Plasma nitriding is considered as a plausible alternate hard-facing technology for enhancing the wear and fatigue resistance of large size and intricate fast reactor components. Plasma nitriding is an environmentally clean process that can be adopted in principle, to produce high surface hardness in a controlled manner, with little distortion of finished components. In this regard, an attempt has been made to adopt plasma nitriding for the casing ring of the secondary sodium pump of PFBR.
A model with reduced thickness and another scaled-down version of the casing ring of the pump made of SS316L were fabricated. These rings were stress relieved, followed by chrome plating to a thickness of ~100 μm. Subsequently, the rings were plasma nitrided at 500°C for about 48 h under standardised process conditions (Working gas: H2:N2 ratio in 1:3 at 4mbar, 600V pulsed DC, 18A). No gross dimensional distortion of the component was found as a result of plasma nitriding. Since the rings have a step like discontinuity in geometry, the possible variation in microstructure and hardness profiles across the thin and thick sections of the scaled down version of the ring were characterized to assess the effectiveness of the plasma nitriding in providing a nitrided layer of reasonable case depth. It was found that a hardness of ~800VHN with a case depth of ~100 μm and 55 μm for thicker and thinner sections respectively could be achieved. X-ray diffraction analysis showed the formation of CrN and Cr2N phases in both the sections after plasma nitriding.
In order to assess the stability of plasma nitrided stainless steel component in liquid sodium, long term exposure to static sodium at 550°C up to 5000h was performed. Only a minor reduction in hardness, in the range of 560-700 VHN was observed due to thermal ageing induced from prolonged high temperature exposure in sodium. However, no significant change in microstructure or nitride layer case depth was observed. These results suggest the retention of long term integrity of the plasma nitrided stainless steel in static sodium.
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|Affiliation/Organization||Indira Gandhi Centre for Atomic Research, Kalpakkam|