High Temperature Sliding Wear Behavior of Ni Alloys Under Helium Environment

Md Saifur Rahman, Andreas A. Polycarpou

Department of Mechanical Engineering, Texas A&M University, College Station, TX, 77843

INTRODUCTION: The very-high-temperature reactor (VHTR), or high-temperature gas-cooled reactor (HTGR), is a Generation IV reactor concept for producing electricity and hydrogen economically. HTGR/VHTR uses Helium (He) as the coolant to transfer the heat generated in the reactor core to the power conversion unit or the hydrogen-production plant. The VHTR is a type of high-temperature reactor (HTR) that can conceptually have an outlet temperature of 1000°C and coolant gas pressure of up to 9MPa. Nickel-based superalloys, 800HT and Inconel 617 are believed to be very good candidate materials for such environment as they show relatively good resistance against various types of environmental degradation such as oxidation, carburization and decarburization under high temperature.^1,2 It is mainly due to the protective external oxide that prevents the direct interaction of the metal with the He coolant gas containing the impurities.

The objective of this research project is to systematically evaluate the tribological response of 800HT and 617 alloys at relevant reactor operating temperatures (700-950^oC) and in the presence of He coolant.

High Temperature Tribometer (HTT): The HTT is a specialized tribometer (shown in Fig.1a) with pin-on-disk (ball-on-disk, flat pin-on-disk and cylinder pin-on-disk) configuration with sample diameter up to 95 mm. It can perform wear/friction experiments at temperatures up to 1000°C and has the capability of unidirectional and oscillatory motion: rotational speeds up to 1000 rpm and oscillation frequencies up to 5 Hertz. The force transducer records the in situ normal (up to 45N) and friction forces which is used to calculate the in-situ coefficient of friction (COF). It has an enclosed chamber which enables the control of the test atmosphere. Using a vacuum pump and ports, we are able to evacuate the chamber and then gas (He with different compositions) can be introduced.

Fig. 1. High temperature tribometer (a) Tribometer setup, (b) Bell jar chamber for vacuum and controlled environment

Samples and experimental conditions: Bulk materials of both Inconel 617 and alloy 800HT are received from Idaho National Laboratory (INL) and Oak Ridge National Laboratory (ORNL). The disk (25.4mm x 25.4mm x 3.8mm) and flat pin are machined using wire EDM (Electric Discharge Machining) process from the bulk material. The disk and pin contact surfaces are then polished to reach surface roughness of approximately 0.3 µm (Rq). As described in Table I, several sets of experiments were designed to cover room and high temperature air and helium (VHTR) environments. To better understand the wear resistance as well as the dominant wear mechanism, the normal load is varied along with the sliding speed and sliding distance fixed.

Results and DISCUSSION: Fig. 2a represents the in-situ COF of Inconel 617 at room and high temperatures in air as well as in helium environments. At high temperature in air, the COF of both alloys drops significantly (about 60% reduction) after few cycles and it is stabilized at lower values. However, the COF at room temperature in air and high temperature in helium shows high and stable values for both samples. The COF at higher temperature in air remains constant with sliding time after the initial run-in period of about 200s for Inconel 617 and 400s for alloy 800HT at low loads.



Fig. 2. (a) In situ COF experiments: Inconel 617 pin on Inconel 617 disk. SEM images of 20N Inconel 617 (b) 950^oC in air, (c) 950^oC in He.

Further observation by SEM in Fig. 2b reveal the formation of the glazed surface of the oxides at HT air which is not present on the samples at HT in helium environment, as shown in Fig. 2c. The observed lower COF at HT in air is attributed to the compact glazed oxide layers separating the two metallic surfaces.^3,4 Fig. 3 shows the average COF and wear rate for different loads. As seen in Fig. 3a, for Inconel 617, in He environment, the COF is minimum at an intermediate temperature and highest at 950^oC for all loads. Alloy 800HT behaves similarly in He environment, showing very high friction at 750^oC for all loads and lowest COF at the intermediate temperature.


Fig. 3. The influence of temperature and load on Inconel 617 (a) COF and (b) Wear rate.

Cross section SEM and EDS analysis of the alloys show that the surface oxide is mostly Cr-rich. Microhardness test reveals that although both alloys have fairly same hardness value outside of the wear track, the hardness of Inconel 617 is more than that of alloy 800HT after sliding showing more hardening effect on Inconel 617 leading to better wear resistant property.

REFERENCES:  1. Cabet C, Mater Corros (2006), 2. Wright RN.,  INL/EXT-06-11494 (2006), 3. Birol Y, Wear (2010), 4. D. Y. Li, Wear (2005).