【作者】 刘刚；

【导师】 王艳丽；

【作者基本信息】 北京科技大学 ， 材料科学与工程， 2018， 博士

【摘要】 双相不锈钢因铁素体相和奥氏体相的双相组织而具有很好的耐蚀性和优良的机械性能,如强度高,耐腐蚀性能和良好的可焊接性良好等。因此,双相不锈钢被广泛应用于压水核反应堆冷却水主管道等重要组件。然而,双相不锈钢在280 ℃至450 ℃温度范围长期服役容易发生热老化脆化现象。冷却水主管道一旦破裂将可能引起严重的核泄露事故,因此近年来核电双相不锈钢的热老化脆化一直是核材料领域的研究热点。本文针对国内压水堆核电站的主管道材料进行热老化研究。双相不锈钢材料经过400 ℃热老化10 000 h后,分析了其在室温拉伸、高温拉伸、夏比冲击、纳米压痕测试下的变形行为,并通过电子背散射衍射(EBSD)与透射电子显微镜(TEM)研究了热老化前后材料在不同变形区域铁素体相和奥氏体相的组织演变。在此基础上讨论了铁素体相和奥氏体相两相间微观应力/应变分布与裂纹萌生之间的关系,揭示了热老化双相不锈钢的形变机制与微观断裂机制。研究成果将有助于掌握核电双相不锈钢材料的失效规律,对我国核电站的国产化与安全运行都有重要意义。双相不锈钢在400 ℃热老化10 000小时后进行了室温下原位拉伸试验,研究长期热老化对其塑性变形机制和裂纹萌生的影响。热老化后,材料的抗拉强度增加而塑性明显降低,断裂模式由浅韧窝的韧性断裂转变为铁素体解理断裂和奥氏体相撕裂的混合模式。通过电子背散射衍射方法来研究变形后三种不同变形程度区域内奥氏体和铁素体晶粒的晶体取向变化。EBSD分析结果表明,多个高应变区出现在奥氏体晶界和铁素体/奥氏体相界,这些高应力集中的局部应变区域和裂纹萌生密切相关。长期热老化会影响裂纹萌生机制,解理裂纹萌生于热老化铁素体晶粒内。研究了热老化对铸造双相不锈钢的高温塑性变形行为和断裂机制的影响。长期热老化后,双相不锈钢的抗拉强度明显增加而屈服强度略有增加。微裂纹在铁素体内萌生后沿相界扩展,导致铁素体相在试样拉伸断裂之前而发生断裂。纳米压痕测试表明,热老化后铁素体相的纳米压痕硬度明显增加,但随着变形量的增加变化不明显。电子背散射衍射观察表明,铁素体/奥氏体相界处应力集中是相界产生裂纹的原因。铁素体相内调幅分解组织与G相对位错运动的阻碍是导致铁素体的塑性变形能力降低的主要原因。研究了热老化和相应相变对双相不锈钢的室温冲击变行行为和断裂机制的影响。长期热老化后,材料的夏比冲击功显著降低,裂纹更容易萌生和扩展,冲击断口表面变得平坦,这与铁素体相的热老化硬化有关。热老化后铁素体相塑性变形能力的退化导致冲击性能的降低,热老化材料中高应力集中区域主要出现在铁素体/奥氏体相相界和奥氏体晶界附近。由于铁素体晶粒的热老化硬化,裂纹萌生于铁素体晶粒内。通过对热老化处理后Z3CN20-09M双相不锈钢进行纳米压痕硬度和电子背散射衍射测试,研究了纳米压痕过程中的塑性变形行为,以及晶体取向对纳米压痕硬度和压痕模量的影响。纳米压痕硬度和压痕模量随取向因子的变化规律基本一致。热老化后,铁素体的显微硬度和压痕模量随晶体取向变化明显。对于铁素体相和奥氏体相,显微硬度和压痕模量的最大和最小值均出现在<111>晶粒取向附近和<001 >晶粒取向附近。透射电镜分析结果表明,热老化后铁素体晶粒的塑性变形影响区域有所减少,而调幅分解组织和析出相对位错运动的阻碍作用,是造成铁素体相的塑性变形能力退化的主要原因。更多还原

【Abstract】 Duplex stainless steels (DSSs) have high strength, corrosion resistance,and good weldability as their duplex structures (austenite and ferrite). They are widely used in major components such as the primary coolant piping of pressurized water nuclear reactors (PWRs). However, DSS are susceptible to thermal aging embrittlement after long-term service at temperatures ranging from 280 to 450 ℃.This embrittlement may cause the brittle fracture of the primary circuit pipes and even the serious nuclear leakage accidents. For these reasons, thermal aging embrittlement in DSS has been concerned for serveral decades.In the present work, the DSS materials from the domestic PWR nuclear power plants were analyzed after thermal aging at 400 ℃ for as long as 10,000 h in order to understand the deformation behavior of these steels. Microstructural evolution in ferrite and austenite in different deformation regions were observed by the electron backscattered diffraction (EBSD) technique and a transmission electron microscope (TEM). The effect of localized strain incompatibility and high stress concentration on the crack initiation were investigated in order to understand the plastic deformation behaviors and fracture mechanisms. These research results are useful to understand the failure mechanism of DSS, and also important to the localization and safe operation of the domestic nuclear power plants.In situ tensile tests at room temperature were conducted on the unaged and aged DSS to investigate both the plastic deformation mechanisms and the effect of long-term thermal aging on the crack initiation. After thermal aging, the ultimate tensile strength of DSS increase and the plasticity has a significant decrease. The fracture morphologies change from ductile fracture with shallow dimples to the mixture of cleavages in ferrite and tearing in austenite. The EBSD technique was used to determine the crystallographic orientations of austenite and ferrite grains on the three different deformation degree areas. The EBSD analysis results indicate that multiple high strain areas exist near the austenite grain boundaries and the ferrite/austenite phase boundaries. This localized strain incompatibility is considered to be related with high stress concentration and crack initiation.Long-term thermal aging affects the crack initiation mechanism and cleavage cracks initiate in the ferrite grains of the aged DSS.The plastic deformation behaviors and fracture mechanisms of the unaged and aged DSS at high temperature were also investigated. Comparing with that at room temperature, the tensile strength at high temperature has an obvious improvement, but the yield strength has a slightly increase after long-term thermal aging. Microcracks initiate in the ferrite and extend to the phase boundaries,leading to the fracture of ferrite before the tensile failure of the specimen. The nano-indentation test results indicate that hardness in ferrite continuously increases with aging time, but not strongly affected by the deformation degree.High stress concentration on the phase boundaries causes the phase boundary separating. The dislocations pile-up at spinodal decomposition precipitates and G-phase in ferrite phases results in the reduction of the micromechanical property of the ferrite.The effects of thermal aging and the associated phase transformations on the impact toughness of DSS were also investigated at room temperature. After long-term thermal aging, the impact energy decreases significantly and the cracks initiate and propagate more easily. The plastic deformation of ferrite decreases and the wavy profiles of impact fracture become flatter. The effect of thermal aging on deformation ability of ferrite leads to the degeneration of the impact property.High stress concentration areas are observed near the phase boundaries and the austenite grain boundaries in the aged materials. The cracks initiation at the phase boundaries have more opportunities to expand to the ferrite grains due to the hardening in the aged ferrite grains.The nanoindenter and the EBSD technique have been conducted to investigate both the plastic deformation behavior and the influence of the crystal orientation on the nanohardness H and the indentation modulus E. Both nanohardness H and indentation modulus E are correlated with the orientation factor averaged over three normal directions of the contact surface. After thermal aging, the dependence of hardness and indentation modulus on the crystallographic orientation obviously changes for the ferrite phase. For the ferrite and austenite phases, the maximum and minimum values of hardness H and indentation modulus E are observed in the near-<111> and near-<001>-oriented grains, respectively. The TEM results indicate that the area of plastic deformation decreases in the ferrite grain. The interactions of a’ phases and G-phases with the dislocations are considered to be responsible for the degradation of plastic deformation ability in ferrite.更多还原