采用宏观观察、化学成分分析、金相检验、扫描电镜和能谱分析、显微硬度测试等方法分析了40Cr钢油缸断裂的原因。结果表明：油缸断裂主要与硫元素含量超标、硫化物聚集、表面凹坑缺陷及热处理不均匀有关；硫元素易产生“热脆”，聚集的条状夹杂物破坏了基体的连续性，使油缸产生微裂纹，在表面凹坑处产生应力集中，热处理不均匀使组织产生内应力，萌生的裂纹及表面凹坑在以上因素的作用下不断扩展，最终导致油缸断裂。

The fracture reasons of 40Cr steel cylinder were analyzed by macroscopic observation, chemical composition analysis, metallographic examination, scanning electron microscope and energy spectrum analysis, microhardness test. The results show that cylinder fracture was mainly related to excessive sulfur content, sulfide aggregation, surface pit defects and uneven heat treatment. Sulfur element was easy to produce "hot brittle", aggregated strip inclusions destroyed the continuity of the matrix, so that the cylinder produced micro-cracks, stress concentration was produced at the surface pits, internal stress was produced by uneven heat treatment. Initiated cracks and surface pits continued to expand under the action of the above factors, eventually led to cylinder fracture.