Rolling in Vacuum and Protective Atmosphere
Introduction
Rolling is the process of reducing the thickness of a metal workpiece by passing it through a pair of rotating metal rolls (rollers); when heated and subjected to amplification, it also increases its length. It is one of the most widely used metallic process technologies and plays a major role in metal production activities. It is also used to produce tube or pipe products which can be hollow and can have variable wall thickness.
In general, metal rolling processes are carried out either on hot or cold materials. Hot rolling usually takes place in higher temperatures because the speed of hot rolling processes is usually faster than that of cold rolling. Hot rolling is normally limited to a maximum process temperature below the re-crystallization temperature of the material, while cold rolling processes are normally performed at room temperature or slightly above. However, rolling in vacuum (Vacuum Rolling) and protective atmosphere (Atmosphere Rolling)are special processes that occur at low temperatures, usually near room temperature.
Overview of Rolling in Vacuum and in Protective Atmosphere
Rolling in vacuum and in protective atmosphere have many advantages compared to conventional rolling, such as improved fatigue strength, better formability, improved surface finish, and improved mechanical properties. Typically, vacuum and protective atmosphere rolling operations use low-viscosity, non-oxidizing lubricants and roll Formidable materials such as titanium in a protective atmosphere increases the process efficiency since there is less oxidation compared to conventional rolling.
Compared to cold rolling, vacuum/atmosphere rolling has the advantage of increased formability because it does not require the workpiece to have a high level of stress. This allows for higher reduction ratio in fewer passes. Also, atmosphere rolling ensures the formation of homogeneous microstructures with improved properties. Vacuum/atmosphere rolling also provides the benefit of higher surface quality and improved fatigue strength in rolled products.
Vacuum and atmosphere rolling are also more applicable to certain materials that contain high levels of inherent oxide content due to their elemental composition. These include titanium, nickel, aluminum and copper alloys, as well as certain stainless steels. Vacuum and atmosphere rolling processes also offer an additional advantage of lower processing costs due to the lower washover rate in some cases.
Processes Involved in Vacuum and Atmosphere Rolling
Vacuum Rolling
The vacuum rolling process involves passing a metal workpiece between two rotating rollers in a sealed chamber, where the atmosphere is pumped out to create a vacuum. The material is then heated until it reaches a temperature sufficient for the rolling process and is rolled to the desired thickness.
Atmosphere Rolling
Atmosphere rolling is similar to vacuum rolling. The workpiece is heated and then passes between two rotating rollers, which are enclosed in a controlled atmosphere of nitrogen or other protective gases. The atmosphere controls the oxidation rate of the material, allowing for a more uniform roll, as well as improved surface finish.
Conclusion
Rolling in a vacuum or protective atmosphere has several advantages over more conventional rolling processes, including improved fatigue strength and formability, improved surface finish, and improved mechanical properties. These processes are particularly useful for materials that contain high levels of inherent oxide content, such as titanium, nickel, aluminum and copper alloys, as well as certain stainless steels. Vacuum and atmosphere rolling also offer the benefit of lower processing costs due to the lower washover rate in some cases.
