International Journal of Machine Tools and Manufacture
White layers and thermal modeling of hard turned surfaces
Introduction
Surface structural change in workpieces introduced by a material removal process is an inevitable but important consequence of any finishing process. This surface modification occurs because of intense, localized and rapid thermal mechanical working resulting in metallurgical transformation and, perhaps, chemical interactions. The worked surface can show an extremely different structure from the bulk. This type of problem has attracted substantial effort in the field of so-called “surface integrity”, a term including all aspects of surfaces such as surface finish, metallurgical change and residual stresses. For more than a decade, cutting of hardened steels using advanced ceramic tool materials such as alumina–titanium carbide ceramic (Al2O3–TiC) and polycrystalline cubic boron nitride (CBN) has been developed and, in some circumstances, has proved an economically attractive alternative to grinding [1]. It is, therefore, of practical importance to understand and characterize the surface integrity in finish cutting of hardened steels. This paper concentrates on one microstructural perspective of machined surfaces, namely, the white layer.
White layer is a result of microstructural alteration. It is called “white” layer because it resists standard etchants and appears white under an optical microscope (or featureless in a scanning electron microscope). In addition, the white layer has high hardness, often higher than the bulk. White layers are found in many material removal processes such as grinding 2, 3, 4, electrical discharge machining [5] and drilling [6]. In grinding, white layers have been suggested to have an untempered martensitic structure [4]. Large plastic deformation [3] and/or rapid heating–cooling [4] are possible formation mechanisms. White layers seem to be detrimental to product performance, and therefore require a post-finishing process.
Surface integrity in hard turning is a relatively new subject. In cutting of hard steels, there are some reports of white layers 1, 7, 8, 9, 10. Most noted that white layer occurs when cutting tools wear out to a certain level, but did not provide an in-depth explanation. Tönshoff et al. [10] studied the influence of hard turning on workpiece properties and reported that retained austenite is the major composition of white layer structures. Surface chemistry was also investigated; the concentration of trace elements is constant with depth, implying no chemical reaction. A higher thrust force component seems to accompany white layer occurrence, as does tensile residual stress. They further showed that the white layer decreases bending fatigue strength probably due to associated tensile residual stresses. In contrast, König et al. [1] and Abrao and Aspinwall [11] reported that, despite white layer occurrence, hard turned steels have greater fatigue resistance than ground steels. Abrao and Aspinwall [11] considered that fine surface finish of hard turned parts resulted in longer fatigue life than ground counter parts even though the former had a deeper white layer. König et al. [12] further explained that strain-induced hardening could suppress the formation of a thermally damaged soft skin and could consequently show high levels of rolling strength even with a white layer. Tool wear was suggested as the most influential parameter on white layer formation, though frequently it was the only variable studied. However, the explanation of white layer formation was rather qualitative and, thus, there was no implication that optimization of surface structures or minimization of white layers is possible.
Several factors may limit tool life and therefore affect machining cost. In a finishing process, surface integrity is often of great concern because of its impact on product performance; indeed, it may be used as a tool-changing criterion. Thus, understanding tool wear and cutting parameter effects on surface integrity is of practical significance. The objectives of our research are to characterize microstructural changes in hard turned surfaces and to study their effect on workpiece performance. In this paper the focus is on: (1) characterization of white layer structures, (2) thermal aspects of white layer formation, and (3) experimental and theoretical analyses of process effects on white layer depth.
Section snippets
An example of white layer
A 62 Rc vacuum induction melt and vacuum arc remelt (VIMVAR) AISI 52100 steel workpiece from machining tests was prepared for surface microstructural examination. Cutting conditions were 3 m/s cutting speed, 50 μm/rev feed rate and 200 μm depth of cut in dry conditions. The cutting tool was an Al2O3–TiC insert with about 210 μm flank wear. After sectioning from the workpiece, the sample was plated with electroless nickel to protect the machined surface, and then mounted in epoxies with the
Theoretical study
Fig. 2(a) is a cross-section view of a turning process. Since the machining-affected zone is extremely shallow (order of 10 μm), the local tool–workpiece contact can be simplified on a rectangular coordinate as Fig. 2(b). Now, considering some finite volume, ΔU, of material ahead of the cutting tool and below the machined surface, in region I, the material is subjected to plastic compression and perhaps some heat propagated from shear zones. When the material enters into region II, where tool
Cutting test
Effects of two process parameters, cutting speed and flank wear, on white layer depth were investigated. They were selected because cutting forces are insensitive to speed, and thus most mechanical effects are similar, and because most literature suggested flank wear as a critical variable. 25 mm diameter bars made of VIMVAR 52100 steel with a hardness of 61–63 Rc were turned on outside diameter. Table 1 shows the chemical composition of the workpiece material and Table 2 (Set 1) heat treatment
Discussion
The white layer phenomenon in hard turned surfaces has been identified by classical metallographic methods. Though white layers are phase transformation products, microstructural evolution during white layer formation is not fully understood. Martensite, the starting microstructure in hard turning, is a metastable structure that will decompose to ferrite and cementite when heated (a tempering process). However, the high heating rate encountered during cutting may prevent martenite from
Conclusions
White layer formation in hard turned surfaces has been theoretically and experimentally studied by turning hardened 52100 steel with worn ceramic tools. Experimental results show the same trend of cutting speed effect on white layer depth as predicted in the thermal model. Thus, white layer formation seems to be dominantly a rapid heating–cooling process. However, it is also believed that plastic deformation will assist grain refinement and phase transformation processes. A more comprehensive
Acknowledgements
We would like to thank our colleagues, R. Polvani, A. Donmez, M. Davies and D. Gilsinn for their encouragement, suggestions and always helpful discussion. Especially, R. Polvani gave many insightful comments through the course of this work. H. Soons at the National Institute of Standards and Technology (NIST) reviewed the manuscript and gave many invaluable comments. The Metallurgy Division at NIST is gratefully acknowledged for access to the facility. The Torrington Company supplied some steel
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