Electric Heating of Lead Baths, Salt Baths, and Induction Heating

1. Electric Heating of Lead Baths and Salt Baths

Lead has relatively high electrical conductivity, so it cannot be used as an electric heating element. If a lead bath is electrically heated, the heating elements should be arranged around the lead-bath crucible and supported on the side walls of the furnace. The arrangement and calculation of these heating elements are almost the same as those used in other types of furnaces. The main difference is that heating elements are not arranged near the bottom of the crucible, because this area is often damaged after being corroded.
Workpieces placed into molten lead are usually small and heat up quickly. The hourly heat required by a lead-bath crucible is equal to the heat absorbed by the workpieces, their containers, and the immersed tools, plus the radiant heat loss from the lead-bath surface and the heat loss through the furnace wall.
The operating temperature range of a lead bath is generally 350-900°C. The crucible functions like a muffle, and the temperature of the electric heating element is higher than the lead-bath temperature. Lead baths used for tool heat treatment are usually covered with petroleum coke or charcoal. Many lead-bath crucibles are heated by combustion products, but local overheating should be avoided as much as possible. Large galvanizing tanks are also heated by combustion products. Because the container temperature is close to a critical condition, forced air circulation is beneficial.
Salt baths may be heated internally or externally. When a salt bath is heated by combustion products through the crucible, the molten salt temperature can reach 870°C. However, when it is heated by electric heating elements through the crucible, the molten salt temperature is usually limited to 555°C or 570°C or below. The calculation and arrangement of the heating elements are the same as those for a lead-bath crucible.
Most electrically heated salt-bath furnaces now use internal heating, where the molten salt itself acts as the heating element. This is possible because the conductivity of the electrodes is much higher than that of the molten salt. With internal heating, the molten salt temperature can reach 1320°C. Ceramic containers may have a service life of up to eight years. The voltage between electrodes is low, usually 5-20 V, while the current passing through the electrodes is generally 1000-8000 A. Rectangular electrode sections are often used because two flat opposing surfaces can concentrate magnetic flux better than curved surfaces.
Experience shows that, to ensure effective directional circulation, the distance between the lower end of the electrode and the bottom of the molten bath should be no less than 100 mm and no more than 460 mm. The maximum immersion depth of a single electrode is about 1.5 m. For deeper molten baths, two or three groups of electrodes are arranged vertically in rows.
Cold salt, like cold glass, is non-conductive. Therefore, solidified salt blocks or salt particles must be started by external heating, commonly with spiral resistance heating coils. During night shutdowns or weekend stoppages, the bath must not be allowed to solidify. To prevent solidification, a lower transformer tap may be selected so that the input power is reduced to about one-third of the working power. This night-time power input is not entirely wasted, because the furnace can be restarted faster the next day.
The electrical energy consumed, which is converted into heat in the salt-bath resistance, is equal to the heat required to heat the workpieces, plus furnace-wall heat loss, heat loss through the electrodes, and radiant heat loss from the salt-bath surface. The last item is usually the largest and often exceeds all the others combined. Because the proportion of time during which the salt bath remains uncovered is difficult to determine, the radiant heat loss cannot be accurately known. For this reason, some salt-bath furnace manufacturers do not determine the blackness coefficient of the salt bath, but calculate radiant heat loss by treating it as a black body. In carburizing baths, the surface is covered with a layer of carbon powder, which reduces radiant heat loss. Other salt-bath furnaces are generally equipped with suspended or movable furnace covers.
Although the following point is not directly related to electric heating, it is worth noting for completeness: different salts have different temperature ranges between their freezing points and decomposition temperatures.
Like lead-bath crucibles, salt-bath crucibles can also be heated by combustion products. In this method, the combustion products flow around the crucible so that both the lower and upper parts are heated evenly. After a long shutdown, uniform heating can help prevent crucible cracking. Lower-temperature salt baths can be heated with siliconized U-shaped radiant tubes inserted into the bath. In some designs, the bottom side of the U-shaped tube is extended so that a straight section is close to the bottom of the container. A few salt baths also have a combustion chamber above the bath to heat the salt with combustion products, but this method has not been widely adopted in industry.

2. Induction Heating

Induction heating can be used for local or overall heating and offers certain advantages in mass production. When the current in a conductor changes, the magnetic field around the conductor also changes. By using this changing magnetic field, current can be induced in a properly positioned material, thereby heating it.
A typical induction heating device has a coil wound with conductive wire, through which alternating current passes. The square or round-section material to be heated is placed inside the coil. The frequency of the alternating current has a major influence on the temperature distribution inside the heated material.
In some countries, the basic power frequency is 50 Hz. In the United States, the standard low frequency is 60 Hz, while high frequencies are generally 960-3000 Hz. For special applications, even higher frequencies may be used, such as 9600 Hz or radio frequency.
Skin effect exists both in the workpiece and in the conductor of the induction coil. The higher the frequency, the stronger the skin effect. At high frequency, almost all heating occurs in the surface layer of the billet, and the heating rate decreases rapidly toward the center according to an exponential curve. At low frequency, the surface still receives the greatest amount of heat, and the heat also decreases toward the center exponentially, but the decrease is slower, meaning deeper heat penetration.
Therefore, for large-section bars or billets that require deep heating before forging or rolling, low-frequency heating is normally used. This principle applies to the heating of any metal. High frequency is suitable for deep heating of small-diameter bars, or for surface-layer heating of steel bars of any cross section.
When deep heating is required, the designer must carefully select the frequency to achieve both high efficiency and low cost.
The metal billet must be thermally insulated from the induction coil. If the diameter of the bar or billet is small, for example less than 63 mm for mild steel, air can act as the insulating medium because the heating time is short and heat loss from the hot surface is small. Although the surface temperature of the billet may be relatively high at the end of heating, it drops during transfer to forming equipment such as a forging press, bending machine, or rolling mill, and the billet temperature becomes more uniform.
How large a bar diameter can still rely on air insulation depends on the type of metal being heated. Different metals have different softening temperatures, electrical conductivity, emissivity, and required frequencies.
For large-section bars or billets, the air between the billet and the induction coil provides electrical insulation, but it does not provide sufficient thermal insulation. The surface temperature of the billet is high, and without an insulating layer the heat loss will be excessive. One insulation method is to attach refractory material, such as sillimanite, to the inner side of the induction coil by ramming or pressing. The refractory material is pressed into the gaps between the coil turns and keeps them separated from each other.
From the standpoint of reducing heat loss, the insulation layer should be thick. However, magnetic flux passing through the refractory material does not heat the billet, so the insulation layer should also be thin. If the refractory layer is too thick, both the power factor and electrical efficiency will decrease. The usual insulation thickness is about 13 mm.
To ensure uniform heating, it is best to keep the billet and induction coil concentric. If tongs are used to transport the billet, sufficient clearance should be left. If the billet is placed on rails, each rail and only a very small portion of the surrounding area should be arranged to avoid overheating or even induction melting.
There are two conflicting effects in induction heating. If the power supplied to the load is low, the heating time is long and the billet temperature is more uniform, but heat loss is large. If the power supplied to the load is high, heat loss is smaller, but temperature differences within the billet may become greater.