Fan Junkai, Chen Huitao, Zhao Wu, Yan Liang
School of Mechanical and Power Engineering, Henan Polytechnic University, Jiaozuo 454000, China.
Materials (Basel). 2018 Apr 27;11(5):690. doi: 10.3390/ma11050690.
A flake is a crack that is induced by trapped hydrogen within steel. To study its formation mechanism, previous studies mostly focused on the formation process and magnitude of hydrogen pressure in hydrogen traps such as cavities and cracks. However, according to recent studies, the hydrogen leads to the decline of the mechanical properties of steel, which is known as hydrogen embrittlement, is another reason for flake formation. In addition, the phenomenon of stress induced hydrogen uphill diffusion should not be neglected. All of the three behaviors are at work simultaneously. In order to further explore the formation mechanism of flakes in steel, the process of flake initiation and growth were studied with the following three coupling factors: trap hydrogen pressure, hydrogen embrittlement, and stress induced hydrogen re-distribution. The analysis model was established using the finite element method, and a crack whose radius is 0.5 mm was set in its center. The cohesive method and Bilinear Traction Separate Law (BTSL) were used to address the coupling effect. The results show that trap hydrogen pressure is the main driving force for flake formation. After the high hydrogen pressure was generated around the trap, a stress field formed. In addition, the trap is the center of stress concentration. Then, hydrogen is concentrated in a distribution around this trap, and most of the steel mechanical properties are reduced. The trap size is a key factor for defining the critical hydrogen content for flake formation and propagation. However, when the trap size exceeds the specified value, the critical hydrogen content does not change any more. As for the crack whose radius is 0.5 mm, the critical hydrogen content of Cr5VMo steel is 2.2 ppm, which is much closer to the maximum safe hydrogen concentration of 2.0 ppm used in China. The work presented in this article increases our understanding of flake formation and propagation mechanisms in steel.
白点是由钢中 trapped hydrogen 引起的裂纹。为研究其形成机制，以往研究大多集中在诸如空洞和裂纹等氢陷阱中氢压力的形成过程和大小。然而，根据最近的研究，氢导致钢力学性能下降，即所谓的氢脆，是白点形成的另一个原因。此外，应力诱导氢上坡扩散现象也不应被忽视。这三种行为同时起作用。为了进一步探究钢中白点的形成机制，通过陷阱氢压力、氢脆和应力诱导氢再分布这三个耦合因素研究了白点的萌生和扩展过程。采用有限元方法建立分析模型，并在其中心设置半径为0.5mm的裂纹。使用内聚方法和双线性牵引分离定律（BTSL）来处理耦合效应。结果表明，陷阱氢压力是白点形成的主要驱动力。在陷阱周围产生高氢压力后，形成了一个应力场。此外，陷阱是应力集中的中心。然后，氢在该陷阱周围呈集中分布，钢的大部分力学性能降低。陷阱尺寸是确定白点形成和扩展临界氢含量的关键因素。然而，当陷阱尺寸超过规定值时，临界氢含量不再变化。对于半径为0.5mm的裂纹，Cr5VMo钢的临界氢含量为2.2ppm，这与中国使用的最大安全氢浓度2.0ppm非常接近。本文所做的工作增进了我们对钢中白点形成和扩展机制的理解。
