This paper describes the developments that have taken place during the past 40 years in the manufacture of the large rotor forgings that are required for the turbine and generator sets in electric utility, central station applications. Modifications in steelmaking practices—such as the changes from open hearth to the electric practice, the introduction of vacuum pouring, and changes in ingot mold design—are discussed, as well as evaluations of newer remelt techniques such as vacuum consumable electrode melting, electroslag remelting, and electroslag hot topping. Changes in alloy content and heat-treating practices and the improvements that have resulted in the mechanical properties and fracture toughness are illustrated.
The progress that has been made in reducing the incidence of internal flaws and the improvements that have been made in nondestructive testing and evaluation to quantitatively appraise the effect of such flaws on the probable performance are discussed.
It is concluded that in low-pressure rotors, as a result of the improved cleanliness, detection methods, and fracture toughness, the probability of an undetected critical size defect has been decreased by several orders of magnitude. In the case of the high-temperature rotors, the improvements in properties, together with improved designs and operating procedures, have substantially improved the life expectancy of these rotors in spite of increases in forging size and unit ratings.
The important contributions that ASTM committees have made to this progress are described and acknowledged.