Mechanical Behaviour of SS 316 (N) Weld after Long Term Exposure to Service Temperatures

The composition of the welding consumable for the SS 316 L(N) components of prototype fast breeder reactor is adjusted such that the weld deposit contains 3 to 7 % delta ferrite, to balance between resistance to hot cracking during welding, and embrittlement of the weld metal during service. The operating temperature range for these components is 623-823 K, which is the range for the so-called 748 K embrittlement, noticed in cast duplex stainless steel. Therefore, a study was taken up to assess the degradation, if any, in the mechanical properties of this weld due to long term service exposure. Tensile, charpy-impact and fracture (J-R curves) toughness tests were done at ambient temperature, in the as-welded condition and after thermal ageing at 643, 748 and 823 K for durations in the range 1000 to 20000 hours. A decrease in toughness, both charpy energy and J-R curves, was observed after ageing at 643, 748 and 823 K, after long ageing times. The variations in properties are discussed in the light of microstructural changes in the delta ferrite phase observed using optical and TEM methods and the fractographic observations on the fracture tested specimens. (C) 2011 Published by Elsevier Ltd. Selection and peer-review under responsibility of ICM11