Materials Science and Engineering A 490 (2008) 16–25
Crack initiation mechanisms for low cycle fatigue of type 316Ti stainless steel in high temperature water S. Xu a,∗ , X.Q. Wu a , E.H. Han a , W. Ke a , Y. Katada b a
Environmental Corrosion Center, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, PR China b National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
Received 27 August 2007; received in revised form 19 December 2007; accepted 21 December 2007
Abstract Low cycle fatigue (LCF) tests were performed for a type 316Ti stainless steel (SS) in high temperature water. Fatigue crack initiation behaviors in high temperature water were investigated. It was found that there existed several kinds of Ti-bearing precipitates, consisting of isolated TiN or duplex (Al, Mg)O/TiN, Mo-rich (Ti, Mo)C and Ti(N,C) in the steel. Fatigue cracks were mainly initiated at Ti-bearing precipitates, phase boundaries of austenite/␣-ferrite phases and persistent slip bands (PSBs) in austenite. It is believed that synergism between the mechanical factors and electrochemical reactions played a key role in the process of fatigue crack initiation in high temperature water. Related fatigue crack initiation mechanisms for the 316Ti SS are discussed. © 2008 Elsevier B.V. All rights reserved. Keywords: Type 316Ti stainless steel; High temperature water; Low cycle fatigue; Crack initiation mechanism; Ti-bearing precipitates
1. Introduction Type 316Ti stainless steel (316Ti SS) has been widely used as sheet and tube materials in the chemical industry and nuclear power industry. Homogeneous and molten salt reactors require sheet for circulation through the core chamber and heat exchangers, while many heterogeneously fuelled reactors use tube and sheet materials for cladding or canning of fuel elements [1–3]. The choice for using this material is mainly based upon the advantages of titanium improved post-irradiation ductility and good mechanical properties at elevated temperatures. Titanium is added to the stainless steels due to its strong carbide-forming ability, so that they preferentially combine with the available carbon and thus lessen the nucleation opportunity of chromium bearing carbides, which significantly improve the intergranular corrosion resistance of the stainless steels [4,5]. 316Ti SS is also one of main candidate materials for the primary and secondary circuit piping of the light water reactors (LWR), which serves in high temperature water [6]. Up to now, various environmentally assisted cracking (EAC) issues in connection with the LWR
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environments have been investigated and reported for materials used in the circuit piping system and should be cautiously considered for purposes of safe operations and managements as well as remaining life assessment of nuclear power plants, among which corrosion fatigue is of great importance [7,8]. Though the irradiation behaviors [9–12], mechanical properties [3,13–16] and potential intergranular corrosion resistance [4,17,18] of type 316Ti SS have been extensively investigated, little work was focused on the corrosion fatigue properties of type 316Ti SS in LWR environments. It is well known that the initiation mechanisms of corrosion fatigue cracks are the most important for the cyclic lifetime of structural members made from stainless steels. Once initiated, a crack cannot be prevented from propagating and the total cyclic lifetime is usually dominated by the formation time of fatigue cracks. It is thus of considerable significance to study the corrosion fatigue crack initiation mechanisms of 316Ti SS in high temperature water environments. The purpose of this paper is to investigate the low cycle fatigue (LCF) property and EAC behavior of 316Ti SS in high temperature water. All the tests were performed in a simulated boiling water reactor (BWR) environment which typical service temperature is 561 K (288 ◦ C). Main attention is paid to the initiation mechanisms of LCF cracks.
S. Xu et al. / Materials Science and Engineering A 490 (2008) 16–25 Table 1 Chemical composition (wt.%) of 316Ti stainless steel investigated C Si Mn P S Ni Cr Mo Ti N Fe
0.07 0.63 1.76 0.02 0.01 13.17 18.35 2.34 0.62 0.03 Bal.
Fig. 1. Illustration of LCF specimens used.
defined as a number of cycles at which the peak tensile stress descended to 75% of the level of the maximum peak stress. For the microstructure examination, the specimens were sectioned longitudinally and ground successively with silicon carbide paper up to 2000 grit, polished with 1.5 m diamond powers, and then ultrasonically cleaned in acetone. The solution of 10% oxalic acid in distilled water was used for electrolytic etching to reveal the steel microstructure. An optical microscope was employed to observe metallographic structure and an X-ray diffraction analyzer (XRD) was used to verify the metallographic phase. The characteristics of fatigue crack on specimen surfaces and longitudinal sections in gauge length were carefully examined using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDX). Some specimens were broken apart in liquid nitrogen and the corresponding fracture surfaces were examined. To clarify preferential sites for pitting initiation in the 316Ti SS, a potentiodynamic anodic polarization test was conducted in 3.5% chloride solution at room temperature. The potential was scanned from the corrosion potential toward the noble potential at a rate of 20 mV/min, and the scanning was stopped after reaching the pitting potential. After cleaning in distilled water, the specimen was examined by the optical microscope to reveal the pitting initiation sites.
2. Experimental
3. Results
The material used for the present work was a type of 316Ti SS, whose chemical compositions were given in Table 1. Smooth cylindrical LCF specimens having gauge length and diameter of 16 and 8 mm, respectively, were machined along the rolling direction (Fig. 1). The specimens were solution annealed at 1100 ◦ C for 40 min in an argon atmosphere and water quenched. For maximum resistance to sensitization, the specimens were also given a stabilizing heat treatment at 800 ◦ C for 2 h to precipitate Ti-bearing precipitates and prevent the precipitation of chromium carbides during subsequent lower temperature exposure. The equipments for LCF tests in high temperature water, as used in previous studies [19,20], comprised a fatigue machine of 100 kN in dynamic load, an austenitic stainless steel autoclave of 6 l in capacity and a water loop with 30 l per hour in flow rate. Fatigue tests were performed in an axial strain control mode with fully reversed triangular waveform. The test conditions and water chemistry are summarized in Table 2. Fatigue life N25 was
3.1. Microstructure and precipitates Fig. 2 is XRD analysis of the 316Ti SS. The microstructure was mainly composed of austenite (γ) and small amounts of rod-like ␣-ferrite phase (␣). The ␣-phase was mainly located at boundaries of the original austenite grains, as shown in Fig. 3. In addition, based on the quantitatively metallographic measurement, the volume fraction of ␣-ferrite phase was about 4.19%. Randomly dispersed Ti-bearing precipitates were frequently observed on the polished surface of the 316Ti SS (Fig. 4). The size of these precipitates ranged from submicron to several microns. There are four types of precipitates and inclusions. Namely, the precipitates A in Fig. 4a; the precipitates B in Fig. 4a
Table 2 Test conditions and high temperature water chemistry Control mode Wave form Strain range Stain rate Temperature Pressure Dissolved oxygen pH Conductivity
Strain Triangle 1.5% 0.1, 0.001% s−1 561 K 8.0 MPa 100 ppb 6.2–6.5
