TRANSITION-METAL JOINTS FOR STEAM-GENERATORS - AN OVERVIEW

The transition metal joint (TMJ) between an austenitic stainless steel and a chromium-molybdenum (Cr-Mo) ferritic steel used widely in steam generators of power plants has for a long time presented problems rel ating to premature failures in service. The direct (bimetallic) TMJ pr esently in use is designed for a service life of about 200,000 h; but such TMJs with iron-base weld metals have been failing in service with in about one-third of their design lifetime, while their counterparts with nickel-base weld metals fail within about one-half of their desig n lifetime. The causes for such premature service failures of these TM Js are discussed in detail, leading to the development of improved TMJ s. One of the improved TMJs with a trimetallic configuration of austen itic stainless steel/Alloy 800/Cr-Mo ferritic steel is discussed in de tail, covering its development, characterisation and evaluation. Accel erated performance tests in the laboratory have indicated a four-fold improvement in the service life of the TMJ with this trimetallic confi guration compared to the bimetallic configuration. The metallurgical d etails of these studies are also discussed in this paper.