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Thermal cracks in cross-sectional transitions, stiffening ribs and casting defects increasingly come to light when HP and IP turbine casings and valve chests of steam turbines of fossil-fired thermal power stations built in the 1950's are inspected during overhauls. These cracks are primarily attrributable to the fact that the turbines, originally designed as base-load units, are being operated as medium-load and peak-load units. Continued safe operation of most casings can only be ensured if the cracks are repaired by welding. Experience has shown that non-cracked areas of the casings still feature material properties which come within the scatter bands of the new material. Good weldability has invariably been established. The geometry, dimensions and purpose of the castings and the location and size of the cracks dictate whether the weld repairs can be made in the power station or whether they must be carried out in the workshop. The best conditions for a good weld repair will be found in a steel foundry with post-weld heat treatment in a stress-relieving furnace. A further extension of the useful life span can be achieved by installing a preheating system for startup.