About forging, die forging, cold forging.

July 13,2022

Forging is a processing method that uses a forging machine to apply pressure to a metal blank to cause plastic deformation to obtain a forging with certain mechanical properties, certain shapes, and sizes. Forging and stamping are both plastic processing properties, collectively referred to as forging. Forging is a commonly used forming method in machine manufacturing. Through forging, the as-cast porosity and welding holes of the metal can be eliminated, and the mechanical properties of forgings are generally better than those of castings of the same material. For important parts in machinery with high loads and severe working conditions, forgings are mostly used in addition to rolling plates, profiles, or welded parts with simple shapes.


Forging can be divided into cold forging and hot forging according to the temperature of the blank during processing. Cold forging is generally performed at room temperature, while hot forging is performed at a recrystallization temperature higher than that of the billet metal. Sometimes it is also in a heated state, but the forging performed when the temperature does not exceed the recrystallization temperature is called warm forging. However, this division is not completely uniform in production. The recrystallization temperature of steel is about 460 °C, but 800 °C is generally used as the dividing line, and hot forging is higher than 800 °C; between 300 and 800 °C, it is called warm forging or semi-hot forging.


Forging can be divided into free forging, die forging, cold heading, radial forging, extrusion, forming rolling, roll forging, rolling, and so on according to the forming method. The deformation of the billet under pressure is basically free from external restrictions, which is called free forging, also called open forging; the billet deformation of other forging methods is limited by the die, which is called closed mode forging.


There is a relative rotational motion between the forming tools such as forming rolling, roll forging, rolling and expanding the billet and the billet is pressurized and formed point by point and asymptotically, so it is also called rotary forging. Forging materials are mainly carbon steel and alloy steel of various compositions, followed by aluminum, magnesium, copper, titanium, etc., and their alloys. The raw state of the material is bar, ingot, metal powder and liquid metal. Generally, small and medium-sized forgings use round or square bars as blanks. The grain structure and mechanical properties of the bar are uniform and good, the shape and size are accurate, and the surface quality is good, which is convenient for mass production. As long as the heating temperature and deformation conditions are reasonably controlled, forgings with excellent performance can be forged without large forging deformation. Ingots are only used for large forgings. The ingot is an as-cast structure with large columnar crystals and a loose center. Therefore, it is necessary to break the columnar crystals into fine grains through large plastic deformation and compact them loosely to obtain excellent metal structure and mechanical properties. The powder metallurgy preforms that have been pressed and sintered can be made into powder forgings by forging without flash in the hot state. The forging powder is close to the density of general die forgings, has good mechanical properties and has high precision, which can reduce subsequent cutting operations.


Powder forgings have a uniform internal structure and no segregation, and can be used to manufacture small gears and other workpieces. However, the price of powder is much higher than that of general bars, and its application in production is limited. Applying static pressure to the liquid metal poured in the die cavity makes it solidify, crystallize, flow, plastically deform and form under the action of pressure, and then die forgings with the desired shape and properties can be obtained. Liquid metal die forging is a forming method between die casting and die forging, and is especially suitable for complex thin-walled parts that are difficult to form in general die forging. Different forging methods have different processes. Among them, hot die forging has the longest process. The general sequence is: blanking of forging billet; heating of forging billet; roll forging preparation billet; die forging forming; trimming; intermediate inspection, an inspection of forgings size and surface defects; heat treatment of forgings to eliminate forging stress and improve metal cutting performance; cleaning, mainly to remove surface oxide scale; correction; inspection, general forgings are subject to appearance and hardness inspection, and important forgings are also subject to chemical composition analysis , mechanical properties, residual stress and other inspection and non-destructive testing. Forging is a collective term for forging and stamping. It is a forming and processing method of using a hammer, anvil, punch of a forging machine or applying pressure to a blank through a die to plastically deform it to obtain the desired shape and size. .


In the forging process, the billet has obvious plastic deformation as a whole, and there is a large amount of plastic flow; in the stamping process, the billet is mainly formed by changing the spatial position of the area of each part, and there is no plastic flow with a large distance inside it. Forging is mainly used to process metal parts, and can also be used to process some non-metals, such as engineering plastics, rubber, ceramic blanks, brick blanks and the forming of composite materials.


Rolling, drawing, etc. in forging and metallurgical industry belong to plastic processing, or pressure processing, but forging is mainly used to produce metal parts, while rolling and drawing are mainly used to produce sheet, strip, pipe, General-purpose metal materials such as profiles and wires. At the end of the Neolithic Age, human beings have begun to make decorations and small articles by hammering natural red copper. China has used the cold forging process to manufacture tools since about 2000 BC. For example, the red copper utensils unearthed from the Qijia Cultural Site of Huangniangniangtai in Wuwei, Gansu, have obvious signs of hammering. In the mid-Shang Dynasty, weapons were made of meteorite iron, and the heating and forging process was adopted. The block wrought iron that appeared in the late Spring and Autumn Period was formed by repeated heating and forging to extrude oxide inclusions. At first, people used hammers for forging, and later, people pulled ropes and pulleys to lift the heavy hammers and then dropped them freely to forge billets. After the 14th century, animal power and hydraulic drop hammer forging appeared. In 1842, Nesmith in England made the first steam hammer, which brought forging into the era of applied power. Later, forging hydraulic presses, motor-driven plywood hammers, air forging hammers and mechanical presses appeared one after another.


The splint hammer was first used in the American Civil War (1861-1865) to die forge parts of weapons, and then steam die forging hammer appeared in Europe, and the die forging process was gradually promoted. By the end of the 19th century, the basic categories of modern forging machinery had been formed. In the early 20th century, with the mass production of automobiles, hot die forging developed rapidly and became the main process of forging. In the mid-20th century, hot die forging presses, flat forging machines and anvilless forging hammers gradually replaced ordinary forging hammers, increasing productivity and reducing vibration and noise.


With the development of new forging processes such as less forging and non-oxidative heating technology, high-precision and long-life dies, hot extrusion, forming and rolling, and the development of forging manipulators, robots and automatic forging production lines, the efficiency and economic effects of forging production continue to improve. . The appearance of cold forging precedes hot forging. Early red copper, gold, and silver flakes and coins were cold forged. The application of cold forging in machinery manufacturing was promoted in the 20th century. Cold heading, cold extrusion, radial forging, and swing rolling were successively developed, gradually forming an efficient forging process that can produce precision parts that do not require cutting. Early stamping only used simple tools such as shovels, shears, punches, hand hammers, and anvils to form metal plates (mainly copper or copper alloy plates, etc.) Manufacturing gongs, cymbals, cymbals and other musical instruments and cans. With the growth of medium and thick plate production and the development of stamping hydraulic presses and mechanical presses, stamping processing also began to be mechanized in the mid-19th century. In 1905, the United States began to produce coiled hot continuous rolled narrow strip, in 1926 began to produce wide band, and later cold rolled strip appeared. At the same time, the output of plates and strips is increased, the quality is improved, and the cost is reduced. Combined with the development of production of ships, railway vehicles, boilers, containers, automobiles, cans, etc., stamping has become one of the most widely used forming processes. Forging is mainly classified by forming method and deformation temperature. Forging can be divided into forging and stamping according to the forming method; forging can be divided into hot forging, cold forging, warm forging and isothermal forging according to the deformation temperature. Hot forging is forging performed above the recrystallization temperature of the metal. Increasing the temperature can improve the plasticity of the metal, which is conducive to improving the intrinsic quality of the workpiece and making it less prone to cracking. High temperature can also reduce the deformation resistance of the metal and reduce the tonnage of the required forging machinery. However, there are many hot forging processes, the workpiece accuracy is poor, the surface is not smooth, and the forgings are prone to oxidation, decarburization and burning.


Cold forging is forging at a temperature lower than the recrystallization temperature of the metal. Generally speaking, cold forging refers to forging at room temperature, while forging at a temperature higher than normal temperature but not exceeding the recrystallization temperature is called warm. Forging. Warm forging has higher precision, smoother surface and less deformation resistance. The workpiece formed by cold forging at room temperature has high shape and dimensional accuracy, smooth surface and few processing steps, which is convenient for automatic production. Many cold forging and cold stamping parts can be directly used as parts or products without cutting. However, during cold forging, due to the low plasticity of the metal, cracking is easy to occur during deformation, and the deformation resistance is large, which requires a large tonnage of forging machinery. In isothermal forging, the billet temperature is kept constant throughout the forming process. Isothermal forging is to make full use of the high plasticity of certain metals at isothermal temperatures, or to obtain specific structures and properties. Isothermal forging needs to keep the mold and the blank together at a constant temperature, and the cost is high. It is only used for special forging processes, such as superplastic forming.


Forging can change the metal structure and improve the metal properties. After the ingot is hot forged, the original as-cast looseness, pores, micro-cracks, etc. are compacted or welded; the original dendrites are broken to make the grains finer; at the same time, the original carbide segregation and unevenness are changed. Distributed to make the structure uniform, so as to obtain forgings with internal compactness, uniformity, fineness, good comprehensive performance and reliable use. After the forging is deformed by hot forging, the metal is a fibrous structure; after being deformed by cold forging, the metal crystal is ordered. Forging is a workpiece that plastically flows metal into the desired shape. After the metal is subjected to plastic flow by external force, the volume remains unchanged, and the metal always flows to the part with the least resistance. In production, the shape of the workpiece is often controlled according to these laws to achieve deformation such as upsetting and drawing, reaming, bending, and deep drawing. The size of the forged workpiece is accurate, which is conducive to the organization of mass production.


The dimensions of die forging, extrusion, stamping and other applications are accurate and stable. High-efficiency forging machinery and automatic forging production lines can be used to organize specialized mass or mass production. The production process of forging includes blanking, heating and pretreatment of forging blanks before forming; heat treatment, cleaning, calibration and inspection of workpieces after forming. Commonly used forging machines include forging hammers, hydraulic presses and mechanical presses.


The forging hammer has a large impact speed, which is conducive to the plastic flow of the metal, but it will produce vibration; the static forging of the hydraulic press is conducive to forging through the metal and improving the structure, and the work is stable, but the productivity is low; the stroke of the mechanical press is fixed, which is easy to realize mechanization. and automation. In the future, the forging process will improve the intrinsic quality of forging parts, develop precision forging and precision stamping technology, develop forging equipment and forging production lines with higher productivity and automation, develop flexible forging and forming systems, and develop new forging materials and forging processing methods. develop. To improve the intrinsic quality of forgings is mainly to improve their mechanical properties (strength, plasticity, toughness, fatigue strength) and reliability. This requires better application of metal plastic deformation theory; application of materials with better intrinsic quality; correct pre-forging heating and forging heat treatment; more rigorous and extensive non-destructive testing of forgings. Less and no cutting processing is the most important measure and direction for the machinery industry to improve material utilization, improve labor productivity and reduce energy consumption. Less forging blanks, no oxidative heating, and the development of high-hardness, wear-resistant, long-life mold materials and surface treatment methods.