18 - Welding technologies for advanced ultra-supercritical power plants materials

Abstract

Similar to manufacturing power plants using USC steels, fusion arc welding will be the key fabrication technology for A-USC materials, as well. The shape of semifinished products, welding details, and strategy of welded manufacturing are not expected to depart significantly from established concepts. In this field, as of today, welding technology and associated filler materials are sufficiently mature at least for some specific austenitic steels, nickel-based alloys, and limited types of arc welding methods. Difficulties still exist mainly for A-USC ferritic steels because promising base material candidates emerged only very recently (now commonly known as SAVE12AD and VM12-SHC, for thick- and thin-walled members, respectively). Also recently emerged are nickel-based alloys due to delicate weldability of the most performing grades (considered to be the alloys 617B and 740H). However, most A-USC austenitic steels are considered of good weldability and fabricability (the most promising grades are 304HCu and HR6W for tubing and thick components, respectively), and their use has been already introduced in USC designs. Control of the weld metal chemistry, its purity, and cleanliness requirements are often very tight and not easy to be consistently achieved during field welding; this is to say welding technologies for A-USC power plants require the highest care in the process and the selection of adequate filler material, which have to be carefully coordinated to the base materials as well as to the operation conditions. As consequence of this behavior, so far the gas tungsten arc method has been the main choice on all A-USC material classes, thus accepting compromises on the achievable deposition rates. Slag-forming methods have been confined to few specific alloys with more tolerant weldability. Also, dissimilar metal welding is an area of great complexity and concern, with qualification of the various solutions currently being undertaken. R&D in this sector has been intense in the last decade, but it is obviously needed to accumulate experience in large A-USC welded fabrication and long-time service at high temperature. This chapter presents recent developments for the most promising weldable A-USC materials, the challenges and the strategies for the proper control of the welding activity, the already mature welding methods suitable for the delicate weldability of such materials, and a selection of applications for different kinds of plant equipments already at a demonstrative stage.