N. G. Muralidharan

Study of Microstructure and Property Changes in Irradiated SS316 Wrapper of Fast Breeder Test Reactor

This paper presents the results of a study on mechanical properties and microstructure in a SS316 wrapper irradiated in a 40MWt/13MWe fast breeder test reactor at Kalpakkam, India. Transmission electron microscopy (TEM) examination and mechanical property evaluation were carried out on the hexagonal wrapper subjected to different displacement damages up to a maximum of 83 dpa at an operating temperature of about 673 K. The steel irradiated to 83 dpa showed an increase in the yield strength and ultimate tensile strength, with a reduction in uniform elongation to about 8 %–10 % from about 20 % in the unirradiated material. Density measurements on the specimen from different portions of the irradiated wrapper showed a peak volumetric swelling of about 3.5 % at a damage of 83 dpa. TEM studies showed extensive void formation at 40 dpa and beyond in addition to extensive precipitation and formation of dislocation loops. The void density and size showed a progressive increase with displacement damage. The precipitates were identified to be of nickel and silicon enriched M6C type of η phase, while radiation induced G phase was also observed at 83 dpa. The increase in strength and reduction in ductility with increase in dpa is attributed to irradiation hardening, which is supported by the increase in defect density as observed from the dislocation substructure and increase in the void density and size. The paper would discuss the degradation in mechanical property in terms of the microstructural changes.

Failure analysis of an ammonia refrigerant condenser tube

A detailed failure analysis was conducted on an ammonia refrigerant condenser tube component that failed catastrophically during its initial hours of operation. Evidence collected clearly demonstrated that the weld between a pipe and a dished end contained a sharp unfused region at its root (lack of penetration). Component failure had started from this weld defect. The hydrogen absorbed during welding facilitated crack initiation from this weld defect during storage of the component after welding. Poor weld toughness at the low operating temperature facilitated crack growth during startup, culminating in catastrophic failure as soon as the crack exceeded critical length.

Failure analysis of carbonate reboiler heat exchangers

Unacceptably high leakage rates from a large number of type 304 stainless steel tubes forced the scrapping of two carbonate reboiler heat exchangers from the carbon dioxide removal system of a fertilizer plant. Extensive failure investigation carried out on the failed heat exchangers established that the main factor responsible for premature failure of the heat heat exchangers was partial expansion of the stainless steel tubes into the carbon steel tube sheet as against the design specification for the expansion of almost the full length of the tubes inside the tube sheet. Partial expansion of the tubes into the tube sheet left undesirable tube-to-tube sheet gaps, which subsequently acted as crevices and led to localized corrosion of the tube sheet as well as the tubes.