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Why Forging is Better?

In the process of forging, not only the appropriate shape and size must be required, but also the performance requirements of the parts during the use must be met, which includes strength index, plasticity index, impact toughness, fatigue strength, fracture toughness, resistance against stress corrosion and so on. This makes forging process always better than other metal working processes. Below we would like to introduce forging under different conditions.
 

Drawing

 

After many tests, it proved that the steel ingot forging ratio has little effect on the strength index of the metal but has significant influence on the plasticity index and impact toughness value of the steel, especially on the axial center of the steel ingot. For example,the quality of the steel ingot becomes good (with the dense and uniform structure ), the forge piece has high mechanical properties and the forging ratio can be reduced. On the contrary, the quality of the ingot is poor (with the poor compactness and nonuniform structure ) and the mechanical properties of the forge piece are poor. In order to improve the cast structure and the mechanical properties, the forging ratio is increased.

Upsetting

 

There is no fundamental difference between the effect of upsetting on the quality of metal and the drawing. However, due to the different stress-deformation states, especially the difference in metal flow , the effect of upsetting on the macroscopic structure and mechanical properties of the metal is different from that of the drawing. As the upsetting ratio increases, the plasticity index along the fiber flow to the metal and the impact toughness is increased, and the value perpendicular to the flow direction of the fiber is decreased. Besides, the mechanical properties of the metal in the rigid region in the upset body are lower than the mechanical properties of the metal in the strongly deformed region.  

Hot Deformation



The raw material for forging is ingot. The defects of the foundry structure are mainly: the internal grains are coarse and nonuniform, the structure is loose and there are bubbles, shrinkage holes , micro-cracks, chemical composition segregation and nonuniform distribution of non-metallic impurities. The hot deformation process can greatly improve the cast structure,and the coarse columnar grains become a new equiaxed fine grain structure after plastic deformation and recrystallization; the defects such as looseness, voids and micro-cracks are compacted or welded with the three-direction compressive stress,the high melting point compound is distributed in a granular or chain shape along the direction of metal deformation after it was broken and the intergranular low melting point impurities are distributed in a strip shape along the deformation direction. As a result, the plasticity of the metal is increased and the mechanical properties are improved.
   

Cooling and Heat Treatment

 

In order to achieve the mechanical properties required for forging technical conditions, the cooling after forging and heat treatment is a key process except the above factors  .

Cooling After Forging

The various internal stresses generated by the forge piece during the cooling process are the key factors that affecting the quality of the forge piece or scrapping the forge piece. The stresses mainly include residual stress caused by the deformation of various forge parts during the forging process;the tensile stress on the forge piece surface layer and the temperature stress of the compressive stress in its core region generated by the forging volume shrinks due to the different cooling rates of the surface layer and the core of the forge piece when the cooling process is conducted after the steel forging is finished and the structural stress (in the cooling process, owing to the different specific volume in different structure , the steel forge piece will cause volume increase during the cooling process). When the superposition value of the above three stresses exceeds the yield limit of the material, the axial line of the fore piece will be bent and lead to deformation. Temperature stress and structural stress can also cause cracks and white spots. Forge pieces with white spots will lead to the dramatically reduced plasticity index and cause accidents owing to the forge pieces’ sudden damage, so the forge pieces with white spots are waste products. The way to prevent white spots is to take the appropriate cooling process, a full dehydrogenization during cooling should be conducted and minimize retained austenite. In addition, improper cooling treatment can also lead to the forge pieces too hard, which makes the cutting process difficult.

We can adopt the following cooling methods under normal circumstances: direct pit cooling, furnace cooling, isothermal cooling, undulating isothermal cooling, normalizing and tempering, isothermal retraction and undulating isothermal annealing after forging.

Heat Treatment 

The purpose of heat treatment is to prevent white spots, improve the structure after forging , eliminate forging stress, adjust hardness to facilitate cutting, improve internal structure of forgings, refine grains, and prepare for final heat treatment. Firstly, the main way to prevent heat treatment from white spots is to create conditions that are most conducive to hydrogen diffusion. Since the solubility of hydrogen in steel in α-Fe is smaller than that in γ-Fe, while the diffusion rate is much higher than the latter, the heat treatment specification for dehydrogenation is to make the austenite transform rapidly and the heat can be preserved for a long time under the temperature of the fastest hydrogen diffusion. Secondly, the purpose of refining and adjusting the structure heat treatment is to subject the forge piece to one or more recrystallizations. What's more, there are many types of forging cooling and heat treatment specifications for large forge piece and it depends on the alloy content of the steel and the cross-sectional size of the forge piece.

The heat treatment of forgings mainly adopts the following methods: one is complete annealing, the other is spheroidizing annealing, the third is isothermal annealing, the fourth is normalizing or normalizing and high temperature tempering, the fifth is thermal refining of forgings.

With the heat treatment, the mechanical properties of the forge piece can be greatly improved. But not all forgings can be heat treated. The austenitic and ferritic in forge piece heat resistant stainless steels, high-temperature alloys, aluminum alloys, magnesium alloys etc..Because there are no homogeneous transformation materials in the process of heating and cooling,the structure defects in some copper alloys and titanium alloys generated in the process of forging cannot be solved by heat treatment. On the contrary, materials with isomeric transformation during heating and cooling, such as structural steel and martensitic stainless steel, some structure defects caused by improper forging process or some innate defects in raw materials can be solved mostly after heat treatment and satisfactory structure and properties can still be obtained after the final heat treatment of the forge piece.For example, in the coarse-grained and widmanstatten structure of common overheated structural steel forgings, reticulated carbides will be generated slightly due to the improper cooling of hypereutectoid steels and bearing steels.Some forge pieces have structural defects that are difficult to eliminate with normal heat treatment,but it can be eliminated by measures such as high temperature normalizing, repeated normalizing, low temperature decomposition, and high temperature diffusion annealing. However, the structural defects of some forge pieces cannot be removed by the common heat treatment process and as a result, the performance of the forge pieces is declined or even unqualified after the final heat treatment. There are also some structural defects in forge pieces that will worsen further during the final heat treatment and even cause cracking. For example,if the coarse-grained structure in alloy structural steel forgings can not be improved after forging heat treatment, it will cause martensite needle coarseness and unqualified performance after carbon, nitrogen co-infiltration and quenching. Frequently, coarse banded carbide in high-speed steel will be crack when quenching.

So, compared with other processes (casting or machining), forging is always better. CFS Forge is the professional drop forging company, we can deal with drop forging process with different materials: steel, aluminum and copper. Contact us if you have any forging source from China.

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