Adam c log
Wed Sept 14th, 2016
Disclaimer: I haven’t posted at the introduction section yet, but my name is Adam Chadek and I am a metallurgical engineer who graduated from UMR in 2007 and I have worked in copper mining, aluminum rolling, and iron green sand casting systems. I am by no means an expert with induction furnaces as I have only been around the one currently used at my workplace for about a year, so I will be the first to admit a mistake in my writings if one is found, but I do have several years of experience conducting research in multiple collegiate studies and have a talent for tracking down arcane pieces of scientific information and then compiling and summarizing that information.
With that in mind, here is my current level of understanding for a summary of induction furnaces:
Induction Furnaces Overview
There are two types of induction furnaces: channel/cored and coreless.
Channel furnace – This is an induction unit consisting of an iron core that has a coil wound around it. Metal passes around one or two channels of refractory that surround the induction unit, but channel furnaces require a source of molten metal to start up an empty furnace. These appear to be used mostly for low melting point alloys, and with the requirement to melt many different alloys and the need for a source of molten metal to start it up while cold, the choice may be easy to that we need to select a coreless furnace for our needs (just my initial findings).
Coreless furnace – These essentially consist of a cylindrical refractory area that is surrounded by copper coils. The copper coils have electricity run through them, creating a magnetic field that interacts with the metal to be melted, creating enough heat in the process to melt the metal.
There are also 3 separate classifications of induction furnaces based on their range of frequencies: 1) Low (Mains?) frequency (60-100 Hz) – I have seen mention online of using these for both low temperature non-ferrous alloys and for cast irons 2) Medium frequency (150-700 Hz) – steel, but I have also seen medium frequency furnaces listed as melting any metal. 3) High frequency (>701 Hz) – super alloys
So, it would appear that we would need either a low-frequency or medium-frequency coreless induction furnace, but from Benny’s comment on the induction furnace overview page, the low-frequency induction furnace may be more practical to start with. If that statement is true, then there appear to be greater technical challenges with melting steel solely from the required frequency levels. This requires more research on my part to better define which metals can be melted by each frequency range along with knowing the specific types of alloy classes we are trying to melt.
Basic parts of a coreless induction furnace: 1) Induction coil – water cooled, typically with a closed-loop recirculating system.
2) Refractory – the thickness of the refractory layer is a critical design variable. Thicker refractory linings lead to less heat loss, but also reduces furnace capacity and electrical efficiency while thinner walls increase furnace capacity and increase electrical efficiency at the cost of thermal efficiency, refractory life, and safety margin. So, there is an optimal balancing point that needs to be found and it sounds like relinings are typically done in industry anywhere from once a week to once a month or longer depending on how aggressively the refractory lining is worn down.
a. Refractory floor – First, the refractory for the floor of the furnace is installed. b. The steel shell (sometimes aluminum?) is then put into place. c. Then, the refractory lining is installed around the shell. d. A “top cap” material is then used for the area above where the molten level rises to in typical operation (we called this the “belly” band in the aluminum operation I worked at, but there are likely several different names for this area where the dross or slag sits in contact with the refractory). e. There is also refractory in the top lid that is movable in order to help contain heat.
3) Hydraulic system – I believe many coreless induction furnaces are tilting, so a hydraulic system would be needed to tilt the furnace and possibly to open and close the lid (I believe the lid we use is simply a swivel joint that is allowed to open and close).
4) Grounding system – this is crucial. In order to prevent the operator from becoming the grounded part of the furnace’s electrical circuit, first of all the power must absolutely be turned off while skimming the furnace as a precautionary measure. This is the best practice to prevent electrocution. Another vital aspect of this protection device is the grounding wire. These are a set of stainless steel wires connected to ground that are also in electrical contact with the base of the steel shell (surrounded by refractory) that must be checked at least once a day to make sure the ground connection is still working. This is another factor of safety to ensure that any stray electrical current from the furnace is not allowed to travel through the operator to the ground.
There is more to come on each topic as I have time to keep summarizing information. After each topic has been expanded upon in detail, I can help dig down into specific design requirements as needed, especially on the materials side.