Strength and toughness of 3·5Ni-Cr-Mo-V steam turbine and generator rotor forgings

Abstract

In order to guarantee the integrity of large monobloc 3·5Ni-Cr-Mo-V rotor forgings, a comprehensive knowledge of their toughness is required. Experimenta heat treatments have been used to define the inherent toughness behaviour of the steel, and a survey of production rotors has been made in order to define the variation of toughness with both position and direction. The lower bainite-martensite structure of the rim and the upper bainite of the core had identical strength and ductility tempering characteristics. The inherent toughness of the upper bainite was less than that of the lower bainite-martensite, the difference being equivalent to some 50°C fracture appearance transition temperature (FATT). However, the difference between the two was much reduced by temper embrittlement, since the rim martensite embrittled more than the core bainite. Toughness was insensitive to strength up to 800 N/mm² proof strength, the unembrittled toughness (FATT) being −100°C (rim) and −60°C (core) at this strength level. In production, however, the toughness of rotor cores can be lower than that which resulted from the simulation heat treatments, not only because of temper embrittlement, but also because the grain size can be larger, resulting in a toughness (FATT) drop, typically 10°–15°C/unit increase in ASTM grain size. A rim FATT of −60°C was typical for the complete range of generator and low-pressure rotors surveyed, the corresponding core FATT being O°C. More recent embrittlement-controlled rotors had improved toughness (rim −90°C, core −10°C), the greater improvement at the rim reflecting the higher sensitivity of martensite and lower bainite to temper embrittlement. The systematic variation in toughness with axial position or test direction was small (10°–15°C FATT). The random scatter between positions was much higher, however, reaching some 40°C FATT. Fracture-toughness K_IC measurements correlate well with the FATT, enabling the latter to be used not only for the present experimental work but also as a quantitative design and production control criterion.