Fundamental studies of environmentally assisted surface cracking in pipeline steels

在加拿大和世界范围内，研究界在处理环境辅助裂化问题时存在许多困境，这是许多设施和部件故障的主要原因。已经进行了调查，以了解故障机制，无论是使用光滑的酒吧标本与积极的机械负荷，如慢应变速率测试，或使用紧凑的拉伸试样，代表长和贯穿厚度的裂纹的增长行为，这是非常不同的裂纹在敏感的结构在现场服务。在后一种情况下，表面裂纹或拇指甲状缺陷开始并生长。在表面裂纹扩展为穿透裂纹之前，某些结构，例如用于石油和天然气输送的钢管道，可能已经失去其使用性能。因此，研究表面裂纹的基本行为是十分必要的。** 通过对现场运行产生的裂纹的基本分析，已经确定：1）管线钢中的环境辅助表面裂纹通过在现有裂纹附近但独立于现有裂纹的新的小裂纹而不是通过在其裂纹尖端直接生长来延长现有裂纹的长度，从而沿着其表面长度方向优先生长; 2）裂纹深度通常保持静止，因为其生长对环境不利，3）然而，在长度方向上的生长增加了深度尖端处的机械驱动力，即使深度尖端在几何上保持静止。这项研究的目的是提供一个基本的了解表面裂纹的增长沿着其长度方向，特别是通过启动小裂纹附近，但独立于现有的裂纹。它已被分析，在其长度方向上的表面裂纹的生长优先通过恒定的小裂纹萌生强烈影响的屈服行为的管线钢和/或其微观结构，这取决于类型的腐蚀环境和可变的压力波动期间遇到的服务。这些因素对表面裂纹扩展的影响是相互关联的。本建议将单独和协同地处理上述因素，以提供对问题的基本理解。预计这些发现对于开发具有更好的抗环境开裂能力的新材料以及制定更好的管线钢操作策略，实现更高的操作完整性和管道安全性至关重要。

There exist many dilemmas in the research community when it comes to dealing with environmentally assisted cracking, a major cause of failures of many facilities and components, in Canada and worldwide. Investigations to understand failure mechanisms have been conducted, either using smooth bar specimens with aggressive mechanical loading, such as slow strain rate testing, or using compact tension specimens, representing the growth behavior of long and through-thickness cracks, which are very different from the cracks formed in susceptible structures during field service. In the latter case, surface cracks or thumbnail-shaped flaws are initiated and grown. Before the surface cracks are growing into a through-thickness crack, some structures, for example, steel pipelines for oil and gas transmission, may have already lost their serviceability. Therefore, it is very desirable to study the fundamental behavior of surface cracks. ******From a fundamental analysis of cracks developed from field service, it has been identified that 1) the environmentally assisted surface cracks in pipeline steels started with preferential growth along their surface length direction by initiating new small cracks in the vicinity of but independent from an existing crack, rather than by extending the length of the existing crack through a direct growth at their crack tips; 2) the crack depth often remains dormant because its growth is not environmentally favorable, 3) however, the growth in length direction increases the mechanical driving forces at the depth tip even though the depth tip remains dormant geometrically. This research is targeted to provide a fundamental understanding of the growth of surface cracks along their length direction, especially by initiating small cracks in the vicinity of but independent from existing cracks. It has been analyzed that the growth of surface cracks in their length direction preferentially through constant small crack initiations is affected strongly by the yielding behavior of pipeline steels and/or their microstructures, depending on the type of corrosion environments and variable pressure fluctuations encountered during service. All these factors are mutually related to each other in terms of their effect on surface crack growth. This proposal will address the above factors individually and synergistically to provide a fundamental understanding of the problem. It is expected that the findings will be critical in the development of new materials with improved resistance to environmentally assisted cracking and in the formulation of better operating strategies for pipeline steels, achieving higher operation integrity and pipeline safety.**