Direct evidence of cementite dissolution in drawn pearlitic steels observed by tomographic atom probe

doi:10.1016/S0921-5093(98)00529-2

Materials Science and Engineering: A

Volume 250, Issue 1, 15 July 1998, Pages 8-13

https://doi.org/10.1016/S0921-5093(98)00529-2 Get rights and content

Abstract

The dissolution of cementite during cold-drawing of pearlitic steels has been directly observed with the tomographic atom probe (TAP). Because of the nanometric interlamellar spacing and size of cementite lamellae, the TAP is shown to be extremely well suited for such a study. Analysis conditions and mass spectra peak deconvolution are shown to lead to quantitative analysis of both ferrite and cementite. It is also shown that specimen preparation always aligns the cementite lamellae habit plane with the analysis direction so that the analysed area is always perpendicular to their habit plane. Local magnification effects are also shown not to affect the carbon concentration measurement in the cementite. The first direct atomic scale quantitative concentration data across a few nanometer-thick cementite lamellae are given, and confirm the dissolution of cementite after cold-drawing. The derived compositions of ferrite, cementite and interfacial areas are obtained, giving information on the cementite dissolution mechanism as well as on its extent.

Introduction

It is now ascertained, at least qualitatively, that cementite undergoes partial dissolution during heavy cold-working of pearlitic steels 1, 2, 3, 4, 5, 6. The extent of the dissolution was first qualitatively estimated using Mössbauer spectrometry. The comparison of the intensities of the carbides characteristic emission peaks before and after drawing indicated a dissolution rate in the range of 20–50 vol.% of the initial cementite proportion [3]. More recently, Araujo [4]found a dissolution rate close to 80 vol.%. Languillaume [6]obtained similar results from the neutron diffraction patterns of cold-drawn and of reannealed specimens. In addition to this decrease in the amount of cementite, it has been emphasized that structure-insensitive properties are also affected by the cold-drawing [2], indicative of a change in the composition of the phases present. It is clear that, consequently to the observed dissolution of cementite, ferrite must be carbon-enriched. Indeed, as cementite partly dissolves, a large amount of carbon atoms is released, and must therefore partition in the ferrite. Owing to the importance of the dissolution, the average carbon content in the ferrite may reach 1–2 at.%. The solubility of carbon in equilibrium ferrite being very small (a few ppm at room temperature), the mechanism for solubilization of carbon in the ferrite is a very interesting point and may be of primary importance with respect to the mechanical aspects of cementite deformation during cold-drawing. Concerning the cementite, there is no indication as to whether the dissolution takes place at a constant composition, only affecting the phase volume fraction, or if this phase experiences a global carbon depletion.

The main purpose of this work is to give direct evidence of cementite dissolution after heavy cold-drawing, using direct imaging and composition measurement of the pearlite by means of the tomographic atom probe (TAP). The TAP has been used because of its unique capability to provide quantitative reconstructions of small volumes of metallic materials with a subnanometric spatial resolution. The first step of this work is to assess TAP composition measurement accuracy in both ferrite and cementite. This has been realised on undrawn specimens, on which the size of each phase is large enough for unmixed analysis. Heavily cold-drawn specimens have then been analyzed, first to identify cementite lamellea and then to observe the distribution of carbon atoms in the steel.

Section snippets

Material

The material studied is a eutectoid steel, the composition of which is given in Table 1. The steel is first drawn to an initial diameter d₀=1.75 mm, and then patented for pearlite formation from the austenite. The mean interlamellar spacing at this stage is 200 nm. The final wire is obtained after cold-drawing the patented wire down to a final diameter d=0.30 mm. The mean interlamellar spacing is reduced down to 20 nm after cold-drawing, while the mean cementite lamellae thickness is reduced

The tomographic atom probe (TAP)

Due to the very fine scale of the pearlite microstructure, the TAP is a unique instrument in the investigation of the chemical fluctuations in both ferrite and cementite. Indeed, thanks to its high spatial resolution, the TAP makes it possible to study the partitioning of carbon atoms within the pearlitic structure with subnanometric precision. The basic principle of the instrument relies on field evaporation of atoms from a sharply pointed needle tip. Chemical identification of detected atoms

Results

Both patented and cold-drawn specimens have been analyzed in this work. The first state has been investigated in order to verify the quantitativity of our analyses for both the ferrite and the cementite and to validate the used carbon peak assignation. Drawn specimens were analyzed to investigate the cementite dissolution and to determine the cementite lamellae concentration after drawing and the carbon concentration gradient in the interlamellar domain.

Conclusion

This work unambiguously shows the capability of the TAP to study the evolution of pearlitic steels after cold-drawing. In particular, quantitative concentration data across a few nanometer-thick cementite lamellae with no convolution effect were obtained for the very first time. It has been shown that cementite dissolution takes place through a global mechanism involving the whole volume of each individual lamellae, resulting in a carbon concentration gradient from cementite to ferrite. Because

Acknowledgements

JC would like to thank Dr J.H. Schmitt, Head of the Mechanical and Physical Metallurgy Department at IRSID for permission to publish part of the work started in his group.

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Direct evidence of cementite dissolution in drawn pearlitic steels observed by tomographic atom probe

Abstract

Introduction

Section snippets

Material

The tomographic atom probe (TAP)

Results

Conclusion

Acknowledgements

Nanostruct. Mater.

Acta Mater.

Appl. Surf. Sci.

Surf. Sci.

Fiz. Met. Metall.

Phys. Met.