Surface Behavior and Temperature of Plug in Piercing of Seamless Steel Pipe

Prediction of tool temperature is very important to improve its life, since tool life is strongly influenced by increase in temperature of tool surface in piercing and rolling process of seamless steel pipes. The temperature of plugs under some piercing conditions of cross roll and press roll piercers was calculated. Actual temperature of plugs were measured and compared with the calculated results to confirm the validity of calculation. In addition, thin layer of actual plug surface after piercing campaign was extracted by solvent method and structure of the thin layer was observed by using an electron microscope. It was clarified that temperature of plug surface becames over 1 200°C in piercing and that a metallic surface plugs were unsuitable for a piercer. Black scale film on plug surface showed a good protective effect against heat conduction. Temperature of a plug treated with scale film became from 700°C to 800°C in piercing. The black scale film on plug surface under piercing condition was enriched in SiO2 and it was presumed that the surface of plug was covered with a thin layer of melted scale, namely, scale lubricant. An amorphous thin layer was confirmed on the surface of a plug after piercing campaign through observation by electron microscope and electron diffraction patterns of the extracted thin layer.