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Through Optimum Use and Innovation of Welding and Joining Technologies
Improving Global Quality of Life
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
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.
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.
9
Needs and challenges of major industry sectors for future applications
