Influence of Cr, C and Ni on intergranular segregation and precipitation in Ti-stabilized stainless steels

Influence of Cr, C and Ni on intergranular segregation and precipitation in Ti-stabilized stainless steels

Available online at www.sciencedirect.com Scripta Materialia 63 (2010) 449–451 www.elsevier.com/locate/scriptamat Influence of Cr, C and Ni on interg...

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Available online at www.sciencedirect.com

Scripta Materialia 63 (2010) 449–451 www.elsevier.com/locate/scriptamat

Influence of Cr, C and Ni on intergranular segregation and precipitation in Ti-stabilized stainless steels Jeong Kil Kim,a,b,* Yeong Ho Kimb and Kyoo Young Kima a

Pohang University of Science and Technology (POSTECH), Graduate Institute of Ferrous Technology, Pohang 790-784, Republic of Korea b POSCO Technical Research Laboratories, Pohang 790-704, Republic of Korea Received 12 April 2010; revised 29 April 2010; accepted 4 May 2010 Available online 6 May 2010

The influence of Cr, C and Ni on intergranular segregation and precipitation has been investigated with various Ti-stabilized stainless steels. TiC precipitates with Cr segregation were observed along the grain boundaries in ferritic and austenitic stainless steels with a carbon content of more than 0.0060 wt.%, but not in ferritic stainless steel with extremely low carbon (0.0023 wt.%). Based on the results of the analysis, practices to avoid intergranular corrosion of Ti-stabilized stainless steels are proposed. Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Stainless steels; Segregation; Precipitation; TEM

Intergranular corrosion (IGC) in stainless steels is an extensively studied subject because it can result in severe loss of corrosion resistance, strength and ductility of steels. It is widely accepted that IGC in stainless steels is induced by the electrochemical potential difference between the matrix and a Cr-depleted zone adjacent to the intergranular precipitation of Cr compounds [1–7]. Generally the Cr compounds are Cr carbides and Cr carbonitrides, formed along the grain boundaries, causing the zone around them to become Cr-depleted. Some studies have reported that other compounds, such as sigma (a Cr compound) and chi (a Cr–Mo–Fe compound) phases, also deplete the Cr concentration in the matrix adjacent to the compounds and develop IGC [2,7]. Furthermore, it has been suggested that martensite and the segregation of phosphorous formed along the grain boundaries leads to IGC [8–10]. Therefore, to prevent these harmful compounds, stainless steels employed in corrosive media such as automotive exhaust systems are stabilized with strong carbide formers and their C and N contents are reduced to low levels. Such methods are known to be effective in * Corresponding author at: Pohang University of Science and Technology (POSTECH), Graduate Institute of Ferrous Technology, Goedong-dong, Nam-gu, Pohang 790-784, Republic of Korea; e-mail: [email protected]

preventing IGC, and many kinds of commercial steels, such as STS 409L and 439 stabilized with Ti, have been widely used for long time. However, recent studies asserted that, in Ti-stabilized ferritic stainless steels (FSS) with low interstitials of 0.001 wt.%, IGC could be induced by Cr depletion due to segregation of Cr in the vicinity of intergranular TiC precipitates [11,12]. In addition, the studies suggest that stabilizing with Ti and reducing the interstitials to 0.001 wt.% cannot prevent IGC any more. Thus, understanding the effects of alloying elements on the segregation of Cr and precipitation of TiC along the grain boundaries is of critical importance in avoiding the risk of IGC in stainless steels. In this article, the authors investigated the effect of Cr and C, which were thought to be the most significant elements affecting IGC behavior, on the segregation and TiC precipitation. An 18 wt.% Cr–8 wt.% Ni austenitic stainless steel (ASS) was also employed, to compare the austenitic matrix with the ferritic one, because precipitation and segregation in ASS occur more slowly than in FSS [13–16]. As given in Table 1, four stainless steels were used in this study: three were Ti-stabilized FSS, with 11 or 18 wt.% Cr and carbon contents from 0.006 to 0.002 wt.% (F1–F3). The other was 18 wt.% Cr–9wt.% Ni ASS stabilized with Ti (A4). Their stabilizing ratios (Ti/(C + N)) matched the guidelines of previous studies

1359-6462/$ - see front matter Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2010.05.002

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Table 1. Chemical compositions of stainless steels. Specimen

C

Cr

Ti

N

Fe

Ni

Ti/(C + N)

F1 F2 F3 A1

0.0054 0.0060 0.0023 0.0109

11.1 17.64 11.30 17.91

0.246 0.290 0.212 0.312

0.0047 0.0064 0.0021 0.0112

Bal Bal Bal Bal

– – – 9.23

24.35 23.38 48.18 14.12

and the standard specification (ASTM A 240/a 240M08) well [17–21]. The materials were vacuum-melted, rolled into a 1.2 mm thick sheet and annealed at 950 °C for 5 min in a laboratory. The as-annealed sheets were solution treated at 1300 °C for 10 min and quenched in water, then aged at 500 °C for various times. The precipitates on carbon extraction replicas were analyzed using a Philips CM-200 transmission electron microscope equipped for energy-dispersive X-ray spectroscopy (EDS), operated at 200 kV. Figure 1 presents the precipitates along the grain boundaries of 1 h-aged F1 and F2 specimens. In EDS analysis on F1 with 11 wt.% Cr, where IGC occurred in corrosion resistance test [11], the intergranular precipitates include the elements Ti, Cr and C (Fig. 1b). However, electron diffraction pattern analysis reveals that these precipitates are TiC carbides with a face-centered cubic structure (lattice parameter = 0.433 nm) and that there are no second phases, such as carbides or intermetallic phases containing Cr (Fig. 1a). The Cr peak in the EDS analysis was reported to be due to Cr segregation, which was the reason for the Cr depletion around intergranular TiC precipitates in previous studies [11,12]. In 18 wt.% Cr FSS, the same TiC carbides with a Cr peak as those observed in 11 wt.% Cr FSS are precipi-

Figure 1. TEM characterization of the intergranular precipitates in the F1 and F2 specimens aged at 500 °C for 1 h. (a) Carbon extraction replica for the intergranular precipitates and electron diffraction pattern (inset) with a zone axis of [0 0 1]; (b) EDS spectra of the intergranular precipitates in F1; (c) carbon replica for the intergranular precipitates and electron diffraction pattern (inset) with a zone axis of [0 1 1] and (d) EDS spectra of the intergranular precipitates in F2.

tated, as shown in Figure 1c and d. From these results, it can be clarified that in Ti-stabilized FSS with 11–18 wt.% Cr, TiC carbides with Cr segregation are formed along the grain boundaries. Here, IGC is induced for the same reason – the Cr segregation around the intergranular TiC – regardless of the Cr content. In addition, the results indicate that an increase in the Cr content cannot prevent IGC completely, although Cr does improve the IGC resistance of stainless steels [15,17,22]. Transmission electron microscopy (TEM) and EDS analyses on the 10 h-aged F3 specimen, which had extremely low carbon, were performed to investigate the effect of C content on intergranular Cr segregation and TiC precipitation. The result is depicted in Figure 2. Neither TiC nor any other kind of precipitate was observed along the grain boundary, whereas the precipitates found in the matrix were identified to be Ti sulfides by EDS analysis, instead of TiC and TiN, which were generally found in the matrices of Ti-stabilized stainless steels [2,11,12]. Sulfur was not added, but its content in all specimens measured 0.003 wt.%. Although the F3 specimen was not extensively evaluated for IGC, it is expected that this steel has good IGC resistance. It is still not clear whether the intergranular segregation of Cr around TiC develops in an ASS. Ferritic and austenitic stainless have the same IGC mechanism, but ASS is known to be less susceptible to IGC than FSS,

Figure 2. TEM characterization of the F3 specimen aged at 500 °C for 10 h. (a) Carbon extraction replica around the grain boundary; (b) higher magnification of “A” marked in (a) and (c) EDS spectra of Ti sulfides in the matrix.

J. K. Kim et al. / Scripta Materialia 63 (2010) 449–451

because the kinetics of precipitation and segregation in ASS is even slower due to its higher solubility for solute elements such as C and Cr. Therefore, TEM and EDS analyses were conducted on replicas of the 1 and 4 haged A1 austenitic specimens to compare the precipitation and segregation of ASS with those of FSS. The analysis of the 1 h-aged A1 specimen indicates that no precipitation was formed along the grain boundary (Fig. 3). However, in the analysis of the 4 h-aged A1 specimen (Fig. 4), precipitation develops along the grain boundaries, and Cr as well as Ti and C were detected in this intergranular precipitates. Electron diffraction pattern analysis reveals that these precipitates are the same TiC carbides with Cr segregation as those found in F1 and F2 FSS. Therefore, it is confirmed that, in comparison with FSS, Cr segregation also occurs in the vicinity of TiC precipitates, even though it takes longer for this phenomenon to occur in ASS at the same aging temperature due to the slower kinetics of ASS, as mentioned above. Pardo et al. [2] suggested that M23C6 carbides were precipitated along the grain boundaries, and that this was the reason for IGC in Ti-stabilized ASS with a similar chemical composition to that of the A1 specimen of the present study, although the solution and aging conditions were different. The different intergranular precipitation and segregation characteristics of these two studies likely resulted from the difference in the heat treatment conditions, since soluble carbon is generated by the dissolution of TiC carbides in Ti-stabilized stainless steels during solution treatment, with the soluble carbons then causing the Cr segregation and the TiC precipitation during aging treatment [11,12]. Therefore, an investigation of the effect of the heat treatment condition on this phenomenon will be performed in a future study. In summary, the intergranular TiC precipitation with Cr segregation – the reason for IGC – was observed in stainless steels with a C content of more than 0.0060 wt.% regardless of Cr content for both FSS and ASS, while it was not formed along the grain boundaries of 11 wt.% Cr FSS with 0.0023 wt.% C. This result also means that an increase in the Cr content and changes in the type of stainless steel do not actually prevent intergranular corrosion of Ti-stabilized stainless steels despite the slower kinetics of TiC precipitation with Cr segregation in ASS. Finally, two practices should be used to avoid IGC in these kinds of stainless

Figure 3. TEM characterization of A1 specimen aged at 500 °C for 1 h. (a) Carbon extraction replica around the grain boundary and (b) higher magnification of “A” marked in (a).

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Figure 4. TEM characterization of A1 specimen aged at 500 °C for 4 h. (a) Carbon extraction replica around the grain boundary; (b) higher magnification of “A” marked in (a); (c) EDS spectra and (d) electron diffraction pattern with a zone axis of [1 1 1] of the intergranular precipitates.

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