To facilitate the heating of large workpieces, trolley-type furnaces suitable for heating steel ingots and large billets have been developed. For heating long bar-shaped components, well-type furnaces have also been introduced. After the 1920s, various mechanized and automated furnace designs emerged, which not only enhanced furnace productivity but also improved working conditions.
With the development of fuel resources and advances in fuel conversion technologies, the fuels used in hearth-type annealing furnaces have gradually shifted from solid fuels such as lump coal, coke, and coal powder to gaseous and liquid fuels such as producer gas, city gas, natural gas, diesel, and fuel oil. Moreover, various combustion devices tailored to the fuels being used have been developed.
The structure, heating technology, temperature control, and furnace atmosphere of a carriage-type annealing furnace all directly affect the quality of the processed products. In casting heating furnaces, increasing the metal’s heating temperature can reduce deformation resistance; however, excessively high temperatures may lead to grain growth, oxidation, or overburning, severely compromising the workpiece quality. During heat treatment, if steel is heated to a point above its critical temperature and then rapidly cooled, its hardness and strength will increase. Conversely, if steel is heated to a point below its critical temperature and then slowly cooled, its hardness will decrease while its toughness will improve.
To obtain workpieces with accurate dimensions and smooth surfaces, or to reduce metal oxidation for purposes such as protecting molds and minimizing machining allowances, various low-oxidation or oxidation-free heating furnaces can be employed. In open-flame, low-oxidation heating furnaces, the incomplete combustion of fuel generates a reducing atmosphere; heating workpieces in such an atmosphere can reduce the oxidation burn-off rate to below 0.3%.
A controlled-atmosphere furnace uses artificially prepared atmospheres that are introduced into the furnace to carry out various heat treatments, such as gas carburizing, carbon-nitrogen co-diffusion, bright quenching, normalizing, and annealing, thereby altering the microstructure and improving the mechanical properties of workpieces. In a fluidized-bed furnace, combustion gases from fuel or other externally supplied fluidizing agents are forcibly passed through a bed of graphite particles or other inert particles on the furnace floor. Workpieces buried within this particle bed can undergo intensified heating, as well as various oxidation-free heating processes, including carburizing and nitriding. In a salt-bath furnace, molten salts serve as the heating medium, effectively preventing oxidation and decarburization of the workpieces. When melting cast iron in a cupola furnace, the process is typically affected by factors such as coke quality, air supply method, charge composition, and ambient air temperature, making it difficult to maintain stable melting conditions and challenging to produce high-quality molten iron. A hot-air cupola furnace, however, can effectively raise the molten iron temperature, reduce alloy burn-off, and lower the oxidation rate of the molten iron, thus enabling the production of high-grade cast iron.
With the advent of coreless induction furnaces, cupola furnaces have gradually been phased out. The melting process in these induction furnaces is not constrained by any specific cast iron grade; they can swiftly switch from melting one grade of cast iron to another, thereby helping to improve the quality of the molten iron. Certain special alloy steels—such as ultra-low-carbon stainless steels and steels used for rolls and turbine rotors—require that the molten steel produced in open-hearth furnaces or conventional arc furnaces undergo further refining in ladle furnaces. In these ladle furnaces, vacuum degassing and argon stirring are employed to remove impurities, resulting in high-purity, large-capacity, premium-quality molten steel.
Flame furnaces have a wide range of fuel sources, low costs, and can be tailored to adopt different structural designs, which helps reduce production expenses. However, flame furnaces are difficult to control precisely, cause severe environmental pollution, and have relatively low thermal efficiency. Electric furnaces, on the other hand, feature uniform furnace temperatures and easy implementation of automatic control, resulting in high-quality heating. According to the method of energy conversion, electric furnaces can be further classified into resistance furnaces, induction furnaces, and arc furnaces. The heating capacity of a furnace—calculated per unit time and per unit furnace floor area—is referred to as the furnace's production rate. The faster the furnace heats up and the greater its loading capacity, the higher its production rate will be. Under normal circumstances, the higher the furnace’s production rate, the lower the specific heat consumption required to heat each kilogram of material. Therefore, to reduce energy consumption, it is advisable to operate the furnace at full load, maximizing its production rate as much as possible. At the same time, the combustion equipment should be equipped with automatic adjustment mechanisms for the fuel-to-combustion-air ratio, ensuring that there is neither an excess nor a shortage of air. In addition, it is important to minimize heat losses due to furnace wall heat storage and radiation, heat loss from water-cooled components, radiative heat loss through various openings, and heat carried away by flue gases leaving the furnace.
The ratio of the heat absorbed by the metal or material during heating to the heat supplied to the furnace is known as the furnace thermal efficiency. Continuous furnaces have higher thermal efficiency than batch furnaces because continuous furnaces feature higher production rates and operate continuously, maintaining a stable furnace thermal regime with no periodic heat losses due to thermal storage in the furnace walls. Additionally, continuous furnaces include a section within the furnace chamber specifically designed to preheat the charge materials; some of the residual heat from the flue gases is absorbed by the cold workpieces entering the furnace, thereby reducing the temperature of the flue gases exiting the furnace.
To achieve active control of furnace temperature, furnace atmosphere, or furnace pressure.
The gases include liquefied petroleum gas, natural gas, coke oven gas, city gas, converter gas, mixed gas, producer gas, and blast furnace gas, among others.
