Abstract: Analysis on the failure reasons of cracks in high-strength martensitic 9Cr18 stainless steel internal gears through the methods such as chemical composition analysis, mechanical property testing, metallographic structure analysis, macroscopic inspection, and SEM morphology observation, the nature of the crack was finally determined and possible causes of cracking were analyzed.The analysis results show that the internal gears undergoes quenching and low-temperature tempering to achieve a hardness of HRC60±2. During the martensitic transfomation process,the structure stress is relatively high,and the material structure is in the state of tempered martensite+carbides.The presence of carbides greatly reduces the material’s ability to resist crack propagation, increases the brittleness of the material,and greatly increases the sensitivity of the part to stress. In addition,there are stress concentration areas in the parts themselves,and the above comprehensive factors have led to cracking of the internal gears. The improvement methods is to add small rounded corners at the sharp corners of the structure, and at the same time, there shoule be no burrs or protrusions on the surface of the transitionor sharp corners to reduce the risk of stress concentration.After heat treatment, magnetic particle inspection is used to detect surface cracks in the product.Finally,all these methodscan ensure the machining of the parts without cracksinside or outside.