Characterisation of Microstructure and Properties of a Transition Weld

Abstract

Transition welds represent a challenge for the assessment of structural integrity of nuclear plant due to the complexity of the microstructure, properties and local stress state. This paper presents the initial findings of a study aimed at characterising the local microstructure and properties of a transition weld between SA508-4N ferritic steel and SS316LN austenitic stainless steel using a nickel-base filler of Alloy 82. The local microstructures and local composition of the material interfaces are characterised using backscattered electron imaging and Energy-dispersive X-ray spectroscopy. The ferritic steel shows significant grain refinement in the heat affected zone compared to the base metal. This refinement is also observed in the heat affected zone of the austenitic stainless steel although not as significant. Micro-hardness testing has also been incorporated to provide an indication of the influence of local microstructure on flow properties across the weld region. The results indicate a hardness range of between 180–340HV across the weld with the highest value in the heat affected zone of the ferritic steel and the lowest in the austenitic stainless steel. Yield and flow properties derived from flat transweld tensile tests incorporating digital image correlation are related to the micro-hardness results and microstructural characterisation, and an initial assessment of the fracture mechanism performed using fractography.

In-situ measurements of stress during thermal shock in clad pressure vessel steel using synchrotron X-ray diffraction

“…Poisson's ratio was assumed temperature independent whereas the modulus was reduced by 15% from 20°C to 450°C [21]. The small grain size in SA508 Grade 4N steel (2.5 -6 micrometre) [22] allowed for acquisition of bright enough rings to be analysed using the procedure detailed above while the size of the Nickel cladding (of the order of 1mm) grains were prohibitively large [23] compared with the slit size used (see below). The nickel alloy had an elastic modulus of 172.4 GPa and a yield stress of 310 MPa [24].…”