The Effect of Carbon Content and Thermal Treatment on the SCC Behavior of Inconel Alloy 600 Steam Generator Tubing

An attempt has been made to improve the SCC performance of Inconel Alloy 600 steam generator tubing by modifying the grain boundary microstructure. Changes in the grain boundary microstructure were initiated either by a thermal treatment in the carbide precipitation range or a high temperature purification anneal in dry hydrogen. SCC performance was evaluated in 10% sodium hydroxide and lead doped water environments. Purified material had similar SCC resistance to the conventional mill annealed tubing, whereas the thermally treated tubing had significantly better SCC resistance in the caustic solution. Also, in the lead doped water environment, failure was predominantly intergranular in mill annealed and purified tubing and transgranular in thermally treated material. Since carbon was the principal element removed by the purification anneal, the purified tubing provided suitable material for assessing the effect of carbon content on stress corrosion performance and on response to thermal treatment.