Prevention of crack initiation in valve bodies under thermal shock

Conference · Sun Dec 01 04:00:00 EST 1996

OSTI ID:401971

Delmas, J; Coppolani, P

On site and testing experience has shown that cracking in valves affects mainly the stellite hardfacing on seats and discs but may also be a concern for valve bodies. Metallurgical investigations conducted by EDF laboratories on many damaged valves have shown that most of the damage had either a chemical, manufacturing, or operating origin with a strong correlation between the origins and the type of damage. The chemical defects were either excess ferritic dilution of stellite or excess carburizing. Excess carburizing leads to a too brittle hardfacing which cracks under excessive stresses induced on the seating surfaces, via the stem, by too high operating thrusts. The same conditions can also induce cracks of the seats in the presence, in the hardfacing, of hidden defects generated during the welding process. Reduction of the number of defects results first from controls during manufacturing, mainly in the thickness of stellite. On the other hand, maintenance must be fitted to the type of defect. In-situ lapping may lead to release of cobalt, resulting in contamination of the circuit. Furthermore, it is ineffectual in the case of a crack through the seating surface, as is often found on globe valves. The use of new technologies of valves with removable seats and cobalt-free alloys solves permanently this kind of problem.

• Research Organization: American Society of Mechanical Engineers, New York, NY (United States)
• OSTI ID: 401971
• Report Number(s): NUREG/CP--0152; CONF-9607103--; ON: TI96013645
• Country of Publication: United States
• Language: English

Related Subjects

22 GENERAL STUDIES OF NUCLEAR REACTORS
CRACK PROPAGATION
FAILURES
FRACTURE PROPERTIES
NUCLEAR POWER PLANTS
STELLITE
THERMAL SHOCK
VALVES