Working within metallurgical limitations in the successful repair welding of power-plant components

"The need to make repairs is seldom a major consideration in the design, fabrication and installation of major components of electrical generation plants. Repair welding, however, is a fact of life in the electrical generation industry, for restoration of degradation in service, from cavitation of hydraulic turbine runners, to corrosion and fatigue damage in pressure vessels. In making weld repairs to carbon and low-alloy steels, the largest source of trouble is hydrogen embrittlement and related post-weld cracking. Various strategies have been developed to control problems from this source, including post-weld heat treatment, the use of nickel-base alloy filler metals and the temper bead welding technique. Each of these basic strategies is being applied as circumstances require; each strategy must, however, be implemented in a manner that reconciles weldment structure and hardness, degree of restraint and service environment with the consequences of unsatisfactory performance of the repaired component.IntroductionMajor working components of electric generating plants are specified and installed on the basis that, wherever technically and economically feasible, they will retain functional capacity without major repairs for the twenty-five-plus years of normal plant lifetime. Nevertheless, repairs are often required, and welding is usually an essential feature of the work. Remedial welding may be performed to correct design or fabrication errors which become evident only at the time of component installation. The most common causes of requirements for repair involve corrosion, fatigue or related processes which prove more aggressive than expected during service. Increasingly, our planning for future repair requirements is related to continual improvements which are occurring in nondestructive evaluation technology. As components are periodically reinspected during their service life, fabrication induced defects may be revealed for the first time quite late in the life of the component. When this occurs, repair must be considered wherever inaccessibility of a flaw makes it impossible to prove its inactivity. Finally, there are certain components, for instance in hydraulic turbines, which cannot be economically made or used without the expectation of service induced wear. Here, periodic maintenance welding must be regarded as a normal event."