焊接热循环及时效处理对一种Ni-Fe基高温合金的组织和力学性能的影响

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

采用焊接热模拟实验,研究了焊接热循环对一种700℃超超临界火电机组高温部件候选材料—Ni-Fe基高温合金组织和力学性能的影响.结果表明,固溶态Ni-Fe基高温合金在经过峰值温度为1249℃的焊接热循环后,25和700℃屈服强度和抗拉强度都降低,延伸率升高.对经过焊接热循环后的合金再进行时效处理发现,在25℃时,焊接热循环后再时效样品的屈服强度和抗拉强度与母材时效态相当;在700℃时,焊接热循环后再时效样品的强度高于母材时效态的强度.Ni-Fe基高温合金在高温焊接热循环过程中,强化相γ'以及难溶相MC发生溶解,导致强度下降.经过时效处理后,强化相γ'再次大量析出,同时晶界析出了M23C6.M23C6存在于晶界,并没有造成拉伸性能的弱化.高温焊接热循环使MC发生部分溶解,为M23C6的时效析出提供了C元素.

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 phmts. 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 Ni- Fe 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 y＇ 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 temperaatre welding thermal cycle, y＇ and M23C6 carbide were precipitated. The precipitation of M233C6 at 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.