EFFECTS OF WELDING THERMAL CYCLE AND AGING TREATMENT ON THE MICROSTRUCTURE AND MECHANICAL PROPERTY OF A Ni-Fe BASE SUPERALLOY

Abstract

Increasing the steam temperature and pressure of boilers in super-ultracritical power plant is an important approach to increase the plant efficiency. The steam temperature of the most efficient coal power plant is now around 620 ℃, representing an increase of about 80 ℃ in the past 40 years, which owes to the high temperature properties improvement of boiler components, such as the superheater and the reheater. Nickel base superalloy, for example Inconel 740 and Inconel 617, is being developed by some countries for the material requirement of 700 ℃ super-ultracritical power plants. Meanwhile, weldability investigation is necessary for the developing materials since welding plays a key role on the construction of coal power plant boilers. In this work, the weldability of a kind of Ni-Fe base superalloy, one of the candidate materials for the high temperature components of 700 ℃ ultrasupercritical coal plant is studied. By welding thermal simulator(Gleeble 1500) experiments, the variation and evolution of mechanical properties and microstructure were analyzed for this Ni- Fe base superalloy, under welding thermal cycle treatment condition and aging treatment condition after welding thermal cycle. After the welding thermal cycle with a peak temperature of 1249 ℃, both the yield strength and tensile strength for solutioned Ni-Fe base superalloy at 25 and 700 ℃ were decreased, along with the increasing of ductility. After aging treatment to the NiFe base superalloy experienced a welding thermal cycle, the yield strength and tensile strength at 25 ℃ were similar with those of the aged base metal. At 700 ℃, the strength of the heat affected zone(HAZ) after aging treatment is higher than that of the aged Ni-Fe base superalloy. Microstructure analysis showed that the γ' phase and MC carbide in Ni-Fe base superalloy dissolved during the high temperature welding thermal simulation experimental process. The solution of carbides in the grain boundaries caused a loss of a pinning effect on the migration of grain boundary and a decreasing of the strength. After the aging treatment to the Ni-Fe base superalloy experienced a high temperature welding thermal cycle, γ' and M23C6carbide were precipitated. The precipitation of M23C6at the grain boundaries during aging treatment was mainly due to the supply of the carbon from the MC which had been dissolved partially during former welding thermal cycle.