) surface reaction with the environment, e.g. in nitriding processes. Several investigators have shown that, irrespective of the process used to generate them, white layers are harder than the parent material [4–8,11–12]. Akcan et al. [4] attributed this to the very fine grain size of the white layer. The very fine

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grain size (<100 nm) in the white layer has also been reported by others [4,5,9,13–15]. With the exception of a few investigators [4,8–9], much of the current literature suggests or assumes that white layers formed in grinding and machining of hardened steels are due to a thermally-induced martensitic (␥–␣) phase transformation effect [3,4,7]. Brinksmeier and Brockhoff [7] discussed evidence of martensitic transformation in ground AISI 52100 and SAE 5045 hardened steels. However, white layers are known to form even under conditions where the temperature rise is too low for re-austenitization to occur [8,11] or in relatively soft materials such as brass [16]. Therefore, it is apparent that white layers may be formed by phenomena other than thermally-driven martensitic phase transformation. Such cases can arise in hard turning as well, since white layers have been reported to form at fairly low speeds and feeds, where temperatures may not be very high. Abrao and Aspinwall [17] and Thiele and Melkote [18] reported white layer formation in AISI 52100 steel while turning with chamfered and new tools with hone (tools with a finite cutting edge radius but negligible wear). An over-tempered layer was observed just below the white layer produced. Similar results were observed when cutting hardened AISI 4340 steel [19]. On the other hand, white layers are also known to form under conditions of large flank wear and high cutting speeds, where temperatures reached may be sufficient for ␣–␥ transformation to occur [3,4,7,15,20–22]. In other words, white layers formed in cutting of hardened steels have been reported under conditions where temperatures and strain fields are very different. This suggests that the mechanisms of formation of white layers under different cutting conditions and their corresponding mechanical properties are not likely to be the same. This paper analyzes the properties and mechanisms of formation of white layers on the machined surface in single point cutting of hardened AISI 52100 steel (62 HRC) at different cutting speeds. It also seeks to shed light on the roles of grain refinement due to severe plastic deformation and thermally-driven martensitic phase transformation in white layer formation in hard turning. This is accomplished through a detailed experimental investigation involving transmission electron microscopy (TEM), X-ray diffraction (XRD) and nano-indentation hardness measurements. 2. Experimental work 2.1. Workpiece material The workpiece material used in this study was AISI 52100 steel. The nominal chemical composition of 52100 steel is shown in. Table 1 Bars of 35 mm diameter (1.375 in.) were heat treated by holding at 829 ◦ C (1525 ◦ F) for 1.5 h, quenching in oil, then tempering at 163 ◦ C (325 ◦ F) for 2 h, which yielded a tempered martensitic bulk structure with a hardness of approximately 62 HRc.

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Table 1 Composition of AISI 52100 Element

Percent weight

C Cr Fe Mn Si P S

0.98–1.1 1.4 97.05 0.35 0.25 <0.25 <0.25

2.2. Machining conditions White layers were generated by turning the hardened steel at three different cutting speeds of 300 SFPM (91.4 m/min), 600 SFPM (182.9 m/min), and 900 SFPM (274.3 m/min). The feed and depth of cut were kept constant at 0.127 mm/rev and 0.254 mm, respectively, and all cutting was performed dry. Since machining with cutting tools is a thermo-mechanical process, it is impossible to completely isolate the mechanical and thermal effects. Consequently, the choice of cutting speeds from low-to-high was designed to emphasize the expected dominant role of thermal phenomena at the highest cutting speed. In contrast, the lowest cutting speed was selected to emphasize the expected dominant role of mechanical (plastic) deformation. Each of the three conditions employed a low CBN-content insert (Kennametal KD050 grade, ANSI TNG-432 geometry) mounted in a standard tool holder (Kennametal DTGNL164D). Hard turning was carried out on a rigid super precision CNC lathe (Hardinge T-42SP). In addition to the hard turned surfaces, a wire-EDM surface exhibiting white layer was also included for comparison purposes. Note that the EDM white layer is produced by a purely thermal process. Optical micrographs created using taper sectioning of the four white layers are shown in Fig. 1. All surfaces exhibit a distinct white layer region above by a darker etching transition (over-tempered) region, which is followed by the bulk structure. 2.3. Transmission electron microscopy (TEM) TEM was used to analyze the white layers generated under different machining conditions in terms of their morphology, chemical composition (using energy dispersive spectroscopy (EDS)), and crystallographic structure. The TEM sample preparation process involved cutting 0.254 mm thick disks from the machined bars using wireEDM. These disks were then cut into 2 mm × 2 mm squares to expose the white layer along one of the edges. The samples were subsequently mechanically polished down to about 50 ␮m thickness. Focused ion beam milling (FIB) was used on the samples to further reduce the thickness of the samples to approximately 2 ␮m. A Gallium-liquid metal source was used to generate the ion beam. Throughout the FIB process, the surface of the sample was protected with a layer of tungsten to

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Fig. 1. White layers generated at (a) 300 SFPM, (b) 600 SFPM, (c) 900 SFPM, and by (d) EDM.

avoid damaging the areas of interest. Once sample preparation was complete, an HF 2000 Field Emission TEM was used for analysis. The camera factor for the microscope was ˚ first determined to be 999.148 A-pixel using a standard sili˚ con sample with a d-spacing of 3.14 A.

removed material [23]. The impact of these corrections was found to be negligible (<0.5%). Measurements were made up to a total depth of approximately 250 ␮m.

2.4. Retained austenite and residual stress measurement

The hardness of the white layers was measured to find mechanical property differences, and to explain these differences in the context of results obtained from TEM and XRD measurements. Nano-indentation was chosen as the hardness measurement method. This method allows sampling of a very small volume of the material [24], thus making it ideal for measuring the hardness of white layers, which are generally very thin (∼2–3 ␮m in the current experiments). Samples for hardness testing were first plated with nickel in order to enhance contrast between the white layer and the surrounding epoxy. Since the actual white layer depth in the samples was approximately only 2–3 ␮m, tapered crosssections of the samples were created in order to maximize the visible width of the white layer. The specimens were then mounted in a thermosetting compound (Buehler EpometTM ) designed to minimize edge loss while polishing. The samples were then rough polished using 240 and 320 grit papers. After obtaining a level surface, they were polished to a mirror finish using Buehler UltraPadTM , TEXMET 2000TM and TEXMET 1000TM polishing cloths using 9, 3 and 1 ␮m diamond slurries, respectively. They were then etched with Nital (2% nitric acid in ethanol) solution.

XRD measurements were performed on an un-machined sample, the 300 and 900 SFPM samples, and the EDM sample to determine the retained austenite levels in the white layer region, as well through-thickness residual stresses. A four-axis goniometer (Scintag X2000) was used for the residual stress and retained austenite measurements. A 1.4 kW Cr K␣ beam was used with a source-to-specimen distance of 290 mm. For the estimation of retained austenite, only the peaks at 106◦ and 155◦ corresponding to the [2 0 0] and [2 1 1] planes for ferrite/martensite, and 78◦ and 128◦ corresponding to the [2 0 0] and [2 2 0] planes for austenite were considered. The determination of retained austenite involved measurement calibration using National Bureau of Standards (NBS) samples (SRM 485a, SRM 486 and SRM 487) with predetermined amounts of retained austenite. For the estimation of residual stresses, only the ferrite/martensite peak at 155◦ corresponding to the [2 1 1] planes was considered. The through-thickness residual stress measurements were biaxial. The layer removal technique was used to measure through-thickness stresses, with corrections being applied for

2.5. Hardness measurements

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Displacement-controlled indentation tests were carried on polished and etched samples using the MTS-NanoXPTM and MTS Nanoindentor IITM instruments to penetration depths of 200 nm at loads of approximately 10 mN. In order to account for possible effects such as etching, composition variation from location to location, out-of-flatness of the sample due to inconsistencies in polishing, ambient vibrations etc., a large number of indents (up to 40) were taken for each test condition. 3. Results and discussion 3.1. Morphology and phase of white layers Inspection of the bright-field TEM images of the four white layer samples (see Fig. 2) shows that the white layers in all samples possess a highly refined grain structure compared to the bulk microstructure. Relatively clear demarcations exist between the white layer and the bulk material in all samples. Dark field images of the hard turned white layers shown in Fig. 3a–c clearly indicate that the white layer generated at all three cutting speeds consists of a very fine structure compared to the bulk material. In fact, for the 300 SFPM and 600 SFPM samples, it was not possible to reliably resolve the

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grain size even at a magnification of 60,000. The grain size of the white layer in these samples is estimated to be no larger than about 5 nm. The grain size of the white layer in the 900 SFPM sample is estimated to be about 20 nm. The grain size of the bulk region adjacent to the white layers in all machined samples is estimated to be in the 50–200 nm range. The fine grain size of the EDM white layer is due to the rapid quench of the material that has been melted by the high localized temperatures produced during the sparking process. It is well-known that cooling rate is an influential factor in determining grain size, with a high cooling rate resulting in small grain size. The wire-EDM process was carried out in a bath of de-ionized water, which cools the molten steel very rapidly leading to formation of very fine grains. The above observations regarding white layer grain size are further validated by the analysis of selected area diffraction patterns for the white layer, interface and bulk regions of the samples shown in Fig. 4a–f The relevant inter-planar ˚ which spacings for this analysis are 2.0705 and 1.5087 A, correspond to the [1 0 1] and [0 0 2] planes of martensite, re˚ corresponding to the [2 0 0] plane of spectively, and 2.547 A cementite. It can be seen that the [1 0 1] and [0 0 2] martensite rings and the [2 0 0] cementite ring are most prominent in the selected area diffraction patterns of the 300 and 600 SFPM samples (Fig. 4a–b).

Fig. 2. TEM images of white layers at 20,000× magnification: (a) 300 SFPM, (b) 600 SFPM, (c) 900 SFPM, and (d) wire-EDM.

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Fig. 3. Darkfield images of machined samples: (a) 300 SFPM, (b) 600 SFPM, and (c) 900 SFPM.

Examination of the diffraction patterns in Fig. 4a–d shows rings that are mostly continuous and, to varying degrees, diffuse. This is characteristic of very fine cell structures and hence confirms the refined grain structure in the white layer regions of the four surfaces. In contrast, the selected area diffraction pattern of the bulk material in Fig. 4f is relatively spotty, indicating a coarser grain structure. Note also that the ring corresponding to the [1 0 1] plane of martensite ˚ in the 900 SFPM sample (Fig. 4c) is markedly (∼1.961 A) sharper than the corresponding rings in the 300 and 600 SFPM samples. This confirms the relative coarseness of the grain structure in the white layer region of the 900 SFPM sample compared to the lower cutting speed sample and is in qualitative agreement with the grain size estimates derived from the dark field TEM images. The differences in grain size between the white layer and bulk structures of the hard turned samples is attributed primarily to severe plastic deformation accompanying the chip formation process. Engineering strains of 4–6 are common in conventional machining operations. Temperature rise during cutting causes thermal softening of the material, which contributes to further deformation of the surface layers causing fragmentation of the grains. Similar grain refinement has been observed in large deformation processes such as equal channel angular extrusion [25,26]. The somewhat larger grain size in the white layer region of the 900 SFPM sample (compared

to the 300 SFPM sample) is attributed to a grain coarsening effect arising from the higher cutting temperatures that typically accompany higher cutting speeds. Prangnell et al. [26] note that nanocrystalline structures are formed by severe deformation only at homologous temperatures below 0.2Tm and only in the presence of large hydrostatic pressures. Chip formation in machining is characterized by a very large compressive stress acting in the shear zone, which tends to enhance the ductility of the material and hence the severity of strain experienced by the material. In addition, it is reasonable to assume that the temperatures generated at the 300 SFPM cutting speed will be lower than those produced at 900 SFPM. This can then explain the nanometric grain size estimates obtained for the hard turned white layer samples. Several fibrous and plate-like substructures can be seen in the bright-field TEM images of the white layers in the 300 and 600 SFPM samples (see Fig. 5). The EDS scan (see Fig. 6) reveals these substructures to be carbon-rich areas suggesting the possibility of cementite clusters. Evidence of plate-like cementite formation due to tempering in martensitic medium carbon steels has been reported by Thomas [27]. The selected area diffraction patterns (Fig. 4a–b) confirm the presence of cementite in the white layer region of these samples. In contrast, these sub-grain structures are not evident in the bright-field TEM images of the 900 SFPM, EDM and bulk samples. This observation is consistent with the absence

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Fig. 4. Selected area diffraction patterns of white layer region in (a) 300 SFPM, (b) 600 SFPM, (c) 900 SFPM, (d) EDM, (e) interfacial region, and (f) bulk.

of the cementite ring in the 900 SFPM (Fig. 4c) and bulk samples (Fig. 4e). The absence of cementite clusters/plates in the 900 SFPM sample can be explained if all carbon is retained in solution. This would be the case if re-austenitization of the martensitic structure (␣–␥ transformation) followed by rapid quenching (␥–␣ transformation) had occurred. On the other hand, in the white layers formed at lower cutting speeds, the presence of carbon-rich plates and cementite rings in the selected area diffraction patterns suggests that temperatures, although

high, were not high enough to cause phase transformation. This produces a tempering-like operation, which results in the precipitation of carbon in the form of cementite from the parent martensitic structure. It should be noted that the selected area diffraction pattern for the EDM white layer also exhibits a ring that appears ˚ (close to correspond to a d-spacing of approximately 2.5 A to the cementite ring diameter). However, the EDS scan for the EDM white layer showed negligible amounts of carbon (see Fig. 6b). Since the EDM process involves vaporizing

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Fig. 5. (a) Fibrous and (b) plate-like substructures in white layer generated at 600 SFPM.

a brass wire in front of the workpiece material, some zinc contamination is to be expected, as confirmed by the EDS scans. It is thought that this ring corresponds to a complex intermetallic compound of Fe, Cu, and Zn. Based on the foregoing analysis, it is concluded that the predominant mechanism of white layer formation at the highest cutting speed (900 SFPM) is martensitic phase transformation, which is aided by plastic deformation accompanying chip formation/surface generation. On the other hand, the predominant mechanism of white layer formation at the lower cutting speeds is believed to be grain refinement due to severe plastic deformation, a mechanism similar to that known to take place in other severe deformation processes such as equal channel angular extrusion [26]. It should also be pointed out that Akcan et al. [4] have observed white layers in hard turned AISI 52100 steel samples and in the resulting segmented chips. Their TEM analysis of white layers in the chips revealed a refined grain structure that was attributed to phase transformation. Their observa-

tions further support the above conclusions of the present study, i.e. white layers formed under conditions promoting high temperatures at the tool–workpiece interface (e.g. high cutting speed) are predominantly due to phase transformation, as are white layers formed in the hard turned chips where similar thermal conditions exist due to frictional heating at the tool–chip interface and plastic deformation. 3.2. Retained austenite content of white layers Fig. 7 shows that the retained austenite level increases with increasing cutting speed and hence cutting temperature. Barry and Byrne [21] observed similar trends in going from fresh cutting tools to worn cutting tools. From a viewpoint of the workpiece surface temperature, worn tools should have a similar effect as with increasing cutting speed. The significant difference in retained austenite content between the 300 SFPM and 900 SFPM samples can be explained by the mechanism of martensitic phase transformation. It is

Fig. 6. EDS spectra for (a) substructure in 600 SFPM white layer and (b) EDM white layer.

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Fig. 7. Retained austenite content on sample surfaces. The error bars denote minimum and maximum measured values.

known that mechanical working of martensitic steel causes the amount of retained austenite to decrease due to strain induced transformation [28]. Heating the steel below its austenitization temperature has a similar effect. It appears that both these phenomena occur in the 300 SFPM sample. On the other hand, the 900 SFPM sample exhibits a significantly higher retained austenite level. Since exposure to high temperatures causes the retained austenite level to diminish, and since temperatures at high cutting speeds are generally higher, the 900 SFPM sample should have lower retained austenite level than the 300 SFPM sample. On the other hand, martensitic phase transformation in the surface layers of the 900 SFPM sample would lead to an increase in the amount of retained austenite. On this basis, phase transformation appears to be the dominant mechanism in the 900 SFPM sample since re-transformed surface layers are not tempered after transformation, resulting in higher retained austenite values. The above reasoning is further strengthened by examining the retained austenite content in the EDM sample (the sample subjected to the highest temperature). As noted earlier, the white layer in this case is entirely due to a thermal phenomenon. The retained austenite content here is 32.5%, which is significantly higher than the un-machined condition. This can happen only due to re-austenitization followed by a rapid ␥–␣ transformation. Note that the retained austenite content of both machined samples in Fig. 7 is significantly lower than the un-machined sample. This is because of the effect of mechanical deformation of the workpiece surface during machining, which tends to lower the amount of retained austenite [28]. 3.3. Through-thickness residual stresses Axial and circumferential through-thickness profiles (see Fig. 8) show the magnitude of residual stress as a function of surface depth. The un-machined sample is nearly stress-free to a depth of approximately 50 ␮m. Measurements on this sample were made to ensure that there were no significant preexisting residual stress gradients through the sample depth.

Fig. 8. Axial and circumferential through-thickness residual stress profiles.

Residual stress profiles for the 300 SFPM sample show compressive stresses to a depth of about 200 ␮m. At this depth, the stresses decay to approximately zero. It is expected that residual stresses would become tensile with increasing depths to attain equilibrium. The peak compressive stress occurs at a depth of about 30 ␮m below the surface. This type of residual stress profile is typical of hardened steel subjected to high plastic deformation. Thiele and Melkote [29] observed similar residual stress profiles for AISI 52100 steel machined with chamfered and honed tools under similar machining conditions. Using backscatter SEM they showed the presence of heavy plastic deformation below the workpiece surface. Residual stresses in the 900 SFPM sample start out tensile near the surface and rapidly become very compressive at approximately 30 ␮m below the surface. This has

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Fig. 9. Nano-indentation hardness from white layers produced under different machining conditions.

been also observed in steels with white layers generated by tools with large flank wear land [1]. These results show that residual stress profiles associated with white layer samples generated under different conditions are considerably different. From the preceding observations, the residual stress profiles for the 300 SFPM sample correspond closely to those associated with heavy plastic deformation. Similarly, the profiles for the 900 SFPM sample correspond to those associated with large thermal effects. The EDM sample exhibits a residual stress profile that is markedly tensile at the surface indicating the dominant influence of thermal effects in the process. These results lend further support to the idea that mechanical effects (such as grain refinement due to heavy plastic deformation) are dominant in generating the hard turned surface at the lower cutting speed, whereas the surface generated at the highest cutting speed is predominantly due to thermal effects, such as martensitic phase transformation. 3.4. Hardness A box plot showing the hardness of white layers tested in different samples is shown in Fig. 9. The top of the box, called the upper fourth, is set at the 75th percentile of the data range. The bottom of the box, known as the lower fourth, is set at the 25th percentile of the data, such that 50% of the data falls within the box. The horizontal line through the box represents the median. The vertical lines are called whiskers and the ends represent the largest and smallest values not identified as outliers. Finally, an outlier is defined as a value that is smaller (or larger) than 1.5 inter-quartile ranges from the lower fourth (or upper fourth). Based on statistical tests of significance (t-tests of difference in the means), the white layers generated at 900 and 600 SFPM and by EDM were found to be significantly harder than the white layer generated at 300 SFPM at confidence levels of 99% and 94%, respectively. Further, the white layers in all samples were harder than the corresponding over-tempered

(transition) layers at confidence levels of 99% or higher. A similar trend was observed when comparing white layer hardness to the bulk region hardness. These results are consistent with the observations of Akcan et al. [4], Smith [30], and Barbacki et al. [5]. Smith [30] also showed that white layers generated by tools with large amounts of flank wear were harder than those generated with unworn tools. Flank wear, like higher cutting speed, generates higher temperatures due to friction and can re-austenitization of the martensitic structure followed by rapid quenching. The following paragraphs explain these observations in greater detail. Results presented in Fig. 9 show that the hardness of white layers generated by predominantly thermal effects differs from that generated by predominantly mechanical effects. This is consistent with the explanations for white layer formation based on TEM results discussed earlier. Results from TEM and XRD presented earlier suggested that white layer in the 300 SFPM sample was largely due to a mechanical process, such as grain refinement, while the white layer in the 900 SFPM sample was predominantly due to martensitic phase transformation. It is well known that the carbon content determines the hardness of martensitic steel. Martensitic steels with a higher percentage of carbon will have greater hardness than those with a lower carbon content [31]. This can explain why the white layer formed under the 900 SFPM machining condition is harder than that formed under the 300 SFPM machining condition. The EDM white layer is also formed entirely by martensitic phase transformation and would be expected to be significantly harder than the other samples, since the extent of heating would guarantee phase transformation. However, this was not observed (see Fig. 9) due to the large fraction of retained austenite, and the presence of zinc in the white layer. The hardness of the white layer in the 600 SFPM sample is seen to be comparable to the 900 SFPM and EDM samples, although TEM data suggests that grain refinement was the primary mechanism responsible for formation of white layer. The small difference in the nano-indentation hardness of these samples (only about 1 GPa) coupled with errors characteristic of low load measurements on metallic samples makes this result somewhat difficult to interpret.

4. Conclusions 1. TEM revealed that the grain sizes of white layers formed in AISI 52100 steel (62 HRC) were considerably smaller than the bulk. It was also shown that white layers generated at higher machining speeds were coarser (∼20 nm) than those generated at lower speeds (∼5 nm). 2. TEM results also suggest that all carbon in the white layer generated at the highest cutting speed (900 SFPM) is retained in solution. In contrast, a strong carbide presence was determined in the white layers formed at lower speeds. These differences and those in (1) suggest the occurrence

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of martensitic phase transformation at the highest machining speed (900 SFPM), and large degree of mechanicallyinduced grain refinement due to severe plastic deformation at the lower machining speed (300 SFPM). 3. Retained austenite content of the white layers was found to increase with greater thermal effects at the workpiece surface. This trend may be explained by phase transformation in the region of the white layer with increased thermal effects. 4. Residual stress profiles revealed a trend of increased tensile stresses with increased thermal effects at the workpiece surface. This confirms that higher machining speeds produce greater thermal loads at the workpiece surface. 5. Nano-indentation hardness data revealed a general trend of increased hardness of white layers with increase in cutting speed. This was explained by the predominance of phase transformation effects at high speeds (and thus higher temperatures), which causes all carbon to be retained in solution and thereby increases the hardness.

Acknowledgements The first two authors would like to acknowledge the support of the National Science Foundation (Grant No. DMI0100176) and the NIST ATP Award No. 70NANBOH3045. The authors are also grateful to the HTML Users Program in the Oak Ridge National Lab for access to XRD, TEM and nanoindentation equipment for the experiments.

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