超高温气冷堆用Haynes230合金高温蠕变机制及显微组织演变

High-temperature creep mechanism and microstructure evolution of Haynes230 alloy for ultrahigh-temperature gas-cooled reactors

  • 摘要: 超高温气冷堆(HTGR)是第四代核能系统重点研发的堆型之一,堆芯出口温度达到800-950℃,使得中间换热器(IHX)工作温度显著提升。Haynes230合金是IHX管材极具竞争力的候选材料,为了使管材在高温高压工况下长期安全运行,对合金抗蠕变性能评估具有重要意义。以Haynes230合金为研究对象,开展了800~950℃高温蠕变试验、蠕变行为分析和显微组织表征。通过分析稳态蠕变速率和应力关系,获得了800~950℃下的蠕变指数n介于3~7之间,据此推断合金蠕变机制为位错滑移和攀移。基于稳态蠕变速率和蠕变断裂时间的关系,建立了Haynes230合金Monkan-Grant关系模型。由Haynes230合金蠕变损伤容限与蠕变断裂时间关系可知,蠕变过程中合金内部裂纹空洞长大导致有效面积减少而引起断裂。显微组织表征显示,固溶后Haynes230合金中主要析出相是富W的M6C,分布在晶界附近;蠕变过程中合金中析出颗粒状的M23C6和M6C;蠕变裂纹起源于晶界上粗大碳化物,沿垂直于受力方向扩展或沿条带碳化物扩展。透射电镜(TEM)分析显示,位错由晶内向晶界滑移,大量塞积在晶界上引起局部应力集中,萌发裂纹。

     

    Abstract: High-temperature gas-cooled reactor (HTGR) is one of the Generation IV nuclear energy systems, and its core outlet temperature is up to 800-950°C. The operating temperature of the intermediate heat exchanger (IHX), therefore, increases significantly. Haynes230 alloy is a very competitive candidate for IHX tubing, and it is important to evaluate the creep properties of Haynes230 alloy to maintain the long-term performance in high temperature and pressure. Creep tests from 800 to 950°C, data processing for creep behavior and microstructural analysis were performed. The Norton model of steady state creep stages at 800°C, 850°C, 900°C and 950°C was established to obtain the steady state creep rate and stress equation, and the creep exponent n was obtained, and the creep mechanism was inferred to be the dislocation slipping and climbing. The Monkan-Grant relation for Haynes 230 alloy was obtained by fitting the derivatives of the steady state creep rate and the time to creep rupture. Through the creep damage calculation, the creep damage tolerance and creep rupture time of Haynes230 alloy were plotted. It shows that the fracture was mainly caused by the growth of internal crack cavities which reduced the effective area and induced the fracture. SEM, EDS, EBSD and EPMA were used to analyze the microstructure and fracture mechanism of the alloy, and the results show that the precipitated phase of Haynes 230 alloy in solid solution condition was W-rich M6C in the vicinity of grain boundaries, and granular M23C6 and M6C re-precipitated at grain boundaries during creep. Creep cracks originated at coarse intergranular carbides, followed by expanding along a direction perpendicular to the creep stress or strip carbides. Dislocations slipped from the intracrystalline to the grain boundary, accumulated, induced a local stress concentration and microcracks.

     

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