The factor that has a greater influence on the malleability of the metal is the shape of the metal itself, the better the plasticity, the less likely it is to crack when forged.
The plasticity of the metal is closely related to the organization of the metal, the finer the grain, the more uniform the structure, the better the plasticity.
So the malleability of the metal can be improved by refining the grain, uniforming the organization.
The metal material can change its shape without cracking during pressure processing.
It includes the ability to perform hammer forging, rolling, stretching, extrusion and other processing in the hot or cold state.
The malleability is mainly related to the chemical composition of metal materials.
Ⅰ The essence of metal
1.1 Effects of chemical composition
Metals of different chemical compositions have different malleability.
In general, pure metals have better malleability than alloys. The lower the mass fraction of carbon in carbon steel, the better the malleability.
When steel contains more carbide forming elements (chromium, tungsten, molybdenum, vanadium, etc.), its malleability is significantly reduced.
1.2 Impact of metal organizations
The malleability of metals varies greatly depending on their tissue construction.
The alloy is malleable when it is a single-phase solid-soluble tissue (e.g. Austenitic).
Whereas, metals with the metallic compound organization (e.g., carburizing bodies) have poor malleability.
The cast columnar tissue and coarse grain are not as malleability as the uniform and fine tissue after pressure processing.
Ⅱ Processing conditions
2.1 Deformation temperature
Increasing the temperature at which the metal deforms is an effective measure to improve the malleability of the metal.
During the heating process, as the temperature of the metal increases, the mobility of the metal atoms increases and the attraction between the atoms decreases, making them prone to slip.
Thus plasticity improves, deformation resistance decreases, malleability improves significantly, so forging is generally carried out at high temperatures.
2.2 Deformation speed
Deformation speed is the degree of deformation per unit time, and the effect of deformation speed on the malleability of the metal is shown in Figure 1.
As can be seen from the figure, its effect on malleability is contradictory. On the one hand, as the deformation speed increases, the recovery and recrystallization can not be carried out in time to overcome the processing hardening phenomenon, so that the plasticity of the metal decreases, deformation resistance increases, and malleability deteriorates (point a in the figure to the left).
On the other hand, in the process of deformation, some of the energy consumed in plastic deformation is converted into heat, which is equivalent to heating the metal, so that the plasticity of the metal improves, the deformation resistance decreases, and the malleability becomes better (point A in the figure to the right).
The greater the rate of deformation, the more pronounced the thermal effect.
Figure 1 Effect of deformation speed on plasticity and deformation resistance
2.3 Mode of deformation (stress state)
The stress state within the deformed metal varies depending on the deformation method.
For example, in the case of extrusion deformation, it is in a three-way compression state;
While pulling, it is in a state of being compressed in two directions and pulled in one direction;
When upsetting, the stress state of the central part of the blank is three-way compressive stress, the surrounding part up and down and radial is compressive stress, tangential is tensile stress, as shown in Figure 2.
Figure 2 Stress states for several forging methods