微区氢浓度与敏感组织耦合作用下X80钢焊接HAZ的开裂机理

Welding heat affected zone (HAZ) of hydrogen gas transmission pipeline is prone to hydrogen enrichment and hydrogen embrittlement, which is a serious threat to the safety of pipeline. Reported studies focused on the effect of hydrogen concentration on the sensitivity of material to hydrogen embrittlement, while we found that the co-effect of microzone hydrogen concentration and microstructure is the key factor inducing the failure of pipeline. In present study, intercritically reheated coarse grained heat affected zone (ICCGHAZ) will be investigated. Secondary ion mass spectroscopy (SIMS) will be used to study the hydrogen distribution in different microstructures and grain boundaries, combined with high-pressure hydrogen permeation tests, the hydrogen permeation parameters for different microstructures and grain boundaries can be calculated. According to these results, mathematical model for hydrogen permeation will be established to reveal the mechanism of hydrogen distribution in ICCGHAZ specimen. Slow strain rate tension (SSRT) tests at different charging pressure will be performed on ICCGHAZ specimens, hydrogen embrittlement index at different charging pressure for different microstructures would be then calculated considering the effect of microstructure type and its grain size and fraction. Combined with hydrogen permeation model, the relationship between hydrogen embrittlement index and hydrogen concentration can be determined for each microstructure. Thus, microstructure, which is sensitive to hydrogen embrittlement will be determined. At last, electron backscattering diffraction (EBSD) and environment transmission electron microscope (TEM) are used to analyse the crack initiation and propagation behavior in ICCGHAZ, combined with microzone hydrogen concentration and microstructure analysis, the cracking mechanism of HAZ will be determined. This research will help provide theoretical basis for safety control of hydrogen transmission pipelines at high pressure.

临氢管线焊接热影响区组织极不均匀，极易发生氢富集和氢致脆化，严重威胁管线服役安全。以往研究集中于平均氢浓度下材料整体的氢脆敏感性，而申请人发现氢在材料中的微区分布与敏感组织的耦合作用是导致整体结构脆性断裂的关键。本项目以临氢管线X80钢焊接接头中的临界粗晶区为研究对象，采用二次离子质谱研究充氢后试样各组织及晶界中的相对氢含量，结合高压氢渗透试验，计算各种组织及晶界的氢扩散参数，建立氢扩散数学模型，解析组织特征影响微区氢分布的机理。进行不同氢压下的慢拉伸试验，考虑组织类型及其晶粒尺寸和体积分数的影响，计算不同氢压下各种组织的氢脆系数，结合氢扩散模型，定量表征各种组织氢脆系数随微区氢浓度变化的规律，确定敏感组织特征。采用电子背散射衍射和环境透射电镜分析试样中的裂纹萌生和扩展行为，结合微区氢浓度和敏感组织特征，阐明两者耦合作用下热影响区局部开裂的机理。研究成果可为临氢管线安全控制提供理论依据。

临氢管线焊接热影响区组织极不均匀，极易发生局部氢富集和氢致脆化，严重威胁管线服役安全。氢在热影响区的微区分布与敏感组织的耦合作用是导致整体结构脆性断裂的关键。本项目以临氢管线X80钢焊接接头粗晶区为研究对象，采用调控焊接热循环参数的方法制备典型热影响区试样，通过高压气相氢渗透试验获得热模拟试样的表观氢扩散参数，结合微观组织特征分析，解析微观组织与氢扩散参数之间的内在关系，建立氢扩散理论模型，确定晶界（GBs）、粒状贝氏体（GB）、贝氏体铁素体（BF）和马氏体-奥氏体组元（M-A）的氢扩散参数。此外，提出氢脆影响因子的概念，结合高压慢应变速率拉伸试验研究各组织对热影响区整体氢脆程度的影响。结果发现，晶界的氢扩散系数最低，具有强大的氢捕获能力，但在12MPa的氢气压力范围以内，晶界对试样整体氢脆的影响因子为负值，说明其具有良好的抗氢脆失效能力。粒状贝氏体和贝氏体铁素体的氢渗透参数相差不大，3MPa～12MPa的氢气压力范围内，二者的氢脆影响因子处于0.30～0.55之间。与贝氏体组织相比，M-A组元的氢捕获能力更强，同等充氢压力下具有更高的氢浓度和氢脆敏感性，在3MPa充氢压力下，氢脆影响因子达到了2.191。结合断口显微组织分析证实，裂纹主要沿M-A组元与周边贝氏体的边界扩展，这主要是由于M-A组元的氢捕获能力较强，使得M-A组元与邻近的粒状贝氏体或贝氏体铁素体界面产生氢富集，造成局部氢致脆化，但是由于M-A组元的脆硬程度高，使裂纹主要沿强度相对较低的粒状贝氏体和贝氏体铁素体扩展。本项目的研究明确了高压临氢管线热影响区的氢富集和氢致脆化机制，为管线钢焊接工艺优化提供了理论支持，有利于促进管道完整性管理措施的完善。