Sensitisation identification of stainless steel to intergranular stress corrosion cracking by atomic force microscopy

Sensitisation identification of stainless steel to intergranular stress corrosion cracking by atomic force microscopy

Available online at www.sciencedirect.com Materials Letters 62 (2008) 1863 – 1866 www.elsevier.com/locate/matlet Sensitisation identification of sta...

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Available online at www.sciencedirect.com

Materials Letters 62 (2008) 1863 – 1866 www.elsevier.com/locate/matlet

Sensitisation identification of stainless steel to intergranular stress corrosion cracking by atomic force microscopy Huang Yanliang a,b,⁎, Brian Kinsella a , Thomas Becker a a

Department of Applied Chemistry, Curtin University of Technology, Kent Street, Bentley, WA 6102, Australia b Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China Received 18 June 2007; accepted 11 October 2007 Available online 25 October 2007

Abstract A new technique was developed for characterisation of stainless steel to intergranular stress corrosion cracking by atomic force microscopy. The technique proved to be effective in sensitisation identification of AISI 304 stainless steel and might be promising in sensitisation identification of other stainless steels. © 2007 Elsevier B.V. All rights reserved. Keywords: Microstructure; Sensitisation; Intergranular stress corrosion cracking; Stainless steel; Atomic force microscopy

1. Introduction Oil and gas flow lines made from 13 Cr weldable martensitic grade stainless steel (WMSGSS) offers a very cost effective alternative to more expensive corrosion resistant alloys, in particular, 22 Cr duplex stainless steel [1,2]. However, laboratory tests and field experience indicates the 13 Cr steel undergoes intergranular stress corrosion cracking in the heataffected zone of the welds [1–3]. The use this steel has been placed on hold pending a greater understanding of the stress corrosion mechanisms [3]. The 13% Cr steel is produced in three grades, Lean grade, Middle grade and Rich grade [3]. Lean grades have undergone corrosion cracking in the heat-affected zone of the welds [2,3]. The mechanism is attributed to intergranular stress corrosion cracking caused by chromium depletion at grain boundaries. For medium and rich grades, the cracking has only been found in laboratory tests. For these grades the mechanism has not been clarified, although the most likely mechanism is considered to be similar to that of the lean grades [3]. Chromium carbide has not been detected at the grain boundaries but it is ⁎ Corresponding author. Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China. Fax: +86 532 82880498. E-mail address: [email protected] (H. Yanliang). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.10.040

speculated that it may exist in narrow bands at the nano scale level. Atomic Force Microscopy (AFM) has been extensively used in the study of microstructures of materials and environmentally induced fracture [4–6]. The images can be obtained at a high resolution by it. The focus of this paper was to develop a technique using AFM to identify sensitisation, particular attention was paid to identify carbides at grain boundaries. During the study, AISI 304 austenitic stainless steel was used. The mechanism of AISI 304 stainless steel to intergranular corrosion/SCC has been well established. It had been known that it is a result of Cr depletion adjacent to the carbides with high Cr content (M23C6, M7C3 etc.) formed at grain boundaries [7,8]. The technique developed in this paper was hoped promising in identifying carbides at grain boundaries in heataffected zone of rich grade martensitic stainless steel welds. 2. Experimental 2.1. Material information The material used in the study was AISI 304 Austenitic Stainless Steel (0.034% C, 0.54% Si, 1.22% Mn, 0.023% P, 0.001% S, 18.33% Cr, 0.31% Mo, 9.40% Ni, 0.026% Ti, 0.01% Nb, 0.022% Al, 0.03% N, 0.007% As, 0.0033% B, 0.16% Co,

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Fe balance) with mean planar grain diameter of 16 μm. The steel samples were heat treated at 700 °C for 5, 15, 30, 60, 300 and 600 min respectively to obtain different degree of sensitisation. The nominal grain size of the material. 2.2. Sample preparation and observation method For surface preparation for AFM observation, samples were polished with water proof emery paper step by step from 150# to 4000# applying faint forces to avoid transformation and

damage to the samples. Then the samples were mechanically polished using 6 μm, 3 μm, 1 μm diamond suspension to a mirror finish. Finally the samples were electrochemically polished and etched basically according to ASTM Standards E1558-93 [9], A 262-93a [10]. The electropolishing solution consisted of 8 vol.% perchloric acid, 10 vol.% butoxyl ethanol, 70 vol.%, and 12 vol.% distilled water. The temperature of the solution was 273 K. The polishing was conducted at an electric voltage of 40 V for 20 s. The same solution was used by Masao Hayakawa to polish carbon steel to reveal carbides by AFM [4].

Fig. 1. AFM images of the electrochemically etched sample surfaces. From (a) to (f), samples were heat-treated at 700 °C for 5, 15, 30, 60, 300 and 600 min respectively.

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Fig. 2. Three dimensional (3D) AFM image of 304 austenitic stainless steel sample sensitised at 700 °C for 60 min, showing carbides projecting out of the surface along grain boundary.

The surface etching for AFM observation was controlled just enough to reveal grain boundary and suitable to be analysed using AFM because of the height of the tip. The AFM observation was conducted using Dimension 3000 in tapping mode where cantilever with a spring constant auto resonated at about 352 kHz. The scanning speed was 0.2 Hz. 3. Results and discussion Fig. 1 shows AFM micrographs of the six samples heat-treated at 700 °C for different time durations. The contrast in the images shows the differences in the level on the surfaces. The brighter the higher, and the darker the lower. The brighter dots or nodules are the carbides formed at grain boundaries. The carbide particles in 304SS contain more Cr and more corrosion resistant. This characteristic makes the particles projecting out of the surface after etching. Fig. 2 is one of the 3D images. The carbides projected out of the surface along grain boundary like spikes. If we measure the width of the grain boundary carbides and the length they occupy the grain boundaries, and calculate the ratio of the occupancy, we can then evaluate the sensitisation to some extent quantitatively. Table 1 lists the number of carbides along measured length of grain boundaries for the six samples. Figs. 3 and 4 show the width of grain boundary carbides and the length ratio they occupy along the grain boundaries. The mechanism of 304 SS to intergranular corrosion/SCC is the result of Cr depletion adjacent to carbides with high Cr content (M23C6, M7C3 etc.) formed at grain boundaries [7,8]. With the increase of heat treatment duration,

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Fig. 3. Width of the grain boundary carbides. No. 1 to No. 6 represent samples heat-treated at 700 °C for 5, 15, 30, 60, 300 and 600 min respectively.

more Cr at grain boundaries was consumed to form larger Cr containing carbides, making grain boundaries less corrosion resistant. The carbide size and the ratio of the occupancy were arbitrarily set to zero for the sample sensitised for 5 min, because the carbides are rare to find. The carbides were only found occasionally at triple grain boundaries joints. The size of the carbides increases with the increase of sensitisation heat treatment duration, and the nodules of the carbides become almost continuous along some boundaries, indicating severe degree of sensitisation. The results here show that AFM technique is effective in evaluating the sensitisation of AISI 304 stainless steel samples to intergranular corrosion/SCC.

4. Conclusion AFM technique is effective in evaluating the sensitisation to intergranular corrosion/SCC of AISI 304 stainless steels. The technique is hoped promising in evaluation of the sensitisation at HAZ of 13 Cr supermartensitic stainless steels weld if the mechanism is also carbide formation at grain boundaries. Acknowledgements The sensitised 304 SS samples were supplied by Prof. Thomas L. Ladwein and Jens Maier, Aalen University of Applied Sciences, Aalen, Germany. Prof. Masao Hayakawa, National Institute for Material Science, Japan gave some suggestions on sample preparation through E-mail communications. Prof. Garry Leadbeater, Curtin University of Technology,

Table 1 The number of carbides along measured length of grain boundaries Sample number

No. 1 No. 2

No. 3

No. 4

No. 5

No. 6

Number of carbides and its characteristics

10 22 17 23 17 (larger, (small) (small) (medium) (large) continuous nodule)

7.8 Measured length of grain boundaries, μm Number of carbides per 10 μm grain boundaries

5.2

7.2

6.4

8.1

10.3

1.9

3.1

2.7

2.8

1.7

Fig. 4. Ratio of carbides occupancy along grain boundaries, Width of the grain boundary carbides. No. 1 to No. 6 represent samples heat-treated at 700 °C for 5, 15, 30, 60, 300 and 600 min respectively.

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also gave some constructive suggestions for sample etching. The help given by Dr. Doug John, Shandelle Bosenberg and other members in WCRG, Curtin University of Technology were also acknowledged. References [1] S. Huizinga, R.K. Ohm, Qualification and Application Limits of Weldable Supermartensitic 13 Cr Linepipe Steels, NACE Corrosion Paper, vol. 01093, 2001. [2] S. Huizinga, R.K. Ohm, Experiences with Qualification of Weldable Martensitic Stainless Steel Pipe, NACE Corrosion Paper, vol. 03092, 2003. [3] Wiillem van Gestel, Girth Weld Failures in 13 Cr Sweet Wet Gas Flow Lines, NACE Corrosion Paper, vol. 04141, 2004. [4] Masao Hayakawa, Saburo Matsuoka, Kaneaki Tsuzaki, Mater. Trans. 43 (2002) 1758.

[5] Masao Hayakawa, Saburo Matsuoka , Yoshiyuki Furuya , Yoshinori Ono, Development of observation method for tempered martensite microstructure using chemical mechanical polishing technique, Mater.Trans. 46 (11) (2005) 2443–2448. [6] Kohji Minoshima, Yoshitaka OIE, Kenjiro KOMAI, In Situ AFM Imaging System for the Environmentally Induced Damage Under Dynamic Loads in a Controlled Environment, ISIJ International, vol. 43 (4), 2003, pp. 579–588. [7] M.A. Thomas, G.S. Was, Metall. Trans. A 21A (1990) 2097–2107. [8] Metals handbook, Ninth Edition, vol. 13, Corrosion, ASM International, Metals Park, Ohio, 44073, Handbook Committee, TA459.M43, 1978, 669 78-14934, ISBN 0-87170-007-7(V.1), SAN 204-7586, p.551. [9] Standard guide for electrolytic polishing of metallographic specimens, ASTM standard E 1558-93, Annual Book of ASTM Standards, vol. 03.01, 1999, pp. 930–941. [10] Standard practices for detecting susceptibility to intergranular attack in Austenitic stainless steels, ASTM Standard A 262-93a, Annual Book of ASTM Standards, vol.01.01, 1999, pp. 42–57.