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What is hot forging

What is hot forging?

The forging process carried out above the recrystallization temperature of the metal is called hot forging. Hot forging is also known as hot die forging, forging deformation metal flow is intense, forging and die contact time is longer. Therefore, the die metal material is required to have high thermal stability, high temperature strength and hardness, impact toughness, heat fatigue and wear resistance and easy processing. Lighter working load of hot forging die can be used to manufacture low-alloy steel.

Advantages and disadvantages of hot forging


If the processing is completed, the temperature of the forging material is still much higher than the recrystallization temperature, the grain has a longer time to grow, will get coarser grains, can increase the fracture toughness (crack expansion rate is lower).


  • (1) Because of high temperature operation, the danger to personnel safety and material safety is taught to be large.
  • (2) The metal material is prone to oxidation at high temperature, resulting in oxide skin, resulting in surface scale accumulation and poor finish and flatness.
  • (3) After the completion of hot forging, the forging material in the cooling process has the phenomenon of cold shrinkage, affecting the accuracy of the forging size.
  • (4) High temperature operation requires high cost of equipment and maintenance.

How to hot forging?

Different forging methods have different processes, including the longest process of hot die forging, the general order is:

Forging billet feeding → forging billet heating → roll forging preparation → die forging forming → cutting edge → punching → correction → intermediate inspection, inspection of forging dimensions and surface defects → forging heat treatment. Used to eliminate forging stress. Improve metal cutting properties → cleaning the surface of forgings (mainly to remove surface oxide) → correction → inspection (- general forgings to discuss the appearance and hardness inspection, important forgings also after chemical composition analysis, mechanical properties, residual stress and other tests and non-destructive testing).

Forging billet feeding method

In addition to large and medium-sized free forgings using ingots as billet, general forgings are used for various metal bars as billet. Before forging, the bars are generally cut into breaks on special undercutting equipment according to the required size, commonly used undercutting methods are as follows.


Sawing undercutting is often carried out on the circular saw and bow saw and high-speed band saw.
The circular saw is driven by the motor with teeth saw disk slow rotation and movement, the bar material will be cut off. The maximum diameter of the saw disk can reach 2m, and the diameter of the bar material that can be cut is below 750mm.
The bow saw is driven by an electric motor to move the saw blade with teeth reciprocally to cut off the bar material. The diameter of the bar material that can be sawed is less than 100mm. For particularly small diameter bar material, it is also possible to saw off in bundles.
Sawing is characterized by a flat cut section, accurate size; but the productivity is low, there is kerf loss, and the loss of the saw blade and saw plate is also relatively large. High-speed band saw is a more advanced undercutting equipment, with high productivity and regular shape of the blank.


Shear undercutting is generally carried out on shears, which can cut billets with a diameter of 200mm or less. When cold shear billet, the sheared place will produce a lot of stress and cracks, therefore, high carbon steel, alloy steel and section size of the larger billet in shear before the need to preheat to 350-700 ℃; for low carbon steel, medium carbon steel and other section size of the smaller billet can be cold shear.
Shear under the characteristics of the machine can be installed automatically feed the material out of the mechanism, better labor conditions for workers, high production efficiency, saving metal, improve the utilization of materials. However, the shear end is not flat, slightly skewed, especially in the hot state when cutting large diameter billets more serious. Shears are commonly used in high-volume production of die forging shops.
Shear downs can also be performed on equipment such as cutting edge presses and friction presses.


Fracture undercutting is carried out on hydraulic presses or crank presses. The pressure is transmitted to the material through the punch, which causes the material to break along a pre-cut. The principle is shown in Figure 1. pre-cutting is generally made by sawing or gas cutting before fracture, the purpose of which is to create a large stress concentration at the cut to ensure that the material does not produce large plastic deformation and affect the quality of the section when fractured in a certain area.
20220609141308 98613 - What is hot forging
Figure.1 fracture principle
Fracture is applicable to high carbon steel and high alloy steel with high hardness. Such as GCr15, GCr15SiMn, GSiMnMo, GSiMnV and other bearing steels, the heating temperature is 300-400℃.

Grinding Wheel Cutting

Grinding wheel cutting is carried out on a grinding wheel cutting machine. The wheel cutting machine is driven by an electric motor to rotate a thin abrasive wheel (generally below 3mm in thickness) at high speed and cut the billet by making it move up and down manually or by motor. The wheel cutter can cut metal blanks of any hardness up to 40mm in diameter.
Grinding wheel cutting is characterized by high productivity and flat cutting section; however, the loss of thin slices of grinding wheel is large, the labor conditions of workers are poor, and good ventilation is required.

Flame cutting

Flame cutting is also called gas cutting. Its principle is to use gas cutter or ordinary welding torch, using oxygen airflow to partially heat the steel to melting temperature, so that it gradually fuses. It is mainly used for large section cutting of large billets and forgings (the thickness of the cut section can be more than 1500mm), and also can be used for cutting the edges of large die forgings produced in small batches.

Anodic cutting

The principle is to cut through the metal material by using the effect of electric corrosion and electrochemical corrosion. The size of the material to be cut is 30-300mm. anodic cutting is characterized by high productivity, low scrap, and the ability to cut any hardness of the metal material with a clean section.

The purpose and method of pre-forging heating of billets

The purpose of pre-forging heating of metal billets is to improve metal plasticity, reduce deformation resistance, make it easy to flow into shape and obtain good post-forging organization. Therefore, pre-forging heating has a direct impact on improving forging productivity, ensuring the quality of forgings and saving energy consumption. According to the different heat sources used, the heating method of metal blanks can be divided into two categories: flame heating and electric heating.

Flame heating

Flame heating is the use of fuel (coal, coke, heavy oil, diesel fuel and gas) in the flame heating furnace burning to produce high temperature gas (flame) containing a large amount of heat energy, convection, radiation to the surface of the blank, and then from the surface to the center of heat conduction and heating of the metal blank.
When the heating temperature is below 600-700°C, the billet is heated mainly by convection heat transfer. The so-called convection heat transfer is through the continuous flow of flame around the blank, with the help of heat exchange between the high temperature gas and the surface of the blank, the heat energy is transferred to the metal blank. When the heating temperature exceeds 700-800°C, the billet is heated mainly by radiation heat transfer. The so-called radiation heat transfer is through the high-temperature gas and the furnace chamber to change the heat energy into radiation energy, the radiation energy propagated in the way of electric microwave is absorbed by the metal billet, and then the radiation energy is transformed into heat energy and the billet is heated. General common forging heating furnace in high temperature heating, radiation heat transfer accounted for more than 90%, convection heat transfer only 8%-10%.
The advantages of flame heating method are convenient fuel source, simple furnace repair, low heating cost, and wide range of adaptation to the blank. However, poor labor conditions, slow heating speed, low efficiency, heating quality is difficult to control, etc.. This heating method is widely used for the heating of various billets.

Electric heating

Electric heating is used to heat metal billets by converting electrical energy into thermal energy. Among them, there are induction electric heating, contact electric heating, resistance furnace heating and salt bath furnace heating.
1. Induction heating
Under the action of alternating magnetic field generated by the inductor through alternating current, alternating eddy current is generated inside the metal blank. Due to the eddy current and magnetization heat (below the magnetic transition point), the metal blank is directly heated.
When the blank is inductively heated, the internal current density generated is unevenly distributed along the section, with a small current density in the center and a large current density in the surface layer, a phenomenon known as the skin effect. As a result, the surface layer of metal is heated mainly by the passage of current, while the heart metal is heated by the conduction of heat from the outer layer to the inner layer. For large diameter billets, in order to improve the heating speed, a lower current frequency should be used to increase the current penetration depth; while for small diameter billets, due to the small cross-sectional size, a higher current frequency can be used, which can improve the electrical efficiency.
2. Contact heating
The principle of contact electric heating is to directly pass through the metal blank with low voltage and high current, as the metal has a certain resistance, the current will generate heat through, thus heating it.
For a certain size of the blank, to be heated to the specified temperature, it is necessary to generate a certain amount of heat. The resistance value of metal is generally small, so in order to improve productivity and shorten the heating time, it is necessary to pass a large electric current through the blank. In order to avoid short circuit, often take the approach of reducing the voltage to get a low voltage of high current. Therefore, the no-load voltage at the vice end of the transformer used for contact heating is only 2-15V.
The characteristics of contact electric heating are: fast heating speed, less metal burning loss, unrestricted heating temperature range, high thermal efficiency, low power consumption, low cost, simple equipment and easy operation. However, the surface roughness and shape size of the blank are strictly required, especially the end of the blank must be regular and must not produce distortion. In addition, the measurement and control of heating temperature is difficult. It is suitable for the whole or partial heating of long blanks.
Determination of forging temperature range
Hot forging is carried out within a certain temperature range. The forging temperature range of steel is the temperature interval between the beginning forging temperature (start forging temperature) and the end forging temperature (end forging temperature).
The basic principle of determining the forging temperature range is to ensure that the steel has high plasticity, low deformation resistance, and high quality forgings, while the forging temperature range is as wide as possible in order to reduce the number of heating fires and improve forging productivity.
The basic method to determine the forging temperature range is: based on the steel equilibrium diagram, and then refer to the steel plasticity diagram, resistance diagram and recrystallization diagram, by plasticity, quality and deformation resistance of the three aspects of the comprehensive analysis, so as to determine the starting and final forging temperature.
Generally, the forging temperature range of carbon steel can be directly determined according to the iron and carbon balance diagram. For most of the alloy structural steel forging temperature range, you can refer to the same carbon content of carbon steel to consider. But the plasticity of the lower high-alloy steel, as well as the steel does not occur in the phase change (such as austenitic steel, pure iron body steel), it is necessary to pass the test, in order to come up with a reasonable forging temperature range.

Determination of temperature

Determine the starting forging temperature of steel, first of all, must ensure that the steel no overburning phenomenon. Therefore, for carbon steel, the starting forging temperature should be lower than the iron – carbon equilibrium diagram of the starting melt line 150-250 ℃. In addition, should also take into account the billet organization, forging method and deformation process and other factors.

Final forging temperature

In determining the final forging temperature, both to ensure that the steel has sufficient plasticity before the final forging, but also to enable the forgings to obtain good organizational properties. Therefore, the final forging temperature of steel should be higher than the recrystallization temperature, in order to ensure that the forging recrystallization completely, so that the forgings get fine grain organization.
In the case of carbon steel, the final forging temperature should not be lower than the A1 line of the iron-carbon equilibrium diagram. Otherwise, the plasticity is significantly reduced, the deformation resistance increases, the process hardening phenomenon is serious, easy to produce forging cracks.
For sub-eutectoid steel, the final forging temperature should be in the A3 line above 15-50 ℃, because it is located in the single-phase austenite zone. The organization is uniform and good plasticity. But for low carbon steel (carbon content of less than 0.3%), the final forging temperature can be reduced to A3 line below, although in the (γ + α) two-phase area, still has sufficient plasticity, deformation resistance is not too high, and also to expand the forging temperature range.
For over-eutectic steel, the final forging temperature should be below the Acm line and 50-100 ℃ above the A1 line. This is because, if the final forging temperature is selected above the Acm line, it will be forged in the cooling process, along the grain boundary precipitation of secondary mesh carburization, will make the forging mechanical properties greatly reduced. Such as forging between the Acm line and A1 line, due to the mechanical crushing effect of plastic deformation, can make the precipitation of secondary carburized body is diffuse.
It must also be pointed out that the final forging temperature of steel and steel organization, forging process and subsequent processes are also related. For the steel without phase change, because the heat treatment method can not be used to refine the grain, only rely on forging to control the grain size. In order to make forgings to obtain small grains, the final forging temperature of such steel is generally low. When forging immediately after the forging afterheat heat treatment, the final forging temperature should meet the requirements of afterheat heat treatment. If the forging material is low carbon steel, the final forging temperature is slightly higher than the A line.

Heat treatment method of hot forging die

Heat treatment: the general heat treatment temperature and time of the die can be referred to the Heat Treatment Manual or Mechanical Engineering Manual. What needs to be noted is that

  • (1) Heat treatment should use a reasonable process to reduce heat treatment deformation (generally using multi-stage heating process, while preventing heating cracking), while considering the heat treatment methods used, should avoid the evaporation of alloying elements, under the conditions of material hardenability allowed, as far as possible to use vacuum heat treatment, gas quenching technology, reduce heat treatment deformation, avoid heat treatment after a large machining allowance, resulting in surface overheating, affecting Mould life. However, for materials with poor hardenability or materials with volatile elements at high temperatures, such as those containing high Ni, etc., it is appropriate to use salt bath heat treatment.
  • (2) Recommend the use of supersaturated carburizing heat treatment technology, that is, the application of carburizing technology to stop the decarburization of the heat treatment surface, while improving the wear resistance of the surface, and the use of carburizing quenching, the formation of high pressure stress on the surface to improve the fatigue resistance of the mold.
  • (3) The mold material generally contains higher Cr,Mo,V,W,Nb and other high temperature, strong carbide forming elements, so as to improve the mold strength, red hardness and other properties, in the heat treatment and tempering treatment, has obvious secondary hardening characteristics, that is, in the low temperature tempering and high temperature tempering to form two high hardness. Therefore, according to the actual use temperature range of the die, the tempering temperature can be selectively applied, but for hot forging dies should be used high temperature tempering process to avoid the secondary tempering hardening effect resulting in the reduction of die performance during use.

On the other hand, also because the die material generally contains higher Cr, Mo, V, W, Nb and other high temperature, strong carbide forming elements, has a strong tempering resistance, so it needs to be tempered several times to avoid early failure (fracture and cracking) caused by inadequate tempering, generally requires at least two high temperature tempering (more often using three tempering process).

What is hot forging of flanges?

Is it traditionally understood as heating? Forging with fire? What we know in the industrial field is that when hot forging is performed, forging may have complex and large flange shapes due to the low deformation energy and less deformation. Therefore, if flanges are to be manufactured with high dimensional accuracy, hot forging can be used in the temperature range of 900-1000°C! This is very strict! In addition, care should be taken to improve the working environment of hot forging. The life of forging dies is also different compared to forging in other temperature regions, but it has been popular for its wide range of uses and low cost. Why do we need to hot forge flanges? The main purpose of hot forging flanges is to reduce the tonnage by reducing the resistance of the metal to deformation. We can improve the mechanical properties of the steel. It can be concluded that hot forging can improve the performance!

What is the cold forging temperature of the flange? Does it deform?

Cold forging of flanges is a slight change in flange dimensions during low temperature forging. How many degrees now? Forging at 700°C or lower, the dimensions are small and no surface decarburization can be observed. Therefore, cold forging can easily achieve good dimensional accuracy and surface finish as long as a reasonable range is controlled. Good accuracy can be achieved with hot forging at temperatures and lubrication properties below 700°C. Cold forging is a forming process below the recrystallization temperature of the material, while forging is performed below a certain temperature. We can analyze that cold forged materials have a greater resistance to deformation at room temperature and can sometimes replace cold worked profiles. Cold forging can enhance the metal strength and the strength of the flange will be increased. Often, high value-added products use cold forging to reduce production costs and can also be combined with these processes to form composite processes.

Castings vs. Forgings

Compared to castings, forging improves the structure and mechanical properties of the metal. Casting organization after forging method of thermal processing deformation due to metal deformation and recrystallization, so that the original coarse dendritic and columnar grains into fine grains, uniform size of the equiaxial recrystallization organization, so that the ingot within the original segregation, loosening, porosity, slag and other compaction and welding, the organization becomes more compact, improve the plasticity and mechanical properties of the metal.
The mechanical properties of the casting low dry with the same material forging mechanical properties, in addition, forging processing can ensure the continuity of the entire genus of fiber organization, so that the forging fiber organization and forging shape to maintain consistent, metal flow line integrity, can ensure that the parts have good mechanical properties and long service life using precision die forging, cold extrusion, warm extrusion and other processes to produce forgings, are incomparable to the castings.
Forgings are objects in which metal is applied pressure to shape the required shape or suitable compression force through plastic deformation. This force is typically achieved through the use of a hammer or pressure. The forging process builds a delicate granular structure and improves the physical properties of the metal. In the real world use of parts, a correct design enables the flow of particles in the direction of the main pressure. Casting is a metal forming object obtained by various casting methods, i.e., the smelted liquid metal is injected into a pre-prepared casting mold by pouring, pressure injection, suction, or other casting methods, cooled, and after sanding, cleaning, and post-treatment, resulting in an object with a certain shape, size, and properties.

Source: Network Arrangement – China Forgings Manufacturer:

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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