Correlation between characteristics of grain boundary carbides and creep–fatigue properties in AISI 321 stainless steel

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Abstract

The effects of the interfacial relationships between grain boundary carbides and neighboring grains on the creep–fatigue behaviors have been investigated in AISI 321 stainless steel. The contacting interfacial planes between grain boundary TiC and neighboring grains are found to have lower Miller indices than those between Cr23C6 and neighboring grains. From this observation, it is suggested that the interfacial free energy between grain boundary TiC and grains is lower than that between Cr23C6 and grains. Creep–fatigue life of TiC aged AISI 321 stainless steel is observed to be longer than that of Cr23C6 aged AISI 321 stainless steel. The differences in creep–fatigue life are based on the stronger cavitation resistance of TiC compared with that of Cr23C6. From the interfacial relationships between the grain boundary carbides and the neighboring grains, it is verified that formation and growth of grain boundary cavities at TiC carbides are more retarded than those at Cr23C6 carbides, thus extending the creep–fatigue life of the steel.

Introduction

AISI 321 stainless steel has been widely used in the power-generation industry because it has good corrosion resistance through inhibited grain boundary sensitization [1], [2], [3]. This alloy is used mainly in superheater tubing in conventional coal-fired boilers, as well as other critical applications such as guide tubes, pipes, and pressure vessels in gas-cooled nuclear reactors. The addition of Ti prevents the formation of chromium-rich carbide precipitates in the grain boundaries which are known to be deleterious to the creep life of the alloy [4], [5], [6].

Particularly in austenitic stainless steels, grain boundary cavitation is the most serious detrimental process in degradation under creep–fatigue interaction conditions [7], [8], [9], [10], [11]. Carbide at the grain boundary provides a preferential site for cavity nucleation under creep–fatigue interaction conditions, reducing the creep–fatigue life drastically. Thus, it can be inferred that carbide distribution, carbide morphology and carbide interfacial free energy are important factors in determining creep–fatigue resistance of austenitic stainless steels [9], [10], [11], [12].

Earlier research on grain boundary carbides concentrated on the improvement of corrosion resistance in AISI 321 stainless steels [1], [2], [3]. Although there have been reports on the mechanical properties of steel aged with TiC, which is the main precipitate in AISI 321 stainless steels, these studies only illustrated the mechanical properties of Cr23C6 aged alloys [7], [8], [9], [10], [11], [12], [13], [14].

It is very important to understand the characteristics of grain boundary carbides in austenitic stainless steels, because the carbides become the grain boundary cavitation sites under creep condition.

The purpose of this paper is to investigate the characteristics of TiC and Cr23C6 carbides precipitated at the grain boundary and the correlation between the interfacial properties of the two carbides (TiC and Cr23C6) and the creep–fatigue properties of AISI 321 stainless steel. Creep–fatigue tests are conducted using specimens with TiC carbide, which is the main carbide, and with Cr23C6 carbide, which is precipitated as a reference for comparison with the effects of TiC carbide in AISI 321 stainless steel.

Section snippets

Experimental procedures

The chemical composition and heat treatment conditions of the investigated AISI 321 stainless steel are shown in Table 1. After solution heat treatments, two different aging treatment processes are designed to independently form TiC and Cr23C6 carbides at the grain boundary. Even though the particles of TiC and Cr23C6 have similar size and density at the grain boundary, the different lattice parameters and different interfacial energies of the two carbides could uniquely affect the cavitation

Carbide distribution and morphology of TiC and Cr23C6 at grain boundary

Fig. 1 shows the carbide distributions and morphologies of TiC and Cr23C6 at grain boundaries after the heat treatments. In Fig. 1(a), it is observed that TiC particles are precipitated at grain boundaries and the size of the TiC particles is about 1–3 μm. The precipitates of Cr23C6, having similar size as TiC, are also precipitated at the grain boundaries as shown in Fig. 1(b). The morphologies of TiC and Cr23C6 are observed to be planar along the grain boundaries.

To quantify the density of

Conclusions

  • 1.

    The interfacial planes between TiC and neighboring grains show lower indices than those between Cr23C6 and neighboring grains. It is suggested that the interfacial free energy between TiC and grains is lower than that between Cr23C6 and grains in AISI 321 stainless steel. Also, the morphologies of carbides are determined by the minimization of interfacial free energy during the carbide growth.

  • 2.

    Creep–fatigue life of TiC aged AISI 321 stainless steel is longer than that of Cr23C6 aged AISI 321

Acknowledgements

This research was sponsored by POSCO (Pohang Steel Making Co. Ltd.) as a project of BK21 (Brain Korea 21). The authors would like to express their appreciation for the financial support.

References (20)

  • M. Schwind et al.

    Acta Mater.

    (2000)
  • S.W. Nam

    Mater. Sci. Eng. A

    (2002)
  • H.U. Hong et al.

    Mater. Sci. Eng. A

    (2001)
  • W. Lojkowski et al.

    Acta Metall.

    (1988)
  • S. Hirth et al.

    Acta Mater.

    (1998)
  • H.U. Hong et al.

    Mater. Sci. Eng. A

    (2002)
  • R. Raj et al.

    Acta Metall.

    (1975)
  • J.H. Payer et al.

    Corrosion

    (1975)
  • C. Hoffmann et al.

    Metall. Trans. A

    (1982)
  • A.S. Grot et al.

    Metall. Trans. A

    (1975)
There are more references available in the full text version of this article.

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