Abstract: The macroscopic observation and microstructure analysis of the cracked middle cylinder were conducted by metallographic microscope,scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). The cracking cause were comprehensively analyzed based on the results of chemical composition analysis and hardness testing. The results showed that the chemical composition of middle cylinder met the composition requirements of 30CrMnSiA steel specified in GB/T 3077-2015 standard. From macroscopic and microscopic morphological analysis, it was evident that the fracture originated from the base material at the fusion zone of the molten copper in the sample, indicating brittle cracking. According to the results of metallographic examination and energy dispersive spectroscopy, there were local cracks in the fracture source area, and the main component of the filler inside the cracks was copper, which was consistent with the molten copper layer and should have been generated during the molten copper welding process, namely copper penetration cracks. From the hardness test results, it was indicated that the martensite near the fusion line had a significantly higher hardness than that of the normal position. Its presence could significantly reduce the overall toughness of the material and increase the risk of fracture. Based on the above analysis, the failure of the middle cylinder was mainly caused by the infiltration cracks and hot cracks during the copper melting process. During the cooling process of the weld seam, the formation and propagation of cracks would be aggravated by the formation of structural stress due to phase transformation. It was recommended to preheat the parts of steel to be welded during copper melting, prevent excessive welding heat input, and shorten the time of molten copper to wet austenite grain boundaries, which would play a positive role in avoiding the production of cracks in the middle cylinder.