At present there are over 10 000 dissimilar welds operating in the creep range in the UK alone. These have a history of creep related failures arising from internal thermally induced stresses.
This project aims at the development of an improved method for the production of these joints. This method requires the production of a transition piece having a composition which varies along its length. In effect this replace the dissimilar weld with two similar welds at each of the transition piece.
The main aim of this work was to demonstrate that a transition piece, with a graded composition along its length, could give improved mechanical properties when compared with traditional dissimilar metal welds made with high nickel fillers. Not only have the properties of these joints exceeded those of the dissimilar welds they were designed to replace, but they have also equalled, or exceeded, the properties of normal like/like welds in the weaker of the materials within the joint. As a result failure of such joints will not occur in normal service during the life of the plant.
The oxidation, toughness and tensile properties of the joints have proved to be equal to those expected of normal wrought products. The bi-axial pressurised and thermally cycled testing has confirmed the high resistance of these joints to thermally induced damage under the most severe of simulated operating conditions.
These joints therefore offer improved joint efficiency particularly for the new 9% and 12% Cr creep resistant steels currently being developed for use at high temperatures. They offer a ferritic/austenitic joint capable of sustaining continued thermal cycling during service in the creep range. It is believed these joints can make a contribution in the design of new power plants with improved efficiency.
Three material combinations will be investigated 2 1/4Cr-316, mod 9Cr-316 and 2 1/4Cr-mod 9Cr. These will be fabricated using Hot Isostatic Pressing (HIP) and Vacuum Plasma Spraying. The technical and economic merits of the two fabrication routes will be compared.
Trial joints in a number of designs of compositional gradient will be produced and subjected to uniaxial creep testing, Finite Element Modelling and corrosion testing.
On the basis of this modelling and testing work optimized compositional gradients will be selected for each material combination. These will be subjected to long term uniaxial and biaxial creep testing.
By the end of this project there will be sufficient design data available for the rapid adoption of this technology for tube size joints in both the Power Generation and Petrochemical Industries.
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