Effects of high temperature aging in an impure helium environment on low temperature embrittlement of Alloy 617 and Haynes 230 Daejong Kim, Injin Sah, Changheui Jang * Department of Nuclear and Quantum Engineering, KAIST, 335 Gwahangno, Yuseong-gu, Daejeon 305-701, Republic of Korea article info Article history: Received 8 April 2010 Accepted 20 July 2010 abstract The effects of high temperature environmental damage on low temperature embrittlement of wrought nickel-base superalloys, Alloy 617 and Haynes 230 were evaluated. They were aged in an impure helium environment at 1000 °C for up to 500 h before tensile tested at room temperature. The tensile test results showed that the loss of ductility was associated with the increase in the inter-granular fracture with aging time. For Alloy 617, inter-granular oxidation and coarsening of grain boundary carbides contributed to the embrittlement. The significant loss of ductility in Haynes 230 was only observed after 500 h of aging when the globular intermetallic precipitates were extensively formed and brittle inter-granular cracking began to occur. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Wrought nickel-base superalloys, such as Alloy 617 and Haynes 230 are the materials considered for the intermediate heat exchan- ger (IHX) applications in a very high temperature gas cooled reactor (VHTR) with an outlet gas temperature of 900 °C and above [1,2]. Based on the operating experience of the earlier high temperature gas cooled reactors, the helium coolant in the primary circuit is likely to contain small amounts of gaseous impurities such as CO, CO 2 , CH 4 ,H 2 O and H 2 in operating condition [3,4]. It has been known that the high temperature environmental degradations such as oxidation, decarburization and carburization in the impure he- lium environment could be extensive depending on temperature and the levels of impurities [5–8]. Such environmental damage can significantly degrade the mechanical properties of the materials for the IHX applications [9], and eventually the life and perfor- mance of the components. Especially, carburization and decarburi- zation can result in the low temperature embrittlement and the reduction in creep or creep-fatigue strength [10]. In this study, the low temperature embrittlement behaviors of Alloy 617 and Haynes 230 were investigated to understand the relationship be- tween environment and microstructure, and their effects on the mechanical properties. Specimens were aged in an impure helium environment and subsequently tensile tested at room temperature. The microstructure evolution such as internal oxide growth along grain boundary and coarsening of inter- and intra-granular carbides was discussed in view of their effects on the embrittlement. 2. Experimental Two commercial grade wrought nickel-base superalloys, Alloy 617 and Haynes 230 were used in this study. Chemical composi- tions of the materials are listed in Table 1 and typical microstruc- tures are shown in Fig. 1. As shown in the figures, the as-received microstructures contain twins, well-distributed primary carbides and grain boundary carbides. Grain boundary carbides were pri- marily M 23 C 6 type carbides precipitated during solution anneal heat treatment. The small-size plate-type tensile specimens with 0.5 mm in thickness, shown in Fig. 2, were machined and aged at 1000 °C in the impure helium containing 5.0 Pa CO, 1.0 Pa CO 2 , <0.1 Pa CH 4 , and 0.07 Pa H 2 O up to 500 h. During the aging treat- ment, gas flow rate was maintained at 500 cc/min. Aged specimens were tensile tested at room temperature. For the tested specimens, metallographic analyses were carried out using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). 3. Results and discussion 3.1. Microstructure evolution Fig. 3 shows the cross-sectional area of Alloy 617 and Haynes 230 aged for 500 h at 1000 °C in the impure helium environments. It is clear from the figures that there are three distinct zones such as an oxidation region, narrow band of the carbide-free zone, and a matrix in which an increased volume fraction of carbides, grain boundary carbides in particular, were observed. Overall, the micro- structure evolution during the aging treatment in the impure 0022-3115/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2010.07.026 * Corresponding author. Tel.: +82 42 350 3824; fax: +82 42 350 3810. E-mail address: chjang@kaist.ac.kr (C. Jang). Journal of Nuclear Materials 405 (2010) 9–16 Contents lists available at ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat