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

Abstract

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 623-823K, which is the range for the so-called “748K 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 823K for durations in the range 1000 to 20000hours. A decrease in toughness, both charpy energy and J-R curves, was observed after ageing at 643, 748 and 823K, 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.