采用电化学充氢及慢应变速率拉伸(SSRT)实验研究了真空渗碳热处理后20Cr2Ni4A齿轮钢的氢脆敏感性，并与常规淬火+回火处理(QT)的20Cr2Ni4A齿轮钢进行了对比。结果表明，渗碳试样渗碳层中的残余奥氏体含量(约13.8%，体积分数，下同)远高于渗碳试样心部和QT试样(约4.6%)，前者主要呈多尺度的块状分布在原奥氏体晶界及板条界处。渗碳试样与QT试样中的室温可扩散性H含量相当，但前者组织中较多的残余奥氏体和渗碳体含量使得其室温非扩散性H含量明显高于后者，H扩散系数明显低于后者。QT试样呈现出优异的强塑性配合，以相对断后伸长率损失表征的氢脆敏感性指数(HEI)为54.3%。与QT试样相比，渗碳试样的抗拉强度提高了34.6%，但塑性显著降低，断后伸长率及断面收缩率分别降低了66.5%和92.4%；充氢后在屈服之前就发生了脆性断裂，呈现出很高的氢脆敏感性，HEI高达90.9%。SSRT断口分析表明，充氢QT试样与最大H扩散距离大体相当的表层脆性区为沿晶+准解理的混合断裂，而充氢渗碳试样则在距表面一定距离的渗碳层内呈现一定宽度的沿晶断裂脆性区，且在接近有效渗碳层深度处出现了一条大体沿渗碳层圆周方向扩展的长裂纹。造成渗碳试样与QT试样氢脆敏感性显著差异和独特氢脆断裂特征的主要原因与2者的微观组织、强度水平及渗层残余压应力等因素有关。

Carburized gear steel has a high-hardness case layer with excellent wear and fatigue resistance and a low-hardness core with high toughness. Such different microstructures imply different susceptibilities to hydrogen embrittlement (HE). However, a few or no studies have explored the HE behavior of carburized gear steel. Herein, the HE behavior of a vacuum-carburized gear steel 20Cr2Ni4A was investigated via an electrochemical hydrogen-charging and slow strain rate tensile test. For comparison, another group of specimens was prepared by a conventional quenched and tempered (QT) treatment. The volume fraction of retained austenite was significantly higher in the case layer of the carburized specimen (13.8%) than in the core and the QT specimen (4.6%). The retained austenite in the case layer showed a mainly irregular block-type morphology with wide size distribution. The room-temperature diffusible hydrogen content in the hydrogen-charged carburized specimen were almost identical to the QT specimen but the nondiffusible hydrogen content was significantly higher in the former than in the latter. Meanwhile, the hydrogen diffusion coefficient was notably lower in the hydrogen-charged carburized specimen than that in the QT sepcimen because the former retained higher fractions of austenite and cementite. The QT specimen exhibited superior strength and ductility. After hydrogen charging, the strength of the QT specimen remained almost unchanged but the total elongation notably decreased, causing the HE index (HEI), as evidenced using the relative total elongation loss, being 54.3%. Relative to the QT specimen, the carburized specimen achieved a higher tensile strength (increase by 34.6%) but a much lower ductility (total elongation and reduction of area reductions by 66.5% and 92.4%, respectively). The carburized specimen underwent premature brittle fracture before yielding, indicating susceptibility to HE. In fact, the HEI was as high as 90.9%. Mixed intergranular and quasi-cleavage fractures were observed in the surface embrittled region of the hydrogen-charged QT specimen. This region roughly corresponded to the maximum hydrogen diffusion distance. Meanwhile, the hydrogen-charged carburized specimen exhibited an embrittled internal-surface region with a certain width of intergranular fracture, and a long crack had propagated along the circumferential direction near the effective case depth. The microstructure, strength level, and residual stress are thought to mainly explain the abovementioned differences between the carburized and QT specimens.