Elsevier

Micron

Volume 32, Issue 8, 2001, Pages 817-824
Micron

Microstructural analysis of carbonitrided austenitic steels

https://doi.org/10.1016/S0968-4328(00)00089-5Get rights and content

Abstract

An austenitic heat resisting steel containing 25% Cr and 19% Ni was internally carbonitrided in an atmosphere of 0.5% C3H6, 9.5% H2 and 90% N2. Immediately below the alloy surface, globular precipitates were observed. At greater depth, two zones of lamellar morphology precipitates were observed to form. M7C3 was found directly beneath the surface and M23C6 was found closer to the reaction front. The lamellar carbide morphology is more typically characteristic of the precipitates formed during nitriding rather than carburising, suggesting that nitrogen plays some role in the precipitation process. A focused ion beam (FIB) miller was used to create thin, electron transparent locations at specific regions in the precipitation zone in order to characterise, via transmission electron microscopy, the crystallography and composition of the phases present. In this study, thin areas were produced in the lamellar region at the transition between the M23C6 and M7C3 regions. The composition, crystal structure, and orientation relationships are reported and compared to those found previously at both the internal precipitation front and the sample surface.

Introduction

Exposing alloys to oxidant gases at high temperature can lead to either internal precipitation or external scale formation. Internal precipitation occurs when the less noble alloy constituent is of insufficient concentration or mobility to sustain a protective external scale. For example, when austenitic heat-resistant steels are exposed to an oxidant gas with low oxygen potential and high carbon activity, they tend to form deep subsurface zones of carbide precipitation with only a thin, non-protective carbide scale on the surface. The formation of the subsurface precipitates depletes these alloys of chromium and can severely reduce component life spans.

The internal carburisation of heat resistant steels has been extensively studied (Lewis, 1968, Schnaas and Grabke, 1978, Ledjeff et al., 1983, Bennett and Price, 1981, Kane, 1981, Meier et al., 1982, Smith et al., 1982, Ramanarayanan and Petkovic-Luton, 1983, Smith et al., 1985, Barnes et al., 1985, Mitchell and Young, 1993, Young and Watson, 1995) and usually results in the formation of numerous small chromium-rich carbides of spheroidal or needle-like morphology due to the rapid inward carbon diffusion. An exception is provided by high chromium, ferritic Fe–Cr alloys which develop lamellar carbides in a discontinuous precipitation process involving a phase change from ferrite to austenite.

Previous work by Udyavar and Young (1997) examined the microstructure of austenitic steels under carburising, nitriding and carbonitriding conditions. They found that carburising resulted in the formation of a high volume fraction of carbides with globular and needle-like morphology. Nitriding resulted in the formation of a much lower volume fraction than that formed under carburising conditions and the nitrides exhibited lamellar morphology. Carbonitriding resulted in the formation of a high volume fraction of carbides, but they occurred in a number of different morphologies. The fully austenitic alloys, with the lowest nickel content (19% Ni, 25% Cr by weight) formed globular carbides immediately below the surface and lamellar morphology deeper in the precipitation zone. In contrast an austenitic steel with a higher nickel content (>37% Ni, 25% Cr by weight) formed globular carbides only.

The formation of the lamellar carbides during carbonitridation increases the rate of penetration of the precipitation zone. This occurs because the preferentially oriented carbide plates provide easy paths for carbon diffusion to the precipitation front, and this results in an increase in the rate of alloy destruction. That this morphology occurs when carburising is combined with nitridation, but not when carburising alone indicates that it is the presence of nitrogen that alters the precipitate morphology. It has been suggested that nitrogen is present at the reaction front, but this has not yet been confirmed (Udyavar and Young, 1997, Ford et al., 2000).

In order to study the microstructure of the reaction zone at the highest resolution it is necessary to produce thin, electron transparent regions suitable for examination in a transmission electron microscope (TEM). However, the area of interest in the carbonitrided samples is restricted to a region a few hundred microns below the surface. Although it is possible to produce TEM specimens through conventional electropolishing or ion beam milling methods (Horiuchi, 1994), these techniques are slow and very tedious and do not allow thin areas to be precisely produced at specific regions, particularly at the edge of a sample.

However, the focused ion beam miller (FIB) allows thin film specimens to be routinely produced at specific sites with very high positional accuracy. Focused ion beam milling has previously been utilised to produce thin film specimens from carbonitrided austenitic steels by Ford et al. (2000). Thin film specimens were prepared immediately below (<10 μm) the sample surface and also at the precipitation front allowing both crystallographic and chemical characterisation of the precipitates and the surrounding austenite. Analysis of these areas revealed carbides of different stoichiometry and crystal structure. Between the surface and precipitation front, within the lamellar precipitate zone, there is a region where the carbide crystallography transforms. The aim of this study was to prepare thin film specimens from carbonitrided austenitic steel in order to characterise the region of carbide crystallography transformation.

Section snippets

Materials and methods

A ternary Fe–Ni–Cr austenitic alloy, containing 19% nickel and 25% chromium (by weight), was prepared using an argon atmosphere arc-melting furnace. The alloy buttons were remelted several times and subsequently annealed at 1000°C for 48 h under a flowing Ar-10% H2 atmosphere to improve chemical homogeneity. Alloy coupons 10×10×2 mm were cut and ground to a 1200 grit finish and ultrasonically degreased in acetone immediately prior to reaction.

The alloy coupons were reacted at 1000°C for 20–120 min

Results and discussion

Previous work on this alloy by Ford et al. (2000) reported the precipitation zone microstructure, kinetics and precipitate volume fraction. TEM examination of both the particles within 10 μm of the external surface and at the precipitation front was conducted to characterise the crystallographic features and chemical composition of these areas. The findings of that work are briefly summarised below to provide a background for the current work.

Optical microscopy revealed the presence of two

Conclusions

The focused ion beam miller was successfully used to prepare site-specific electron transparent areas for TEM examination from an internally carbonitrided austenitic steel. Examination of the interface between the hexagonal M7C3 and cubic M23C6 carbides was undertaken using a FEGTEM. At this interface the M23C6 carbides were found to have retained the lamellar morphology and strong, cube-on-cube orientation relationship with the matrix developed when they first formed at the precipitation

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