The Effect of Service Aging on the Creep-Fatigue Properties of Alloy 617 Parent Metal and Welds

In order to improve power plant efficiency, a number of projects are currently underway across the world with the intention of increasing the operating temperature to 700 degrees C (1292 degrees F) and beyond. These temperatures are outside of the effective operating range for conventional ferritic and austenitic steel alloys, necessitating the need for nickel based alloys, for which experience is very limited for the thick section size and long service lives required. The current study examined the fatigue and creep-fatigue performance of Alloy 617, parent metal and welds, at 650 degrees C (1202 F). The testing was carried out on virgin material, service aged samples (up to 25,000 hours) and material over-aged at 800 degrees C (1472 degrees F) for 1,000 hours. The results show that, under the conditions chosen, the pure fatigue properties of Alloy 617 were only marginally reduced as a result of service aging. However, a much larger reduction in life was observed following service aging under creep-fatigue conditions. The creep-fatigue life of ex-service welds was reduced further, to less than 1/3 of the life seen by virgin parent metal. The limited data reviewed suggests that the reduction in life as a result of the introduction of a tensile hold period is more significant within Alloy 617 than Alloy 263, and less significant than seen within Alloy 740. This has been tentatively linked to the cyclic strength of the alloys investigated. The reduction in life observed within Alloy 617 is also significantly greater than that seen within conventional ferritic alloys.