Abstract: Steering knuckles are one important component of steering units in vehicles. They can ensure vehicles run and turn around smoothly. After a vehicle ran about 10 000 kilometers, its steering knuckle fractured. In order to determine its failure mode and cause, macro and micro observation, microstructure examination, hardness testing, chemical composition analysis and hydrogen content testing were carried out. The results show that the failure mode of the steering knuckle is intergranular fracture due to hydrogen embrittlement. The fracture of the steering knuckle has little to do with the hydrogen content, and it was mainly caused by the higher hardness and larger thickness of the quenched layer. The tested hardness of the quenched layer is about HRC 57.0, significantly higher than the required value(HRC 45~52), and the thickness of the quenched layer is also obviously larger than the required value(2~3 mm). The higher hardness and larger thickness of the quenched layer increased the sensitivity to hydrogen embrittlement, resulting in the final hydrogen embrittlement fracture. It is proposed that the quenching process should be adjusted to prevent such failure.