IIW White Paper

9 Needs and challenges of major industry sectors for future applications

Welding Challenges CA-6NM/GX4CrNi13-4 castings are commonly used for turbine runners and wicket gates. It is a 13Cr-4Ni martensitic stainless steel developed in the early 1960s. An improved version “Super Martensitic S.S” is very popular these days in the off-shore oil & gas industry due to vastly improved resistance to Stress Corrosion Cracking (SCC). CA6-NM is a material of relatively high strength, and has a good cavitation resistance. Due to the relatively low chromium and nickel content, this material is subject to pitting in salt water or a similarly corrosive environment. While welding CA-6NM, relatively higher preheat and post weld heat treatment (stress relief) is required to prevent cracking when welded with matching martensitic welding consumables. Field repairs with austenitic weld materials are feasible with minimal preheat and no post weld heat treatment. The deposited weld material does not have the same strength as the CA-6NM base metal, however. Tragic failures in different engineering disciplines in history were one of the main driving forces for accurate investigations of failure mechanisms and the development of improved techniques. In the field of hydropower plants the recent case, Cleuson-Dixence, was a significant failure, which clearly showed the necessity of understanding the mechanisms and the consequences of not doing things correctly. To avoid such accidents in future, new materials and their processing have to be understood in advance. Therefore accurate scientific investigations have to be performed with respect to the basic understanding of the weldability of the materials selected and their service behaviour such as: Fracture mechanics and fatigue properties. Quantitative description of the relation between materials properties, namely strength, fracture toughness and fatigue and NDT procedures and the evaluation of its results. Influence of modern welding processes (including filler material) and parameters on the microstructure and their properties. Definition of significant parameters for the characterisation of All these efforts should result in modern standards and recommendations for the selected materials and welding processes which enable welding engineers to contribute to effective renewable energy production for a better future of the world. Future Developments Materials selection, welding and fabrication technology are very mature for hydroelectric turbines. As the demand increases for hydroelectric plants, turbines with higher capacity may be required in future. Development of steels with high strength and extremely good resistance to cavitations are of paramount importance, accordingly the welding challenges required to be dealt with. With the present day available resources such challenges are very often met by the Original Equipment Manufacturer (OEM) and the end users/utilities. 9.1.7 Hot topics Research and development of light weight high strength water resistant materials for turbine runners and associated welding processes and procedures. welding processes and their influence on possible failure mechanisms. Corrosion in weld metal and heat affected zone under service conditions. Reliable QA measures in production and during service.

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Through Optimum Use and Innovation of Welding and Joining Technologies

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