For many years, gas diffusers, also known as porous plugs, have been used in ladles to allow gas purging of steel melts before pouring the metal into molds. The purging step degasses the melt and produces cleaner steel, resulting in a reduced scrap rate in castings.
Capital Refractories (www.capital-refractories.com), designs and produces gas diffusers for induction furnaces, as well as ladles, and according to the company, the correct use of gas diffusers offers a range of advantages in induction melting. These include lower scrap rates, a more efficient melt, and a significant improvement in furnace campaign life.
For a gas diffuser to function safely and effectively, a number of factors have to be considered. The refractory material selected must be able to resist penetration when in contact with liquid metals. The design must ensure that the gas diffuser is able to deliver small quantities of inert gas to the induction furnace melt in a controllable manner. Also, it must be compatible with the induction furnace lining materials. The gas diffuser must be easy to install and operate, and it must be able to last for the life of the induction furnace lining. Ultimately, it must be cost effective, too.
Where possible, a diffuser should be installed in the center of the furnace base, or as close as possible to the center.
The furnace must have a gas supply to connect to the gas diffuser, and a there must be a suitable gas-flow control system. Such a system may be as simple as a pressure regulator on an argon-gas bottle with an inlet needle valve and flow meter. Or, it may be as sophisticated as a PLC package linked to a computer-controlled process control system.
The induction furnace lining must be sintered before the gas diffuser is used, so that the gas can pass through the lining without disturbing it. Using a gas diffuser early in the melting process is not recommended; it’s necessary for adequate sintering to take place first. Experience indicates that the best results are obtained from introducing gas to the diffuser during the third melt and onward.
Gas flow is turned on at “full melt” and the flow is increased until a gentle bubbling motion is seen on the surface of the melt. This is generally at a rate of around 10 liters/minute, for example, in a onemetric- ton melt. Gas expands as the temperature rises, so there may be more bubbling as the temperature increases prior to tapping. Gas flow can be reduced at this stage to 6-8 liters/minute.
Gas purging should be continued for the duration of the melt, right up to the point of tapping. It is not necessary to purge fully for every melt.