Fatigue life reduction of steels in pressurized hydrogen as a consequence of changes in short crack propagation mechanisms

尽管有许多研究活动，但氢引起的材料损伤仍不是很清楚。一方面，氢致损伤过程受材料、载荷和环境等多种因素的影响。另一方面，氢脆的主要机制(Help Hede、AIDE、HESIV以及压力理论)可以同时发生，从而使不同的脆化机制在材料损伤演化的不同阶段起主导作用。此外，循环载荷下的氢脆还缺乏基础性的研究。在所提出的研究项目的框架内，将描述不同类型的钢在低周和高周疲劳状态下的氢致材料损伤机理，并以可用于模拟计算的基于微观组织的模型来表示。出于实际应用的需要，选择了两种不同应用领域、不同组织和吸氢敏感性的钢，即亚稳奥氏体钢X2CrNi19-11和软马氏体钢X3CrNiMo13-4。这些钢有一个广泛的数据库，描述了在氢气条件下不同参数对机械性能的影响。真空中的疲劳试验将作为在加压氢和氦中获得试验结果的参考。详细的金相和断口显微结构研究旨在显示氢对材料损伤过程(位错排列的变化、短裂纹扩展和相变)的影响。因此，在从微结构短裂纹形成到长裂纹扩展的整个疲劳寿命过程中，将监测损伤的起始位置和发生频率的演变。实验表征的氢致损伤机制将被转化为基于物理的模拟模型。通过将已识别的损伤机制纳入已有的基于边界元方法的短裂纹模型中，系统地描述了氢对微结构损伤演化的影响。通过这种方式，建模有助于更好地理解氢影响下的材料损伤。实验结果和模拟模型的开发都将为更好地利用暴露在氢中的部件提供良好的材料利用基础，并将有力地促进材料向抗氢微结构的无误调整。

Material damage resulting from hydrogen is still not clearly understood despite many research activities. On the one hand, the hydrogen-induced damage processes are influenced by many factors which are specific to material, loading and environment. On the other hand, the main mechanisms of hydrogen embrittlement (HELP HEDE, AIDE, HESIV as well as pressure theory) can occur in a combined manner, so that different embrittlement mechanisms are dominating in different stages of material damage evolution. Moreover, hydrogen embrittlement in the case of cyclic loading is still lacking fundamental investigations.Within the framework of the proposed research project, the mechanisms of hydrogen-induced material damage of different types of steel in the low and high cycle fatigue regime will be characterized and represented in a microstructure-based model usable for simulation calculations. For practical reasons, two steels from different application areas were selected with different microstructures and susceptibility to hydrogen absorption, i.e., the metastable austenitic steel X2CrNi19-11 and the soft-martensitic steel X3CrNiMo13-4. An extensive database for these steels is available, describing the effect of different parameters on the mechanical properties under hydrogen conditions. Fatigue tests in vacuum will serve as reference for the test results obtained in pressurized hydrogen and helium. Detailed metallographic and fractographic microstructural investigations are intended to show the effect of hydrogen on the material damage process (change in the dislocation arrangement, short crack growth and phase transformation). Thus, the damage evolution will be monitored regarding initiation site and frequency of occurrence of damage throughout the major part of fatigue life starting from the formation of microstructurally short cracks until the transition to long crack propagation.The experimentally characterized hydrogen-induced damage mechanisms will be transferred into a physically-based simulation model. By including the identified damage mechanisms into an already existing short crack model, which is based on the boundary element method, the effect of hydrogen on the microstructural damage evolution will be described systematically. In this way, the modeling contributes to the development of an improved understanding of material damage under the influence of hydrogen. The experimental results and the development of the simulation model will both provide a sound basis for a better material utilization in components being exposed to hydrogen and will strongly contribute to an unerring material adjustment towards hydrogen-resistant microstructures.