As early as 1925, it was already known that steel could be deformed through extrusion, similar to non-ferrous metals. Due to the low cost of equipment and tools, as well as the wide variety of product sections available, the hot-forming method for steel gained increasing attention. In 1941, SejournetJ, in collaboration with Renn Electric Furnace Corporation, successfully tested glass lubricants, making hot extrusion of steel feasible and quickly spreading its application worldwide.
Traditionally, graphite—often a mixture of flake graphite with engine oil, tar, or grease—has been used as a lubricant in steel extrusion. However, this approach comes with several disadvantages: first, graphite has high thermal conductivity, leading to rapid cooling of the tool and increased wear, which makes it difficult to extrude long products. Second, it can cause carbon enrichment in the final product, especially when extruding stainless steel, which may lead to intergranular corrosion. As a result, the carburized layer must be removed post-extrusion, increasing metal consumption. Third, graphite lubricants often lead to environmental pollution due to their composition and handling.
In comparison, glass lubricants offer significant advantages. First, they have low thermal conductivity, which helps prevent the billet from cooling too quickly and also protects the mold. Second, they provide excellent lubrication, allowing for higher extrusion ratios and faster extrusion speeds, thereby expanding the range of products that can be manufactured. Third, glass lubricants are chemically stable and do not introduce defects into the material. Fourth, their properties such as softening point and viscosity can be easily adjusted by changing the composition, making them adaptable to different materials. Lastly, using glass lubricants keeps the process clean and reduces contamination.
Overall, the benefits of glass lubricants are substantial. Numerous studies from abroad confirm that glass lubricants are the most effective for steel extrusion.
When extruding seamless steel tubes, three key parts require lubrication: the extrusion die, the extrusion cylinder, and the mandrel.
(1) Lubrication of Extrusion Dies: A common method involves pressing glass powder into a glass mat, which is placed between the mold and the billet before extrusion. To ensure continuous softening on the friction surface, the shape of the glass mat must match the entrance cone of the die hole and the end of the billet. Typically, the outer diameter of the glass mat is 3–5 mm smaller than the barrel's diameter, while the hole in the mat is less than or equal to the die hole's diameter. The thickness of the mat is usually 4–8% of the billet’s length.
(2) Lubrication of the Extruder Barrel and Mandrel: This is done by coating the inner and outer surfaces of the billet with fine glass powder that has a low softening point. Alternatively, glass cloth can be wrapped around the billet and the mandrel to provide additional lubrication.
(3) Removal of the Glass Film: After extrusion, a thin glass film (20–50 μm thick) remains on the inner and outer surfaces of the steel tube. This film is hard and brittle, making it unsuitable for further processing. It must be removed using either mechanical or chemical methods. Mechanical methods include shot peening, water cooling, and tension straightening. Chemical removal typically involves alkali or acid solutions, though these can be corrosive, especially to carbon steel. As a result, the acid-base joint removal method is now commonly used to minimize damage.
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