Structure and composition of built-up layers on coated tools during turning of Ca-treated steel

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Abstract

Built-up layers formed on CVD coated cutting tools during turning of Ca-treated steel were investigated. The built-up layers were analysed using scanning electron microscopy and transmission electron microscopy (TEM) in combination with energy dispersive X-ray spectroscopy, electron diffraction, energy-filtered TEM and high resolution TEM. This is the first paper where the inclusion layer is studied and analysed using TEM. The structure and composition of the inclusion layer is reported in detail and compared with the inclusion population in the workpiece material. The part of the inclusion layer that was subjected to the highest temperature during cutting was crystalline, and is composed of round sulphide grains (Ca0.85Mn0.15S) and deformed elongated aluminate grains (CaO·6Al2O3). These are the high-melting point phases of the inclusions present in the steel and it appeared that only these compounds can contribute to the formation of the solid built-up layer.

Introduction

Ca-treatment can be used to enhance the machinability of Al-killed steels [1], [2], [3], [4], [5], [6], [7], [8]. The Ca-treatment modifies the sharp-edged Al2O3 inclusions found in conventional steels into globular calcium-aluminates surrounded by soft rims of (Ca,Mn)S. These duplex inclusions are, due to the more favourable shape and lower hardness, less abrasive to the tool than the Al2O3 inclusions in conventional steels. During cutting of Ca-treated steels, adhering built-up layers, also referred to as inclusion layers or protective layers, with compositions similar to that of the inclusions in steel are often formed on the tool [3], [4], [5], [6]. Inclusion layers are believed to reduce tool wear, especially crater wear. As far as crater wear is concerned, the inclusion layers are often believed to act as diffusion barriers between the chip and the tool surface. This hypothesis is based on the assumption that crater wear is a result of dissolution/diffusion processes [9].

Wear of coated tools during cutting of conventional steels have been the subject of several investigations [9], [10], [11], [12], and a variety of coatings and workpiece combinations have been studied. The wear mechanisms are, however, not clear. As far as crater wear on the rake face is concerned, two different mechanisms have been suggested, dissolution and plastic deformation. Furthermore, it has been suggested that the wear mechanisms when cutting Ca-treated steels are different from the ones active when cutting conventional steels. This is especially the case when Al2O3 coatings are involved [3], [4]. Evidence has been presented that Al2O3 coatings should react with the molten inclusion layer formed on the tool. Ruppi et al. [7] did not observe any evidence for this drastic dissolution of Al2O3 and instead showed that the Al2O3 coatings behaved in a similar manner in both Ca-treated and conventional martensitic quenched and tempered steels. Plastic deformation and abrasion were suggested to be responsible for crater wear in accordance with Stjernberg et al. [10] and, consequently, the inclusion layer was not considered as a diffusion barrier between the coating and the chip. Better machinability was explained mainly in terms of the less abrasive inclusions in the Ca-treated steel, and the inclusion layer was seen to be the result of a successful inclusion control.

Independent of the active wear mechanism, the composition of the adhering inclusion layer must be of importance, especially with respect to its formation processes and suspected chemical interactions. However, the inclusion layers formed as a result of turning Ca-treated steels have been studied only using techniques that all lack the spatial resolution required to determine the actual layer composition [3], [4], [6], [13]. The purpose of this work is to determine the structure and composition of built-up inclusion layers formed on CVD coated cutting tools when turning Ca-treated quenched and tempered steel. The machinability of this steel, using TiC, Ti(C,N), TiN and Al2O3 coated cutting tools, is described in detail elsewhere [7]. Due to the controversies, as far as the behaviour of Al2O3 is concerned, this material was studied thoroughly in this investigation.

The inclusion layer is studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in combination with energy dispersive X-ray spectroscopy (EDS), electron diffraction, energy-filtered TEM (EFTEM) and high resolution TEM (HREM). The spatial resolution of imaging and microanalysis in the TEM enables full characterisation of the microstructure of the investigated layers. The results are compared with the inclusion composition in the steel and the mechanisms behind the build up of an adhering layer are discussed. To our knowledge, this is the first paper where the inclusion layer is studied and analysed using TEM.

Section snippets

Materials

The workpiece material was a Ca-treated 42CrMo4 steel, with the composition given in Table 1. The steel was quenched and tempered to an average hardness of HB 284. CVD α-Al2O3 coated and CVD TiN coated cemented carbide inserts (SNUN120408), with composition 94 wt.% WC and 6 wt.% Co, were used in the cutting tests. The machining tests were performed as a continuous turning operation on a cylindrical bar. All cutting tests were performed dry. The cutting conditions are given in Table 2.

Materials characterisation

SEM

Structure and composition of inclusions

The effect of Ca-treatment on the morphology and composition of the inclusions in the steel is well described in the literature [3], [4], [5], [6], [8], [13]. The inclusions in the present steel were examined using backscattered electron imaging and X-ray mapping in the SEM. The samples were prepared perpendicular to the rolling direction of the steel. Fig. 1, Fig. 2, Fig. 3 show the results of electron and X-ray imaging from the three main types of inclusions in the steel. The X-ray maps show

Discussion

The part of the inclusion layer subjected to the highest temperatures during cutting is composed of two crystalline phases having the same composition as the high-melting point phases of the inclusion species in the steel. The sulphide, with the composition Ca0.85Mn0.15S, is reported to have a melting point of approximately 1800°C, which is much higher than the melting points of pure MnS and Ca poor mixed sulphides [17]. Eriksson and Pelton [18] studied the CaO–Al2O3 system and reported

Conclusions

In this work, the composition and structure of built-up inclusion layers formed on the rake face of coated cemented carbide inserts have been studied using SEM and cross-sectional TEM. The high spatial resolution in the structural and compositional analyses allowed an extensive characterisation of the microstructure. The following was concluded:

  • The inclusions in the Ca-treated steel occurred as rounded calcium-aluminate and elongated sulphide inclusions. The calcium-aluminate inclusions were

Acknowledgements

The collaboration with Imatra Steel Oy Ab is acknowledged. The authors wish to thank Drs Jenni Zackrisson, Bengt Högrelius and Mats Halvarsson for valuable discussions. Financial support was provided by the Swedish Research Council for Engineering Sciences (TFR).

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