Failure analysis of a MARVAL18 steel sample holder in contact with liquid lead-bismuth eutectic

Transcription

1 Failure analysis of a MARVAL18 steel sample holder in contact with liquid lead-bismuth eutectic Xing Gong a,b, Pierre Marmy b, Serguei Gavrilov b, Rui Li a, Qisen Ren a, Tong Liu a a Department of ATF R&D, Nuclear Fuel Research and Development Center, China Nuclear Power Technology Research Institute, China General Nuclear Power Corporation (CGN), Shenzhen, , China b SCK CEN (Belgian Nuclear Research Centre), Boeretang 200, B-2400 Mol, Belgium Abstract This paper reports about the failure of a testing machine component. A hollow cylindrical sample holder made of MARVAL18 steel was fractured into two pieces after a total service time of h in low oxygen liquid lead-bismuth eutectic (LBE) under monotonic tensile loading. The failure analysis by means of scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS) indicates that this fracture event is likely caused by liquid metal embrittlement (LME). Ni dissolution and subsequent LBE penetration at grain boundaries ultimately lead to crack initiation in an intergranular way, followed by unstable and rapid crack propagation. Keywords: Steel; LBE; Intergranular cracking; LME 1

2 1. Introduction MARVAL 18 is the name of the maraging steel containing 18% Ni. Maraging steels have ultrahigh strength, good ductility and excellent creep resistance, resulting from very fine intermetallic precipitates which can reduce dislocation mobility. These impressive properties enable this type of steels to have been widely used in aerospace, military and nuclear industries for bearing loads up to high temperatures. In this paper, a hollow cylindrical sample holder made of MARVAL18 steel was fractured into two pieces after total service time of h in low oxygen liquid lead-bismuth eutectic (LBE) under monotonic tensile loading. The fractured sample holder was subject to failure examination, showing that the failure was mainly due to LME. 2. Materials and loading history The MARVAL18 steel was first solution-treated at 825 followed by air cooling. Afterwards, it was subject to ageing at 480 for 4h. The chemical composition of this steel is given in Table 1. It can be seen that this steel is highly alloyed with Ni, Co and Mo. The failed sample holder belongs to a LIMETS1 tensile testing machine located at SCK CEN, Mol, Belgium. This machine is used to test LME susceptibility of MYRRHA candidate materials in contact with a stagnant LBE environment under monotonic tensile loading. Figure 1 is the schematic illustration of the fractured sample holder and the location of the cracking site. 2

3 Performing a tensile test with this machine basically involves a two-step process, including preexposure of a tensile specimen in low oxygen LBE at for h and subsequent running of the tensile test at 350. The latter step generally requires 10 to 20 h to break a specimen. During the entire process, the bottom sample holder is fully submersed in LBE and thus has the same pre-exposure/loading history as the tensile specimen. The total service time of this sample holder is h. During one half of this service time, the sample holder was subject to tensile forces in presence of LBE, while forces were absent during the rest of the service time, i.e. during the LBE pre-exposure. The loading history of the latest tensile test which directly caused the failure of the sample holder is shown in Figure 2. The tensile specimen was a tempering-hardened T91 steel and had unexpectedly high strength. Finally, the force reached the safety limit which is 10 kn and is used to protect the machine. Then, the force dwelled at this level for 2.5 h, followed by abrupt stress drop indicating failure of some component. The postexamination showed that the failed component was the bottom sample holder rather than the tensile specimen, as seen in Figure Failure analysis and discussion In order to examine the fracture surface, one half of the sample holder was cleaned with a chemical solution consisting of CH 3 COOH, CH 3 CH 2 OH and H 2 O 2 with a volume ratio of 1:1:1. Afterwards, the fracture surface was carefully examined in SEM and the results are presented in Figures 4 and 5. 3

4 Figure 4 shows that the fracture surface at the outer periphery of the sample holder is characterized by intergranular facets, forming a ring-like zone of intergranular cracking. The width of this intergranular cracking zone is 100 to 200 µm. The rest of the fracture surface is dominantly covered by intergranular ductile fracture features (Figure 5a), accompanied with a small fraction of mixed intergranular and transgranular brittle fracture features (Figure 5b). The EDS elemental mapping of the periphery area of the fractured sample holder shows that the surface has been severely corroded and the corroded area is mainly composed of two layers the outer porous iron oxide and the inner LBE penetration zone (i.e. Ni depletion zone), see Figure 6. Underneath the inner LBE penetration zone, there are some small cracks (denoted with the black arrows) filled with LBE and these cracks seem to be grain boundaries which could serve as preferential paths for LBE to penetrate along. It is known that Ni has high solubility in LBE particularly at high temperature [1]. When exposed to low oxygen LBE, the surface protective oxides are no longer thermodynamically stable, leading to decomposition of the oxides and subsequent dissolution of Ni into LBE. Generally, the Ni dissolution occurs preferentially at grain boundaries, immediately followed by LBE penetration along such sites. The LBE penetration causes decohesion of the grain boundaries, easing the crack initiation under stresses. When the stress intensity factor at the penetration front, which acts as a crack tip, reaches a critical value, the crack will propagate into the bulk. This is a special case of LME, involving significant atomic diffusion during the crack initiation stage[2]. The LBE penetration along grain boundaries could be assisted by the external load, which has been reported by Gordon and An [3, 4]. The mechanism governing the formation of the 4

5 intergranular ductile cracking zone (Figure 5) is likely not influenced by LBE due to the very rapid crack front propagation. 4. Summary The failure of the hollow cylindrical sample holder made of MARVAL18 steel is a special LME case. The crack initiation was caused by the Ni dissolution and subsequent LBE penetration at grain boundaries. Under the external load, the stress intensity factor at the crack tip reached a critical value, leading to the unstable and fast crack propagation. Acknowledgements The work is financially supported by the MYRRHA project, SCK CEN, Belgium and partly funded by the European Atomic Energy Community s (Euratom) Seventh Framework Programme FP7/ under grant agreement No (MatISSE project) and in the framework of the EERA (European Energy Research Alliance) Joint Programme on Nuclear Materials. References 1. Zhang, J.S. and N. Li, Review of the studies on fundamental issues in LBE corrosion. Journal of Nuclear Materials, (1-3): p S. Hémery, et al., Liquid metal embrittlement of an austenitic stainless steel in liquid sodium. Corrosion Science, : p

6 3. Gordon, P. and H.H. An, THE MECHANISMS OF CRACK INITIATION AND CRACK- PROPAGATION IN METAL-INDUCED EMBRITTLEMENT OF METALS. Metallurgical Transactions a-physical Metallurgy and Materials Science, (3): p Lynch, S.P., Mechanisms and Kinetics of Environmentally Assisted Cracking: Current Status, Issues, and Suggestions for Further Work. Metallurgical and Materials Transactions A, : p

7 Figure captions Figure 1 Schematic illustration of the cracking site at the bottom sample holder. Figure 2 The latest loading history of the sample holder. Figure 3 Photos of the fractured sample holder. Figure 4 SEM images of the fracture surface at the outer periphery of the fractured sample holder. Figure 5 SEM pictures of the fracture surface in the center of the fractured sample holder. Figure 6 EDS elemental mapping of at the surface of the cracked sample holder. The black arrows indicate small cracks. 7

8 Figures Figure 1 8

9 Figure Load, kn Time, h 9

10 Figure 3 10

11 Figure 4 11

12 Figure 5 12

13 Figure 6 13

14 Table 1 Chemical composition of the MARVAL18 steel. C Ni Co Mo Ti Fe Bal. 14