Basic knowledge of forging-related processes

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What is forging? Forging is a processing method in which forging machinery is used to apply pressure to metal blanks, causing them to undergo plastic deformation in order to produce forgings with specific mechanical properties as well as certain shapes and dimensions. It is one of the components of forging processes (forging and stamping). Forging can eliminate defects such as as-cast porosity that occur during metal smelting, and optimize the microstructural layout; moreover, since the complete metal flow patterns are preserved, the mechanical properties of forgings are generally superior to those of castings made from the same material. For the important components in related machinery that are subject to high loads and harsh operating conditions, forgings are often used, except for those with relatively simple shapes for which rolled sheets, profiles, or welded parts can be employed. The deformation temperature: The onset recrystallization temperature of steel is around 727°C, but 800°C is commonly used as the threshold value; temperatures above 800°C correspond to hot forging ； Forging between 300 and 800°C is called warm forging or semi-hot forging, while forging at room temperature is called cold forging. Forgings used in most industries are hot-forged, while warm forging and cold forging are mainly used for forging parts in the automotive and general machinery sectors; warm forging and cold forging can help save materials effectively. Based on the forming mechanism, forging can be divided into free forging, die forging, ring rolling, and special forging. 01 Free forging refers to a forging process in which simple, general-purpose tools are used, or external force is applied directly to the billet between the upper and lower anvil plates of the forging equipment, causing the billet to deform in order to achieve the desired geometric shape and internal quality. Forgings produced by the free forging method are called free forgings. Free forging is primarily used to produce forgings in small batches; forging equipment such as hammers and hydraulic presses is employed to shape the blanks in order to obtain qualified forgings. The basic processes of free forging include upsetting, drawing, punching, cutting, bending, twisting, shifting, and forging together. Free forging always employs hot forging methods. 02 Forging: Forging is further divided into open forging and closed forging. The metal billet is deformed under pressure within a forging die of a specific shape to produce the forged part; forging is generally used to manufacture parts that are not very heavy but are produced in large quantities. Die forging can be divided into hot die forging, warm die forging, and cold die forging. Warm forging and cold forging represent the future development directions of die forging, as well as reflecting the level of forging technology. Based on the materials used, die forging can also be divided into ferrous metal die forging, non-ferrous metal die forging, and powder product forming. As the name implies, the materials in question are black metals such as carbon steel, non-ferrous metals such as copper and aluminum, and powder metallurgy materials. Extrusion falls under die forging and can be divided into heavy metal extrusion and light metal extrusion. Closed die forging and closed die heading are two advanced processes in die forging; due to the absence of flash, they offer a higher material utilization rate. The fine finishing of complex forgings can be accomplished with one or a few processing steps. Since there are no flash, the stress-bearing area of the forging is reduced, and the load required also decreases. However, it should be noted that the blank must not be completely constrained; to this end, the volume of the blank must be strictly controlled, the relative position of the forging die must be regulated, and the forged part must be measured, in order to minimize wear on the forging die. 03 Ring Rolling: Ring rolling refers to the production of ring-shaped parts of various diameters using specialized equipment known as ring rolling machines; it is also used to manufacture wheel-shaped components such as automobile wheels and train wheels. 04 Special Forging: Special forging includes processes such as roll forging, wedge rolling, radial forging, and die casting; these methods are well suited for producing parts with specific shapes. For example, roll forging can serve as an effective pre-forming process that significantly reduces the forming pressure required later on ； Wedge rolling can be used to produce parts such as steel balls and drive shafts ； Radial forging can produce large forgings such as cannon barrels and stepped shafts. The choice of pre-forging heating method should be determined based on the specific requirements of forging, as well as factors such as investment efficiency, energy availability, and environmental protection. The heating methods for metal blanks before forging can be divided into electric heating and fuel heating, as detailed below. 01 Electric heating: Electric heating converts electrical energy into thermal energy to heat metal blanks. Electric heating has advantages such as fast heating speed, accurate furnace temperature control, good heating quality, less oxidation of the workpiece, favorable working conditions, and easy automation ； However, due to the high cost of equipment investment, expensive electricity bills, and high heating costs, its widespread use in our country remains limited. Resistive heating and induction heating can be classified as the conversion of electrical energy into thermal energy. 1. Resistance heating: Based on the heating element that generates resistive heat, there are resistance furnace heating, contact electric heating, and salt bath furnace heating. Resistive furnace heating: A resistive furnace uses the current generated by its electric heating elements to heat metal ； Contact electric heating: Contact electric heating involves connecting the electric heater directly to the electrical circuit. When current passes through the billet, the billet is heated as the resistance of the billet itself generates thermal energy ； Salt bath heating: The working principle of an internal-electrode salt bath furnace is to pass low-voltage alternating current between the electrodes; the conductivity of the salt solution generates a large amount of resistive heat, which is used to heat the salt solution to the desired operating temperature. 2. Induction heating: Under the action of the alternating magnetic field generated by an alternating current, the metal blank placed in this magnetic field develops an alternating electromotive force, resulting in the formation of alternating eddy currents. The metal blank is heated by eddy current heating and hysteresis loss heating caused by its resistance. 02 Fuel Heating: Fuel heating involves using the thermal energy generated by the combustion of solid fuels (coal, coke, etc.), liquid fuels (heavy oil, diesel, etc.), or gaseous fuels (gas, natural gas, etc.) to heat the billets. The fuel burns in the fuel furnace to produce high-temperature flue gas (flames), which heats the metal billets through flue gas convection, radiation from the furnace walls and roof, and heat conduction from the furnace bottom. In a furnace, metal heating relies mainly on convective heat transfer. In furnaces at medium temperatures (650–1000°C) and high temperatures (above 1000°C), metal heating is primarily by radiation. Radiative heat transfer can account for over 90% of the total heat transfer in conventional high-temperature forging furnaces. Fuel heating furnaces are highly versatile, require low investment, are easy to construct, can use locally available fuels which are easy to obtain, have low fuel costs, and are widely used. The production of small and medium-sized forging sprockets mainly relies on chamber furnaces fueled by oil, gas, natural gas, or coal. Steel is heated in a continuous furnace or a rotary bottom furnace. Oil, gas, and natural gas are often used for large forging sprocket blanks or steel ingots. The disadvantages of fuel heating are poor working conditions, and it is difficult to control the furnace atmosphere, temperature, and heating quality. Before forging, the metal billet is heated in order to improve the plasticity of the metal, reduce its resistance to deformation – in other words, to increase its malleability and make it easier to shape through forging – so as to achieve a favorable microstructure and mechanical properties after forging. With the continuous emergence of new materials and forging processes, the requirements for metal heating techniques are increasing, and pre-forging heating has become a very important step in the forging production process.