利用直读光谱仪、红外碳硫仪、光学显微镜和硬度计，对40Cr钢飞轮进行了化学成分分析、宏观低倍和高倍显微组织观察、硬度试验等检测，并结合飞轮的制造加工工艺过程对飞轮开裂的原因进行了分析。分析结果表明：飞轮调质硬度和轴颈淬火硬度均不符合要求，飞轮宽轴颈表面淬火不均，部分区段未进行表淬，在未淬区与淬火层界面形成应力集中，同时飞轮材料中存在大尺寸超宽硫化物夹杂，导致飞轮开裂。

The chemical composition, macroscopical low magnification observation, high magnification microstructure observation and hardness test of 40Cr steel flywheel were carried out by means of direct reading spectrometer, infrared carbon sulfur analyzer, optical microscope, and hardness tester. Combined with the manufacturing technology of flywheel, the causes of cracking were analyzed. The results showed that the quenching and tempering hardness of flywheel and the quenching hardness of shaft neck could not meet the requirements. The surface quenching hardness of the wide shaft neck of flywheel was not uniform, and local areas were unquenched. There was stress concentration between this unquenched area and the quenching layer. Moreover, some sulfide inclusions with large size and width existed in the flywheel material, which caused the cracking of flywheel.