MATERIALS CHARACTERIZATION 24:199-204 (1990)
SHORT COMMUNICATION Effect of TiC on the Stress Corrosion Cracking of Austenitic Stainless Steel
WEN-TA TSAI,* CHI-MING LIAW,t AND JU-TUNG LEE*
*Department of Materials Engineering, National Cheng Kung University, Tainan, Taiwan, R.O.C., and tResearch and Development Department, China Steel Corporation, Kaohsiung, Taiwan, R.O.C.
Because of its good corrosion resistance, austenitic stainless steel has a wide range of engineering applications. However, this material is susceptible to intergranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC) when it is subjected to asensitizing heat treatment, which results in the formation of chromium carbide along the grain boundaries. As a consequence of chromium carbide precipitation, chromium depletion may occur in the grain boundary regions. This eventually leads to the decrease of corrosion resistance in this region. In order to prevent sensitization, low-carbon-content stainless steel, or titanium- or niobiumstabilized stainless steel is recommended . Although titanium-stabilized austenitic stainless steel is immune from sensitization, it is still susceptible to stress corrosion cracking (SCC) in certain environments. An example of the stress corrosion cracking occurring in titanium-stabilized austenitic stainless steel was the failure of a fractionator in an ammonia recovery plant. In the fractionator, liquid ammonia was extracted from the ammonium hydroxide, to which sodium hydroxide had been added to neutralize the residual acid. The operating temperature and pressure were about 200°C and 2 Mpa, respectively. The micrographs of the stress corrosion cracks in the failed fractionator are shown in Fig. 1. Figure l(a) (not polished, not etched) shows the throughthickness crack, while Fig. l(b) (etched in 10 mL HNO3 + 20 mL HCI + 30 mL H20 etchant) depicts the transgranular mode of cracking. The cause of the SCC mentioned is examined and discussed below. The chemical composition (wt%) of the material of the failed fractionator was: 0.076 C, 0.537 Ti, 17.96 Cr, 10.40 Ni, 0.640 Mo, 0.522 Si, 1.56 © Elsevier Science Publishing Co.. Inc.. 1990 655 Avenue of the Americas, New York, NY 10010
W.-T. Tsai et al. ::i!iil
FI6. 1. Stress corrosion cracks in titanium-stabilized austenitic stainless steel, following use in an ammonia recovery system at 2 Mpa and 200°C: (a) Macrograph (not polished, not etched) showing the treelike pattern of SCC crack; (b) optical micrograph of an etched (in 10 mL HNO3 + 20 mL HCI + 30 mL H20) cross section, showing the transgranular mode of cracking.
Mn, 0.032 P, 0.018 S, 0.215 Cu, and balance Fe. This material, with a Ti/ C ratio of 7.066, was equivalent to AISI 321 stainless steel. Microstructural examination was performed using electrolytic oxalic acid etching (ASTM A-262 Practice A), a test used to detect the susceptibility to IGC or the degree of sensitization. As shown in Fig. 2, a step structure is clearly revealed, which indicates that the stainless steel was not sensi-
Effect of TiC on Stress Corrosion Cracking
Oxalic acid etch structure of the as-received stainless steel,
tized. Large holes are also seen on the specimen surface after it is etched electrolytically in oxalic acid. Some of these holes had a rectangular shape, possibly resulting from the detachment of second-phase particles. Further analysis of these holes was performed with scanning electron microscopy (SEM). The results are given in Fig. 3. As can be seen in Fig. 3(a), numerous holes with a rectangular shape were formed on the surface examined. Most of these holes were less than 10 Ixm in size, but there were also some that were considerably larger. As shown in Fig. 3(b) with higher magnification, two large particles, with the largest one having an approximate length of 20 txm, were seen sitting on the surface examined. In a comparison of the geometries of the holes and the particles observed, it is thought that the holes were formed as a result of the detachment of these second-phase particles from the stainless steel matrix. Energy dispersive spectroscopy (EDS) of the surface particles indicated that these second-phase particles had high Ti content, as revealed in Fig. 3(c). X-ray mappings of Ti and C elements are given in Figs. 3(d,e), respectively, which demonstrate that the particles shown in Fig. 3(b) were TiC. The angular shape of TiC precipitated in austenitic stainless steel had been recognized previously by Streicher  and others [2, 3]. The extraordinarily large TiC particles were probably formed during cooling from the melt as primary carbides  and could not be redissolved in the austenitic matrix in the subsequent heat treatment. The existence of these massive incoherent TiC precipitates could promote the SCC of the titanium-stabilized stainless steel in an ammonia-containing environ-
W.-T. Tsai et al.
FIG. 3. SEM micrograph showing the TiC morphology,and EDS and x-ray mapping showing the phase and the distribution of constituents.
ment in two ways. First, preferential corrosion could occur at the interface between the TiC particle and the matrix on the surface, and, thus, a crack nucleus was developed. Second, void formation would be facilitated because of the incoherent nature of the massive TiC particles, which then accelerated the cracking process. The observation of a stream of voids located along the crack edges, as shown in Fig. 4, clearly demonstrates that TiC precipitates assisted the crack propagation process. The possible sequence for the crack propagation process is shown schematically in
Effect of TiC on Stress Corrosion Cracking
SEM of the SCC cracks showing the voids located along the crack sides.
Fig. 5. As demonstrated in Fig. 5, the combined effects of the aggressive environment and the applied stress ensure that the SCC proceeds once the crack is initiated. Meanwhile, void formation resulting from the decohesion between the TiC particles and the matrix, caused by stress concentration ahead of the crack tip, assists the crack extension.
STRESS FIG. 5.
Schematic drawing illustrating the effect of TiC on the SCC crack propagation.
W.-T. Tsai et al.
The e n c o u r a g e m e n t o f the China Steel Corporation f o r this investigation is appreciated.
References 1. M. A. Streicher, Effect of heat treatment, composition and microstructure on corrosion of 18Cr-8Ni-Ti stainless steels in acids, Corrosion, 20:57t-72t (1964). 2. C. H. Samans, K. Kinoshita, and I. Matsushima, Further observations on sensitization of chemically stabilized stainless steels, Corrosion, 33:271-279 (1977). 3. S. M. Box and F. G. Wilson, Effect of carbide morphology and composition on the intergranular corrosion of titanium-stabilized austenitic stainless steels, J. Iron Steel Inst., 210:718-722 (1972). 4. M. Ozel and J. Nutting, Plastic deformation and recrystallization of TiC particles during creep of an austenitic steel, J. Iron Steel Inst., 207:92-94 (1969).
Received Dec. 6, 1988; accepted June 25, 1989.