K417G服役涡轮导向叶片的组织性能及热疲劳损伤机理分析

Investigation on Microstructure, Mechanical Properties and Thermal Fatigue Behaviors of K417G alloy used in Turbine Guide Vane

摘要:

涡轮导向叶片在服役过程中承受的热疲劳损伤是影响其使用寿命的主要因素。本文对服役后低压涡轮导向叶片的组织性能及热疲劳行为进行了系统研究,结果表明,K417G合金在服役后组织发生明显的弱化,γ′相的网状组织长大、粗化严重；高温拉伸与室温拉伸试验结果的对比表明,K417G合金在高温下力学性能出现大幅降低,这主要是由于合金内析出相之间的相界面和晶界在高温下成为合金的薄弱环节,易成为裂纹的起源位置,从而降低了强度,断裂方式也从室温下的韧性断裂逐渐转变为沿晶特征的脆性断裂；叶片在热疲劳应力作用下表面的涂层发生开裂、脱落,基体合金被氧化,氧化物在应力作用下开裂、脱落而形成疲劳裂纹源；热疲劳试验数据的拟合结果表明随着温度循环周次的增加,裂纹扩展速率呈减小的趋势,这是由于随循环周次的增加,二次裂纹出现并生长,释放了热应力,从而降低了裂纹扩展速率。

Abstract:

Low pressure turbine guide serves in extreme conditions. Repeated heating/cooling cycles have a significant impact on microstructure and properties of alloy used in turbine guide vane. In this paper, the microstructure, mechanical properties and thermal fatigue behaviors of the serviced K417G guide vane were systematically investigated. The results show that coarsening of γ′ phase for the serviced K417G alloy has significantly degraded the microstructure. The strengthening effect of γ′ phase in the serviced K417G alloy has significantly weakened. Due to low bonding strength of precipitate interfaces and grain boundaries at high temperature, the tensile strength and yield strength of the K417G guide vane have decreased with the increase of temperature. The thermal fatigue crack forming process of K417G alloy under thermal stress has been observed with time. With increase of thermal cycles, the thermal barrier on surface of the vane has fallen off due to thermal fatigue stress, which leads to oxidation of surface of the alloy. The thermal fatigue crack sources can be initiated in the place where the oxide particles fractured and fell off. The a~N and da/dN~N curves obtained from thermal fatigue tests indicate that the crack growth rate decreases with the increase of thermal cycles, which is resulted from the release of thermal stress due to presence of secondary cracks.