IIW White Paper

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Needs and challenges in welding and joining technologies 4.1 New materials, weldability and testing T raditionally welding science and technology are associated with joining of metals and alloys as they form a substantial part of the material consumed by mankind. Steels, aluminium alloys, nickel-base super alloys and titanium alloys constitute the major share of these metals and alloys and development in welding science and testing have been in improving the weldability of these materials and for understanding the basic metallurgy of their welds. Similarly, processes employed to weld these materials have been confined predominantly to the arc welding processes and, as in automotive industries, to resistance welding. Today, however, many more new materials and design methods (e.g multi-material) are available and accordingly the science and technology of joining them have also undergone revolutionary changes. The need to improve the quality of life, conserve the natural resources, protect the environment etc. has been the major driving force for innovations in the field of materials and their application. Some of these were driven by the desire to take the conventional structural materials like steels, Al alloys etc. up to the upper limits of their performance. Transformation Induced Plasticity (TRIP) steels used in automobile industries, advanced ferritic steels in fossil power plants, super austenitic and super duplex stainless steels for corrosion resistance applications, Al-Li alloys and maraging steels for aerospace applications are successful examples of these innovations. A paradigm shift in the way the materials are chosen for different application is another factor that triggered innovation in this field. The choice of ceramics, composites, fibre reinforced plastic etc. for structural applications, often for very extreme operating conditions, resulted in significant advances and developments in these new structural materials. These materials now find applications in gas turbines (structural ceramics), space vehicles (composites) windmills (composites and fibre reinforced plastics) etc. Development of new technologies, information, communication and bio-technology, triggered the development in a new generation of materials called advanced materials which are used more for their special physical properties like biocompatibility, magnetic, electrical and optical properties. Weldability and performance of the welds in service have been important issues that have to be considered while developing new structural materials. Often, the welding processes employed for their traditional counterparts may not be suitable for these new alloys. A good example for this case is the new generation oxide dispersion strengthened (ODS) ferritic martensitic steels being developed for high temperature applications in nuclear (both fusion and fission) reactor and fossil power plants. In the case of conventional ferritic martensitic steels (Cr-Mo steels) weld metals produced by any of the fusion welding processes possess the desired properties. In contrast, one has to necessarily use solid state welding processes to join the ODS alloys to ensure that fused weld metal is avoided so that weld joints retain the properties of the base metal, inherently achieved through dispersion of Y 2 O 3 based nano particles in the steel. Similarly in materials like TRIP steels and maraging steels, improved properties are attained by controlled thermo- mechanical processes which produce the desired microstructure in the base metal. Such a microstructure is destroyed in the weld metal and the heat affected zone (HAZ) during welding and this has necessitated use

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Through Optimum Use and Innovation of Welding and Joining Technologies

Improving Global Quality of Life