Analysis of Causes for Crack of DZ55 Cold-drawn Steel Tube and Related Preventive Measures

Causes for the transverse cracks of the DZ55 cold-drawn pipe as occurrred during the drawing process are analyzed by means of metallographic examination, hardness testing, scanning electron microscopy, and energy spectrum analysis as well as heat treatment tests.Accordingly preventive measures are proposed.The analysis result identifies that owing to element segregation, it becomes easier for the martensite matrix with plasticity lower than that of the base metal to produce in the Mn-aggregation area that close to the inner surface, which leads to reduction of the plasticity of the material, and thus causes the development of the said crack during the cold drawing process. Therefore taking low temperature casting of the billet to expand the central equiaxed zone of the billet can reduce or even eliminate the segregation so as to reduce development of the inner crack.Moreover adopting the heat treatment process of hi-temperature tempering+slow-cooling can decrease strength of the DZ55 steel, and eliminate the martensite band so as to enhance plasticity of the material, which is helpful for the cold-work process as follows.