Effects of oxidation and surface roughening on drawing limit in dieless drawing process of SUS304 stainless steel microtubes

• Effect of surface roughening and oxidation on drawing limit for microtubes was clarified.
• Appropriate heating temperature with maximum drawing limit can be clarified.
• Drawing limit in air condition is lower than that in Ar gas at low temperature due to oxidization.
• Drawing limit under air condition at high temperature is higher than that under argon gas condition.
• Roles of oxidation and surface roughening are different in response to the heating temperature.

In this study, we investigate the effects of surface roughening and oxidation, which are dependent on the heating temperature, on the drawing limit in the dieless drawing process of SUS304 stainless steel microtubes. The microtubes have an outer diameter of 0.5 mm and a thickness of 0.13 mm. Dieless drawing apparatus with a chamber filled with argon gas to prevent oxidation is used in the experiments. It was found that the limiting reduction in area increases to a maximum value and then decreases with increasing heating temperature in both argon gas and air. An appropriate heating temperature range from 950 to 1100 °C with the maximum drawing limit is obtained from the dieless drawing experiment. In addition, the drawing limit in air is lower than that in argon gas at low temperatures of below 950 °C owing to oxidization. In contrast, the drawing limit in air at high temperatures of over 1100 °C is higher than that in argon gas, because the oxidation inhibits free surface roughening due to grain growth. Consequently, the drawing limit in dieless drawing strongly depends on the heating temperature. The roles of oxidation and surface roughening, which affect the drawing limit in the dieless drawing of microtubes, also vary widely depending on the heating temperature.