Fundamental studies of environmentally assisted surface cracking in pipeline steels

在加拿大和世界范围内，当涉及到处理环境辅助开裂时，研究界存在许多困境，这是许多设施和部件失效的主要原因。为了了解破坏机制，已经进行了研究，要么使用具有剧烈机械载荷的光滑棒试件，如慢应变速率试验，要么使用代表长裂纹和贯穿厚度裂纹的紧致拉伸试件，这与现场服务中敏感结构中形成的裂纹非常不同。在后一种情况下，表面裂纹或指甲状的缺陷被启动和生长。在表面裂缝成长为贯穿厚度的裂缝之前，一些结构，例如石油和天然气输送的钢制管道，可能已经失去了其可用性。因此，研究表面裂纹的基本行为是非常必要的。******通过对现场服务中产生的裂纹的基本分析，已经确定：1)管道钢中的环境辅助表面裂纹首先沿着表面长度方向优先扩展，在现有裂纹附近启动新的小裂纹，但与现有裂纹无关，而不是通过裂纹尖端的直接扩展来延长现有裂纹的长度；2)裂纹深度的发育不符合环境条件，裂纹深度往往处于休眠状态；3)纵使深度尖端几何上处于休眠状态，但长度方向的发育增加了深度尖端的机械驱动力。本研究的目的是对表面裂纹沿其长度方向的扩展提供一个基本的理解，特别是通过在现有裂纹附近启动小裂纹，但独立于现有裂纹。根据腐蚀环境的类型和使用过程中遇到的可变压力波动，管道钢的屈服行为和/或其显微组织强烈地影响着表面裂纹在长度方向上的扩展，这些扩展优先通过恒定的小裂纹萌生。这些因素对表面裂纹扩展的影响是相互联系的。本建议将个别和协同处理上述因素，以提供对问题的基本了解。预计这些发现将对开发具有更好的抗环境辅助开裂性的新材料以及制定更好的管道钢操作策略至关重要，从而实现更高的操作完整性和管道安全性

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.**