某40Cr钢加力缸后盖轴在正常服役条件下发生断裂。通过对断口进行宏观观察、微观观察、非金属夹杂物评定、金相检验及硬度测试，分析了加力缸后盖轴断裂的原因。结果表明：该轴材料热处理工艺控制不当，导致材料表面组织不正常，减弱了材料的强度和硬度，条带状夹杂物严重超标并破坏了金属基体的连续性；两方面的综合因素导致加力缸后盖轴在交变应力作用下于轴的变径处出现疲劳裂纹，从而引起早期疲劳断裂失效。

The fracture failure happened to the 40Cr steel rear cover shaft of a reinforcing cylinder. The fracture causes of the rear cover shaft of the reinforcing cylinder were analyzed by macroscopic observation, microscopic observation, nonmetallic inclusion evaluation, metallographic inspection and hardness testing. The results show that the heat treatment technology of the shaft material was unreasonable, leading to the abnormal surface microstructure and the strength and hardness reduction of the shaft material; stripped nonmetallic inclusions were seriously overproof and they destroyed the continuity of the metal base; the above factors of two aspects resulted in the generation of fatigue cracks in the variable diameter area of the rear cover shaft of the reinforcing cylinder under the action of alternating stress, and then caused the early fracture failure.