An experimental investigation of work material microstructure effects on white layer formation in PCBN hard turning

An experimental investigation of work material microstructure effects on white layer formation in PCBN hard turning

International Journal of Machine Tools & Manufacture 45 (2005) 211–218 www.elsevier.com/locate/ijmactool An experimental investigation of work materi...

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International Journal of Machine Tools & Manufacture 45 (2005) 211–218 www.elsevier.com/locate/ijmactool

An experimental investigation of work material microstructure effects on white layer formation in PCBN hard turning G. Poulachona,*, A. Alberta,1, M. Schluraff a,1, I.S. Jawahirb,2 a

ENSAM, LaBoMaP, Rue Porte de Paris, Cluny 71250, France b University of Kentucky, Lexington, KY 40506-0108, USA Received 23 October 2003; accepted 22 July 2004 Available online 23 September 2004

Abstract White layers formed in machining of hardened alloys are known to be very hard and resistant to standard etchants used in metallographic studies. Many studies have been performed on this subject, but only with little progress showing definite results concerning the actual effectiveness of white layer formation. Hence, the basic question that remains unanswered is: are the white layers a tribological advantage for the manufacturing industry producing parts/components from hard alloys? The focus of this study is to investigate the evolution of white layers produced during progressive tool flank wear in dry hard turning with CBN (cubic boron nitride) tools, and to correlate this with the surface integrity of the machined surface. The following four materials were machined: X160CrMoV12 cold work steel (AISI D2), X38CrMoV5 hot work steel (AISI H11), 35NiCrMo16 high toughness steel and 100Cr6 bearing steel (AISI 52100). Samples of chips were metallographically processed and observed under an electronic microscope to determine whether white layers are present or not. More specifically, chipforms/shapes were studied to determine how they developed during machining with potential appearance of white layers, with a view to correlating the chip-forms/shapes with the white layer formation. Finally, by using scanning electron microscopy and EDS techniques on these chip samples, properties and microstructures of white layers were deduced in order to verify some of the prevalent theories. q 2004 Elsevier Ltd. All rights reserved. Keywords: White layer; Hard turning; Microstructure; Chip-forms; Surface integrity

1. Introduction The lack of knowledge concerning surface quality and integrity of the machined surfaces, especially with the appearance of white layers, in hard turning, has severely limited the study of the effectiveness of white layers in machining of parts/components such as bearing rolls, pinions. In this study, the appearance of white layers and the associated effects of cutting parameters at varying tool-wear rates have been studied and compared for

* Corresponding author. Tel.: C33-3-8589-5330; fax: C33-3-85595370. E-mail addresses: [email protected] (G. Poulachon), [email protected] (A. Albert), [email protected] (M. Schluraff), [email protected] (I.S. Jawahir). 1 Tel.: C33-3-8589-5330. 2 Tel.: C1-859-257-6262; fax: C1-859-257-1071. 0890-6955/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2004.07.009

various work materials commonly used in industry. The results clearly show that the thickness of these layers depends on the nature of the microstructure of the work material. The experience obtained during this study involving the white layer formation in various work materials also allows a special procedure to coat the chip specimen with a nickel coating in order to take care of the polishing direction, thus avoiding the usual damage and possible breakage of the white layers. In a finish turning, surface quality and integrity are often of great concern because of their impact on product performance in terms of functional behavior and dimensional stability. It has been shown that these factors can also be used as a tool-changing criterion. Thus, the understanding of how the appearance of white layers is related to tool-wear and cutting parameter variations affecting surface quality and integrity, is of practical significance.

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2. State-of-the-art on white layer formation in hard machining White layer is a result of micro-structural alteration on a martensite structure. It is called ‘white’ because it appears white under an optical microscope or featureless in a scanning electron microscopy (SEM). White layers can be found in many material removal processes such as turning, reaming, grinding and electrical discharge machining. The generation of a white layer and its quantification would indicate the amount of surface energy brought into the part/component. Currently, three different theories are prevalent to explain the structure of white layer formation. According to Barry and Byrne [1] and Chou and Evans [2], the high austenite content of the surface white layer clearly confirms the occurrence of the reverse martensite transformation during machining. A rapid increase in temperature, combined with high pressure generated by the action of the tool, transforms the machined surface to the austenitic state. When the tool leaves, the surface cools down and the critical speed of martensite formation is reached by convection of heat into the air and by conduction into the workpiece material. As a result of the high speed, (Chou and Evans [2] have estimated the surface cooling rate in hard turning to be of the order of 104 8C/s), some austenite has no time to transform and some retained austenite traces can be found in the surface layer. Mybokwere et al. [3], Cho et al. [4] and Zhang et al. [5] show that dynamic recovery is the dominant process in the formation of surface white layers and internal white adiabatic shear bands, which are internal non-etching white bands in steels, deformed at high strain rates (from 103 to 106 sK1). Dynamic recovery can be explained simply as the beginning of dislocations, arranged in cell boundaries. Assisted by the local increase in temperature, due to rapid localized deformation, dislocations concentrate into tangles, producing regions of high and low dislocation density and forming sub-grain boundaries. A new hypothesis has been developed by Zurecki et al. [6]. It is postulated that there is an almost complete dissolution of carbides due to the high temperature generated by plastic deformation. The more the amount of carbon in the matrix increases, the more the melting point of the steel decreases. As the tool leaves the material, the white layer cools down quickly which induces freezing of its microstructure. A small quantity of non-quenched martensite or retained austenite may develop within the white layer. There are several problems concerning the white layers’ microstructure, and two major theories have been proposed. The white layer microstructure is recognized as an ‘abnormal martensite’ composed of nanocrystalline, partially transformed material with a high density of dislocations considered to be a very fine martensite lath (misoriented cells between 30 and 100 nm) with fine dispersed carbides and high rate of retained austenite. According to To¨nshoff et al. [7], retained austenite is

the major composition of white layer structure. Also, To¨nshoff [7] and Chou and Evans [2] show that the volume fraction of retained austenite in white layers increases threefold compared to the one in virgin work material. However, more experiments are needed to confirm it. Indeed, the authors of this paper found no publications correlating the retained austenite to grain size. The microstructural evolution during white layer formation appears to be unknown. Matsumoto et al. [8] claim that the white layer has a mixed martensite a 0 and austenite g structure. Indeed, martensite in steels is a metastable structure. Moderate heat will lead to its decomposition to cementite and ferrite, in a tempering process. The machined surface encounters an extremely short cycle thermo-mechanical process. With such a high heating rate such as over 106 8C/s, as shown by Chou and Evans [2], if the austenite transformation starting temperature is reached, martensite could transform into austensite by reverse martensitic transformation. In the subsequent rapid cooling stage (once the machined surface leaves the tool flank contact), austenite can transform back into martensite, if the starting martensite transformation temperature is reached. In order to understand the mechanics of the revolution of white layers, it is important and necessary to know the effects of various process factors. Some studies have been performed to determine the effect of tool-wear, cutting speed and hardness on white layer depth. Results of those experiments are summarized below. Chou and Evans [2] investigated the effect of cutting speed. This study was performed on 100Cr6 steel with a hardness of HRC 61–63. The cutting tool was a 558 diamond-shape Al2O3–TiC insert with K308 rake angle, 58 clearance angle and 0.8 mm nose radius. Cutting conditions were 50 mm/rev feed rate and 200 mm depth of cut. The cutting speed ranged from 0.5 to 4.5 m/s for three different levels of flank wear: 110, 210, and 300 mm. This study shows that, in general, white layer depth increases by increasing tool-wear, but not significantly at the low speed of 0.5 m/s. This increase is attributed to the associated higher cutting forces and the increased contact time. In another study, Chou and Evans [10] chose the reference cutting condition to be 3 m/s cutting speed, 50 mm/rev feed rate, 200 mm depth of cut and 300 mm flank wear land. Data points were used to represent the average values of tests, and error bars showed the ranges of values for each test. It was shown that the white layer depth also increases with cutting speed, but eventually seems to approach an asymptote at high cutting speeds. They also showed that the depth of cut does not affect the white layer depth and that there is a slight increase with feed rate. Yang et al. [9] studied the wear characteristics of the white layer using a pin-on-disc machine. As shown in Fig. 1, the thicker the white layer, the lower the wear resistance of the material. This may be due to the occurrence of micro-cracks. Now, to specifically answer the question if the white layers are a tribological advantage or inconvenience for machining industry, the following can

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3. Experimental set-up 3.1. Work materials

Fig. 1. Thickness of the white layer as a function of the matrix hardness [9].

be stated. As to the metallurgical changes associated with the generation of white layers, there exist two conflicting opinions: (i) hardened surface and the thermal stability of the white layer are considered to be of tribological advantage. Also, the gradual transition in microstructure has been shown to increase wear resistance; (ii) microcracks and voids formed by adiabatic shearing are harmful to surface wear resistance. Also, it is known to decrease the fatigue resistance and stress-induced corrosion. Since the white layer is hard and brittle, cracks can easily nucleate and propagate within.

The three major selection criteria for the work materials used in the current work are: industrial relevance, hardenability, and microstructure variations as shown in Fig. 2. Four different work materials have been studied: (i) X160CrMoV12 (AISI D2) with a ferritic and cementite matrix and coarse M7C3 primary carbides (z20 mm); (ii) X38CrMoV5 (AISI H11) with a martensitic matrix (grain size z25 mm) and few MC carbides; (iii) 35NiCrMo16 with a martensitic matrix (grain size z10 mm) and rare carbides (no tempering); and (iv) 100Cr6 (AISI 52100) with a martensitic matrix (grain size z10 mm) and plenty of small M7C3 primary carbides (1 mm). Heat treatment conditions shown in Table 1 helped to maintain hardness for all four work materials at HRC 54. The work material samples were prepared in a tube shape, 100 mm outside diameter with 12 mm thickness. This was in essence to maintain homogenous hardness across the tube thickness. 3.2. Test conditions Tool-wear tests were conducted on SOMAB T400 turning lathe dedicated to hard turning. A low CBN (cubic boron nitride) content tool with a TiN coating was used because of

Fig. 2. Microstructure of the workpiece materials.

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Table 1 Tool-wear tests, preparation and observation of the samples Cutting tool

Cutting operation Cutting parameters

Work piece materials

Tool holder CBN tool insert Cutting edge angle Rake angle Inclination angle Longitudinal turning Dry cutting Cutting speed Feedrate Depth of cut X38CrMoV5 (AISI H11)

X160CrMoV12 (AISI D2)

35NiCrMo16 100Cr6 (AISI 52100)

Sample preparation

CTGNL 2525M 161D (Sandvik) TNGA 160408 S01020 A7020 (Sandvik), six cutting edges, PVD-TiN coated krZ918 gZK68 lZK68

VcZ230 m/min fZ0.12 mm/rev apZ0.2 mm Austenitization 1020 8C, vaccum atmosphere Air quenching (nitride brewed TZ24 8C and PZ5 bars) Tempering, TZ200 8C, tZ1 h, cool down with brewed nitride gas Austenitization 1030 8C, vaccum atmosphere Air quenching (nitride brewed TZ24 8C and PZ5 bars) Double tempering, TZ560 8C, tZ1 h, cool down with brewed nitride gas Austenitization 850 8C, vaccum atmosphere Oil quenching, TZ21 8C Austenitization 830 8C, vaccum atmosphere Oil quenching, TZ21 8C Tempering, TZ300 8C, tZ1 h, cool down with oil

Cutting

Coating Resin Polishing Etching Nital—3%

Macro-observations

Micro-observations

Workpiece

Chips

Parting: tool N 151.2-400-30 EG CB20, tool holder L151.21-2525 30, VcZ150 m/min, fZ0.08 mm/rev Cutting along the radius: round saw tool 1 mm of nickel Workpiece: thermosetting resin Disc 15, 9, 3, 1 mm, FZ180 N AISI H11: tZ1 min 15 s AISI D2: tZ25 s AISI 52100: tZ23 s 35NiCrMo16: tZ31 s

Chips: resin 25 gC hardener 3 g Disc 200, 9, 3 mm, OP-S FZ180 N AISI H11: tZ15 s AISI D2: tZ5 s AISI 52100: tZ15 s 35NiCrMo16: tZ5 s

Forces: Ff, Fc, Fp (frequency acquisition: fZ500 Hz) Tool-wear Roughness Hardness Visual observation Microscope Micro-hardness SEM EDS

Tool

Flank wear: Vb Crater width Ra, Rt, Rz Rockwell Depth of white layer Microstructure Weight: 25 g measure every 500 mm along the radius Microstructure Composition

its high percentage of binder providing reduced thermal conductivity. In this case, thermal softening is possible due to high temperature. The machining tests were conducted under dry conditions with no coolants. All four work materials were machined at the same cutting parameters: cutting speed, VcZ230 m/min, and feed, fZ0.12 mm/rev. The depth of cut was kept constant at apZ0.2 mm. The tool-wear tests are shown in Table 1. These tests were conducted with

Shape Chips geometry Microstructure

Geometry Second body

regular measurement of progressive tool-wear, and chips were collected at all stages. Also, the forces were recorded with a piezoelectric dynamometer for each stage of the toolwear tests. Machined workpiece specimens were saved to study the surface integrity subsequently. These surfaces were metallographically processed: embedded, polished, and etched. Table 1 details the tool-wear operation, the preparation of the samples and the observations made.

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Fig. 3. Tool material removal rate during the machining of X38CrMoV5 steel.

4. Results and analysis 4.1. Flank wear In previous studies, Poulachon et al [11,12] presented the results of a tool-wear study in CBN tools, and showed the interdependence of tool-wear with the workpiece microstructure. Fig. 3 shows the variation of tool material loss, with the total length of machined work material for four sets of cutting conditions for one of the four work materials used in the current work. The progression of the tool material loss as a function of the involved total cutting length is shown in a straight line. In the follow-up work presented in this paper, Fig. 4 shows that the materials can be classified into two distinct groups according to their

tool-wear rates. The first group, which includes 100Cr6 and X160CrMoV12 work material, has a high tool-wear rate whereas the second group with the other two work materials (35NiCrMo16 and X38CrMoV5) has a lower tool-wear rate. The tool-wear rate increases with increasing cutting parameters (f and Vc,) and it has been previously shown that Vc is the most influencing parameter. On the flank face, some grooves can be noticed with the same size as the carbides for the 100Cr6 and X160CrMoV12 work materials and as the martensite grain for 38CrMoV5 and 35NiCrMo16 work materials. Harder grains of microstructures have a high influence on the tool-wear process. 4.2. White layer In general, it can be said that white layers are enbrittled, damaged, and faded during the polishing as shown in Fig. 5. The white layers are not only hard, but also brittle, making it more difficult to work with. To counter this problem there are two solutions: (i) place and orient the resin sample on

Fig. 4. Comparison of CBN tool-wear according to the work material.

Fig. 5. SEM observation of X160CrMoV12 steel.

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Fig. 6. Nickel coating on a 100Cr6 bearing steel above the white layer.

the polishing plate in order to push the white layer onto the bulk material; and (ii) polish only with fine grains, since coarse ones break white layers down. Also, this problem can be overcome by laying a nickel coating on the part specimen as shown in Fig. 6. This technique allows good polishing of the workpiece without removal of the white layer, and provides a good observation of the precise boundary of the part specimen. When studying the microstructure of white layers, another problem is encountered: 3% Nital solution does not reveal the white layer’s microstructure. This observation confirms the previous findings reported in the state-of-theart review of literature: white layers have a good resistance to corrosion. Three percent Nital reveals only a difference between matrix and white layer microstructure as the latter is finer as shown in Fig. 7. If the presence of carbides must be revealed, it is better to use etchants such as Murakami or Le Chatelier Igevski as shown in Fig. 8, where it is possible to see that there are as much carbides in the white layer as in the matrix. The most important fact encountered and established during the studies of white layers is the appearance of some orientation lines for X38CrMoV5 as shown in Fig. 9, which contradicts a common opinion currently held that white

Fig. 7. SEM observation of 35NiCrMo16 steel etched with a 3% Nital solution.

Fig. 8. 100Cr6 saw-tooth chip etched with Murakami for 5 min.

layers are not misorientated. The SEM pictures show that some cracks are generated on these lines. White layers have been observed and examined at each stage of the progressive tool-wear. Using an optical microscope, the white layer thickness was measured. It is so possible to draw a graph of the latter as a function of flank wear as shown in Fig. 10 for each step corresponding to a wear stage. On this graph, as a general remark it can be said that white layer thickness grows with increasing flank wear. Also, two steel families can be easily distinguished: one with fine microstructure (35NiCrMo16 hot work steel and 100Cr6 bearing steel) and the others with coarse microstructure (X160CrMoV12 cold work steel and X38CrMoV5 hot work steel). The first family develops a thicker white layer than the second one. All theories about white layer formation point out the importance of high temperature. If the steel is not too abrasive, then tool-wear is progressive, hence the temperature field is more and more important, generating all necessary conditions for white layers formation. Another remark also can be made in that the white layer thickness is not very important. Indeed, the maximum value of 6.2 mm is obtained for 35NiCrMo16 at the flank wear of 355 mm. According to published literature, the depth should be between 3 and 50 mm; thus, the values found in the study correspond to only a small thickness of white layer. The maximum value is obtained for a wear-criterion commonly

Fig. 9. SEM observation of X38CrMoV5.

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Fig. 10. White layer thickness as a function of flank wear Vb combined 35NiCrMo16 roughness profiles.

Fig. 11. Correlation between roughness profile, tool edge and microstructure for X160CrMoV12 steel.

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5. Conclusions

Fig. 12. 35NiCrMo16 (right) and X38CrMoV5 (left) surface profile at tZ5 min.

accepted as a rough machining criterion. However, hard turning is rarely performed under rough machining conditions, but more often under finish conditions. Besides, this graph also shows that for a wear-criterion commonly accepted as a finish criterion, white layer thickness is less than 1 mm. It is also very difficult to measure it with high precision. An EDS (energy dispersive spectrometry) analysis of the machined surface shows that carbon content in the white layer increases with time, and with increasing layer thickness. It probably diffuses from the surrounding environment. 4.3. Roughness profiles On each work material, the roughness profiles measured at different levels of progressive tool-wear show regular modulations on the roughness of order 4 as shown in Fig. 11. The wave length of the modulations has the same size as the carbides for X160CrMoV12 and 100Cr6 steels, and as the martensitic grain for the 35NiCrMo16 and X38CrMoV5 steels. It also corresponds to the groove size found on the tool flank wear (see Section 4.1). According to the surface characteristics, the material can be divided into two categories: materials with a fine microstructure (100Cr6 and 35NiCrMo16), which has little influence on the surface profile; and, materials with a coarse microstructure (X160CrMoV12 and X38CrMoV5), which produce high modulations on the surface. In Fig. 12, it can be seen that for material with a similar tool-wear rate, the surface profile can be different due to the size of the microstructure. 35NiCrMo16 steel has nearly no surface roughness of the order of 4 whereas X38CrMoV5 has large variations in surface roughness. Ra, which gives an arithmetic average of the roughness, is nearly constant with time and equals to the geometry-based theory involving feed and tool nose radius. Rt has higher values and changes with time and as a function of the work material properties. The failure of the tool edge makes the surface roughness increase consequently.

This study reveals new findings and has given some clues and new ideas on the surface sub-layer characteristics as a function of the microstructure of the workpiece and the progressive tool-wear. First, the white layers raise a problem due to their brittleness and the thickness of white layers increase with increasing flank wear. The major findings of white layers are: the microstructure and the relative orientation of the white layers with respect to machined surface, and the increasing carbon content with time resulting from carbides not being dissolved. The surface profile is influenced by the workpiece microstructure, especially if it is coarse-grained. The carbides in the matrix create grooves on the flank wear, which are transferred into corresponding geometric replica on the machined surface. Two work material categories have been found in this study. Those with a fine microstructure have little influence on the surface profile but generate a larger rate of white layer formation. The materials with coarse microstructure have a heavy influence on the roughness of the order of 4, increasing the surface roughness significantly. These materials generate little white layer and can generate segmented (elemental) chips. References [1] J. Barry, G. Byrne, TEM study on the surface white layer in two turned hardened steels, J. Mater. Sci. Eng. A325 (2002) 356–364. [2] Y.K. Chou, C.J. Evans, White layers and thermal modeling of hard turned surfaces, Int. J. Mach. Tools Manufact. 39 (1999) 1863–1881. [3] C.O. Mybokwere, S.R. Nutt, J. Duffy, Shear band formation in 4340 steel: A TEM study, Mech. Mater. 17 (1994) 97–110. [4] K.M. Cho, S. Lee, S.R. Nutt, J. Duffy, Adiabatic shear band formation during dynamic torsional deformation of an HY-100 steel, Acta Metall. Mater. 41 (3) (1993) 923–932. [5] B. Zhang, W. Shen, Y. Liu, X. Tang, Y. Wang, Microstructures of surface white layer and internal white adiabatic shear band, Wear 211 (1997) 164–168. [6] Z. Zurecki, R. Ghosh, J.H. Frey, Investigation of white layers formed in conventional and cryogenic hard turning steels, ASME International Mechanical Engineering Congress and Exposition, Washington, DC, USA, IMECE Proceedings, November vol. xx (2003) pp. 16–21. [7] H.G. Wobker, D. Brandt, H.K. To¨nshoff, Hard turning: influences on the workpieces properties, Trans. NAMRI/SME 23 (1995) 215–220. [8] Y. Matsumoto, C.R. Liu, M.M. Barash, Residual stress in the machined surface of hardened steel, Proceedings of ASME Winter Annual Meeting, High Speed Machining (1984) pp. 193–204. [9] Y. Yang, H. Fang, W. Huang, A study on wear resistance of the white layer, Tribol. Int. 29 (1996) 425–428. [10] Y.K. Chou, C.J. Evans, Process effects on white layer formation in hard turning, Trans. NAMRI SME 26 (1998) 117–122. [11] G. Poulachon, B.P. Bandyopadhyay, I.S. Jawahir, S. Pheulpin, E. Seguin, The influence of the microstructure of hardened tool steel workpiece on the wear of PCBN cutting tools, Int. J. Mach. Tools Manufact. 43 (2003) 139–144. [12] G. Poulachon, B.P. Bandyopadhyay, I.S. Jawahir, S. Pheulpin, E. Seguin, Wear behavior of CBN tools while turning various hardened steels, Wear 256 (2003) 302–310.