高强度管线钢带状组织中应力腐蚀裂纹萌生与扩展微观机理的实验研究

Banded structure is a common defect in high strength pipeline steel, which is closely related to the stress corrosion cracking behavior of steel. At present, few studies have taken into account the coupling effect of banded microstructure change process, micro deformation process and micro electrochemical process. Therefore, it is difficult to reveal the mechanism for the influence of banded structure on the stress corrosion cracking behavior of high strength pipe line steel. This project first develops a stress corrosion cracking image test system based on the three-dimensional micro-scale digital image correlation method. The influence of corrosive solution and corrosion products on the accuracy of the digital image correlation will be reduced. During the process of stress corrosion cracking, micro-region deformation field test, microstructure change and micro-crack initiation and propagation monitoring can be realized by the developed system. Then, combined with the scanning vibrating electrode technique, the effect of the type, grade and orientation of banded structure on the initiation and propagation of stress corrosion crack under near-neutral pH and high pH in high-strength pipeline steel will be clarified, and the deformation condition and electrochemical condition for the initiation and propagation of stress corrosion crack in banded structure will be investigated. This project is expected to reveal the micro-mechanism of initiation and propagation of stress corrosion crack in banded structure of high strength pipeline steel, which is of great significance to scientifically control microstructure of high strength pipeline steel and effectively prevent stress corrosion cracking.

带状组织是与高强度管线钢应力腐蚀开裂行为密切相关的一种常见缺陷组织。目前研究很少考虑带状组织微区微观结构变化过程、微区变形过程、微区电化学过程三者的耦合作用，因而无法全面揭示带状组织对高强度管线钢应力腐蚀开裂行为的影响机理。本项目首先基于三维微观数字图像相关方法开发一套应力腐蚀开裂图像测量系统，降低腐蚀溶液及腐蚀产物对数字图像相关方法精度的影响，实现带状组织在应力腐蚀开裂过程中的微区变形场测试、微观结构形貌变化及微裂纹萌生扩展的监测。然后将该系统与扫描振动电极技术相结合，明确高强度管线钢中带状组织的类型、级别、取向对近中性pH和高pH两种腐蚀环境下应力腐蚀裂纹萌生扩展规律的影响，探究应力腐蚀裂纹在带状组织中萌生扩展所需的变形条件及电化学条件。本项目预期较为全面地揭示应力腐蚀裂纹在高强度管线钢带状组织中萌生扩展的微观机理，对科学控制高强度管线钢微观结构，有效防止应力腐蚀开裂具有重要意义。

带状组织是与高强度管线钢应力腐蚀开裂行为密切相关的一种常见缺陷组织。本项目首先利用多尺度数字图像相关(Digital image correlation, DIC)探究了非腐蚀环境下带状组织的微观变形行为及其对高强度管线钢力学性能的影响，结果表明：单轴拉伸试验中带状组织将引起X80管线钢拉伸性能的各向异性，而在单边缺口拉伸试验中，并未发现试样取向与拉伸曲线的明显关联；在拉伸曲线弹性阶段，试样表面微区变形场往往存在与带状组织形貌相对应的带状分布规律，而随着塑性变形程度的增加，变形场的带状分布规律逐渐消失。接下来，开发了一套应力腐蚀开裂图像测量系统，主要工作包括：提出了二次相减法的图像预处理方法，减少了实验中气泡对DIC变形测量精度的影响；提出了二次折射模型，降低了气-固-液折射率不匹配引起的DIC变形测量误差。最后，结合扫描振动电极技术，在近中性pH和高pH两种腐蚀环境下，进行了含带状组织X80管线钢的应力腐蚀开裂实验，结果表明：珠光体条带相对于铁素体条带不易发生腐蚀；高pH环境中，X80管线钢的腐蚀活性随着钝化膜的生成降低，不易产生裂纹；近中性pH环境中，X80管线钢试样表面易形成腐蚀产物，裂纹通常萌生于腐蚀坑边缘并平直扩展。本项目的研究成果为金属材料的应力腐蚀机理研究提供了重要实验手段，同时也阐明了应力腐蚀裂纹在高强度管线钢带状组织中的萌生扩展规律，对科学控制高强度管线钢微观结构、有效防止应力腐蚀开裂具有重要意义。