Metallographic study of hongwu cannon casted in the early ming dynasty

The purpose of this study is to analyze the microstructure and chemical composition of the oldest heavy section cast iron cannon in China, which was manufactured at the early Ming dynasty in the year of 1377 with the inscription of Hongwu. It is worthwhile to study any characteristic casting processes and advancement of the heavy section casting technology during the Ming dynasty because the cannon technology could be developed based upon the leading technology of that age. This cannon has been preserved and displayed at Shanxi Province Art Museum. Some microstructural analysis was performed on site by use of portable grinding and polishing tools for the metallurgical examination. In addition, more detailed metallurgical examination by EDX, XPS, high temperature optical microscope (OM) and DSC analysis was performed on a small sample taken from the cannon. Chemical analysis was also performed on the grinding chips and powder collected during the metallurgical surface preparation. A thermodynamic phase diagram was calculated by CALPHAD method for the material with the same chemical composition as measured. After the detailed metallurgical examination and chemical composition analysis, it turned out that the cannon has a chemical composition range of minor elements as: C: 1.36-1.8 mass%, S: 0.58-0.78 mass%, P: 0.61-0.81 mass%, Si: 0.45-0.61 mass%, and shows a uniform hypereutectoid microstructure. Taking into account the evidence of no microstructural gradient from a surface to inside which is not common in ancient decarburized cast iron, the cannon is highly likely to be made by a cast steel rather than by a cast iron. Non-metallic inclusions are observed both by OM and by a scanning electron microscope (SEM) on the sample. A high temperature optical microscope with an in-situ heating capability was used to directly observe any phases with a lower melting temperature. Also DSC analysis was performed on the sample to examine any phase change at some specific temperature. Based upon these observations and analysis, it was concluded that the Hongwu cannon was made by cast steel with a hypereutectoid microstructure. This microstructure contained a eutectic phase in pro-eutectoid cementite, which suggests a formation of steadite Fe-Fe₃C-Fe₃P with a low melting temperature of 1227 K. This finding strongly suggests that, instead of cast iron technology, cast steel technology had already existed and was put in use for heavy-section cannon production during early Ming Dynasty in China.